Signal transducer and activator of transcription 1 (STAT1) mediates interferon gamma signaling which activates the expression of various genes related to apoptosis, inflammation, cell cycle and angiogenesis. Several experimental and clinical studies have investigated the role of STAT1 in primary tumor growth in breast cancer; however, its role in tumor metastasis remains to be determined. To determine the role of STAT1 in breast cancer metastasis, we analyzed growth and metastasis in WT or STAT1−/− mice orthotopically implanted with metastatic 4T1.2 cells. Primary tumor development was faster in STAT1−/− mice and these mice developed significantly bigger primary tumors and displayed more lung metastasis compared with WT counterparts. STAT1−/− mice showed elevated Ly6G+CD11b+ granulocytic MDSC infiltration in their primary tumors and spleens with concomitant upregulation of Mmp9 and Cxcl1 expression in tumors compared with WT counterparts. Blockade of IL-17A in primary tumor-bearing STAT1−/− mice suppressed accumulation of Ly6G+CD11b+ cells and markedly reduced lung metastasis. These data show that STAT1 is an important suppressor of primary breast tumor growth and metastasis. Importantly, we found anti-IL-17 treatment can rescue STAT1 deficient animals from developing exacerbated metastasis to the lungs which could be important for immunotherapies for immunocompromised breast cancer patients.

STAT4 is critical for the production of IFN-γ during the generation of T1 immune responses. We investigated the role of STAT4 in mediating T1-inducing activity of a vaccine adjuvant monophosphoryl lipid A (MPL-A) using the standard antigen ovalbumin (OVA) in STAT4KO mice. Our results show that splenocytes from STAT4KO mice displayed lower OVA-specific T-cell proliferation and IL-2 production compared with wild-type (WT) mice. Further, IFN-γ production was diminished in STAT4KO-derived splenocytes but the levels of IL-12 and TNF-α were similar compared with WT mice. Interestingly, STAT4 deficiency also led to a decrease in IL-10 and T2 cytokines such as IL-4 and IL-13 upon MPL-A immunization, although IL-17 production was similar between WT- and STAT4KO-derived splenocytes. Our observations for defective T1 and T2 responses in STAT4KO mice were further supported by the low levels of T1-associated IgG2a and T2-associated IgG1 in the sera of these mice. Taken together, our results show that STAT4 plays a critical role in mediating both T1 and T2 responses upon immunization with MPL-A. Our study provides a better understanding of how MPL-A mediates T-cell activation which will be critical for future vaccine development.

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