Subclinical or
asymptomatic infection is documented in individuals living in endemic areas for
leishmaniasis suggesting that the development of an appropriate
immune response can control
parasite replication and maintain
tissue integrity. A low
morbidity indicates that intrinsic factors could favor resistance to
Leishmania infection. Herein, leishmanial
T-cell responses induced in subjects with low susceptibility to
leishmaniasis as asymptomatic subjects were compared to those observed in cured
cutaneous leishmaniasis (CCL)
patients,
who controlled the
disease after antimonial
therapy. All of them have shown
maintenance of specific long-term
immune responses characterized by expansion of higher proportions of CD4+ as compared to CD8+
Leishmania reactive
T-lymphocytes. Asymptomatic subjects had lower indexes of
in vitro Leishmania induced lymphoproliferative responses and
interferon-gamma (IFN-gamma)
production in comparison to CCL
patients. On the other
hand,
interleukin (
IL-10)
production was much higher in asymptomatics than in CCL, while no differences in
IL-5 levels were found. In conclusion, long lived
T-cell responses achieved by asymptomatic individuals differed from those
who had developed symptomatic
leishmaniasis in terms of intensity of
lymphocyte activation (proliferation or IFN-gamma) and regulatory mechanisms (
IL-10). The absence of the
disease in asymptomatics could be explained by their intrinsic
ability to create a balance between immunoregulatory (
IL-10) and effector
cytokines (IFN-gamma), leading to
parasite destruction without producing
skin tissue damage. The establishment of profiles of
cell-mediated
immune responses associated with resistance against
Leishmania infection is likely to make new inroads into
understanding the long-lived immune
protection against the
disease.