BACKGROUND: Pharyngeal colonisation by the commensal bacterium Neisseria lactamica inhibits colonisation by Neisseria meningitidis and has an inverse epidemiological association with meningococcal disease. The mechanisms that underpin this relationship are unclear, but could involve the induction of cross-reactive immunity. In this study, we aimed to evaluate whether colonisation with N lactamica induces N lactamica-specific B-cell responses that are cross-reactive with N meningitidis. METHODS: In this randomised, placebo-controlled, human infection trial at University Hospital Southampton Clinical Research Facility (Southampton, UK), healthy adults aged 18-45 years were randomly assigned (2:1) to receive intranasal inoculation with either 105 colony-forming units of N lactamica in 1 mL phosphate-buffered saline (PBS) or 1 mL PBS alone. Participants and researchers conducting participant sampling and immunological assays were masked to allocation. The primary endpoint was the frequency of circulating N lactamica-specific plasma cells and memory B cells after N lactamica inoculation (day 7-28) compared with baseline values (day 0), measured using enzyme-linked immunospot assays. The secondary endpoint was to measure the frequency of N meningitidis-specific B cells. In a second study, we measured the effect of duration of N lactamica colonisation on seroconversion by terminating carriage at either 4 days or 14 days with single-dose oral ciprofloxacin. The studies are now closed to participants. The trials are registered with ClinicalTrials.gov, NCT03633474 and NCT03549325. FINDINGS: Of 50 participants assessed for eligibility between Sept 5, 2018, and March 3, 2019, 31 were randomly assigned (n=20 N lactamica, n=11 PBS). Among the 17 participants who were colonised with N lactamica, the median baselines compared with peak post-colonisation N lactamica-specific plasma-cell frequencies (per 105 peripheral blood mononuclear cells) were 0·0 (IQR 0·0-0·0) versus 5·0 (1·5-10·5) for IgA-secreting plasma cells (p<0·0001), and 0·0 (0·0-0·0) versus 3·0 (1·5-9·5) for IgG-secreting plasma cells (p<0·0001). Median N lactamica-specific IgG memory-B-cell frequencies (percentage of total IgG memory B cells) increased from 0·0024% (0·0000-0·0097) at baseline to 0·0384% (0·0275-0·0649) at day 28 (p<0·0001). The frequency of N meningitidis-specific IgA-secreting and IgG-secreting plasma cells and memory B cells also increased signficantly in participants who were colonised with N lactamica. Upper respiratory tract symptoms were reported in ten (50%) of 20 participants who were inoculated with N lactamica and six (55%) of 11 participants who were inoculated with PBS (p>0·99). Three additional adverse events (two in the N lactamica group and one in the PBS group) and no serious adverse events were reported. In the second study, anti-N lactamica and anti-N meningitidis serum IgG titres increased only in participants who were colonised with N lactamica for 14 days. INTERPRETATION: Natural immunity to N meningitidis after colonisation with N lactamica might be due to cross-reactive adaptive responses. Exploitation of this microbial mechanism with a genetically modified live vector could protect against N meningitidis colonisation and disease. FUNDING: Wellcome Trust, Medical Research Council, and NIHR Southampton Biomedical Research Centre.