Group V Secreted Phospholipase A2 Is Upregulated by IL-4 in Human Macrophages and Mediates Phagocytosis via Hydrolysis of Ethanolamine Phospholipids

Abstract

Studies on the heterogeneity and plasticity of macrophage populations led to the identification of two major polarization states: classically activated macrophages or M1, induced by IFN-γ plus LPS, and alternatively activated macrophages, induced by IL-4. We studied the expression of multiple phospholipase A2 enzymes in human macrophages and the effect that polarization of the cells has on their levels. At least 11 phospholipase A2 genes were found at significant levels in human macrophages, as detected by quantitative PCR. None of these exhibited marked changes after treating the cells with IFN-γ plus LPS. However, macrophage treatment with IL-4 led to strong upregulation of the secreted group V phospholipase A2 (sPLA2-V), both at the mRNA and protein levels. In parallel with increasing sPLA2-V expression levels, IL-4–treated macrophages exhibited increased phagocytosis of yeast-derived zymosan and bacteria, and we show that both events are causally related, because cells deficient in sPLA2-V exhibited decreased phagocytosis, and cells overexpressing the enzyme manifested higher rates of phagocytosis. Mass spectrometry analyses of lipid changes in the IL-4–treated macrophages suggest that ethanolamine lysophospholipid (LPE) is an sPLA2-V–derived product that may be involved in regulating phagocytosis. Cellular levels of LPE are selectively maintained by sPLA2-V. By supplementing sPLA2-V–deficient cells with LPE, phagocytosis of zymosan or bacteria was fully restored in IL-4–treated cells. Collectively, our results show that sPLA2-V is required for efficient phagocytosis by IL-4–treated human macrophages and provide evidence that sPLA2-V–derived LPE is involved in the process.

Secreted PLA2 group X orchestrates innate and adaptive immune responses to inhaled allergen

“…Prior studies have also demonstrated that sPLA 2 -V is necessary for the development of allergic inflammation in the HDM (68) and OVA/alum (69) models of allergic airways disease, and that while sPLA 2 -V is involved in maturation and antigen processing in DCs (68), macrophages expressing sPLA 2 -V are sufficient to transfer antigen sensitization mice (20). Studies in human macrophages support a role for sPLA 2 -V in phagocytosis and antigen processing (70). Given our findings of a marked alteration in the adaptive immune response to inhaled allergen, it will be important in future studies to clarify the respective roles of these sPLA 2 s in macrophage function.…”

A new era of secreted phospholipase A2

“…Nonetheless, several functions of sPLA 2 s in mice, as depicted in Fig. 2 , appear to be conserved in humans ( 55,70,74,93,117,134 ). Further advances in this research fi eld and their integration for therapeutic applications are expected to benefi t from improved lipidomics that will allow monitoring of individual sPLA 2 s and associated lipid metabolisms within specifi c tissue niches.…”

“…This transgenic phenotype is attributable to aberrant hydrolysis of surfactant phospholipids (dipalmitoyl-PC) and is apparently eicosanoid-independent. Although sPLA 2 -V was previously thought to be upregulated by pro-infl ammatory stimuli (as in the case of sPLA 2 -IIA) ( 90,91 ), it has recently become obvious that its expression is induced by the Th2 cytokines, IL-4 and IL-13, much more potently than by pro-infl ammatory stimuli including LPS, zymosan, and Th1 cytokines ( 74,92,93 ). Indeed, sPLA 2 -V is expressed in IL-4-driven M2 macrophages and Th2 cells, which facilitate Th2-type immunity while attenuating Th1-or Th17-type immunity.…”

“…Rather, sPLA 2 -X might be expressed in a limited subset of these cells or supplied from proximal or even distal cells in a paracrine manner. As sPLA 2 -X is abundantly expressed in the gut epithelium (a "gastrointestinal sPLA 2 "), it is likely that the decreased digestion and through production of LPE, which fully restores defective phagocytosis of zymosan and bacteria in sPLA 2 -V-knockdown cells ( 93 ). Because hydrolysis of phospholipids in LDL by sPLA 2 -V is capable of promoting foam cell formation by macrophages in vitro ( 98 ), sPLA 2 -V (and several other sPLA 2 s) has currently been implicated in the development of atherosclerosis and related cardiovascular disorders.…”

Foamy Monocytes Are Enriched in cis -7-Hexadecenoic Fatty Acid (16:1n-9), a Possible Biomarker for Early Detection of Cardiovascular Disease

“…Accumulation of 16:1n-9 was also examined in monocytes differentiated toward macrophages with recombinant macrophage colony-stimulating factor (Rubio et al, 2015). While the level 16:1n-9 greatly increased along the differentiation process, 16:1n-7 content did so to a much lesser extent ( Figure S1).…”