Interleukin-36γ is expressed by neutrophils and can activate microglia, but has no role in experimental autoimmune encephalomyelitis

Background: Experimental autoimmune encephalomyelitis (EAE) is a model of inflammatory demyelinating diseases mediated by different types of leukocytes. How these cells communicate with each other to orchestrate autoimmune attacks is not fully understood, especially in the case of neutrophils, whose importance in EAE is newly established. The present study aimed to determine the expression pattern and role of different components of the IL-36 signaling pathway (IL-36 alpha, IL-36 beta, IL-36 gamma, IL-36R) in EAE. Methods: EAE was induced by either active immunization with myelin peptide, passive transfer of myelin-reactive T cells or injection of pertussis toxin to transgenic 2D2 mice. The molecules of interest were analyzed using a combination of techniques, including quantitative real-time PCR (qRT-PCR), flow cytometry, Western blotting, in situ hybridization, and immunohistochemistry. Microglial cultures were treated with recombinant IL-36 gamma and analyzed using DNA microarrays. Different mouse strains were subjected to clinical evaluation and flow cytometric analysis in order to compare their susceptibility to EAE. Results: Our observations indicate that both IL-36 gamma and IL-36R are strongly upregulated in nervous and hematopoietic tissues in different forms of EAE. IL-36 gamma is specifically expressed by neutrophils, while IL-36R is expressed by different immune cells, including microglia and other myeloid cells. In culture, microglia respond to recombinant IL-36 gamma by expressing molecules involved in neutrophil recruitment, such as Csf3, IL-1 beta, and Cxcl2. However, mice deficient in either IL-36 gamma or IL-36R develop similar clinical and histopathological signs of EAE compared to wild-type controls. Conclusion: This study identifies IL-36 gamma as a neutrophil-related cytokine that can potentially activate microglia, but that is only correlative and not contributory in EAE.