Host and parasite determinants of Leishmania survival following phagocytosis by macrophages

The obligate intracellular protozoan, Leishmania infantum chagasi (Lic) is the causative agent of visceral leishmaniasis in South America. The flagellated promastigote life stage of the parasite undergoes receptor-mediated phagocytosis by macrophages. This process is followed by a transient delay in phagolysosome maturation that allows for conversion into the amastigotes, a stage that is resistant to degradation inside host cells. We hypothesized that events occurring early during parasite-host interaction influence whether the pathogen ultimately survives or is eliminated in the intracellular environment, and that these processes are facilitated by determinants from both the macrophage and the incoming Leishmania. We found differences in the pathway through which virulent Lic metacyclic promastigotes or avirulent logarithmic promastigotes are phagocytosed by human monocyte-derived macrophages (MDMs). The macrophage surface receptors that ligated the two forms of promastigotes differed, guiding metacyclic promastigotes into a compartment that supported their replication and logarithmic promastigotes into a vacuole that rapidly assembled its microbicidal machinery. Survival of metacyclic promastigotes following their phagocytosis also varied greatly on characteristics of the host macrophage. U937 cells, a model monocytic cell line lacking the third complement receptor (CR3) on their surface, took up parasites via a unique "coiling" mode of pseudopod extension, leading to a formation of a phagosome that did not fully mature. Since the parasites never demonstrated escape into the macrophage cytosol, it is logical to predict that they synthesize and release virulence factors that localize within the parasitophorous vacuole (PV) in order to establish communication with the host cell. Using a previously assembled bioinformatic catalogue of putatively secreted or excreted (E/S) proteins encoded in the Leishmania infantum genome, we chose four candidate proteins for further analysis. Two of these, serine carboxypeptidase (CBP) and a flavodoxin domain-containing protein (HP) coding sequences, were overexpressed or removed in Lic. Parasites lacking one allele of either CBP or HP were defective in survival within MDMs. Furthermore, recombinant overexpressed HP was detected from parasite lysate in a stage-specific manner, paralleling expression in wild type Lic. This implies that the regulatory elements within the protein coding sequence remain functional outside of their native locus. Taken together, our study shows that quiescent entry of virulent Leishmania spp. into macrophages is accounted for by i) the ability of metacyclic promastigotes to selectively bypass macrophage components leading to deleterious pathways, as well as ii) tightly regulated parasite virulence factors for deliberately enhancing intracellular survival.