Tolerance of thymic cytotoxic T lymphocytes to allogeneic H-2 determinants encountered prethymically: evidence for expression of anti-H-2 receptors prior to entry into the thymus

The role of hematogenous and intrinsic precursor cells in lymphocyte production in murine bone marrow and thymus

It is well recognized that the bone marrow contains cells that can repopulate a depleted thymus as well as cells that can be induced to express phenotypic markers characteristic of T cells. It is not known, however, to what extent thymocytopoiesis in the normal thymus relies on immigrant, bone marrow-derived cells, nor whether some T cell precursors have entered the bone marrow from the circulation. We used the parabiotic system to test whether thymocytopoiesis relies on progenitors intrinsic to the thymus or on cells that enter the organ from the circulation. In the same system, we have also investigated whether Thy-1- bone marrow lymphocytes that respond to phytohemagglutinin (PHA) by proliferation and Thy-1 expression are produced by myelogenous or hematogenous progenitors. Syngeneic CBA/HT6 and CBA/CaJ mice were joined in parabiotic union at 4-6 weeks of age. Cross circulation between the two partners was verified by the equilibration of Evans' blue dye injected into one partner and by the equilibration of PHA-responsive T cells in the spleen of the parabionts. Chromosome spreads were prepared from the PHA-stimulated T cell-depleted bone marrow and from spontaneously proliferating thymocytes as well as from thymocytes stimulated by PHA or Concanavalin A (Con A). The exchange of spleen colony-forming units (CFU-S) in the femoral marrow was assessed by karyotyping individual spleen colonies. Regardless of the length of parabiotic union, ranging from 4 to 20 weeks, Thy-1-, PHA-responsive bond marrow lymphocytes remained predominantly of the host type with only 3% being derived from the opposite partner.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the thymus in natural tolerance to an autologous protein antigen

C5-deficient mice grafted with thymus from C5-sufficient donors and immunized with C5 failed to make humoral antibody to C5, suggesting that the transfer of thymus had induced tolerance. Irradiated C5-deficient hosts repopulated with lymphoid cells from thymectomized C5-deficient mice grafted with C5-sufficient thymus also failed to respond to immunization with C5, thus showing that the state of tolerance can be adoptively transferred. These results demonstrate that natural tolerance to self-protein antigen is "learned" in the thymus.

Two rare populations of mouse Thy-1lo bone marrow cells repopulate the thymus

Two-color FACS analysis of mouse bone marrow reveals a rare population, comprising 0.1-0.3% of the total, that expresses low levels of the Thy-1 antigen but does not express any of five surface markers that characterize differentiated hematolymphoid cells. We demonstrate here that this fraction of mouse bone marrow is enormously enriched in cells that can home to the thymus and differentiate into mature T lymphocytes, subsequently migrating to peripheral lymphoid organs. Only a subset of the FACS-isolated fraction (1/90 after intrathymic injection) is capable of responding to the thymic microenvironment with a productive commitment to the T cell lineage. A second fraction of mouse bone marrow, which expresses how levels of Thy-1 but is also positive for at least one of five hematolymphoid lineage-specific markers, also contains cells that home to the thymus and establish colonies of thymocytes. The two fractions each contribute approximately equal amounts of thymic colony-forming units (CFUt) to the bone marrow, and together can account for at least half of the CFUt in whole bone marrow.

Studies on the recovery from tolerance to tumor antigens. I. Bone marrow cells from tolerant hosts are not rendered tolerant, but provide potential to reconstitute tumor-specific effector T cell clones

The present study investigates the potential of bone marrow cells from mice tolerant to tumor antigens to repopulate tumor-specific effector T cells. C3H/He mice were inoculated i.v. with 10(6) 10,000 R X-irradiated syngeneic X5563 plasmacytoma tumor cells three times at 4-day intervals. This regimen abrogated the ability of spleen cells from these mice to develop anti-X5563 cytotoxic and in vivo protective (tumor-neutralizing) T cell-mediated immunity as induced by i.d. inoculation of viable X5563 cells followed by surgical resection of the tumor. Since such suppression was induced in a tumor-specific way, this represented a state of antitumor tolerance. When bone marrow cells from normal or X5563-tolerant mice were transferred i.v. into 950 R X-irradiated syngeneic C3H/He mice, both groups of recipient mice generated anti-X5563 tumor immunity over a similar time course and to almost the same degree. Anti-X5563 tumor immunity induced in (C3H/He X C57BL/6) F1 mice which had been transferred with bone marrow cells from normal or X5563-tolerant C3H/He mice were mediated by T cells expressing the Ly phenotype of C3H/He, but not of C57BL/6, excluding the possibility that the antitumor effector cells were derived from recipient mice. It was also demonstrated that C3H/He mice which had been reconstituted with normal marrow were rendered tolerant when the tolerance regimen was started 7 weeks, but not 1 week after the bone marrow reconstitution. These results indicate that bone marrow cells from antitumor tolerant mice are not rendered tolerant to the tumor but can provide the potential to repopulate antitumor CTL and in vivo protective effector T cells.

Studies on the recovery from tolerance to tumor antigens. II. Accelerated recovery of tumor-specific effector T cells in tolerant mice by applying T-T cell interaction mechanism

C3H/He mice were injected i.v. with heavily X-irradiated syngeneic X5563 tumor cells three times at 4-day intervals. This regimen resulted in the abrogation of the potential to generate X5563 tumor-specific T cell-mediated immunity as induced by i.d. inoculation of viable X5563 tumor cells followed by surgical resection of the tumor, representing the tolerance induction. Although such a tumor-specific tolerant state was long-lasting, the recovery of anti-X5563 effector T cell responses was observed when the above ordinary immunization procedure was performed 6 months after the tolerance induction. The present study investigated whether the recovery from the tolerance can be accelerated by applying a helper-effector T-T cell interaction model in which enhanced anti-X5563 immunity is obtained by priming mice with BCG and by immunizing X5563 tumor cells modified with BCG cross-reactive MDP hapten (designated as L4-MDP) in the presence of anti-L4-MDP helper T cells preinduced with BCG. The results demonstrated that BCG-primed mice which received the tolerance regimen failed to generate anti-X5563 immunity when the ordinary immunization was performed 2 or 3 months after the tolerance induction. In contrast, the immunization of BCG-primed and X5563-tolerant mice with L4-MDP-coupled X5563 tumor cells at comparable timing to that of the ordinary immunization were capable of generating potent X5563-specific in vivo protective T cell-mediated immunity. As control groups, BCG-primed or unprimed tolerant mice did not develop anti-X5563 immunity when immunized with L4-MDP-uncoupled or L4-MDP-coupled tumor cells, respectively. These results indicate that immunization of BCG-primed, tumor-tolerant mice with L4-MDP-modified tumor cells results in accelerated recovery from the tumor tolerance.

A single micromanipulated stem cell gives rise to multiple T-cell receptor gene rearrangements in the thymus in vitro

The extensive range of specificities of T-cell receptors is generated, as for immunoglobulins, by rearrangement of genetic information. Much valuable information about rearrangement processes has been inferred by comparing DNA from (monoclonal) lymphoid lines with germ-line DNA and, for B cells, from rearrangements in some Abelson murine leukaemia virus-transformed cell lines. However, because it is difficult to isolate and grow precursor populations, it has not proved possible to study rearrangements occurring in normal untransformed cells in vitro. Here we show that a single T-cell precursor colonizing an alymphoid thymus lobe in organ culture can generate multiple receptor beta-chain gene rearrangements. These observations provide unequivocal evidence for the intra-thymic diversification of the T-cell repertoire. They also offer the possibility of investigating rearrangement and its control in the clonal progeny of a single normal T-cell precursor without the perturbations involved in the use of viral transformation or the production of T-cell hybridomas.

Intrathymic differentiation: thymocyte heterogeneity and the characterization of early T-cell precursors

Multiparameter FCA of unfractionated or isolated subpopulations of thymocytes reveals at least seven subpopulations in the mouse thymus. One of these subpopulations, designated dLy1 has been isolated and characterized extensively. The data reviewed here indicate that the dLy1 thymocyte subpopulation, whether derived from the adult or fetal thymus, represents an early stage in intrathymic differentiation. The immature status of dLy1 cells was suggested by the extensive similarity to a predominant cell type that occurs early in fetal thymic ontogeny. Its precursor role was demonstrated by its capacity to generate cortical and medullary-type thymocytes in vivo. Its expression of Ly1, Thy-1 and mRNA specific for the beta-chain of the T-cell antigen receptor support its commitment to a T-cell developmental pathway. In summary, dLy1 thymocytes appear to be the earliest committed T cells yet to be described, isolated and characterized. Further investigation should reveal whether this subpopulation of thymocytes contains subsets of cells in earlier states of maturation and/or precursors already committed to more than one T-cell lineage.

Thymus dictates major histocompatibility complex (MHC) specificity and immune response gene phenotype of class II MHC-restricted T cells but not of class I MHC-restricted T cells

Athymic H-2b nude mice received grafts from C57BL/6 (Sendai virus and H-Y antigen cytotoxic T lymphocyte [CTL] responder type), bm1 (H-2Kb mutant, Sendai CTL nonresponder type), or bm12 (H-21-A mutant, H-Y CTL nonresponder type) neonates. In observations of the CTL response to H-Y, both recipients and thymus donors were female. All types of thymus engraftment resulted in mature H-2b splenic T lymphocyte surface phenotype in nude hosts. T cell immunocompetence (as measured by major histocompatibility complex [MHC] CTL responses to allogeneic cells) was restored, and induced nonresponsiveness to the MHC determinants of the engrafted thymus in the nude host. The CTL reaction to Sendai virus in both responder type C57BL/6 and nonresponder type bm1 neonatal thymuses allowed maturation of Sendai-specific, H-2Kb-restricted CTL. For the CTL reaction to H-Y, only responder type C57BL/6 thymuses restored the CTL response, whereas this was not achieved with thymuses from nonresponder type bm12 neonatal females. Results of double thymus (B6 and bm12) engraftment excluded the possibility that this latter effect was caused by suppression. In addition, athymic bm1 mice were engrafted with thymuses from either B6 (Sendai CTL responder type) or syngeneic bm1 neonates (Sendai CTL nonresponder type). Again, both types of neonate thymuses restored T cell competence as measured by MHC/CTL responses to allogeneic cells. However, neither responder B6 nor nonresponder bm1 neonate thymus grafts allowed maturation of Sendai-specific CTL. In conclusion, the thymus dictates MHC specificity and immune response gene phenotype of T cells restricted to class II MHC molecules but not of T cells restricted to class I MHC molecules.

Irreversible inactivation of activated cytotoxic T lymphocyte precursor cells by "anti-self" suppressor cells present in murine bone marrow T cell colonies

When added to a mixed lymphocyte culture, cells in T cell colonies grown from bone marrow (BM) suppressed the development of cytotoxic activity against H-2 antigens shared by the colony cells and the stimulator cells, apparently by inactivating cytotoxic T lymphocyte precursor cells (CTLP). From the point of view of the added suppressor cells, the suppression was against self-reactive cells. The suppressor cells were resistant to gamma irradiation (1500 rds) but sensitive to UV irradiation. Inactivated CTLP separated from the suppressor cells by cell sorting could not be reactivated on being recultured with fresh stimulator cells, suggesting the suppression is irreversible. There was a critical time window, extending roughly from 20 to 40 h after culture initiation, during which the suppressor cell had to be present if CTLP were to be inactivated. During the first 20 h and after 40 h of exposure to stimulator cells, CTLP were resistant to the suppressor cell. Direct experimental evidence is presented against the possibility that the suppressor cells derived from BM colonies act by augmenting the production of Lyt-2+ suppressor cells from the responder population which then produce the suppression, or that the suppressor cells interfere with an early interaction between CTLP and stimulator cells. We conclude that the suppressor cells in T cell colonies grown from BM act directly on activated CTLP and permanently inactivate them.

Early events in T lymphocyte genesis in the fetal thymus

There is considerable uncertainty about the nature and level of maturation of the stem cells which colonize the thymus. Arguments are presented here which raise doubts about the claims that these cells have undergone substantial maturation along the T-lineage pathway prior to migration to the thymus. Instead, emphasis is placed on the role of thymic stromal cells in T-lymphocyte maturation. The heterogeneous nature of these cells is well established, but progress is described in analyzing the various cell types and their embryological origins. In particular, the expression of the major histocompatibility complex (MHC) antigens on thymic stromal cells might be relevant to the understanding of restriction and tolerance. The early phases of thymus lymphocyte differentiation are described; but no clear account of the generation of T-cell subsets from immature cells can, as yet, be offered.

The double cascade of lymphoid proliferation: current challenges and problems areas

Two largely but not entirely separate cascades of proliferation and differentiation are involved in immunity. First, processes that are mainly independent of the entry of antigen into the body generate the two types of lymphocytes, T cells and B cells. This lymphoneogenesis proceeds in the thymus and the bone marrow, respectively. It creates families and subfamilies of nondividing cells that do nothing but patrol the body until antigen enters. Each lymphocyte set represents a repertoire of recognition units preequipped with receptors for antigen, and each individual lymphocyte carries its own particular specificity as a result of a somatic rearrangement of genes. When an antigenic stimulus is given, the second and antigen-dependent wave of proliferation and differentiation occurs. This involves clonal selection of particular lymphocytes within the repertoire with specificity for the antigen, and the generation of both immunologic effector cells and memory cells. This second cellular cascade takes place chiefly in the secondary lymphoid tissues, e.g., lymph nodes or spleen. The developing immune system learns to discriminate self from not-self. The paper outlines the chief mechanisms operative in immunologic tolerance and also briefly addresses the fact that T lymphocytes must learn to "see" antigens in association with a "self" major-histocompatibility-complex antigen. Both proliferative cascades depend on intercellular interactions, but much more is known about those operative in antigen-driven lymphocyte activation. Here there is a cyclical process in which antigen-trapping cells stimulate helper/inducer T cells, which in turn stimulate cytotoxic/suppressor T cells or B cells. Some T-cell products can augment the antigen-trapping cells' inductive role.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathymic presentation of circulating non-major histocompatibility complex antigens

Intrathymic selection of T-cell specificity has been shown to be influenced by self-major histocompatibility complex (MHC) antigens encoded by radioresistant thymic stromal cells. The role of non-MHC antigens in intrathymic T-cell differentiation, in particular induction of antigen-specific tolerance, has been unclear and the access of non-MHC antigens to the thymus is controversial. Here we present evidence that circulating protein antigens enter the thymus and are presented by thymic stromal cells. At least three distinct types of stromal cells are thought to be associated with intrathymic lymphopoiesis; after intravenous (i.v.) injection of antigen only I-A/E-positive medullary dendritic cells, but not I-A/E-negative macrophages or I-A/E-positive cortical epithelial cells co-purified with antigen-specific stimulation of cloned T-helper cells in vitro. Antigen presentation by thymic stromal cells was dependent on the dose of antigen injected and the time interval after injection.

T cell tolerance to non-H-2-encoded stimulatory alloantigens is induced intrathymically but not prethymically

The present report has evaluated the differentiation compartment in which T cells are tolerized to non-major histocompatibility complex (MHC)-encoded minor lymphocyte-stimulating locus (MLS) alloantigens. It was observed that T cell precursors are not tolerized prethymically to MLS alloantigens but are tolerized intrathymically and postthymically to MLS alloantigens. The failure of prethymic T cells to be tolerized indicates either that T cell precursors are unable to be tolerized to MLS alloantigens or that cells in the prethymic compartment are unable to induce MLS-specific tolerance. In either case, these results demonstrate that the thymus is the initial site in which T cell tolerance to MLS alloantigen is induced. The present results also demonstrate a striking disparity in the reactivity of thymocytes to MHC and MLS alloantigens expressed in the extrathymic host through which their precursors had migrated. In the experimental mice constructed for these studies, intrathymic T cells were tolerant to the MHC alloantigens but were reactive to the MLS alloantigens expressed by the extrathymic host. This observation is consistent with the concept that T cell precursors may be tolerized to MHC alloantigens at an earlier point in their differentiation than they are tolerized to non-MHC-encoded MLS alloantigens.

The ontogeny of T lymphocytes

We have provided a speculative overview of some of the questions concerning the nature of thymic stem cells and the role of the thymus in their maturation, and have suggested experimental approaches which might provide solutions to some of the questions. Throughout, we have stressed the complexity of the cellular constitution of the thymus and have suggested roles for particular cell types which seem plausible based on present evidence.

Functional clonal deletion of cytotoxic T-lymphocyte precursors in chimeric thymus produced in vitro from embryonic Anlagen

Chimeric thymus, formed by fusing the prelymphoid third pharyngeal pouches of fetal mice with fetal liver, have been allowed to develop entirely in vitro. Syngeneic and allogeneic chimeras were prepared and both types of thymus were shown to contain substantial numbers of functional cytotoxic T lymphocyte precursors reactive against "third party" alloantigens. However, alloreactivity specific for H-2 antigens present on either the third pharyngeal pouch or the fetal liver was minimal. In three different allogeneic chimeric thymuses, the frequencies of cytotoxic T lymphocyte precursors reactive to H-2 antigens present on the third pharyngeal pouches were reduced to 1%, 4%, and 0% of control values, whereas, in the one allogeneic chimera tested for alloreactivity to H-2 antigens present on the fetal liver, the cytotoxic T lymphocyte precursor frequency was reduced to less than 1% of control values. The phenotype of the H-2 tolerance is shown to be one of functional clonal deletion of the cytotoxic T lymphocyte precursor.