Professor University of Maryland School of Medicine Baltimore, Maryland, United States
Introduction/Rationale: The IL-33/ST2 cytokine/receptor axis is well known to regulate various disease processes, including pulmonary fibrosis (PF). We and others have also demonstrated that the intranuclear, precursor form of IL-33 regulates diseases in an ST2-independent fashion. We have recently reported that genetic deletion of IL-33 nearly completely abrogates bleomycin (BLM)-induced PF, whereas attenuation of PF in ST2 knockout (ST2KO) mice was only partial. The goal of this study was to identify the mechanism that prevents ST2 deficiency from fully attenuating PF.
Methods: Mice genetically deficient of both IL-33 and ST2 (double-knockout, DKO) were tested along with ST2KO and wild-type (WT) mice in two BLM models of PF, including single-hit intratracheal BLM injury as well as double-hit IL-33 overexpression combined with BLM injury models. Pulmonary levels of cytokines were screened by Luminex multiplex and Olink approaches, with findings confirmed by ELISAs. In vivo blockade of cytokines with specific antibodies was utilized to assess the effect on severity of PF.
Results: In both single-hit and double-hit models of PF, the levels of TSLP and IL-9 were elevated in WT, DKO, and ST2KO mice based on Luminex, Olink, and ELISA measurements. Neither DKO and ST2 mice were protected from PF. In vivo blockade of TSLP potently attenuated pulmonary levels of IL-9 and collagen in all tested strains. Flowcytometric and immunocytochemical assessment of lung tissue cells identified CD4+ T cells as the main producers of IL-9, with ILC2 cells producing a fraction of IL-9.
Conclusion: The antifibrotic effect of ST2 deficiency is partially negated by the concurrent activation of the profibrotic TSLP—IL-9 axis. Therapeutic targeting of ST2 alone may represent a suboptimal strategy, and needs to be combined with simultaneous targeting of TSLP. Of note, these findings echo observations made by others in a mouse model of asthma outside of the context of PF, suggesting a more universal applicability of this mechanism.
