The humoral response to BK polyomavirus in kidney transplant recipients is dominated by IgM antibodies that use a distinct repertoire compared to IgG against the same antigen

1.The BK polyomavirus (BKPyV) persists asymptomatically in the kidney and active replication is only seen in immunosuppressed individuals, such as kidney transplant (KTx) recipients, in whom BKPyV reactivation can cause significant morbidity. KTx recipients with BKPyV reactivation mount a robust humoral response, but this often fails to clear the virus. In order to characterize the BKPyV-specific B-cell receptor (BCR) repertoire in KTx recipients, we used fluorescence-labeled BKPyV virus-like particles (VLPs) to sort with BKPyV-specific B-cells, then single-cell RNAseq to obtain paired heavy and light chain antibody sequences, and gene transcriptome data. The BCR repertoire was highly diverse in terms of both V-gene usage and clonotype diversity, with approximately 3% repertoire overlap between patients. The BKPyV-specific response was characterized by the presence of both memory IgG and memory IgM B-cells with extensive somatic hypermutation, which expressed distinct BCR repertoires within the same patient. The gene expression profile of IgG and IgM memory B-cells was highly similar, with only 19 genes, including CD83, CD79A and PARP1 showing significant differential expression. These results confirm that the IgM memory B-cells are a significant component of the BKPyV-specific humoral response, and show for the first time that IgG and IgM repertoires directed against the same antigen can have significant differences.