Plasma cell infiltrates in polyomavirus nephropathy

Polyomavirus (PV) associated nephropathy (PVAN) has become an important cause of allograft dysfunction. We studied plasma cells (PCs) – which have not yet been characterized – present in the cellular infiltrate of 20 PVAN cases using immunohistochemistry and morphometry. The results were correlated with morphological, clinical and anti‐BK virus serological findings. PC‐rich cellular infiltrates occurred in 50% of cases (>15% PCs in the cellular infiltrate) and in these IgM producing PCs were commonly seen (70%): IgM PC predominance in 50% of cases and a comparable number of IgM and IgG PCs in 20% of cases. We found a significant correlation not just between the absolute numbers (P < 0.034) and the percentage values of IgM PCs (P < 0.004 in relation to all cells) and the serum IgM‐Ab anti‐BKV activity, but also between the ratio of IgG/IgM PCs and the ratio of serum IgG/IgM‐Ab activities (P < 0.0001). We showed that IgM PC counts in biopsies correlate with titers of circulating anti‐BK virus IgM antibodies. Every case except one was C4d negative in peritubular capillaries (PTC). As IgG PCs characterize PC‐rich rejection cases, we suggest that in the presence of IgM PCs in PC‐rich infiltrate with PTC C4d negativity, a search for possible PVAN infection should be initiated.