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.