Recruitment of thymocytes from G1 into S phase by a thymic factor.

A thymic factor previously shown to stimulate the DNA synthesis of immature thymocytes was found to recruit G1 cells synchronously into S phase within 1.5-2 h. The S + G2 + M duration of the cultured thymocytes was not affected. In the absence of the thymic factor, the responding thymocytes seemed to be blocked in G1 at a putative restriction point 1.5 h prior to S phase. When the addition of the thymic factor to cultured cells was delayed, the responsiveness rapidly declined, indicating that target cells were transferred into an unresponsive state. A stimulating effect of the DNA synthesis of cultured thymocytes was obtained also after a transient exposure to the thymic factor for 2-3 h. The stimulating activity was completely absorbed from the medium by incubation with thymocytes at both 4 and 37 degrees C, but not by lymph node lymphocytes which are unresponsive to the thymic factor. Our interpretation of the results is that the thymic factor acts on thymocytes in G1, triggering the responsive cells via membrane receptors to proceed from G1 into S phase. The thymic factor is proposed to be a progression factor for the rapidly cycling immature thymocytes and to be involved in the regulation of the intense thymic growth in vivo.