Colonization of the upper respiratory tract with Streptococcus pneumoniae is the precursor of pneumococcal pneumonia and invasive disease. Following exposure, however, it is unclear which human immune mechanisms determine whether a pathogen will colonize. We used a human challenge model to investigate host-pathogen interactions in the first hours and days following intranasal exposure to Streptococcus pneumoniae. Using a novel home sampling method, we measured early immune responses and bacterial density dynamics in the nose and saliva after volunteers were experimentally exposed to pneumococcus. Here, we show that nasal colonization can take up to 24 h to become established. Also, the following two distinct bacterial clearance profiles were associated with protection: nasal clearers with immediate clearance of bacteria in the nose by the activity of pre-existent mucosal neutrophils and saliva clearers with detectable pneumococcus in saliva at 1 h post challenge and delayed clearance mediated by an inflammatory response and increased neutrophil activity 24 h post bacterial encounter. This study describes, for the first time, how colonization with a bacterium is established in humans, signifying that the correlates of protection against pneumococcal colonization, which can be used to inform design and testing of novel vaccine candidates, could be valid for subsets of protected individuals. IMPORTANCE Occurrence of lower respiratory tract infections requires prior colonization of the upper respiratory tract with a pathogen. Most bacterial infection and colonization studies have been performed in murine and in vitro models due to the current invasive sampling methodology of the upper respiratory tract, both of which poorly reflect the complexity of host-pathogen interactions in the human nose. Self-collecting saliva and nasal lining fluid at home is a fast, low-cost, noninvasive, high-frequency sampling platform for continuous monitoring of bacterial encounter at defined time points relative to exposure. Our study demonstrates for the first time that, in humans, there are distinct profiles of pneumococcal colonization kinetics, distinguished by speed of appearance in saliva, local phagocytic function, and acute mucosal inflammatory responses, which may either recruit or activate neutrophils. These data are important for the design and testing of novel vaccine candidates.

Background The COVIH-study is a prospective SARS-CoV-2 vaccination study in 1154 people with HIV (PWH), of whom 14% showed a reduced or absent antibody response after primary vaccination. We evaluated whether an additional vaccination boosts immune responses in these hyporesponders. Methods Consenting hyporesponders received an additional 100µg mRNA-1273 vaccination. The primary endpoint was the increase in antibodies 28 days thereafter. Secondary endpoints were the correlation between participant characteristics and antibody response, levels of neutralizing antibodies, S-specific T-cell and B-cell responses, and reactogenicity. Results Of the 66 participants, 40 previously received two doses ChAdOx1-S, 22 two doses BNT162b2, and four a single dose Ad26.COV2.S. The median age was 63[IQR:60-66], 86% were male, pre-vaccination CD4+ T-cell count was median 650/μL[IQR:423-941] and 96% had HIV-RNA < 50 copies/mL. The mean S1-specific antibody level increased from 35 BAU/mL (95%CI:24–46) to 4317 BAU/mL (95%CI:3275–5360) post-vaccination (p < 0.0001). Of all participants, 97% showed an adequate response (>300 BAU/mL) and the 45 antibody negative participants all seroconverted (>33.8 BAU/mL). A significant increase in the proportion of PWH with detectable ancestral S-specific CD4+ T-cells (p = 0.04) and S-specific B-cells (p = 0.02) was observed. Conclusion An additional mRNA-1273 vaccination induced a robust serological response in 97% of PWH with a hyporesponse after primary vaccination.

Rationale: Pneumococcal colonisation is key to the pathogenesis of invasive disease, but is also immunogenic in young adults, protecting against re-colonisation. Colonisation is rarely detected in older adults, despite high rates of pneumococcal disease. Objectives: To establish experimental human pneumococcal colonisation in healthy adults aged 50-84 years, to measure the immune response to pneumococcal challenge, and to assess the protective effect of prior colonisation against autologous strain rechallenge. Methods: Sixty-four participants were inoculated with Streptococcus pneumoniae (serotype 6B, 80,000CFU in each nostril). Colonisation was determined by bacterial culture of nasal wash, serum anti-6B capsular IgG responses by ELISA, and anti-protein immune responses by multiplex electrochemiluminescence. Measurements and Main Results: Experimental colonisation was established in 39% of participants (25/64) with no adverse events. Colonisation occurred in 47% (9/19) of participants aged 50-59 compared with 21% (3/14) in those aged 70 years and older. Previous pneumococcal polysaccharide vaccination did not protect against colonisation. Colonisation did not confer serotype-specific immune boosting: GMT (95% CI) 2.7ug/mL (1.9-3.8) pre-challenge versus 3.0 (1.9-4.7) four weeks post-colonisation (p = 0.53). Furthermore, pneumococcal challenge without colonisation led to a drop in specific antibody levels from 2.8ug/mL (2.0-3.9) to 2.2ug/mL (1.6-3.0) post-challenge (p = 0.006). Anti-protein antibody levels increased following successful colonisation. Rechallenge with the same strain after a median of 8.5 months (IQR 6.7-10.1) led to recolonisation in 5/16 (31%). Conclusions: In older adults, experimental pneumococcal colonisation is feasible and safe, but demonstrates different immunological outcomes compared with younger adults in previous studies.

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