Distinct Roles for CD4⁺ Foxp3⁺ Regulatory T Cells and IL-10-Mediated Immunoregulatory Mechanisms during Experimental Visceral Leishmaniasis Caused by Leishmania donovani

The outcome of intracellular parasitic infection can be determined by the immunoregulatory activities of natural regulatory CD4⁺ Foxp3⁺ T (Treg) cells and the anti-inflammatory cytokine IL-10. These mechanisms protect tissue but can also suppress antiparasitic CD4⁺ T cell responses. The specific contribution of these regulatory pathways during human parasitic diseases remains unclear. In this study, we investigated the roles of Treg cells and IL-10 during experimental visceral leishmaniasis caused by Leishmania donovani infection of C57BL/6 mice. We report only a limited contribution of Treg cells in suppressing antiparasitic immunity, but important roles in delaying the development of splenic pathology and restricting leukocyte expansion. We next employed a range of cell-specific, IL-10- and IL-10R-deficient mice and found these Treg cell functions were independent of IL-10. Instead, conventional CD4⁺ T cells and dendritic cells were the most important cellular sources of IL-10, and the absence of IL-10 in either cell population resulted in greater control of parasite growth but also caused accelerated breakdown in splenic micro-architecture. We also found that T cells, dendritic cells, and other myeloid cells were the main IL-10-responding cells because in the absence of IL-10R expression by these cell populations, there was greater expansion of parasite-specific CD4⁺ T cell responses associated with improved control of parasite growth. Again, however, there was also an accelerated breakdown in splenic micro-architecture in these animals. Together, these findings identify distinct, cell-specific, immunoregulatory networks established during experimental visceral leishmaniasis that could be manipulated for clinical advantage.