Relationships between terminal transferase expression, stem cell colonization, and thymic maturation in the avian embryo: studies in thymic chimeras resulting from homospecific and heterospecific grafts.

Terminal deoxynucleotidyl transferase (TdT) can be detected in 11- to 12-day-old embryonic chick thymuses 5 to 6 days after the first influx of lymphoid stem cells into the thymic rudiment. To identify the main factors of TdT induction, grafting experiments were devised in such a way that the age of the grafted thymus and that of the host were different. Uncolonized embryonic chick thymuses were grafted into chick hosts of different ages. Under these conditions, lymphoid differentiation arose from host lymphoid stem cells (LSC) invading the thymic rudiment. TdT immunofluorescent detection in the first wave of thymocytes showed that the percentages of TdT+ cells were related to the total age of the explant and not to the age of the host (11 to 17 days). Similar results were obtained when the chick thymic rudiment was transplanted into quail embryos, showing that quail LSC have TdT inducibility similar to that of chick LSC while developing in a chick thymic environment. Colonized chick thymuses were also grafted into quail embryos to compare the TdT inducibility of the first lymphoid generation (of chick type) and of the second (of quail origin), taking advantage of the different chromatin structure of quail and chick cells. In these experiments, the majority of chick cells remained TdT negative for as long as 10 days, whereas most lymphocytes of the second generation became TdT+ soon after their arrival in the grafted thymus. Therefore, during embryonic life, most TdT+ cells were derived from the second wave of stem cells, but some early stem cells were also able to acquire the enzyme. In a final series of experiments, early thymic rudiments were cultured in vitro with 14- to 16-day-old bone marrow and then grafted into 3-day-old host embryos. Under these conditions, bone marrow LSC contributed to a variable proportion of the first generation of thymocytes. The percentage of TdT+ cells among the progeny of these bone marrow stem cells was found to be two times higher than that of thymocytes derived from host LSC. These results suggest that, in addition to intrathymic environmental factors, the origin of LSC influences the frequency of TdT expression in their progeny.

Isolation and partial characterization of messenger RNA, from murine T cell hybrids, coding for suppressive immunoglobulin G-binding factor.

Poly A RNA has been isolated from a murine T cell hybridoma ( T2D4 ) that spontaneously secretes suppressive immunoglobulin G-binding factor ( IgGBF ). Translation products, obtained from a rabbit reticulocyte lysate translation system and after injection into Xenopus laevis oocytes, contain material with the biologic activity, the affinity, and the m.w. of murine IgGBF ; it suppresses secondary in vitro IgG antibody production in a dose-dependent fashion. The suppressive factor binds to IgG but not to IgM immunoadsorbents and, after mild NaDodSO4 treatment, dissociates in NaDodSO4 polyacrylamide gels into two peaks at 78 and 40 kD. Translation products from two non- IgGBF -secreting cell lines (BW-5147, a T lymphoma line, and A9, a fibroblast cell line) fail to exert any suppressive activity. On sucrose gradients, the RNA responsible for the biologic activity was found in one major peak located at 11S. IgGBF synthesized in a cellfree translation system by using poly A RNA and sucrose gradient fractions was also characterized by immunoprecipitation with Fc fragments of [35S]methionine-labeled proteins. On NaDodSO4 polyacrylamide gels, it migrates in one peak located at 37 kD. We conclude that IgGBF is coded for by 11S poly A RNA and that no post-translational modifications (other than proteolytic cleavage) are necessary to obtain a biologically active factor with Ig-binding properties.

Molecular mass analysis of murine immunosuppressive immunoglobulin G-binding factors (IgG-BFs) produced by T-cell hybrids.

Induced and constitutive murine IgG-binding factors (IgG-BFs) have been purified by affinity chromatography from supernatants of T-cells preincubated with or without murine monoclonal IgG1 and IgG2b, respectively. IgG-BF Mr values have been studied by SDS-polyacrylamide gel electrophoresis (PAGE) after treatment with SDS under conditions which do not noticeably alter their immunosuppressive activities on the secondary in vitro IgG antibody response. Suppression was recovered at Mr values of 80000, 40000 and 20000. When induced IgG-BF was tested, the isotype-specific suppressive activity was found only at 40 kDa. The 20-kDa moiety appeared to derive from the 40-kDa component and the material found at 80 kDa exerted non-specific immunosuppressive effects. We conclude therefore that isotype-specific IgG-BF has an apparent Mr of 40000.