Immature Thymocytes Employ Distinct Signaling Pathways for Allelic Exclusion versus Differentiation and Expansion

Abstract

T cell receptor (TCR) beta chain allelic exclusion occurs at the thymocyte CD4- 8- (double-negative, or DN) to CD4+ 8+ (double-positive, or DP) transition, concurrently with differentiation and cellular expansion, and is imposed by a negative feedback loop in which a product of the first rearranged TCRbeta allele arrests further recombination in the TCRbeta locus. All of the major events associated with the development of DP cells can be induced by the introduction of TCRbeta or activated Lck transgenes. Here, we present evidence that the signaling pathways that promote thymocyte differentiation and expansion of RAG-deficient DN cells but not those that suppress rearrangements of endogenous TCRbeta genes in normal DN cells are engaged by activated Ras. We propose that TCRbeta allelic exclusion is mediated by effector pathways downstream of Lck but independent of Ras.

Transgenic Expression of RasGRP1 Induces the Maturation of Double-Negative Thymocytes and Enhances the Production of CD8 Single-Positive Thymocytes

“…Expression of a constitutively active V12 form of H-Ras enables the development of DP thymocytes from DN thymocytes lacking the TCR␤ component of pre-TCR (10,11). Activated H-Ras does not provide equivalence to all signals derived from pre-TCR or TCR, as the RAG-deficient DP thymocytes expressing V12 H-Ras failed to undergo allelic exclusion at the TCR␤ locus and failed to develop further into SP thymocytes (10,11). In DP thymocytes the expression of N17 H-Ras, a dominant negative mutant that sequesters Ras-specific exchange factors (24), impedes positive selection while having no effect on negative selection (25).…”

Metabolites from apoptotic thymocytes inhibit thymopoiesis in adenosine deaminase–deficient fetal thymic organ cultures

“…When productive TCR β-chain rearrangement takes place and a functional pre-TCR is expressed, the cells transit to the DNIV stage (14), a process known as β selection. Approximately four of nine early thymocytes die because they fail to make a productively rearranged TCR-β chain (15). The consequences of β selection include rapid proliferation, phenotypic maturation, allelic exclusion at the TCR Vβ locus, and germline transcription of the TCR-α locus.…”

“…We believe that late DN thymocytes are the primary targets because this is the first stage of differentiation when substantial numbers of cells die and provide a source of toxic metabolites. Indeed, it is estimated that four of nine DN thymocytes die because they do not receive a survival signal through the pre-TCR as a consequence of failing to productively rearrange their TCR Vβ genes (15). Based on the early work of Chan (4) and Smith and Henderson (5), macrophages probably ingest apoptotic nuclei and secrete purines derived from degraded DNA.…”

TCRβ Allelic Exclusion in Dynamical Models of V(D)J Recombination Based on Allele Independence

“…cell expansion and differentiation (46,47), feedback inhibition may well depend on a series of non-mutually exclusive processes impinging on the activity of the recombination apparatus and on the epigenetic status of the Vb versus DJb chromosomal templates (38,(48)(49)(50). Additional molecular mechanism(s), most of which remain incompletely elucidated, likely also act beyond chromosomal accessibility to impair recombination between sites eluding epigenetic silencing at the TCRb locus (51)(52)(53).…”