Factor H Binds to Extracellular DNA Traps Released from Human Blood Monocytes in Response to Candida albicans

Abstract

Upon systemic infection with human pathogenic yeast Candida albicans (C. albicans), human monocytes and polymorph nuclear neutrophilic granulocytes are the first immune cells to respond and come into contact with C. albicans. Monocytes exert immediate candidacidal activity and inhibit germination, mediate phagocytosis, and kill fungal cells. Here, we show that human monocytes spontaneously respond to C. albicans cells via phagocytosis, decondensation of nuclear DNA, and release of this decondensed DNA in the form of extracellular traps (called monocytic extracellular traps: MoETs). Both subtypes of monocytes (CD14++CD16−/CD14+CD16+) formed MoETs within the first hours upon contact with C. albicans. MoETs were characterized by the presence of citrullinated histone, myeloperoxidase, lactoferrin, and elastase. MoETs were also formed in response to Staphylococcus aureus and Escherichia coli, indicating a general reaction of monocytes to infectious microbes. MoET induction differs from extracellular trap formation in macrophages as MoETs are not triggered by simvastatin, an inhibitor of cholesterol synthesis and inducer of extracellular traps in macrophages. Extracellular traps from both monocytes and neutrophils activate complement and C3b is deposited. However, factor H (FH) binds via C3b to the extracellular DNA, mediates cofactor activity, and inhibits the induction of the inflammatory cytokine interleukin-1 beta in monocytes. Altogether, the results show that human monocytes release extracellular DNA traps in response to C. albicans and that these traps finally bind FH via C3b to presumably support clearance without further inflammation.

Macrophage Extracellular Traps: A Scoping Review

“…For instance, Bonne-Année et al [32] noted that although human monocyte-derived macrophages produced METs in response to Strongyloides stercoralis infection, mouse peritoneal macrophages did not, despite the fact that both human and mouse neutrophils produced NETs during infection. Similarly, Schorn et al [10] reported that neutrophils, basophils, and eosinophils responded to monosodium urate crystals in gouty arthritis [22] were not able to demonstrate similar responses with human peripheral-blood monocytes. Current evidence is insufficient to determine if the differential responses are due to experimental conditions or if some types of macrophages are inherently more prone to METosis.…”

“…Yousefi et al [9] described DNA release from eosinophils within 5 min of stimulation with complement component C5a or lipopolysaccharide (LPS), and the maximum effect, measured as fluorescence of a cell-impermeable DNA-staining dye, occurred within 30 min. METosis has been described as a rapid process that can occur in <30 min [22,23].…”

“…Additional verification that proposed structures are ETs can be obtained by staining for known ET components such as histones, elastase, or MPO. Elastase, often considered a neutrophil-specific marker, has been identified in the METs of human peripheral-blood monocytes and in THP-1 macrophage-like cells [22,26]. Similarly, MPO has been identified in the METs of diverse macrophage populations including human glomerular macrophages, human peripheral-blood monocytes, THP-1 macrophage-like cells, murine J774A.1 macrophage-like cells, bovine monocytes, and caprine monocytes [22,23,[26][27][28][29] (Table 1).…”

“…To date, there are reports of potential MET microbicidal activity against pathogens including Staphylococcus aureus, S. agalactiae, E. coli, and C. albicans [28,[38][39][40]. Halder et al [22] isolated monocyte ETs by digesting immobilized traps with the restriction enzyme AluI, and then added ET material to C. albicans culture; ET material, but not monocyte chromosomal DNA, was able to inhibit fungal growth even at 30 h of incubation. Instead of employing direct counting of microorganisms, Chow et al [31] used SYTO TM 9 and propidium iodide staining of S. aureus cells trapped within METs to demonstrate the decreased viability of bacterial cells.…”

“…Halder et al [22] demonstrated that human monocyte release of METs was enhanced in the presence of human serum. During infection with C. albicans in media containing serum, they used immunofluorescence staining to establish that complement factors C3b and C5b-9 were deposited onto METs.…”