LAUSR

Multiple myeloma is a malignancy of terminally differentiated plasma cells that are primarily resident in the bone marrow. Despite novel therapies, myeloma remains a fatal disease. Through interactions with immune cells in the bone marrow microenvironment, myeloma cells have been shown to facilitate their own proliferation and drug resistance. Therefore, targeting the immune support cells in the myeloma bone marrow microenvironment could add further value to existing treatment regimens. ZFP91, a C2H2-type zinc-finger protein, is a substrate of IMID drugs (An et al, Nat. Commun. 2017). The downstream target of ZFP91 in myeloma has been elusive so far. Our myeloma-specific ZFP91 interactome data and recent publications indicate an immune related function for ZFP91. A recent study also indicated that treatment of healthy volunteers with iberdomide, impaired the ability of monocytes to secrete IL-1β in response to LPS (Schafer PH, et al. Ann Rheum Dis 2018). To further investigate this, we generated CRISPR knockouts of IKZF1, IKZF3 and ZFP91, three major substrates of lenalidomide and iberdomide, in THP-1 monocytic cells. Upon LPS induction, loss of ZFP91 impaired the ability of THP-1 monocytic cells to secrete IL-1β (WT 70 pg/ml of IL-1β vs ZFP91-/- 4.5 pg/ml of IL-1β). Loss of IKZF1/3, the other two substrates of lenalidomide and iberdomide did not impair IL-1β secretion. IL-1β is a known activator of IL-6, a critical cytokine for myeloma cell proliferation. As a next step, we created a co-culture model to mimic a minimal myeloma tumor microenvironment with ANBL-6 (IL-6 dependent) myeloma cell line and primary bone marrow stromal cells in the presence of pre-conditioned media from LPS stimulated THP-1 monocytes. Preconditioned media from ZFP91-/- monocytes impaired proliferation of myeloma cells in the co-culture model due to lack of IL-1β, the inducer of IL-6 from the stromal cells. Mechanistically, loss of ZFP91 impairs the TLR4 signaling pathway. Thus, targeting ZFP91 impairs proliferation of myeloma cells within the bone marrow microenvironment by acting in trans from monocytes. Our findings for the first time identify a tumor cell proliferative role for ZFP91 in the myeloma tumor microenvironment. In addition, it establishes ZFP91 as an immune substrate of lenalidomide and iberdomide and could be a key player that mediates the immune functions observed. Jeyaraju: Celgene Corporation: Employment, Equity Ownership. Kumari:Celgene Corporation: Employment. Amatangelo:Celgene Corporation: Employment, Equity Ownership. Bjorklund:Celgene Corporation: Employment, Equity Ownership. Hsu:Celgene Corporation: Employment. Thakurta:Celgene: Employment, Equity Ownership.