Banff Classification for Renal Allograft Pathology, 2019

Introduction

Since its first consensus meeting in 1991,¹ the Banff Classification Of Allograft Pathology has provided a framework for the reporting of renal allograft biopsies. It was the first classification system of its kind and answered the need for an international consensus on renal transplant biopsy reporting, providing guidance for clinical diagnosis and enabling meaningful comparison between research studies and clinical trials investigating the diagnosis, treatment and outcome in kidney transplantation. The Banff Classification has since been further strengthened by evidence-informed biannual updates elaborated during open international expert meetings.² As a result, the Banff Classification of Allograft Pathology has become the predominant classification system used worldwide.³ A total of 15 meetings reported in 10 articles reflect the developments of the Banff Classification from the first consensus meeting in 1991 to the recently published consensus after the 2017 meeting in Pittsburgh, USA.^1,4-13 Each of these iterations provides a short summary of the meeting and contributes to the classification in a cumulative fashion. The dispersal of both relevant and outdated content over 10 articles could make access to the Banff Classification difficult for beginners and experts and has created ambiguities in the past.³ Yet, accessibility and clarity are of utmost importance not only for clinical practice and research but also for the Banff Classification itself to evolve through accountability, critique, and change. To improve on these aspects, the Rules and Dissemination Banff Working Group was initiated at the last Banff meeting held in Barcelona, Spain in March 2017. With a scope beyond the helpful syllabus provided by the Banff group in the online supplement of the 2015 update¹¹ and incorporating the latest changes introduced in the 2017 update,¹² the aim of this Working Group is to collate all current content of the Banff Classification and improve its accessibility. A systematic inventory of the content is given in Figure 1.

This practical guide is based on the 2018 review, updated with the 2019 update. Since the 2019 update contained some minor errors, this on-line document supersedes and replaces the 2019 update. The content of this on-line document is divided into the following sections: a brief guide about the histopathological and serological work-up; a list of Banff Lesion Scores (previously known as components, e.g. Banff t for tubulitis) with their current definitions; practical tips for their application and illustrative figures (see definitions below, thresholds in Table 1 and all Figures); a list of Banff Diagnostic Categories in Table 2; and a list of the Additional Diagnostic Parameters in Table 2. Finally, we provide in Table 3 the Banff Diagnostic Categories and the underlying algorithms. A glossary of terms is provided in the appendix below, explaining important concepts and terminology underlying the Banff Classification. Lastly, we provide a critical appraisal of areas of the Banff Classification that require clarification and provide an outlook for future developments. All terms from the Banff Classification will be given in capitals for clarity. All abbreviations for Banff lesion scores will be given in italic typeface.

We hope this Banff 101 will serve as a handy reference for the clinicians and the pathologists. Updated content will appear online with the next update of the Banff Classification of Renal Allograft Pathology.

DIAGNOSTIC WORK-UP OF BIOPSIES

A kidney transplant biopsy should fulfil the criteria for specimen adequacy (see Glossary of Terms, in the appendix below) detailed in the Banff 1997 update.⁵ C4d staining is considered indispensable, either as immunofluorescence (IF) on fresh frozen or immunohistochemistry (IHC) on paraffin-embedded tissue. The paraffin block should be cut in several numbered level sections examined with hematoxylin-eosin, periodic acid-Schiff (PAS), trichrome-elastic and Jones or methenamine silver stains. Immunohistochemistry staining for simian virus-40, cross-reacting with BK virus is highly recommended when indicated. Where available, minute portions of cortex should be embedded for transmission electron microscopy (EM).

Depending on clinical and histopathological findings a complete nephropathological work-up including staining for immunoglobulin heavy and light chains and complement split products might be necessary to rule out or confirm a diagnosis of glomerulonephritis. Other ancillary staining might be necessary as for native kidney biopsies to establish specific recurrent or de novo kidney diseases (eg, Congo red stain).

Serological testing for donor-specific antibodies (DSAs) should be performed as described in consensus documents.¹⁴ Ancillary molecular tests, based on tissue and body fluids, are emerging.

Preimplantation biopsies should be obtained, processed, and reported as described by the Banff Working Group on Preimplantation Biopsies.¹⁵

BANFF LESION SCORES

Banff Lesion Scores assess the presence and the degree of histopathological changes in the different compartments of renal transplant biopsies, focusing primarily but not exclusively on the diagnostic features seen in rejection. These Banff Lesion Scores are not by themselves sufficient to reach the various Banff Diagnostic Categories in Table 1; the Additional Diagnostic Parameters—histopathological, molecular, serological and/or clinical—may be required to determine the diagnosis. For each Banff Lesion Score we give the current consensus definitions below. As new knowledge emerges, these might be refined for the next Banff update. A synopsis of their semiquantitative thresholds is given in Table 2. However, use of this threshold table without knowledge of the precise definitions and regulatory statutes underlying each Banff Lesion Score is strongly discouraged.

Banff Lesion Score i (Interstitial Inflammation)

This score evaluates the degree of inflammation in non-scarred areas of cortex, which is often a marker of Acute T Cell–Mediated Rejection (TCMR). As per the Banff update from 1997, areas that must not be considered for Banff Lesion Score i are “fibrotic areas, the immediate subcapsular cortex, and the adventitia around large veins and lymphatics”.⁵ As can indirectly be derived from the definition of Banff Lesion Score ti in the 2007 update of the Banff classification, nodular infiltrates, if in unscarred cortex, are also⁸ considered for Banff Lesion Score i. An asterisk shall be added to Banff Lesion Score i (e.g., i1*), “if there are more than 5% to 10% of eosinophils, neutrophils or plasma cells”.⁵ Exemplary lesions are shown in Figure 2.

i0—No inflammation or in less than 10% of unscarred cortical parenchyma.
i1—Inflammation in 10 to 25% of unscarred cortical parenchyma.
i2—Inflammation in 26 to 50% of unscarred cortical parenchyma.
i3—Inflammation in more than 50% of unscarred cortical parenchyma.¹¹

Banff Lesion Score t (Tubulitis)

This Banff Lesion Score evaluates the degree of inflammation within the epithelium of the cortical tubules. As per the Banff 2003 update “Tubulitis—the presence of mononuclear cells in the basolateral aspect of the renal tubule epithelium” is one of the defining lesion of TCMR in kidney transplants.⁶ According to Banff 1997, in tubules cut longitudinally, the score shall be determined as the number of mononuclear cells per 10 tubular epithelial cells, which is the average number of epithelial cells per tubular cross-section (Figure 3).

Tubulitis must be present in at least 2 foci. We have emphasized this by rephrasing the criteria for Banff Lesion Score t0 below; the most severely affected tubule determines the score.^5,11 Please note also that we have returned from the altered definition with “leukocytes” in the Banff 2015 update¹¹ to “mononuclear cells” as given in the 1997 update.⁵ Also revoked is the change introduced with Banff 2017 that for Acute TCMR Grade IA, IB and Chronic Active TCMR Grade IA and IB but not Borderline (Banff Diagnostic Category 3), tubulitis is considered in all but severely atrophic cortical tubules. Thus, the old rule introduced in Banff 1997 that Banff Lesion Score t must only be scored in cortical tubules with less than 50% reduction in caliber ⁵ is valid again. The reason for this reversal is that with the introduction of Banff Lesion score t-IFTA (see below) in Banff 2019¹³ this 2017 expansion of Banff Lesion Score t became obsolete and is best be avoided in order not to compromise one of the oldest and diagnostically most important Banff Lesion Scores. According to Banff 2019, lesion score t should only be scored in “preserved” areas of cortex, without interstitial fibrosis/tubular atrophy. An example of tubulitis in various stages of tubular atrophy is shown in Figure 4.

t0—No mononuclear cells in tubules or single focus of tubulitis only.
t1—Foci with 1 to 4 mononuclear cells/tubular cross section (or 10 tubular cells).
t2—Foci with 5 to 10 mononuclear cells/tubular cross section (or 10 tubular cells).
t3—Foci with >10 mononuclear cells/tubular cross section or the presence of ≥2 areas of tubular basement membrane destruction accompanied by i2/i3 inflammation and t2 elsewhere. ¹²

Banff Lesion Score v (Intimal Arteritis)

This Banff Lesion Score evaluates the presence and the degree of inflammation within the arterial intima. Arteries are defined as having at least 2 layers of smooth muscle cells in the media (Glossary of Terms, see in the appendix below). Note that intimal arteritis (also referred to as endothelialitis and endarteritis) is defined by the presence of inflammatory cells, mainly lymphocytes and monocytes, in the subendothelial space of 1 or more arteries.¹⁰ One such cell suffices. Examples of this lesion are shown in Figure 5. Intimal arteritis is a feature seen in both Acute TCMR and Active AMR. For Banff Lesion Score v, the most severely affected artery dictates the score.⁵ Similar lesions in arterioles are only coded as an asterisk behind the Banff Lesion Score ah and are disregarded for Banff Lesion Score v. Infiltrates buried deeper in the intima are not considered for the v Banff Lesion Score but have been recognized as Chronic Active TCMR since the 2005 update⁷ and graded in the 2017 update as Grade II.¹² In the presence of tubulointerstitial hemorrhage (see Glossary of Terms in the appendix below) and/or and infarct (see Glossary of Terms in the appendix below) an asterisk “*” is attached to the Banff Lesion Score v (e.g. Banff v0*, v2*).⁵

v0—No arteritis.
v1—Mild to moderate intimal arteritis in at least 1 arterial cross section.
v2—Severe intimal arteritis with at least 25% luminal area lost in at least 1 arterial cross section.
v3—Transmural arteritis and/or arterial fibrinoid change and medial smooth muscle necrosis with lymphocytic infiltrate in vessel.¹¹

Banff Lesion Score g (Glomerulitis)

This Banff Lesion Score evaluates the degree of inflammation within glomeruli (Figure 6). Glomerulitis is a form of microvascular inflammation (MVI) and is a feature of activity and antibody interaction with tissue in AMR. It can also be seen in recurrent or de novo glomerulonephritis which must be excluded by appropriate immunostains and EM.

Banff Lesion Score g is determined by the proportion of glomeruli showing glomerulitis defined as “complete or partial occlusion of 1 or more glomerular capillary by leukocyte infiltration and endothelial cell enlargement.”¹⁰ Leukocytes include polymorphonuclear cells and mononuclear cells. Both endothelial cell enlargement and leukocyte(s) must contribute to the complete or partial occlusion. Of note, glomerulitis must be scored even in glomeruli with segmental glomerulosclerosis. The denominator in this proportion is the number of non-sclerosed glomeruli in the biopsy.

g0—No glomerulitis.
g1—Segmental or global glomerulitis in less than 25% of glomeruli.
g2—Segmental or global glomerulitis in 25 to 75% of glomeruli.
g3—Segmental or global glomerulitis in more than 75% of glomeruli.¹¹

Banff Lesion Score ptc (Peritubular Capillaritis)

This Banff Lesion Score evaluates the degree of inflammation within peritubular capillaries (PTCs). Together with glomerulitis, peritubular capillaritis constitutes MVI as a feature of Active AMR or Chronic Active AMR. Peritubular capillaritis can be observed with pure Acute TCMR or Borderline as well. According to the Banff 2005 update, the Banff Lesion Score ptc is determined by the most severely involved PTC (Figure 7). Peritubular capillaries are by definition found in the cortex, their medullary equivalent are medullary vasa recta. The number of luminal inflammatory cells includes polymorphonuclear and mononuclear leukocytes, with an asterisk “*” used to indicate only mononuclear cells and absence of neutrophils. The extent of the PTC inflammation in the biopsy should be documented, either as focal (10-50% of cortical area) or diffuse (>50% of cortical area), but this does not contribute to the score. The presence of associated PTC dilatation may also be noted. Areas affected by acute pyelonephritis or necrosis and subcapsular cortex with nonspecific inflammation should not be scored. Inflammatory cells within PTCs must be distinguished from interstitial inflammation by careful examination of basement membrane stains (PAS, silver). Inflammatory cells within veins and medullary capillaries (vasa recta) should not be scored. Consequently, peritubular capillaritis and Banff Lesion Score ptc can only be assessed in the cortex after exclusion of areas of pyelonephritis and infarcted areas and exclusion of areas close to lymphoid aggregates to avoid confusion with lymphatic vessels. Banff Lesion Score ptc should not be based on longitudinally cut PTCs. Peritubular capillaries in areas affected by tubular atrophy and interstitial fibrosis must explicitly be considered for this Banff Lesion Score. Note that we have simplified the definition of ptc0 from the original version in the Banff 2017 update.¹²

ptc0—Maximum number of leukocytes <3.
ptc1—At least 1 leukocyte cell in ≥10% of cortical PTCs with 3-4 leukocytes in most severely involved PTC.
ptc2—At least 1 leukocyte in ≥10% of cortical PTC with 5-10 leukocytes in most severely involved PTC.
ptc3—At least 1 leukocyte in ≥10% of cortical PTC with >10 leukocytes in most severely involved PTC.¹¹

Banff Lesion Score C4d

This score evaluates the extent of staining for C4d on endothelial cells of PTCs and medullary vasa recta by IF on snap frozen sections of fresh tissue or IHC on formalin-fixated and paraffin-embedded tissue. Although Banff 2007 states that areas of tubular atrophy and interstitial fibrosis have reduced PTC density that could affect the extent of staining,¹⁶ scoring of C4d in such cortical areas is not excluded.⁸ Scoring of C4d staining is based on the percentage of peritubular capillaries and vasa recta that has a linear, circumferential staining pattern (Figure 8). The minimal sample for evaluation is 5 high-power fields of cortex and/or medulla without scarring or infarction. C4d must not be scored in areas of infarction. On IF, staining should be at least 1+ in intensity.⁸ Strong staining is not required for a positive reading for IHC.¹¹ In terms of extent of staining, with IF, Banff Lesion Score C4d ≥ 2 is considered positive and a criterion for antibody interaction with tissue and as equivalent to DSA (see Table 1 and SDC, Glossary of Terms in the appendix below), whereas with IHC, Banff Lesion Score C4d ≥ 1 is counted as positive already.¹¹ Note that the definition below deviates from the one provided in the Banff 2015 update,¹¹ in that it explicitly allows scoring in medullary vasa recta as originally intended, not only PTCs. The thresholds remain unchanged.

C4d0—No staining of PTC and medullary vasa recta (0%).
C4d1—Minimal C4d staining (>0 but <10% of PTC and medullary vasa recta).
C4d2—Focal C4d staining (10-50% of PTC and medullary vasa recta).
C4d3—Diffuse C4d staining (>50% of PTC and medullary vasa recta).

Banff Lesion Score ci (Interstitial Fibrosis)

This lesion score evaluates the extent of cortical fibrosis. The Banff Classification has never given a precise definition for individual areas of interstitial fibrosis (Figure 9). The reason for this is that Banff Lesion Score ci was meant to purely reflect the cortex composed of fibrous tissue, which does not necessarily correspond to areas that a pathologist would pick up as a patch of pathological tubulointerstitial fibrosis. The fraction of fibrous tissue in the cortex was considered as up to 5% for normal kidneys, hence the difference in cut-offs between ci1 and ct1.

A Working Group on this topic has produced useful reference guides (Figures 10 and 11).¹⁷

Of note, determination of Banff Lesion Score ci (as well as Banff Lesion Scores ct, ti, i-IFTA, t-IFTA) must also take into consideration the subcapsular cortex.

ci0—Interstitial fibrosis in up to 5% of cortical area.
ci1—Interstitial fibrosis in 6 to 25% of cortical area (mild interstitial fibrosis).
ci2—Interstitial fibrosis in 26 to 50% of cortical area (moderate interstitial fibrosis).
ci3—Interstitial fibrosis in >50% of cortical area (severe interstitial fibrosis).¹¹

Banff Lesion Score ct (Tubular Atrophy)

This Banff Lesion Score evaluates the extent of cortical tubular atrophy which is usually tightly associated with the areas affected with interstitial fibrosis (Figure 9). Both correlate with time posttransplantation in the setting of progressive disease of any cause. Accordingly, neither Banff Lesion Scores ct nor ci have diagnostic specificity, but both have significant correlation with allograft function and prognosis.

Historically, the Banff classification has defined tubular atrophy as reflected in the Banff Lesion Score ct in the 1995 update⁴ as tubules with a thickened basement membrane or a reduction of greater than 50% in tubular diameter. Banff Lesion Score ct is still based on this definition of tubular atrophy. The definitions of moderate and severe atrophy from the Banff 2017 update¹² are irrelevant for Banff Lesion Score ct. In the following definition, we have omitted the designation as “mild” for ct1, “moderate” for ct2 and “severe” for ct3 which was still included in the Banff 2015 update to avoid confusion between the definition of atrophy for an individual tubule as described above and the extent of tubular atrophy reflected in the Banff Lesion Score ct. Of note, Banff Lesion Score ct must be determined including the subcapsular cortex.

ct0—No tubular atrophy.
ct1—Tubular atrophy involving up to 25% of the area of cortical tubules.
ct2—Tubular atrophy involving 26 to 50% of the area of cortical tubules.
ct3—Tubular atrophy involving in >50% of the area of cortical tubules.¹¹

Banff Lesion Score cv (Vascular Fibrous Intimal Thickening)

This Banff Lesion Score reflects the extent of arterial intimal thickening in the most severely affected artery (see Definition of Terms in the appendix below), not the average of all arteries.⁵ It does not discriminate between bland arterial intimal fibrosis and fibrosis containing leukocytes (Figure 12), although the latter is more likely to reflect chronic rejection (AMR and/or Chronic Active TCMR Grade II).¹² A visual analogue scale for application in daily practice is provided in Figure 13.

cv0—No chronic vascular changes.
cv1—Vascular narrowing of up to 25% luminal area by fibrointimal thickening.
cv2—Vascular narrowing of 26 to 50% luminal area by fibrointimal thickening.
cv3—Vascular narrowing of more than 50% luminal area by fibrointimal thickening.¹¹

Banff cg Score (Glomerular Basement Membrane Double Contours)

Banff Lesion Score cg is based on the presence and extent of glomerular basement membrane (GBM) double contours or multilamination in the most severely affected glomerulus (Figure 14). Scoring should be carried out on PAS or silver stains; a designation as cg1a requires transmission EM to exclude cg0. With Banff Lesion Score cg > 0 (including both cg1a and cg1b), a diagnosis of transplant glomerulopathy (TG) (see Glossary of Terms in the appendix below) can be made, if other causes can be excluded. Banff Lesion Score cg > 0 can be a feature of Chronic AMR or Chronic Active AMR, but can also be seen in association with thrombotic microangiopathy of other causes than AMR, e.g. hepatitis C virus infection,18 hypertensive glomerulopathy,19 and glomerulonephritis. In analogy to Banff Lesion Score g, even in the presence of an explanation other than rejection for GBM double contours, Banff Lesion Score cg shall still be applied. Of note, Banff Lesion Score cg is not scored in ischemic or segmentally sclerosed glomeruli.1,11 Late ischemic glomerulopathy is defined as “thickening, wrinkling and collapse of glomerular capillary walls associated with or extracapillary fibrotic material”.1 As stated above, the earliest lesion of TG (cg1a) requires transmission EM for diagnosis. To detect such lesions, it is recommended that at centers with EM capability, “ultrastructural studies should be performed in all biopsies from patients who are sensitized, have documented DSA at any time post-transplantation and/or who have had a prior biopsy showing C4d staining, glomerulitis and/or peritubular capillaritis”. It is also advised that EM be considered in all biopsies performed from 6 months post-transplantation onward and in for-cause biopsies done from 3 months post-transplantation onward to determine if early changes of TG are present, prompting testing for DSA.10 Electron microscopy is also recommended for any biopsy done for the indication of increasing or new onset proteinuria.

cg0—No GBM double contours by light microscopy (LM) or EM.
cg1a—No GBM double contours by LM but GBM double contours (incomplete or circumferential) in at least 3 glomerular capillaries by EM, with associated endothelial swelling and/or subendothelial electron-lucent widening.
cg1b—Double contours of the GBM in 1-25% of capillary loops in the most affected nonsclerotic glomerulus by LM; EM confirmation is recommended if EM is available.
cg2—Double contours affecting 26 to 50% of peripheral capillary loops in the most affected glomerulus.
cg3—Double contours affecting more than 50% of peripheral capillary loops in the most affected-glomerulus.¹¹

Banff Lesion Score mm (Mesangial Matrix Expansion)

This score evaluates the percentage of glomeruli with “moderate mesangial matrix expansion” in relation to all non-sclerosed glomeruli. Banff 1997 defines moderate mesangial matrix increase as “expansion of the matrix in the mesangial interspace to exceed the width of 2 mesangial cells in the average in at least 2 glomerular lobules”.⁵ An example is shown in Figure 15. Banff Lesion Score mm is currently not used to reach a Diagnostic Category and is purely descriptive.

mm0—No more than mild mesangial matrix increase in any glomerulus.
mm1—At least moderate mesangial matrix increase in up to 25% of nonsclerotic glomeruli.
mm2—At least moderate mesangial matrix increase in 26% to 50% of nonsclerotic glomeruli.
mm3—At least moderate mesangial matrix increase in >50% of nonsclerotic glomeruli.¹¹

Banff Lesion Score ah (Arteriolar Hyalinosis)

This score evaluates the extent of arteriolar hyalinosis (Figure 16). The first edition of the Banff Classification defined ah as “nodular hyaline afferent arteriolar thickening suggestive of cyclosporine toxicity”; however, in Banff 1997 and later updates, Banff Lesion Score ah is defined simply as PAS-positive arteriolar hyaline thickening, as a finding of “uncertain significance”. An asterisk “*” is added to the ah score when arteriolitis is present (e.g. ah0*, ah2*).⁵ Banff Lesion Score ah is currently not used to reach a diagnostic category and is purely descriptive.

ah0—No PAS (PAS)-positive hyaline arteriolar thickening.
ah1—Mild to moderate PAS-positive hyaline thickening in at least 1 arteriole.
ah2—Moderate to severe PAS-positive hyaline thickening in more than 1 arteriole.
ah3—Severe PAS-positive hyaline thickening in many arterioles.¹¹

Banff Lesion Score aah (Hyaline Arteriolar Thickening)

This Banff Lesion Score provides an alternative way of quantifying arteriolar hyalinosis. It was proposed in the 2007 update, because of the insufficient reproducibility of the Banff Lesion Score ah.⁸ This alternative tries to reach better reproducibility by focusing on circumferential or non-circumferential hyalinosis and the number of involved arterioles. Still, this lesion cannot be considered specific, that is, diagnostic for calcineurin inhibitor-related arteriolopathy. The use of this Banff Lesion Score aah has been left as optional since its introduction in 2007, no final decision has been reached whether it shall replace Banff Lesion Score ah. Banff Lesion Score aah is currently not used to reach a diagnostic category and is purely descriptive.

aah0—No typical lesions of calcineurin inhibitor-related arteriolopathy.
aah1—Replacement of degenerated smooth muscle cells by hyaline deposits in only 1 arteriole, without circumferential involvement.
aah2—Replacement of degenerated smooth muscle cells by hyaline deposits in more than 1 arteriole, without circumferential involvement.
aah3—Replacement of degenerated smooth muscle cells by hyaline deposits with circumferential involvement, independent of the number of arterioles involved.¹¹

Banff Lesion Score ti (Total Inflammation)

This lesion score evaluates the extent of total cortical inflammation. According to the Banff 2007 update and in contrast to the Banff Lesion Score i, all of the cortical parenchyma, including areas of interstitial fibrosis and tubular atrophy (IFTA), subcapsular cortex and perivascular cortex including nodular infiltrates are considered for ti scoring.⁸ Mengel et al. found Banff Lesion Score ti to be better predictive of poor graft outcomes than the Banff Lesion Score i in cases where at least mild IFTA was present.¹⁸ The association between interstitial inflammation in areas of IFTA as reflected in Banff Lesion Score i-IFTA and decreased graft survival was noted by Mannon et al.¹⁹ and subsequently confirmed by others.^20,21 As a consequence, Banff Lesion Score ti became part of the criteria for a diagnosis of Chronic Active TCMR Grade IA and IB;¹² both Banff Lesion Scores ti and i-IFTA must be at least 2 to consider a diagnosis of Chronic Active TCMR Grade IA or IB.¹²

ti0— No or trivial interstitial inflammation (<10% of total cortical parenchyma).
ti1— 10-25% of total cortical parenchyma inflamed.
ti2— 26-50% of total cortical parenchyma inflamed.
ti3— >50% of total cortical parenchyma inflamed.¹¹

Banff Lesion Score i-IFTA (Inflammation in Area of IFTA)

This score evaluates the extent of inflammation in scarred cortex, i.e. areas that qualify for Banff Lesion Scores ci and ct (Figure 17). The Banff Lesion Score i-IFTA was first introduced to the Banff Classification in 2015.¹¹ Both Banff Lesion Scores ti and i-IFTA must be at least 2 to consider a diagnosis of Chronic Active TCMR Grade IA or IB.¹²

i-IFTA0—No inflammation or less than 10% of scarred cortical parenchyma.
i-IFTA1—Inflammation in 10% to 25% of scarred cortical parenchyma.
i-IFTA2—Inflammation in 26% to 50% of scarred cortical parenchyma.
i-IFTA3—Inflammation in >50% of scarred cortical parenchyma.¹¹

Banff Lesion Score t-IFTA

This Banff Lesion Score was fully introduced in Banff 2019.¹³ It refers to scoring of tubulitis in areas of IFTA. t-IFTA is graded according to presence of mononuclear cell infiltrates in the moderately but not severely atrophic cortical tubules in areas of IFTA. An example is given in Figure 4. Moderately atrophic cortical tubules are those that show a reduction in diameter of more than 50% but are not severely atrophic. Severely atrophic tubules are defined as tubules of diameter <25% of that of unaffected or minimally affected tubules in the biopsy, often with an undifferentiated- appearing, cuboidal or flattened epithelium (or in some cases even loss of epithelium with denudation of the tubular basement membrane), and pronounced wrinkling and/or thickening of the tubular basement membrane. This definition of severely atrophic tubules also includes very small, endocrine-like tubules with very narrow lumens, although the basement membranes of the latter may not be thickened.¹²

Of note, Banff Lesion Score t-IFTA must be determined including the subcapsular cortex.

t-IFTA0 –  no mononuclear cells in tubules
t-IFTA1 – 1-4 mononuclear cells/tubular cross section
t-IFTA2 – 5-10 mononuclear cells/tubular cross section
t-IFTA3 –  >10 mononuclear cells/tubular cross section

Banff Lesion Score pvl

Developed in the report of the Banff Working Group on Polyomavirus,²² this Banff Lesion Score was formally introduced into the Banff Classification in the 2019 update.¹³ Banff Lesion Score pvl is determined over the entire biopsy sample (cortex, medulla, scarred or unscarred). Tubular epithelial nuclei are considered positive (viral replication) with the typical viral inclusions and/or positive immunohistology (usually SV-40 large T antigen). Note that pvl is not determined by the ratio of positive nuclei to all nuclei but by the ratio of tubules/ducts with at least one positive nucleus to all tubules/ducts. Banff Lesion Scores pvl and ci yield the Class of Polyomavirus Nephropathy 1 (pvl1 and ci≤1), 2 (all other combinations of pvl and ci not qualifying for class 1 or 3) or 3 (pvl3 and ci≥2).²² Polyomavirus Nephropathy is formally recognized among the Category 6 diagnoses.

pvl0 – no positive nuclei in any tubules/ducts
pvl1 – ≤1% of all tubules/ducts
pvl2 – >1% to ≤10% of all tubules/ducts
pvl3 – >10% of all tubules/ducts

Table 1: This is a synopsis of the thresholds for all Banff Lesion Scores. The user of this table should be familiar with the exact definitions underlying each individual Banff Lesion Score. Reliance on these thresholds alone without consideration of the regulatory statutes behind these scores is strongly discouraged. Abbreviations: EM: electron microscopy, LM: light microscopy, max: maximum, PTC: peritubular capillary

Additional Diagnostic Parameters

The only Banff Additional Diagnostic Parameter with a proper and explicit definition so far is Severe Peritubular Capillary Basement Membrane Multilayering (ptcml). It was defined in the Banff 2013 update as 1 PTC with ≥7 layers + 2 PTC with ≥5 layers.²³ The Banff Electron Microscopy Working Group met at the 2019 Banff meeting and discussed whether or not to introduce a scoring system. As the members of the group used different thresholds based on their individual review of the evidence published so far, a consensus opinion was reached not to include a scoring system until further evidence was available. It was also agreed that, as a minimum, the report should state the number of PTC examined; the number of basement membrane layers in the 3 most affected PTC; and whether the Banff 2013 threshold for chronic ABMR (1 PTC with ≥7 layers + 2 PTC with ≥5 layers) is met.

Severe Peritubular Capillary Basement Membrane Multilayering is considered a Histologic Feature of AMR Chronicity for a diagnosis of Chronic AMR or Chronic Active AMR.

Table 2: These Additional Diagnostic Parameters, some histopathologic, some clinical, are derived from the diagnostic algorithms in Table 1. Depending on the constellation of findings they may be required in addition to the Banff Lesion Score to determine the Banff Diagnostic Categories.

BANFF DIAGNOSTIC CATEGORIES

Table 3 presents the Banff Diagnostic Categories and is based on the original table of the most recent Banff update from 2017,¹² incorporating retained changes from 2019.¹³ The Banff 2019 update suggests to diagnose both Borderline and Active TCMR diagnoses together with the Chronic Active TCMR diagnosis could “improve… reporting“.¹³ Readers should stay alert to future updates on the Banff Foundation website (www.banfffoundation. org) informed by updates to the Banff Classification from 2019 onward.

Table 3: Banff Diagnostic Categories form the core of the Banff Classification of Renal Allograft Pathology. We refer to the Banff Lesion Scores in the main body of this review as well as to the Additional Diagnostic Parameters listed in Table 3. Note that diagnoses from various Banff Diagnostic Categories can coexist in a given biopsy, e.g. Acute TCMR Grade IB, Chronic Active ABMR, moderate Interstitial Fibrosis and Tubular Atrophy and Calcineurin Inhibitor Toxicity. From each Banff Diagnostic Category except for 6, only one diagnosis must be made. Banff also suggests that giving both Borderline (Cat. 3) and Acute TCMR diagnoses together with Chronic Active TCMR diagnoses from Cat. 4 could improve reporting.

CRITICAL APPRAISAL

Since 1991, the Banff classification has undergone several amendments, reflecting the growing body of knowledge in transplant pathology. These amendments have been based on a consensus reached at the biannual Banff meetings. This constant refinement based on emerging data is a strength of the Banff process and has led to the worldwide dominance of the Banff Classification for diagnostic practice, research and clinical trials. However, the iterative fashion in which the definitions and rules were published has dispersed the relevant content and created ambiguities. This has led to the creation of the Banff Rules and Dissemination Working Group in the aftermath of the Banff Meeting in Barcelona in March 2017. The aim of the Working group is not to alter the content of the Banff Classification. Rather, it shall collate all relevant Banff content in a central repository under the auspices of the Banff Foundation for Allograft Pathology, with a single updatable content, similar to the Union for International Cancer Control’s TNM Classification. Changes in the content of the Banff Classification must only be made through review of evidence and expert consensus at the Banff meetings or within the relevant other Working Groups. Like the collation of content above, the following critical appraisal is based on this mission and does not touch on the content of the Banff Classification itself.

Although the Banff Lesion Scores required for a diagnosis of AMR have recently undergone a partial overhaul¹⁰ and although a dedicated Working Group is reexamining the Banff Lesion Scores for TCMR, no or little effort has been devoted to the Additional Diagnostic Parameters in Table 3. For example, “Acute Tubular Injury In The Absence Of Any Other Cause” as a criterion for active AMR is as important as Banff Lesion Scores v, g or ptc,¹² yet this feature is still imperfectly defined, the last definition dating back to the 1995 update.⁴ Another example is “Infection,” which precludes the use of Banff Lesion Score ptc alone as a criterion for AMR.¹² Use of the isolated term “infection” is ambiguous in the context of whether inflammation in the transplant should be considered as evidence for rejection or not. We would recommend treating these Additional Diagnostic Parameters like the Banff Lesion Scores, presenting them in clear and consistent wording, and, whenever necessary, by providing guidance through meaningful definitions elaborated over time through Working Groups and in alignment with the respective diagnostic criteria applied.

Among the Banff Lesion Scores, the Banff Lesion Score cv has a confusing array of terminologies, appearances and diagnostic implications. “Arterial fibrointimal thickening” or “vascular fibrous intimal thickening” imply a chronic fibrous change, whereas arterial intimal thickening can be cellular and nonfibrous in “transplant vasculopathy” or “chronic allograft arteriopathy”. As a manifestation of chronic TCMR, it is defined as “arterial intimal fibrosis with mononuclear cell infiltration in fibrosis, formation of neointima¹² whereas, as a criterion for AMR chronicity, it is defined as “arterial intimal fibrosis of new onset, excluding other causes; leukocytes within the sclerotic intima favor chronic AMR if there is no prior history of biopsy-proven TCMR with arterial involvement but are not required”.¹² In clinical practice, it might not always be possible to exclude prior TCMR or to precisely diagnose “Arterial intimal fibrosis of new onset” as a criterion for AMR chronicity.¹² A related problem is attached to Banff Lesion Score cg: “evidence of chronic thrombotic microangiopathy (TMA)” excludes the use of Banff Lesion Score cg > 0 as a criterion for AMR chronicity, whereas Active AMR can be diagnosed with TMA, as long as it is “in the absence of any other cause [than AMR]”. Because Active AMR causing TMA can lead to glomerular lesion qualifying as TG, it would make sense to change the cg criterion to only exclude chronic TMA of any other cause than AMR.

The use of asterisks (“*”) attached to Banff Lesion Scores v, i, ah and ptc^5,7is problematic and widely neglected. Their reproducibility and diagnostic value are unknown, and they are ambiguous: an asterisk behind the Banff Lesion Score ptc signifies only mononuclear cells and absence of neutrophils, whereas the asterisk behind Banff Lesion Score i denotes a significant neutrophilic, eosinophilic or plasmacellular component in the infiltrate, and these different cell types can have widely differing implications. We suggest the Banff community should reassess these modifiers, either by improving their definitions and assigning them a significance or by abandoning them.

Inevitably, the Banff Classification has focused mainly on features of rejection, but with Banff Lesion Scores developed for other features with little or no guidance on their contribution to diagnosis. An example for this is Banff Lesion Score aah, originally intended to replace the poorly reproducible Banff lesion score ah.⁷ However, its use is still optional, and it has neither been widely adopted nor used in any of the Banff Diagnostic Categories. The Banff community should reassess arteriolar hyalinosis lesion scores, and clarify grading and diagnostic implications.

Regarding the Banff Diagnostic Categories, a clear diagnostic pathway should be recommended when dealing with Borderline or Acute TCMR (Banff Diagnostic Categories 3 and 4) in the presence of Polyomavirus Nephropathy, Pyelonephritis or other infectious diseases of the transplant, as well as AMR with glomerulitis in the presence of recurrent or de novo glomerulonephritis. These issues could be referred to the Banff TCMR and Glomerulonephritis Working Group respectively. The definition of Banff Borderline with regards to the Banff Lesion Score i threshold (i0 or i1) is still ambiguous¹¹ but should be resolved by the TCMR Working Group.

There are uncertainties around the application of transmission EM in the diagnosis of AMR which are currently being addressed by the Electron Microscopy Working Group. These issues include precise guidelines for indications and methods for application of EM in transplant biopsies; perhaps also the introduction of a new Banff Lesion Score for multilamination of the basement membranes of peritubular capillaries which we have covered as an Additional Diagnostic Parameter for now.

Another critical issue is related to the molecular diagnostics of AMR and TCMR. Although the current Banff classification endorses the use of molecular diagnostics in the definition of AMR, there is limited guidance regarding methods and diagnostic cut-offs, which could be elaborated by the Molecular Working Group. Lastly, the introduction of the new diagnostic categories of Chronic Active TCMR is likely to undergo changes informed by the TCMR Working Group. Before Banff 2017, there were no specific criteria for Chronic Active TCMR outside of arteries, and tubulitis was only scored in non-atrophic and mildly atrophic tubules, effectively excluding moderately and severely atrophic tubules. To avoid having two separate criteria for Banff Lesion Score t in Acute versus Chronic Active TCMR, it was decided that for both diagnoses tubulitis would be scored in all tubules except severely atrophic tubules. The difference between Banff 2017 and previous versions of the classification with respect to Acute TCMR is that tubulitis in moderately atrophic tubules is now counted toward Banff Lesion Score t. Because the latter was done for clarity and to avoid confusion rather than on the basis of specific evidence, it would be beneficial that future studies be done to address the most clinically relevant threshold for the level of atrophy permitted in scoreable tubules, especially for diagnosis of Acute TCMR. In addition, the 2017 changes to the TCMR criteria also suggest future work be aimed at examining the response of Chronic Active TCMR to steroids and other anti–T cell therapies (e.g. thymoglobulin), determining if there are differences in this response between: (1) Grade IA versus Grade IB Chronic Active TCMR and (2) biopsies with Chronic Active TCMR that would otherwise meet criteria for acute TCMR (i.e. with Banff Lesion Score i ≥ 2) and those that would not (with Banff Lesion Score i ≤ 1). The alignment of diagnoses from the spectrum of Acute TCMR with those from the spectrum of Chronic Active TCMR of different compartments could be problematic. For example, a biopsy with Banff Lesion Score v1 fulfilling also the criteria for chronic active TCMR grade IB would be diagnosed as the latter only,¹² as according to Banff 2017, a diagnosis of Chronic Active TCMR precludes the diagnosis even of higher grade Acute TCMR. In such cases, however, the use of modifying text independent from Banff diagnostic categories should be considered (e.g. Acute TCMR Grade II with a chronic active tubulointerstitial component; Acute TCMR Grade II with isolated intimal arteritis [isolated v]).

PROSPECTS

Going forward, this web resource will serve as the continuously updated go-to resource for the relevant Banff content, with the regular Banff update manuscripts providing the detailed reasoning for changes. Depending on the progress in the definitions and diagnostic rule sets we are aiming to develop web-based resources such as diagnostic algorithms to further strengthen standardization and reproducibility of the Banff Classification for clinical practice and research. It should be emphasized that the Banff Classification of Kidney Allograft Pathology does not cover all relevant aspects of transplantation medicine. Allograft transplantation only reaches 10% of patients needing new organs. Through regenerative medicine and tissue engineering and other optimizing initiatives we will eventually be able to provide organs to everyone in need. For this, we will need a new Banff Classification of Tissue Engineering Pathology^24,25 reflecting the new challenges of delivering the right cells to the right places in a bioengineered organ and having them function normally. Rejection will no longer be the primary threat in bioengineered organs. For a decade or more the new Banff Classification of Tissue Engineering Pathology will be used concurrently with the existing Banff Classification of Allograft Pathology. Getting the right cells in the right places sounds simple, but in fact, we have poor knowledge of what all the normal cell types in transplanted organs are. For instance, in the kidney, we have traditionally taught that there are 26 cell types,²⁶ but in fact, high throughput single cell analysis in the Human Cell Atlas Project shows many more than that and can determine not only cell identity but also lineage and activation state.^27-29 The transplantation and transplantation pathology community needs to embrace Human Cell Atlas technology, so we are not blindsided by this new technology. The scale of the likely impact of the Human Cell Atlas Project on nephrology and transplantation is currently being analyzed.³⁰

ACKNOWLEDGEMENTS

The authors would like to acknowledge the help in the preparation of the visual analog scales from Christopher Bellamy, Alton “Brad” Farris and Daniel Serón as well as the web design by Devansh Bhatt.

REFERENCES

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3           Becker, J. U., Chang, A., Nickeleit, V., Randhawa, P. & Roufosse, C. Banff borderline changes suspicious for acute T-cell mediated rejection: where do we stand? American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons, doi:10.1111/ajt.13784 (2016).

4           Solez, K. et al. Report of the Third Banff Conference on Allograft Pathology (July 20-24, 1995) on classification and lesion scoring in renal allograft pathology. Transplantation proceedings 28, 441-444 (1996).

5           Racusen, L. C. et al. The Banff 97 working classification of renal allograft pathology. Kidney international 55, 713-723 (1999).

6           Racusen, L. C., Halloran, P. F. & Solez, K. Banff 2003 meeting report: new diagnostic insights and standards. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 4, 1562-1566 (2004).

7           Solez, K. et al. Banff ’05 Meeting Report: differential diagnosis of chronic allograft injury and elimination of chronic allograft nephropathy (‘CAN’). American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 7, 518-526, doi:AJT1688 [pii] 10.1111/j.1600-6143.2006.01688.x (2007).

8           Solez, K. et al. Banff 07 classification of renal allograft pathology: updates and future directions. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 8, 753-760, doi:AJT2159 [pii] 10.1111/j.1600-6143.2008.02159.x (2008).

9           Mengel, M. et al. Banff 2011 Meeting Report: New Concepts in Antibody-Mediated Rejection. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 12, 563-570, doi:10.1111/j.1600-6143.2011.03926.x (2012).

10         Haas, M. et al. Banff 2013 meeting report: inclusion of c4d-negative antibody-mediated rejection and antibody-associated arterial lesions. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 14, 272-283, doi:10.1111/ajt.12590 (2014).

11         Loupy, A. et al. The Banff 2015 Kidney meeting report: Current challenges in rejection classification and prospects for adopting molecular pathology. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 17, 28-41, doi:10.1111/ajt.14107 (2017).

12         Haas, M. et al. The Banff 2017 Kidney Meeting Report: Revised Diagnostic Criteria for Chronic Active T Cell-Mediated Rejection, Antibody-Mediated Rejection, and Prospects for Integrative Endpoints for Next-Generation Clinical Trials. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons, doi:10.1111/ajt.14625 (2017).

13         Loupy, A. et al. The Banff 2019 Kidney Meeting Report (I): Updates on and clarification of criteria for T cell- and antibody-mediated rejection. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons, doi:10.1111/ajt.15898 (2020).

14         Gebel, H. M., Liwski, R. S. & Bray, R. A. Technical aspects of HLA antibody testing. Current opinion in organ transplantation 18, 455-462, doi:10.1097/MOT.0b013e32836361f1 (2013).

15         Liapis, H. et al. Banff Histopathological Consensus Criteria for Preimplantation Kidney Biopsies. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 17, 140-150, doi:10.1111/ajt.13929 (2017).

16         Ishii, Y. et al. Loss of peritubular capillaries in the development of chronic allograft nephropathy. Transplantation proceedings 37, 981-983, doi:S0041-1345(04)01742-7 [pii] 10.1016/j.transproceed.2004.12.284 [doi] (2005).

17         Farris, A. B. et al. Banff fibrosis study: multicenter visual assessment and computerized analysis of interstitial fibrosis in kidney biopsies. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 14, 897-907, doi:10.1111/ajt.12641 (2014).

18         Mengel, M. et al. Scoring total inflammation is superior to the current Banff inflammation score in predicting outcome and the degree of molecular disturbance in renal allografts. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 9, 1859-1867, doi:10.1111/j.1600-6143.2009.02727.x (2009).

19         Mannon, R. B. et al. Inflammation in areas of tubular atrophy in kidney allograft biopsies: a potent predictor of allograft failure. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 10, 2066-2073, doi:10.1111/j.1600-6143.2010.03240.x (2010).

20         Lefaucheur, C. et al. T cell-mediated rejection is a major determinant of inflammation in scarred areas in kidney allografts. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 18, 377-390, doi:10.1111/ajt.14565 (2018).

21         Nankivell, B. J. et al. The causes, significance and consequences of inflammatory fibrosis in kidney transplantation: The Banff i-IFTA lesion. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 18, 364-376, doi:10.1111/ajt.14609 (2018).

22         Nickeleit, V. et al. The Banff Working Group Classification of Definitive Polyomavirus Nephropathy: Morphologic Definitions and Clinical Correlations. Journal of the American Society of Nephrology : JASN 29, 680-693, doi:10.1681/ASN.2017050477 (2018).

23         Roufosse, C. et al. A 2018 Reference Guide to the Banff Classification of Renal Allograft Pathology. Transplantation, doi:10.1097/TP.0000000000002366 (2018).

24         Solez, K. et al. The bridge between transplantation and regenerative medicine: Beginning a new Banff classification of tissue engineering pathology. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons, doi:10.1111/ajt.14610 (2017).

25         Solez, K. Kim Solez, Edmonton, Alberta, Canada Banff: A Unique Start Setting Standards for Consensus Conferences. Transplantation 101, 2264-2266, doi:10.1097/TP.0000000000001901 (2017).

26         Al-Awqati, Q. & Oliver, J. A. Stem cells in the kidney. Kidney international 61, 387-395, doi:10.1046/j.1523-1755.2002.00164.x (2002).

27         Regev, A. et al. The Human Cell Atlas. Elife 6, doi:10.7554/eLife.27041 (2017).

28         Stubbington, M. J. T., Rozenblatt-Rosen, O., Regev, A. & Teichmann, S. A. Single-cell transcriptomics to explore the immune system in health and disease. Science 358, 58-63, doi:10.1126/science.aan6828 (2017).

29         Rozenblatt-Rosen, O., Stubbington, M. J. T., Regev, A. & Teichmann, S. A. The Human Cell Atlas: from vision to reality. Nature 550, 451-453, doi:10.1038/550451a (2017).

30         Moghe, I., Loupy, A. & Solez, K. The Human Cell Atlas Project by the numbers: Relationship to the Banff Classification. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 18, 1830, doi:10.1111/ajt.14757 (2018).

31         Regele, H. et al. Capillary deposition of complement split product C4d in renal allografts is associated with basement membrane injury in peritubular and glomerular capillaries: a contribution of humoral immunity to chronic allograft rejection. Journal of the American Society of Nephrology : JASN 13, 2371-2380 (2002).

32         Bonsib, S. M. in Heptinstall’s Pathology of the Kidney Vol. 1  (eds C. J. Jennette, F. G. Silva, J. L. Olson, & V. D. D’Agati) Ch. 1, 1-66 (Lippincot Williams & Wilkins, 2015).

33         Tait, B. D. et al. Consensus guidelines on the testing and clinical management issues associated with HLA and non-HLA antibodies in transplantation. Transplantation 95, 19-47, doi:10.1097/TP.0b013e31827a19cc (2013).

34         Noris, M. & Remuzzi, G. Thrombotic microangiopathy after kidney transplantation. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons 10, 1517-1523, doi:10.1111/j.1600-6143.2010.03156.x (2010).

35         Hill, G. S. et al. Donor-specific antibodies accelerate arteriosclerosis after kidney transplantation. Journal of the American Society of Nephrology : JASN 22, 975-983, doi:10.1681/ASN.2010070777 (2011).

Glossary of Terms

Acute Tubular Injury

There is no current definition of acute tubular injury endorsed by the Banff classification. Acute Tubular Injury In the Absence Of Any Other Apparent Cause is included as one of the criteria for histological evidence of acute tissue injury in the diagnosis of Active AMR.¹² Moreover, Acute Tubular Injury without other specification is included as a diagnosis in Banff Diagnostic Category 6.

Antibody Mediated Rejection (AMR)

Antibody-mediated rejection (AMR) refers to a rejection process believed to be primarily driven by antibodies against graft epitopes. Diagnostic criteria are listed under Banff Diagnostic Category 2 of the 2017 Banff update.¹² Diagnostic subcategories within include the following; C4d Staining Without Evidence of Rejection, Active AMR, Chronic Active AMR, Chronic AMR. AMR can coexist with additional diagnoses from Banff Diagnostic Categories 3-6.

Adequacy of Specimen

Since Banff 1997 a biopsy has been considered adequate if it contains at least 10 or more glomeruli and at least 2 arteries; the threshold for a “minimal sample” is 7 glomeruli and 1 artery.⁵ It is also recommended that at least 2 separate cores containing cortex be obtained or that there be 2 separate areas of cortex in the same core.¹¹ In the recent consensus manuscript on polyomavirus nephropathy for the determination of Banff Score pvl adequacy requires medulla in the biopsy in addition to the two cores.²²

Arteriole

Derived from the definition of arteries below, the definition of arterioles is consequently arterial vessels having less than two smooth muscle layers.¹¹Changes in arterioles are currently not recognised as contributory to Banff Lesion Score v or cv.¹² Arteriolar inflammation is noted with an asterisk behind Banff Lesion Score ah, e.g. ah2*.⁵

Arteritis, Intimal

Arteritis is synonymous with endarteritis or arterial endothelialitis. According to Banff 2015, intimal arteritis as per Banff Lesion Score v1, v2 is defined as mononuclear cell infiltration beneath the arterial endothelium. Severe arteritis v3 is defined by inflammation in the media and/or fibrinoid necrosis of the vessel wall; the total number of arteries in the biopsy and the number of arteries affected should be noted.¹¹ Marginated leukocytes alone are insufficient to diagnose arteritis.

Artery

Since Banff 2013⁴ an artery has been defined by “having a continuous media with two or more smooth muscle layers”.¹¹

Borderline Changes

This refers to Banff Diagnostic Category 3 “Suspicious (Borderline) for Acute TCMR”.¹² This can be used in conjunction with other Banff categories with the exception of Category 1 (Normal Biopsy Or Nonspecific Changes).

C4d

Complement component 4 fragment d (C4d) is a degradation product of complement component C4, which can bind covalently to the endothelial cell surface.³¹ Positive immunostaining indicates local complement activation and is scored with Banff Lesion Score C4d.¹¹ The clinical significance of these findings is different in grafts exposed to anti-blood group antibodies (ABO-incompatible allografts), in which isoagglutinin-mediated complement activation does not appear to be injurious to the graft and may represent accommodation. However, when caused by donor-specific antibodies against HLA or other antigens, it may indicate AMR activity. Based on the high specificity of C4d positivity, it is considered equivalent to the serological proof of donor-specific antibody (DSA) for a diagnosis of AMR, although it is still recommended to test for DSA.

Banff Diagnostic Categories

This refers to the six consensus-based and empirically validated Banff Diagnostic Categories (1 to 6) for renal allograft biopsies.¹² Any diagnosis from Categories 2-6 excludes Banff Diagnostic Category 1 (Normal Biopsy or Nonspecific changes). Multiple diagnoses from a single Banff Diagnostic Category can only be made from Category 4 and 6.

Banff Lesion Scores

Banff Lesion Scores are descriptive, consensus-based, standardised terms to grade important active and chronic histopathological findings in the different morphological compartments of the renal transplant. Although they are an integral part of the minimal dataset, they are by themselves not necessarily sufficient to determine Banff Diagnostic Categories.

Chronic Allograft Arteriopathy

Chronic Allograft Arteriopathy is a feature of Chronic Active TCMR and Chronic Active AMR. It is defined as arterial intimal fibrosis with mononuclear cell infiltration in fibrosis and/or formation of neointima. The Banff Lesion Score cv is used to grade it, based on the extent of luminal occlusion in the most severely affected artery. Lack of prior biopsy-proven TCMR favours a diagnosis of Chronic Active AMR.¹²

Cortex

The outer part of the renal parenchyma, beneath the capsule and peripheral from the medulla, including the columns of Bertin. It is characterised by the presence of glomeruli and arteries.³²

Donor-specific Antibody (DSA)

Circulating antibodies directed against donor human leukocyte antigen (HLA) class I and/or class II and/or non-HLA antigens. For the definition and technical details see the recent and forthcoming consensus recommendations by The Transplantation Society.³³ DSA are relevant for Banff Diagnostic Category 2 (AMR) diagnoses. For Active and Chronic Active AMR only DSA present at the approximate time of biopsy and against the current transplant are considered positive for the diagnosis; for Chronic AMR DSA against the current transplant present at or prior to the biopsy are considered.¹²

Fibrointimal Thickening, Arterial See ->Transplant Arteriopathy.

Gene Transcripts/Classifiers in the Biopsy Tissue strongly associated with AMR

Increased expression of these, if thoroughly validated, are the only molecular markers considered in the Banff classification. Since Banff 2013¹⁰ they have been included in Banff Diagnostic Category 2 for the diagnosis of Active AMR and Chronic Active AMR.¹¹ Along with C4d positivity, they are considered equivalent to positive test for DSA, although serological testing for DSA is still encouraged.¹²

Glomerulonephritis

Recurrent or de novo glomerulonephritis can occur in renal transplant biopsies. The diagnosis is made by a combination of conventional histology, immunohistochemistry or immunofluorescence, electron microscopy and correlation with clinical data. It should be diagnosed under Banff Diagnostic Category 6, and can be used in combination with categories 2-5 and any other diagnosis from Category 6. The presence of glomerulonephritis precludes the use of Banff Lesion Score g as histologic evidence of acute tissue injury for the diagnosis of Active AMR and of Banff Lesion Score cg as evidence of chronic tissue injury in the diagnosis of Chronic Active or Chronic AMR.¹²

Haemorrhage, Tubulointerstitial

Extravasation of red blood cells into the tubulointerstitium. There is no special Banff Lesion Score for this lesion. Instead, the lesion is noted as an asterisk “*” attached to Banff Lesion Score v (e.g. v0* or v3*). It shares this coding with Infarct.⁵

Infarct

Zone of tissue which is non-viable due to insufficient blood supply. It qualifies as “necrosis” listed in the supplement of Banff 2015 and since Banff 1997⁵ has been coded with an asterisk (*) behind the Banff Lesion Score v (e.g. v1*, v2*) accordingly.¹¹ Infarcted areas must not be scored for Banff Lesion Score C4d.⁸

Infection

In the presence of Infection (Table 5, Banff 2017) Banff Lesion Score g>0 greater is required when using moderate microvascular inflammation (sum of Banff g+ptc≥2) as a criterion from Criteria Group 3 (antibody interaction with tissue) for a diagnosis of Active AMR or Chronic Active AMR.¹² I.e. Banff Lesion Score ptc2 with Banff Lesion Score g0 is insufficient for this criterion when disease caused by infection of the transplant is present.

Interstitial Fibrosis

Banff Lesion Score ci for interstitial fibrosis was initially meant to describe the area fraction of fibrous tissue in the cortex only regardless of whether it represented fibrous tissue as the normal constituent of the renal cortex (considered to be 5%) or fibrous tissue within a cortical scar. Hence, a precise definition for individual patches of interstitial fibrosis in a scar has never been given. This purely morphometric approach runs counterintuitive to routine histopathological assessment for atrophic tubules separated by interstitial fibrosis in a tubulointerstitial scar. Guides for the assessment of Banff Lesion Score ci can be found in the main body of this manuscript.

Microvascular Inflammation, Moderate

Microvascular inflammation is scored by calculating the sum of the two Banff Lesion Scores g and ptc. Microvascular inflammation is considered moderate with [Banff Lesion Scores g + ptc] ≥2. In the presence of Acute TCMR, Borderline Changes or Infection such as Pyelonephritis, Banff Lesion Score g must be equal or greater than 1 in this calculation to fulfil the criteria for moderate microvascular inflammation.

Peritubular Capillary

Only capillaries within the renal cortex are peritubular capillaries (PTCs) which can be used for scoring peritubular capillaritis as in Banff Lesion Score ptc. Capillaries next to lymphoid follicles must be disregarded to avoid confusion with lymphatic vessels. Banff Lesion Score C4d can be scored not only in PTCs but also in medullary vasa recta.

Peritubular Capillary Basement Membrane Multilayering

Severe Peritubular Capillary Basement Membrane Multilayering (ptcml) is defined as “seven or more layers of basement membrane in one cortical peritubular capillary and five or more in two additional capillaries, avoiding portions cut tangentially“.¹² Only Severe Peritubular Capillary Basement Membrane Multilayering is considered sufficient as one possible criterion for chronicity of AMR. This relative high threshold has been established because mild PTCML can be seen in other diseases like hypertension. In cases where electron microscopy (EM) is carried out in transplant biopsies, cortical peritubular capillary multilayering should be assessed and reported. The number of layers of basement membrane should be counted in the most affected PTC as well as in at least two additional PTCs. Tangentially cut PTC should be avoided. The biopsy report should state the number of PTC examined; the number of basement membrane layers in the 3 most affected PTC; and whether the Banff 2013 threshold for chronic ABMR (1 PTC with ≥7 layers + 2 PTC with ≥5 layers) is met. See also recommendations for performing EM cited from the Banff 2015 report under Banff Lesion Score cg for more detail.¹¹

Pyelonephritis

Inflammation of the transplant parenchyma caused by bacterial or fungal infection, usually ascending from the bladder. This falls within the remit of Banff Diagnostic Category 6. As an -> Infection, the presence of Pyelonephritis has important bearings on the diagnosis of Active AMR and Chronic Active AMR, as then Banff Lesion Score g must be equal or greater than 1 to fulfil the criteria for -> Microvascular inflammation, Moderate ([Banff Lesion Score g + ptc] ≥2).¹²

Polyomavirus Nephropathy

Replicative infection of the kidney by polyoma virus as indicated by viral inclusions seen on histology, immunohistochemistry and/or electron microscopy. A Banff Working Group for Polyomavirus Nephropathy has developed and validated a respective scoring schema, which was presented at the meeting in 2017 and has been published recently. The percentage of tubules expressed in the Banff Lesion Score pvl and Banff Lesion Score ci yield the classification as 1,2 or 3.²² Polyomavirus nephropathy is included in Banff Diagnostic Category 6.

Scarred Cortical Parenchyma

Characterised by cortical tubular atrophy, usually but not always accompanied by interstitial fibrosis –see ->Tubular Atrophy. Banff Lesion Score i must not be scored in these areas, instead Banff Lesion Score i-IFTA is assessed in the areas.

Subcapsular Cortex

This term refers to areas immediately beneath the capsule, which may show areas of non-specific scarring or inflammation thought to be related to surgical ‘healing in’.¹ The extent of the cortex that qualifies as “subcapsular” has not been more clearly defined in the Banff Classification. Banff Lesion Scores ptc⁷ and i¹¹ must not be scored in these areas.

T Cell-mediated Rejection (TCMR)

The spectrum of TCMR is defined as Banff Diagnostic Category 4 which contains Acute TCMR Grade IA, IB, IIA, IIB, III as well as Chronic Active TCMR Grade IA, IB and II. Suspicious (Borderline) For Acute TCMR is the only diagnosis in Banff Diagnostic Category 3. Banff Diagnostic Categories 3 and 4 but can be rendered together for the combination of borderline with chronic active TCMT, or with other diagnoses from Categories 2, 5 and 6.

Thrombotic Microangiopathy, Thrombotic Microangiopathy In The Absence Of Any Other Cause

Thrombotic microangiopathy (TMA) can be caused by recurrent disease (usually atypical haemolytic-uremic syndrome), AMR, Calcineurin Inhibitor Toxicity (included in Banff Diagnostic Category 6) and other causes.³⁴ If there is evidence of chronic TMA, Banff Lesion Score cg must not be used as a criterion for histological evidence of chronic tissue injury in the diagnosis of either Chronic Active AMR or Chronic AMR.¹²

Transplant Arteriopathy

Transplant Arteriopathy is defined as arterial fibrointimal thickening, also referred to as vascular fibrous intimal thickening, and is graded based on the extent of luminal occlusion in the most severely affected artery. Transplant arteriopathy is scored with the Banff Lesion Score cv. Features that suggest Chronic Active TCMR Grade II or Chronic Active AMR versus other causes such as conventional arteriosclerosis or hypertension are disruption of the elastica, inflammatory cells in the fibrotic intima, proliferation of myofibroblasts and/or foam cells in the expanded intima, formation of a neointima.¹¹ However, it should be noted that bland but progressive intimal fibrosis may be associated with the presence of DSA.³⁵

Transplant Glomerulopathy

Transplant glomerulopathy is defined as a Banff Lesion Score cg greater than 0, after the exclusion of chronic thrombotic microangiopathy, recurrent or de novo Glomerulonephritis. It is not synonymous to AMR, but is frequently associated with it.

Tubular Atrophy

Moderately atrophic tubules are defined as tubules having a diameter <50% of that of unaffected or minimally affected tubules on the biopsy, with wrinkling and/or thickening of the tubular basement membrane.⁵
Severely atrophic tubules are defined as tubules of diameter <25% of that of unaffected or minimally affected tubules on the biopsy, often with an undifferentiated- appearing, cuboidal or flattened epithelium (or in some cases even loss of epithelium with denudation of the tubular basement membrane), and pronounced wrinkling and/or thickening of the tubular basement membrane. This definition of severely atrophic tubules also includes very small, endocrine-like tubules with very narrow lumens, although the basement membranes of the latter may not be thickened.¹² According to Banff 2019, severely atrophic tubules should be disregarded for Banff Lesion Scores t and t-IFTA.¹³