T-cell costimulation typically occurs in a defined microenvironment that is not recapitulated by agonistic antibody therapy (Abstracts, Heat Biologics, JUN 30, 2016, View Source [SID1234517429]). To deliver such stimulation under more favorable conditions, we investigated whether an allogeneic cell-based vaccine that secreted Fc-OX40L, Fc-ICOSL, or Fc-4-1BBL would activate and expand T cells comparably to systemically administered agonist antibodies. Among these costimulators, locally secreted Fc-OX40L provided superior priming of antigen-specific CD8+ T cells, compared to combinations with OX40 antibodies or vaccine alone. Vaccine-expressed Fc-OX40L also stimulated IFNγ, TNFα, granzyme B, and IL2 by antigen-specific CD8+ T cells similarly to OX40 antibodies, without off-target consequences such as proinflammatory cytokine induction. Vaccine-secreted Fc-OX40L increased CD127+KLRG-1- memory precursor cells during the contraction phase, resulting in improved proliferation upon secondary antigen challenge, as compared to OX40 antibody. A cell-based vaccine co-secreting gp96-Ig and Fc-OX40L led to even more pronounced tumor control, complete tumor rejection, and increased tumor antigen-specific T-cell proliferation, including in TILs, as compared to combinations of gp96-Ig vaccine and OX40 antibodies, in mice with established melanoma or colorectal carcinoma. These data suggest that local modulation of the vaccine microenvironment has unexpected advantages over systemic costimulation with agonistic antibodies, which may simplify the clinical translation of such combination immunotherapies into humans.

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