Reference Guide to the Banff Classification

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 16 meetings with changes to the classification reported in 11 articles reflect the developments of the Banff Classification from the first consensus meeting in 1991 to the recently published consensus after the 2022 meeting in Banff, Canada.^1,4-14 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 The dispersal of both relevant and outdated content over 11 articles could make access to the Banff Classification difficult for beginners and experts and has created ambiguities in the past.³ Yet,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 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 changes introduced in the 2019 and 2022 meeting reports. 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); and a list of the Additional Diagnostic Parameters in Table 2. 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.

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 1. 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 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).

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,¹⁹ 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 3 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, whereas any atrophic tubule is considered abnormal, leading to a difference in cut-offs between ci1 and ct1: ci0 allows 0% to 5% of cortical fibrous tissue as normal, whereas ct1 starts anywhere above 0%. A useful definition for interstitial fibrosis proposed at the Banff 2022 meeting was: cortical areas where tubules are not lying back to back, and are separated from each other by fibrous tissue, best appreciated using a silver or trichrome stain. Care should be taken to distinguish separation of tubules by fibrous tissue versus oedema and/or inflammation.

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 (defined as tubules with a thickened basement membrane or a reduction of greater than 50% in tubular diameter).
ct1—Tubular atrophy (see ct0) involving up to 25% of the area of cortical tubules.
ct2—Tubular atrophy (see ct0) involving 26 to 50% of the area of cortical tubules.
ct3—Tubular atrophy (see ct0) 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,²¹ hypertensive glomerulopathy^,22 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 nonsclerotic glomerulus.
cg3—Double contours affecting more than 50% of peripheral capillary loops in the most affected nonsclerotic 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-sclerotic 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.²¹ 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.²² and subsequently confirmed by others.^23,24 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 cortical parenchyma with interstitial fibrosis and tubular atrophy.
i-IFTA1—Inflammation in 10% to 25% of cortical parenchyma with interstitial fibrosis and tubular atrophy.
i-IFTA2—Inflammation in 26% to 50% of cortical parenchyma with interstitial fibrosis and tubular atrophy.
i-IFTA3—Inflammation in >50% of cortical parenchyma with interstitial fibrosis and tubular atrophy.¹¹

Banff Lesion Score t-IFTA (Tubulitis in areas of interstitial fibrosis)

This Banff Lesion Score was fully introduced in Banff 2019.¹³ It refers to scoring of tubulitis in areas of interstitial fibrosis. t-IFTA is graded according to presence of mononuclear cell infiltrates in non-atrophic, mildly atrophic, and moderately atrophic tubules in areas of interstitial fibrosis. Tubulitis in severely atrophic tubules should not be considered. Like for the t lesion score, tubulitis must be present in at least 2 foci. 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 in areas of interstitial fibrosis, or single focus of tubulitis only.
t-IFTA1 – 2 or more foci with 1-4 mononuclear cells/tubular cross section (or 10 tubular cells) in the most affected tubule of the focus, in areas of interstitial fibrosis.
t-IFTA2 – 2 or more foci with 5-10 mononuclear cells/tubular cross section (or 10 tubular cells) in the most affected tubule of the focus, in areas of interstitial fibrosis.
t-IFTA3 –  2 or more foci with >10 mononuclear cells/tubular cross section (or 10 tubular cells) in the most affected tubule in the focus, in areas of interstitial fibrosis.

Banff Lesion Score pvl

Developed in the report of the Banff Working Group on Polyomavirus,²⁵ 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).²⁵ 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

BANFF DIAGNOSTIC CATEGORIES

Table 3 presents the Banff Diagnostic Categories and is based on a table in the Banff update from 2017,¹² incorporating changes from 2019 and 2022.^13-14 Banff consensus supports giving both a diagnosis of Borderline (Cat. 3) or Acute TCMR alongside a diagnosis of Chronic Active TCMR, if both are present.¹³ Banff 2022 introduces a flow-chart, reasoning framework and clinical interpretation figure and tables, to help with implementation of Category 2. These are included below Table 3. Readers should stay alert to future updates on the Banff Foundation website (www.banfffoundation. org) informed by updates to the Banff Classification from 2024 onward.

Flowchart of the Banff 2022 Classification for Category 2: Antibody-mediated rejection and microvascular inflammation/injury (AMR/MVI).

This can be used as companion for disease classification but does not modify the detailed Banff Classification for Category 2 AMR/MVI.

a. Other lesions can be observed in AMR and strengthen the diagnosis but are not diagnostic by themselves: arterial intimal fibrosis (cv) of new onset, excluding other causes; leukocytes within the sclerotic intima favour chronic AMR if there is no prior history of TCMR; acute tubular injury, in the absence of any other apparent cause.
b. Definitions of “diagnostic features of AMR/MVI”: g>0 in the absence of glomerulonephritis; ptc > 0 in the absence of acute TCMR or borderline (suspicious) for acute TCMR; v>0; acute thrombotic microangiopathy (TMA) in the absence of any other cause; cg>0 by LM, or EM if available, if no evidence of chronic TMA and if absence of recurrent or de novo glomerulonephritis; ptcml = seven or more layers in 1 cortical peritubular capillary and 5 or more layers in 2 additional capillaries, avoiding portions cut tangentially by EM, if available.
c. [g + ptc ≥ 2] in the absence of recurrent or de novo glomerulonephritis. If borderline (suspicious) for or acute TCMR, or infection are present, [g + ptc ≥ 2] is not sufficient and Banff lesion score g≥1 is required.
d. Biopsy-based transcript diagnostics for AMR/MVI above a defined threshold, if thoroughly validated for use a substitute for AMR/MVI and available.
e. In cases of MVI below threshold, biopsy-based transcript diagnostics can be applied, if thoroughly validated for use as a substitute for AMR/MVI and available.
f. C4d deposition should be evaluated in peritubular capillaries and vasa recta (C4d positive = C4d2 or C4d3 by IF on frozen sections, C4d >0 by IHC on paraffin sections).
g. If thorough testing for DSA (anti-HLA or other specificity) has not yet been performed, this should be done, following the STAR guidelines. Detection of non-HLA antibodies (including ABO antibodies in ABO-incompatible transplantation) can be used as serologic Banff criterion for diagnosis of AMR, if the testing protocols are sufficiently standardized and clinically validated for the appropriate clinical context. At present, no non-HLA antibodies (apart from ABO antibodies) have been validated sufficiently for inclusion into the routine clinical classification of kidney transplant biopsies.
h. Upon diagnosis of AMR, further differentiation of disease stage is as follows. Active AMR: presence of only active features (including C4d positivity) (cg=0; ptcml=0); Chronic active AMR: presence of both active (including C4d positivity) and chronic (cg>0 and/or severe ptcml) features.
i Cases with “Probable AMR” and histological chronic lesions (cg or ptcml) can be labelled as “chronic AMR”. For these cases, prior documented diagnosis of active or chronic active AMR, or documented prior evidence of DSA, also count as DSA positivity.

Clinical interpretation of Banff Category 2: Antibody-mediated rejection and microvascular inflammation/injury (AMR/MVI)

The text in this Table can be used in the Comments section of a transplant renal biopsy pathology report, to support standardised communication between pathologists and clinicians around the meaning of the terminology of diseases covered in Category 2.

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 and Celina Schulz.

Glossary of Terms

Acute Tubular Injury

There is no current definition of acute tubular injury endorsed by the Banff classification. 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

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.

Biopsy-based transcript diagnostics for AMR/MVI above a defined threshold, if thoroughly validated for use a substitute for AMR/MVI and available.

Increased expression of these, if thoroughly validated, is the only molecular marker 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

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.³⁴ 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.

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.³⁵

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.^15-17 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.

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.³⁶ 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.³⁷

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