The role of regulatory T and regulatory B cells is to suppress CMI enough to prevent tissue damage. However, an excessive regulatory response can prevent clearance of Leishmania and could explain the anergy of VL, poor response to drug treatment, development of PKDL, and relapses. A role for regulatory cells in VL has long been suspected. A variety of regulatory T and B cells have been implicated in VL, including Type 1 T helper cells that secrete IL-10 in addition to IFN-γ, natural T reg, Tr1, CD8+ T reg, and B reg. All of these lymphocytes act, at least in part, by secreting IL-10 and other suppressive cytokines.CD4+ T regs are present at increased frequency in the bone marrow of VL patients, are one source of IL-10, and proliferate in response to Leishmania antigen. Levels of FoxP3 mRNA were also up-regulated in lesional tissue from PKDL patients. However, T regs are not elevated in spleen cells from VL patients nor does depletion of T regs increase Leishmania antigen specific IFN-γ secretion The highest levels of IL-10 mRNA in spleen cells is in CD8+ and other non-FoxP3+ T cells. White blood cell CD8+ T cells from VL patients have elevated IL-10 levels. There is a 9.6 fold increase in IL-10 expressing CD8+ T cells among PBMC lymphocytes from PKDL patients. In the one study of T cell clones from VL patients, the clones isolated from VL PBMC were 100% CD8+.
Some clones secreted soluble factors that caused the death of CD8+ regulatory T cells but not CD4+ T cells from VL patients, which might explain the strong protective immunity of asymptomatic patients. Non-protective immunity
VL patients are unable to clear their infections because they lack CMI. This anergy may be limited to Leishmania antigens or extend to mitogens and other antigens as the disease progresses. In addition to skin test negativity, VL patient PBMC do not proliferate or secrete IL-2 or IFN-γ in response to Leishmania antigens. Memory T cells may be depleted in VL patient PBMC. Since IL-10 is known to suppress innate and acquired immunity and prevent IFN-γ from activating macrophages, its role in VL has been studied extensively and elevated IL-10 production is often used as a marker of non-protective immunity in VL. Elevated levels of IL-10 in the plasma, infected tissues, and PBMC of VL patients accompany the anergy of VL. PKDL patients also have elevated IL-10 levels. VL patients with the highest IL-10 levels are more likely to be unresponsive to treatment and progress to PKDL. PBMC secretion of IL-10 without the addition of Leishmania antigen (endogenous) is inversely correlated with antigen specific IFN-γ secretion but Leishmania antigen specific IL-10 and IFN-γ secretion are not correlated, suggesting that endogenous secretion is more important in pathology. Addition of anti-IL-10 increases proliferation and IFN-γ secretion by PBMC from some patients. Both CD4+ and CD8+ T cells have been shown to contribute to IL-10 secretion by VL PBMC.