All successful microbial pathogens evade or alter host immune responses. Arguably the most well-known example of immune evasion by parasites is antigenic variation of the variant surface glycoprotein (VSG) coat by the African trypanosomes, which prevents immune elimination of the organisms from vascular and- extra vascular tissue sites of infection. VSG-specific B cell responses are associated with, and largely limited to, control of Parasites in the blood, but are VSG-specific Th1 cell responses that are functionally and genetically linked to relative resistance against trypanosomes. This resistance is mediated by IFN-? activation of macrophages for production of the trypanocidal factors RNI, ROS and TNFa In the extra vascular tissues. Thus, IFN-? production is critical for host survival but is largely dependent on Th1 cell recognition of VSG-specific epitomes. Ultimately, however, antigenic variation of the VSG coat leads to immune escape and trypanosome infections are invariably fatal for their hosts. Two recent achievements in our laboratory have opened a window of opportunity for immunological control or cure of African trypanosomiasis. The first is a discovery that temporally protective VSG-specific T cell responses are directed exclusively at highly variable N-terminal micro domains within the VSG molecule, and are not directed at residues within the highly conserved -terminal domain. Thus, VSG specific T cell responses are variant specific and do not cross-react with other VSGs that express homologous C-terminal domain sequences. This information provides a unique opportunity to artificially direct Th1 cell responses to conserved VSG sequences and to promote control or cure of infection. The second discovery is that by expressing an exogenous eGFP gene from a stable transcription site in trypanosome DNA, we are able for the first time to visualize viable trypanosomes within the extra vascular tissues and to monitor the efficacy of immunotherapeutic treatments for host resistance. These novel elements form the infrastructure for this proposal. The central hypothesis to be tested is that activation of T cell responses to conserved C-terminal residues of the trypanosome variant surface glycoprotein (VSG) molecule will promote variant cross- protective immunity to infection, leading to control or cure of disease.
Aim 1 encompasses experiments in which natural, altered natural, recombinant and synthetic conserved C-terminal VSG subsequences will be used to provoke variant cross-specific Th1 cell responses in na?ve mice. Immunized animals subsequently will be infected with different trypanosome variants expressing the conserved VSG residues.
This aim will provide """"""""proof of principle"""""""" for the central hypothesis.
Aim 2 extends the work plan to experiments that have clinical translational potential. Treatment of mice already infected with African trypanosomes will be accomplished by immunization with invariant VSG cross- protective protein sequences identified in Aim 1. Animals will be monitored for resolution of infection coupled with activation of appropriate T cells with specificity for variant cross-protective sequences.
This aim provides a hypothetical """"""""real world test"""""""" for the central hypothesis, in which patients diagnosed with the disease could be rescued by intervention therapy. The ability to induce cross-variant specific T cell responses and parasite elimination in individuals already infected would have tremendous impact on clinical disease in Africa. Thus, the experiments proposed here provide, for the first time, a novel approach to the control or cure of a Third World disease that has proven intractable to every form of preventive or clinical therapy to date.
African trypanosomiasis (Human Sleeping Sickness) is a devastating Third World disease that is endemic throughout Sub-Saharan Africa. There is no natural cure for infection, primarily due to extensive antigenic variation of the parasite variant surface glycoprotein (VSG) coat. Further, all preventive and therapeutic strategies aimed at controlling the disease have been unsuccessful to date. Therefore the present proposal tests a novel approach and uses innovative new tools aimed at the induction and monitoring of variant cross-protective immunity to African trypanosomiasis, and findings relative to VSG structure/function will be applicable to other microbial infections in which infected hosts preferentially respond to variant rather than invariant residues of protective antigens.