Cutting edge: Leukotriene C4 activates mouse platelets in plasma exclusively through the type 2 cysteinyl leukotriene receptor.

Leukotriene C4 (LTC4) and its extracellular metabolites, LTD4 and LTE4, mediate airway inflammation. They signal through three specific receptors (type 1 cys-LT receptor [CysLT1R], CysLT2R, and GPR99) with overlapping ligand preferences. In this article, we demonstrate that LTC4, but not LTD4 or LTE4, activates mouse platelets exclusively through CysLT2R. Platelets expressed CysLT1R and CysLT2R proteins. LTC4 induced surface expression of CD62P by wild-type mouse platelets in platelet-rich plasma (PRP) and caused their secretion of thromboxane A2 and CXCL4. LTC4 was fully active on PRP from mice lacking either CysLT1R or GPR99, but completely inactive on PRP from CysLT2R-null (Cysltr2(-/-)) mice. LTC4/CysLT2R signaling required an autocrine ADP-mediated response through P2Y12 receptors. LTC4 potentiated airway inflammation in a platelet- and CysLT2R-dependent manner. Thus, CysLT2R on platelets recognizes LTC4 with unexpected selectivity. Nascent LTC4 may activate platelets at a synapse with granulocytes before it is converted to LTD4, promoting mediator generation and the formation of leukocyte-platelet complexes that facilitate inflammation.

Critical role for phosphoinositide 3-kinase gamma in parasite invasion and disease progression of cutaneous leishmaniasis.

Obligate intracellular pathogens such as Leishmania specifically target host phagocytes for survival and replication. Phosphoinositide 3-kinase γ (PI3Kγ), a member of the class I PI3Ks that is highly expressed by leukocytes, controls cell migration by initiating actin polymerization and cytoskeletal reorganization, which are processes also critical for phagocytosis. In this study, we demonstrate that class IB PI3K, PI3Kγ, plays a critical role in pathogenesis of chronic cutaneous leishmaniasis caused by L. mexicana. Using the isoform-selective PI3Kγ inhibitor, AS-605240 and PI3Kγ gene-deficient mice, we show that selective blockade or deficiency of PI3Kγ significantly enhances resistance against L. mexicana that is associated with a significant suppression of parasite entry into phagocytes and reduction in recruitment of host phagocytes as well as regulatory T cells to the site of infection. Furthermore, we demonstrate that AS-605240 is as effective as the standard antileishmanial drug sodium stibogluconate in treatment of cutaneous leishmaniasis caused by L. mexicana. These findings reveal a unique role for PI3Kγ in Leishmania invasion and establishment of chronic infection, and demonstrate that therapeutic targeting of host pathways involved in establishment of infection may be a viable strategy for treating infections caused by obligate intracellular pathogens such as Leishmania.

Cytokines and their STATs in cutaneous and visceral leishmaniasis.

Cytokines play a critical role in shaping the host immune response to Leishmania infection and directing the development of protective and non-protective immunities during infection. Cytokines exert their biological activities through the activation and translocation of transcription factors into the nucleus whether they drive the expression of specific cytokine-responsive genes. Signal transducer and activator of transcription (STATs) are transcription factors which play a critical role in mediating signaling downstream of cytokine receptors and are important for shaping the host immune response during Leishmania infection. Here we discuss the signature cytokines and their associated STATs involved in the host immune response during cutaneous and visceral leishmaniasis.

PI3Kgamma (PI3Kgamma) is essential for efficient induction of CXCR3 on activated T cells.

The gamma isoform of PI3Kinase (PI3Kgamma) controls leukocyte chemotaxis by participating in GPCR signaling, and by regulating cellular polarization. Here we show that PI3Kgamma is required for efficient induction of CXC chemokine receptor 3 (CXCR3) on T cells upon activation. T cells from PI3Kgamma(-/-) mice up-regulated CXCR3 less efficiently than wild-type controls both upon activation in vitro as well as during Leishmania mexicana infection. Inhibition of PI3Kinases using wortmannin and LY294002 or blockade of PI3Kgamma activity using a selective inhibitor or PI3Kgamma siRNA suppressed induction of CXCR3 on T cells following activation. Levels of CXCR3 and T-bet mRNA were significantly lower in PI3Kgamma inhibitor-treated T cells, indicating that PI3Kgamma may control CXCR3 expression in part through induction of T-bet. These results reveal a novel role for PI3Kgamma in the induction of CXCR3 on T cells and suggest that PI3Kgamma may regulate leukocyte chemotaxis by controlling the expression of chemokine receptors.

Role of phosphatidylinositol-3-kinase-gamma (PI3Kgamma)-mediated pathway in 17beta-estradiol-induced killing of L. mexicana in macrophages from C57BL/6 mice.

We recently demonstrated that 17beta-estradiol (E2) enhances killing of Leishmania mexicana in macrophages from both male and female DBA/2 mouse by increasing nitric oxide (NO) production. Here, we analyzed the effect of E2 on leishmanicidal activity and cytokine production by bone marrow-derived macrophages (BMDMs) from male and female C57BL/6 mice in vitro, specifically examining the role of phosphatidylinositol-3-kinase-gamma (PI3Kgamma) in E2-induced parasite killing. Unlike its effect on macrophages from both male and female DBA/2 mice, E2 only increased leishmanicidal activity in macrophages from female C57BL/6 mice, which was evident by a significant reduction in both infection rates and infection levels compared to sham controls. E2-treated BMDMs from female C57BL/6 mice expressed higher levels of interferon-gammaRalpha, and also produced more interleukin (IL)-12, IL-6 and NO than both the sham controls and E2-treated male-derived macrophages. Sham-treated BMDMs from female PI3Kgamma-/- C57BL/6 mice displayed lower infection rates and infection levels compared to sham-treated wild-type (WT) macrophages. However E2, unlike its effect on macrophages from female WT C57BL/6 mice, failed to reduce infection rates and infection levels in BMDMs from female PI3Kgamma-/- mice. Interestingly, E2-treated BMDMs from female C57BL/6 mice produced significant amounts of inflammatory cytokines and NO in levels comparable to those observed in sham-treated PI3Kgamma-deficient macrophages as well as E2-treated macrophages from WT mice. These findings show that E2 exerts a distinct effect on leishmanicidal activity of macrophages from male versus female C57BL/6 mice. In addition, they suggest that PI3Kgamma is not required for E2-induced cytokine and NO production in L. mexicana-infected macrophages from female C57BL/6 mice but it may be involved in parasite clearance from these cells.