Transcription of the TCR-β locus initiates in adult murine bone marrow

Successful expression of the TCR β-chain gene is a multistep process that involves: 1) initial transcription of multiple, unrearranged gene segments, 2) rearrangement of V, D, and J gene segments to form a complete β-chain gene, and 3) transcription of the fully rearranged β gene. All of these events have been shown to occur in the thymus, where the majority of T cell development takes place; however, the extent to which any of these events may occur prethymically has not been established. To examine prethymic TCR-β gene expression, RNA was isolated from a precursor T cell-enriched population (Thy 1(low) CD3^-) of C58/J mouse bone marrow, and analyzed by reverse transcriptase-PCR. A transcript containing TCR-β constant (C) region sequences but not variable (V) region sequences was amplified, suggesting that an unrearranged TCR-β gene locus is transcriptionally active in this bone marrow population. The same product was detected in Thy 1⁺ CD3^- bone marrow cells from nude mice, indicating that the thymic microenvironment is not necessary for initiation of TCR-β gene transcription. This Cβ transcript is not confined to pre-B cells, as it was identified in RNA isolated from Thy 1(low) CD3^- B220^- bone marrow cells. Germline Vβ transcripts were also detected in RNA from this bone marrow population. Furthermore, Sca-1⁺ Lin^- and Sca-1⁺ Lin⁺ bone marrow populations from both C58/J mice and nude mice also expressed the Cβ transcript. DNA-PCR analyses with Dββ-Jβ primer sets revealed that partial rearrangement of the β locus had occurred in all bone marrow populations analyzed. These data suggest that both transcription and partial rearrangement of the TCR-β locus can initiate in bone marrow cells of adult mice, before exposure of these cells to the thymus.