Dissemination of bacteria in multiple organs associated with apoptosis and macrophage activity in different stages of experimental sepsis

Synergistic effects of streptolysin S and streptococcal pyrogenic exotoxin B on the mouse model of group A streptococcal infection

Streptococcus pyogenes is a group A streptococcus (GAS) and an important human pathogen that causes a variety of diseases. Streptococcal pyrogenic exotoxin B (SPE B) and streptolysin S (SLS) are important virulence factors involved in GAS infection, but it is not clear which one is more virulent. Using an air pouch infection model, the wild-type strain NZ131, its isogenic mutants, and complementary mutants were used to examine the effects of SPE B and SLS on GAS infection. The results of the skin lesion and mouse mortality assays showed that although SPE B and SLS had a synergistic effect on GAS infection, SPE B played a more important role in local tissue damage while SLS had a more prominent effect on mouse mortality. Surveys of the exudates from the air pouch revealed that the expression of inflammatory cytokines was significantly inhibited in the sagB/speB-double-mutant JM4-infected mice. Furthermore, in vivo and in vitro studies showed that the isogenic mutant strains were more susceptible to the immune cell killing than the wild-type strain and that the sagB/speB-double-mutant JM4 was the most sensitive among these strains. Moreover, infection with the sagB/speB-double-mutant JM4 strain caused the least amount of macrophage apoptosis compared to infection with the wild-type NZ131 and the other complementary strains, which express only SPE B or SLS or both. Taken together, these results indicate that both SPE B and SLS contributed to GAS evasion from immune cell killing, local tissue damage, and mouse mortality.

Steroid receptor coactivator 3 is required for clearing bacteria and repressing inflammatory response in Escherichia coli-induced septic peritonitis

Steroid receptor coactivator 3 (SRC-3) is a multifunctional protein that plays an important role in regulation of bacterial LPS-induced inflammation. However, its involvement in host defense against bacterial infection remains unclear. In this study, we used SRC-3 knockout mice to assess the role of SRC-3 in antibacterial defense in Escherichia coli-induced septic peritonitis. After E. coli bacteria were injected i.p., SRC-3-deficient mice exhibited excessive local and systemic inflammatory responses and more severe bacterial burdens, leading to a significantly higher mortality compared with wild-type mice. Peritoneal macrophages of SRC-3-deficient mice showed a decrease in bacterial phagocytosis in culture and an increase in apoptosis, which was consistent with the defective bacterial clearance observed in SRC-3-deficient mice. Accordingly, SRC-3 null macrophages expressed much lower levels of scavenger receptor A, the antioxidant enzyme catalase, and antiapoptotic gene Bcl-2. Collectively, our data demonstrate that SRC-3 is important not only in modulating the local and systemic inflammation but also in intensifying bacterial clearance, which highlights a pivotal role of SRC-3 in the host defense system against bacterial infection.

Aspartic protease and caspase 3/7 activation are central for macrophage apoptosis following infection with Escherichia coli

Macrophages are vital for host defense against microbial infections. We have previously shown that infection of macrophages with a nonpathogenic strain of Escherichia coli induces apoptosis rapidly. Here, we demonstrate that infection of macrophages results in the activation of caspases prior to the induction of the intrinsic apoptosis pathway. Caspases 9 and 3 are activated prior to the release of intermembrane mitochondrial protein cytochrome C into the cytosol in infected macrophages. Treatment with an inhibitor to caspase 9 has no effect on the death of macrophages and does not prevent activation of the downstream effector caspase 3/7. In contrast, an inhibitor to caspase 3/7 reduces cell death in E. coli-infected macrophages. Although caspase 9 is not required, activation of aspartic proteases, of which cathepsin D is one of the central members, is essential for activation of caspase 3/7. Treatment with pepstatin A, an inhibitor of aspartic proteases, markedly diminishes the activation of cathepsin D and caspase 3/7 and reduces death in E. coli-infected macrophages. Collectively, these data suggest that cathepsin D activation of caspase 3/7 may be required for inducing one of the death pathways elicited by E. coli.