Origin of T lymphocyte colony-forming cells in cell populations depleted of sheep erythrocyte rosette forming cells.

Cell populations depleted of sheep erythrocytes (E) rosette-forming T cells (E-cells) contain cells capable of giving rise to T cell colonies. We have characterized the T cell colony-forming cell from human bone marrow, blood and tonsil E- cells using a T-cell colony assay. Depletion of CD2+, CD3+ or CD4+ cells from E- cells reduced colony formation by 70-100%. Removal of CD8+ cells did not reduce, but rather enhanced colony formation by 50% or more. The most effective reduction (100%) in colony formation was obtained with anti-CD4, indicating that CD4 is a marker of all colony-forming T cells. The CD4+ lymphocytes generated two types of colonies, types I and II, in the presence or absence, respectively, of CD8+ lymphocytes. Type I were small and compact, reached a peak on days 5-7, and contained CD4+ and CD8+ cells. Type II were large and diffuse, reached a peak on days 9-10 and contained CD4+ cells. In continuous culture of single type II colony cells, we observed a consistent increase of CD8+ cells. In one colony the combined percentage of CD4+ and CD8+ cells exceeded 100% (averaging 83% CD4+ and 72% CD8+), indicating the presence of dual markers on some cells. We suggest that colony forming T cells are CD2+ CD3+ CD4+, the CD4+ antigen being the most consistent marker of such precursor cells.