Interferon-γ- and glucocorticoid-mediated pathways synergize to enhance death of CD4(+) CD8(+) thymocytes during Salmonella enterica serovar Typhimurium infection.

Thymic atrophy is known to occur during infections; however, there is limited understanding of its causes and of the cross-talk between different pathways. This study investigates mechanisms involved in thymic atrophy during a model of oral infection by Salmonella enterica serovar Typhimurium (S. typhimurium). Significant death of CD4(+) CD8(+) thymocytes, but not of single-positive thymocytes or peripheral lymphocytes, is observed at later stages during infection with live, but not heat-killed, bacteria. The death of CD4(+) CD8(+) thymocytes is Fas-independent as shown by infection studies with lpr mice. However, apoptosis occurs with lowering of mitochondrial potential and higher caspase-3 activity. The amounts of cortisol, a glucocorticoid, and interferon-γ (IFN-γ), an inflammatory cytokine, increase upon infection. To investigate the functional roles of these molecules, studies were performed using Ifnγ(-/-) mice together with RU486, a glucocorticoid receptor antagonist. Treatment of C57BL/6 mice with RU486 does not affect colony-forming units (CFU), amounts of IFN-γ and mouse survival; however, there is partial rescue in thymocyte death. Upon infection, Ifnγ(-/-) mice display higher CFU and lower survival but more surviving thymocytes are recovered. However, there is no difference in cortisol amounts in C57BL/6 and Ifnγ(-/-) mice. Importantly, the number of CD4(+) CD8(+) thymocytes is significantly higher in Ifnγ(-/-) mice treated with RU486 along with lower caspase-3 activity and mitochondrial damage. Hence, endogenous glucocorticoid and IFN-γ-mediated pathways are parallel but synergize in an additive manner to induce death of CD4(+) CD8(+) thymocytes during S. typhimurium infection. The implications of this study for host responses during infection are discussed.

Roles of Salmonella enterica serovar Typhimurium encoded Peptidase N during systemic infection of Ifnγ-/- mice.

Pathogen encoded peptidases are known to be important during infection; however, their roles in modulating host responses in immunocompromised individuals are not well studied. The roles of S. typhimurium (WT) encoded Peptidase N (PepN), a major aminopeptidase and sole M1 family member, was studied in mice lacking Interferon-γ (IFNγ), a cytokine important for immunity. S. typhimurium lacking pepN (ΔpepN) displays enhanced colony forming units (CFU) compared to WT in peripheral organs during systemic infection in C57BL/6 mice. However, Ifnγ(-/-) mice show higher CFU compared to C57BL/6 mice, resulting in lower fold differences between WT and ΔpepN. Concomitantly, reintroduction of pepN in ΔpepN (ΔpepN/pepN) reduces CFU, demonstrating pepN-dependence. Interestingly, expression of a catalytically inactive PepN (ΔpepN/E298A) also lowers CFU, demonstrating that the decrease in CFU is independent of the catalytic activity of PepN. In addition, three distinct differences are observed between infection of C57BL/6 and Ifnγ(-/-) mice: First, serum amounts of TNFα and IL1ß post infection are significantly lower in Ifnγ(-/-) mice. Second, histological analysis of C57BL/6 mice reveals that damage in spleen and liver upon infection with WT or ΔpepN is greater compared to ΔpepN/pepN or ΔpepN/E298A. On the other hand, Ifnγ(-/-) mice are highly susceptible to organ damage by all strains of S. typhimurium used in this study. Finally, greater survival of C57BL/6, but not Ifnγ(-/-) mice, is observed upon infection with ΔpepN/pepN or ΔpepN/E298A. Overall, the roles of the host encoded IFNγ during infection with S. typhimurium strains with varying degrees of virulence are highlighted.