EP4 and EP2 receptor activation of protein kinase A by prostaglandin E2 impairs macrophage phagocytosis of Clostridium sordellii.

PROBLEM: Clostridium sordellii causes endometrial infections, but little is known regarding host defenses against this pathogen. METHOD OF STUDY: We tested the hypothesis that the immunoregulatory lipid prostaglandin (PG) E2 suppresses human macrophage clearance of C. sordellii through receptor-induced increases in intracellular cyclic adenosine monophosphate (cAMP). The THP-1 macrophage cell line was used to quantify C. sordellii phagocytosis. RESULTS: PGE2 increased cAMP levels, activated protein kinase A (PKA), and inhibited the class A scavenger receptor-dependent phagocytosis of C. sordellii. Activation of the EP2 and EP4 receptors increased intracellular cAMP and inhibited phagocytosis, with evidence favoring a more important role for EP4 over EP2. This was supported by EP receptor expression data and the use of pharmacological receptor antagonists. In addition, the PKA isoform RI appeared to be more important than RII in mediating the suppression of ingestion of C. sordellii. CONCLUSION: The endogenous lipid mediator PGE2 impairs human innate immune responses against C. sordellii.

Tissue expression of steroid hormone receptors is associated with differential immune responsiveness.

Glucocorticoids have been used as treatments against a number of diseases, especially autoimmune/inflammatory conditions in which the immune system is overactive. These treatments have varying degrees of responsiveness among individuals and in different tissues (including brain); therefore, it is important to determine what could account for these differences. In this study, we evaluated expression of stress hormone receptors in immune cells from lymphoid and non-lymphoid tissues (including brain) as a possible explanation. We analyzed leukocytes (CD45(+)) in kidney, liver, spleen, and thymus tissues from healthy mice for expression of the receptor for stress hormone (glucocorticoid-GR) as well as other steroid hormones (androgen-AR, progesterone-PR) and found that all tissues expressed these steroid hormone receptors but with varying patterns. To determine whether tissue-specific differences were related to immune cell composition, we examined steroid hormone receptor expression in T lymphocytes from each of these tissues and found similar patterns of expression in these cells regardless of tissue source. Because glucocorticoids can also impact brain function, we further examined expression of the stress hormone receptor in brain tissue and found GR expressed in immune cells at this site. In order to investigate the potential impact in an area of neuropathology, we utilized a mouse model of West Nile Virus (WNV). We observed pathological changes in brains of WNV-infected animals and T lymphocytes in the areas of inflammation; however, these cells did not express GR. These data indicate that tissue-specific differences in steroid hormone receptor expression by immune cells could determine responsiveness to steroid hormone treatment.

The class A scavenger receptor, macrophage receptor with collagenous structure, is the major phagocytic receptor for Clostridium sordellii expressed by human decidual macrophages.

Clostridium sordellii is an emerging pathogen associated with highly lethal female reproductive tract infections following childbirth, abortion, or cervical instrumentation. Gaps in our understanding of the pathogenesis of C. sordellii infections present major challenges to the development of better preventive and therapeutic strategies against this problem. We sought to determine the mechanisms whereby uterine decidual macrophages phagocytose this bacterium and tested the hypothesis that human decidual macrophages use class A scavenger receptors to internalize unopsonized C. sordellii. In vitro phagocytosis assays with human decidual macrophages incubated with pharmacological inhibitors of class A scavenger receptors (fucoidan, polyinosinic acid, and dextran sulfate) revealed a role for these receptors in C. sordellii phagocytosis. Soluble macrophage receptor with collagenous structure (MARCO) receptor prevented C. sordellii internalization, suggesting that MARCO is an important class A scavenger receptor in decidual macrophage phagocytosis of this microbe. Peritoneal macrophages from MARCO-deficient mice, but not wild-type or scavenger receptor AI/II-deficient mice, showed impaired C. sordellii phagocytosis. MARCO-null mice were more susceptible to death from C. sordellii uterine infection than wild-type mice and exhibited impaired clearance of this bacterium from the infected uterus. Thus, MARCO is an important phagocytic receptor used by human and mouse macrophages to clear C. sordellii from the infected uterus.

Innate immune recognition of, and response to, Clostridium sordellii.

Clostridium sordellii, an anaerobic pathogen, has recently been associated with rapidly fatal infections following medically induced abortions and injecting drug use. Patients with C. sordellii infection display few signs of inflammation such as fever, or redness and pain at the site of infection. We hypothesized that this could be due to reduced recognition of the organism by Toll-like receptors (TLRs) of the innate immune system. An ELAM-NF-kappaB luciferase reporter system in TLR-transfected HEK cells was used to measure TLR-dependent recognition of washed, heat-killed C. sordellii and other pathogenic clostridial species. Results demonstrated that all clostridia were well recognized by TLR2 alone and that responses were greatest when TLR2 was co-expressed with TLR6. Further, isolated human monocytes produced the pro-inflammatory cytokine TNFalpha and the immunoregulator IL-10 in response to C. sordellii. In addition, C. sordellii-stimulated monocytes produced 30% less TNFalpha following treatment with an anti-TLR2 blocking antibody. These data demonstrate that innate immune recognition of, and response to, cell-associated components of C. sordellii and other clostridial pathogens are mediated by TLR2 in combination with TLR6. We conclude that the characteristic absence of inflammatory signs and symptoms in C. sordellii infection is not related to inadequate immune detection of the organism, but rather is attributable to a species-specific immune system dysfunction that remains to be elucidated.

Misoprostol impairs female reproductive tract innate immunity against Clostridium sordellii.

Fatal cases of acute shock complicating Clostridium sordellii endometritis following medical abortion with mifepristone (also known as RU-486) used with misoprostol were reported. The pathogenesis of this unexpected complication remains enigmatic. Misoprostol is a pharmacomimetic of PGE(2), an endogenous suppressor of innate immunity. Clinical C. sordellii infections were associated with intravaginal misoprostol administration, suggesting that high misoprostol concentrations within the uterus impair immune responses against C. sordellii. We modeled C. sordellii endometritis in rats to test this hypothesis. The intrauterine but not the intragastric delivery of misoprostol significantly worsened mortality from C. sordellii uterine infection, and impaired bacterial clearance in vivo. Misoprostol also reduced TNF-alpha production within the uterus during infection. The intrauterine injection of misoprostol did not enhance mortality from infection by the vaginal commensal bacterium Lactobacillus crispatus. In vitro, misoprostol suppressed macrophage TNF-alpha and chemokine generation following C. sordellii or peptidoglycan challenge, impaired leukocyte phagocytosis of C. sordellii, and inhibited uterine epithelial cell human beta-defensin expression. These immunosuppressive effects of misoprostol, which were not shared by mifepristone, correlated with the activation of the G(s) protein-coupled E prostanoid (EP) receptors EP2 and EP4 (macrophages) or EP4 alone (uterine epithelial cells). Our data provide a novel explanation for postabortion sepsis leading to death and also suggest that PGE(2), in which production is exaggerated within the reproductive tract during pregnancy, might be an important causal determinant in the pathogenesis of more common infections of the gravid uterus.