A CpG-Activated Whole-Cell Vaccine ‘Boost’ Enhances the Anti- Lymphoma Efficacy of Immunotransplant

BACKGROUND: Previously, we demonstrated that an anti-lymphoma vaccine was made more powerful by ‘immunotransplant’ of vaccine-primed donor splenocytes into lethally irradiated, syngeneic bone marrow transplant recipients. Immunotransplant increased the proportion of tumor-specific, memory CD8 T cells by almost ten-fold and cured tumors several times larger than what could be cured by vaccination alone. We demonstrated that this increased anti-tumor effect correlated with the ‘homeostatic proliferative’ signal that tumor-specific donor T cells receive in lymphodepleted recipients. If the ‘‘homeostatic proliferative’ signal could synergize with other T cell activating signals, it is possible that the anti-tumor effect could be even further enhanced. METHODS: We asked whether the simultaneous transmission of ‘signal 1+2’ (the TCR-recognized antigen + co-stimulation) along with the ‘homeostatic proliferative’ signal could further enhance the anti-tumor immunity induced by immunotransplant. To optimize ‘signal 1+2’, lymphoma cells were incubated with a TLR9 agonistic CpG oligodeoxynucleotide, which has been shown to upregulate both surface MHC (signal 1) and co-stimulatory molecule (signal 2) expression. This CpG-NHL was irradiated and administered as a vaccine boost along with the previously described immunotransplant maneuver (i.e. vaccine-primed donor splenocytes and bone marrow transferred into lymphodepleted, syngeneic recipients). RESULTS: CpG-NHL ‘boosting’ increased the proportion of tumor-specific, memory CD8 T cells approximately four fold. In tumor protection experiments, addition of CpG-NHL boost to lower doses of transferred, vaccine-primed splenocytes, increased the proportion of surviving recipients from 0% to 100% in both subcutaneous and systemic tumor challenge models. The CpG-NHL boosting effect was specific to the immunotransplant of vaccine-primed splenocytes and did not induce significant tumor protection in combination with ‘sham’ immunotransplant of normal donor splenocytes. Conversely, the CpG-mediated pre-activation was essential, as ‘boosting’ with non-treated, irradiated NHL cells induced inferior tumor protection. Our prior work demonstrated that myeloablative conditioning of immunotransplant recipients was necessary for significant tumor protection. As a primary goal of this work is the modeling of the immunotransplant system for clinical translation, we asked whether the addition of the CpG-NHL boost could allow non-myeloablative conditioning approaches to be effective in protecting immunotransplant recipients from tumor challenge. We demonstrated that both sub-lethal irradiation and chemotherapeutic conditioned recipients developed significant anti-tumor immunity only when the CpG-NHL boost was combined with immunotransplant. CONCLUSIONS: Several clinical trials using adoptive transfer of anti-tumor lymphocytes have demonstrated objective clinical responses in patients with lymphoma and solid tumors. Our work suggests that such clinical approaches could be enhanced by post-transfer vaccine boosting and also provides a basis to study the interaction of the ‘homeostatic proliferative’ signal with ‘signal 1+2’.