Selective expression of class II MHC isotypes by MLC-activated human T lymphocytes

Although activated human T cells express class II MHC molecules, the biologic significance of this event is not understood. Using two-color flow cytometry, we have analyzed the expression of HLA-DR, -DQ, and -DP isotypes by T cells following activation by allogeneic lymphoblastoid B-cell lines. Within the CD3+ population, transient expression was observed at 1 day following initiation of culture, which preceded a dramatic and sustained increase around 6-7 days. DR expression was always highest, followed by DP and DQ with DP expression usually somewhat higher than DQ. At day 8, three populations were observed consisting of DR+DP+DQ+ (60%), DR+DP+ (69%), and DR+ (75%) T cells. Interestingly, DQ+ or DP+ but DR- T cells were not observed. These patterns of class II isotype expression were similar in CD2+, CD4+, and CD8+ subgroups and suggest that class II molecules are selectively expressed on T cells and may play a role in the regulation of T-cell responses to alloantigens.     

Equine T lymphocytes express MHC class II antigens

Six anti-HLA class II mouse monoclonal antibodies (mAbs) were used in conjunction with a rat monoclonal antibody raised against horse lymphocytes to define class II major histocompatibility complex (MHC) molecules in the horse. By utilizing an ELISA assay and complement dependent lymphocytotoxicity assay, five out of the six anti-HLA class II antibodies and the rat anti-horse monoclonal antibody were found to react with a high percentage of peripheral blood lymphocytes. Flow cytometry demonstrated a variable antigen density on peripheral blood lymphocytes and clear evidence for expression by lymphocytes that carried no detectable surface immunoglobulin. None of the antibodies reacted with equine platelets. The mAbs immunoprecipitated an antigenic complex of Mr 29,000-33,000 from horse lymphocytes. It appears that the distribution of MHC class II antigens in the horse is different from that in man but is similar to that in the dog, since MHC class II antigens are expressed on resting peripheral blood lymphocytes which lack membrane-bound immunoglobulins. Correlations between the distribution of MHC class II antigens on lymphocyte subpopulations and their role in immunological phenomena may contribute to our understanding of the functional properties of these molecules.     

Inhibition of T cell activation by blockade of MHC class II molecules

Autoimmune diseases result from the activation of self-reactive T cells induced by autoantigens or by foreign antigens cross-reactive with an autoantigen. A striking characteristic of autoimmune diseases is the increased frequency of certain HLA alleles in affected individuals. Moreover, as demonstrated for example in rheumatoid arthritis and insulin-dependent diabetes mellitus, class II alleles positively associated with autoimmune diseases share amino acid residues in the hypervariable HLA regions involved in peptide binding. Therefore, it is likely that disease-associated HLA class II molecules have the capacity to bind the autoantigen and present it to T cells, thereby inducing and maintaining, under appropriate conditions, the autoimmune disease. The data reviewed here demonstrate MHC-selective inhibition of antigen-induced T cell responses in vivo by parenterally administered soluble, MHC-binding peptide competitors, under conditions in which the competitor is not immunogenic. This suggests the feasibility of a therapeutic approach based on MHC blockade in the treatment of HLA-linked autoimmune diseases.     

Endogenous antigen presentation by MHC class II molecules

T cell recognition of antigen requires that a complex form between peptides derived from the protein antigen and cell surface glycoproteins encoded by genes within the major histocompatibility complex (MHC). MHC class II molecules present both extracellular (exogenous) and internally synthesized (endogenous) antigens to the CD4 T cell subset of lymphocytes. The mechanisms of endogenous antigen presentation are the subject of this review. Isolation and amino acid sequencing of peptides bound to the class II molecule indicate that a very high proportion (70–90%) of the total peptides presented by the class II molecule are in fact derived from the pool of proteins that are synthetized within the antigen-presenting cell (APC). This type of sequence information as well as the study of model antigens has indicated that proteins expressed in a diversity of intracellular sites, including the cell surface, endoplasmic reticulum and cytosol can gain access to the class II molecule, albeit with different efficiencies. The main questions that remain to be answered are the intracellular trafficking patterns that allow colocalization of internally synthesized antigens with the class II molecule, the site(s) within the cell where peptide: class II molecule complex formation can take place and whether presentation of ‘foreign’ as well as ‘self’ antigens takes place by mechanisms that vary from one cell type to another or that vary with the metabolic state of the APC. If such variability exists, is would imply that the array of peptides displayed by class II molecules at the cell surface has similar variability, a possibility that would impact on self tolerance and autoimmunity.     

Mutations of class II MHC molecules

It remains unclear how the tertiary interaction of T-cell receptor, la molecule and foreign antigen results in the extensive diversity of the helper T cell repertoire. Here Laurie Glimcher and Irwin Griffith focus on what has been learned about the relationship between structure and function of the la molecule from the use of mouse strains with mutations in the genes coding for these glycoproteins.     

Peptides recognized by class I restricted T cells also bind to MHC class II molecules

The mechanisms of antigen recognition employed by both class I and class II MHC-restricted T cells are very similar, yet many of the T cell determinants described to date are recognized in the context of a single class of MHC molecules, and generally with only one or a very few different MHC alleles. To determine whether this might be due to a structural difference between class I and class II restricted T cell determinants, peptides previously shown to be recognized in the context of MHC class I proteins by mouse or human CD8+ T lymphocytes were tested for their capacity to bind to HLA-DR molecules on the surface of B lymphoblastoid cell lines (B-LCL). Four out of five class I restricted T cell determinants tested bound to a panel of B-LCL, and the binding was inhibited by anti-HLA-DR mAb. The peptides did not bind to the class II-negative B-LCL RJ2.2.5 nor to mouse L cells, but did bind to L cells transfected with HLA-DR1.     

Mixed-haplotype MHC class II molecules select functional CD4+ T cells

MHC class II molecules are formed from polymorphic alpha and beta chains. While pairing of chains is most efficient within class II isotypes and haplotypes, limited pairing and surface expression of mixed-haplotype and -isotype class II molecules is common. The function of such molecules in antigen presentation has been established by the unique restriction of responses in F1 mice. However, it has not been established whether mixed class II molecules are able to mediate selection of functional T cells and how the reduced avidity of the TCR/MHC interaction influences the repertoire. In this report we have addressed these issues through the production of mice expressing solely mixed-haplotype class II molecules. The mixed class II molecules promote selection of a small CD4+ T cell repertoire with modified TCR use. The selected CD4+ T cells are functional in vivo and in vitro.     

Sheep MHC class II molecules. II. Identification and characterization of four distinct subsets of sheep MHC class II molecules

A panel of seven monoclonal antibodies, sequential immunoprecipitation and two-dimensional NEPHGE/SDS-PAGE analyses were used to identify and characterize subsets of sheep MHC class II molecules. Using sequential immunoprecipitation four distinct subsets of class II molecules were identified by the monoclonal antibodies SBU.II 28-1, 37-68, 38-27 and 42-20, while another monoclonal antibody, SBU.II 49-1, recognized all four subsets of class II molecules. These four subpopulations of sheep class II molecules displayed different two-dimensional gel profiles and, using splenocytes from four outbred sheep, the class II molecules recognized by SBU.II 28-1, 37-68 and 42-20 showed structurally detectable allelic polymorphism in their beta polypeptides, but no detectable variation in their alpha polypeptides. In contrast, the class II molecules recognized by SBU.II 38-27 showed allelic variation in both their alpha and beta polypeptides. Two-dimensional (2-D) gel analyses of non-glycosylated class II molecules immunoprecipitated by SBU.II 49-1 suggested that approximately 10-12 different class II molecules were expressed by a single sheep. The results of this study show that sheep express class II molecules that can be divided into four structurally and serologically distinct subsets, and provide additional evidence for the subdivision of the sheep MHC class II genetic region into at least three distinct subregions.     

Intracellular transport of MHC class II molecules.

MHC class II molecules associate, during biosynthesis, with peptides derived from endocytosed antigen. Here, Jacques Neefjes and Hidde Ploegh describe the intracellular transport of MHC class II molecules and its relationship to the binding of peptides in endosomal compartments. They discuss alternative routes for the delivery of antigen to sites at which peptides associate with MHC class II molecules and raise the possibility of cell type-specific differences in the handling of MHC class II molecules, and hence in antigen presentation.     

Targeting of human cytotoxic T lymphocytes to MHC class II-expressing cells by staphylococcal enterotoxins.

The staphylococcal enterotoxins (SE) comprise a family of structurally related phage-encoded bacterial proteins, which are the most potent mitogens known for murine and human T lymphocytes. In this report we describe a novel cytotoxic mechanism, where SE directs human CD3+ T lymphocytes to mediate strong cytotoxicity against target cells of irrelevant nominal specificity. The SE-dependent cellular cytotoxicity (SDCC) occurred at picomolar concentrations of SE and involved the initial binding of the SE to the target cells and subsequent triggering of the cytotoxic T cells. SDCC was induced by SEA, SEB, SEC1 and SED, which indicates that this is a common property conserved among all SE. Certain antibodies to the HLA-DR molecule efficiently blocked SDCC. Major histocompatibility complex (MHC) class II+ RAJI cells and HLA-DR-transfected murine L cells were sensitive to SDCC, whereas the MHC class II- RJ.2.2.5 RAJI cell mutant and untransfected L cells were completely resistant to SDCC. These results demonstrate that the MHC class II antigen is the target molecule in SDCC. HLA-DR molecules acted as receptors for SE and the complex was recognized by T lymphocytes in a polyclonal fashion. SDCC was mediated by allospecific cytotoxic CD8+ T cells, by cloned CD8+ T cells and by fresh human peripheral blood mononuclear cells. The SDCC phenomenon provides a rapid, potent and specific mechanism for elimination of HLA-DR+ target cells. We suggest that SDCC is an important combat strategy, employed by the bacteria to avoid specific MHC class II antigen-dependent immune recognition, by inducing T-cell dependent autologous lysis of MHC class II-expressing cells.     

Spatial correlation between thyroid epithelial cells expressing class II MHC molecules and interferon-gamma-containing lymphocytes in human thyroid autoimmune disease

In this immunohistochemical study we addressed the question whether aberrant class II MHC expression by thyroid epithelial cells (thyrocytes) in established thyroid autoimmune disease is the result of release of interferon-gamma (IFN-gamma) by adjacent lymphocytes. Thyroids from eight cases of Hashimoto's thyroiditis, 13 cases of Graves' disease and 10 cases of focal thyroiditis were studied. Both thyrocytes expressing class II MHC and lymphocytes containing immunoreactive IFN-gamma were found in all 31 autoimmune thyroids. In a serial section study of these thyroids, IFN-gamma-expressing lymphocytes were found within 50 microns of class II MHC-positive thyrocytes in 89% of 282 randomly selected fields. Conversely, class II MHC-positive thyrocytes were found within 50 micron of aggregates of IFN-gamma-positive lymphocytes in 82% of 272 randomly selected fields. These findings support the view that in established thyroid autoimmune disease expression of class II MHC by thyrocytes is the result of local release of IFN-gamma.     

Accessory molecules for MHC class II peptide loading

Accessory molecules, such as HLA-DM and invariant chain, modulate the ligands bound to MHC class II molecules in antigen-presenting cells. Recent investigations, including gene targeting experiments, have shed light on the functions of these molecules, their mechanisms of action, interactions with class II molecules, and the relationships with associated molecules such as tetraspanins and HLA-DO.     

Multiple proteases process viral antigens for presentation by MHC class I molecules to CD8(+) T lymphocytes

Recognition by CD8(+) cytotoxic T lymphocytes of any intracellular viral protein requires its initial cytosolic proteolytic processing, the translocation of processed peptides to the endoplasmic reticulum via the transporters associated with antigen processing, and their binding to nascent major histocompatibility complex (MHC) class I molecules that then present the antigenic peptides at the infected cell surface. From initial assumptions that the multicatalytic and ubiquitous proteasome is the only protease capable of fully generating peptide ligands for MHC class I molecules, the last few years have seen the identification of a number of alternative proteases that contribute to endogenous antigen processing. Trimming by non-proteasomal proteases of precursor peptides produced by proteasomes is now a well-established fact. In addition, proteases that can process antigens in a fully proteasome-independent fashion have also been identified. The final level of presentation of many viral epitopes is probably the result of interplay between different proteolytic activities. This expands the number of tissues and physiological and pathological situations compatible with antigen presentation, as well as the universe of pathogen-derived sequences available for recognition by CD8(+) T lymphocytes.     

Cellular studies on antigen presentation by class II MHC molecules.

Particularly prominent during the past year was the analysis of the subcellular compartment in which MHC class II molecules are located. Some investigators also analyzed the site where peptides are generated for MHC class II binding. Studies of invariant chain were particularly important in trying to establish the functional significance of this molecule.     

Presentation of cytosolic antigens via MHC class II molecules

Major histocompatibility (MHC) class II molecules function to present antigenic peptides to CD4 T lymphocytes. The pathways by which these molecules present exogenous antigens have been extensively studied. However by contrast, far less is known about the processing and trafficking of cytosolic antigens, which can also serve as an alternative source of ligands for MHC class II molecules. Self-proteins, tumor antigens, as well as viral proteins found within the cytosol of cells, can be presented via MHC class II molecules, resulting in the activation of specific CD4 T cells. Studies have begun to reveal unique steps as well as some similarities in the pathways for cytosolic and exogenous antigen presentation. Recent developments in this area are summarized here.     

CD40 associates with the MHC class II molecules on human B cells

The MHC class II and CD40 molecules are two major components of the immune system that are involved in cell-cell interactions and signal transduction. Data obtained in the course of the present investigation show that these two molecules are physically associated on the surface of various human B cell lines and on normal tonsilar B cells. The CD40 / MHC class II complexes were not detected on the germinal center B cell line Ramos. However, stimulation of these cells via CD40 or MHC class II triggered their association, suggesting that the formation of the complex is related to the activation status of the cells. The formation of these complexes did not alter the interaction of MHC class II molecules with one of their natural ligands, the staphylococcal enterotoxin A (SEA), as evidenced by the ability of SEA to bind MHC class II / CD40 complexes. Cross-linking of MHC class II or CD40 molecules leads to the association as well as the co-association of both molecules to the NP-49-insoluble cellular matrix. Such association allowed us to demonstrate that only a fraction of these molecules can be physically associated on the cell surface. Based on previous observations and those presented here, it is highly possible that the CD40 / MHC class II complexes may have an important role in signal(s) induced via both molecules and during T / B cells interactions.