| There is an unmet need for novel and effective treatments for lymphoma, the fifth most common malignancy in the US. In situ vaccination---an immunotherapeutic maneuver involving local irradiation, intratumoral (i.t.) injections of Flt3L, a hematopoietic growth factor, and Toll-like receptor (TLR) agonist---has been shown in recent clinical trials to induce partial and complete remissions in patients with low-grade lymphoma. The strength of anti-tumor response correlates with the potency of immunogenic dendritic cells (DCs) to efficiently uptake and present tumor antigens to T cells. While the latest clinical trial employs Poly-ICLC---a synthetic TLR3 agonist---to activate DCs, we hypothesize that "natural" TLR agonists (nTLRa) contained within prophylactic vaccines could simultaneously target multiple TLRs and be repurposed as clinical- grade DC activators for the in situ vaccination maneuver. We screened twenty-four prophylactic vaccines for TLR ligand activity using in vitro assays. DC subsets were gated for: (i.) activation markers, (ii.) ability to co-stimulate T cells in the context of T cell receptor activation, and (iii.) ability of nTLRa-activated DCs to induce T cell proliferation. We identify a combination of vaccines---Typhim, BCG, and MMR---that possesses the ability to induce high expression of costimulatory molecules on DCs in vitro, as well as co-stimulating in vitro T cell activation and proliferation. Cohorts of mice implanted with A20 lymphoma tumors that received i.t. injections of Typhim, BCG, MMR demonstrated slower tumor growth status as compared to the control cohort receiving only Flt3L and local irradiation. Combinations of the best nTLRa candidates display synergistic activation of DCs and long term in vivo anti-A20 murine lymphoma immunity. Our results demonstrate that prophylactic vaccines contain natural ligands to TLRs and are immediately translatable as sources of clinical-grade stimulators of dendritic cells. |
