Our study demonstrates that EOS can enhance the production of IL-17 in CD4+ T lymphocytes through the release of IL-1β. Our data suggest that EOS interaction with effector/memory CD4+ T cells can contribute to inflammation and remodeling via IL-17 production during an adaptive immune response. Because IL-17 is often associated with neutrophilia or neutrophil activation, eosinophilia might be a prelude to a neutrophilic inflammation, at least in some individuals with allergic asthma.
The potential impact of EOS on lymphocyte phenotypes has been studied in the context of Th1/Th2-type responses. We previously reported that EOS co-cultured with staphylococcal enterotoxin B-activated CD4+
T cells enhance production of type 1 and type 2 cytokines [22
]. Using a co-culture model whereby CD4+
T cells are “primed” with anti-CD3/CD28 and then activated again with anti-CD3 in the presence of EOS, we now show that EOS can augment the expression of the type 17 cytokine, IL-17, by CD4+
T lymphocytes. In this model, CD4+
T cells displayed a similar phenotype as activated memory/effector T cells (CD25+
) that are predominant in the airways of asthmatics [32
]. The enhancement of IL-17 correlated with EOS release of IL-1β. The release of IL-1β is consistent with reports that EOS are a source of IL-1β [33
] and express a constitutive, inducible, active IL-1β converting enzyme protease(s) [35
In previous studies, IL-1β release was due to EOS activation with either toll-like receptor ligands or uric acid [33
]. In our co-culture model, the mediators used by CD4+
T cells to rapidly activate EOS remain unknown but might involve a variety of cytokines. Spencer and colleagues demonstrated that EOS can quickly respond to TNF-7agr;, IFN-γ, IL-10, or IL-4 to release multiple cytokines [37
]. Surprisingly enough, in our study, EOS polarization by lymphocytes required CD4+
T cells to be activated again on anti-CD3 (not shown). This suggests that a secondary T cell activation leads to a fast (within several hours) release of mediators that cannot involve newly transcribed proteins but might rather implicate post-transcriptional or post-translational mechanisms.
Our data are consistent with results by Liu et al.
demonstrating that IL-1β increases IL-17 production by activated Th17+
memory T cells [31
]. In that study and others, IL-23 and IL-6 amplified IL-1β-induced IL-17. In our study, IL-23 was never detected in EOS at the mRNA level or as protein in EOS supernatant. IL-6 is also known to be released by EOS [30
]; however, rhIL-6 alone did not increase IL-17 in our activated CD4+
T cells. We have not ruled out the possibility that EOS release of IL-6 enhances IL-17 induction by other factor(s). Also, in accordance with published studies of differentiated human Th17 cells [38
], TGF-β1 was not required for IL-17 production in our model. It is important to note that our complete medium contained 10% FBS in which exogenous TGF-β1 may be present and could thus interfere with the effects of rhTGF-β1.
The heterogeneity and plasticity of Th17 is an area of active research and controversy. IL-17 can be expressed in Forkhead box P3+
T cells [40
] and co-expressed with both IFN-γ (Th17/Th1) and IL-4 (Th17/Th2) [41
]. In addition to IL-17, our CD4+
T cells that were activated again on anti-CD3 also produced high levels of Th2 (IL-13) and Th1 (IFN-γ) (data not shown). Future studies are required to determine the effect of EOS on various “subsets” of IL-17+
The biological significance of EOS regulation of IL-17+
cells during asthma remains speculative. We demonstrated that despite high levels of BAL EOS, antigen-induced increases in IL-17 mRNA only occurred in a subset of subjects. Notably, antigen-induced IL-17 was mitigated after administration of mepolizumab and subsequent reduction in Ag-induced BAL EOS. It is not clear why EOS would increase IL-17 expression in only a subset of asthmatics. Our in vitro
study suggests that the amount of IL-1β produced, stored, and released by EOS could explain the expression and the heterogeneity of IL-17 expression. The cause for elevated IL-1β in EOS and its association with IL-17 expression remain unknown. Importantly, it is plausible that EOS-induced IL-17 is an important link to a more severe asthma phenotype that is observed in only 5–10% of the asthmatic population. As an important inducer of neutrophil chemoattractants, IL-17 is associated with neutrophilia in the airway [42
]. While mild allergic asthma is often associated with eosinophilia, more severe asthma can be characterized by the presence of both EOS and neutrophils [43
Patients with nocturnal asthma, have elevated levels of EOS, IL-1β, and CD4+
T lymphocytes in the airway at nighttime [44
], suggesting a link between eosinophilia and IL-1β in symptomatic asthma. Other reports have demonstrated elevated levels of IL-1β in BAL and sputum of asthmatics [48
] and that IL-1R1 is a potential therapeutic target for asthma [50
]. Also importantly, IL-1β and IL-17 have synergistic functions on epithelial cells and airway smooth muscle cells by enhancing mucin production and the neutrophil chemoattractant CXCL-8 and therefore participate in airway obstruction and inflammation [51
In conclusion, we demonstrated that EOS augment IL-17 during CD4+ T lymphocyte activation via IL-1β release. The increase of pathogenic cytokines such as IL-1β and IL-17 during an immune response could have dramatic effects in vivo. Understanding IL-1β processing in EOS could further elucidate EOS function in pathologies such as asthma.