The tumor microenvironment (TME) significantly influences antitumor immunity, with monocytes and macrophages playing pivotal roles both in pro- and anti- tumoral functions. Tumor-associated macrophages (TAMs) often adopt immunosuppressive phenotypes that promote tumor progression by inhibiting cytotoxic T and NK cells. This study investigates the antitumor mechanisms of intravenous (IV) Bacillus Calmette-Guerin (BCG) in a B16-F10 lung melanoma mouse model, focusing on its impact on monocyte/macrophage populations. Single-cell RNA sequencing revealed that IV BCG reprograms tumor-associated monocyte-derived macrophages (mo-macs), shifting them from immunosuppressive to pro-inflammatory phenotypes enriched in interferon-response signatures. BCG treatment increased the recruitment of classical (Mon Iigp1) and non-classical (Mon Fcgr4) monocytes, which exhibited enhanced antigen presentation and pro-inflammatory cytokine production, while reducing immunosuppressive subsets prevalent in untreated controls. These BCG-induced mo-macs established robust interactions with NK and T cells, promoting their activation and enhancing cytotoxic function, as validated by functional assays. Notably, transfer of BCG reprogrammed bone marrow progenitors into naive recipients elicited a sustained generation of immunostimulatory mo-macs that enhanced NK and T cell responses upon tumor challenge. These findings highlight IV BCG potential as a cancer immunotherapy that targets the myeloid compartment to foster a pro-inflammatory TME, offering durable antitumor immunity by engaging both innate and adaptive immune responses.
Moreo et al. (2025) studied this question.