• TLR-stimulated macrophages synthesize, release, and hydrolyze ATP via CD39 to regulate their own activation state.• The loss of macrophage CD39 prevents regulatory macrophage development and leads to lethal inflammatory responses and septic shock in mice.Sepsis is a highly fatal disease caused by an initial hyperinflammatory response followed by a state of profound immunosuppression. Although it is well appreciated that the initial production of proinflammatory cytokines by macrophages accompanies the onset of sepsis, it remains unclear what causes the transition to an immunosuppressive state. In this study, we reveal that macrophages themselves are key regulators of this transition and that the surface enzyme CD39 plays a critical role in self-limiting the activation process. We demonstrate that Toll-like receptor (TLR)-stimulated macrophages modulate their activation state by increasing the synthesis and secretion of adenosine triphosphate (ATP). This endogenous ATP is paradoxically immunosuppressive due to its rapid catabolism into adenosine by CD39. Macrophages lacking CD39 are unable to transition to a regulatory state and consequently continue to produce inflammatory cytokines. The importance of this transition is demonstrated in a mouse model of sepsis, where small numbers of CD39-deficient macrophages were sufficient to induce lethal endotoxic shock. Thus, these data implicate CD39 as a key "molecular switch" that allows macrophages to self-limit their activation state. We propose that therapeutics targeting the release and hydrolysis of ATP by macrophages may represent new ways to treat inflammatory diseases. (Blood. 2013;122(11):1935-1945 IntroductionFailure to control inflammatory macrophage activation responses can lead to pathological diseases, best exemplified by sepsis. Despite our growing understanding of its pathogenesis, sepsis continues to affect more than 200 000 people annually in the United States, with a mortality rate as high as 50%.1,2 The severe pathology associated with sepsis occurs in response to the hyperproduction of macrophagederived inflammatory cytokines, which can lead to vascular and tissue destruction, multiple organ failure, shock, and death.3 Intriguingly, macrophages isolated from late-stage septic individuals exhibit the phenotype of immunosuppressive, regulatory macrophages, expressing high levels of the anti-inflammatory cytokine interleukin-10 (IL-10) and low levels of tumor necrosis factor a (TNF-a) and IL-12. [4][5][6] These observations suggest the transition from inflammatory to immunosuppressive macrophages may be critical to control initial inflammatory responses and prevent lethal septic shock. 4,7,8 However, the molecular mechanism by which this transition is achieved remains poorly understood.In the present work, we examine the role that endogenous CD39 and adenosine triphosphate (ATP) play in regulating the macrophage inflammatory response. Recently, extracellular ATP (eATP) has been characterized as a "danger signal" that can promote inflammation through ...

Leishmania infection triggers the recruitment of Gr1+ monocytes to the site of infection via platelet-derived PDGF and subsequent CCL2 production.

Macrophages make major contributions to inflammatory immunopathology. In this work, we examine three disease scenarios, in which M1s play a major role early in the disease but eventually transitions into a population of cells with immunoregulatory activity. We propose that the transition from an inflammatory to a regulatory phenotype is a natural progression that regularly occurs in stimulated macrophages and that the timing of this transition is critical to maintaining homeostasis. In the first section of this review, we discuss the exogenous microenvironmental cues that may induce macrophages to enter a regulatory state. In the second half of this review, we discuss a novel mechanism, whereby TLR-stimulated macrophages can intrinsically induce their own regulatory activation state. They do so by secreting and synthesizing endogenous "reprogramming" signals that work in an autocrine fashion to promote a regulatory phenotype. We propose that these endogenous regulatory mechanisms exist to prevent macrophage-mediated immunopathology. Thus, macrophages can respond to endogenous and exogenous cues to regulate their activation state, and without these controlled regulatory responses, M1 would persist to the detriment of the host.

The priming of macrophages with IFN-γ prior to TLR stimulation results in enhanced and prolonged inflammatory cytokine production. Here, we demonstrate that following TLR stimulation, macrophages up regulate the adenosine 2b receptor (A2bR) to enhance their sensitivity to immunosuppressive extracellular adenosine. This up-regulation of A2bR leads to the induction of a macrophage with an immunoregulatory phenotype and the down regulation of inflammation. IFN-γ priming of macrophages, selectively prevents the induction of the A2bR in macrophages to mitigate sensitivity to adenosine and prevent this regulatory transition. IFN-γ-mediated A2bR blockade leads to a prolonged production of TNFα and IL-12 in response to TLR ligation. The pharmacological inhibition or the genetic deletion of the A2bR results in a hyper-inflammatory response to TLR ligation, similar to IFN-γ treatment of macrophages. Conversely, the overexpression of A2bR on macrophages blunts the IFN-γ effects and promotes the development of immunoregulatory macrophages. Thus, we propose a novel mechanism whereby IFN-γ contributes to host defense, by desensitizing macrophages to the immunoregulatory effects of adenosine. This mechanism overcomes the transient nature of TLR activation, and prolongs the anti-microbial state of the classically activated macrophage. This study may offer promising new targets to improve the clinical outcome of inflammatory diseases in which macrophage activation is dysregulated.