Parenteral BCG vaccine induces respiratory mucosal-resident memory macrophages and trained innate immunity via the gut-lung axis. Jeyanathan et al.
Besides centrally induced innate immune memory/trained immunity in the bone marrow/peripheral blood by parenteral vaccination or infection, recently emerging evidence suggests that the barrier mucosal tissue-resident innate immune memory may develop via a local inflammatory pathway following mucosal immunologic exposure. However, it remains unclear whether the mucosal-resident innate immune memory may result from integrating distally generated immunological signals following parenteral vaccination/infection. We show here that subcutaneous Bacillus Calmette-Guérin (BCG) vaccination is able to induce memory alveolar macrophages (AM) and trained immunity at the respiratory barrier mucosa. Although parenteral BCG vaccine can centrally train bone marrow progenitors and circulating monocytes, induction of memory AM is entirely independent of circulating monocytes. Rather, parenteral BCG vaccination, via distal mycobacterial dissemination, causes a time-dependent alteration in gut microbiome, barrier function and microbial metabolites including short-chain fatty acids, and subsequently the changes in circulating and lung tissue metabolites, leading to induction of tissue-resident memory macrophages and trained innate immunity in the lung. Our study thus reveals a novel gut microbiota-mediated pathway for innate immune memory development at distal barrier mucosal tissues. Our findings have far-reaching implications in developing next-generation parenteral vaccine strategies against respiratory pathogens such as M.tb.