Thy-1-expressing CD34+ human cells express multiple hematopoietic potentialities in vitro and in SCID-hu mice

We have detected a minor subset of fetal liver and bone marrow CD34+ cells that coexpress the Thy-1 antigen at their surface. In immune-deficient SCID mice engrafted with human fetal hematopoietic tissues, CD34+ Thy-1+ reconstitute the full range of thymic lymphocytes and yield a progeny of myeloid and B cells in the human bone marrow. The frequency of cells initiating long-term human myeloid and B lymphocytes in vitro cultures at the contact of the Sys-1 stromal cell line was also found to be 10-15 times higher among CD34+ Thy-1+ cells than in the Thy-1- counterpart population. We propose that CD34+ Thy-1+ cells include candidate human hematopoietic stem cells.