T cells from fully H-2 allogeneic (A replaced by B) radiation bone marrow chimeras are functionally competent and host restricted but are alloreactive against hybrid Ia determinants expressed on (A x B)F1 cells

A nondeletional mechanism of thymic self tolerance

T cells become tolerant of self antigens during their development in the thymus. Clonal deletion of thymocytes bearing self-reactive T cell receptors is a major mechanism for generating tolerance and occurs readily for antigens expressed by bone marrow-derived cells. Tolerance to antigens expressed on the radioresistant thymic stromal elements is demonstrated here to occur via a nondeletional mechanism. For minor lymphocyte stimulatory (Mls-1a) and major histocompatibility complex (MHC) antigens, this alternate form of tolerance induction results in clonal anergy.

Recognition requirements for the activation, differentiation and function of T-helper cells specific for class I MHC alloantigens

The present review has focused on the specificity of the T-helper cell populations initiating MHC class I alloreactions. In contrast to conventional immune responses against soluble antigens, responses against membrane-bound class I alloantigens are initiated by two distinct antigen-specific T-helper cell populations that can be distinguished by their Lyt phenotype, MHC restriction specificity, antigen specificity, and requirement for thymically determined self-recognition. Alloresponses were shown to be a composite consisting of two distinct components: one mediated by L3T4+ Th cells and very similar to conventional self + X responses; and one mediated by Lyt2+ Th cells and unique to alloresponses against MHC class I antigens. As would befit an unusual Th cell population, the recognition/response spectrum of Lyt2+ Th cells was highly unusual and was found to be the basis for much of the uniqueness we attribute to immune alloreactions, including rapid rejection of tissue allografts in vivo.

A naturally occurring bone marrow-chimeric primate. II. Environment dictates restriction on cytolytic T lymphocyte-target cell interactions

Restriction on cytolytic T lymphocyte (CTL)-target cell-interactions are studied in the primate S. oedipus, a naturally occurring A + B----A bone marrow-chimeric species. We show that the T cell, B cell, and myelomonocytic progenitor cell populations are chimeric in this species. We selected animals for study that are populated by fully major histocompatibility complex (MHC)-disparate hematopoietic cell populations, using a functional assay system. We then developed an in vitro system for analyzing at the clonal level the genetic restrictions on the trinitrophenyl-specific CTL-target cell interactions of this species. In this system, we have shown that tolerance to foreign MHC determinants was not, of itself, sufficient to facilitate the generation of CTL specific for target cells expressing those foreign MHC determinants. Rather, a marked preference for the expansion of CTL clones with a restriction for target cells bearing the host animals' MHC determinants was seen. Hematopoietically derived cells did not affect the repertoire of these T lymphocytes. These studies represent the first demonstration of the phenomenon of an environment dictating interactional restrictions on CTL in a naturally occurring bone marrow-chimeric animal. This is also the first demonstration of the profound influence of the environment on the repertoire of the T lymphocyte in a primate species.

Graft-vs.-host reactions: mechanisms and contemporary theories

A graft-vs.-host (GvH) reaction can be initiated by injection of immunocompetent lymphocytes into a histoincompatible host that is unable to reject these cells. The reaction is characterized by splenomegaly, hepatomegaly, lymph node atrophy, body weight loss, dermatitis, and diarrhea, often leading to mortality. The onset and severity of the GvH reaction are determined by differences in histocompatibility antigens between the donor and the acceptor, and by the number and nature of the transplanted allogeneic cells. Many different in vivo and in vitro systems have been devised for experimental studies of the GvH reaction. In several of these models, however, different parameters are measured. Furthermore, the conclusions drawn from these investigations sometimes contradict each other. This paper reviews the experimental data, and discusses the mechanisms underlying the GvH reaction.

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.

Ontogeny of primary lymphoid organs and lymphoid stem cells

Cells of the immune system go through a series of important developmental steps that begin early in embryonic life and include, first, the various waves of hemopoietic-cell production in the embryo and, second, the homing of these cells to the hemopoietic organs, which are the sites of hemopoiesis and lymphopoiesis in embryonic and adult life. The avian embryo is an important model for investigating these early steps; and this paper presents a comprehensive review of the work done on the early ontogeny of the avian immune system.

Cytotoxic T lymphocyte responses in allogeneic radiation bone marrow chimeras. The chimeric host strictly dictates the self-repertoire of Ia-restricted T cells but not H-2K/D-restricted T cells

The present report has used fully H-2 allogeneic radiation bone marrow chimeras to assess the role of host restriction elements in determining the self-specificity of Ia- and H-2K/D-restricted T cells that participate in the generation of trinitrophenyl (TNP)-specific cytotoxic T lymphocytes (CTL). It was demonstrated that there exists a stringent requirement for the recognition of host thymic-type Ia determinants, but there exists only a preference for host thymic-type H-2K/D determinants. Indeed, once the stringent requirement for recognition of host Ia determinants was fulfilled, anti-TNP CTL were generated in response to TNP-modified stimulators that expressed either donor-type or host-type H-2K/D determinants. The CTL that were generated in response to TNP-modified donor-type stimulators were shown to be specific for TNP and restricted to the non-thymic H-2K/D determinants of the chimeric donor. Thus, these results demonstrate in a single immune response that the thymic hypothesis accurately predicts the self-specificity expressed by Ia-restricted T cells, but does not fully account for the self-specificity expressed by H-2K/D-restricted T cells. These results are consistent with the concept that H-2K/D-restricted T cells, but not Ia-restricted T cells, can differentiate into functional competence either intrathymically or extra-thymically. The present results are also informative for understanding the cellular interactions that are required for the generation of antigen-specific CTL responses. The Ia-restricted T cells that are required for the generation of H-2K/D-restricted anti-TNP CTL were shown to be helper T (TH) cells since (a) like TH cells functioning in antibody responses, they were specific for Ia determinants expressed by accessory cells, and (b) their function could be replaced by either TNP-primed, irradiated TH cells or by nonspecific soluble helper factors. It was also shown that the T-T cell interaction between Ia-restricted TH cells and H-2K/D-restricted precursor CTL (pCTL) is not Ia restricted. Rather, the results demonstrate that the generation of anti-TNP CTL responses involve two parallel sets of major histocompatibility complex-restricted cell interactions, an Ia-restricted TH-accessory cell interaction required for TH cell activation, and an H-2K/D-restricted pCTL-stimulator cell interaction required for pCTL stimulation. The interaction between activated TH cells and stimulated pCTL is mediated, at least in part, by nonspecific soluble helper factors.