An NK1.1+CD4+8-Single-Positive Thymocyte Subpopulation That Expresses a Highly Skewed T-Cell Antigen Receptor VβFamily

In the present report we describe a CD4+8-heat stable antigen-negative (HSA-) thymocyte subpopulation that expresses a distinguishably low density of αβ T-cell antigen receptors (TCRlo) from the majority of CD4+8-high-density TCR (TCRhi) mature-type thymocytes. This subpopulation appears relatively late in life. Analysis of MEL-14, Pgp-1 (CD44), ICAM-1 (CD54), and NK1.1 expression on this subpopulation revealed that the CD4+8-TCRlopopulation was a population having unique characteristics (MEL-14-, CD44+, ICAM-1+, and NK1.1+) compared to the CD4+8-TCRhlthymocytes, most of which are MEL-14+, CD44-, ICAM-1-, and NK1.1-. When TCR β-chain variable region (Vβ) usage was analyzed, this thymic population expressed predominantly products of Vβ7 and Vβ8.2 TCR gene families. Interestingly, cells with Vβ8.1 TCRs, which are reactive to Mls-1aantigens, were not eliminated from the CD4+8-HSA-TCRlosubpopulation but had been eliminated from the major CD4+8-HSA-TCRhisubpopulation in Mls-1astrains. A subset with a phenotype similar to the CD4+8-HSA-TCRlothymocytes was also identified primarily in bone marrow, and this subset constituted approximately half of the CD4+T cells in the bone marrow. The CD4+8-HSA-TCRlocells showed extremely high proliferative responses to immobilized anti-TCR antibody but generated negligible responses to allogeneic H-2 antigens compared to the responses generated by the major CD4+8-HSA-CD3hicells. However, the CD4+8-HSA-TCRlocells in Mls-1bmice mounted vigorous proliferative responses to Mls-1aantigens but not in Mls-1amice. The properties of this T-cell subset suggest that these cells belong to a lineage distinct from the major T-cell population.