Maturation modulates caspase-1-independent responses of dendritic cells to Anthrax lethal toxin.

Anthrax lethal toxin (LT) contributes to the immune evasion strategy of Bacillus anthracis by impairing the function of cells of the immune system, such as macrophages and dendritic cells (DCs). Macrophages from certain inbred mice strains undergo rapid death upon LT treatment mediated by caspase-1 activation dependent on Nalp1b, an inflammasome component. Rapid LT-induced death is however, not observed in macrophages from human and many mouse strains. Here, we focused on the responses of various murine DCs to LT. Using a variety of knockout mice, we found that depending on the mouse strain, death of bone marrow-derived DCs and macrophages was mediated either by a fast Nalp1b and caspase-1-dependent, or by a slow caspase-1-independent pathway that was triggered by the impairment of MEK1/2 pathways. Caspase-1-independent death was observed in cells of different genetic backgrounds and interestingly occurred only in immature DCs. Maturation, triggered by different types of stimuli, led to full protection of DCs. These studies illustrate that the cellular damage inflicted by LT depends not only on the innate responses but also on the maturation stage of the cell, which modulates the more general caspase-1-independent responses.

Enhancing immunogenicity by limiting susceptibility to lysosomal proteolysis.

T cells recognize protein antigens as short peptides processed and displayed by antigen-presenting cells. However, the mechanism of peptide selection is incompletely understood, and, consequently, the differences in the immunogenicity of protein antigens remain largely unpredictable and difficult to manipulate. In this paper we show that the susceptibility of protein antigens to lysosomal proteolysis plays an important role in determining immunogenicity in vivo. We compared the immunogenicity of proteins with the same sequence (same T cell epitopes) and structure (same B cell epitopes) but with different susceptibilities to lysosomal proteolysis. After immunizing mice with each of the proteins adsorbed onto aluminum hydroxide as adjuvant, we measured serum IgG responses as a physiological measure of the antigen's ability to be presented on major histocompatibility complex class II molecules and to prime CD4+ T cells in vivo. For two unrelated model antigens (RNase and horseradish peroxidase), we found that only the less digestible forms were immunogenic, inducing far more efficient T cell priming and antibody responses. These findings suggest that stability to lysosomal proteolysis may be an important factor in determining immunogenicity, with potential implications for vaccine design.