Selective killing of hepatitis B envelope antigen-specific B cells by class I-restricted, exogenous antigen-specific T lymphocytes

Specific B lymphocytes can act as very efficient antigen-presenting cells. They bind antigen with high affinity via their immunoglobulin receptors, process it through the class II major histocompatibility complex (MHC) pathway, and present its fragments to class II-restricted T lymphocytes. In general, exogenous antigens and noninfectious viral particles enter the class II pathway and are selectively associated with class II MHC molecules. The presentation of an exogenous antigen in association with class I molecules has been reported for only a few antigens, including the hepatitis B envelope antigen (HBenvAg). Here we demonstrate that antigen-specific B cells can efficiently deliver HBenvAg to the class I pathway, presenting its fragments to class I-restricted cytotoxic T lymphocytes (CTLs) which kill the specific B cells. This could represent a mechanism of suppression of neutralizing anti-hepatitis B virus (HBV) antibody response, a phenomenon that accompanies the development of the chronic HBV-carrier state.     

Processing of a minimal antigenic peptide alters its interaction with MHC molecules

Before their recognition by T lymphocytes, protein antigens generally require processing by antigen-presenting cells. In a poorly understood series of events, the protein antigen is internalized, transformed and re-expressed on the surface of the antigen-presenting cell in association with gene products of the major histocompatibility complex (MHC). Small peptides derived from the native protein can be recognized in the absence of antigen processing, suggesting that processing involves proteolytic degradation. These peptides are thought to mimic the naturally produced peptide fragment. We describe here a synthetic peptide antigen of this type which does not require processing but which is nevertheless further processed by splenic antigen-presenting cells. Interestingly, this processing event specifically alters the interaction of the peptide with the class II MHC (Ia) molecule, markedly affecting both its potency as an antigen in vitro and its immunogenicity in vivo (IR gene control).     

Specific binding of antigenic peptides to cell-associated MHC class I molecules

T lymphocytes recognize antigen in the form of peptides that associate with specific alleles of class I or class II major histocompatibility (MHC) molecules. By contrast with the clear MHC allele-specific binding of peptides to purified class II molecules purified solubilized class I molecules either bind relatively poorly or show degenerate specificity. Using photo-affinity labelling, we demonstrate here the specific interaction of peptides with cell-associated MHC class I molecules and show that this involves metabolically active processes.     

弱鞅的一类极小值不等式

在弱鞅序列{S_n, n≥1}的背景下, 给出了序列{c_nS_n, n≥1}和{c_ng(S_n), n≥1}的一类极小值不等式。 所得结论推广了已有文献中的相关结果。     

Characterization of a naturally processed MHC class II-restricted T-cell determinant of hen egg lysozyme

Compelling evidence indicates that T cells recognize complexes formed by major histocompatibility complex-encoded molecules and antigenic peptide fragments. This is based largely on the ability of small synthetic peptides to substitute for naturally processed antigen in stimulating T cells. Naturally processed fragments of exogenous antigen are thought to arise by limited proteolytic degradation of native antigen inside acidic compartments of antigen-presenting cells, but until now no physiologically processed antigen has been directly analysed. Here we report the characterization of physiologically processed antigen eluted from mouse class II major histocompatibility complex I-Ed molecules. The antigenic material corresponds to a previously described antigenic determinant of hen egg lysozyme (HEL 107-116) and has a relative molecular mass Mr of about 2,000. HPLC analysis identified at least two or three separate molecular species, suggesting limited, albeit significant, heterogeneity of naturally processed peptides. Finally, under our experimental conditions, it was calculated that a substantial proportion (10-40%) of I-Ed molecules were occupied by these HEL-derived antigenic determinants.     

可逆可对角化矩阵最小多项式的行列式表示法

用同构映射与初等变换研究矩阵的最小多项式问题,提出一种新的求矩阵最小多项式的简便方法.进一步地,对可逆的可对角化矩阵,用行列式建立其最小多项式的表达公式.     

Processing pathways for presentation of cytosolic antigen to MHC class II-restricted T cells.

Antigens presented to CD4+ T cells derive primarily from exogenous proteins that are processed into peptides capable of binding to class II major histocompatibility complex (MHC) molecules in an endocytic compartment. In contrast, antigens presented to CD8+ T cells derive mostly from proteins processed in the cytosol, and peptide loading onto class I MHC molecules in an early exocytic compartment is dependent on a transporter for antigen presentation encoded in the class II MHC region. Endogenous cytosolic antigen can also be presented by class II molecules. Here we show that, unlike class I-restricted recognition of antigen, HLA-DR1-restricted recognition of cytosolic antigen occurs in mutant cells without a transporter for antigen presentation. In contrast, DR1-restricted recognition of a short cytosolic peptide is dependent on such a transporter. Thus helper T-cell epitopes can be generated from cytosolic antigens by several mechanisms, one of which is distinct from the classical class I pathway.     

A shared antigenic determinant between natural killer cells and nervous tissue

Considerable evidence for shared antigenic determinants between nervous elements and lymphocytes has accumulated. It has also been suggested that this cross-recognition may be involved in the pathogenesis of human neurological diseases such as myasthenia gravis and multiple sclerosis. We report here evidence that a marker for natural killer (NK) cells, anti-Leu-7 (HNK-1), specifically binds to components of human and rodent central nervous tissue as well as peripheral nervous tissue, especially to myelin sheaths. In contrast, another NK-cell marker (VEP13) did not react with nervous tissue. Since NK-cell function is impaired in a population of multiple sclerosis patients, the observed cross-reactivity indicates that autosensitization against myelin may simultaneously cause a defect of NK-cell function. Furthermore, the shared antigenic determinant may help to identify a hitherto undefined nervous tissue antigen and simultaneously increase the knowledge about the nature of NK-cell antigens.     

Segregation of MHC class II molecules from MHC class I molecules in the Golgi complex for transport to lysosomal compartments.

Traffic of MHC molecules dictates the source of peptides that are presented to T cells. The intracellular distribution of MHC class I and class II molecules reflects the dichotomy in presentation of antigen from endogenous and exogenous origin, respectively. In human B lymphoblastoid cells, class I molecules are present in compartments constituting the biosynthetic pathway, whereas class II molecules enter structures related to lysosomes during their biosynthesis.     

T-cell recognition of a chimaeric class II/class I MHC molecule and the role of L3T4

In addition to expressing clonally distributed antigen-specific and major histocompatibility complex (MHC)-restricted receptors, T cells also express non-clonally distributed surface molecules that are involved in T-cell function. Among the most intriguing of the latter are L3T4 and Lyt 2, which are expressed on individual T lymphocytes in striking, though not absolute, concordance with their restriction by either class II or class I MHC determinants, and which are thought to contribute to the overall avidity of T-cell interactions by binding to monomorphic determinants on class II and class I MHC molecules, respectively. To examine the ability of T cells to recognize a single class II domain in the absence of the remainder of the Ia molecule, as well as to evaluate the structural basis for the putative interaction of L3T4 with Ia, a recombinant class II/class I murine MHC gene was constructed and introduced into mouse L cells. Here we demonstrate that a subset of class II allospecific cytotoxic T lymphocytes (CTL) can specifically recognize and lyse L-cell transfectants expressing an isolated polymorphic A beta 1 domain, and that anti-L3T4 antibody can block such killing, a result inconsistent with the highly conserved membrane-proximal domains of Ia acting as unique target sites for L3T4 binding.     

Major histocompatibility complex class I molecules internalized via coated pits in T lymphocytes

Endocytosis of the major histocompatibility complex (MHC)-encoded class I and class II molecules has been the subject of recent investigations. Class I molecules, which are key elements in T cell-mediated cytotoxicity, are differentially endocytosed by different cell types. Fibroblasts internalize their class I molecules via uncoated cell surface vesicles and tubular invaginations when these molecules are cross-linked with multivalent ligands. T lymphocytes internalize their class I molecules spontaneously, but B lymphocytes do not internalize them at all. Here we describe a morphological investigation of the mechanism by which class I molecules are endocytosed by T lymphocytes. We show that, unlike fibroblasts, T lymphocytes spontaneously internalize 20-40% of their class I molecules in a process involving coated pits and coated vesicles. Thus, the endocytic pathway of class I molecules in T lymphocytes is similar to those of other more classical cell-surface receptors involved in receptor-mediated endocytosis. In contrast, the same class I molecules remained on the cell surface in B lymphocytes. These data show that class I molecules are differentially regulated in T and B lymphocytes and fibroblasts.     

A novel class II MHC molecule with unusual tissue distribution.

The repertoire of mature class II-restricted T cells is generated through a complex process of selection whereby early T cells confront class II molecules in the thymus, especially on epithelial cells. Expression of class II molecules on such cells is prominent both in the cortex and in the medulla. We have identified a novel class II molecule, H-20, which is expressed only in epithelial cells of the thymic medulla and in B cells. The unusual tissue distribution and the nonpolymorphic nature of H-20 suggest that its function is different from that of classical class II molecules.     

Expression of the T-cell-specific gamma gene is unnecessary in T cells recognizing class II MHC determinants

Subtractive complementary DNA cloning combined with partial protein sequencing has allowed identification of the genes encoding the alpha and beta subunits of T-cell receptors. The subtractive cDNA library prepared from the cytotoxic T lymphocyte (Tc) clone 2C has been found to contain a third type of clone encoding the gamma chain. The gamma gene shares several features with the alpha and beta genes: (1) assembly from gene segments resembling immunoglobulin V, J and C (respectively variable, joining and constant region) DNA segments; (2) rearrangement and expression in T cells and not in B cells; (3) sequences reminiscent of transmembrane and intracytoplasmic regions of integral membrane proteins; (4) a cysteine residue at the position expected for an interchain disulphide bond. The alpha and beta genes are expressed at equivalent levels in both Tc cells and helper T cells (TH). The gamma gene, obtained from 2C, has been found to be expressed in all Tc cells studied. Here we present evidence that strongly suggests that TH cells do not require gamma gene expression.     

Epithelial cells expressing aberrant MHC class II determinants can present antigen to cloned human T cells

The first step in the induction of immune responses, whether humoral or cell mediated, requires the interaction between antigen-presenting cells and T lymphocytes restricted at the major histocompatibility complex (MHC). These cells invariably express MHC class II molecules (HLA-D region in man and Ia in mouse) which are recognized by T cells of the helper/inducer subset in association with antigen fragments. Interestingly, in certain pathological conditions, for example in autoimmune diseases such as thyroiditis and diabetic insulitis, class II molecules may be expressed on epithelial cells that normally do not express them. We speculated that these cells may be able to present their surface autoantigens to T cells, and that this process may be crucial to the induction and maintenance of autoimmunity. A critical test of this hypothesis would be to determine whether epithelial cells bearing MHC class II molecules (class II+ cells) can present antigen to T cells. We report here that class II+ thyroid follicular epithelial cells (thyrocytes) can indeed present viral peptide antigens to cloned human T cells.     

Thy-1 functions as a signal transduction molecule in T lymphocytes and transfected B lymphocytes

Thy-1, a glycoprotein of relative molecular mass 25,000 (25K), is a major constituent of the cell surface of mouse thymocytes, peripheral T cells and neurones. In man, Thy-1 is present on neurones and on a small percentage of thymocytes, but is absent from peripheral T cells. The amino-acid and complementary DNA sequences of Thy-1 indicate that it has a structure similar to an isolated V (variable region) domain of immunoglobulin. Although the function of Thy-1 is unknown, the ability of different anti-Thy-1 monoclonal antibodies to activate murine T cells or induce functional changes in neuronal cells in vitro suggests that Thy-1 is involved in transmembrane signalling. We now show that crosslinking of murine Thy-1 triggers a rapid rise in the cytoplasmic free calcium concentration ([Ca2+]i), not only in murine T cells and Thy-1.2-transfected human T cells, but also in murine B-lymphoma cells transfected with the murine thy-1.2 gene. These results indicate that the generation and transduction of the signal leading to the rise in [Ca2+]i is independent of the T-cell receptor and other T-cell-specific molecules. The preservation of the [Ca2+]i-modulating function of Thy-1 in various lymphoid cells of two species further suggests that the necessary signal either originates in the Thy-1 molecule itself or is generated in concert with a highly conserved molecules(s) associated with Thy-1.     

MHC class II interaction with CD4 mediated by a region analogous to the MHC class I binding site for CD8.

Interactions between major histocompatibility complex (MHC) molecules and the CD4 or CD8 coreceptors have a major role in intrathymic T-cell selection. On mature T cells, each of these two glycoproteins is associated with a class-specific bias in MHC molecule recognition by the T-cell receptor. CD4+ T cells respond to antigen in association with MHC class II molecules and CD8+ T cells respond to antigen in association with MHC class I molecules. Physical interaction between the CD4/MHC class II molecules and CD8/MHC class I molecules has been demonstrated by cell adhesion assay, and a binding site for CD8 on class I has been identified. Here we demonstrate that a region of the MHC class II beta-chain beta 2 domain, structurally analogous to the CD8-binding loop in the MHC class I alpha 3 domain, is critical for function with both mouse and human CD4.