Distinct mechanisms of dysfunctional antigen-presenting DCs and monocytes by single-cell sequencing in multiple myeloma

Multiple myeloma (MM) is an incurable plasma cell malignancy with the hallmark of immunodeficiency, including dysfunction of T cells, NK cells, and APCs. Dysfunctional APCs have been reported to play a key role in promoting MM progression. However, the molecular mechanisms remain elusive. Here, single-cell transcriptome analysis of dendritic cells (DC) and monocytes from 10 MM patients and three healthy volunteers was performed. Both DCs and monocytes were divided into five distinct clusters, respectively. Among them, monocyte-derived DCs (mono-DC) were shown to develop from intermediate monocytes (IM) via trajectory analysis. Functional analysis showed that, compared with healthy controls, conventional DC2 (cDC2), mono-DC, and IM of MM patients exhibited impaired antigen processing and presentation capacity. Moreover, reduced regulon activity of interferon regulatory factor 1 (IRF1) was found in cDC2, mono-DC and IM of MM patients according to single-cell regulatory network inference and clustering (SCENIC) analysis, while the downstream mechanisms were distinct. Specifically in MM patients, cathepsin S (CTSS) was markedly downregulated in cDC2, major histocompatibility complex (MHC) class II transactivator (CIITA) was significantly decreased in IM, in addition both CTSS and CIITA were downregulated in mono-DC based on differentially expressed genes analysis. In vitro study validated that knockdown of Irf1 downregulated Ctss and Ciita respectively in mouse DC cell line DC2.4 and mouse monocyte/macrophage cell line RAW264.7, which ultimately inhibited proliferation of CD4(+) T cells after being cocultured with DC2.4 or RAW264.7 cells. This current study unveils the distinct mechanisms of cDC2, IM, and mono-DC function impairment in MM, offering new insight into the pathogenesis of immunodeficiency.