Tumor cells upregulate CCL22 in a STING-dependent manner in response to paracrine factors released by STING-activated myeloid cells and type I interferons.

Immunosuppressive elements within the tumor microenvironment include both regulatory T cells (Tregs) and M2 macrophages. A well-described mechanism of Treg recruitment occurs via the chemokine CCL22, and CCL22 has also recently been implicated in the polarization of tumor-associated macrophages to the M2a subtype. Our lab and others have shown that CCL22 is upregulated in cancer cells following activation of the stimulator of interferon genes (STING). STING triggers immune responses against pathogenic nucleic acids as well as self-DNA mislocalized to the cytoplasm, which can accumulate in cancer cells due to chromosomal instability, damaged mitochondria, and increased expression of LINE-1 retrotransposons. STING activation has been associated with both anti-tumor and pro-tumor immune responses, and a potential mechanism of STING-mediated immune evasion is through CCL22 upregulation. CCL22 was first characterized in macrophages, and here we investigate the effects of STING activation on CCL22 expression in macrophages and monocytes. We report that human macrophages and monocytes are resistant to CCL22 upregulation by STING, but that STING-activated macrophages and monocytes release unidentified paracrine factor(s) that dramatically increase CCL22 upregulation in cancer cells in a manner that remains STING-dependent, as evidenced by the inability of STING knockout cells to upregulate CCL22 in response to these factors. We further found that exogenous type I interferons, a major downstream product of STING activation, also upregulate CCL22 in cancer cells via a STING-dependent mechanism and that exogenous IFN-β can directly activate STING.