Francisella tularensis Induces Ubiquitin-Dependent Major Histocompatibility Complex Class II Degradation in Activated Macrophages

The intracellular bacterium Francisella tularensis survives and replicates within macrophages, ultimately killing the host cell. Resolution of infection requires the development of adaptive immunity through presentation of F. tularensis antigens to CD4(+) and CD8(+) T cells. We have previously established that F. tularensis induces macrophage prostaglandin E-2 (PGE(2)) production, leading to skewed T-cell responses. PGE(2) can also downregulate macrophage major histocompatibility complex (MHC) class II expression, suggesting that F. tularensis-elicited PGE(2) may further alter T-cell responses via inhibition of class II expression. To test this hypothesis, gamma interferon (IFN-gamma)-activated reporter macrophages were exposed to supernatants from F. tularensis-infected macrophages, and the class II levels were measured. Exposure of macrophages to infection supernatants results in essentially complete clearance of surface class II and CD86, compromising the macrophage's ability to present antigens to CD4 T cells. Biochemical analysis revealed that infection supernatants elicit ubiquitin-dependent class II downregulation and degradation within intracellular acidic compartments. By comparison, exposure to PGE(2) alone only leads to a minor decrease in macrophage class II expression, demonstrating that a factor distinct from PGE(2) is eliciting the majority of class II degradation. However, production of this non-PGE(2) factor is dependent on macrophage cyclooxygenase activity and is induced by PGE(2). These results establish that F. tularensis induces the production of a PGE(2)-dependent factor that elicits MHC class II downregulation in IFN-gamma-activated macrophages through ubiquitin-mediated delivery of class II to lysosomes, establishing another mechanism for the modulation of macrophage antigen presentation during F. tularensis infection.