Tumour cell-intrinsic complement C1r and C1s regulate cancer cell fitness and shape the immune microenvironment in triple-negative breast cancer.

Emerging evidence positions tumour cell-intrinsic complement proteins as regulators of cancer cell behaviour and immune crosstalk, yet their role in triple-negative breast cancer (TNBC) remains undefined. Here, transcriptomic profiling across breast cancer molecular subtypes reveals that C1R, C1S and C3 are preferentially expressed in TNBC cells, both in patient tumours and in vitro, and that their expression is upregulated by pro-inflammatory cytokines. In situ mRNA hybridization and hyperplex SeqIF immunofluorescence demonstrate that C1R and C1S share more closely matched expression patterns with one another than with C3 within TNBC cell populations. These patterns associate with distinct transcriptional programs: metabolic and cell cycle gene sets are enriched in C1R-, C1S- and C3-high cells, whereas inflammatory and apoptotic signatures are specifically linked to C1R and C1S. Functionally, silencing of C1r or C1s modestly impairs TNBC cell proliferation, viability and tumoursphere formation — effects not rescued by extracellular protein supplementation, supporting an intracellular mechanism. C3 silencing exerts minimal effects. C1R- and C1S-high tumour cells are enriched in chemoattractant chemokines in patient tumours, and their secretion decreases upon complement gene silencing in vitro. C1R and C1S expression further correlates with cytotoxic T cell infiltration and improved patient survival. Together, these findings indicate that whilst cell-intrinsic C1r and C1s enhance modestly tumour cell fitness, their more consequential contribution lies in shaping the tumour immune microenvironment through regulation of chemokine secretion and cytotoxic T cell infiltration, highlighting their role as enhancers of anti-tumour immunity in TNBC.