Allergy-Inducing Chromium Compounds Trigger Potent Innate Immune Stimulation Via ROS-Dependent Inflammasome Activation

Abstract

Chromium allergy is a common occupational skin disease mediated by chromium (VI)-specific T cells that induce delayed-type hypersensitivity in sensitized individuals. Additionally, chromium (VI) can act as an irritant. Both responses critically require innate immune activation, but if and how chromium (VI) elicits this signal is currently unclear. Using human monocytes, primary human keratinocytes, and murine dendritic cells we show that chromium (VI) compounds fail to trigger direct proinflammatory activation but potently induce processing and secretion of IL-1β. IL-1β release required priming by phorbol-ester or toll-like receptor stimulation and was prevented by inhibition of K efflux, NLRP3 depletion or caspase-1 inhibition, identifying chromium (VI) as a hapten activator of the NLRP3 inflammasome. Inflammasome activation was initiated by mitochondrial reactive oxygen species production triggered by chromium (VI), as indicated by sensitivity to treatment with the ROS scavenger N-acetyl cysteine and a coinciding failure of K efflux, caspase-1, or NLRP3 inhibition to prevent mitochondrial reactive oxygen species accumulation. IL-1β release further correlated with cytotoxicity that was secondary to reactive oxygen species, K efflux, and NLRP3 activation. Trivalent chromium was unable to induce mitochondrial reactive oxygen species production, inflammasome activation, and cytotoxicity, suggesting that oxidation state-specific differences in mitochondrial reactivity may determine inflammasome activation and allergic/irritant capacity of different chromium compounds.

Improved metal allergen reactivity of artificial skin models by integration of Toll‐like receptor 4‐positive cells

“…we observed high mRNA levels in the dermal FTSE isolates, BJ-terts and reportedly TLR4-positive HUVECs 5 (included as reference), but barely detected TLR4 mRNA expression in the epidermal FTSE component; the level of mRNA expression was comparable to that of functionally TLR4-negative HEK293 cells 5 (Figure 2A 30 (B), IL-8 ELISA of culture supernatants from MALP2-exposed, NiCl 2 -exposed, CoCl 2 -exposed, K 2 Cr 2 O 7 -exposed ( Considering that dendritic cell (DC) activation and maturation is a key event during sensitization to contact allergens, and that DCs are well known to express functional TLR4, [33][34][35] ( Figure 3A,B) and by the IL-8 secretion observed upon nickel exposure ( Figure 3C). However, we could not find significant differences in nickel-induced CD86 surface expression or total IL-8 production from those in equally seeded iDC monocultures ( Figure 3B,C).…”

“…To determine whether the lack of functional TLR4 expression in epidermal keratinocytes might account for the poor metal responsiveness of RhE, we measured release of the TLR4‐triggered cytokine IL‐8 after exposure to the TLR4‐stimulating metals nickel and cobalt, the TLR4 agonist lipopolysaccharide (LPS), or dichromate. The last of these is unable to trigger TLR4 activation, but can induce potent inflammasome activation upon proinflammatory priming . None of the metals or LPS was capable of inducing IL‐8 secretion in NHEKs (Figure A), although we readily detected induction of the hypoxia‐induced transcription factor hypoxia‐inducible factor 1α (HIF‐1α), an IKK2/nuclear factor‐κB (NF‐κB)‐independent nickel target, with the nickel and cobalt concentrations used (Figure S1A).…”

“…The last of these is unable to trigger TLR4 activation, but can induce potent inflammasome activation upon proinflammatory priming. 30 None of the metals or LPS was capable of inducing IL-8 secretion in NHEKs ( Figure 1A), although we readily detected induction of the hypoxia-induced transcription factor hypoxia-inducible factor 1α (HIF-1α), an IKK2/nuclear factor-κB (NF-κB)-independent nickel target, 31 with the nickel and cobalt concentrations used ( Figure S1A). In contrast, treatment with the TLR2/TLR6 agonist MALP2 (included as a positive control) induced robust IL-8 production ( Figure 1A).…”

“…(A) , Interleukin (IL)‐8 enzyme‐linked immunosorbent assay (ELISA) of normal human epithelial keratinocyte (NHEK) culture supernatants, showing release of IL‐8 upon stimulation with the Toll‐like receptor (TLR) 2/TLR6 agonist MALP2 (100 ng/mL), but not upon stimulation with increasing doses of lipopolysaccharide (LPS) or the TLR4‐activating metals Ni 2+ and Co 2+ . K 2 Cr 2 O 7 (100 µM) was included as a TLR4‐independent metal allergen . (B) , IL‐8 ELISA of culture supernatants from MALP2‐exposed, NiCl 2 ‐exposed, CoCl 2 ‐exposed, K 2 Cr 2 O 7 ‐exposed (8 hours or 16 hours) RhE and full‐thickness skin equivalent (FTSE).…”

Ugonin U stimulates NLRP3 inflammasome activation and enhances inflammasome-mediated pathogen clearance

“…The most characterized inflammasome is the NLRP3 inflammasome, which is composed of a sensing apparatus (NOD-like receptor pyrin domain 3, NLRP3), an adaptor (apoptosis-associated speck-like protein containing a caspase recruitment domain, ASC) and the pro-form of cytokine converting enzyme (pro-caspase-1) [7]. Numerous structurally diverse stimulators activate the NLRP3 inflammasome through different signaling pathways, including K + efflux [7], [8], [9], [10], [11], reactive oxygen species (ROS) production [12], [13], [14], [15], Ca 2+ mobilization [11], [16], [17], [18], [19], mitochondrial destabilization [17], [20], [21], [22], [23], and lysosome rupture [24], [25]. Therefore, it is critical that the molecular mechanisms by which novel stimulators activate the NLRP3 inflammasome are delineated in a context-specific manner.…”