The Roles of Phospholipase A₂ in Phagocytes

Phagocytic cells, such as macrophages, neutrophils, and dendritic cells, ingest particles larger than about 0.5 μM and thereby clear microbial pathogens and malignant cells from the body. These phagocytic cargoes are proteolytically degraded within the lumen of phagosomes, and peptides derived from them are presented on Major Histocompatibility Complexes (MHC) for the activation of T cells. Mammalian PLA2 isozymes belong to a large family of enzymes that cleave phospholipids at the second position of the glycerol backbone, releasing a free fatty acid and a lysolipid moiety. In human macrophages, at least 15 different PLA2 forms are expressed, and expression of many of these is dependent on pathogenic stimulation. Intriguing questions are why so many PLA2 forms are expressed in macrophages, and what are the functional consequences of their altered gene expression after encountering pathogenic stimuli. In this review, we discuss the evidence of the differential roles of different forms of PLA2 in phagocytic immune cells. These roles include: lipid signaling for immune cell activation, initial phagocytic particle uptake, microbial action for the killing and degradation of ingested microbes, and the repair of membranes induced by oxygen radicals. We also discuss the roles of PLA2 in the subsequent digestion of ingested phagocytic cargoes for antigen presentation to T cells

Focal exocytosis of Syntaxin 3 and TRPML1 at pseudopodia of nascent phagosomes

Macrophages clear invading pathogens by phagocytosis. Phagocytosis is a complex mechanism involving the local expansion of the membrane, cytoskeletal remodeling, and the delivery of phagosomal proteins to the nascent phagosomes. However, the organelle trafficking events underlying this are largely unclear. Here, we show in human blood monocyte-derived macrophages that TRPML1, a calcium channel involved in the phagocytic process, is delivered to phagosomes in Syntaxin 3-positive vesicles. Syntaxin 3 is a SNARE protein previously shown to mediate the secretion of IL-6 by macrophages. Total Internal Reflection Microscopy (TIRF) revealed that Syntaxin 3 positive compartments carry TRPML1 to pseudopodia for focal exocytosis at the nascent phagosomes during E. coli uptake. Using siRNA knockdown, we show that both Syntaxin 3 and TRPML1 are required for E. coli uptake. Moreover, using TRPML1 agonists we show that increased TRPML1 activity leads to increased E. coli uptake, whereas calcium chelation decreased intracellular E. coli load. Understanding the membrane trafficking pathways is critical for understanding how macrophages clear invading pathogens

Antigen Cross-Presentation by Macrophages

The contribution of dendritic cell (DC) antigen cross-presentation to the activation of CD8(+) T lymphocytes for immune defense against tumors, viruses, and intracellular pathogens has been recognized widely. Although originally thought to be an exclusive characteristic of DCs, recently also other immune cells, particularly macrophages, have been shown capable of cross-presentation. Here we provide an overview of in vitro and in vivo evidence on cross-presentation by macrophages. As we discuss, it is now firmly established that various types of tissue-resident macrophages are able to cross-present via similar cellular pathways as DCs. This is based on a wide range of antigens in macrophages from many different tissue origins such as blood, tumors, and lymphoid tissue. However, the physiological relevance of macrophage cross-presentation with potential contributions to activation of CD8(+) T lymphocytes is still mostly unknown. While cross-presentation by various types of proinflammatory macrophages might be involved in cross-priming of naive CD8(+) T lymphocytes, it might also be involved in local reactivation of memory and/or effector CD8(+) T lymphocytes. Moreover, cross-presentation by anti-inflammatory macrophages could be related to immune tolerance. Because cross-presentation promotes the initiation and potentiation of antigen-specific CD8(+) T lymphocyte responses, stimulating macrophages to cross-present antigen might be a promising strategy for antitumor or antiviral therapies

Membrane trafficking as an active regulator of constitutively secreted cytokines

Item does not contain fulltextImmune-cell activation by inflammatory stimuli triggers the transcription and translation of large amounts of cytokines. The transport of newly synthesized cytokines to the plasma membrane by vesicular trafficking can be rate-limiting for the production of these cytokines, and immune cells upregulate their exocytic machinery concomitantly with increased cytokine expression in order to cope with the increasing demand for trafficking. Whereas it is logical that trafficking is rate-limiting for regulated secretion where an intracellular pool of molecules is waiting to be released, the reason for this is not obvious for constitutively secreted cytokines, such as interleukin-6 (IL-6), interleukin-12 (IL-12) and tumor necrosis factor-alpha (TNF-alpha). These constitutively secreted cytokines are primarily regulated at the transcriptional and/or translational level but mounting evidence presented here shows that cells might also increase or decrease the rate of post-Golgi cytokine trafficking to modulate their production. Therefore, in this Hypothesis, we ask the question: why is there a need to limit the trafficking of constitutively secreted cytokines? We propose a model where cells monitor and adjust their production rate of cytokines by sensing the intracellular level of cytokines while they are in transit to the plasma membrane. This self-regulation of cytokine production could prevent an overshooting response of acute-phase cytokines, such as IL-6, IL-12 and TNF-alpha, upon acute infection

What makes (hydroxy)chloroquine ineffective against COVID-19:Insights from cell biology

Since chloroquine (CQ) and hydroxychloroquine (HCQ) can inhibit the invasion and proliferation of SARS-CoV-2 in cultured cells, the repurposing of these antimalarial drugs was considered a promising strategy for treatment and prevention of COVID-19. However, despite promising preliminary findings, many clinical trials showed neither significant therapeutic nor prophylactic benefits of CQ and HCQ against COVID-19. Here, we aim to answer the question of why these drugs are not effective against the disease by examining the cellular working mechanisms of CQ and HCQ in prevention of SARS-CoV-2 infections

Radical Stress Is More Cytotoxic in the Nucleus than in Other Organelles

Contains fulltext : 209354.pdf (publisher's version ) (Open Access

Putative regulation of macrophage-mediated inflammation by catestatin

Catestatin (CST) is a bioactive cleavage product of the neuroendocrine prohormone chromogranin A (CgA). Recent findings show that CST can exert anti-inflammatory and antiadrenergic effects by suppressing the inflammatory actions of mammalian macrophages. However, recent findings also suggest that macrophages themselves are major CST producers. Here, we hypothesize that macrophages produce CST in an inflammation-dependent manner and thereby might self-regulate inflammation in an autocrine fashion. CST is associated with pathological conditions hallmarked by chronic inflammation, including autoimmune, cardiovascular, and metabolic disorders. Since intraperitoneal injection of CST in mouse models of diabetes and inflammatory bowel disease has been reported to be beneficial for mitigating disease, we posit that CST should be further investigated as a candidate target for treating certain inflammatory diseases