The evolution of the Banff classification schema for diagnosing renal allograft rejection and its implications for clinicians

Hyperacute rejection

Hyperacute rejection occurs within 10 minutes to 1 hour after perfusion with the recipient's blood. This occurs since the recipient is presensitized to alloantigens on the surface of the graft endothelium.[9] These alloantigens include:

• ABO incompatibility: Primarily IgM antibodies.

• Anti-HLA class I antibodies: Primarily IgG3.

• Anti-HLA class II antibodies: IgG/IgM antibodies in glomeruli and peritubular capillaries (where class II is prominent).

• Anti-endothelial-monocyte antibodies.

During surgery, the initial pink kidney becomes soft, flabby, mottled purple or cyanotic and anuric. Subsequently, it swells with widespread interstitial hemorrhage and cortical necrosis. The histological features include the presence of thrombi in the microvasculature, interstitial hemorrhages, and prominence of neutrophils in the glomeruli. Currently, it is well known that C4d staining in peritubular capillaries is the diagnostic feature, which helps in differentiating it from vascular thrombosis. Hyperacute rejection is now rare, seen in nearly 0.5% of transplants. In addition, some cases of primary nonfunction of the graft may be due to hyperacute rejection. As it is well known that the treatment of hyperacute rejection is nephrectomy.

Borderline changes

Borderline changes was characterized by infiltration of mononuclear cells (<25% of the parenchyma) or foci of mild tubulitis (1–4 mononuclear cells/tubular cross-section). Borderline changes might be considered suggestive of ‘very mild acute rejection’, but which were nondiagnostic. Banff opined that it was not mandatory to treat Borderline rejection. This has been somewhat controversial.

Acute rejection

It was felt that tubulitis was a better measure of severity of rejection than the intensity or extent of interstitial lymphocytic infiltration. Thus, tubulitis (infiltration by > 4 mononuclear cells / tubular cross-section) and intimal arteritis (subendothelial infiltration by mononuclear cells) were taken as defining features for acute rejection. Glomerulitis, although included in the scoring system, was not taken for diagnosing or grading of acute rejection. Acute rejection was graded as follows:

Grade I acute rejection: Moderate (>25%) to severe mononuclear cell interstitial infiltrate (i2/i3) and moderate tubulitis (4 mononuclear cells/tubular cross section i.e., t2).

Grade II acute rejection: Severe tubulitis (t3) and/or intimal arteritis (v1/v2).

Grade III acute rejection: Transmural arteritis (v3).

Chronic allograft nephropathy

Banff introduced the category ‘chronic allograft nephropathy’ (CAN) as a histopathological correlate of chronic allograft dysfunction. CAN was thought to include at least four entities at that period of time viz. chronic rejection, chronic cyclosporine toxicity, hypertensive changes, and chronic infection. The features suggestive of chronic rejection were: a) chronic transplant glomerulopathy: Glomerular basement membrane duplication and mesangial cell proliferation, and b) vasculopathy: Fibrous intimal thickening often with fragmentation of internal elastic lamina. Chronic changes in the interstitium (ci), tubules (ct), vessels (cv), and glomerulus (cg) were likewise graded into 0, 1, 2, and 3. The severity of interstitial fibrosis and tubular atrophy, as also chronic transplant glomerulopathy and vasculopathy were used to grade CAN into mild, moderate, and severe.Two other classification systems which developed around this time deserve mention.

Chronic allograft damage index

Chronic allograft damage index (CADI) score was first developed in 1994.[7] The CADI score was obtained by scoring each of the following from 0–3: Diffuse or focal inflammation, interstitial fibrosis, increase in mesangial matrix, glomeruloscerosis, intimal proliferation of vessels and tubular atrophy. CADI score < 2 is associated with a good graft survival, while high CADI score > 4 is associated with a poor outcome.[10] This scoring system is still used occasionally both to study protocol and indication biopsies.

Cooperative clinical trials in transplantation system

In 1997, pathologists in centers participating in the cooperative clinical trials in transplantation (CCTT) sponsored by the National Institutes of Health developed a scoring system (based on Banff) for the diagnosis of acute rejection. This was named as the CCTT system,[8] which was more simple and objective. It replaced ‘grades of rejection’ in the Banff schema by ‘types of rejection’. There was no category of ‘borderline rejection’; and all cases of borderline rejection as per Banff met the criteria for type I rejection in the CCTT system. Type I rejection was defined by mononuclear interstitial infiltrate (>5% of parenchyma) and tubulitis. It recognized the importance of vasculitis per sec, as it has implications for response to therapy and graft survival. Type II rejection was defined by arterial or arteriolar endothelialitis and type III by fibrinoid necrosis or transmural inflammation. These criteria were subsequently incorporated in the Banff '97 schema. Currently, the CCTT system is no longer used independently.

Reasons for eliminating the terminology ‘CAN’

CAN is not a homogenous entity. It is the end result of various injurious processes to the glomerulus, tubulointerstitium, and the microvascular compartments; resulting in interstitial fibrosis and tubular atrophy. The various such causes, which may be present either singly or in combination are shown in [Table 2].[11] During the last 12 years since the introduction of the term, researchers used ‘CAN’ indiscriminately as a diagnostic entity by itself rather than making attempts to pinpoint the exact disease causing CAN. Hence, in 2003, the Banff consensus conference decided to eliminate the term CAN from the Banff schema. Instead all efforts should be made to diagnose the etiology of the chronic allograft injury. If attempts to find the exact cause fail, then the biopsy report should be labelled as ‘interstitial fibrosis and tubular atrophy-not otherwise specified’ as discussed below.

The re-classified categories are as follows:

• Category 2: Antibody mediated rejection: This category includes acute and chronic active antibody mediated rejection

• Acute antibody-mediated rejection: Acute antibody-mediated rejection (also called acute humoral rejection) occurs when acute rejection occurs due to antidonor antibodies. AMR may occur on the operating table, or even weeks to months later. The histological features include ATN, presence of neutrophils in the peritubular capillaries, thrombi and fibrinoid necrosis, along with C4d deposition.[12] The grading of acute AMR remains the same as mentioned earlier. The term-hyperacute rejection-is not mentioned in the Banff schema since 2003, but still described in the standard textbooks.[9] However, the term accelerated acute rejection is confusing and should no longer be used.

AMR may be super-imposed on acute T-cell-mediated rejection. AMR needs stronger immunosuppression. Plasmapheresis, IVIg, and ant-CD 20 antibody rituximab have all been tried with variable success.

• Chronic active antibody-mediated rejection: This is also called chronic humoral rejection. In biopsies with interstitial fibrosis and tubular atrophy, the presence of additional glomerular and vascular features is suggestive of immune-mediated chronic rejection. The association between antidonor HLA antibodies and the development of chronic allograft dysfunction and obliterative vasculopathy was actually first demonstrated way back in 1970.[13] In the subsequent years, this idea remained in hibernation. It was three decades later that C4d deposition in peritubular capillaries was strongly associated with transplant glomerulopathy.[14] The features of chronic active antibody mediated rejection are shown in [Table 3]. On light microscopy, the important histopathological features are glomerular basement membrane duplication, multilamination of the peritubular capillary basement membrane and arterial intimal fibrosis. Two other conditions in which duplication of the glomerular basement membrane may occur are thrombotic microangiopathy and membranous glomerulopathy. It has been postulated that denovo membranous glomerulopathy may be related to chronic antibody-mediated rejection. The postulated stages of CHR are:[15]

• Development of donor specific antibodies

• Deposition of C4d in the allograft

• Graft pathology (subclinical rejection)

• Chronic graft dysfunction

Most cases of CHR are of insidious onset without past history of acute humoral rejection. CHR is present in about 10% of indication biopsies.

• Category 4: T-cell-Mediated Rejection: This category includes Acute and Chronic active T-cell-mediated rejection

  • Acute T-cell-mediated rejection: Acute T-cell mediated rejection occurs after 5–6 days. This time is required for the antigen presenting cells to present the alloantigens in the spleen and lymph nodes. There is interstitial infiltrate of mononuclear cells with interstitial edema and very occasionally hemorrhage. The infiltrate consists of mainly t-lymphocytes and macrophages. The presence of lymphoblasts (activated T cells) indicate increased proliferative activity.[9] The mononuclear cells invade tubules and can be seen between the tubular epithelial cells. This is termed as tubulitis and is best seen in slides stained with the PAS stain. t- cells express cytotoxic molecules viz. perforin, Fas ligand, granzyme A, and TNFβ. The classification of acute TCR is the same as ‘acute rejection’ mentioned in Banff '97 schema. Endarteritis (type II acute rejection) is manifested by the infiltration of mononuclear cells under the vascular endothelium. It affects arteries and arterioles. While under normal conditions the arterial endothelial cells express HLA class I antigens, during acute rejection they express increased HLA DR, ICAM-I and VCAM-I.[9] Glomerular lesions are uncommon in acute rejection. Very occasionally, ‘transplant glomerulitis’ may be seen. The endothelial cells are enlarged and hypercellular, with infiltration by mononuclear cells.

Clinical significance of other types of cells in the interstitium

Although occasional eosinophils may be present, the presence of abundant eosinophils is associated with higher grades of rejection and a worse prognosis. The differential diagnosis consists of drug induced acute interstitial nephritis and infection. Plasma cells are associated with a poor graft survival.[16] Similarly the presence of CD 20 + B cells in the interstitium is associated with a poor response to steroids.[17] Neutrophils are suggestive of AMR or pyelonephritis. Mast cells correlate with interstitial edema.

• Chronic active T-cell-mediated rejection: The defining feature is ‘chronic allograft arteriopathy’ characterized by arterial intimal fibrosis with mononuclear cell infiltration, and formation of neo-intima.

• Category 5: Interstitial fibrosis and tubular atrophy: IFTA replaces the term CAN in category 5. The diagnosis of IFTA should only be given if attempts to find the exact cause eg. antibody mediated, cell mediated, CNI toxicity [Figure 3], etc., of chronic histopathological changes in the allograft fail. The grading of IFTA is mild, moderate and severe [Figure 4].

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Figure 3

Photomicrograph showing a glomerulus with its afferent arteriole demonstrating a transmural hyaline nodule suggestive of CNI toxicity (H and E stain×200)

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Figure 4

Low-power photomicrograph showing extensive tubular atrophy and interstitial fibrosis (IFTA Grade III) (H and E stain×40)

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Impact of Banff classification

Mueller et al. confirmed a significant association between the revised Banff '97 classification and graft outcome.[18] Intimal arteritis was the only significant predictor of a poor survival probability. An area of some controversy is the ‘borderline change’. Although Banff recommends that treatment may not be indicated, studies have shown beneficial effect of treating this group with ant-rejection therapy.[1819] Recently, SWOT (strengths, weaknesses, opportunities and threats) analysis of Banff was done. They found that Banff had universal impact on clinical practice and research.[20] Currently, for routine clinical purposes, pathologists mention only the broad Banff category. Detailed scoring of the various compartments of the renal parenchyma is not mentioned in the report.

Future perspectives

During rejection certain genes-g interferon inducible or cytotoxic T-cell associated are upregulated.[2122] A study of the transcriptional profile of these genes is called transcriptomics, which can improve the biopsy diagnosis. One area in which it is especially useful is-the Banff ‘Borderline rejection’. These cases can be resolved into two distinct classes viz., rejection and nonrejection.[22] It is expected that future Banff updates will integrate the histopathological and molecular parameters.