Signal transduction via MHC class II antigens on B lymphocytes

The role of the MHC class II antigens in the activation of resting human B lymphocytes (B-Go) was examined with respect to both early and late events in the activation process. The (Ca2+)i induced by anti-IgM was enhanced in the presence of, or following pre-incubation with, an anti-MHC class II DR antibody (D1.12). Pre-incubation with a sepharose conjugated antibody (Seph.-D1.12) augmented the proliferation of B-Go in response to a sub-optimal concentration of anti-IgM. The 2D PAGE profile of B-Go differed from that of in vivo activated B lymphocytes. The 2D PAGE profile of B-Go activated by Seph.-D1.12 was not identical to the profile of B-Go activated by either anti-IgM or PMA. These data suggest that the activation of B-Go via the class II antigens shares part of the pathway of anti-IgM induced activation but does not follow an identical pathway.     

Intracellular signal transduction mediated by ligation of MHC class I molecules

Abstract: A great deal of knowledge has accumulated regarding signal transduction after ligation of MHC class I (MHC-I) molecules. In recent years focus has been given to delineation of the intracellular signal pathways activated after MHC-I ligation. Activation of tyrosine kinases leading to a rise in the intracellular free calcium concentration ([Ca²⁺]_i) is the major initial event occurring after MHC-I ligation of T cells. Curiously, the MHC-I-induced signaling is not dependent upon the cytoplasmic tail of the MHC-I molecule, suggesting that the MHC-I molecule induces intracellular signaling through association with other membrane-embedded molecules. More distal signaling events after MHC-I ligation includes activation of the Jak/Stat pathway leading to Stat-3 activation, and activation of the PI3-kinase leading to JNK activation and apoptosis. This review will sum up what is currently known about signaling induced by ligation of MHC-L     

Simian virus 40 infection via MHC class I molecules and caveolae

Summary: MHC class I molecules are a necessary component of the cell surface receptor for simian virus 40 (SV40). After binding to class I molecules, SV40 enters cells via a unique endocytic pathway that involves caveolae, rather than clathrin-coated pits. This pathway is dependent on a transmembrane signal that SV40 transmits from the cell surface. Furthermore, it delivers SV40 to the endoplasmic reticulum, rather than to the endosomal/lysosomal compartment, which is the usual target for endocytic traffic. The glycosphingolipid and cholesterol-enriched plasma membrane domains that contain caveolae are also enriched for class I molecules, relative to whole plasma membrane. Nevertheless, although class I molecules bind SV40, they do not enter with SV40, nor do they enter spontaneously into uninfected SV40 host cells. Instead, they are shed from the cell surface by the activity of a metalloprotease. These results imply the existence of a putative secondary receptor for SV40 that might mediate SV40 entry. It is not yet clear whether class I molecules are active in transmitting the SV40 signal. Monoclonal antibodies against class I molecules also induce a signal in the SV40 host cells. However, the antibody-induced signal is mediated by mitogen-activated protein kinase (MAP kinase), whereas the SV40 signal is independent of MAP kinase.     

Signal transduction in lymphocyte activation through crosslinking of HLA class I molecules

The inhibitory effect of anti-HLA class I monoclonal antibodies on lymphocyte proliferation has been well documented. However, recent data suggest that anti-HLA class I monoclonal antibodies can enhance lymphocyte proliferation via both anti-CD3-induced {1,2} and anti-CD2-induced {3} activation pathways. Here we demonstrate that both inhibition and activation can be regulated by the degree of aggregation of HLA class I antigens. Crosslinking of monoclonal antibodies specific for HLA-A, HLA-B, or monomorphic determinants (using anti-IgG2 and/or anti-Ig “second step” monoclonal antibodies) increased the capacity of the anti-HLA class I monoclonal antibodies to inhibit phytohemagglutinin-induced proliferation. However, the cytosolic free calcium concentration was increased in CD4+ cells, CD8+ cells, B cells, and CD16+ cells when anti-HLA class I monoclonal antibodies were crosslinked, suggesting that an activation signal was generated by aggregation of the corresponding antigens. Indeed, inositol 1,4,5-trisphosphate could be detected in peripheral blood lymphocytes following crosslinking of anti-HLA class I monoclonal antibodies. Class I aggregation also induced proliferation of peripheral blood mononuclear cells in the presence of submitogenic doses of phorbol 12-myristate 13-acetate. Strong conditions of crosslinking (monomorphic monoclonal antibody plus both anti-IgG2 and anti-Ig ) induced CD25 expression and responsiveness to recombinant interleukin 2. Our results suggest that aggregation of HLA class I antigens primed cells to become activated in the presence of progression signals including phorbol 12-myristate 13-acetate, recombinant interleukin 2, or anti-CD5 plus anti-CD28 monoclonal antibodies.     

Conformational changes in MHC class I molecules

In this article we review the role of MHC conformation, including peptide-induced MHC conformation, in forming antibody (Ab), T-cell receptor (TCR), and natural killer (NK) cell receptor epitopes. Abs recognize conformational major histocompatibility (MHC) epitopes that often are influenced by the identity of MHC-bound peptide. Diverse TCRs recognize a common docking site on peptide/MHC complexes and directly contact peptide. Human NK cell inhibitory receptors (KIR) appear to recognize limited regions of the HLA α₁ helix. DX9+ KIR specifically focus on HLA-B residues 82 and 83. However, NK cells recognize much broader regions of HLA class I molecules and are sensitive to bound peptides. Thus, several classes of lymphocyte receptors are peptidespecific. Peptide specificity could be the result of direct contact with the receptor, or to conformational shifts in MHC residues that interact with both receptor and bound peptide.     

NK cells, MHC class I molecules and the missing self

This article is based on a lecture presented at the Novartis Prize ceremony at the International Congress of Immunology in July 2001. It gives a personal and historical perspective on the research performed by the author and his colleagues during the development and pursuit of the model of 'missing-self recognition' for natural killer (NK) cells. This model is based on the idea that one important function of NK cells is to detect and eliminate cells because they fail to express normal self markers. Further mechanistic models predicted the existence of inhibitory major histocompatibility complex (MHC) class I specific receptors, later identified by the fellow Novartis laureates contributing in this issue. The article covers the first decade (1980-1990) of research on this concept. It discusses factors contributing to the formulation of a hypothesis, the use of predictions and experimental test models, the importance of international collaborations and reagent exchange, and several other aspects that allowed the progression of this research project. Finally, the perspective of today's knowledge is used to discuss some surprising findings where the missing-self hypothesis made the wrong predictions, or at least failed to make the correct ones.     

MHC class I molecules, structure and function

MHC class I molecules (MHC-I) are cell surface recognition elements expressed on virtually all somatic cells. These molecules sample peptides generated within the cell and signal the cell's physiological state to effector cells of the immune system, both T lymphocytes and natural killer (NK) cells. In addition, molecules structurally related to MHC-I, collectively known as MHC-Ib, are more specialized and, in some cases, interact with more limited subsets of lymphoid cells. Using the recently determined structure of the classical MHC-I molecule, H-2Dd, as a paradigm for structure and function, we review other MHC-I and MHC-Ib molecules, with an emphasis on how the same basic structural fold is employed by classical MHC-I molecules to bind specific peptides and T cell receptors, and is exploited by the MHC-Ib molecules in more stringent molecular interactions. It is instructive that structurally related molecules have evolved to perform a number of unique and distinct functions in immune and non-immune recognition.     

Antigen degradation or presentation by MHC class I molecules via classical and non-classical pathways

Major histocompatibility complex (MHC) class I molecules usually present endogenous peptides at the cell surface. This is the result of a cascade of events involving various dedicated proteins like the peptide transporter associated with antigen processing (TAP) and the ER chaperone tapasin. However, alternative ways for class I peptide loading exist which may be highly relevant in a process called cross-priming. Both pathways are described here in detail. One major difference between these pathways is that the proteases involved in the generation of peptides are different. How proteases and peptidases influence peptide generation and degradation will be discussed. These processes determine the amount of peptides available for TAP translocation and class I binding and ultimately the immune response.     

Peptide loading of nascent MHC class I molecules

A critical molecular interaction during assembly of the major histocompatibility complex (MHC) class I molecules takes place between the heavy chain and the transporter-associated with antigen-processing (TAP) complex. The recent mapping of regions of the heavy chain involved in the binding to TAP suggests a complex molecular interaction essential for the cell surface expression of the MHC class I. The advances made in understanding the TAP-MHC class I interaction are reviewed and discussed here.     

Expression of inhibitory receptors for MHC class I molecules on T cells

Inhibitory receptors (IRs) specific for MHC class I molecules and originally described on natural killer (NK) cells are also expressed on a fraction of peripheral T cells. The presence of these receptors on T cells is poorly understood. In this review, the different antigen specificities described to date for IR+ T cells and the expression pattern of these receptors on T cells are analyzed. This analysis indicates that the population of T cells defined by IR expression is heterogeneous and that different IRs (or families of IRs) may play different roles in T-cell biology.     

NK-cell receptors and recognition of MHC class I molecules

Two families of natural killer (NK)-cell receptors exist that internet with major histocompatibility complex (MHC) molecules, thereby preventing cytotoxic cells from killing target cells. The expanding knowledge of these receptors provided the focus for a recent meeting¹.     

Functions of nonclassical MHC and non-MHC-encoded class I molecules

Fascinating recent discoveries have focused attention on the nonclassical class I molecules. They can exert their function at most levels of the immune response, being part of both innate and adaptive immune systems. They not only have specialized antigen-presentation functions but also play important immunoregulatory roles: HLA-E regulates natural killer cells by interacting with CD94/NKG2 receptors; the MIC (MHC class I chain related) glycoproteins appear crucial to the activation of gammadelta T cells in the gastrointestinal epithelium; HLA-G may play a role in controlling the immune response to the fetus; and CD1 molecules are important in defense against bacterial infections, as well as in the development and regulation of a subset of NKT cells expressing a highly restricted TCR repertoire; however not all nonclassical class I molecules have an immunological function, as demonstrated by HFE which is implicated in iron metabolism.     

Intracellular recycling and cross-presentation by MHC class I molecules

Cross-presentation of internalized antigens by dendritic cells requires efficient delivery of Major Histocompatibility Complex (MHC) class I molecules to peptide-loading compartments. Strong evidence suggests that such loading can occur outside of the endoplasmic reticulum; however, the trafficking pathways and sources of class I molecules involved are poorly understood. Examination of non-professional, non-phagocytic cells has revealed a clathrin-independent, Arf6-dependent recycling pathway likely traveled by internalized optimally loaded (closed) class I molecules. Some closed and all open MHC class I molecules travel to late endosomes to be degraded but might also partly be re-loaded with peptides and recycled. Studies of viral interference revealed pathways in which class I molecules are directed to degradation in lysosomes upon ubiquitination at the surface, or upon AP-1 and HIV-nef-dependent misrouting from the Golgi network to lysosomes. While many observations made in non-professional cells remain to be re-examined in dendritic cells, available evidence suggests that both recycling and neo-synthesized class I molecules can be loaded with cross-presented peptides. Recycling molecules can be recruited to phagosomes triggered by innate signals such as TLR4 ligands, and may therefore specialize in loading with phagocytosed antigens. In contrast, AP-1-dependent accumulation at, or trafficking through, a Golgi compartment of newly synthesized molecules appears to be important for cross-presentation of soluble proteins and possibly of long peptides that are processed in the so-called vacuolar pathway. However, significant cell biological work will be required to confirm this or any other model and to integrate knowledge on MHC class I biochemistry and trafficking in models of CD8⁺ T-cell priming by dendritic cells.     

Intracellular signal transduction mediated by ligation of MHC class 1 molecules

Abstract: A great deal of knowledge has accumulated regarding signal transduction after ligation of MHC class I (MHC-I) molecules. In recent years focus has been given to delineation of the intracellular signal pathways activated after MHC-I ligation. Activation of tyrosine kinases leading to a rise in the intracellular free calcium concentration ([Ca²⁺]i) is the major initial event occurring after MHC-I ligation of T cells. Curiously, the MHC-I-induced signaling is not dependent upon the cytoplasmic tail of the MHC-I molecule, suggesting that the MHC-I molecule induces intracellular signaling through association with other membrane-embedded molecules. More distal signaling events after MHC-I ligation includes activation of the Jak/ Stat pathway leading to Stat-3 activation, and activation of the PI3-kinase leading to JNK activation and apoptosis. This review will sum up what is currently known about signaling induced by ligation of MHC-I.     

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.     

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.