MHC restriction, self-tolerance and the thymus

Cellular and molecular signals in T cell differentiation

The thymic epithelium exerts its differentiative effects through several mechanisms, involving direct contact with stem cells as well as secretion of various thymic hormones. Indirect and direct evidence suggests that the thymus produces chemotactic factors for the stem cells that colonize the thymus anlage. The epithelium also produces several maturational factors which act upon stem cells that have undergone primary differentiation by contact with thymic epithelium. The chemical characteristics of these hormones and their mode of action at the cellular level (high affinity receptors, metabolic effects, target cells) are partly known. Their relationship with T cell factors such as Interleukin-2--produced in the periphery and endowed with strong differentiative capacity--is intriguing, the more so because thymic hormones not only act within the thymus but also affect peripheral T cells after they have emigrated from the thymus.

Deriving quantitative constraints on T cell selection from data on the mature T cell repertoire

The T cell repertoire is shaped in the thymus through positive and negative selection. Thus, data about the mature repertoire may be used to infer information on how TCR generation and selection operate. Assuming that T cell selection is affinity driven, we derive the quantitative constraints that the parameters driving these processes must fulfill to account for the experimentally observed levels of alloreactivity, self MHC restriction and the frequency of cells recognizing a given foreign Ag. We find that affinity-driven selection is compatible with experimental estimates of these latter quantities only if 1) TCRs see more peptide residues than MHC polymorphic residues, 2) the majority of positively selected clones are deleted by negative selection, 3) between 1 and 3.6 clonal divisions occur on average in the thymus after completion of TCR rearrangement, and 4) selection is driven by 103-105 self peptides.

Mosaic pattern and lineage analysis in chimeras

Mammalian chimeras have been used in a number of developmental studies over the years. A major limitation in these studies has been the lack of in situ procedures for establishing mosaic pattern in the tissues of these animals. Recently, a number of procedures have become available for the histochemical demonstration of mosaicism in chimeras. These include the elucidation of various enzymes, receptors, or surface antigens, which have variant expression between strains. The observation of pattern in organs of mosaic animals can suggest possible modes of organogenesis and organ maintenance. Experimentation with such animals can be used to establish some mechanisms of pathogenesis as well.

The study of mammalian organogenesis by mosaic pattern analysis

Chimeras are animals derived from more than one zygote and composed of two cell lineages which are distinguishable in some way at the cellular level. Spontaneous mosaic animals are also composed of distinguishable cell lineages but are monozygotic. The tissues of both mono- and multizygotic animals of this type are mosaic arrays in which aggregates of like cells form patches, the size and distribution of which can be useful in the analysis of diverse problems in developmental biology. Both biochemical and in situ methods have been applied to the elucidation of mosaic pattern. Both forms of mosaicism have proven useful in establishing theoretic constructs of the formation and maintenance of mammalian organs. A number of these constructs are discussed: cell fusion as related to myotube formation; mechanisms of coat pigmentation and the cellular origin of melanocytes; and pattern analyses of the retinal pigmented epithelium, the intestine, liver, adrenal cortex and thymus. Pathologic alterations in such animals have also been studied utilizing mosaic pattern analysis. In particular, neoplastic tumors and their associated preneoplastic lesions have been shown to be clonal.

Graft-versus-host reactions in F1 hybrid mice: MHC-restriction-independent generalized depression of virus-specific cytotoxic T cell response

A graft-versus-host reaction (GVHR) was induced in F1 hybrid mice by intravenous injection of parental spleen cells. A possible effect generated by recipient F1 mice of a GVHR on the restriction specificities of anti-viral cytotoxic T cells was investigated. (A/J X C57BL/6) F1 recipients were injected with 1 X 10(8) or 2 X 10(7) spleen cells from either parent. Zero to two weeks later during an acute GVHR or 12 to 26 weeks later during a chronic GVHR, spleen cells from these F1 recipients were assayed for their capacity to generate vaccinia (or lymphocytic choriomeningitis) virus-specific cytotoxic T cells. No effect was seen when parental cells were injected with virus on the same day. During the acute phase of the GVHR, recipients of spleen cells of both doses or from either parent injected 7 or 14 days previously generated markedly fewer cytotoxic T cells with respect to both parental restriction specificities. In the more chronic situation only, recipients of 1 X 10(8) parental spleen cells showed depressed virus-specific cytotoxic responses, again both restriction specificities were affected comparably. Therefore, a GVHR depresses generation of virus-specific cytotoxic T cells in a dose- and time-dependent way, but does not measurably disturb the balance of parental 1 versus parental 2 restriction specificity in an F1 recipient.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Alteration in lymphocyte recognition repertoire during aging. II. Changes in the expressed T-cell receptor repertoire in aged mice and the persistence of that change after transplantation to a new differentiative environment

Changes in the T-lymphocyte alloreceptor repertoire associated with aging by exploring the frequency of cytotoxic T-lymphocyte precursors (CTLp) available for activation by various major histocompatibility complex (MHC) haplotypes in mice of different ages have been investigated. There was no consistent pattern of change in CTLp frequencies. Thus, for instance, while the frequency of responder C57B1/6 CTLp for ATH alloantigen decreased with age, the frequency for C3H alloantigen increased. There was no significant change in the overall frequency of splenic CTLp (assessed irrespective of antigen specificity). No evidence was found that CTL produced by activated CTLp of aged mice were less specific in their lytic capacity that CTL produced by CTLp of young mice. However, by assaying responder CTLp cultures at limiting dilution we obtained evidence that the "burst size" (mean lytic capacity per responder well assayed at limiting dilution) was diminished with age of the donor of the CTLp pool. Furthermore, we obtained evidence that the apparent affinity of CTL for their target antigen was consistently decreased when those effector cells were derived from a pool of CTLp of aged mice. All of these changes reflected in mature T cells derived from aged mice were already apparent in the bone marrow stem cell pool of aged individuals and were not due to environmental influences alone, as assessed by the phenotype of T cells derived from young or old bone marrow stem cells transplanted to young or aged recipient mice. A final study has examined evidence for more subtle changes in the T-cell alloreceptor repertoire, reflecting heterogeneity in young or aged mice in the recognition repertoire associated with a given antigenic specificity. By preparing F1 anti (parent anti-F1)-suppressor cells directed against CTL from young parental mice (a, b, c), or aged parental mice (x, y, z), we have explored the heterogeneity in the anti-C3H alloreceptor repertoire in individual young or aged C57B1/6 mice. Suppression by immunized F1 animals was assessed in tissue culture (inhibition of mixed lymphocyte culture (MLC) responses) or in vivo (inhibition of lethal GvHD induced by inoculation of parental lymphocytes into sublethally irradiated F1 hybrid mice). Irrespective of the assay system used, the data suggests that the receptor repertoire of aged T lymphocytes uses recognition structures different from those of young individuals, and that there is less individual-to-individual variation in the receptor repertoire of aged mice than in young mice.

Restoration of antibody responsiveness in early thymectomized Xenopus by implantation of major histocompatibility complex-mismatched larval thymus

The restorative potential of isogeneic and fully allogeneic thymic implants to restore in vivo antibody responsiveness to a thymus-dependent antigen has been examined in thymectomized clawed toads. Isogeneic clones (Xenopus laevis/gilli and X. laevis/muelleri hybrids) of known major histocompatability complex (MHC) haplotypes were thymectomized at 5-7 days, implanted with MHC-identical or -mismatched (normal or irradiated ) thymuses from 5-week-old larvae and 4 months later injected with dinitrophenylated keyhole limpet hemocyanin. IgM antibody responsiveness (measured by phage inactivation) was restored in terms of its quantity, affinity and specificity in those thymectomized animals given two haplotype-different thymuses. Isoelectric focusing data revealed that low molecular weight antibody production was also restored with isogeneic and allogeneic thymus and that the antibody spectrotypes were of the host type. The finding that in vivo T-B collaboration in thymectomized animals develops normally following thymus implantation, even when the thymus genotype is disparate from the host, is discussed both in the light of previous experiments revealing MHC restriction of Xenopus T helper cells in vitro and in terms of the role of the thymus in promoting adaptive differentiation of this lymphocyte subset.

Thymocytes can be stimulated to give a strong vaccinia virus-immune cytotoxic T lymphocyte response

The emergence of virus-specific cytotoxic T lymphocyte (CTL) precursors in thymus can be examined directly by stimulating thymocytes for 6 days in irradiated, virus-infected recipients. Responsiveness is a characteristic of the thymocytes themselves and does not reflect the presence of contaminating, blood-bourne T cells. Strong CTL activity may be generated earlier from thymus than from spleen or lymph node of lethally irradiated, bone marrow reconstituted mice. Also, the reconstitution of virus-immune CTL function in these peripheral lymphoid organs is thymus-dependent. Use of this simple experimental protocol offers considerable possibilities for the analysis of the role of the thymus in the ontogeny of the T cell repertoire.