Thymocyte growth factor: A progression growth factor for cycling immature cortical thymocytes

A growth factor for cultured guinea pig thymocytes was isolated from an extract of calf thymus. This thymocyte growth factor (TGF) was purified to apparent homogeneity employing reverse phase high performance liquid chromatography as the final step. TGF was characterized as an N-terminal blocked hydrophilic peptide with an apparent M_r of 1600. The amino acid composition revealed a high content of acidic amino acids but no apparent relationship with previously defined growth factors and thymic differentiation hormones. TGF was active in nanomolar concentration and stimulated the DNA synthesis and mitotic activity specifically in a subpopulation of thymocytes with immature cortical phenotype. The responsive thymocytes were recruited from G₁ into S phase within 2 h after addition of TGF. In the absence of the growth factor, the target cells were blocked at a putative restriction point in G₁, prior to the onset of the S phase. It is proposed that TGF is a progression growth factor participating in the regulation of the intense proliferation of immature thymocytes in the thymus cortex in vivo.