Cytokines in the generation and function of regulatory T cell subsets in leishmaniasis

CD4⁺ T regulatory cells (Tregs) are a group of T lymphocytes that maintain self-tolerance and protect the host from inflammation-induced tissue damage. An interacting network of cytokines and transcription factors influence the origin, differentiation, and function of the Tregs in primary and secondary lymphoid organs. However, following antigenic stimulation, it can also be induced at the sites of infection. Immune cell resident microbial pathogens, such as Leishmania, employ varieties of mechanisms to promote the suppressive functions of Tregs for protective evasion from the host immune system. This establishes a state of immune unresponsiveness in the host, exacerbating the disease in Leishmania infection. Elimination of Leishmania pathogens is accomplished with a strong pro-inflammatory response accompanied by the release of host protective cytokines such as Interleukin-2 (IL-2), Interferon-gamma (IFN-γ), and Tumor necrosis factor-alpha (TNF-α), which functions through suppression of Tregs or making the effector cells recalcitrant to Treg mediated suppression. Nevertheless, during chronic infection, the persistence of unwarranted pro-inflammatory cytokines can trigger self-tissue damage. Tregs limit the consequence of chronic inflammation to restrict self-harm suggesting its mutually opposing role in host protection. Furthermore, Tregs function to prevent complete parasite clearance to provide long-term immunity to re-infection. This review summarizes the roles of pro-inflammatory and anti-inflammatory cytokines involved in the homing, activation, differentiation, and suppression of Tregs in the course of Leishmania infection. We also suggest cytokines that can be modulated as potential therapeutic targets to treat Leishmania infection.