IL-32γ delays spontaneous apoptosis of human neutrophils through MCL-1, regulated primarily by the p38 MAPK pathway

Neutrophil Survival Signaling During Francisella tularensis Infection

Neutrophils are the most abundant and shortest-lived leukocytes in humans and tight regulation of neutrophil turnover via constitutive apoptosis is essential for control of infection and resolution of inflammation. Accordingly, aberrant neutrophil turnover is hallmark of many disease states. We have shown in previous work that the intracellular bacterial pathogen Francisella tularensis markedly prolongs human neutrophil lifespan. This is achieved, in part, by changes in neutrophil gene expression. Still unknown is the contribution of major neutrophil pro-survival signaling cascades to this process. The objective of this study was to interrogate the contributions of ERK and p38 MAP kinase, Class I phosphoinositide 3-kinases (PI3K), AKT, and NF-κB to neutrophil survival in our system. We demonstrate that both ERK2 and p38α were activated in F. tularensis-infected neutrophils, but only p38α MAPK was required for delayed apoptosis and the rate of cell death in the absence of infection was unchanged. Apoptosis of both infected and uninfected neutrophils was markedly accelerated by the pan-PI3K inhibitor LY2094002, but AKT phosphorylation was not induced, and neutrophil death was not enhanced by AKT inhibitors. In addition, isoform specific and selective inhibitors revealed a unique role for PI3Kα in neutrophil survival after infection, whereas only simultaneous inhibition of PI3Kα and PI3kδ accelerated death of the uninfected controls. Finally, we show that inhibition of NF-κB triggered rapid death of neutrophil after infection. Thus, we defined roles for p38α, PI3Kα and NF-κB delayed apoptosis of F. tularensis-infected cells and advanced understanding of Class IA PI3K isoform activity in human neutrophil survival.

Protective immune response mediated by neutrophils in experimental visceral leishmaniasis is enhanced by IL-32γ

Neutrophils are important cells in protection against microbial infections including visceral leishmaniasis (VL). It is well known that IL-32γ increases the protective T helper 17 cell mediated immune response against Leishmania infantum. Thus, in this study we evaluated whether IL-32 γ can increase the protective role of neutrophils against VL. In comparison with wild type (WT) mice, transgenic mice for human IL-32 γ (IL-32 γ Tg) presented a higher frequency and absolute number of neutrophils in both spleen and liver after the establishment of L. infantum infection. The IL-32 concentrations correlated with neutrophil numbers in the infected tissues. The IL-32 γ -induced recruitment of neutrophils was dependent on IL-17, since inhibition of Th17 T cells generation and IL-17 production with digoxin treatment reversed the effects of IL-32 γ. In murine neutrophils, the presence of IL-32 γ enhanced the phagocytosis of L. infantum via CR3. In addition, murine IL-32 γ Tg neutrophils were able to kill L. infantum due to the increased production of ROS when compared with WT neutrophils. In fact, IL-32 γ Tg mice lost their ability to control infection by L. infantum when neutrophils were depleted. In parallel, treatment of human neutrophils with recombinant IL-32 γ increased phagocytosis and ROS-dependent killing of L. infantum, similarly to murine IL-32 γ Tg neutrophils. The data show that IL-32 γ induces neutrophil recruitment to organs affected by VL and increases phagocytosis and killing of L. infantum by neutrophils. Together, data indicate the pivotal axis IL-32 γ -Th17-neutrophils to control VL.

Molecular Mechanisms for Regulation of Neutrophil Apoptosis under Normal and Pathological Conditions

Neutrophils are one of the main cells of innate immunity that perform a key effector and regulatory function in the development of the human inflammatory response. Apoptotic forms of neutrophils are important for regulating the intensity of inflammation and restoring tissue homeostasis. This review summarizes current data on the molecular mechanisms of modulation of neutrophil apoptosis by the main regulatory factors of the inflammatory response-cytokines, integrins, and structural components of bacteria. Disturbances in neutrophil apoptosis under stress are also considered, molecular markers of changes in neutrophil lifespan associated with various diseases and pathological conditions are presented, and data on pharmacological agents for modulating apoptosis as potential therapeutics are also discussed.

Resolution of inflammation: a new therapeutic frontier

Dysregulated inflammation is a central pathological process in diverse disease states. Traditionally, therapeutic approaches have sought to modulate the pro- or anti-inflammatory limbs of inflammation, with mixed success. However, insight into the pathways by which inflammation is resolved has highlighted novel opportunities to pharmacologically manipulate these processes - a strategy that might represent a complementary (and perhaps even superior) therapeutic approach. This Review discusses the state of the art in the biology of resolution of inflammation, highlighting the opportunities and challenges for translational research in this field.

Mcl-1 is vital for neutrophil survival

Upon entry to the systemic circulation, neutrophils exhibit a short mean time to cell death. The viability of most cell types in a steady state is preserved by the interplay of the Bcl-2 family of proteins, wherein the anti-apoptotic members inhibit the action of their pro-apoptotic counterparts. Neutrophils, however, display absent or severely reduced expression of several anti-apoptotic Bcl-2 family proteins. Hence, they rely on the expression of Mcl-1, an anti-apoptotic member of the Bcl-2 family, for survival. This protein is uniquely short-lived relative to related proteins and its loss likely precipitates the induction of apoptosis in neutrophils. This review describes the role of Mcl-1 in the neutrophil in the context of apoptosis and highlights the proteins' importance to the cell. We also address neutrophil apoptosis in the broader context of the cells' response to pathogens, focussing particularly on the strategies used by pathogens to manipulate the apoptotic pathway to their own ends.

IL-32θ gene expression in acute myeloid leukemia suppresses TNF-α production

The proinflammatory cytokine TNF-α is highly expressed in patients with acute myeloid leukemia (AML) and has been demonstrated to induce rapid proliferation of leukemic blasts. Thus suppressing the production of TNF-α is important because TNF-α can auto-regulate own expression through activation of NF-κB and p38 mitogen-activated protein kinase (MAPK). In this study, we focused on the inhibitory effect of IL-32θ on TNF-α production in acute myeloid leukemia. Approximately 38% of patients with AML express endogenous IL-32θ, which is not expressed in healthy individuals. Furthermore, plasma samples were classified into groups with or without IL-32θ; then, we measured proinflammatory cytokine TNF-α, IL-1β, and IL-6 levels. TNF-α production was not increased in patients with IL-32θ expression than that in the no-IL-32θ group. Using an IL-32θ stable expression system in leukemia cell lines, we found that IL-32θ attenuated phorbol 12-myristate 13-acetate (PMA)-induced TNF-α production. IL-32θ inhibited phosphorylation of p38 MAPK, inhibitor of κB (IκB), and nuclear factor κB (NF-κB), which are key positive regulators of TNF-α expression, and inhibited nuclear translocation of NF-κB. Moreover, the presence of IL-32θ attenuated TNF-α promoter activity and the binding of NF-κB with the TNF-α promoter. In addition, IL-32γ-induced TNF-α production has no correlation with inhibition of TNF-α via IL-32θ expression. Thus, IL-32θ may serve as a potent inhibitor of TNF-α in patients with AML.