PROJECT SUMMARY/ABSTRACT
Visceral leishmaniasis is a devastating, neglected tropical disease, ultimately leading to multiorgan failure and
death when left untreated. As with many parasitic infections, efforts to develop an effective vaccine have
remained unsuccessful. In contrast to a number of infectious pathogens which can be controlled by neutralizing
antibodies and/or cytotoxic CD8+ T cells, protection against leishmaniasis is largely mediated by T helper 1
differentiated CD4+ T cells, but how CD4+ T cell activation, differentiation, and function are regulated during
visceral leishmaniasis is still largely unknown. A major limitation to these studies has been the inability to
accurately identify Leishmania-specific CD4+ T cells in experimental model systems in order to assess T cell
longevity and functions during and after the course of the infection. Here, using a recently identified MHC-II
epitope conserved across Leishmania species, we show that in contrast to cutaneous versions of leishmaniasis,
antigen-specific CD4+ T cells become functionally exhausted during visceral leishmaniasis, produce IL-10, and
express high levels of the inhibitory receptors PD-1 and CTLA-4. This suggests that rather than remaining
“hidden”, high parasite burden and/or antigen load overwhelms the adaptive immune system to the point where
antigen-specific CD4+ T cells lose the capacity to produce the necessary pro-inflammatory cytokines to kill
parasites residing within macrophages. Because there is currently no effective vaccine and limited treatment
options for leishmaniasis, we have generated recombinant Vaccinia virus (VacV) vectors expressing the
immunodominant Leishmania epitope targeted for MHC-II presentation to study the CD4+ T cell response against
this parasitic infection. To begin to investigate the mechanisms of CD4+ T cell activation and to identify strategies
to reverse or limit T cell dysfunction during visceral leishmaniasis, we will 1) define the functions and gene
expression profiles of antigen-specific CD4+ T cells that become activated following either cutaneous or visceral
Leishmania infections and 2) determine if therapeutic viral immunization combined with immunotherapy restores
function to CD4+ T cells during visceral leishmaniasis. The overall goal of this project will be to begin to define
the mechanisms that regulate CD4+ T cell function and dysfunction during visceral leishmaniasis, which will
contribute to our long-term goal of improving vaccine design and therapies against this complex parasite.