Partial T-cell receptor gene rearrangement: A source of pseudo-clonal populations in thymomas and other thymic tissues

The differentiation of thymocytes into mature post-thymic T cells requires rearrangement of T-cell antigen receptor genes from germline to a spectrum of spliced configurations encoding functional receptors. In the T-cell receptor beta chain locus, this process occurs via a hierarchical series of recombination events, linking 'diversity' and 'joining' segments, then 'variable' and 'diversity' segments. The authors report Southern blot analysis of the T-cell receptor beta locus in normal human thymic tissue after restriction digestion with Bam HI identifying rearranged DNA fragments in 5 of 6 samples, which probably represent an intermediate deletion joining Dβ 1 to a Jβ 2 segment without rearrangement of the upstream V region. Hybridization intensity was in the range of 5% to 30% of represented DNA. Reduction of signal from bands containing Cβ 1 and Jβ 2 sequences after Eco RI and Hind III digestion was consistent with this model. A probe specific for Jβ 1 did not hybridize with the rearranged fragment while Jβ 2 specific sequences did, indicating a deletion of the region of Jβ 1, but limited to sequences upstream of Jβ 2. Most peripheral lymphoid specimens do not demonstrate similar rearranged DNA fragments, however T-cell-rich populations may do so. Analysis of 31 additional surgically resected thymic tissues and three polyclonal T-cell-rich peripheral lymphoid specimens revealed a similarly rearranged fragment in 13 of 14 thymuses with follicular hyperplasia; 6 of 9 thymomas; 5 of 6 malignant (invasive) thymomas; 0 of 2 thymic carcinomas; and, at very low abundance, in 3 of 3 peripheral T-cell populations. Because this nongermline T-cell receptor gene apparently reflects polyclonal incomplete rearrangement, rather than clonality, awareness of this phenomenon may help prevent erroneous diagnosis of T-cell lymphoma for mediastinal lymphoid lesions with an apparent 'clonal' T-cell receptor gene rearrangement. If the intermediate deletion is derived from an ongoing process, the data suggest that in many cases neoplastic thymic epithelium may retain functions necessary not only to support a resident thymocyte population, but also to activate ongoing T-cell differentiation. Alternatively, the intermediate deletion may derive from abandoned non- productive rearrangements.