A NATURAL MUTATION OF THE AMINO-ACID RESIDUE AT POSITION-60 DESTROYS STAPHYLOCOCCAL-ENTEROTOXIN A MURINE T-CELL MITOGENICITY

A variety of techniques have been used to identify the amino acid resi dues of bacterial superantigens involved in their interactions with ma jor histocompatibility complex (MHC) class II and T-cell receptor (TCR ). In this study, we isolated a naturally mutated staphylococcal enter otoxin A (SEA) from three different Staphylococcus aureus strains, in which the amino acid at position 60 has been changed from aspartic aci d (D) to asparagine (N). We then studied the influence of this change on the immunological activities of SEA. Our results demonstrated that this mutation does not affect the capacity of SEA to bind MHC class II molecules and consequently activates human monocytes and peripheral b lood lymphocytes. In contrast, mutated SEA failed to stimulate the pro liferation of murine splenic lymphocytes of two different strains, and when presented by human MHC class II molecules, it also failed to act ivate murine cell line 3DT, which expresses tbe SEA-specific TCR V bet a element (V beta 1). These results indicate that this mutation alters the interaction between SEA and murine TCR. The reactivity patterns o f the mutated SEA with two specific anti-SEA monoclonal antibodies sug gested that the observed effect of the isolated mutation in the murine system might be due to certain conformational changes in the SEA mole cule introduced upon changing the D at position 60 to N. Site-directed mutagenesis of the N residue to D or to glycine reconstituted the abi lity of SEA to stimulate murine splenic lymphocytes. The different eff ects of this natural mutation at position 60 on the immunological acti vities of SEA with murine and human cells highlight the relevance of t he affinity and avidity in SEA-TCR interactions in the function of dif ferent species or may reflect a difference in epitope specificity.