Thymic lymphoid-stromal cell complexes in mice: in vitro assay and mechanism of the complex formation

A quantitative assay was established to analyze in vitro thymic lymphoid-stromal cell complex formation. Major parameters of this assay were the number of thymic lymphocytes, incubation time, the age of the thymocyte donor, and the source and amount of serum used. The majority of complex-forming lymphocytes from normal young adult mice were found to have a blastlike morphology, indicating their possible origin from the subcapsular zone of the thymus. Changes of complex-forming cells during thymus development seemed to support this concept. The complex formation occurred in two steps: adherence of the lymphocytes to stromal cells and subsequent crawling of the lymphocytes under stromal cell cytoplasm. The first step was competitively inhibited by a serum activity and the second was noncompetitively inhibited by chemicals affecting cytoskeleton. In this assay, the behavior of normal complex-forming thymocytes was shown to be similar to that previously demonstrated for leukemia thymocytes with respect to morphology of the complex as well as the effects of certain inhibitors. This assay should provide the means both to analyze the nature of this cell interaction and to explore the relationship between thymocyte differentiation and a step in thymic leukemogenesis.