Immunophenotypic and immunogenotypic characteristics of TCRγδ⁺ T cell acute lymphoblastic leukemia

Thirty T cell receptor (TCR)γδ⁺ cell acute lymphoblastic leukemias (T-ALL) were analyzed for their immunophenotype, as well as for the rearrangements and junctional regions of the TCRG and TCRD genes. In 15 cases membrane expression of TCRγδ proteins could be studied extensively by flow cytometry with a new Vγ/Vδ antibody panel. Virtually all TCRγδ⁺ T-ALLs expressed TdT, CD2, CD3, CD5, CD6, and CD7, but they were heterogeneous in their CD1/CD4/CD8 immunophenotype. The majority expressed either CD4⁺/CD8^- or CD4⁺/CD8⁺, whereas only 7/30 TCRγδ⁺ T-ALLs lacked both antigens. Despite heterogeneity in the rearranged TCRG and TCRD genes, we found preferential protein expression of VγI (21/30), Jγ2.3 (19/30) and Cγ2 (21/30) gene products in the TCRγδ⁺ T-ALL. Expressed TCRD genes were largely limited to Vδ1-Jδ1, except for six patients who expressed non-Vδ1 TCRδ chains (Vδ2-Jδ1, Vδ2-Jδ3, Vδ3-Jδ1, Vδ6-Jδ2, and two Vα-Jδ1). In spite of the relatively limited combinatorial repertoire of the TCRG and TCRD genes, the junctional region diversity of the expressed genes was extensive. The Vγ/Vδ antibody panel confirmed the predominant, but not exclusive, expression of VγI and Vδ1 proteins. Importantly, not a single T-ALL expressed the common peripheral blood Vγ9⁺/Vδ2⁺ phenotype. These immunogenotypic and immunophenotypic characteristics represent excellent targets for flow cytometric and PCR-based detection of 'minimal residual disease' in all TCRγδ⁺ T-ALL. Comparison of non-Vδ1⁺ TCRγδ T-ALLs with the more common Vδ1⁺ type showed a trend towards a more mature immunogenotype in the former. Firstly, more complete TCRD rearrangements were identified on the non-expressed allele in the non-Vδ1⁺ group (83% vs 43%); secondly, a higher frequency of 'end-stage' Jγ2.3 gene rearrangements was found in non-Vδ1 cases on both TCRG alleles (83% vs 66%); thirdly, a higher frequency of complete TCRB rearrangements was found in non-Vδ1 cases (79% vs 50%).