High Fc Gamma R Expression On Intratumoral Macrophages Enhances Tumor-Targeting Antibody Therapy

Therapy with tumor-specific Abs is common in the clinic but has limited success against solid malignancies. We aimed at improving the efficacy of this therapy by combining a tumor-specific Ab with immune-activating compounds. In this study, we demonstrate in the aggressive B16F10 mouse melanoma model that concomitant application of the anti-TRP1 Ab (clone TA99) with TLR3-7/8 or -9 ligands, and IL-2 strongly enhanced tumor control in a therapeutic setting. Depletion of NK cells, macrophages, or CD8(+) T cells all mitigated the therapeutic response, showing a coordinated immune rejection by innate and adaptive immune cells. Fc gamma Rs were essential for the therapeutic effect, with a dominant role for Fc gamma RI and a minor role for Fc gamma RIII and Fc gamma RIV. Fc gamma R expression on NK cells and granulocytes was dispensable, indicating that other tumoricidal functions of NK cells were involved and implicating that Fc gamma RI, -III, and -IV exerted their activity on macrophages. Indeed, F4/80(+)Ly-6C(+) inflammatory macrophages in the tumor microenvironment displayed high levels of these receptors. Whereas administration of the anti-TRP1 Ab alone reduced the frequency of these macrophages, the combination with a TLR agonist retained these cells in the tumor microenvironment. Thus, the addition of innate stimulatory compounds, such as TLR ligands, to tumor-specific Ab therapy could greatly enhance its efficacy in solid cancers via optimal exploitation of Fc gamma Rs.