Tachyzoite-specific isoform of Toxoplasma gondii lactate dehydrogenase is the target antigen of a murine CD4+ T-cell clone

In two-dimensionally separated Toxoplasma gondii lysate, mouse Th1 clone 3Tx15 detects two proteins of apparent molecular weight 40 000 and pI of 5.8 and 5.9. Microsequencing of peptide fragments from tryptic digestion of one of these proteins yielded partial sequences of T. gondii lactate dehydrogenase (LDH)1. As shown by Western blot, toxoplasmic LDH co-migrates in two-dimensional gel electrophoresis with both T-cell antigenic proteins. With synthetic peptides spanning the complete primary structure of T. gondii LDH1, the T-cell epitope was mapped to a nine amino acid partial sequence which exhibits a motif for binding to I-Ek, the class II restriction element of antigen recognition by clone 3Tx15. From the two known isoforms of T. gondii LDH, clone 3Tx15 specifically recognises tachyzoite LDH1, but not bradyzoite LDH2, as shown with the corresponding epitope peptides and recombinant proteins. Antigen-presenting cells infected with live bradyzoites stimulate 3Tx15 T cells, while killed bradyzoites provide no antigenic stimulus. This finding implies that a transformation into the tachyzoite stage occurs in cells challenged with bradyzoites. Although LDH1 represents one major constituent of the tachyzoite proteome, the protein does not seem to be immunogenic in T. gondii infection of mice. This is evident from the lack of serum anti-LDH immunoreactivity and the failure of adoptively transferred 3Tx15 T cells to protect against lethal challenge. In conclusion, a T-cell-stimulatory Toxoplasma antigen is identified by means of a novel, high-resolution T-cell blot technique, the cloneˈs antigenic fine specificity allowing detection of parasite-stage conversion.