Podocyte Injury in Kidney Allograft with Chronic Antibody-Mediated Rejection: Late Occurrence and Association with Glomerular Basement Membrane Thickening

Background: The histologic hallmarks of chronic antibody-mediated rejection (CAMR) are evidence of endothelial injury and glomerular basement membrane (GBM) thickening and double contour without immune complex depositions. Our previous data show that endothelial injury is the primary injury that causes initial GBM thickening. However, proteinuria is usually present suggesting podocytes are also injured, but this has not been extensively studied. Recently, we described sarcomere-like structures (SLSs), a feature of injured podocytes and cytoskeletons rearrangement that is found in various glomerular diseases. We aim to assess the prevalence of SLSs in various stages of CAMR. Methods: We utilized super-resolution imaging of kidney tissues to investigate the association between GBM changes in transplant glomerulopathy and podocyte injury at various stages of CAMR. Paraffin sections from 9 allograft biopsies with CAMR and 7 nephrectomy controls were analyzed. Immunofluorescence staining for synaptopodin and myosin IIA was performed to visualize SLSs, Nephrin and laminin α5, Col4a1a1a2 and Col4a3a4a5 were used to label the slit diaphragms and GBM. Results: Compared with the control samples, laminin α5 staining shows a significant GBM thickening in CAMR biopsies. In the thickened areas of the GBM, increase in Col4a1a1a2 was observed, especially, on the endothelial side of the GBM with no changes in the Col4a3a4a5. SLSs, defined by presence of alternating synaptopodin and myosin IIA clusters in podocyte foot processes, were observed predominantly in CAMR groups. The extent of SLSs in advanced stages of CAMR (Banff cg2-3) was significantly higher when compared to earlier stages (Banff cg1a-b) and was present in areas with foot process effacement as suggested by low, blunt nephrin density. The chance of SLSs occurrences was significantly higher as GBM thickness increased. Conclusion: The positive correlation between podocyte injury and the degree of GBM thickening in CAMR where the injury starts primarily from antibody-mediated endothelial damage suggests that alterations in the GBM convey injurious signals to podocytes. This finding provides new insight into the progression of CAMR and, if incorporated into the disease classification, may enhance prognostic evaluation.