Complement activation is required for B-1 cells antibody response against F. tularensis LPS 4230

Description Francisella tularensis (Ft), a gram-negative bacterium, possesses an atypical LPS (Ft-LPS) that does not activate TLR4 or inflammation and is considered a type 2 T-independent (TI) antigen. We previously showed that Ft-LPS immunization elicits the production of protective Ft-LPS-specific IgM (IgMFt) and IgG3Ft by B-1 cells in a mechanism that is dependent on the IL-25-ILC2-IL-5 axis. Although devoid of inflammatory activity, Ft-LPS is highly immunogenic through unknown mechanism. Here, we examined the role of complement in the B-1 cells antibody response against Ft-LPS. C3-/- and C4-/- mice vaccinated with Ft-LPS failed to produce IgMFt and IgG3Ft. In contrast, the response of Fb-/- and Mbl1/Mbl2-/- mice to vaccination was comparable to that of WT mice. Thus, activation of the classical complement cascade, but not the alternative or the MBL pathway, is required for activation of B-1 cells and production of Ft-LPS-specific antibodies. Complement activation opsonizes antigens with the C3d fragment and releases C3a and C5a anaphylatoxins. Response to Ft-LPS vaccination was drastically decreased in C3ar1-/-, but not in C5ar1-/- mice. Moreover, our preliminary data showed that C3d opsonizes Ft-LPS and that Cr2-/- mice also failed to respond to Ft-LPS vaccination. Taken together, these results show that the high immunogenicity of Ft-LPS is dependent on activation of the classical complement pathway resulting in Ft-LPS opsonization and capture by CR2 and production of C3a.