Transduction of Malignant Plasma Cells with Three Costimulatory Molecules (TRICOM) Elicits Myeloma-Specific Immune Response in Vitro – a Promising Strategy for Immunotherapy
Authors: Luptakova Katarina , Mills Heidi , Rosenblatt Jacalyn , Stroopinsky Dina , Kufe Turner , Vasir Baldev , Arnason Jon E. , Tzachanis Dimitrios , Joyce Robin , Levine James D. , Kufe Donald , Avigan David 
Published: October 2019

Abstract
Abstract 1908

Introduction:
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.

Methods and results:
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).

Conclusions:
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.

Disclosures:
No relevant conflicts of interest to declare.