Interaction of B cells with intracellular pathogen Francisella tularensis

Immunity to Francisella tularensis is largely mediated by T lymphocytes but an important role of B lymphocytes in early stage of infection was previously uncovered. We wanted to find out if F. tularensis is able to infect B cells and/or influence them by direct contact. To investigate this possibility we infected B cell lines from mouse (A20) or humans (Ramos RA-1), or primary mouse spleen cells, with F. tularensis LVS and F. tularensis FSC200 in vitro. In all cases, we detected bacteria on the cell surface and inside the B cells using transmission electron microscopy. More than 20% cells were infected by microbes after 24 h. The number of bacteria, determined by CFU, increased about 1 log during 24 h. Infection with live bacteria led to apoptosis of Ramos cells and mouse CD19+ spleen cells. Approximately 30% of cells were apoptotic after 24 h and 70% after 48 h, independently of the F. tularensis strain, while only 10% of non-infected cell were apoptotic at either time point. Apoptosis was confirmed by Western blot using anti-PARP antibodies. Thus, this study demonstrates unique phenomenon – namely, the ability of the intracellular pathogen F. tularensis to invade and induce apoptosis in B cells.