Analysis of thymocyte MHC specificity with thymocyte hybridomas

Interface-disrupting amino acids establish specificity between T cell receptors and complexes of major histocompatibility complex and peptide

T cell receptors (TCRs) bind complexes of cognate major histocompatibility complex (MHC) and peptide at relatively low affinities (1-200 microM). Nevertheless, TCR-MHC-peptide interactions are usually specific for the peptide and the allele encoding the MHC. Here we show that to escape thymocyte negative selection, TCRs must interact with many of the side chains of MHC-peptide complexes as 'hot spots' for TCR binding. Moreover, even when the 'parental' side chain did not contribute binding affinity, some MHC-peptide residues contributed to TCR specificity, as amino acid substitutions substantially reduced binding affinity. The presence of such 'interface-disruptive' side chains helps to explain how TCRs generate specificity at low-affinity interfaces and why TCRs often 'accommodate' a subset of amino acids at a given MHC-peptide position.

Characterization of a CD4-positive T-cell line derived from an athymic (nu/nu) mouse

We have isolated a Thy-1+, CD3+, CD4+ T-cell line from the spleen of a 12-week-old nu/nu (nude) BALB/c mouse. The cell line is clonal, and it expresses an alpha beta T-cell antigen receptor. Upon activation, these cells secrete IL-2 but not IL-4, putting them in the Th1 category. The cells can be triggered to proliferate and secrete lymphokines in the presence of irradiated syngeneic or allogeneic splenic feeder cells that express a variety of MHC haplotypes. This response is MHC class II-specific, because it can be blocked by either anti-Ia or anti-CD4 antibodies. From the response pattern of this T-cell line, we conclude that it recognizes a common determinant on class II MHC antigens. This nude mouse T-lymphocyte presumably has not undergone thymic selection. Therefore its unique specificity may reflect both the bias of T-cell antigen receptor genes for encoding receptors that recognize MHC molecules and the requirement for functional thymic epithelial cells for the efficient education of a self-MHC-restricted repertoire.

Selection of T cell receptor expression mutants through the functionally linked Ly-6A

Ly-6A is a glycosyl-phosphatidylinositol (GPI)-anchored molecule that participates in murine T cell activation. Activation of T cell hybridomas with anti-Ly-6A monoclonal antibody (mAb) leads to production of interleukin-2 (IL-2), but also to a paradoxical growth inhibition, which was used to select for signaling mutants. Fifteen subclones derived from two independent mutageneses and anti-Ly-6A selection were characterized. Thirteen subclones responded poorly or not at all to soluble anti-Ly-6A mAb. Although the selective pressure was exerted through Ly-6A, only one mutant did not express the Ly-6A antigen. Interestingly, 10 of the 15 subclones expressed either nondetectable or a very low level of T cell receptor/CD3 complex (TCR/CD3). Preferential expansion of TCR/CD3 expression mutants following anti-Ly-6A selection further established functional linkage between Ly-6A and TCR/CD3 complex. The mechanism of the functional coupling was investigated by analyzing the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), one of the early events in T cell activation. We showed that PIP2 was not hydrolyzed in response to anti-Ly-6A in TCR/CD3-negative mutants. Aluminum fluoride, which activates G protein directly, did induce PIP2 hydrolysis in these cells. These data suggest that activation signals originated from Ly-6A must be transmitted first to TCR/CD3 complex, which then couples to the G protein/phospholipase C system. A similar requirement also applies to the Thy-1 protein and lectin receptors. Thus, the TCR/CD3 complex plays a central role in the integration and transmission of activation signals that originated from several T cell surface molecules.

Tolerization during pregnancy: impact on the development of antigen-specific help and suppression

Analysis of splenic B cells in the offspring of BALB/c mice which were treated with 2,4,6-trinitrobenzenesulfonic acid (TNBS) throughout the gestational period revealed that reduced responsiveness was predominantly reflecting changes in regulatory cells. Since development of T cells takes place in the thymus, postnatal maturation of helper (Th) and suppressor (Ts) T cells was evaluated in the thymus as well as in the spleen. To elaborate the impact of prenatal TNBS-treatment on the trinitrophenyl (TNP) repertoire, TNP-sheep red blood cell- and fluorescein isothiocyanate-specific regulatory elements were determined in parallel. It was found that the thymus contained activated helper and suppressor cells during the first 2 weeks of postnatal life, the majority of which appeared to be self-specific. Self reactivity in thymus and periphery disappeared within 3 weeks. Prenatally TNBS-treated mice differed inasmuch as they apparently recognized TNP-self as a self component, TNP-self reactivity being down-regulated similar to self reactivity in prenatally untreated mice. This explains why prenatally TNBS-treated mice responded poorly to stimulation with conventional T-dependent TNP-carrier conjugates. Yet, they responded transiently to TNP-anti-TNP conjugates. It could be shown that this was due to down-regulation of putatively idiotype-specific Ts, which were highly increased in prenatally TNBS-treated mice. Data are interpreted in the sense that the establishment of self tolerance is preceded by activation of self-reactive Th, which become partially eliminated. Remaining autoreactivity is controlled by the establishment on an equilibrium between idiotypic and anti-idiotypic elements.

Murine hybrid cell lines expressing the NLDC-145 dendritic cell determinant

Dendritic cells play an important role as accessory cells in the immunological defense system. Investigations into the biological functions of this population have been limited by the difficulties encountered in isolating sufficient numbers of cells, and therefore different approaches have been described to immortalize accessory cell populations. In this report, four murine hybrid cell lines (H1-H4) are described that were produced by fusion of in vitro isolated dendritic cells and a tumor cell. The hybrid lines are characterized by the expression of the dendritic cell specific marker NLDC-145, Ia expression and the absence of macrophage or T cell markers. The cell lines have no phagocytic activity and are functionally active as accessory cells. The expression of the NLDC-145 antigen in vitro was found to be inducible. These NLDC-145-positive cell lines will be useful for the further characterization of the NLDC-145 antigen and in the study of dendritic cell and lymphocyte interactions.

Con A-stimulated thymocytes produce interleukin 2 after removal of Lyt-1 positive cells

Mouse lymphocytes produce several lymphokines, including interleukin 2 (IL-2) and colony-stimulating factors (CSFs) following stimulation with T-cell mitogens. However, very little IL-2 is produced by thymocytes upon concanavalin A (Con A) stimulation. Strong selective inhibition of IL-2 production was observed when fresh spleen cells were mixed with Con A-activated thymocytes. Sorting of populations on the basis of antigenic phenotype showed that the cell mediating the blockage in IL-2 secretion is a large T cell expressing markers for both Lyt-1 and Lyt-2. This specific inhibition of IL-2 accumulation was not mediated by a soluble product, or by absorption on expressed IL-2 receptors on the activated thymocytes. Removal of the Lyt-1 positive cells from a thymocyte population renders it capable to produce IL-2 upon Con A stimulation, indicating a functional role of these cells.

Functional differentiation and repertoire diversification of T cells derived from single progenitor cells

Functions of T cells derived from single progenitor cells were investigated. B10. Thy-1.1 recipient mice were either whole body-irradiated and marrow reconstituted or thymus-shielded, irradiated and marrow reconstituted, and limited numbers (3 x 10(3) or 6 x 10(3] of a 1:1 mixture of bone marrow cells from C57BL/6 and B6.Lyt-2.1 mice were transferred intrathymically (i.t.). Donor-type (Thy-1.2+) cells of the thymus of a small portion of recipients which expressed the phenotype of either Ly-2.2 or Ly-2.1 but not both were regarded to be a clone of T cells derived from a single progenitor cell, and such clones were assayed for polyclonal helper (Th) and polyclonal cytolytic (CTL) activities as well as alloantigen-specific proliferative (mixed lymphocyte reaction; MLR) and CTL activities. Clones taken 4 weeks after transfer (4-week-old clones) which were generated in the thymus of whole body-irradiated recipients showed polyclonal CTL but not polyclonal Th activity, whereas 4-week-old clones generated in the thymus of thymus-shielded recipients showed both polyclonal CTL and Th activities. Similarly, 4-week-old clones generated in whole body-irradiated recipients responded with CTL to alloantigens when induced in the presence of T cell growth factors but not with MLR, whereas 4-week-old clones generated in thymus-shielded recipients showed both MLR and CTL to alloantigens. Repertoire diversification of 4-week-old clones, however, was incomplete, since clones generated in whole body-irradiated recipient did not necessarily respond with CTL to all alloantigens examined, and those generated in thymus-shielded recipients did not necessarily respond with MLR to all the antigens. On the other hand, T cell clones were shown to fully mature by 7 weeks after transfer in terms of cell function as well as repertoire diversification.

'Autoreactivity' of some hybridomas may be caused by a three-cell interaction

We have produced hybridomas by the fusion of BALB/c (H-2d) bone marrow cells, bone marrow cells from BALB/c-nu/nu mice, BALB/c fetal liver cells, and BALB/c fetal thymocytes with the AKR (H-2k) thymoma BW5147. The hybridomas were selected for the expression of the Thy-1.2 antigen of the normal cell donor and for their ability to produce IL-2 upon co-culture with irradiated normal spleen cells. The hybridomas produce IL-2 when co-cultured with H-2k, H-2u, H-2j, or H-2v cells of some strains but not in mixtures with H-2p, H-2s, H-2f, H-2b, H-2q, or H-2d cells. An investigation into the nature of these differences revealed a novel type of interaction between hybridomas, mature T lymphocytes and class II-positive spleen or lymph node cells. The experiments described in this communication suggest that irradiated L3T4+, Ly-1 High T cells recognize syngeneic class II-positive spleen or lymph node cells, but only in some strains. The ability to recognize the syngeneic cells depends both upon the H-2 complex and on the non-H-2 genes. The recognition leads, in the absence of proliferation, to the secretion of an as yet unidentified and largely hypothetical factor which then acts on the hybridoma cells. Upon stimulation with the T lymphocyte-derived factor, the hybridoma cells begin to secrete IL-2, which can then be measured by the proliferation of the IL-2-dependent CTLL line. The IL-2 production by the hybridoma cells is independent of their proliferation. The described interaction apparently does not involve the T-cell receptor of the hybridoma cells. The interaction resembles the autoreactivity of thymocyte hybridomas described by other investigators, and therefore it is possible that this 'autoreactivity' may in fact be generated by a similar mechanism to the one described here.

Cyclosporin A and the thymus. Immunopathology

Cyclosporin A (CsA) is known to diminish the size of the thymus, especially the thymic medulla. The significance of these changes is not presently understood. This study reveals several immunopathologic changes induced in the thymic medulla by CsA (15 mg/kg/day). The weight and relative size of the thymus dramatically and rapidly involutes, with marked changes observed in 1 week. The medullary thymocytes show segregation of rat T-cell phenotypes as seen in control rats, but the number of such cells is markedly reduced in accordance with the medullary remnant. This is consistent with a maturational arrest of thymocytes. The medullary epithelium was assessed directly by irradiating the control or CsA-treated rats 2 days prior to sacrifice. The epithelium of Hassall's corpuscles was essentially absent in CsA-treated rats but prominent in control rats. The cortical epithelial cells were preserved. Stains for Ia antigen with the anti-OX4 antibody show little change in expression by cortical epithelium, but a marked reduction in the Ia+ medullary cells in the thymocyte purged rats. All of these changes were reversible in the normal rat after cessation of CsA, with near normal recovery in 3 weeks. No morphologic or immunopathologic changes were noted in the cortical thymocytes. These cells did, however, acquire CsA receptors, as detected by the binding of fluorescent dansylated CsA.

Alteration of the T cell self-specificity repertoire by treatment with anti-Ia antibody during embryonic life

The effect of anti-Ia antibody on the development of murine neonatal self-Ia-reactive T lymphocytes in the thymus was studied. A C57BL/6 (B6) mouse pregnant with (B6 X C3H/He)F1 (B6C3F1) embryos was injected with monoclonal anti-I-Ek (m alpha I-Ek) antibody every other day starting from the 12th day of gestation. The mixed leukocyte culture reaction (MLR) responses of thymocytes from m alpha I-Ek-treated neonatal mice were assayed against adult B6 (H-2b), C3H/H3 (H-2k) or BALB/c (H-2d) splenic stimulator cells. The syngeneic MLR response of m alpha I-Ek-treated B6C3F1 neonatal thymocytes against C3H stimulator cells was augmented compared to that of untreated neonatal thymocytes. Experiments utilizing recombinant inbred strains of mice as well as monoclonal anti-Ia antibody blocking experiments suggested that the antigen(s) responsible for this augmentation of syngeneic MLR resided in the I-Ek molecule. The MLR response against B6 or BALB/c stimulator cells was not influenced by m alpha I-Ek treatment. We speculate that m alpha I-Ek antibody blocked the expression of I-Ek molecules in B6C3F1 embryos and this resulted in the alteration of T cell self-specificity repertoire in the thymus. Our results support the concept that Ia molecules play important roles for the elimination of self-reactive T lymphocyte clones in the thymus.

Cotransfer of the Ed alpha and Ad beta genes into L cells results in the surface expression of a functional mixed-isotype Ia molecule

Ia molecules play a key role in antigen recognition by T lymphocytes. To analyze the structural features of the individual alpha and beta chains relevant to the assembly of intact Ia molecules, mouse fibroblasts were cotransfected with various combinations of haplotype- and isotype-mismatched Ia alpha/beta gene pairs. Two important points emerged. First, the level of surface expression of a given haplotype-mismatched A alpha A beta pair appears to depend upon the alpha and beta chain alleles involved. Second, transfection with some isotype-mismatched combinations such as Ed alpha Ad beta results in a significant level of surface expression of a stable mixed-isotype dimer, which also appears to be normally expressed at a low level by an Iad-positive B lymphoma. Moreover, a T-cell hybridoma specific for human gamma globulin and restricted by the Ed molecule was found to be efficiently stimulated by the Ed alpha Ad beta-positive transfectant in the presence of antigen. The stimulation was specifically inhibited by monoclonal antibodies directed to either the Ia or the L3T4 molecule. These findings suggest that the estimates of the potential number of Ia molecules available in an animal for restricting T-lymphocyte recognition of antigens must be revised.

Proliferative responses of T cells from SJL----F1 and F1----SJL bone marrow chimeras to SJL lymphoma cells

RCS tumor cells induce marked proliferation of syngeneic SJL T cells in vivo and in vitro. Certain F1 hybrids of SJL mice give high proliferative responses to gamma-RCS, while other F1 hybrids give low responses. SJL----"non-responder" F1 and "non-responder" F1----SJL semiallogeneic bone marrow chimeras were prepared to study how the host environment affects the ability of T cells to give a proliferative response to gamma-RCS. The results indicate that T cells educated in an SJL host become responsive to RCS cells, while T cells educated in an (SJL X BALB/c)F1 host become unresponsive. This finding applies to both thymus and lymph node T cells. The unresponsiveness in F1 mice is not due to suppressor cells, since added F1 cells do not affect the proliferative response of SJL cells to gamma-RCS. Instead, it appears that RCS-specific T cells are either deleted in (SJL X BALB/c)F1 mice, or expanded in SJL mice as they develop. These findings are discussed in relation to the specificity of the responding T cells, for LPS activated syngeneic B cell blasts as well as RCS cells, and to the presence of a "leaky" thymus barrier in SJL mice for B cells.

A role for self-recognizing T cells in the regulation of antibody-forming and cell-mediated cytotoxic responses in the mouse

Mitogen- or antigen-activated murine spleen and thymus cell blasts have been used as inducer cells to stimulate production of suppressor T cells (Ts) in autologous mixed leucocyte culture. Ts cells were induced in an MHC-restricted fashion and produced suppression of cell-mediated lympholysis and antibody synthesis from fresh spleen cells sharing MHC haplotypes with the Ts pool. Ts were Thy 1+, Lyt 1+ cells. Induction of Ts depended upon expression of class II MHC antigens on stimulating cells, but the level of Ts induced by different populations of suppressor inducer cells was not correlated with the level of Ia antigen expression. Activity of suppressor inducer cells was correlated with the functional activity of the inducer pool. The data are interpreted to favour a model in which Ts are induced by associative recognition on stimulator cells of class II MHC antigens and lymphocyte recognition receptors as nominal antigen (i.e. Ts are auto anti-idiotypic in nature). Using as stimulator/responder cells in AMLR lymphocytes taken from normal (8-week) or aged (greater than 24-month) mice, data were obtained consistent with this model and with earlier evidence for an altered expression of both lymphocyte recognition receptors and/or class II antigens in senescence.

Role of the Ly 1 antigen in interleukin 1-induced thymocyte activation

Interleukin 1 (IL 1), but not IL 2-induced thymocyte proliferation was augmented by monoclonal antibodies (mAb) against the Ly 1 antigen. The anti-Ly 1 mAb could also co-stimulate thymocytes with phytohemagglutinin (PHA) alone, mimicking the effect of IL 1. Moreover, overnight culture with PHA of thymocytes, but not of spleen T lymphocytes led to selectively enhanced expression of the Ly 1 antigen on the cell surface. Similar data were generated at the clonal level. Thus, a thymocyte hybridoma (22A6) could be stimulated to release IL 2 in response to PHA plus IL 1, but also to PHA plus anti-Ly 1 mAb. In addition, 22A6 exhibited enhanced cell surface expression of the Ly 1 antigen after overnight culture with PHA. These data suggest a critical role for the Ly 1 antigen in thymocyte proliferation, perhaps by serving as an IL 1-receptor.

Expression of genes of the T-cell antigen receptor complex in precursor thymocytes

The antigen receptor on T lymphocytes has recently been characterized as a heterodimeric, transmembrane glycoprotein consisting of disulphide-linked alpha (acidic) and beta (basic) subunits of relative molecular mass (Mr) 40,000-45,000 each. The genes encoding these proteins have been cloned and shown to resemble immunoglobulin genes in both overall structure and the requirement for DNA rearrangement before expression. In humans, three additional proteins, termed the T3 complex, are found associated with the clonotypic receptor, and a role for T3 in receptor expression has been proposed. Despite these recent advances in characterizing the antigen receptor complex, there is as yet little understanding of T-cell maturation, particularly the stage of T-cell ontogeny at which the genes encoding the antigen receptor and its associated structures are expressed and assembled. In the adult, stem cells destined to differentiate into T cells arise in the bone marrow and migrate to the thymus, where T-cell precursors proliferate, develop a preference for recognizing antigens in the context of self MHC molecules and are released to the periphery. Recently, cells that have the properties of immature murine thymocytes have been isolated and described. We have now analysed these cells with a series of molecular probes and we describe three distinct patterns of T-cell antigen receptor gene rearrangements in developing thymocytes.