The differentiation function of T cell‐replacing factor in nu/nu spleen cell cultures

The effect of T cell‐replacing factor (TRF) on B cells was studied in spleen cell cultures from athymic nu/nu mice with particular emphasis on its possible role in proliferation and/or differentiation of precursor cells. In contrast to B cell mitogens, such as lipopolysaccharide, TRF does not significantly increase thymidine incorporation nor does it lead to the development of an increased number of plaque‐forming cells in the absence of test antigen. It was found that under conditions of maximal induction of plaque‐forming cells by TRF, i.e. TRF administration 1–2 days following commencement of tissue culture and antigenic stimulation, there was a proportional increase in levels of IgM production relative to total protein synthesis by the cells. In such cultures, Ig formation represented up to 37 % of total protein synthesis. Although the background level of Ig production varied in different cultures, TRF reproducibly led to a 1.5 to 4‐fold increase in proportional Ig synthesis. The results suggest that some proliferation takes place before TRF is added and indeed before it can act. This is supported by the finding that conditions of reduced cell proliferation (lowered temperature) before the addition of TRF prevent subsequent plaque responses. In contrast, the same treatment applied later on and simultaneously with TRF addition still permits a significant increase in the number of PFC. X‐irradiation of the culture on day 2, immediately prior to TRF addition, still permits an increase in proportional Ig synthesis. Thus, it appears that a major component of the function of TRF, i.e. the nonantigen‐specific soluble T cell factor, is a differentiation signal favoring Ig production. Mitosis is required before the differentiation signal can be received, and some controlled concomitant proliferation is not excluded.