Aberrant single-cell phenotype and clinical implications of genotypically defined polyclonal plasma cells in myeloma

Multiple myeloma (MM) is driven by clonal plasma cell (cPC)-intrinsic factors and changes in the tumor microenvironment (TME). To investigate whether residual polyclonal PCs (pPCs) are disrupted, single-cell (sc) RNA sequencing (scRNA-seq) and sc B-cell receptor analysis were applied in a cohort of 46 samples with PC dyscrasias and 21 healthy donors (HDs). Of 234 789 PCs, 64 432 were genotypically identified as pPCs with frequencies decreasing over different disease stages, from 23.66% in monoclonal gammopathy of undetermined significance to 3.23% in MMs (P = .00012). Both cPCs and pPCs had a comparable expression of typical lineage markers (ie, CD38 and CD138), whereas others were more variable (CD27 and ITGB7). Only cPCs overexpressed oncogenes (eg, CCND1/2 and NSD2), but CCND3 was often expressed in pPCs. B-cell maturation antigen was expressed on both pPCs and cPCs, whereas GPRC5D was mostly upregulated in cPCs with implications for on-target, off-tumor activity of targeted immunotherapies. In comparison with HDs, pPCs from patients showed upregulated autophagy and disrupted interaction with TME. Importantly, interferon-related pathways were significantly enriched in pPCs from patients vs HDs (adjusted P < .05) showing an inflamed phenotype affecting genotypically normal PCs. The function of pPCs was consequently affected and correlated with immunoparesis, driven by disrupted cellular interactions with TME. Leveraging our scRNA-seq data, we derived a "healthy PC signature" that could be applied to bulk transcriptomics from the CoMMpass data set and predicted significantly better progression-free survival and overall survival (log-rank P < .05 for both). Our findings show that genotypic sc identification of pPCs in PC dyscrasias has relevant pathogenic and clinical implications.