Vaccine-induced antibodies can limit <i>Salmonella</i> infection in the absence of complement or macrophages

Antibodies against <i>Salmonella</i> Typhimurium (STm) can provide protection against infection. Understanding how antibodies, complement, and leukocytes interact is essential and can help advance vaccine development. To evaluate the <i>in vivo</i> role of STm-specific antibodies, mice were immunized with an outer membrane vesicle (OMV) vaccine and subsequently challenged with STm. Immunohistology and intravital microscopy revealed that OMV-induced antibodies promoted STm uptake by macrophages in the spleen and liver, whereas bacteria were only infrequently associated with neutrophils. Depletion of monocytic cells using clodronate liposomes demonstrated that these cells help prevent antigen dissemination. Immunization and challenge experiments in mice deficient in C1q, C3, C4, or C5 showed that OMV immunization conferred protection in all groups except C3-deficient mice. Mice deficient in C3 failed to develop robust germinal center and plasma cell responses following OMV immunization. Nevertheless, C3-deficient mice that received immune sera by adoptive transfer prior to infection had significantly reduced bacterial burdens compared to mice receiving non-immune control sera. <i>In vitro</i> studies using human sera and THP-1 cells confirmed that antibodies alone are sufficient to promote bacterial capture by macrophages. Complement-particularly C1q and C3, but not C5-further enhanced bacterial uptake. Overall, under the conditions tested, antibodies are sufficient to reduce STm burdens <i>in vivo</i> and facilitate bacterial uptake <i>in vitro</i>, with these processes supported by complement and macrophages.IMPORTANCEBacterial infections remain a significant global challenge, further complicated by the growing issue of antimicrobial resistance (AMR). In this context, vaccination provides a cost-effective means of preventing infections and combating the emergence of AMR strains. Antibodies are key mediators of vaccine-induced protection. However, their mechanisms of action <i>in vivo</i> are not fully understood. In this study, we demonstrate how antibodies enhance the capture of <i>Salmonella</i> by macrophages. Although these cells may be dispensable for controlling bacterial numbers, they are crucial for limiting the spread of bacterial antigens. Additionally, we find that the classical complement cascade is not necessary for bacterial clearance but does facilitate bacterial capture by macrophages. Overall, our findings reveal that multiple antibody-mediated pathways operate <i>in vivo</i>, exhibiting some redundancy in their combined efforts to control infection, underscoring the importance of antibody functions in limiting bacterial spread.