In mice with EG.7-OVA tumors, efficacy of adoptively transferred OT-I T cells plus OVA peptide vaccination with TLR3/9 agonists was supported by blockade of CTLA-4, but not PD-1. Antibodies against LAG-3 or VISTA demonstrated similar or superior efficacy to anti-CTLA-4. In a prostate tumor model, combining a DNA vaccine against androgen receptor with TLR3/9 agonists and either anti-LAG-3 or anti-CTLA-4 also improved efficacy, more so than with anti-PD-1. With PD-1 blockade, a Treg-depleting antibody increased tumor control, suggesting that Tregs may constrain efficacy of this combination.

Contributed by Alex Najibi

BACKGROUND: T cell checkpoint receptors are expressed when T cells are activated, and modulation of the expression or signaling of these receptors can alter the function of T cells and their antitumor efficacy. We previously found that T cells activated with cognate antigen had increases in the expression of PD-1, and this was attenuated in the presence of multiple toll-like receptor (TLR) agonists, notably TLR3 plus TLR9. In the current report, we sought to investigate whether combining TLR agonists with immune checkpoint blockade can further augment vaccine-mediated T cell antitumor immunity in murine tumor models. METHODS: TLR agonists (TLR3 plus TLR9) and immune checkpoint inhibitors (antibodies targeting PD-1, CTLA-4, LAG-3, TIM-3 or VISTA) were combined and delivered with vaccines or vaccine-activated CD8+T cells to E.G7-OVA or MyC-CaP tumor-bearing mice. Tumors were assessed for growth and then collected and analyzed by flow cytometry. RESULTS: Immunization of E.G7-OVA tumor-bearing mice with SIINFEKL peptide vaccine, coadministered with TLR agonists and _CTLA-4, demonstrated greater antitumor efficacy than immunization with TLR agonists or _CTLA-4 alone. Conversely, the antitumor efficacy was abrogated when vaccine and TLR agonists were combined with _PD-1. TLR agonists suppressed PD-1 expression on regulatory T cells (Tregs) and activated this population. Depletion of Tregs in tumor-bearing mice led to greater antitumor efficacy of this combination therapy, even in the presence of _PD-1. Combining vaccination with TLR agonists and _CTLA-4 or _LAG-3 showed greater antitumor than with combinations with _TIM-3 or _VISTA. CONCLUSION: The combination of TLR agonists and _CTLA-4 or _LAG-3 can further improve the efficacy of a cancer vaccine, an effect not observed using _PD-1 due to activation of Tregs when _PD-1 was combined with TLR3 and TLR9 agonists. These data suggest that optimal combinations of TLR agonists and immune checkpoint blockade may improve the efficacy of human anticancer vaccines.

Author Info: (1) Cancer Biology, University of Wisconsin-Madison, Madison, Wisconsin, USA. (2) Medicine, University of Wisconsin-Madison, Madison, Wisconsin, USA. (3) Cancer Biology, University

Author Info: (1) Cancer Biology, University of Wisconsin-Madison, Madison, Wisconsin, USA. (2) Medicine, University of Wisconsin-Madison, Madison, Wisconsin, USA. (3) Cancer Biology, University of Wisconsin-Madison, Madison, Wisconsin, USA. (4) Medicine, University of Wisconsin-Madison, Madison, Wisconsin, USA dm3@medicine.wisc.edu.