P-206: Type I interferon response in monocytes altered monocyte differentiation pathways and promotes multiple myeloma proliferation

Introduction: A growing body of evidence has indicated impaired function or compositional changes of monocytes in inflammatory disorders, such as acute respiratory syndrome and COVID-19. In multiple myeloma (MM) tumor microenvironment, activation of type I interferon pathway and dysregulated expression of major histocompatibility complex type II genes are observed in classical monocytes, which result in loss of antigen presentation of monocytes. Nevertheless, the mechanisms underlying monocyte defects in MM remain poorly addressed, at least in part due to the lack of large-scale single-cell RNA sequencing (scRNA-seq) studies.

Methods:
We performed scRNA-seq on monocytes of 7 newly diagnosed MM (NDMM) patients and 12 HD. Specifically, 3 and 5 BM samples and 9 and 7 PB samples of 12 HD and 7 NDMM patients were obtained and sequenced, respectively.

Results: We constructed a precise atlas of human PB and BM monocytes, identified seven subpopulations in both BM and PB—including S100A12, HLA, ISG15, CD16, proinflammatory, and intermediate in both BM and PB; megakaryocyte-like in PB; and proliferating subset in BM. Differential expression analysis on the BM and PB monocytes showed that a large number of interferon (IFN) signaling pathway genes (e.g. IFI27, IFI6, ISG15) were overexpressed in MM compared with HD. Genes encoding major complement system components and class II major histocompatibility complex molecules (MHC class II) were more highly expressed in MM compared to HD, indicating higher inflammatory and phagocytic potential of MM monocytes. However, relative to HD, T-cell attraction-related genes (e.g. CCL3 and CCL4) were markedly downregulated in MM, and T-cell suppression-related genes (e.g. IDO1, CD274 and PDCD1LG2) were markedly upregulated in MM.

Furthermore, we identified two monocyte differentiation pathways in both BM and PB, and discovered that BM monocyte feature type I IFN-associated alterations in differentiation in patients with MM as well as dysregulated patterns at transcriptome. Quantitative PCR results showed that，human monocytes expressed type I IFN (IFNα and IFNβ), but not type II IFN (IFNγ). Furthermore, we collected conditioned media (CM) from alone culture or cocultured myeloma cell lines and human monocytes at 1:10 ratio, and co-cultured CM significantly promotes myeloma cell line proliferation. Finally, we included 10 MM patients as a validation cohort, by tracking the alterations in transcriptome and differentiation during treatment using scRNA-seq. Our results indicated that type I IFN signaling pathway activation and alterations in differentiation were partially alleviated for BM monocytes in MM by antitumor therapy.

Conclusions: Our results provided further insight into transcriptional and differentiation alterations occurring in the BM and PB monocytes from patients with MM and explored mechanisms of immune evasion associated with monocytes.