Our current study extends previous findings demonstrating that an ongoing Th2 polarized pulmonary inflammation facilitates priming towards unrelated antigens.23,26,29,30
We identify IL-17A as the key cytokine to facilitate priming towards new antigens during a concomitant Th1-polarized airway inflammation and demonstrate that Th17 cells by themselves can facilitate priming of endogenous T cells for IL-17A production. IL-17A seems to be major contributor to both Th1 and Th17 collateral priming: Neutralization of IL-17A in Th1 () and Th17 (see Figure E7, in the Online Repository) collateral priming significantly reduces BAL inflammatory cell influx suggesting that IL-17A is an important contributor to the collateral priming process under both circumstances. However, neutralization of IL-17A during cell transfer and first challenge phase did not affect Th2 collateral priming (see Figure E7, in the Online Repository).
Human studies show that exacerbation and chronicity of asthma are linked to Th1 polarized pulmonary inflammation often occurring due to viral infections,18–20,31
thus underlining an important role not only for Th2 but also Th1 polarized lymphocytes in asthma. Our results revealed that collateral priming can be induced through transfer of Th1 polarized cells, possibly modelling the increased risk of allergic sensitization after viral infections seen in children.18–20
However, in spite of the importance of IFN-γ as the key Th1 cytokine, Th1 collateral priming depends on IL-17A. Since the paradigm of a Th1/Th2 dichotomy in inflammatory disorders has been revised to include Th17 cells, a wealth of data has been generated linking these cells to deleterious effects that were previously thought to be Th1-driven diseases.7,27,32,33
Additionally, a critical role of Th17 cells in host defense, including viral infections, has been demonstrated.34,35
In allergic airway disease IL-17A levels correlate with disease severity and the influx of neutrophils.36–38
Rodent models of allergic airway inflammation revealed a role for IL-17A in the recruitment of neutrophils as well as eosinophils10,39
and complement factor C3a as important regulator of IL-23/Th17-axis in severe asthma.40
Our studies confirm the crucial role of Th17 in pulmonary priming for airway inflammation and reactivity and thus underline the need to address these cells in more detail when searching for new interventional strategies in allergic airway disease.
The development of an IL-17A producing subpopulation of CD4+ under Th1 polarizing conditions was surprising since IFN-γ is described to counterbalance Th17 polarization.41
However, in light of in vivo
circumstances where developing Th17 cells might encounter IFN-γ and other cytokines favoring or opposing Th17 cell development, the presence of a minor IL-17A producing population that is not susceptible to IFN-γ suppression is not inconceivable. In fact, our own in vivo data on cytokine secretion by lung and LN cells after the secondary challenge in Th17 collateral priming () showed secretion of IL-17A and large amounts of IFN-γ by distinct populations of lung cells, suggesting that Th17 cells can escape the suppressive effects of IFN-γ during their development. Indeed, studies have suggested that under certain circumstances, the presence of IL-17A might even be important for recruitment of Th1 cells in bacterial infections.42
Since IL-17A – in contrast to IL-4, which was identified as crucial cytokine for Th2 collateral priming – is not directly acting as a T-cell differentiation factor, the mechanism behind IL-17A-mediated collateral priming might be more indirect. Various cell types including bronchial epithelial cells and fibroblasts are described to secret chemokines and cytokines (e.g. Th17 differentiation factor IL-6) and upregulate leukocyte adhesion molecules like ICAM-1 in response to IL-17A stimulation.43
Together with the presence of other inflammatory cells during the first challenge phase, for instance macrophages 44
which under inflammatory conditions have been described to be sufficient for Th17 polarization of naïve T-cells, this might provide a milieu sufficient for Th17 differentiation.
Contrary to some studies which point towards a pivotal role of IL-17A in neutrophilia,10,34,45
we found a lymphocytic influx into the BAL on day 22 after Th17 collateral priming with few neutrophils. This discrepancy might depend on different protocols for the induction of airway inflammation, in particular with regards to the amount of antigen used. Higher amounts of antigen invariably increase the amount antigen-contaminating LPS which dose-dependently induces neutrophilia.46
Additionally, analysis of BAL cells at different time points after challenges revealed that neutrophils appear in the lung 24h after the first challenge phase (d4) as well as 24h after the second challenge phase (d20), but decrease in number with time as is seen at 72h after the second challenge (d22), suggesting that early neutrophilia might be missed in our protocol (see Figure E4, in the Online Repository). Recent clinical data also suggest a role for IL-17A (and IL-17F) in COPD and asthma, but shows no correlation with neutrophilic airway inflammation.47
Several studies have shown a seminal role for the IFN-γ/IL-12 axis in the induction of AHR,48,49
whereas other studies demonstrate contribution of the Th17/IL-23 axis and neutrophil recruitment in conferring AHR.39,45
However, a recent study describes conversion of Th17 into IFN-γ-producers in vivo as a prerequisite for AHR,50
which constitutes a combination of both axes and is conceivable in the light of potential Th17 cell plasticity.51
Additional Th plasticity has been observed in allergic airway disease where a subset of IL-17A secreting Th2 cells has been detected at increased levels in asthmatic patients and pro-inflammatory cytokine stimulation was shown to induce IL-17A secretion from classical Th2 cells.52
Our studies addressed the role of IL-17A vs. IFN-γ during the collateral priming process where we did observe a role for IL-17A but not for IFN-γ (). However,, at the present time we can neither distinguish between a singular contribution of IL-17A vs. IFN-γ towards the induction of AHR nor exclude a contribution of IL-17A-producing Th2 cells to collateral priming.
Our findings concerning the role of IL-17A in pulmonary priming might be particularly critical with regards to steroid resistant asthma, which has been described to be mediated by Th17 cells.12
Extrapolating our data, patients suffering from this form of asthma would be at a particular risk for developing new sensitizations.
Evolutionarily, collateral priming might have evolved to ensure that during an ongoing immune response bystander cells with different antigen-specificity could be more easily recruited to become polarized effector cells. At least partially, we draw upon this effect when boosting for vaccination to enhance a response against a given pathogen. However, in the context of allergic responses this beneficial effect can have deleterious consequences as any lung inflammation, regardless of its origin (allergic, viral, environmental) or polarization will increase the risk of de-novo sensitization towards unrelated, harmless antigens. Ultimately this process leads to polysensitization, a subtype of allergic disease with a much worse clinical course which is immensely more difficult to treat53–55
and thus needs to become a focus of future research directions.
Our findings bring mechanistic knowledge to the phenomena observed when boostering for vaccination and the phenomenon of polysensitization and primary sensitization in asthma, particularly in the context of concomitant viral infections.
We show that airway inflammation mediated by Th17 cells facilitates sensitization to new antigens and confers increased airway inflammation and airway responsiveness in a mouse model of polysensitization.