Protection against Neisseria meningitidis nasopharyngeal colonization relies on antibody opsonization and phagocytosis by neutrophils

Neisseria meningitidis is a human-restricted pathogen that can cause a rapidly progressing invasive meningococcal disease, yet it is also a regular inhabitant of the human nasopharynx. Vaccines that target N. meningitidis aim to prevent invasive disease, but their ability to interfere with nasal colonization could effectively eradicate this bacteria in a population, and so is an important target for meningococcal vaccine design. While protection against invasive meningococcal disease is classically attributed to IgG-dependent complement activation and bacterial killing, there remains no indication of what confers protection against nasopharyngeal colonization, making it impossible to deliberately target this stage during vaccine development. Moreover, without understanding what confers protection in this tissue site, it is impossible to understand the level of susceptibility within a population. To address this, we have taken advantage of the CEACAM1-humanized mouse model to characterize immune effectors that protect against nasal carriage of N. meningitidis. Protection against nasal colonization could be induced by live mucosal infection or by parenteral immunization with heat-killed bacteria. Mice possessing genetic deficiencies in B cells were used to evaluate the role of B cells and a specific antibody response, while neutrophil and complement depletion were used to evaluate their respective contributions to immunization-induced protection against meningococcal nasal carriage. Despite the essential role for complement killing in preventing invasive meningococcal disease, complement was not required for protection against nasal colonization. Instead, N. meningitidis-specific antibodies and neutrophils were both required to protect mice against the nasal infection. Combined, these data suggest that phagocytic bacterial killing is necessary for protection against mucosal colonization by N. meningitidis, indicating that nasal immunoglobulin with the ability to promote opsonophagocytosis must be considered as a correlate of protection against meningococcal carriage.