Double positive IL-17A+IFN-γ+CCR6+ ILCs contribute towards the immunopathology of lepromatous leprosy

Leprosy is a skin disease caused by Mycobacterium leprae, characterized by both localized and generalized immune responses. Th1/17 lymphocytes play a crucial role in the immune response against M. leprae. However, adaptive immunity alone is not sufficient to completely eradicate the pathogen, suggesting the involvement of other innate immune cells in pathogen removal. Therefore, we investigated innate lymphoid cells (ILCs), which are the innate counterparts of helper T cells in adaptive immunity and are known to produce IFN-γ and IL-17. In the present study, we evaluated the expression of ILC1 and ILC3 in borderline tuberculoid (BT) and lepromatous leprosy (LL) lesional skin by flow cytometry and real time PCR. Further, the expression of various in-situ genes, including cytokines, chemokines, cytokine receptors chemokine receptors, and transcription factors by qPCR in skin lesions of leprosy patients were analyzed. The phenotypes of ILC1 and ILC3 cells were determined as CD3^negCCR6⁺CD19^negIFN-γ⁺ and CD3^negCCR6⁺CD19^negIL-17A⁺, respectively, by flow-cytometry analysis. BT skin lesions represents high CCR6⁺expression on total ILCs as compared to LL patients. Our results clearly indicate that ILC1 and ILC3 were highly expressed in skin lesions of BT as compared to LL leprosy patients. Moreover, we observed that double positive (DP) CD3^negCCR6⁺CD19^negIFN-γ⁺IL-17A⁺ ILCs were up-regulated in LL and showed a pathogenic role. The gene expression of IL-17A and IFN-γ were found to be significantly positively correlated with the percentage of CCR6⁺ ILCs. On the other hand, CCR6^neg ILCs were negatively correlated with ILC1 and ILC3 associated markers. Summarily our results clearly suggest that both ILC1 and ILC3 are important and immune-protective, on the contrary DP (IFN-γ⁺IL-17A⁺) ILCs may promote progression and immunopathology of leprosy.