IL-33 activates LTC4 synthesizing machinery in eosinophils

Eosinophils are circulating and mucosal resident cells that can be activated by stress signals, like the alarmin IL-33 that can activate cytokine release. Yet, little is known about IL-33 ability to induce eicosanoid synthesis by eosinophils. Here, we studied whether IL-33 was capable to trigger leukotriene C₄ (LTC₄) synthesis in vivo and in vitro. To this end, we first developed an in vivo model of eosinophilic inflammation induced by IL-33 in sensitized BALB/c mice [OVA plus Al(OH)₃; s.c.] followed by intrapleural challenge with IL-33 (100 ng/cavity) (CEUA UFRJ 22/23 and 36/23). IL-33 induced a marked pleural accumulation of eosinophils in pre-sensitized mice at 24 h without acute changes (4 h). Besides eosinophil influx, IL-33 also triggered cellular activation of these pleural infiltrating eosinophils, evidenced by an increase in their cytoplasmic content of lipid bodies (LBs). IL-33 also led to an increase in pleural levels of cysteinyl leukotrienes LTC₄/LTD₄/LTE₄, at 24 h. At 4 h, IL-33 triggered an increase in pleural levels of PGD₂ – a prostanoid known to trigger both LB biogenesis within eosinophils and elevated pleural levels of LTC₄ – that returned to basal levels at 24 h. Therefore, we investigate the potential role of endogenous PGD₂ in mediating IL-33-driven effects by pretreating IL-33-challenged mice with HQL-79 (PGD₂ synthesis inhibitor). In contrast to our initial hypothesis, HQL-79 did not impact pleural eosinophil influx, LB biogenesis, or LTC₄ synthesis induced by IL-33 in vivo. In vitro, isolated human eosinophils stimulated with IL-33 (100 ng/mL; 1 h) showed increased LBs associated to LTC₄ synthesis. Collectively, our results confirm our hypothesis that IL-33 indeed can activate the LTC₄ synthetic machinery, both during an ongoing eosinophilic inflammatory reaction in vivo (by mechanisms still not defined) as well as directly in human eosinophils. Financial support: FAPERJ, CNPq, CAPES.