IL-2 family cytokines IL-9 and IL-21 differentially regulate innate and adaptive type 2 immunity in asthma

Asthma is often accompanied by type 2 immunity rich in IL-4, IL-5, and IL-13 cytokines produced by T_H2 lymphocytes or type 2 innate lymphoid cells (ILC2s). IL-2 family cytokines play a key role in the differentiation, homeostasis, and effector function of innate and adaptive lymphocytes.

IL-9 and IL-21 boost activation and proliferation of T_H2 and ILC2s, but the relative importance and potential synergism between these γ common chain cytokines are currently unknown.

Using newly generated antibodies, we inhibited IL-9 and IL-21 alone or in combination in various murine models of asthma. In a translational approach using segmental allergen challenge, we recently described elevated IL-9 levels in human subjects with allergic asthma compared with nonasthmatic controls. Here, we also measured IL-21 in both groups.

IL-9 played a central role in controlling innate IL-33-induced lung inflammation by promoting proliferation and activation of ILC2s in an IL-21-independent manner. Conversely, chronic house dust mite-induced airway inflammation, mainly driven by adaptive immunity, was solely dependent on IL-21, which controlled T_H2 activation, eosinophilia, total serum IgE, and formation of tertiary lymphoid structures. In a model of innate on adaptive immunity driven by papain allergen, a clear synergy was found between both pathways, as combined anti-IL-9 or anti-IL-21 blockade was superior in reducing key asthma features. In human bronchoalveolar lavage samples we measured elevated IL-21 protein within the allergic asthmatic group compared with the allergic control group. We also found increased IL21R transcripts and predicted IL-21 ligand activity in various disease-associated cell subsets.

IL-9 and IL-21 play important and nonredundant roles in allergic asthma by boosting ILC2s and T_H2 cells, revealing a dual IL-9 and IL-21 targeting strategy as a new and testable approach.