B cell subsets are activated and produce cytokines during early phases of Francisella tularensis LVS infection

• Francisella tularensis LVS infects peritoneal CD19+ B1a cells after in vivo infection.
• CD19+ cell subsets express activation markers after F. tularensis LVS infection.
• CD19+ cells produce sets cytokines in response to F. tularensis LVS infection.

Francisella tularensis, a facultative intracellular Gram-negative bacterium, causes the illness tularemia. The infection of mice with live vaccine strain is considered to be a model of human tularemia. F. tularensis infects predominantly such phagocytic cells as macrophages or neutrophils, but it also infects non-phagocytic hepatocytes, epithelial cells, and murine and human B cell lines. Based on work with the murine tularemia model, we report here that F. tularensis LVS infects peritoneal CD19+ cells – exclusively B-1a cells – early after intraperitoneal infection in vivo. The peritoneal and consequently spleen CD19+ cells are activated by the F. tularensis LVS infection to express the activation markers from MHC class II, CD25, CD54, CD69, and the co-stimulatory molecules CD80 and CD86. As early as 12 h post-infection, the peritoneal CD19+ cells produce IFN-γ, IL-1β, IL-4, IL-6, IL-12, IL-17, IL-23, and TNF-α. The spleen CD19+ cells respond to infection with some delay. Moreover, the F. tularensis infected A20 B cell line activates CD3+ spleen cells isolated from naïve mice. Thus, the data presented here suggest that B cells have all the attributes to actively participate in the induction and regulation of the adaptive immune response during early stages of F. tularensis infection.