Smallpox vaccination induces a substantial increase in commensal skin bacteria that promote pathology and enhance immunity

Interactions between pathogens, host microbiota and the immune system influence many physiological and pathological processes. In the 20th century, widespread dermal vaccination with vaccinia virus (VACV) led to the eradication of smallpox but how VACV interacts with the microbiota and whether this influences the efficacy of vaccination are largely unknown. Here we report that intradermal vaccination with VACV induces a large increase in the number of commensal bacteria in infected tissue, which enhance recruitment of inflammatory cells, promote tissue damage and increase immunity. Treatment of vaccinated specific-pathogen-free (SPF) mice with antibiotic, or infection of genetically-matched germ-free (GF) animals caused smaller lesions without alteration in virus titre. Tissue damage correlated with enhanced neutrophil and T cell infiltration and levels of pro-inflammatory tissue cytokines and chemokines. One month after vaccination, GF mice had reduced VACV-neutralising antibodies compared to SPF mice; while numbers of VACV-specific CD8+ T cells were equal in all groups of animals. Thus, skin microbiota may provide an adjuvant-like stimulus during vaccination with VACV. This observation has implications for dermal vaccination with live vaccines. Author Summary Smallpox was caused by variola virus and was eradicated by widespread dermal vaccination with vaccinia virus (VACV), a related orthopoxvirus of unknown origin. Eradication was declared in 1980 without an understanding of the immunological correlates of protection, or knowledge of the effect of smallpox vaccination on the local microbiota. Here we demonstrate that intradermal infection of mice with VACV induces a ∼1000-fold expansion of commensal skin bacteria that influence the recruitment of inflammatory cells into the infected tissue and enhance the size of the vaccination lesion. Antibiotic treatment reduced lesion size without changing virus titres. The bacterial expansion also contributes to the level of neutralizing antibodies at one month post vaccination, because genetically matched germ-free mice developed lower neutralizing antibodies than specific pathogen free controls. Thus, dermal infection by VACV enhanced bacterial growth and these bacteria promote pathology and enhance the antibody response. This finding has implication for dermal vaccination with live vaccines. ### Competing Interest Statement The authors have declared no competing interest.