Basophils Promote Tumor Rejection via Chemotaxis and Infiltration of CD8+ T Cells

Elevated numbers of regulatory T cells (Treg) in patient tumors are known to inhibit efficient antitumor T-cell responses. To study the mechanisms controlling tumor rejection, we assessed different mouse models for Treg depletion. In Foxp3DTR knockin mice, about 99% Treg depletion was achieved, resulting in complete rejection of transplanted HCmel12 melanomas in a CD8(+) T-cell-dependent way. In contrast, about 90% Treg depletion obtained in BAC transgenic Foxp3. LuciDTR4 mice failed to induce complete rejection of HCmel12 melanomas, demonstrating that residual Tregs were able to control CD8(+) T-cell responses against the tumor. Ninety-nine percent of Treg depletion pro-voked drastic changes in the tumor microenvironment, such as strong infiltration of CD8(+) T cells and basophils. Intratumoral basophils enhanced CD8(+) T-cell infiltration via production of chemokines CCL3 and CCL4; antibody-based blocking of these chemokines inhibited CD8(+) T-cell infiltration. Therapeutic induction of basophilia by IL3/anti-IL3 antibody complexes, combined with transfer of CD8(+) T cells, resulted in enhanced T-cell infiltration and tumor rejection. Our study identifies a critical role basophils play in tumor rejection and that this role can be exploited for therapeutic intervention. (C) 2016 AACR.