Transduction of Malignant Plasma Cells with Three Costimulatory Molecules (TRICOM) Elicits Myeloma-Specific Immune Response in Vitro – a Promising Strategy for Immunotherapy

Tumor vaccines hold promise as a means of eliciting anti-myeloma immunity and controlling disease that may be resistant to chemotherapy and biologic therapy. We have developed a whole cell tumor vaccine, whereby patient derived plasma cells are transduced with an attenuated vaccinia vector that contains transgenes for the costimulatory molecules B7.1 (CD80), ICAM-1 (CD54), and LFA-3 (CD58), designated TRIad of COstimulatory Molecules (TRICOM). In this manner, a broad array of tumor antigens, including those which may be specific to a given patient, are presented in the context of costimulatory molecules that have been shown to be synergistic in the stimulation of the effector T-cells. In the present study, we evaluated the phenotype and functional characteristics of TRICOM transduced primary myeloma cells.

Plasma cells were isolated from bone marrow aspirates obtained from patients with multiple myeloma following Ficoll density centrifugation. Bone marrow derived mononuclear cells were infected with a replication-defective poxviral vector, the modified vaccinia Ankara strain (MVA), encoding TRICOM, or a control empty MVA vector. The expression of costimulatory molecules was assessed using flow cytometric analysis 3 hrs following viral infection. Viral transduction using the TRICOM vector at the dose of 20 MOI (multiplicity of infection) increased the mean percentage of CD38+ cells expressing CD80, CD54 and CD58 from a minimal baseline level (below 5%) to 70%, 56% and 47%, respectively (n=4). Transduction with control MVA vector did not augment expression of costimulatory molecules on plasma cells (mean percent expression of CD80, CD54 and CD58 of 2.6%, 2.7% and 3.8%, respectively, n=4). Of note, compared to CD38+ plasma cells, the CD38 negative fraction of bone marrow derived mononuclear cells demonstrated a significantly lower TRICOM transduction efficiency (mean percent expression of CD80, CD54 and CD58 of 16%, 17% and 16%, respectively, n=4, p<0.05 compared to CD38+ plasma cells).

The ability of MVA-TRICOM transduced plasma cells to stimulate autologous T cell populations in vitro was assessed. Patient derived T-cells were purified from the non-adherent portion of PBMC by magnetic bead separation. MVA-TRICOM or empty MVA vector infected plasma cells were irradiated with 20Gy and co-cultured with autologous T cells at a 10:1 ratio of effector cells to vaccine for 7 days. MVA-TRICOM transduced plasma cells potently stimulated activated T cell responses, as assessed by the percentage of CD4+/CD25+/CD69+ T-cells (mean 7.8% of activated T-cells with TRICOM vaccine vs. 2.7% with control vaccine, n=3, p<0.05). In contrast, vaccine stimulation did not result in regulatory T-cell expansion, assessed as the percentage of cells co-expressing CD4,CD25 and FoxP3 (2.4% vs. 2.3%, for TRICOM and control vaccine, respectively, n=3). In concert with these findings, vaccine stimulation resulted in a polarization towards Th1 cytokine secretion, with 7.9% of CD4+ T-cells expressing intracellular IFN-γ after stimulation with TRICOM vaccine as compared to 5.4% after stimulation with the control vaccine (n=3, p<0.05). To further assess the expansion of tumor specific T cell populations, the ability of vaccine stimulated T cells to kill autologous tumor was assessed in a cell-based fluorogenic cytotoxicity assay. MVA-TRICOM transduced plasma cells potently stimulate the expansion of myeloma specific CTLs with the capacity to lyse autologous tumor targets. Mean CTL lysis was 20% and 8% for vaccine stimulated and unstimulated T cells respectively (n=2).

Malignant plasma cells transduced with MVA-TRICOM strongly express costimulatory molecules, and potently stimulate activated, tumor reactive T cell populations. This preclinical data serves as a platform for developing a phase 1 clinical trial evaluating the use of MVA-TRICOM transduced autologous plasma cells in patients with multiple myeloma.

No relevant conflicts of interest to declare.