Adoptive Cell Therapy for Lymphoma: Use of CpG-Loaded Tumor Cells to Generate Potent Anti-Tumor CD4 T Cell Immunity.
Authors: Goldstein Matthew J , Varghese Bindu , Rajapaksa Ranjani , Brody Joshua , Levy Shoshana , Levy Ronald 
Published: October 2019

Abstract
Abstract 929

Background:
Recently, we have investigated adoptive cell therapy for treating lymphoma. The efficacy of this maneuver has been demonstrated by curing large established tumors. Specifically, we use active immunization to generate anti-tumor T cells in vivo and transfer these T cells into lymphodepleted recipient mice (Brody J, Goldstein MJ, Czerwinski DK, and Levy R; Blood, 2009). A major challenge in adoptive therapy is the method of generating anti-tumor T cells. Traditionally, tumor-specific T cells are expanded to large numbers ex vivo. Herein, we describe a new, whole-cell vaccine that is effective in inducing anti-tumor T cells in vivo. This vaccine combines tumor antigens with an immune stimulant: irradiated-tumor cells (a source of tumor antigens) are loaded with the TLR agonist CpG (an immune stimulant). Our vaccine approach has several potential advantages: (1) anti-tumor immunity generated by our CpG-loaded, whole-cell vaccine is poly-antigenic and thus, not limited by the expression of a single antigen target on tumor cells; (2) ex vivo expansion may generate large numbers of effector T cells that can induce tumor regression in the short-term, but have a limited ability to maintain a persistent anti-tumor response. Our model avoids ex vivo manipulation of anti-tumor T cells and thus, may preserve and enhance a memory T cell population that sustains the anti-tumor response.

Methods:
We derived a new pre-B cell lymphoma cell line in the C57BL/6 background. Primary bone marrow cells were isolated from C57BL/6 donor mice and transfected with a recombinant retrovirus containing the Bcr-Abl oncogene. The emerging transformed cell line was designated H11. This cell line expressed the B lineage marker CD19 but was negative for MHC II and surface Ig. Irradiated H11 tumor cells were pre-loaded with CpG for 24 hours and administered to donor mice by daily, sub-cutaneous injections for five days. Donor splenocytes were harvested seven days following vaccination and adoptively transferred into lethally irradiated recipient mice that were subsequently challenged with a lethal dose of H11 tumor cells.

Results:
Vaccination with CpG-loaded H11 tumor cells (CpG-H11) generated anti-tumor T cells that are effective in adoptive cell therapy. 100% of mice receiving adoptive therapy with vaccine-induced T cells were protected from tumor challenge. In contrast, vaccination of donor mice with untreated H11 tumor was insufficient for generating anti-tumor T cells. Only 20% of mice treated with T cells from these donors were protected from tumor challenge. In spite of the H11 tumor being MHC Class II−, we observed that anti-tumor immunity generated by the CpG-H11 vaccine was CD4 T cell mediated. CD4 T cells were isolated from CpG-H11 vaccinated donors by flow cytometry. Fewer than 1.8×106 CD4 T cells were sufficient to protect 80% of recipient mice from tumor challenge. In contrast, equivalent numbers of donor CD8 T cells provided no benefit. These results strongly suggest that the CpG-H11 vaccine induced cross-presentation of tumor antigens by antigen-presenting cells (APCs). We have demonstrated that CpG-loaded H11 tumor cells can leak CpG into the immediate environment activating nearby APCs. These APCs have greater phagocytic potential and express higher levels of co-stimulatory molecules such as CD40. Ongoing studies will determine whether APCs which encounter the CpG-H11 vaccine but not untreated H11 tumor cells, can stimulate proliferation of anti-tumor T cells.

Conclusions:
Here we describe a novel, whole-cell vaccine approach that induces anti-tumor T cells for adoptive therapy to treat lymphoma. This vaccine is superior to vaccination with tumor cells alone. We are currently developing this therapy for evaluation in a clinical trial to treat mantle cell lymphoma.

Disclosures:
No relevant conflicts of interest to declare.