New data in Nature Communications suggests local complement component C3 can shape how tumors respond to checkpoint blockade. Researchers reported that C3’s impact is highly context-dependent—driven by where C3 is produced within the tumor microenvironment—and that it affects myeloid infiltration patterns tied to immunotherapy efficacy. The study links complement biology to immune checkpoint outcomes using mechanistic work that distinguishes local complement effects from systemic signals. The findings point to tumor-specific immune circuitry as a determinant of responsiveness, rather than relying on a single biomarker axis like tumor stage. For translational efforts, the work supports the idea that therapies combining checkpoint inhibitors with agents that modulate complement pathways may need to account for spatial biology—specifically, whether and where complement proteins are expressed in the tumor bed. Overall, the research adds another lever to immunotherapy stratification, with C3 emerging as a candidate target for precision modulation of innate immune cells within tumors.