Effect of thymocyte-stimulating factor-containing supernatants on the surface antigens of murine thymocytes

Immunological aspects of aging: early changes in thymic activity

Age related changes in immune function are preceded by changes in thymic function. One index of thymic function is the ability of the thymus to induce the differentiation of bone marrow pre-T cells into active T cells. Movement of precursors of PHA-responsive T cells into the thymus ceases prior to adolescence in long-lived, autoimmune disease resistant mice as determined by studies utilizing chromosome markers in parabiotic mice. The cause for this age-dependent, cell traffic phenomenon is not known, but it would appear that an extrathymic regulatory mechanism may be operating, since these precursor T cells have the capacity to migrate into the thymus, and even a grafted, involuting thymus is capable of accepting precursor T cells, under extraordinary conditions. Results indicate that shortly after adolescence, immature T cell precursors of PHA-responsive cells in the thymus, and perhaps the periphery, are the sole source of PHA-responsive T cells. T cell migration and maturation is discussed in relation to thymic hormones and the thymic-neuroendocrine regulatory axis.

Separation of spleen colony-forming units and prothymocytes by use of a monoclonal antibody detecting an H-2K determinant

The density of H-2K antigens was determined on both the mouse hemopoietic stem cell, using an assay for spleen colony-forming units (CFU-S), and the prothymocyte, using a thymus repopulation assay. This was done by light-activated cell sorting of bone marrow cells labeled first with a biotinylated antibody against H-2Kk and then with avidin-fluorescein isothiocyanate. Almost all CFU-S were found to be present among the 4% bone marrow cells with high forward light scatter (FLS), low perpendicular light scatter (PLS), and bright immunofluorescence. Thymus regeneration by this brightly fluorescent fraction was delayed 3 days compared to thymus regeneration by unsorted cells, although the same number of CFU-S was present in each cell suspension. This delay indicates that differentiation from CFU-S to prothymocytes takes 3 days. The fraction of cells in the FLS/PLS window with dull anti-H-2Kk fluorescence contained few CFU-S and gave rise to a transient thymus regeneration. These findings indicate that the prothymocyte carries fewer H-2K antigens than does the CFU-S. The H-2K antigen is a marker with which CFU-S and prothymocytes can be separated. Therefore, during early T-cell differentiation, the number of H-2K molecules on the cell surface decreases (CFU-S----prothymocyte----cortical thymocyte). During maturation of T cells, a reexpression of H-2K molecules occurs, since lymph node cells and spleen cells were shown to be brightly positive for H-2K antigen.

Thymic lymphocytes. II. Phenotypic modifications of thymocytes after concanavalin A stimulation in the presence of interleukin 2: early modifications of Lyt 1+2+ subset and later proliferation of cells with more mature phenotypes

Cortical thymocytes are devoid of any immune function, as tested by presently available techniques. The ability of this subpopulation to respond to mitogens or antigens in the presence of interleukin 2 (IL-2) produced by activated mature T lymphocytes has been claimed but is still questioned. In an attempt to study the participation of the different thymocyte subsets and especially that of the cortical type, phenotypic modifications were examined during concanavalin A activation in the presence of IL-2. An immunofluorescent double labeling technique with anti-Lyt 1 and anti-Lyt 2 antibodies was used which led to the determination of four different phenotypes: Lyt 1+2+, Lyt 1+2-, Lyt 1-2+, and Lyt 1-2-. Careful analysis of cell viability in culture and expression of the results in absolute numbers of living cells per culture allowed us to follow modifications of small cellular subsets. Cultures of total thymocytes and PNA-agglutinated (enriched in Lyt 1+2+ cells) and non-PNA-agglutinated cells (enriched in Lyt 1+2-, Lyt 1-2+, and Lyt 1-2- cells) were studied. It was shown that thymocyte activation began by early phenotypic modifications which took place within the first 2 hr of culture but only when Con A plus IL-2 were used. These modifications imply the reduction of the Lyt 1+2+ pool and a compensatory enhancement of Lyt 1-2+ and Lyt 1-2- cells, without modification of the total cell number or [3H]thymidine incorporation. These early phenotypic changes are interpreted as the modulation of antigens on the surface of Lyt 1+2+ cells. The second phase of thymocyte activation implies cell death (essentially Lyt 1+2+ cells) and cell proliferation. The cells which specifically proliferate in the presence of Con A and IL-2 are Lyt 1+2- and Lyt 1-2+, the latter always being present in greater number. Cell survival and absolute number of Lyt 1+2- and Lyt 1-2+ cells in the activated PNA- -enriched population are always higher than in total thymocyte and PNA+ cells cultures. Thus, if Lyt 1+2+ cortical thymocytes do not proliferate by themselves, they seem to intervene by providing Lyt 1-2+ cells which proliferate secondarily.

Functional and phenotypic properties of subpopulations of murine thymocytes. I. The bulk of peanut agglutinin-positive Lyt-1,2,3 thymocytes lacks precursors of cytotoxic T lymphocytes responsive to interleukin 2 (T cell growth factor)

Thymocytes were separate according to their surface phenotype and tested for cytotoxic T lymphocyte (CTL) precursor function by stimulation with allogeneic or hapten-coupled cells, with or without addition of T cell growth factor (interleukin 2). The data show that only a minor subpopulation of thymocytes agglutinated by peanut agglutinin, expressing relatively high amounts of H-2K antigen, contained CTL precursors. The remaining population, approximately 80%, could not be induced to generate CTL, even in the presence of interleukin 2.

A specific biosynthetic marker for immature thymic lymphoblasts. Active synthesis of thymus-leukemia antigen restricted to proliferating cells

Large cortical thymocytes from C57BL/6-Tla(a) mice have been prepared rapidly and in high yield by a combination of centrifugal elutriation and differential binding to peanut agglutinin (PNA)-coated plates. The cells in these lymphoblast-rich fractions were clearly distinct from the majority of thymocytes, with up to 70 percent in the S or G(2) + M phases of the cell cycle and an average rate of [(35)S]methionine incorporation per cell up to 20 times higher than that of the majority population. The populations of cells resolved in this fractionation were characterized by monitoring their rates of synthesis of specific glycoproteins, thymus- leukemia antigen (TL) and the Lyt-2, Lyt-3 complex (Lyt-2/3), relative to their total protein synthesis. Cells that bound to PNA synthesized high levels of Lyt-2/3, consistent with their identification as cortical thymocytes. Those that failed to bind made little or no Lyt-2/3, as expected for medullary cells, The fraction of dividing lymphoblasts that bound to PNA was enriched in cortical thymocyte precursors, including all the large cells detectably active in synthesizing Lyt-2. It differed sharply from the small cortical cells, however, in the synthesis of TL. Although both populations displayed abundant surface TL, the TL glycoprotein was produced actively in fractions containing dividing cells but made at a drastically reduced rate by the nondividing majority of cortical thymocytes. Thus, TL seems to be made at a narrowly circumscribed stage of early thymocyte development that is correlated with rapid proliferation. In most of the descendants of such blast cells, the TL glycoprotein is presumably retained on the cell surface as long as no substantial membrane turnover takes place. Ongoing TL synthesis may therefore serve as a marker for a unique developmental state which terminates rapidly in normal differentiation but may be extended by agents that give rise to TL(+) thymic lymphomas.

Identification and characterization of immature thymocytes responsive to T cell growth factor

The surface antigenic phenotype of peanut lectin agglutinin (PNA)-separated, high-density ("immature") thymocytes which are the target for the T cell growth factor (TCGF) enabling this otherwise unresponsive cell population to mount a strong proliferative response to concanavalin A (Con A) has been studied. The results obtained indicate that (a) the major population of cells bearing the typical phenotype of immature thymocytes, i.e. low content of H-2, high content of Thy-1. TL, Lyt-1 and Lyt-2 antigens, is unable to respond to Con A in the presence of TCGF, and (b) that the responsive population constitutes a minor fraction (about 15%) of PNA+, high-density thymocytes with surface antigenic phenotypes similar to those of mature T cells, i.e. high content of H-2, low content of Thy-1 and differentiated expression of Lyt antigens. Such a unique combination of properties and the presence of TL antigen on some members of the responding population suggest that it contains cells at intermediate stage(s) of differentiation between immature thymocytes and T cells.

Mitogenic effects of partially purified interleukin 2 on thymocyte subpopulations and spleen t cells of the mouse

Partially purified interleukin 2 (IL-2) promotes proliferation of mouse spleen T, but not B cells, and of peanut-agglutinin-negative (PNA-), and cortisone-resistant "mature" thymocytes, but not of PNA+ "immature" thymocytes. Within the cortisone-resistant thymocyte population, IL-2-responsive cells were found in the blast cell fraction. Proliferation was measured by [3H] thymidine incorporation and subsequent increase in viable cells. The mitogenic effect of IL-2 could not be inhibited by 50 mM methyl-alpha-D-mannoside which excludes contaminating concanavalin A (Con A) as a cause of mitogenicity. The relative increase in viable cells in IL-2 vs. control cultures was abrogated by 1.5 mM hydroxyurea. A possible effect of IL-2 on cell survival is thus ruled out. IL-2, when acting as comitogen with Con A, affected only PNA- and cortisone-resistant thymocytes. These cells also showed high intrinsic IL-2 release when stimulated with Con A such that a comitogenic effect of externally added IL-2 was only seen at low cell concentrations. PNA+ thymocytes could neither be induced to release IL-2 nor did these cells become Con A-responsive under the influence of IL-2, thereby excluding an IL-2-mediated maturation.