Cholera toxin B induced activation of murine macrophages exposed to a fixed bacterial immunogen

Objectives: Previous studies have demonstrated that intranasal administration of inactivated (fixed) Francisella tularensis (iFt) live vaccine strain (LVS) in conjunction with the mucosal adjuvant, cholera toxin B (CTB), provides full protection against subsequent lethal challenge with Ft LVS and partial protection against the more virulent Ft SchuS4 strain. Understanding the mechanisms of CTB-induced immune stimulation that confer protection against Ft will be valuable to the development of an effective vaccine against this highly virulent fatal pathogen. In this study, an in vitro system was utilized to further elucidate the immunologic adjuvant effect of CTB when administered with the fixed bacterial immunogen iFt. Methods: The murine macrophage cell line (RAW264.7) was treated with combinations of iFt and CTB. The treated RAW264.7 cells and their supernatants were collected and assessed for cell surface marker expression and cytokine secretion. In addition, the ability of RAW264.7 cells to present bacterial antigens (iFt or LVS) to an Ft-specific T-cell hybridoma cell line, following exposure to CTB, was analyzed. Results: We found that RAW264.7 cells responded to treatment with iFt + CTB by an increased secretion of the proinflammatory cytokines interleukin 6 and tumor necrosis factor α and upregulation of the surface expression of toll-like receptor 4 and the costimulatory molecules CD80 and CD86. Furthermore, the experimental vaccine treatment iFt + CTB enhanced the ability of macrophages to present iFt antigens to an FT-specific T-cell hybridoma cell line, although they failed to do so with LVS. Conclusion: The adjuvant CTB administered in conjunction with iFt showed evidence of enhancing an antigen-specific proinflammatory response in vitro. These observations allow us to define, in part, the mechanisms of immune activation conferred by mucosal administration of iFt + CTB against lethal F. tularensis challenge.