IL-15 in human visceral leishmaniasis caused byLeishmania infantum

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Abstract
Interleukin (IL)‐15 is a recently discovered cytokine with the ability to stimulate the proliferation activity of Th1 and/or Th2 lymphocytes. Here, we investigated the involvement of IL‐15 in the immune response to Leishmania infantum infection by studying patients with visceral leishmaniasis (VL). We found that IL‐15 is produced by leishmanial antigen (LAg)‐stimulated peripheral blood mononuclear cells (PBMC) from active VL patients at a significantly higher level than those produced by cells from healed VL subjects or healthy controls. A significant increase in IL‐15 serum blood levels was also observed in acute VL patients compared with healed ones. Furthermore, recombinant IL‐15 had an appreciable effect in vitro in reducing IL‐4 and increasing the production of IL‐12 in response to LAg, but it was ineffective in altering the production of interferon‐γ (IFN‐γ). The production of endogenous IL‐15 in acute VL patients appeared to be insufficient to activate both IFN‐γ and IL‐12, as attested by the absence of modification of these two cytokines by neutralization experiments in the presence of anti‐IL‐15 monoclonal antibodies (MoAB). On the contrary, the neutralization of IL‐15 increased IL‐4 production. Together, these results indicate that endogenous IL‐15 plays a role in the suppression of Th2‐type cytokines, even though it does not enhance the production of Th1 cytokines in acute VL patients. Since IL‐15, in the presence of anti‐IL‐4 MoAb, caused a further increase in IL‐12 production and led to a significant production of IFN‐γ, one of its indirect effects on Th1 cell activation could be due to the latter’s effect on Th2 cytokines such as IL‐4. Therefore, our observations indicate that there is a potential for IL‐15 to augment the T‐cell response to human intracellular pathogens.