Dual Face of Immune Microenvironment in Myeloma

An important property of human multiple myeloma (MM) is the growth of tumor cells and their precursors in the bone marrow microenvironment. This specialized microenvironment includes a distinct component of immune cells that includes cells of innate as well as adaptive immune system. Growing body of evidence points to a central role for cross-talk between this immune microenvironment and tumor cells in regulating the growth of tumor cells in myeloma. These interactions play a dual role-with some interactions promoting tumor growth while others suppress growth of tumor cells. Cross talk between T and B cells has been implicated in normal B/plasma cell differentiation and may also play a role in pathogenesis of myeloma, explaining increased risk of plasma cell malignancies in certain settings such as Gaucher disease. The bone marrow of MM patients is often infiltrated with innate cells such as dendritic cells (DCs) and cross-talk between myeloid/plasmacytoid DCs or macrophages may promote growth and survival of MM cells. This cross-talk may also have fundamental implications for inducing genomic instability in tumor cells, such as via the induction of cytidine deaminases in tumor cells. Genomic instability is an early feature of human MM and many of the mutations found in MM tumor cells are also present in the precursor states. The immune system has the capacity to recognize these precursor lesions and even in the setting of clinical MM, the tumor bed contains T cells that can be recruited to kill tumor cells. Specific targets of tumor immunity may differ between preclinical and clinical malignancy. Immunity to certain stem cell-associated antigens are in particular associated with reduced risk of progression to clinical MM. With advanced disease, tumor infiltrating T cells encounter several immune inhibitory checkpoints which leads to suppression of function of tumor-infiltrating T cells. In addition to T cells, cells of the innate immune system such as natural killer and natural killer T cells may also be attractive targets of protective tumor immunity in MM. Immunologic approaches may also be combined with other strategies such as other immunomodulatory drugs (IMiDs) already in clinical use in MM. Recent advances in mouse modeling provide new opportunities to better understand how to harness the immune system against human MM. Such approaches have the advantage of the potential for durable control of tumor cells via immunologic memory. Such approaches are now entering the clinic as attractive targets to restore anti-tumor immunity, both in the context of therapy or prevention of MM.