The interleukin 7 receptor is required for T cell receptor gamma locus accessibility to the V(D)J recombinase

Defects in the interleukin (IL)-7 signal transduction pathway lead to sever e 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 diffe rent loci, being arrested in the T cell receptor (TCR)-gamma locus, aberran t in the immunoglobulin heavy chain (IgH) locus, and delayed in the TCR)-ga mma locus. Here, we analyze the recombination defect of the TCR-gamma locus . Using ligation-mediated polymerase chain reaction, we sought intermediate s of the recombination process. In the absence of the IL-7 signal, no initi ation of recombination of the TCR-gamma locus was observed, whereas recombi nation intermediates at the TCR-beta locus could be detected. Thus, the fai lure to rearrange the TCR-gamma locus is due to a failure to initiate cleav age 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, her e we show that both TCR-gamma and -beta recombination intermediates are rea dily detectable in normal T1 cells, but only TCR-beta intermediates were de tected in IL-7R(-/-) T1 cells, supporting a mechanistic role for IL-7 in TC R-gamma locus rearrangement. Since reduced recombination activating gene (r ag) expression has been reported in the absence of the IL-7 signal, we dire ctly tested whether the TCR-gamma locus is accessible to cleavage by recomb inant Rag proteins in vitro. We found a reduction in chromatin accessibilit y for Rag-mediated cleavage in IL-7R(-/-) thymocytes compared with wild-typ e. Thus, IL-7 controls recombination at the TCR-gamma locus by regulating l ocus accessibility.