ATTACHMENT OF TOXOPLASMA-GONDII TO HOST-CELLS INVOLVES MAJOR SURFACE PROTEIN, SAG-1 (P-30)

Previous observations have demonstrated that monoclonal and polyclonal antibodies directed at SAG-1, the major surface protein of Toxoplasma gondii, decreased the number of T. gondii that infected fibroblast mo nolayers. Direct evaluation of parasite-host cell attachment using glu taraldehyde-fixed human fibroblasts and live tachyzoites was performed to determine whether SAG-1 was a ligand for the host cell receptor. T he interaction between the fixed cells and T. gondii was specific and saturable as determined by a radioisotope competitive binding assay. M oreover, the specificity of this interaction was confirmed by comparis on to another member of the Apicomplexa, Besnoitia jellisoni. Treatmen t of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 s erum inhibited parasite attachment to host cells by 71%. A monoclonal antibody (6A8) directed at SAG-1 was able to inhibit parasite binding to fixed host cells by 65%. Other mAb's directed at SAG-1 failed to in hibit parasite attachment in this assay. Fab derived from 6A8 mAb show ed dose-dependent inhibition of parasite attachment. At an Fab concent ration of 25 mu g/ml, 47% inhibition was observed. Attachment assays u sing mutant parasites with defective SAG-1 (PTgA and PTgC) showed sign ificantly reduced binding (26 and 39%) when compared to wild-type (SAG -1+) parentals. Taken together, these observations suggest that SAG-1 is an important parasite ligand that binds to the host cell in the pro cess of T. gondii invasion. (C) 1994 Academic Press, Inc.