Terminal deoxynucleotidyl transferase expression during neonatal life alters D-H reading frame usage and Ig-receptor-dependent selection of V regions

During neonatal life, Ig diversity is limited in many respects. The absence of terminal deoxynucleotidyl transferase (TdT) expression with the consequ ent lack of nontemplated addition during the neonatal period, coupled with the predominant usage of a single D-H reading frame (RF), leads to severe l imitations of diversity in the CDR3 region of Ig heavy (H) chains. The neon atal Ig H chain repertoire is also characterized by restricted V-H usage, w ith predominant expression of certain V-H segments, such as V(H)81x, that a re rarely evident during adult life. In this report, we examine the effect of enforced TdT expression on the neonatal repertoire of V(H)81xDJ(H) rearr angements. We find that TdT synthesis abrogates D-H RF bias during the feta l/neonatal period through a Ig-receptor-independent mechanism. These findin gs suggest that D-H RF bias during neonatal life is determined largely by h omology-directed joining. We also find that TdT synthesis alters the select ion of productively rearranged V(H)81xDJ(H) alleles in the neonatal spleen through a Ig-receptor-dependent mechanism. Analysis of predicted CDR3 amino acid sequences indicates that positive selection of V(H)81x-encoded H chai ns is correlated with the presence of a consensus sequence immediately adja cent to the V-H segment. These data support the hypothesis that the CDR3 re gion is critical in determining the ability of V(H)81x-encoded H chains to form functional receptors that support positive selection of B lymphocytes. Together, our results demonstrate that TdT can indirectly influence the Ig repertoire by influencing both receptor-dependent and receptor-independent selection processes.