Effect of macrophages on the go-gl and gl-S transition of thymocytes.

Thymocytes freed of adherent cells and stimulated by mitogens yield only a poor proliferative response ([3H]-thymidine uptake). However, the activation process assessed fluorocytometrically (RNA-synthesis) is not reduced to the same extent. Addition of adherent cells caused full restoration of both activation and proliferation, whereas 2-mercaptoethanol (2-ME) restores activation completely, but proliferation only partially. Cell viability is not influenced by the absence or presence of adherent cells or of 2-ME during the first 24 hr of incubation, which is the time period used to assess cell activation. Macrophages on the other hand, can be replaced completely by the addition of 2-ME and Interleukin (IL-1), or IL-2 containing supernatants. The addition of these substances even appears to be superior, suggesting that macrophages can as well suppress as enhance the thymocyte response to mitogens. The results presented further support the current concept of the multifunctional role of macrophages: depending on the selected mitogen, it appears that only the IL-1 production is an absolute, but indirect requirement for mitogen-stimulated thymocytes, in order to proceed through the cell cycle.