Discussion
Here we show that IL-25 has an important role in type 2 allergic immunity through boosting the ability of eosinophils to initiate Th2 cells. Our data demonstrated that IL-25 promotes not only eosinophils migration to lung, but antigen uptake by eosinophils, along with eliciting the expression of surface costimulatory on eosinophils, which together activate naïve CD4+ T cells polarization to Th2 cells, resulting in allergic airway inflammation ultimately.
Previous studies showed that IL-25 is critical in type 2 immune response by stimulating Th2 and ILC2 cells28, 29. Using an in vivo mouse model of airway inflammation, we showed that Il25-/-mice displayed mitigated inflammatory responses and Th2 responses in HDM-induced asthma model, which was consistent with the previous OVA-induced studies or in anti-IL-25 administrated mice model28, 30. Our results stressed the importance of IL-25 in allergic airway inflammation again. The contribution of IL-25 on asthma symptoms by its direct action on lung endothelial cells, DCs and Th2 cells has been well documented28, 31-33. During allergic airway inflammation, the receptor for IL-25 was shown to be expressed by eosinophils, suggesting a direct function of IL-25 on it34. Diverse functions of IL-25 on eosinophils have been elucidated, including prompting cell activation, cytokine release and survival maintenance12, enhancing its role as effector cells in immunity responses.
It is now clear that eosinophils are more than just terminally differentiated effector cells35 and have antigen-presenting cells-like functions, such as expression of co-stimulatory molecules36, migration toward regional lymph nodes37and assistant in the priming of naïve T cells to initiate Th2 responses7, 38, 39. The evolving recognition of the capacity of eosinophils to engage in interaction with CD4 lymphocytes and other immunologic cells suggests that eosinophils could function as antigen-presenting cells to elicit antigen-specific responses, which places eosinophils upstream of Th2 cell-mediated effector responses40, 41.
Antigen present cells must be able to internalize foreign antigen efficiently and coordinately regulate antigen uptake, migration and T cell stimulatory capacity42. Antigen uptake and process is an important step in the initiation of antigen-specific responses. Enhanced allergen capturing and processing by airway APC populations increases T cell activation and contributes to allergic airways disease43. In our study, IL-25 significantly promoted antigen uptake by eosinophils, supporting the idea that IL-25 can directly modulate antigen present capability of eosinophils. We here meanwhile investigated that IL-25 is implicated in the priming and activation of eosinophils on their accumulation to lung. We also found that IL-25 treatment upregulated cell surface expression of HLA-DR, PD-L1 and OX40L on eosinophils in peripheral blood from allergic asthma patients. PD-L1, a co-stimulatory molecule of the CD28/B7 family, is highly expressed on dendritic cells and upregulates upon antigen uptake. PD-L1 plays a crucial role in migration and activation of antigen present cells44. OX40L is the tumor necrosis factor receptor (TNFR) superfamily, expressed on professional and non-professional APC. It has been shown that interaction between membrane OX40L on APCs and OX40 on naive T cells contributes to the induction T cells polarization into the Th2 subset45, 46. It was reported in several studies that eosinophils could express CD40, CD80 and CD8647, 48, but we did not observe any significant increase in the expression of these surface molecules upon IL-25 stimulation, which indicates probably the role of IL-25 in promoting the antigen presentation of eosinophils is independent of these molecules.
In co-culture studies aimed at understanding the role of IL-25 on the antigen present capacity for eosinophils toward T cells, we determined that IL-25 activated-eosinophils promote T cells differentiation only to Th2 cell polarization but not Th1, Th17 and Th9 cells from naive CD4+ T cells. When T cells culture alone, it hasn’t seen IL-25 activated directly to T cells, which is consistent with that reported by Yui-Hsi et al, in which they observed that IL-25 alone could induce neither Th2 polarization nor phenotypic changes of Th2 memory cells, but could induced the expanded Th2 cells driven by DCs49. Besides, we have observed that when HDM was added alone in EOS-T from asthmatics co-cultures system, eosinophils were not capable to induce T cells polarization. This result is consistent with the previous study that human peripheral blood eosinophils pulsed with HDM drove Th cell proliferation but the cytokine responses were not biased towards any specific Th subset50. However, these results differ slightly in co-culture of mouse derived EOS and spleen T cells, in which HDM pulsed eosinophils promote Th1 and Th9 polarization. This discrepancy may be explained by species difference and technical differences in the manner of culturing such as mouse bone-marrow derived eosinophils culture with SCF, FLT and rmIL-5 in the process prior to co-culture with T cells.
Our study suggested that IL-25 can facilitate naïve CD4+T cells differentiation to Th2 cells as we demonstrated employing mouse-derived cells and human peripheral blood eosinophils both. However, we were unable to obtain eosinophils from human airway or local lung tissues to prove it directly. Although we have uncovered multiple evidences for IL-25 enhancing eosinophils capacity in antigen presentation, the powerful role of DC as the classic antigen-presenting cell during airway inflammation is still the main pathway in initiating Th2 response. Further studies are required to define the relationship in antigen presenting function of eosinophils with or without the condition of excluding DCs in vivo.
In summary, we demonstrated a novel role of IL-25 in enhancing eosinophils antigen presenting function in allergic airway inflammation. Identifying diverse roles of eosinophils playing in the immune responses and its related triggers provides important insights and precise targets to eosinophil-associated diseases.