Self H-2 antigens influence the specificity of alloreactive cells

Factors influencing the patterns of T lymphocyte allorecognition

BACKGROUND

Strong alloreactive T cell responses are a menace in transplantation surgery and their menagement requires understanding the basis of alloreactivity. Alloantigen recognition can be peptide independent, peptide specific, or peptide dependent. The mechanisms influencing each recognition pattern are largely unknown.

METHODS

Peptide dependence was examined in vitro by adding peptides to antigen processing-deficient cell line used as target in cytotoxic T cell assays. Responses to major histocompatibility complex (MHC) alleles most homologous to self were recently shown to be more peptide dependent than to those with lesser homology to self. Hence, peptide reactivity in vivo was estimated based on relative strengths of alloreactive responses to more homologous and less homologous MHC alleles.

RESULTS

Alloreactive CD8+ TCR repertoire in beta2-microglobulin-deficient mice is preferentially peptide independent. The peptide-specific component is acquired as a function of wild-type thymic epithelium grafting. Irrespective of the presence of the peptide-specific component, in vivo alloantigenic priming was associated with a greater sensitivity to the MHC structure than was in vitro priming.

CONCLUSIONS

Thymic positive selection and the mode of alloreactivity induction are the major independent factors determining the patterns of alloantigen recognition.

The repertoire of T cells shaped by a single MHC/peptide ligand

Although the thymus produces many immature thymocytes, few of these cells mature. Positive selection has been thought to limit thymocyte development. In thymuses expressing a single MHC/peptide combination, however, surprisingly large numbers of thymocytes are selected to mature. Many of these react with the selecting MHC, bound to other self-peptides. Therefore, the number of thymocytes that mature is limited by the fact that positively selected cells die because they react too well with MHC bound to self-peptides that are not identical to those involved in positive selection. T cells that mature in thymuses expressing a single MHC/peptide ligand react frequently with foreign MHC, suggesting that the repertoire of alpha beta receptors may be more biased toward reaction with MHC than was previously thought.

Immune recognition by cytotoxic T lymphocytes of minor histocompatibility antigens expressed on a murine colon carcinoma line

We have characterized in vivo and in vitro responses of mice to the BALB/c-derived carcinoma, C26. BALB/c mice were highly susceptible, in a dose-dependent fashion, to local tumor development following subcutaneous injection of C26. Other strains of mice, including allogeneic strains and major histocompatibility complex compatible strains of different minor histocompatibility (H) backgrounds, were resistant to C26-induced tumors. The basis for resistance of mice to C26 was studied using an in vitro-derived C26 line as target cells in microcytotoxicity assays, and as a source of antigen for in vivo priming. An H-2d-specific alloreactive cytotoxic T lymphocyte (CTL) line was isolated from C57BL/6 mice primed with C26, demonstrating the expression, and immune recognition, of MHC class I antigens on C26. C26 also expressed minor H antigens of BALB background as demonstrated by the ability of CTL specific for BALB minor H antigens to selectively lyse C26. Conversely, minor H antigens on C26 were immunogenic across a minor H barrier as demonstrated by the ability to raise anti-minor H CTL to C26 from minor H disparate strains. Collectively, those experiments indicate that C26 may be useful for immunologic and biochemical studies of murine minor H antigens, and for in vivo and in vitro studies of local immunity.

The cytotoxic T-cell response to Ia antigens: lack of correlation between the level of specific cytotoxicity obtained in primary mixed lymphocyte reaction and the frequency of specific cytotoxic T lymphocyte

We have investigated the cytotoxic response to allogeneic I-E and I-A antigens in bulk culture and limiting dilution experiments. A high degree of specific killing could be generated from unprimed T cells of I-E-negative strains upon stimulation with cells expressing I-E. In such conditions, cytotoxic T lymphocytes (CTL) were generated in the absence of a specific proliferative response. The frequency determinations by means of limiting dilution experiments showed that I-E-specific CTL precursors were much less frequent than the K and D specific precursors. The results suggest the existence of a population of I-E-specific CTL that failed to grow under conditions that allowed the growth of class I-specific CTL.

Individual differences in the cytotoxic T-lymphocyte response in man to public HLA determinants

The allospecific anti-HLA response of cytotoxic T lymphocytes (CTL) from 22 unrelated individuals and 7 monozygous twin pairs was examined. From each responder, CTL were generated in several responder-stimulator combinations, each mismatched for one HLA-A or -B antigen. The CTL were assayed in the cell-mediated lympholysis (CML) on panels of third-party target cells, comprising cells that express the stimulating antigen (specific target cells), cells that express an antigen cross-reactive with the stimulating antigen (CREG target cells), and cells that do not express either the stimulating or a cross-reactive antigen (nonsharing target cells). Individual variations in the allo-CTL response were observed. We identified individuals (responders) who showed a consistently narrow CTL response and those who showed a broad reaction pattern to various stimulator cells. The narrow response was restricted almost entirely to specific target cells; the broad response comprised lysis of specific, CREG, and nonsharing target cells. These differences were evidently not dependent on the HLA-A, -B, -C phenotype of the responder, because HLA-A, -B, (-C)-identical individuals responded differently to the same stimulator. The identical response of monozygous twins indicates that the allogeneic CTL response is genetically controlled. The CTL response is not regulated by the HLA-DR antigens of the responder, nor is it influenced by the DR mismatch between responder and stimulator. The observed differences were not dependent on sex or age and could not be explained by differences in the T-lymphocyte subsets (OKT3+, OKT4+, OKT8+) or by differences in proliferative reactivity to mitogens (phytohemagglutinin, concanavalin A, phorbol-myristate acetate, pokeweed mitogen, anti-T3 monoclonal antibodies). The IL-2 activity in the supernatants of mixed lymphocyte cultures of broad and narrow responder-stimulator combinations did not differ.

T helper cell receptors: idiotypes and repertoire

Idiotopes displayed by T cell receptors could be regulatory elements in the selection and maintenance of the T cell repertoire. Among these, of particular interest are those structures which allow receptor-receptor interactions between distinct T cell clonotypes. The existence of murine T lymphocytes which can be induced to proliferate by a monoclonal antibody specific for an allogeneic Class II antigen has been demonstrated. Such cells can interact with a fraction of the syngeneic T cells which react to the same alloantigen as the antibody. T cells involved in such an interaction were immortalized as IL-2 secreting T hybridomas. Functional and structural data suggest that some T cell idiotopes are internal images of MHC (Class II) epitopes, recognized by both alloreactive T lymphocytes and Ia specific antibodies. The relation between T cell receptor variable gene polymorphism and the predominance of a family of idiotopes indicates, on the other hand, the ability of a V beta gene family to participate in the encoding of receptors which exhibit a variety of antigen specificities. At least in one instance, topochemical resemblance between non-self antigens and self-MHC limits the T cell repertoire and gives rise to Ir gene controlled patterns of T cell responsiveness to nominal antigens.

Cooperation between major histocompatibility complex mismatched mononuclear cells from a human chimera in the production of antigen-specific antibody

Fetal liver and thymus transplantation can be successfully employed for the treatment of severe combined immunodeficiency disease. In virtually all cases, donor and recipient cells are HLA mismatched. In a patient suffering from a severe combined immunodeficiency disease, full immunological reconstitution was obtained after fetal liver and thymus transplantation. HLA typing revealed that the patient's T cells were of donor origin, while the B cells and monocytes were of host origin. Despite this complete HLA mismatch, the patient was found to mount a subnormal to normal antibody response in vivo. This finding is in contrast with the concept that antigen recognition by T cells is major histocompatibility complex (MHC) restricted. To define the mechanism responsible for this in vivo antibody response, antibody production by peripheral blood mononuclear cells from the patient was tested in vitro after in vivo booster. The in vitro anti-tetanus toxoid antibody production was similar to that of the control group. In addition, specific proliferative responses to tetanus toxoid were obtained. Immunoglobulin allotype determination showed that antibodies were synthetized by host B cells. The results of the present study indicate that transplanted T lymphocytes and recipient cells cooperate despite complete HLA mismatch.

I-J epitopes are adaptively acquired by T cells differentiated in the chimaeric condition

I-J has been defined as a locus mapped in the murine major histocompatibility complex (MHC) which encodes serological markers found primarily on the surface of suppressor T cells (TS) and soluble suppressor factors (TSF). Recent studies have, however, revealed that there is no such specialized locus within the MHC at the DNA level. As the existence of I-J determinants at the protein level on functional T cells, T-cell clones and hybridomas has been confirmed by several serological and biochemical studies, this contradiction has raised serious arguments in the immunological community concerning the nature, origin and expression of I-J determinants. We have raised a number of monoclonal antibodies against the polymorphic structure of I-J molecules, and have studied the expression of I-J epitopes on T cells derived from irradiated bone marrow chimaeras in which stem cells of different genotype differentiated into T cells under the foreign host MHC environment. The results, presented here, indicate that I-J epitopes are not primarily determined by the MHC genes of the stem cells themselves, but are adaptively acquired by T cells differentiated in the chimaeric condition according to the environmental MHC phenotype. Thus, the serologically detectable I-J epitopes are found to be associated with inducible T-cell receptors recognizing self class II MHC antigens.

Involvement of I-J epitopes in the self- and allo-recognition sites of T cells: blocking of syngeneic and allogeneic mixed lymphocyte reaction-responder cells by monoclonal anti-I-J antibodies

Monoclonal anti-I-Jk antibodies (mAbs) were found to inhibit syngeneic and allogeneic mixed lymphocyte reactions by blocking the responder T cells but not the stimulator cells. Only the responses of H-2k and H-2a strains were inhibited. Three different anti-I-Jk mAbs (1G8, 4B11, and KN34) showed different inhibitory patterns in allogeneic mixed lymphocyte reactions of individual H-2k strains, depending on the H-2 and immunoglobulin heavy chain variable region (IgVH) genes possessed by the stimulator strains. The results indicated that I-J epitopes are involved in the self- and allo-recognition sites of T cells, which are clonally distributed and used to recognize Ia plus IgVH-linked products.

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.

Major histocompatibility restriction of antigen recognition by T cells in a recipient of haplotype mismatched human bone marrow transplantation

Immune T cells proliferate in response to antigen that is recognized in association with self-Ia determinants. T cells from a patient with severe combined immunodeficiency that has been successfully reconstituted with haplotype-mismatched, maternal bone marrow were studied in an attempt to understand the development of Ia restriction of antigen recognition in man. All the patient's T cells were of maternal origin as determined by HLA typing. The patient received a series of three immunizations with tetanus toxoid (TT) antigen between the 6th and 14th week posttransplant. TT-specific T cell lines were established from the patient's peripheral blood at 6 and 8 mo posttransplantation and were maintained in culture in the presence of irradiated monocytes from the patient, TT antigen, and interleukin-2. HLA typing of the two T cell lines revealed them to be exclusively of donor origin. Both T cell lines could proliferate to TT in the presence of monocytes derived from either the patient's mother or father. In contrast, a TT-specific T cell line obtained from the patient's mother proliferated to TT in the presence of autologous monocytes, but not in the presence of monocytes derived from the patient's father. Studies using monocytes from a panel of HLA-typed donors indicated that the patient's T cell lines proliferated to TT in the presence of monocytes that expressed the paternal DR antigen (HLA-DR4) inherited by the patient but not in the presence of monocytes that expressed the paternal DR antigen (HLA-DR1) not inherited by the patient or in the presence of monocytes bearing irrelevant DR antigens. Monocytes that expressed either one of the two maternal DR antigens (HLA-DR3 and DR5) could support the proliferation of the patient's T cell lines in response to TT antigen. HLA typing of the patient's monocytes at 6 mo post-transplant revealed only recipient HLA-DR antigens (HLA-DR3 and DR4). At 12 mo posttransplant, the patient's monocytes expressed recipient HLA-DR antigens as well as the non-shared HLA-DR5 antigen of donor origin. The results of the present study indicate that T cells of human bone marrow chimera recognized antigen in the context of Ia determinants of recipient origin. The apparent recognition of antigen by the chimera's T cells in the context of donor Ia determinants that were not shared with the recipient is discussed.

Internal images of antigens within the immune network

The variable regions of antibody molecules react with each other via specific recognition of clonally distributed antigenic determinants, known under the general term of idiotypes. The totality of such interactions determines a special behavior of the immune system, as a whole, and its ability to regulate immune responses has been analyzed by the 'immune network theory' of Jerne [5]. In light of this theory, a particular property of idiotypes appears to be of special interest: as revealed by recent experiments, frequently idiotype-like determinants represent internal images of antigens. These internal images can be topochemical copies of nominal non-self antigenic determinants, of various functional non-lymphoid receptors, or of transplantation antigens. In this article, we present a view according to which internal images are essential structures which mediate cellular interactions leading to the selection and conservation of the immune repertoire of T and B lymphocytes. Our opinion is exposed in the context of experimental data obtained in several laboratories, including ours.

Generation of cytolytic T lymphocytes in thymectomized, irradiated, and bone marrow-reconstituted mice

A model system has been developed to study extrathymic T cell differentiation. Mice have been thymectomized, lethally irradiated, and reconstituted with bone marrow cells depleted of Thy-1-positive cells. After 8 wk, the spleen cells of these 5athymic, bone marrow-reconstituted chimeras contain Thy-1-positive pre-cytolytic T lymphocytes (CTL) that are able to respond to antigen only when exogenous interleukin 2 is added to culture.. The phenotype of these pre-CTL is similar to that of thymocytes, suggesting that they may be an immature T cell. Initial evaluation of the CTL repertoire of these athymic mice demonstrates that the CTL generated to trinitrophenyl-modified syngeneic cells are H-2 restricted and that the CTL generated to alloantigens have many of the cross-reactivities observed in normal but not in nude mice. The discrepancies observed in the CTL repertoire between these thymectomized chimeras and nude mice are discussed.

Tolerance of thymocytes to allogeneic I region determinants encountered prethymically. Evidence for expression of anti-Ia receptors by T cell precursors before their entry into the thymus

The present study has assessed whether precursor T cells express receptors specific for the recognition of allogeneic I region-encoded determinants before their entry into the thymus. Because the ability of thymocytes to proliferate in response to allogeneic stimulator cells was shown to primarily result from the recognition of allogeneic I region determinants, thymocytes must already express anti-Ia receptors. In contrast, the expression of anti- Ia receptors by functionally immature thymocyte precursors could not be directly assessed by mixed lymphocyte reaction reactivity. However, expression of anti-Ia receptors by thymocyte precursors could be assessed by their ability to be specifically tolerized by the allogeneic Ia determinants that they encountered during their differentiation. To determine whether T cell precursors could specifically recognize and be tolerized to allogeneic Ia determinants expressed prethymically, thymus- engrafted radiation bone marrow chimeras were constructed [A {arrow} A x B (Tx + A Thy)] such that strain A T cells would be differentiating within a syngeneic strain A thymus but would have been previously exposed to the allogeneic strain B Ia determinants of the irradiated A x B host. The strain A thymocytes from these experimental animals were indeed tolerant to the extrathymic allogeneic strain B Ia determinants expressed by the irradiated host. Such tolerance was not mediated by detectable suppression and was not explained by the presence intrathymically of extrathymic allogeneic Ia determinants. Thus, these results suggest that T cell precursors can be specifically tolerized entry into the thymus. In addition, the failure to detect the generation of thymocytes with specificity for the allogeneic Ia determinants of the irradiated host, which were not deleted prethymically, argues that novel anti-allo Ia receptor specificities are not generated intrathymically.

Influence of the major histocompatibility complex on the repertoire of allospecific cytolytic T lymphocytes

Superimposed on the heterogeneous anti-H-2Kb cytolytic T lymphocyte (CTL) receptor repertoire of allogeneic murine strains are reactivities that recur with high frequency amongst individuals of any given strain. These receptor specificities represent phenotypic markers of the CTL repertoire and, as such, have been used to compare receptor repertoires of genetically disparate strains. The results demonstrate that congenic strains differing only in the MHC (B10.D2 and B10.BR) differ significantly in their H-2Kb-specific CTL repertoires. This finding clearly demonstrates a role for the MHC in determination of the CTL precursor repertoire. The mechanism by which MHC influences CTL specificity was explored through analysis of the anti-H-2Kb repertoire of (B10.BR X B10.D2)F1 hybrids. Because at least one recurrent parental specificity has found to be recurrent in F1 progeny as well, the findings indicate that MHC-specific tolerance cannot be solely responsible for repertiore differences between MHC-disparate strains. In addition, the F1 repertoire is characterized by the emergence of several nonparental recurrent specificities.

Antigen recognition by cloned cytotoxic T lymphocytes follows rules predicted by the altered-self hypothesis

Radiation chimeras prepared by injecting H-2 heterozygous F1 stem cells into lethally irradiated parental hosts show a marked, but not absolute, preference for host-type H-2 antigens in the H-2-restricted cytotoxic T lymphocyte (CTL) response to minor histocompatibility (minor H) antigens. We have selected for the anti-minor HCTL that are restricted to the parental H-2 type absent from the chimeric host and found that in two out of eight cases, such CTL lysed target cells of either parental H-2 type. From one of these CTL populations that lysed H-2d and H-2k target cells expressing BALB minor H antigens, clones were derived and further analyzed. The results showed that: (a) lysis of both H-2d and H-2k target cells was H-2 restricted; (b) H-2d restriction mapped to Dd, and H-2k restriction mapped to Kk; (c) testing against various H-2d and H-2k strains of different and partially overlapping minor H backgrounds as well as against the appropriate F1 crosses revealed that in Dd- and Kk-restricted killing, different minor H antigens were recognized. In a second system, a CTL population was selected from normal (H-2d x H-2k)F1 mice that was specific for H-2d plus minor H antigens and for H-2k plus trinitrophenylated bovine serum albumin. We interpret these findings in terms of the altered-self hypothesis: The association of one H-2 antigen with one conventional antigen X may be recognized by the same T cell receptor specific for the complex formed by a different H-2 antigen in association with a second conventional antigen Y. The implications of these observations for the influence of self H-2 on the generation of the T cell receptor repertoire are discussed.

Clonal analysis of the specificity of alloreactive cells: "dominance" of E beta reactive clones

The present experiments analyze the specificity of cells proliferating in murine mixed lymphocyte cultures (MLC). Lymphocytes from the B10.A(5R) strain, enriched by repeated restimulations with B10.A cells, proliferate specifically in response to B10.BR, B10.A and [B10.A(4R) X B10.A(5R)]F1 stimulators, while showing very limited responsiveness to B10.A(4R) cells. The preferential recognition of determinants on molecules other than Kk or I-Ak was directly proven by isolating 21 T-cell clones, all of which were Ek beta specific. This "dominance" seems to reflect a high frequency of Ek beta reactive T cells in the unselected B10.A(5R) repertoire because: (1) it is already observed in blasts isolated in a primary MLC, (2) it is typical of several independently raised B10.A(5R) anti-B10.A cultures, and (3) it is not found in parallel C57BL/6 anti-B10.BR MLCs that show dominant I-A specificity. Results showing the importance of determinant concentrations expressed by stimulator cells suggest that uncloned T-cell lines are heterogeneous in their "affinities" and that our cloning conditions select cells with high "functional affinity".

The H-2 restriction specificity of cytotoxic T cells derived from intrathymic precursors

Both cortical and medullary thymocytes could give rise to both allospecific and hapten-specific cytotoxic T lymphocytes (CTLs) if the cultures contained the T-helper-cell-replacing hormone, interleukin 2 (IL-2). The H-2 restriction specificity of these CTLs was tested by using three different specificity determinants that are known to be influenced by the H-2 genotype of the thymus (rather than by the H-2 of the precursor cell itself): the cross-killing of trinitrophenyl (TNP) coupled allogeneic targets in anti-TNP self responses; the dominance of H-2k-restricted CTLs over H-2d-restricted CTLs in the response of (H-2k X H-2d)F1 mice to TNP-self; and the capacity of alloreactive CTLs to cross-reactively lyse TNP-coupled syngeneic cells. The experiments show that, with regard to all of these specificity determinants, CTLs derived from these intrathymic populations are similar to CTLs derived from the adult mouse spleen. These data suggest that the H-2 restriction specificity is dictated before or at the same time that the cell acquires the capacity to respond to allogenic or haptenated syngeneic cells.

Antigen-inducible, H-2-restricted, interleukin-2-producing T cell hybridomas. Lack of independent antigen and H-2 recognition

We developed a method for production of antigen-specific, H-2-restricted T cell hybrids. The tumor cell partner in the fusions was itself a T cell hybrid, FS6-14.13.AG2 (or its derivatives), which could be induced to produce the growth factor, interleukin-2 (IL-2), in response to a challenge with concanavalin A, but had no known antigen specificity. The normal T cell partner in the fusions was a population of lymph node T cell blasts that had been highly enriched in antigen-specific, H-2-restricted T cells by in vivo immunization, followed by in vitro challenge with antigen and clonal expansion in IL-2-containing medium. These fusions produced hybrids that grew constitutively in culture. A sizable proportion of the hybrids demonstrated the ability to produce IL-2 in response to a challenge with specific antigen presented by irradiated spleen cells of the appropriate H-2 type. Four cloned antigen/H-2-specific hybrid lines were produced. AO-40.10 responded to chicken ovalbumin (OVA) when presented by I-A(k)-bearing cells. DC1.18.3 responded to the apo form of beef cytochrome c when presented with I-A(d). AODK-10.4 responded to keyhole limpet hemocyanin (KLH) presented with I-A (d). AODK-1.16 also responded to KLH presented by a product of the I region of H-2(d), but the data were consistent with either a product of the I-J-I-E(d) region or a combinatorial molecule with elements from both I-A(d) and I-E(d)/I-C(d). Coincidentally, AO-40.10 was shown to have an unexpected alloreactivity with a product of H-2(b) mapping to the K-I-A region. These hybrids should prove invaluable as sources of monoclonal material for the study of the receptor(s) on T cells with H-2-restricted antigen specificities. We also generated T cell hybrids with two antigen/H-2 specificities by fusing an azaguanine-resistant clone of AO-40.10 to normal T cells with a different antigen/H-2 specificity. Many of the hybrids retained reactivity to OVA plus H-2(a) and to the second antigen/H-2 combination. None reacted to either OVA plus the second H-2 type or to the second antigen plus H-2(a). One of these hybrids was successfully cloned to produce the line AOFK- 11.11.1. It retained the ability to recognize OVA plus I-A(k) inherited from one parent, and KLH plus IA(f) inherited from the other. It did not recognize OVA plus IA(f) or KLH plus I-A(k). These results have some bearing on models describing the nature of T cell receptors for antigen recognized in association with H-2 products. They do not support models in which antigen and H-2 are recognized separately by two independent T cell receptors.

T cells respond preferentially to antigens that are similar to self H-2

We have constructed bone marrow irradiation chimeras to investigate the influence of self antigens on the specificity of the T lymphocyte receptor repertoire. Bone marrow cells from (A X B)F1 mice heterozygous for the major histocompatibility genes were allowed to mature into T cells in irradiated parent A or parent B strains. More than 8 weeks after irradiation, when the lymphoid system had regenerated from the F1 stem cells, the degree of T cell reactivity to mutant major histocompatibility antigens, A', was assessed. It was found that T cells that had matured in the irradiated A mice, [F1 leads to A] chimeras, responded better to A' antigen than did T cells from the [F1 leads to B] chimeras. Because the mutant histocompatibility antigen A' is very similar in structure to A, differing only by one or a few residues, this suggests that the T cell repertoire in [F1 leads to parent] chimeras reacts preferentially with foreign antigens that are slight variants of the self antigens expressed on radiation-resistant cells--probably cells in the thymus.

Specificity of cytotoxic T cells from athymic mice

In normal mice, self-H-2 antigens in the thymus have a profound influence on T cell specificity. We have therefore investigated the properties of cytotoxic T lymphocyte (CTL) precursors from athymic nude mice (5) with the notion that they may provide a model system for the study of T cells whose receptro specificity is closer to the germ-line-encoded repertoire. It was found that the precursors of nude CTL are, themselves, THy-1+ cells. The possibility that these nude t cells were derived from the phenotypically normal mother by placental transfer was ruled out. In the presence of T cell growth factor, nude CTL can be induced by polyclonal activation with concanavalin A or by stimulation with allogeneic or trinitrophenyl (TNP)-modified syngeneic stimulator cells, but not by stimulation with minor H antigens in the context of self-H-2. Alloreactive, nude CTL--like those from normal mice--recognize H-2K- and H-2D-region-encoded antigens in killer-target cell interactions, but, unlike normal CTL, did not cross-react on third-party target cells. Whereas the anti-TNP response of nude mice is H-2 restricted, it does not seem to be influenced by self-H-2 antigens in the same manner as in normal mice. This is suggested by the finding that the immunodominance of H-2k over H-2d in the anti-TNP-self response of normal (H-2d X H-2b)F1 mice is absent in (H-2d X H-2k)F1 nude mice. These observations are discussed in relation to the role of the thymus in the generation of the normal mature T cell receptor repertoire.