Background <p>Tumor-associated macrophages (TAMs) in the tumor microenvironment (TME) primarily polarize into the M2-phenotype. Our previous study showed that the small GTPase Rab37 mediates IL-6 trafficking in macrophages for M2 polarization. Here, we uncover an unconventional role of Rab37, independent of vesicle trafficking, in promoting M2 polarization of TAMs.</p> Methods <p>The gene profiles in wild-type and <i>Rab37</i> knockout (KO) bone marrow-derived macrophages (BMDMs) were analyzed using cDNA microarray. The mechanism of Rab37 in regulating the interferon (IFN) pathway was confirmed through in vitro/vivo assays and clinical studies.</p> Results <p>Type I IFN signaling was highly enriched in BMDMs from <i>Rab37</i> KO mice. Moreover, Rab37 induction and decreased type I IFN genes were observed in macrophages treated with lung cancer-conditioned medium and epigenetic drugs, indicating an epigenetic regulation of Rab37 in TAMs. Mechanistically, GDP-bound Rab37 interacted with the nuclear localization sequence of STAT1 to sequest it in the cytosol from its transcription activities, thus leading to the downregulation of IFN genes. Clinically, CD163⁺/Rab37⁺/STAT1^cytosol in TAMs expression signature correlated with advanced tumor stages and poor survival of lung cancer patients.</p> Conclusions <p>Our findings highlight the cytosolic interaction of Rab37-STAT1 in M2 TAM polarization, with CD163⁺/Rab37⁺/STAT1^cytosol TAMs as a lung cancer prognosis biomarker.</p> <p></p>