Defects in the interleukin (IL)-7 signal transduction pathway lead to severe immunodeficiency in humans and in mice. In IL-7 receptor–deficient (IL-7R−/−) mice, lymphoid precursors show a reduced survival rate and variable/diversity/joining region V(D)J recombination is variously affected in different loci, being arrested in the T cell receptor (TCR)-γ locus, aberrant in the immunoglobulin heavy chain (IgH) locus, and delayed in the TCR-β locus. Here, we analyze the recombination defect of the TCR-γ locus. Using ligation-mediated polymerase chain reaction, we sought intermediates of the recombination process. In the absence of the IL-7 signal, no initiation of recombination of the TCR-γ locus was observed, whereas recombination intermediates at the TCR-β locus could be detected. Thus, the failure to rearrange the TCR-γ locus is due to a failure to initiate cleavage rather than a failure to religate broken DNA ends. V(D)J recombination was previously thought to begin at the pro-T2 stage of T cell development after the arrest of IL-7R−/− thymocytes at the pro-T1 stage. However, here we show that both TCR-γ and -β recombination intermediates are readily detectable in normal T1 cells,
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