Increased IL-35 producing Tregs and CD19⁺IL-35⁺ cells are associated with disease progression in leprosy patients

Background The clinical forms of leprosy consist of a spectrum that reflects the host's immune response to the M. leprae; it provides an ideal model to study the host pathogen interaction and immunological dysregulation in humans. IL-10 and TGF-β producing Tregs are high in leprosy patients and responsible for immune suppression and M. leprae specific T cells anergy. In leprosy, involvement of IL-35 producing Tregs and Bregs remain unstudied. Objective To study the role of IL-35 producing Tregs and Bregs in the human leprosy. Methods Peripheral blood mononuclear cells from leprosy patients were isolated and stimulated with M. leprae antigen (MLCwA) for 48 h. Intracellular cytokine IL-35 was evaluated in CD4⁺CD25⁺ Tregs, CD19⁺ cells by FACS. Expression of PD-1 on CD4⁺CD25⁺ Tregs, CD19⁺ cells and its ligand (PD-L1) on B cells, CD11c cells were evaluated by flow cytometry (FACS). Serum IL-35 level was estimated by ELISA. Results The frequency of IL-35 producing Tregs and Bregs cells were found to be high in leprosy patients (p < 0.0001) as compared to healthy controls. These cells produced suppressive cytokine IL-35 which showed positive correlation with bacteriological index (BI) and TGF-β producing Tregs, indicating its suppressive nature. We found higher expression of PD-1 on Tregs, B cell and its ligand (PD-L1) on antigen presenting cells in leprosy patients. Conclusion This study point out a shift in our understanding of the immunological features that mediate and regulate the immune suppression and the disease progression in leprosy patients with a new paradigm (IL-35 producing Tregs and Bregs) that is beyond TGF-β and IL-10 producing Treg cells.