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 cell 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 (c mu+)/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).