Clusters of B cell infiltrates are often observed in kidney allografts. Several independent studies have correlated the presence of these clusters with poor graft outcome. Despite their possible contribution to the rejection process, little is known about how these clusters develop. Here, we addressed this question using rejected allograft specimens. By PCR, we characterized the partial B cell repertoires of 5 failed kidney grafts with documented B cell infiltrates. A comparison between the intragraft and peripheral repertoire was also carried out for one case. Rearranged immunoglobulin gene sequences were identified and compared to germline sequences to assess clone frequency, phylogeny, and rate/type of mutation for each heavy chain variable region family. Fixed tissue sections prepared from the same specimens were also assessed via immunohistochemistry (IHC) for the co-localization of CD20+ B cells and several markers indicative of traditional germinal centers (GC). Redundant clones were observed in both blood and graft, but the level of clonal amplification in graft tissue was significantly higher. Several sequences were highly represented, indicating a massive expansion of the corresponding B cell clones in vivo. Multiple sequences showed accumulation of somatic hypermutations (SHM) along the VH segment. SHM appeared more frequent in sequences found in the graft compared to the blood. In addition, we observed a higher rate of non-silent mutations in complementarity determining regions (CDR) compared to framework regions in blood sequences as well as in low frequency graft sequences, indicating an active selective pressure. This preferential distribution was lost in high frequency graft sequences, suggesting a lack of affinity maturation in situ. Lastly, IHC staining showed co-localization of CD20+ and Bcl-6+ (a GC-specific marker) cells in graft tissue. Overall, our findings provide supportive evidence that B cell clones expand and undergo SHM in situ in structures resembling GC. However, the even distribution of non-silent mutations in different segments of highly redundant graft sequences suggests that these ectopic GC-like structures lack the machinery for affinity maturation.

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