A T cell-mediated immune response is mainly determined by the 3-5 aa residues that protrude upwards from a peptide bound to an MHC molecule. Alterations of these peptide residues can diminish, eliminate or radically alter the signal that the T cell receives through its T cell receptor (TCR). We have used peptide immunizations of normal mice and mice carrying alpha or beta chain TCR transgenes to identify three distinct peptide contact points. One, near the carboxyl terminus of the peptide, involves the beta chain CDR3 region; the second was centrally located and interacted with both the alpha and beta chain CDR3 loops; the third was near the amino terminus of the peptide, and affected V alpha gene usage, but not the structure of CDR3 of either TCR chain. Based on these results, we propose an orientation for the TCR of this cloned line and argue for its generality.

Surntnary CD4 T cell receptors (TCRs) recognize antigenic peptides presented by self major histocompatibility complex (MHC) class II molecules as well as non-self MHC class II molecules. The TCR.s can also recognize endogenous retroviral gene products and bacterial toxins known collectively as superantigens (SAGs) that act mainly on the V{3 gene segment-encoded portion of the V{3 domain; most SAGs also require MHC class II for presentation. We have studied the interaction of the TCR from a well-characterized CD4 T cell line with SAGs by mutational analysis of its VI3 domain. This appears to separate viral (v)SAG from bacterial (b)SAG recognition. T cells having a TCR. with a glycine to valine mutation in amino acid residue 51 (G51V) in complementarity determining region 2 of the TCIL V[3 domain fail to respond the bSAGs staphylococcal entertoxin B (SEB), SEC1, SEC2, and SEC3, whereas they retain the ability to respond to non-self MHC class II molecules and to foreign peptides presented by self MHC class II molecules. It is interesting to note that T cells expressing mutations of both G51V and G53D of V[3 regain the response to SEB and partially that to SEC1, but do not respond to SEC2 and SEC3, suggesting that different bacterial SAGs are viewed differently by the same TCR. These results are surprising, because it has been generally believed that SAG recognition by T cells is mediated exclusively by hypervariable region 4 on the exposed, lateral face of the TCR V[3 domain. Response to the vSAG Mtv-7 was generated by mutation in V[3 residue 24 (N24H), confirming previously published data. These data show that the vSAG Mtv-7 and bSAGs are recognized by different regions of the TCR V[3 domain. In addition, various bSAGs are recognized differently by the same TC1L. Thus, these mutational data, combined with the crystal structure of the TCR ~3 chain, provide evidence for distinct recognition sites for vSAG and bSAG. 1The TC1L can recognize superantigens (SAGs) derived .IL from bacterial or endogenous retroviral genes (bSAG or vSAG). Unlike conventional antigen recognition, where the CDR3 regions of the TCR ~ and [3 chains play a critical role, response to these substances involves primarily the V[3 gene segment-encoded portion of the V[3 domain of the TCR. As 2-10% ofT cells express the product of a given V[3 gene segment, SAGs stimulate T cells of many different specificities. Recognition ofbSAGs and vSAGs by the TCR is believed to involve regions distinct from the antigen recognition site, comprising the CD1L 1, 2, and 3 loops of both chains of the TCR, as predicted earlier (1). Many reports have shown that a region known as hypervariable region 4 (HVR4) between the E and F strands of the TCR. V[3 domain is a crucial site for SAG recognition, but is not 1Abbreviations used in this paper: bSAG, bacterial superantigen; CA, conalbumin; HVR4, hypervariable region 4; SAG, superantigens; SEB, Staphyloccal enterotoxin B; TSST-1, toxic shock syndrome toxin 1. involved in responses to foreign peptide-.self MHC (2-4). Althou...