Characterization of the target cell for a thymocyte specific growth factor in guinea pigs.

A growth factor prepared from calf thymus has been described previously (Thymus 3, 289, 1981). The factor was shown to stimulate the DNA synthesis in thymic cells but not in peripheral lymphocytes in guinea pigs. We now present a further analysis of the target cell. Thymus cells were fractionated into subpopulations based on their different adherence to plastic, buoyant density and agglutinability by peanut agglutinin (PNA) and the influence of the growth factor on DNA synthesis was studied. The results showed that the responding cell was enriched in the population of non-adherent PNA-positive (PNA +) cells of low buoyant density. This cell population showed high spontaneous DNA synthesis in vitro, but did not contain the phytohemagglutinin-responsive thymocyte, which resided in the PNA-negative (PNA -) population of low density. The results strongly indicated that the target cell of the thymic growth factor is the immunologically immature proliferating thymocyte of the thymic cortex.