Characterization and optimization of animal and culture models of Leishmania chagasi

Abstract

Leishmania chagasi are one species of Leishmania that cause visceral leishmaniasis, a potentially fatal disease of humans: annual incidence ≈ 0.5 mil. Within the sand fly vector Leishmania replicate as promastigote-form parasites that undergo complex changes in morphology as they progress from early-stage procyclic- promastigotes, to intermediate stage leptomonadand nectomonad-promastigotes, and ultimately to terminal stage metacyclic-promastigotes that are more highly infective to vertebrates. This developmental progression is largely recapitulated in vitro using axenic promastigote cultures that have been passaged only a few times. Within a single passage (which takes about a week), such axenic cultures progress from logarithmic to stationary growth phases, and parasites within those growth phases progress from parasites that do not have metacyclic cell properties to ones that do. Interestingly, repeated serial passage of promastigote cultures results in cell populations that exhibit perturbations in developmental progression, in expression levels of surface macromolecules (Major Surface Protease, MSP, and Promastigote Surface Antigen, PSA), and in virulence 28 properties including resistance to serum lysis. Experiments were performed to determine whether there exists a direct relationship between promastigote developmental form and these perturbations associated with repeated serial passage. Passage 2-4 L. chagasi cultures at stationary growth phase were predominately (> 85%) comprised of metacyclic promastigotes and exhibited high resistance to serum-lysis and high levels of MSP and PSA. Serial passaging 8 or more times resulted in a stationary phase population that was largely (> 85%) comprised of nectomonad promastigotes, almost completely devoid ( 2%) of metacyclic promastigotes, and exhibited low resistance to serum-lysis and low levels of MSP and PSA. This study suggests that the loss of particular cell properties seen in cells from serially passaged cultures is principally due to a dramatic reduction in the proportion of metacyclic promastigotes. Additionally, this study suggests that serially passaged cultures may be a highly enriched source nectomonad promastigotes, a parasite form that has largely been characterized in mixtures containing other promastigote forms.