T‐lymphocyte Maturation: Cell Surface Markers and Immune Function Induced by T‐lymphocyte Cell‐free Products and Thymosin Polypeptides

It has been at least a decade since the first reports that cell-free diffusible factors from the thymus could partially reverse the deficiencies caused by neonatal thymectomy in a variety of species including mice, rats, hamsters, rabbits, and Humoral factors have also been found to be involved in the many pathways that lead from the pluripotent, hemopoietic stem cell ( H E ) to mature elements of the blood.A classic example of the latter is the discovery that the hormone, erythropoietin, interacts with erythroid precursors and leads to the differentiation of mature red blood cells in vitro.6.7 Differentiation of the HSC along the thymusderived (T-cell) lymphoid pathway has been a subject of intensive investigation.The underlying principle in these investigations is the concept that there are at least three distinct steps in the life of a T-cell:1.A precursor stage that is devoid of phenotypic characteristics of a mature T-cell and lacking immunological function ascribed to mature T-cells.2. A differentiated stage that possesses phenotypic (surface markers) characteristic of a mature T-cell.3. Finally, an immunologically mature stage consisting of cells that not only bear phenotypes characteristic of T-cells, but also perform immunological functions.Obviously, this division of stages is arbitrary, and it is likely that certain immunological functions precede the acquisition of certain cell-surface markers.However, it serves as the outline upon which most research has been based.Thus, the phenotypic conversion and acquisition of cell-surface markers and