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B Kyewski

Publications and source records attributed to B Kyewski.

At least 19 recordsLinked to original sources
PubMed2001

Sampling of complementing self-antigen pools by thymic stromal cells maximizes the scope of central T cell tolerance.

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity, yet the identity of cells involved remains elusive. Here we show that antigen expression in a minor fraction of medullary thymic epithelial cells leads to deletion of specific CD4 T cells. Strikingly, this deletion is not dependent on cross-presentation by hemopoietic antigen-presenting cells, which have been ascribed a predominant role in negative selection. By contrast, when the same antigen enters the thymus via the blood stream, negative selection is strictly dependent on antigen presentation by hemopoietic cells. These findings imply that the (re)-presentation of "self" by thymic stromal cells is non-redundant, and that different thymic antigen-presenting cells instead cover complementing sets of self-antigens, thus maximizing the scope of central tolerance

Animals↗
PubMed2001

Promiscuous gene expression in medullary thymic epithelial cells mirrors the peripheral self.

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity. Here we identified medullary thymic epithelial cells as being a unique cell type that expresses a diverse range of tissue-specific antigens. We found that this promiscuous gene expression was a cell-autonomous property of medullary epithelial cells and was maintained during the entire period of thymic T cell output. It may facilitate tolerance induction to self-antigens that would otherwise be temporally or spatially secluded from the immune system. However, the array of promiscuously expressed self-antigens appeared random rather than selected and was not confined to secluded self-antigens.

Animals↗
PubMed2000

"Promiscuous" expression of tissue antigens in the thymus: a key to T-cell tolerance and autoimmunity?

Induction and maintenance of self-tolerance in the developing and mature T cell repertoire is mediated by multiple mechanisms operating both in the thymus ("central tolerance") and in peripheral lymphoid and nonlymphoid organs ("peripheral tolerance"). The thymus is viewed as the prime site of T cell tolerance induction to ubiquitous proteins and abundant blood-borne antigens entering the thymus via the circulation. By contrast, tolerance to self-antigens that are confined to specific tissues has been ascribed to a variety of mechanisms acting on peripheral T cells. Based on the recent finding that intrathymic expression of "tissue-specific" antigens is a common occurrence the prevailing notion that tolerance induction in the thymus applies only to a limited set of "abundant" proteins may have to be revised. Interestingly, this "promiscuous" expression of tissue antigens in the thymus appears to be a unique property of thymic epithelial cells rather than bone marrow derived antigen-presenting cells, implying cell type specific regulation rather than basal leakiness as a mechanism of "promiscuous" gene transcription. We summarize recent experimental evidence supporting this novel concept and discuss implications for autoimmunity.

Amino Acid Sequence↗
PubMed2000

Self-antigen presentation by thymic stromal cells: a subtle division of labor.

Self-antigen-MHC complexes expressed by thymic stromal cells serve as ligands for TCR-mediated positive and negative selection, resulting in a self-MHC-restricted, self-tolerant T cell repertoire. It has recently become apparent that thymic stromal cells differ in their accessibility to antigen as well as their ability to process and present antigen. These differences result in the sampling by thymic stromal cells of largely nonoverlapping self-antigen pools and the display of self-peptide profiles specific for each cell type. In conjunction with single or serial cell-cell interactions between thymocytes and stromal cells, such differences in self-antigen display allow for maximal (re)presentation of 'self' in the thymus and optimize the efficacy of positive and negative selection.

Acute-Phase Proteins↗
PubMed2000

Shaping of the autoreactive T-cell repertoire by a splice variant of self protein expressed in thymic epithelial cells.

Intrathymic expression of tissue-specific self antigens may be involved in immunological tolerance and protection from autoimmune disease. We have analyzed the role of T-cell tolerance to proteolipid protein (PLP), the main protein of the myelin sheath, in susceptibility to experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. Intrathymic expression of PLP was largely restricted to the shorter splice variant, DM20. Expression of DM20 by thymic epithelium was sufficient to confer T-cell tolerance to all epitopes of PLP in EAE-resistant C57BL/6 mice. In contrast, the major T-cell epitope in SJL/J mice was only encoded by the central nervous system-specific exon of PLP, but not by thymic DM20. Thus, lack of tolerance to this epitope offers an explanation for the exquisite susceptibility of SJL/J mice to EAE. As PLP expression in the human thymus is also restricted to the DM20 isoform, these findings have implications for selection of the autoimmune T-cell repertoire in multiple sclerosis.

Alternative Splicing↗
PubMed1998

CD4 T cell tolerance to human C-reactive protein, an inducible serum protein, is mediated by medullary thymic epithelium.

Inducible serum proteins whose concentrations oscillate between nontolerogenic and tolerogenic levels pose a particular challenge to the maintenance of self-tolerance. Temporal restrictions of intrathymic antigen supply should prevent continuous central tolerization of T cells, in analogy to the spatial limitation imposed by tissue-restricted antigen expression. Major acute-phase proteins such as human C-reactive protein (hCRP) are typical examples for such inducible self-antigens. The circulating concentration of hCRP, which is secreted by hepatocytes, is induced up to 1,000-fold during an acute-phase reaction. We have analyzed tolerance to hCRP expressed in transgenic mice under its autologous regulatory regions. Physiological regulation of basal levels (<10(-9) M) and inducibility (>500-fold) are preserved in female transgenics, whereas male transgenics constitutively display induced levels. Surprisingly, crossing of hCRP transgenic mice to two lines of T cell receptor transgenic mice (specific for either a dominant or a subdominant epitope) showed that tolerance is mediated by intrathymic deletion of immature thymocytes, irrespective of widely differing serum levels. In the absence of induction, hCRP expressed by thymic medullary epithelial cells rather than liver-derived hCRP is necessary and sufficient to induce tolerance. Importantly, medullary epithelial cells also express two homologous mouse acute-phase proteins. These results support a physiological role of "ectopic" thymic expression in tolerance induction to acute-phase proteins and possibly other inducible self-antigens and have implications for delineating the relative contributions of central versus peripheral tolerance.

Acute-Phase Proteins↗
PubMed1998

Tolerance and determinant hierarchy.

The overall T cell response to a multideterminant antigen consists of the sum of responses to a limited number of different determinants on the protein. Antigen-presenting cells (APCs) are crucial in delimiting the determinants on the protein to which a response will be mounted. This influence is apparent at two levels. First, the determinants that are generated and displayed by APCs in the thymus are pivotal in shaping the T cell repertoire that will be available for responding to antigen determinants in the periphery. Second, antigen processing affects the selection of determinants that become displayed by the various peripheral APC populations that are involved in inducing and promoting a T cell response. We have studied the effect of the display hierarchy on tolerance induction to individual determinants in transgenic mice expressing different serum levels of hen egg lysozyme. We have also analysed aspects of the processing machinery that contribute to shaping the hierarchy of determinant display on MHC class II molecules: proteolysis and reduction of disulfide bonds.

Amino Acids, Diamino↗
PubMed1997

A filarial cysteine protease inhibitor down-regulates T cell proliferation and enhances interleukin-10 production.

Filarial nematodes are a cause of chronic debilitating diseases in the tropics. A hallmark of filariasis is the marked down-regulation and polarization of host immune responses, yet molecular constituents of parasites causing this state have remained undefined. We describe a 17-kDa antigen (Av17) of the rodent filarial parasite Acanthocheilonema viteae, which shows amino acid homologies to cystatin C, a major cysteine protease inhibitor belonging to family 2 of the cystatin superfamily. Av17 is released by filariae in vitro. Exported molecules of A. viteae worms are shown to markedly suppress mitogen-induced T cell proliferation of mice and jirds. Av17 accounts for 45.5% of this suppressive activity in the murine system. Recombinant Av17 (rAv17), expressed in Escherichia coli, exhibits biological activity as a cysteine protease inhibitor and was used to examine the immunomodulatory effects, rAv17 induces down-regulation of murine T cell responses to mitogens, to T cell receptor cross-linking by anti-CD3 antibodies and to specific antigens, and at the same time up-regulation of interleukin-10. Hence, this filarial cystatin is a likely effector molecule of immunomodulation and a potential target for antifilarial intervention.

Animals↗
PubMed1997

Selection of a broad repertoire of CD4+ T cells in H-2Ma0/0 mice.

According to past reports, H-2Ma0/0 mice express a single major histocompatiblity complex class II molecule, A(b), heavily loaded with a single peptide derived from the invariant chain, CLIP. Despite the highly restricted diversity of the class II:peptide complexes expressed on thymic stromal cells in the mutant animals, a large and diverse population of CD4+ T cells is positively selected. However, two important issues remained unresolved and are addressed here: Just how preponderant is CLIP occupancy of the class II molecules from H-2M0/0 mice? How extensive and functionally competent is the CD4+ population selected in the mutant animals? Our results argue that a single class II:peptide complex can select a very broad, though not complete, repertoire of CD4+ T cells.

Animals↗
PubMed1996

Two genetically separable steps in the differentiation of thymic epithelium.

The development of the thymus depends initially on epithelial-mesenchymal and subsequently on reciprocal lympho-stromal interactions. The genetic steps governing development and differentiation of the thymic microenvironment are unknown. With the use of a targeted disruption of the whn gene, which recapitulates the phenotype of the athymic nude mouse, the WHN transcription factor was shown to be the product of the nude locus. Formation of the thymic epithelial primordium before the entry of lymphocyte progenitors did not require the activity of WHN. However, subsequent differentiation of primitive precursor cells into subcapsular, cortical, and medullary epithelial cells of the postnatal thymus did depend on activity of the whn gene. These results define the first genetically separable steps during thymic epithelial differentiation.

Alleles↗
PubMed1995

Clonal deletion of major histocompatibility complex class I-restricted CD4+CD8+ thymocytes in vitro is independent of the CD95 (APO-1/Fas) ligand.

The CD95 (APO-1/Fas) ligand (CD95L) mediates apoptosis in sensitive target cells, Ca(2+)-independent cytotoxicity of cells from perforin knock-out mice, and peripheral deletion of activated T cells through engagement of its cognate receptor CD95. Double-positive thymocytes show a high constitutive expression of CD95. Therefore, we used a model system and investigated whether negative selection through apoptosis might involve CD95/CD95L. We analyzed whether CD95L may induce antigen-specific deletion of double-positive thymocytes from mice transgenic for a lymphocytic choriomeningitis virus (LCMV)/H2b-specific T cell receptor (TCR). These cells are deleted in vitro upon addition of the LCMV-peptide 33-41 in a major histocompatibility complex-class I-restricted fashion. Deletion was not blocked by soluble mouse and human CD95-Fc receptor decoys. CD95-Fc receptor decoys, however, were effective in blocking apoptosis induced by mouse CD95L-transfected L929 cells in sensitive CD95+ target cells and in thymocytes. These results suggest that TCR-induced deletion of immature thymocytes in vitro is independent of CD95L. Thus, our data argue against a role of CD95L in negative selection of MHC-class I-restricted autoreactive thymocytes.

Animals↗
PubMed1995

Tolerance and immunity to the inducible self antigen C-reactive protein in transgenic mice.

The understanding of immunological tolerance has been greatly aided by the development of transgenic animal models in which expression of a specific T cell receptor (or B cell receptor) and its cognate self antigen is experimentally controlled. In most cases, expression of the self antigen was constitutive and did not allow for variation of its time- and dose-dependent expression pattern, parameters which are known to influence the balance of tolerance versus immunity. We describe a transgenic model in which expression of human C-reactive protein (hCRP), an acute-phase protein, is tightly controlled at basal levels (female mice express around 10(-9) M and male mice 5 x 10(-7) M circulating hCRP) and is highly inducible (induction factor of 25-500). T cells from C57BL/6 mice recognize two epitopes of hCRP termed A (residues 79-95) and B (residues 87-102). Different efficacies of presentation in vitro and in vivo identify epitope A as sub-dominant and epitope B as dominant. T cells of non-induced hCRP transgenic mice are tolerant to the dominant epitope, but reactive to the subdominant epitope. A hCRP-specific IgG antibody response is detectable in transgenic mice, but is weaker than in littermates. Upon induction of hCRP, both T cell epitopes are presented by thymic and splenic antigen-presenting cells (APC) in vivo. Kinetics of presentation by splenic APC closely match serum kinetics of hCRP, whereas presentation in the thymus is considerably prolonged. This model enables epitope-specific T cell tolerance to be studied as a function of time- and dose-dependent expression of the self antigen.

Amino Acid Sequence↗
PubMed1994

Presentation and intercellular transfer of self antigen within the thymic microenvironment: expression of the E alpha peptide-I-Ab complex by isolated thymic stromal cells.

Expression of a self peptide derived from the alpha chain of MHC class II (I-Ed) in association with I-Ab was studied in the murine thymic microenvironment. Previous work using the mAb Y-Ae which specifically recognizes the E alpha-I-Ab complex had reported differential expression between the thymic medulla and the cortex of this peptide-MHC complex: MHC class II-positive stromal cells in the medulla were strongly positive, whereas this complex was barely detectable on cortical epithelial cells (cEpC) in situ. This difference in presentation of an abundant self peptide is intriguing, since the self protein from which this peptide is derived and the presenting MHC molecule are strongly expressed in both compartments. In this report we show by cell surface phenotype and functional assays that isolated cEpC express the E alpha-I-Ab complex at significant although lower levels than medullary dendritic cells (DC), when examined ex vivo. These results support the notion that cEpC and bone marrow-derived stromal cells present a similar set of self peptide-MHC complexes in situ. In addition, we detect intercellular transfer in situ of the E alpha determinant from radioresistant stromal cells to thymic DC, a mechanism which may enhance the efficacy of tolerance induction by spreading self antigens with the thymic microenvironment.

Animals↗
PubMed1994

Programmed cell death is affected in the novel mouse mutant Fused toes (Ft).

We have identified a novel dominant mouse mutant that is characterised by fused toes on the fore limbs and a thymic hyperplasia, in heterozygous animals. Homozygosity of the mutation leads to malformation of the developing brain, lost of the genetic control of left-right asymmetry and to death around day 10 of development. Analysis of both limb development and induction of apoptosis in immature thymocytes in vitro suggest that programmed cell death is affected by the mutation. Since the mutation was caused via a transgene insertion we were able to map it to the D region on mouse chromosome 8. So far, no mutation that affects programmed cell death has been mapped to this chromosome. Thus, this mutation will allow the identification of a novel gene involved in programmed cell death during mammalian development.

Animals↗
PubMed1993

Tolerance induction by clonal deletion of CD4+8+ thymocytes in vitro does not require dedicated antigen-presenting cells.

The cellular requirements of T cell tolerance induction in the thymus by clonal deletion was investigated by using an in vitro assay: thymocytes from mice expressing a transgenic TcR specific for lymphocytic choriomeningitis virus (LCMV) and H-2Db were co-cultured with various H-2b cell types as antigen-presenting cells in the presence of the antigenic LCMV peptide. The results revealed that all cell lines examined including embryonic and transformed fibroblasts, melanoma cells, cortical thymic epithelial cells, lymphomas and neuronal cells induced an antigen dose-dependent deletion of CD4+8+ thymocytes. Similarly, highly enriched accessory cell populations from thymus and spleen (macrophages, dendritic and cortical epithelial cells, i.e. thymic nurse cells) could induce antigen-specific depletion of immature CD4+8+ thymocytes. Depending on the cell type examined micromolar to picomolar concentration of LCMV peptide were required to induce deletion. The effectiveness of deletion by the different cell types did not correlate with their major histocompatibility class I expression level; it was, however, influenced by the presence of ICAM-1 adhesion molecules.

Animals↗
PubMed1992

Taking the thymus to pieces.

Complex in vitro and in vivo techniques are being combined to unlock the remaining secrets of the thymus. In this report from a recent thymus workshop*, Bruno Kyewski and Thomas Hünig describe the genetic manipulations aimed at clarifying the mechanisms of T-cell selection and lineage commitment, and the use of organ culture and immunohistology to identify the thymic microenvironments in which these events take place.

Animals↗