The Effect of Live Attenuated Influenza Vaccine and Experimental Human Pneumococcal Carriage on Human Immunity

Live Attenuated Influenza Vaccine (LAIV) is used in immunisation campaigns but may alter the dynamics of naturally occurring nasal colonisation by Streptococcus pneumoniae (Spn), a common human pathogen. We tested how the attenuated influenza viruses contained in the vaccine and Spn interact in the host’s nasopharynx using for the first time an Experimental Human Pneumococcal Challenge model (EHPC) with multiple live pathogens: LAIV and Spn of serotype 6B. Two double blinded randomised clinical trials represented two scenarios of controlled co-infection: 1) Antecedent LAIV administration followed by nasopharyngeal Spn inoculation or 2) Concurrent LAIV administration during established Spn colonisation, separated by a 3 day interval. We validated non-invasive micro-sampling techniques for mucosal immunity analysis by comparing reliability and reproducibility of available methods. Absorptive matrices and nasal curettes were established as the preferred techniques to investigate lining fluid and immune cells in the nasal mucosa. In addition, we collected nasal wash, BAL and serum from healthy adults to investigate immune cell recruitment, cytokine and influenza-specific antibody responses using flow cytometer, human cytokine 30-plex panel and ELISA analysis. Here, we showed that LAIV-induced inflammation in the nasopharynx was associated with Spn colonisation. Immune responses to Spn and to the attenuated influenza virus were impaired by LAIV, reducing chemoattractant cytokines, recruitment of monocytes, and activation of T-cells and neutrophils. In the lung, our results demonstrated that LAIV induces inflammatory cytokines produced by T-cells and that tissue-resident memory T-cells have an important role in producing specific cytokines against the attenuated influenza virus. In short, LAIV was shown to be immunogenic in healthy adults, but less in Spn colonised individuals, highlighting the significance of nasal microbiota when developing vaccines and assessing its efficacy