Antibody Mediated Rejection (AMR) Lesions in Renal Biopsies: Evidence of Progression with a Specific Microvascular Inflammatory Phenotype. Diagnostic Implications

Abstract

Introduction: The diagnosis of AMR hinges on the combination of DSA, C4d and microvascular injury/inflammation (MVI) but is complicated by temporal lesion variations and the fluctuations of C4d staining and DSA levels. MVI needs therefore to be fully analized for optimal diagnostic use. Methods: 563 consecutive renal biopsies (Bx) from 496 patients were analyzed using current Banff criteria and were stained with C4d, CD68 and CD3. For each biopsy, concurrent DSA measurements were also available. The mean number of biopsies per patient was 1.44 ((+/- 0.91, median 1). Mean patient age was 50.27 yeras (+/-13.79). 63.1% of the patients were male, 8.1% had regrafts and 6.6% were hepatitis C positive. Specific definitions used for analysis: C4d+ ≥10% staining in peritubular capillaries; DSA+ >1000MFI; MVI defined as glomerulitis (g), transplant glomerulopathy (cg) and peritubular capillaritis (ptc) >0 scores. On electron microscopy (EM), TG was defined by duplication of the basement membranes, whereas endothelial injury (EI) was defined as swelling, loss of fenestrations ± expansion of the subendothelial space. The maximum nubmer of glomerular macrophages and lymphocytes were also counted (GM and GL) with CD68 and CD3 stains, respectively. Results: Cases with acute T-cell mediated rejection (ACMR) (n=76) or cases lacking any AMR or ACMR features (n=187) had an average of 5±6 GM/3.4±4.8 GL and 3.67±4.62 GM/1.57±2.52 GL respectively (NS), whereas cases with any combination of AMR parameters that includes MVI displayed a similarly marked increase of GM/GL (up to 16.58±12.19 GM/ 7.94±6.64 GL, p=.000). In contrast cases with only C4d and/or DSA without MVI showed low GM/GL (up to 5.37±6 GM/1.87±2.23 GL) statistically indistinguishable from the ACMR or cases lacking any AMR or ACMR features. The time post Tx was also higher in cases with MVI, irrespective of C4d or DSA presence (34.38± 35.73) than in cases with DSA and/or C4d only (16.96± 34.74)(p=.000), indicating a temporal sequence. This particular glomerular inflammation immunophenotype characterized not only the entire spectrum of MVI but was also associated to EM microvascular pathology, time post Tx and C4d results (Table 1).Table: [Correlations with EM findings]Conclusions: AMR displays a temporal an immunophenotypical spectrum ranging from an earlier DSA± C4d stage without MVI and progression to the full spectum (DSA+C4d+MVI) including a marked increase in GM accompanied by a definite but lesser increase in GL (2:1 ratio). This glomerular immunophenotype is so characteristic and constant that its presence would serve as a surrogate to the more variable C4d staining for the diagnosis of AMR (i.e. C4d negative AMR).