Thymic microenvironment: selective activities of soluble and insoluble components

Thymus hormones do not induce proliferative ability or cytolytic function in PNA+ cortical thymocytes

A variety of thymus hormone preparations, as well as drugs known to perturb cell differentiation, were tested for their ability to induce nonfunctional cortical thymocytes to become functional precursor cells. Murine cortical thymocytes, defined as the high peanut agglutinin (PNA) binding or as the low H-2K, major [86%] thymocyte subpopulation, were isolated by fluorescence-activated cell sorting. Their function was assessed in a high cloning efficiency, growth factor saturated, concanavalin A-stimulated limit-dilution culture system, determining the number of precursors of extended clones (PTL-p), or determining with a lectin-mediated tumor-lysis readout the number of precursors of cytolytic clones (CTL-p). The hormone preparations tested were crude or partially purified culture supernatants from thymus "epithelial" monolayers (TES), soluble extracts of thymic nonlymphoid tissue (STF), semipure thymus humoral factor (THF), and the pure peptides thymopoietin 32-36 (TP5) and "facteur thymique sérique" (FTS). These preparations were either added directly to the limit dilution cultures, or were first preincubated with the cells, which were then subjected to limit-dilution culture. In no case did the hormone preparations cause any increase in the level of PTL-p or CTL-p in the PNA+ or low H-2K thymocyte population, even though a conversion of only a few percent to functional cells could have been detected. Two possible explanations are considered. One is that the main function of these materials is to control post-thymic peripheral T cells, rather than to induce intrathymic differentiation. Another is that the typical cortical thymocyte is beyond the stage at which thymocytes can be induced by hormones, a view that is strengthened by the failure of either 5-azacytidine or the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate to activate these cells. In this latter explanation the true intrathymic target of hormone action may be an earlier, and very minor, thymus subpopulation.

Selective induction of T-cell subsets capable of accelerating and retarding Lewis lung carcinoma

The influence of three T-cell subsets on tumor growth was studied. The first, induced by a thymic microenvironmental fraction, accelerated the appearance of Lewis lung carcinoma in Winn assay. The second subset, derived from the first by incubation with a thymic hormone preparation, had a marked retarding effect on tumor appearance. The third subset, also induced by thymic hormone, but directly from bone marrow precursors, does not appear to influence tumor cells.

Quantitative changes in AKR thymocyte subsets preceding the onset of leukemia

The evolution of T-cell subsets in the thymuses of leukemia-prone AKR mice and of non-leukemia-prone BALB/c mice was compared for a period of 9 months. In AKR mice there is a sharp rise in thymic hormone sensitive thymocytes bearing a high concentration of Thy 1 antigen. While these cells also rise with age in BALB/c mice, they do so more slowly. The response of thymocytes to Con A was also compared between the two strains: the response of AKR cells drops to below significant levels within seven months, while BALB/c cells remain reactive throughout the experimental period. Since these age-related changes in Thy 1 antigen density and Con A reactivity can be reversed after incubation with a thymic hormone preparation, it is concluded that they are the result of a decrease in thymic hormone secretion, that this decrease is much more pronounced in AKR mice than in BALb/c mice and may therefore play a role in the etiology of the AKR leukemia.

Regluating factors in helper T-cell maturation

The effect of soluble and insoluble thymic fractions (STF and ITF, respectively) on the antibody response to sheep red blood cells (SRBC) was tested at different times after injection in mice. In normal mice, a temporary increase in the number of plaque forming cells (PFCs) was observed after injecting either ITF and STF, this increase being more prolonged when both fractions were injected together. In thymectomized, lethally irradiated mice repopulated with syngeneic untreated bone marrow, full reconstitution of T helper function was achieved only if both ITF and SFT were injected, partial reconstitution being obtained when either of these fractions was administered alone. In thymectomized, lethally irradiated mice repopulated with T-depleted bone marrow, reconstitution of the T helper function was only achieved when both ITF and SFT were administered. These results show that ITF and STF together constitute all the essential elements of the thymic microenvironment necessary to support the entire differentiation process of prethymic precurosors into helper T cells.