Malaria and leishmaniasis co-infection : the cardinal role of macrophages

Introduction Malaria and leishmaniasis are vector-borne parasitic diseases responsible for million cases every year, mostly children, in tropical and subtropical countries. In areas where malaria and leishmaniasis are both endemic, co-infections have been reported. During co-infections, immune mediators produced against one parasite can promote or prevent the establishment and the outcome of the other infection. Macrophages are involved in the defense and pathogenesis of both diseases phagocytizing the pathogens and producing immune mediators responsible for the control of the infections. Here, we investigated the effects on macrophages of the concomitant exposure to Leishmania infantum promastigotes and to hemozoin (Hz), the hemoglobin detoxification product of Plasmodium falciparum. In particular, the ability of macrophages to internalize Hz and L. infantum at the same time and the outcome of the infection ware studied. The production of cytokines and Nitric Oxide (NO) by macrophages was also investigated. Materials and Methods: Macrophages from differentiated human THP-1 cells, immortalized bone marrow derived macrophages (BMDM) from C54BL/6 mice, Hz from P. falciparum cultures and L. infantum were used. The growth rate of Leishmania parasites in macrophages was determined by optical microscopy. The co-localization of Hz and Leishmania inside macrophages and their morphology were investigated by electron microscopy. The Griess assay and ELISA were used to determine NO and cytokines production, respectively. Results: Electron microscopy showed that L. infantum amastigotes and Hz can co-localize in the same macrophages. Digested parasites were also observed in some macrophages. The percentage of Leishmania infected macrophages (both human and murine) was reduced by the phagocytosis of Hz indicating either a reduced parasite growth or an increased killing of Leishmania parasites in macrophages treated with Hz. Low levels of NO were produced by BMDM stimulated by L. infantum alone but, in the presence of Hz, the NO production was significantly higher. Moreover, the presence of Hz enhanced the production of TNF-alpha, Interleukin-12 and CCL-5 in L. infantum infected THP-1 derived macrophages. Discussion and Conclusions: These results suggest that Hz may play a protective role versus L. infantum infection inducing an increase in NO production and enhancing the levels of cytokines involved in TH1 polarization, required for the resolution of Leishmania infection.