Broadly reactive human CD4(+) T cells against Enterobacteriaceae are found in the naïve repertoire and are clonally expanded in the memory repertoire

Enterobacteriaceae are a large family of Gram‐negative bacteria that includes both commensals and opportunistic pathogens. The latter can cause severe nosocomial infections, with outbreaks of multi‐antibiotics resistant strains, thus being a major public health threat. In this study, we report that Enterobacteriaceae‐reactive memory Th cells were highly enriched in a CCR6(+)CXCR3(+) Th1*/17 cell subset and produced IFN‐γ, IL‐17A, and IL‐22. This T cell subset was severely reduced in septic patients with K. pneumoniae bloodstream infection who also selectively lacked circulating K. pneumonie‐reactive T cells. By combining heterologous antigenic stimulation, single cell cloning and TCR Vβ sequencing, we demonstrate that a large fraction of memory Th cell clones was broadly cross‐reactive to several Enterobacteriaceae species. These cross‐reactive Th cell clones were expanded in vivo and a large fraction of them recognized the conserved outer membrane protein A antigen. Interestingly, Enterobacteriaceae broadly cross‐reactive T cells were also prominent among in vitro primed naïve T cells. Collectively, these data point to the existence of immunodominant T cell epitopes shared among different Enterobacteriaceae species and targeted by cross‐reactive T cells that are readily found in the pre‐immune repertoire and are clonally expanded in the memory repertoire.