Immunobiological analysis of TCR transgenic mice revealed structural features of TCR CDR3 loops in antigen recognition
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The interactions between the T cell receptor (TCR) and an antigenic peptide presented by the major histocompatibility complex (MHC) molecules primarily determine the specificity of the T cell mediated immune response. To study the important features of the interaction of the CDR3 loops of the TCR with the VSV peptide (RGYVYQGL) presented by the class I MHC molecule H-2K b, we generated TCRalpha and TCRbeta chain transgenic mice using a VSV-specific TCR. We immunized these transgenic mice with the VSV peptide or its variants substituted at TCR contacting positions 1, 4 or 6 and analyzed the CDR3 loop sequence of TCRs from T cells elicited by the different peptides. Previous results from the TCRalpha chain transgenic mice suggested that position 98 of the CDR3beta loop is the key residue responsible for the specific interaction with position 6 of peptide. In this study, we identified a highly conserved residue at position 93 of the CDR3alpha loop and a predominant Jalpha usage in TCRs from peptide-specific CTLs elicited from Tgbeta mice. Furthermore we found that substitution of the VSV peptide at position 4, but not position 1, to some residues can induce the compensatory changes at position 93 or/and in the Jalpha motif sequence. These data suggest that position 93 and the Jalpha region of CDR3alpha regions are important for recognizing the VSV peptide. Interestingly we also found that in peptide variant-induced CTLs that can crossreact with VSV peptide, the same Jalpha15 stretch in the TCRs is used; otherwise a different Jalpha is used. This correlation of the CTL crossreactivity with Jalpha usage demonstrates the key roles of the Jalpha region in peptide-specific recognition. In addition we found that a substitution at position 6 to a negatively charged residue (Asp or Glu) induced a reciprocal change at position 93 of CDR3alpha to a positively charged residue (Lys or Arg). These data suggest there is a salt bridge between the C terminal residue of the peptide and the CDR3alpha loop in Tgbeta mice. Such data help us to refine our understanding of the general rules of TCR specific recognition of peptide-MHC complexes in vivo.