The Gut-Lung Axis and Butyrate-Mediated Immunomodulation: A Study on Pneumococcal Pneumonia Severity and Vaccine Efficacy

Abstract

Lower respiratory tract infections (LRTI) remain a leading cause of mortality worldwide. Community-acquired pneumonia (CAP) accounts for a significant burden of LRTI cases, with Streptococcus pneumoniae being the primary aetiological agent of CAP globally and also causing other forms of invasive disease (IPD), namely otitis media, meningitis, bacteraemia and sepsis. Despite the availability of pneumococcal vaccines, the global burden of pneumococcal pneumonia and IPD remains high. The gut microbiota has emerged as a significant regulator of chronic respiratory diseases and infections through the gut-lung axis of immune regulation. Short-chain fatty acids (SCFA) are highly pleiotropic immunomodulatory microbiota-derived metabolites, produced exclusively by microbial commensals through the saccharolytic fermentation of dietary fibre. Acetate, propionate and butyrate are the three most abundant short-chain fatty acids in the intestinal tract, the peripheral circulation and the lung.

Dysbiosis, the imbalance or depletion of the microbiota, increases susceptibility to respiratory infection and is noted in patients with chronic respiratory disease. However, the exact mechanisms behind how the gut microbiota modulates lung immunity remain to be fully elucidated. SCFAs, as systemic messengers of the microbiota, alleviate respiratory infections in murine models of Influenza, Klebsiella pneumoniae, and recently, S. pneumoniae under eubiotic conditions. Potentially therapeutic links have been identified between intake of the prebiotic agent of SCFA, fermentable fibre and outcomes of respiratory infection and disease. Here, we examine whether the therapeutic potential of butyrate can be harnessed to alleviate pneumococcal pneumonia in a highly susceptible model of antibiotic-induced dysbiosis. Bulk RNA sequencing revealed that butyrate treatment altered the transcriptomic profile of alveolar macrophages (AMs) in vitro, enhancing antimicrobial pathways and significantly upregulating genes associated with several antimicrobial peptides. Gentamicin protection assays in both murine and human AMs further highlighted the ability of butyrate to enhance intracellular containment of S. pneumoniae while limiting IL-6 production. In vivo butyrate treatment effectively alleviated the severity of pneumococcal infection in highly susceptible dysbiotic mice, attenuating manifestations of disease such as hypothermia and reducing local and systemic bacterial burden.

Our current understanding of how gut commensal composition and microbiota-derived metabolites impact vaccine immunogenicity and efficacy could underpin heterogeneity in vaccine performance amongst the global population. Current conjugated pneumococcal vaccines have proved successful in limiting the burden of IPD; however, limitations still remain, such as serotype replacement, limited protection against pneumonia and limited induction of mucosal immunity. The modulatory impact of SCFA on vaccine immunogenicity and efficacy remains to be elucidated and has not been studied in the context of pneumococcal vaccination or intranasal vaccination. Here, the effect of oral butyrate supplementation on the response to the licensed pneumococcal vaccine Prevnar13 is investigated. Additionally, the effects of butyrate supplementation on a trivalent pneumococcal vaccine formulated with heat-killed whole-cell lysed S. pneumoniae combined with the adjuvant double-mutant heat-labile toxin (dmLT) were studied. Conversely to the initial hypothesis, butyrate supplementation limited antigen-specific humoral responses locally in the lung, with reductions in antigen-specific IgA and systemically with reductions in serum antigen-specific IgG subclasses. Additionally, butyrate supplementation modulated antigen-specific cellular-mediated immunity, altering lymphocyte dynamics and functional characteristics. Collectively, these findings open new avenues for harnessing the gut-lung axis against pneumococcal pneumonia, while emphasising the need to optimise and rationalise the use of widely consumed butyrate supplements to selectively boost protective immunity against pneumococcal infection without compromising the efficacy of future novel mucosal pneumococcal vaccines.