Tumor-associated eosinophilia is frequently observed in cancer. However, despite numerous studies of patients with cancer and mouse models of cancer, it has remained uncertain if eosinophils contribute to tumor immunity or are mere bystander cells. Here we report that activated eosinophils were essential for tumor rejection in the presence of tumor-specific CD8(+) T cells. Tumor-homing eosinophils secreted chemoattractants that guided T cells into the tumor, which resulted in tumor eradication and survival. Activated eosinophils initiated substantial changes in the tumor microenvironment, including macrophage polarization and normalization of the tumor vasculature, which are known to promote tumor rejection. Thus, our study presents a new concept for eosinophils in cancer that may lead to novel therapeutic strategies.

Despite the potential efficacy of cancer immunotherapy in preclinical studies, it did not show yet significant positive clinical results in humans with only a small number of cancer patients demonstrating objective tumor regression. This poor clinical outcome can be explained by the generation of sophisticated tumor immune escape mechanism, in particular, abnormalities in the expression of HLA class I antigens. We have studied the expression of HLA class I antigens in ten metastatic lesions obtained from a melanoma patient undergoing immunotherapy. Five lesions were obtained after Interferon-alpha-2b treatment and five after autologous vaccination plus BCG (M-VAX). Eight metastases were regressing after immunotherapy while two were progressing. The eight regressing metastases showed high level of HLA class I expression, whereas the two progressing lesions had low levels as measured by real time PCR and immunohistological techniques. These results indicate a strong association between HLA class I expression and progression or regression of the metastatic lesions. Our data support the hypothesis that the level of HLA class I expression is an important parameter of tumor immune escape that needs to be monitored.

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 knock-in 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 provoked 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. Cancer Res; 77(2); 291-302. ©2016 AACR.