Contribution of non-Leishmania-specific immunity to resistance to Leishmania infection in humans.

Lymphocytes of individuals from a country non-endemic for Leishmania (Sweden), responded with a vigorous interferon-gamma (IFN-gamma) and IL-6 response when exposed to live or dead promastigotes of Leishmania aethiopica. This response was sometimes as strong as when the same cells were exposed to the mitogen (phytohaemagglutinin (PHA)). Furthermore, supernatants of cells exposed to Leishmania promastigotes were able to inhibit the amastigote form of the same parasite. In some few instances there was no such reactivity to Leishmania parasites. It is proposed that most individuals have such a first line cytokine response which is enough to prevent further spread and growth of the parasites. In exposed individuals who display disease symptoms, this non-Leishmania-specific response is overcome (by dose) or is weak (for genetic reasons). In the latter instances curbing of parasite growth would depend on acquired immunity.