Inhibition of the transcription factor PU.1 suppresses tumor growth in mice by promoting the recruitment of cytotoxic lymphocytes through the CXCL9-CXCR3 axis

Tumor-associated macrophages (TAMs) are among the most abundant immune cells associated with tumors, which often exhibit immune regulatory phenotypes that promote tumor growth and confer resistance to anti-tumor immune therapies. Despite extensive efforts in developing immunotherapeutic strategies aimed at controlling the recruitment or reprogramming of TAMs, success has been limited due to strategic caveats, underscoring the need for a novel approach targeting the TAMs. PU.1, a lineage-dependent transcription factor, is highly expressed throughout the lifespan of macrophages. We have found that inhibition of PU.1 by the small molecule DB2313 suppresses melanoma tumor growth in mice through enhanced tumor recruitment of CD4+ T helper cells and cytotoxic T/NK cells mediated by TAMs. Whole transcriptome and targeted gene expression analyses revealed that DB2313 upregulates CXCL9 expression in bone marrow-derived macrophages (BMDMs) and TAMs. The anti-tumor effects of DB2313 were abolished by depleting macrophages with clodronate or inhibiting the CXCL9-CXCR3 chemokine axis using neutralizing antibodies against CXCL9 or CXCR3. Collectively, these results suggest that pharmacological inhibition of PU.1 suppresses tumor growth by promoting tumor infiltrating lymphocytes through the CXCL9-CXCR3 chemokine axis. Our study establishes a framework for developing TAM-modulating immunotherapies by targeting the transcriptional factor PU.1.