Making use of mice deficient for terminal deoxynucieotidyi transferase (TdT) expression and a random pep tide library, we have examined the diversity and pep tide specificity of the neonatal T ceil repertoire specific for a single H-2Db-restricted peptide. Consistent with the predicted decrease in repertoire diversity, poiycional CTL lines and individual clones from different TdT” mice are more similar to each other than those from different wild-type mice in terms of their fingerprints of cross-reactivity to the library and their TCR sequences. We have also found that several TdT” CTL clones cross-react with many more library peptides than wild-type CTL clones. in a few instances, the degree of peptide promiscuity correlates with TCR sequence characteristics such as N region addition and homology-directed recombination, but not CDR3 loop length. Based on epitope titrations for each clone, TCR affinity for antigen is consistently high; thus, this reduced specificity for peptide may coincide with an accentuated affinity for the a heiices of the MHC. Peptide promiscuity in the neonate may allow the relatively small numbers of T ceils in the periphery to protect against a broader range of pathogens. introduction

Lymphocytes are capable of responding to a great diversity of antigens. The multiple specificities of their antigen receptors is generated, in part, from the combinatorial usage of large families of gene segments. Both 6 and T cells produce heterodimeric receptors in which the antigenbinding domain is encoded by variable (V), diversity (D), and joining (J) segments for one chain and V and J segments for the other. Further diversification derives from imprecision in recombination, driven by nucieases that process the gene segment termini and a template-independent poiymerase, terminal deoxynucieotidyi transferase (TdT), that randomly inserts nucieotides between receptor gene segments prior to ligation (for review see Lieber, 1991). Based on predicted and solved T ceil receptor (TCR) structures, several domains of each chain fold together to form the antigen-binding surface (Bentley et al., 1995). Of these, the compiementarity determining region-3 (CDRS) is encoded by the last few nucleotides of the V segment, the J or DJ segments, and the intervening N regions. Thus, the CDR3 possesses much of the heterogeneity. Unlike immunoglobulin, a TCR will only recognize