Recruitment of Thymocytes from G1 into S Phase by a Thymic Factor

Abstract

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 °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.

Isolation of a Thymocyte Growth Peptide from Human Thymus

“…A dose-dependent TGP activity was de tected in an aqueous extract of intact human thymus. Higher activity was recovered from extracts of sus pended thymocytes than from intact thymus, and weaker activity from thymic stroma depleted of thy mocytes, supporting previous results with murine TGP which suggested an autocrine or paracrine growth stimulation of the thymocytes [7], Although hampered by lack of human material, our functional studies aimed to delineate the target cell of human TGP have shown results consistent with those ob tained with bovine TGP [5,15].…”

“…Previously, a similar activity has been puri fied from bovine thymus and shown to reside in a low-Mr peptide [6]. This bovine TGP acts as a progres sion factor for the main proliferating population in the thymic cortex consisting of immature thymocytes [17,18], regulating their entry from the Gi into the S phase [5], Bovine TGP is a 1.6-kD N-terminal blocked hydrophilic peptide with an isoelectric point of 5.7 [4,6]. The amino acid composition has shown a high content of Glx and Asx residues, but their degree of amidation remains to be determined.…”

“…The TGP activity can be expressed as Acpm, i.e. cpm exceeding control cpm, which in rodent systems has been shown to reflect the number of G| cells recruited into the S phase [5].…”

“…One feature of the proliferating thymo cytes is a remarkably rapid cell cycle of 7-10 h, with very short duration of the G| phase [1][2][3]. In previous papers we have proposed that the thymic prolifera tion is regulated by local growth factors, and we have reported on the purification of a bovine thymic pep tide, distinct from previously defined interleukins, peptide growth factors and thymic maturation hor mones, with ability to induce DNA synthesis and mi toses specifically in immature thymocytes [4][5][6], Our results indicated that this thymocyte growth peptide (TGP) acts as a progression factor shortening the G| phase by regulating the S phase entry of cycling pre cursor lymphocytes in the thymus [5]. Recently, we isolated a corresponding activity from tissue culture medium conditioned by a murine thymic leukemia cell line [7], Similar activity has also been detected in extracts of guinea pig [8] and mouse thymus [7].…”

Effect of Growth Factors on the Proliferation of Thymocytes in Different Functional and Anatomical Compartments

“…1] and on the basis of studies of DNA synthesis, mitogen stimulation, rosetting, PNA binding, cell volume and buoyant density, the target cell for this factor was shown to be an immunologically immature cell [2,3]. In vitro, TGP allowed the responsive cells to pass a restriction point in G] and to enter S phase and mitosis [4], In vivo, these cells constitute a major fraction of cycling thymo cytes. Another large population of proliferating cells (in guinea pigs distinct from TGP-responsive cells mainly by their inability to form E rosettes) does not respond to TGP, and no growth factor for these cells is known [5,6].…”