Human Thy-1 antigen: Cell surface expression on early T and B lymphocytes

Human Thy-1 is present on the surface of only a small proportion of haemopoietic cells (thymus - 0.2 - 10%; bone marrow - 0.1 - 0.5%). We have analysed these Thy-1⁺ cells in more detail using immunofluorescence on a wide range of human haemopoietic cells lines and fresh leukaemic blasts; both situations where rare cells can become clonally expanded. The data demonstrate that Thy-1 is confined to the early stages of T- and B-lymphocyte development, and is absent from all myeloid cells. The Thy-1⁺ B cells represent late pre-B (cμ⁺)/early sIgM⁺ B cells. The Thy-1⁺ T cells are situated in the outer thymic cortex. Dual immunofluorescence analysis of FACS separated Thy-1⁺ thymocytes shows them to have the antigenic phenotype and morphology of prothymocytes/early thymocytes (some overlap seen with CALLA, TdT, OKT6 and OKT1).