Abundance of C1q Expressing Macrophages Promotes Resistance to Modern Therapy

Overview: Despite advances in therapies targeting the tumor-intrinsic aspects of the disease, multiple myeloma (MM) remains functionally incurable. Given that new alternatives such as T cell redirected therapy and monoclonal antibodies engage ADCC and CDC by leveraging other elements of the immune system, we believe that deeper studies of the tumor microenvironment are warranted. In particular, we believe that certain macrophages are associated with tumor progression, serve as markers of future relapses, are immunosuppressive, and reduce the effectiveness of otherwise successful therapies.

Methods: We performed 5’ scRNAseq analysis of 189,025 cells collected from spleen or bone marrow of 4 wild type, 14 de novo, and 40 tumor bearing Vk*MYC mice treated with vehicle or effective anti-MM therapies. Discoveries were cross-referenced against a set of 45,626 human bone marrow cells collected from 35 samples at different stages of disease and treatment. We experimentally recreated in vitro the cross-talk between MM cells and macrophages by co-culturing bone marrow derived macrophages (BMDM) with MM cells or their conditioned medium, and correlated changes in gene expression as well as the ability of polarized BMDM to inhibit CAR T mediated killing of MM cells in vitro.

Results: Gene expression analysis of murine in vivo C1q macrophages and in vitro polarized BMDM revealed a common signature identified in lipid-associated macrophages and Kupffer cell-like macrophages¹ (high expression of Fabp5, ApoE, Cd63, C1qa/b/c, Cd163, Cd5l, Vcam1, Fcgr4, Folr2, Maf) which is associated with immunosuppression and disease progression. Ligand-receptor analysis via LIANA identified C1qb ligand in macrophages with C1qbp in tumor among the top interactions in both human and murine single cell analyses. C1qbp is a crucial element of cell adhesion and metastasis,² and this link between tumor and macrophage can severely limit effective therapy: BMDM following stimulation/polarization with MM lines, but not naïve/unstimulated BMDM, strongly inhibit anti-BCMA CAR T in vitro cytotoxic activity. In total, a complement-related connection between tumor and macrophage is capable of directing an immunosuppressive, chemokine response that imparts resistance to therapy.

While effective therapies reduce tumor and C1q expressing macrophage populations,³ retention of C1q expressing cells may be associated with future relapses. Presence of a specific subset of C1Q expressing cells, even in very low numbers, in human samples following ASCT was associated with early relapse (6 of 8 good responding samples had zero C1Q+ cells while 12 of 16 poor responding samples had 1 or more C1Q+ cells–Fisher test p-value < 0.05). In addition to high expression of C1QA/B/C, these cells are also highly upregulated in TYROBP (known to relate to macrophage polarization⁴ and found in a spatial transcriptomic analysis of cells at the periphery of MM and its environment⁵) as well as FCER1G (known to relate to macrophage infiltration⁶).

Conclusion: As advanced therapies manipulate elements of the immune microenvironment to impart effective treatment, the connection between tumor and macrophage should not be overlooked since macrophages likely help create an immunosuppressive niche that assists in disease growth and eventual revival.

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