MRGPRX2 is negatively targeted by SCF and IL‐4 to diminish pseudo‐allergic stimulation of skin mast cells in culture

Abstract

MRGPRX2 was recently uncovered as the "missing link" in clinically relevant mast cell (MC) activation explaining previously puzzling phenomena. It is the receptor for various endogenous ligands and exogenous compounds alike, whose binding evokes rapid degranulation much like allergen-mediated exocytosis. While the perceivable outcomes are similar, the two activation routes differ regarding mechanism and regulation. We recently reported that acute SCF administration curbs responses evoked by MRGPRX2 in human skin MCs. Maintenance of MCs in culture requires the presence of MC supportive factors and renders the cells functionally and molecularly unequal to ex vivo counterparts. Here, we asked whether expansion in culture impacts the pseudo-allergic route, and if so, what contribution SCF and IL-4 play in this scenario. We report that the in vitro micromilieu dampens (but does not erase) pseudo-allergic responses and that this is accompanied by strongly reduced MRGPRX2 expression. Withdrawal of SCF or IL-4 individually, but most potently of both collectively, partially reinstates the MRGPRX2 pathway, revealing that SCF and IL-4 make negative adjustments to the pseudo-allergic pathway. Under all conditions, the FcεRI-triggered route showed the inverse pattern of regulation, substantiating that allergic and pseudo-allergic MC activation can obey opposite rules, hinting at possible competition between them.

Thymic Stromal Lymphopoietin Interferes with the Apoptosis of Human Skin Mast Cells by a Dual Strategy Involving STAT5/Mcl-1 and JNK/Bcl-xL

“…MCs were isolated from human foreskins, whereby material from several donors (typically between 2 and 9) was combined for one experiment to achieve the cell numbers required, as described [31,32,33]. The skin was obtained from circumcisions, with the informed consent of the patients or legal guardians and approval by the university ethics committee.…”

IL-33 and MRGPRX2-Triggered Activation of Human Skin Mast Cells—Elimination of Receptor Expression on Chronic Exposure, but Reinforced Degranulation on Acute Priming

“…The negative regulation of the pseudo-allergic/neurogenic route by long-term IL-33 is notable, because SCF by itself dampens MRGPRX2 responsiveness and expression [16,19], so that on extended exposure, IL-33 further augments the negative effect of SCF and does not reverse the positive effect of SCF (as detected for the allergic route) [19,24,33].…”

“…In fact, potent MC-modulating factors thus far studied (SCF, IL-4, and retinoic acid) have shown a clear-cut separation with opposite patterns of regulation, i.e., enhancing effects on the allergic route, but concomitant suppression of the MRGPRX2 pathway [16,19,20]. IL-33 is thus the first cytokine to support the MRGPRX2 route altogether.…”

“…Surprisingly, we also observed that MRGPRX2- and FcεRI-triggered activation routes are inversely regulated by stem cell factor (SCF) [16], the most crucial supportive factor of the MC lineage [18]. In our studies, SCF did not only attenuate pseudo-allergic/neurogenic responses when provided acutely to skin MCs, i.e., directly before stimulation, but it also dampened MRGPRX2 expression and function during skin MC culture, where it co-operated with IL-4 to inhibit the alternative route [19]. Skin MCs exposed to retinoic acid likewise diminished MRGPRX2 function [20].…”

“…Skin MCs exposed to retinoic acid likewise diminished MRGPRX2 function [20]. Conversely, the allergic route was simultaneously promoted by all of the above stimuli, i.e., SCF, IL-4, and retinoic acid [19,20].…”