Divergent roles for complement components C3 and C4 in controlling Klebsiella pneumoniae gut colonization and systemic dissemination.

Klebsiella pneumoniae is an escalating public health threat driven by the emergence of antibiotic-resistant and hyper-encapsulated strains that spread systemically from the gut. The immune defenses preventing gut colonization and dissemination remain poorly defined. Here, we identify distinct and context-dependent roles for complement proteins C3 and C4 in host defense following K. pneumoniae infection. Following gut colonization, the levels of C3 and C4 significantly increase, and C3/C4 deposition is found on K. pneumoniae within the gut. In vitro, complement from rabbit, mouse, and human sources induced C3b deposition on K. pneumoniae grown under gut-mimetic conditions, even when C4-dependent pathways were inhibited. In addition to promoting opsonization, C3 was found to be critical for recruiting myeloid cells to the gut and for preventing lethal systemic spread. Depletion of systemic C3 revealed mucosal-derived C3 controls K. pneumoniae gastrointestinal colonization, whereas systemic C3 is essential for limiting fatal dissemination. In contrast, C4 is dispensable for controlling gastrointestinal colonization, dissemination, and myeloid recruitment under conditions of natural acquisition. However, C4 is critical for reducing gut burden and systemic disease following antibiotic-induced dysbiosis and supercolonization with antibiotic-resistant K. pneumoniae. Collectively, these findings reveal a dual-layered immune strategy: C3-driven opsonization and phagocyte recruitment, independent of C4, provide a mechanism for rapid containment of K. pneumoniae gastrointestinal colonization and dissemination under baseline conditions, whereas C4 becomes critical for controlling high bacterial burdens associated with antibiotic resistance. This work advances our understanding of complement-dependent mucosal immunity and identifies potential targets to prevent gut-to-bloodstream transition of this formidable pathogen.IMPORTANCEKlebsiella pneumoniae, a major public health threat, resists antibiotics and can spread from the gut to the bloodstream, causing severe infections. Our study reveals how the immune system uses complement proteins C3 and C4 to block this spread. C3 limits bacterial growth in the gut through two potential mechanisms: (i) coating K. pneumoniae with fragments that signal bacteria-eating phagocytic cells to destroy it and (ii) recruiting more phagocytes into the gut. C3 also helps clear bacteria that escape into the blood. However, when antibiotic-resistant strains overgrow, C3 alone is insufficient. In these cases, C4 becomes critical, likely by enhancing C3's ability to tag bacteria for elimination. This two-layered defense highlights new immune pathways that could be targeted to prevent bloodstream infections, especially in vulnerable patients or those colonized with drug-resistant bacteria. These insights open doors to innovative strategies against life-threatening Klebsiella infections.