P-072: Progression and dissemination of experimental multiple myeloma is associated with loss of innate and adaptive immune-mediated tumor control

Background Multiple myeloma (MM) is an incurable plasma cell malignancy and identifying mechanisms driving disease progression and relapse is crucial for development of novel therapies. Although cytotoxic immunity is an important factor in controlling tumor progression, the cells and signals that constitute an effective anti-myeloma immune response within the bone marrow (BM) are incompletely defined. In this study we set out to identify immune-related mechanisms of disease progression in a murine model of MM. Methods C57Bl/6 and KaLwRij mice received 106 5TGM1-GFP murine myeloma cells intravenously. Tumor development was monitored through weekly measurements of M-protein levels. Results Three weeks after tumor injection all KaLwRij mice (18/18) developed myeloma, as defined by >5% tumor cells in BM (“unrestrained tumor”) and serum M-protein >2mg/ml, and in all mice tumor cells had infiltrated the spleen. Interestingly, while 39% (7/18) of C57Bl/6 mice had no tumor, 44% (8/18) had low but detectable levels of MM cells (0.1-5% of BM cells, “restrained tumor”) while 17% (3/18) presented with an unrestrained myeloma with BM tumor load similar to that seen in KaLwRij mice. In all C57Bl/6 mice MM was confined to the bone marrow with no splenic involvement at day 21. When tumor development in 5TGM1-injected C57BL/6 mice was allowed to proceed for a prolonged period of time, the incidence of mice developing a tumor did not change (59%, 7/12 mice). However, the number of mice with unrestrained tumor increased to 42% (5/12 mice), and 2 out of these 5 mice also developed splenic MM involvement. In addition, 17% (2/12) of mice were characterized by restrained tumor, as they contained low but detectable tumor levels up until 42 days post tumor cell transfer. The time-dependent increase in incidence of mice with high tumor burden suggests an initial immune-mediated restraint of MM that eventually fails, leading first to unrestrained BM disease and then to systemic spread of the tumor. The C57Bl/6 mice with restrained bone marrow myeloma had an expansion of activated mature (CD69+ CD11b+) NK cells as well as activated (Lsel-CD44+CD69+) CD8+ cytotoxic T cells. In contrast, high bone marrow tumor burden and spread of MM cells to the spleen was associated with a sharp decline in the absolute numbers of both these cytotoxic immune cell subsets. Conclusion Here we provide evidence that loss of innate and adaptive cytotoxic immune populations is associated with progression and dissemination of experimental MM in mice. Transfer of 5TGM1 myeloma cells into C57Bl/6 mice leads to initial tumor control via expansion of innate and adaptive cytotoxic immune cell populations. However, with time, immune-mediated restraint of MM fails, exemplified by loss of NK cell and CD8+ T cell expansion, culminating in tumor outgrowth in the BM and eventual systemic dissemination. This in vivo model will allow for generation of novel insights into the mechanisms of immune escape by MM cells.