A Prospective cohort study of zoonotic cutaneous leishmaniasis in tunisia: Clinical and Immunological features and immune correlates of protection

BackgroundThis study aimed to define immunological markers of exposure to L. major parasites and identify correlates of protection against infection.MethodsWe analyzed a cohort of 790 individuals at risk of developing ZCL living in endemic areas with varying L. major infection prevalence. One area had a high infection prevalence indicated by high proportions of leishmanin skin test (LST) positive subjects, while the other areas were recent foci with lower infection prevalence. Blood samples were collected before the transmission season to measure Interferon gamma (IFN-gamma), Interleukin 10 (IL-10), and Granzyme B (GrB) levels in response to parasite stimulation in peripheral blood mononuclear cells.A one-year follow-up period involved active detection of new ZCL cases to estimate disease incidence after a transmission season and identify immune correlates of protection.MethodsWe analyzed a cohort of 790 individuals at risk of developing ZCL living in endemic areas with varying L. major infection prevalence. One area had a high infection prevalence indicated by high proportions of leishmanin skin test (LST) positive subjects, while the other areas were recent foci with lower infection prevalence. Blood samples were collected before the transmission season to measure Interferon gamma (IFN-gamma), Interleukin 10 (IL-10), and Granzyme B (GrB) levels in response to parasite stimulation in peripheral blood mononuclear cells.A one-year follow-up period involved active detection of new ZCL cases to estimate disease incidence after a transmission season and identify immune correlates of protection.ResultsThe study population showed heterogeneity in parasite contact, evident from specific scars and/or positive LST results, significantly higher in the old focus compared to recent foci. IFN-gamma and GrB were markers of parasite exposure and reliable indicators of immunity to L. major. Positive correlations were observed between IFN-gamma/IL-10 and GrB/IL-10 ratios and LST results.Unexpectedly, only 29 new ZCL cases (4%) appeared after a transmission season, with 27 cases reported in recent foci and 2 in the oldest focus. Our findings indicate that individuals in L. major endemic areas are likely to develop ZCL regardless of their LST status.We showed that high pre-transmission season levels of IFN-gamma and GrB produced by PBMC, along with a high IFN-gamma/IL-10 ratio, were associated with protection.ResultsThe study population showed heterogeneity in parasite contact, evident from specific scars and/or positive LST results, significantly higher in the old focus compared to recent foci. IFN-gamma and GrB were markers of parasite exposure and reliable indicators of immunity to L. major. Positive correlations were observed between IFN-gamma/IL-10 and GrB/IL-10 ratios and LST results.Unexpectedly, only 29 new ZCL cases (4%) appeared after a transmission season, with 27 cases reported in recent foci and 2 in the oldest focus. Our findings indicate that individuals in L. major endemic areas are likely to develop ZCL regardless of their LST status.We showed that high pre-transmission season levels of IFN-gamma and GrB produced by PBMC, along with a high IFN-gamma/IL-10 ratio, were associated with protection.ResultsThe study population showed heterogeneity in parasite contact, evident from specific scars and/or positive LST results, significantly higher in the old focus compared to recent foci. IFN-gamma and GrB were markers of parasite exposure and reliable indicators of immunity to L. major. Positive correlations were observed between IFN-gamma/IL-10 and GrB/IL-10 ratios and LST results.Unexpectedly, only 29 new ZCL cases (4%) appeared after a transmission season, with 27 cases reported in recent foci and 2 in the oldest focus. Our findings indicate that individuals in L. major endemic areas are likely to develop ZCL regardless of their LST status.We showed that high pre-transmission season levels of IFN-gamma and GrB produced by PBMC, along with a high IFN-gamma/IL-10 ratio, were associated with protection.ConclusionThis study on a large cohort at risk of ZCL confirmed IFN-gamma and GrB as protective factors against the disease. A high IFN-gamma/IL-10 ratio, but not GrB/IL-10 ratio was associated with resistance. These results are valuable for developing and evaluating of a vaccine against human leishmaniasis. A cohort of 790 individuals at risk of developing ZCL, residing in areas endemic to leishmaniasis with different prevalence of L. major infection, was followed for a one-year period to determine the incidence of the disease following a transmission season and to identify immune correlates of protection.Our results showed that the study population in an endemic area exhibited heterogeneity in terms of exposure to the bites of infectious flies due to the characteristics of the houses such as the presence of shelters for animals which attracts the vectors. IFN-gamma and GrB were detected in most participants with significantly higher values in the old focus compared to recent foci and among individuals with LST reaction. Furthermore, a strong correlation was observed between IFN-gamma and GrB as well as a positive association between the IFN-gamma/IL-10 ratios and LST results. Cumulative incidence of infection among the study cohort was 4% after a transmission season, primarily from recent foci. We also showed that high levels of IFN-gamma and GrB produced by PBMCs in response to the parasite, as well as a high IFN-gamma/IL-10 ratio, at the steady state, were associated with protection against disease development.