Human immunome, bioinformatic analyses using HLA supermotifs and the parasite genome, binding assays, studies of human T cell responses, and immunization of HLA-A*1101 transgenic mice including novel adjuvants provide a foundation for HLA-A03 restricted CD8+T cell epitope based, adjuvanted vaccine protective against Toxoplasma gondii.

BACKGROUND: Toxoplasmosis causes loss of life, cognitive and motor function, and sight. A vaccine is greatly needed to prevent this disease. The purpose of this study was to use an immmunosense approach to develop a foundation for development of vaccines to protect humans with the HLA-A03 supertype. Three peptides had been identified with high binding scores for HLA-A03 supertypes using bioinformatic algorhythms, high measured binding affinity for HLA-A03 supertype molecules, and ability to elicit IFN-γ production by human HLA-A03 supertype peripheral blood CD8+ T cells from seropositive but not seronegative persons.

RESULTS: Herein, when these peptides were administered with the universal CD4+T cell epitope PADRE (AKFVAAWTLKAAA) and formulated as lipopeptides, or administered with GLA-SE either alone, or with Pam2Cys added, we found we successfully created preparations that induced IFN-γ and reduced parasite burden in HLA-A*1101(an HLA-A03 supertype allele) transgenic mice. GLA-SE is a novel emulsified synthetic TLR4 ligand that is known to facilitate development of T Helper 1 cell (TH1) responses. Then, so our peptides would include those expressed in tachyzoites, bradyzoites and sporozoites from both Type I and II parasites, we used our approaches which had identified the initial peptides. We identified additional peptides using bioinformatics, binding affinity assays, and study of responses of HLA-A03 human cells. Lastly, we found that immunization of HLA-A*1101 transgenic mice with all the pooled peptides administered with PADRE, GLA-SE, and Pam2Cys is an effective way to elicit IFN-γ producing CD8+ splenic T cells and protection. Immunizations included the following peptides together: KSFKDILPK (SAG1224-232); AMLTAFFLR (GRA6164-172); RSFKDLLKK (GRA7134-142); STFWPCLLR (SAG2C13-21); SSAYVFSVK(SPA250-258); and AVVSLLRLLK(SPA89-98). This immunization elicited robust protection, measured as reduced parasite burden using a luciferase transfected parasite, luciferin, this novel, HLA transgenic mouse model, and imaging with a Xenogen camera.

CONCLUSIONS: Toxoplasma gondii peptides elicit HLA-A03 restricted, IFN-γ producing, CD8+ T cells in humans and mice. These peptides administered with adjuvants reduce parasite burden in HLA-A*1101 transgenic mice. This work provides a foundation for immunosense based vaccines. It also defines novel adjuvants for newly identified peptides for vaccines to prevent toxoplasmosis in those with HLA-A03 supertype alleles.