Successful Implementation of GM-CSF Cellular Immunotherapy for the Treatment of Multiple Myeloma In a Mouse Model of Autologous Bone Marrow Transplantation
Authors: Zilberberg Jenny , Stein Johann , Friedman Thea M , Korngold Robert 
Published: October 2019

Abstract
Abstract 4288
A modified murine model of multiple myeloma (MM), derived from the well established 5T33MM cell line (originated from spontaneously developed MM in aged C57BL/KalwRij (B6Rij; H2b haplotype)), was utilized to mimic minimal residual disease after bone marrow transplantation (BMT). The 5T33MM model has many of the characteristics associated with human MM, including homing and growth in the bone marrow compartment, hypercalcaemia and elevated tumor-associated IgG2bk in the circulation, with diffuse osteolytic lesions. In contrast to human MM however, the 5T33MM cells are also often found in the mouse spleen, since in this species the spleen is a bone marrow-like hematopoietic organ. Due to limitations on the availability of B6Rij mice and the vast array of mutant mice on the closely related C57BL/6 (B6) background, that could be useful in probing mechanisms of action for MM development or immune resistance to the tumor, it is highly desirable to adapt the MM model for B6 mice. However, 5T33MM cells do not cause disease in immunocompetent B6 mice, presumably because of allogeneic differences that cause rejection of the tumor cells. To overcome this problem, we found that 5T33MM cells will develop MM disease in lethally irradiated B6 recipients in a syngeneic BMT setting. Using this model, we tested for the first time the feasibility of using GM-CSF secreting vaccines in combination with tumor MHC-matched BMT from B6 mice for the treatment of MM. To this end, B6 Mice received a single subcutaneous infusion of 2×106 irradiated 5T33MM cells engineered to secrete GM-CSF (GM-5T33MM), or were left untreated. Seven days later, mice were lethally irradiated with 850 cGy and transplanted with 2×106 anti-T cell-depleted B6 BM cells. On day 1 post-BMT, recipients were challenged with 2×106 5T33MM cells (i.v). Mice that presented with the disease, developed hind limb paralysis and signs of morbidity, along with extensive plasmacytoma growth in the bone marrow and the spleen (as determined by H&E staining and flow cytometric analysis of BM and spleen samples). This pathology was similar to aged B6Rij mice that develop MM, as reported in the literature. Statistical comparisons using a non-parametric log-rank test showed enhanced survival rate in the GM-5T33MM treated group 70 days post-BMT (60% survival), compared to 0% survival, MST = 25 d (p = 0.020) in non-vaccinated recipients. These results indicate that a single pre-transplant administration of GM-5T33MM cells can significantly improve survival of transplanted/5T33MM challenged mice, even in the absence of donor lymphocytes. Furthermore, it suggests that immune cells exposed to GM-5T33MM vaccination, that survive host irradiation, constitute a significant component of this anti-tumor effect.
The current experimental model should provide the basis to study the efficacy of GM-CSF secreting tumor vaccination in combination with BMT for the treatment of MM, as well as the cellular mechanisms associated with it.

Disclosures:
No relevant conflicts of interest to declare.