Inhibition of the Transcription Factor PU.1 Suppresses Tumor Growth in Mice by Promoting the Recruitment of Cytotoxic Lymphocytes Through the CXCL9-CXCR3 Axis

<b>Background</b>: Targeting tumor-associated macrophages (TAMs) is a promising immunotherapy for cancers, but current strategies are limited due to strategic caveats. PU.1 is a transcription factor required for macrophage generation and differentiation. To date, the effect of PU.1 inhibition on solid tumors is unknown. <b>Methods</b>: This study examines the anti-tumor effect of PU.1 inhibition and its mechanism using the small-molecule DB2313 in mouse melanoma and breast tumor models. <b>Results</b>: We found that inhibition of PU.1 by DB2313 suppresses B16-OVA melanoma and 4T1 breast tumor growth in mice. In the melanoma tumor model, DB2313 enhanced tumor recruitment of CD4⁺ T helper 1 (Th1) and cytotoxic T/natural killer (NK) cells by targeting TAMs. Transcriptome and targeted gene expression analyses revealed that PU.1 inhibition by DB2313 and small-interference RNAs enhances CXCL9 expression in bulk tumors, TAMs, and bone marrow-derived macrophages. The anti-tumor effects of DB2313 were abolished by depleting macrophages with clodronate or inhibiting the CXCL9-CXCR3 chemokine axis using CXCL9- or CXCR3-neutralizing antibodies. <b>Conclusions</b>: These results suggest that pharmacological inhibition of PU.1 suppresses tumor growth by at least promoting the infiltration of lymphocytes into tumors through the CXCL9-CXCR3 chemokine axis. Our study establishes a framework for developing TAM-modulating immunotherapies by targeting the transcriptional factor PU.1.