Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Molecular Microbiology and Microbial Pathogenesis


English (en)

Date of Award

January 2011

Degree Type


Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Stephen Beverley


Leishmania parasites are the causative agent of leishmaniasis, a neglected tropical disease. An important aspect of Leishmania biology is asymptomatic parasite persistence, which typically occurs after clinical cure. Persistent parasites remain enigmatic despite their importance as reservoirs for transmission, having roles in maintaining protective immunity, and posing the risk of reactivation. I developed methods for assessing parasite replication by BrdU labeling and showed that persistently infected mice harbor two sub-populations of L. major, one labeling similarly to acute-phase parasites, with the other showing much less labeling. That persistent parasite replication occurs without a commensurate increase in parasite number implies parasite killing. Continual parasite replication and destruction within antigen presenting cells provides an attractive model explaining the role of persistent parasites in maintaining immunity, namely through constant presentation of antigens derived from dead parasites and subsequent immune boost. While many of the persistent parasites are within host cells expressing high levels of iNOS, there is no apparent correlation between this and the parasite's survival/replication status. Attenuated lpg2- L. major, a proposed model of parasite persistence, resemble WT persistent parasites for most parameters tested. However, more lpg2- parasites are associated with host cells expressing elevated levels of arginase 1, which further studies implicate as a negative correlate of immunity. While persistent parasites immunize their hosts against pathology from subsequent infection, experiments using marked parasites showed that persistently infected mice could be super-infected. This has implications for the generation of parasite phenotypic diversity, as genetically distinct parasites could be simultaneously transmitted to sand flies, the site of parasite sexual recombination. In addition to my studies of Leishmania persistence, I also identified markers that differentiate amastigote- from metacyclic-stage parasites, and used them to assay parasite differentiation within different host cell types in vitro. Although the markers were induced in the same sequence in all host cell types, the parasites in bone marrow-derived macrophages and dendritic cells were slower to lose LPG expression and resume replication. These data show that invading L. major can retain virulence factors, potentially playing a role in situations where parasites are transferred from one host cell to another.


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