Basic concepts of immune response and defense development

The induction of immune responses requires critical interaction between innate parts of the immune system, which respond rapidly and in a relatively nonspecific manner, and other specific parts, which recognize particular epitopes on an antigen. A critical element in this interaction is the role played by dendritic cells (DCs), which represent "professional antigen-presenting cells." DCs endocytose and process antigen to peptide presented on the cell surface in association with major histocompatibility complex (MHC) molecules. This presentation results in interaction with and stimulation of helper T (Th) lymphocytes, which recognize peptide in association with either MHC class II or cytotoxic T (Tc) lymphocytes, which recognize peptide in association with MHC class I. Stimulation of Th lymphocytes produces the growth and differentiation factors (cytokines) essential for the B lymphocytes that have responded to a more intact form of the antigen and that differentiate into antibody-producing cells. The precise interaction between the cells depends on cognate ligand-receptor recognition between the B and Th lymphocytes. DCs also play a direct role with the stimulation of the B lymphocytes. It appears that DC can deliver antigen to the B lymphocytes in a more intact form than the processed form essential for stimulating T lymphocytes, and can release cytokines that assist the differentiation of the B lymphocytes into antibody-producing cells. This close relationship among the three cell types and the cytokines that are produced ensures the precise control and regulation necessary for immune response development.     

The fine specificity of antigen and Ia determinant recognition by T cell hybridoma clones specific for pigeon cytochrome c

The activation of proliferative T lymphocytes normally involves the simultaneous recognition of a particular foreign antigen and a particular Ia molecule on the surface of antigen-presenting cells, the phenomenon of major histocompatibility complex (MHC) restriction. An analysis of T cell clones specific for pigeon cytochrome c, from B10.A and B10.S(9R) strains of mice, revealed the unusual finding that several of the clones could respond to antigen in association with Ia molecules from either strain. Using these cross-reactive clones, we performed experiments which demonstrated that both the Ia molecule and the T cell receptor contribute to the specificity of antigen recognition; however, MHC-linked low responsiveness to tuna cytochrome c (an immune response gene defect) could not be attributed solely to the efficacy with which the Ia molecules associated with the antigen. These results imply that antigen and Ia molecules are not recognized independently, but must interact at least during the process of T cell activation.     

Antigen presentation and the triggering of the immune response

Immune defense is based on the interaction of nonspecific factors of natural resistance and factors of antigen-specific adaptive immune response. The key event of this interaction is antigen presentation. Its matter is a recognition of antigenic peptide complexed with MHC molecule of class II on the antigen-presenting cell (APC) surface by TCR receptor of T helper cell. The realization of the antigen presentation is connected with some problems: the number of cells in specific T cell clones is too low; the complexes of each peptide with MHC-II is a low part of the general population of APC surface peptide-MHC-II complexes; the bonds forming between these complexes and TCR molecules are too weak and some other molecules must be involved to reach T cell activation. These difficulties and mechanisms of their overcoming are considered in the review.     

Targeted antigen presentation using crosslinked antibody heteroaggregates

We have targeted protein antigens to antigen-presenting cells in vitro by using antibody heteroaggregates containing an antibody against a protein antigen covalently crosslinked to an antibody against a target structure on the surface of the antigen-presenting cells. Antigen presentation was assessed by measurement of lymphokine released by antigen-specific T cell hybridomas. Depending on the experimental conditions, the crosslinked antibodies decreased the amount of antigen required to give a response by the hybridomas by factors of 10(2) to 10(3). Enhanced presentation occurred when antigen was targeted to MHC class I and class II molecules, surface immunoglobulin, or Fc gamma receptors on the surface of the murine B cell lymphoma-hybridoma, TA3. An enhancement of antigen presentation also occurred when antigen was targeted to surface IgD, or class I and class II MHC molecules on murine splenic B cells, and when antigen was targeted to class I and class II molecules on irradiated adherent spleen cells. No response was seen when antigen was targeted to Fc gamma R on B cells or adherent spleen cells. The ability of each crosslinked antibody to enhance presentation paralleled the total amount of each that bound to the surface of the antigen-presenting cells. Antigen presentation, mediated by crosslinked antibody, was antigen-specific and I-A restricted. The presentation of one antigen by using crosslinked antibody did not result in enhanced presentation of a second, bystander antigen. These results suggest that a novel means of stimulating immune responses may be possible in vivo, by targeting antigen to surface structures on antigen-presenting cells.     

B cells extract and present immobilized antigen: implications for affinity discrimination

Binding of antigen to B-cell antigen receptor (BCR) leads to antigen internalization and presentation to T cells, a critical process in the initiation of the humoral immune response. However, antigen internalization has been demonstrated for soluble antigen, in vivo antigen is often encountered in insoluble form or tethered to a cell surface. Here, we show that not only can B cells internalize and present large particulate antigen (requiring a signalling-competent BCR to drive antigen uptake), but they can also extract antigen that is tethered tightly to a non-internalizable surface. The form in which the antigen is displayed affects the B cell's ability to discriminate antigen-BCR affinity. Thus, arraying an antigen on a particle or surface allows efficient presentation of low affinity antigens. However, the presentation efficiency of antigen arrayed on an internalizable particle plateaus at low affinity values. In contrast, extraction and presentation of antigen from a non-internalizable surface depends on antigen-BCR affinity over a wide affinity range. The results have implications for understanding both the initiation and affinity maturation of the immune response.     

Role of antigen-specific B cells in the induction of SRBC-specific T cell proliferation

In this study, we ask whether antigen presentation can be effected by antigen-activated B cells. Antigen-dependent in vitro proliferation of T cells from mice primed with SRBC or HoRBC occurs in the presence of B cells primed to the relevant antigen. B cells prepared from lymph nodes of mice primed with irrelevant antigens are not effective antigen-presenting cells for RBC-specific T cell proliferation over a wide range of SRBC doses. This is true even when both RBC and the antigen to which the B cells are primed are included in the culture. In contrast, B cells specific for a hapten determinant coupled to SRBC are able to support proliferation of T cells specific for SRBC determinants. We conclude from these data that antigen-specific B cells play a role in the induction of T cell proliferative responses to SRBC and HoRBC antigens. Two models are proposed: either B cells, upon antigen interaction with surface antibody, are able to act as accessory cells to induce Ia-dependent proliferation of immune T cells; or B cells augment the T cell proliferative response by secretion of antibody, leading to opsonization of the antigen for macrophage uptake and presentation.     

Membrane specializations and endosome maturation in dendritic cells and B cells

Interest in the cell biology of antigen presentation is centered on dendritic cells (DCs) as initiators of the immune response. The ability to examine primary antigen-presenting cells, as opposed to cell lines, has opened a new window for study of antigen processing and peptide acquisition by Class II major histocompatibility complex (MHC) products, especially where intracellular trafficking of peptide-Class-II complexes is concerned. Here, we review the dynamics of Class II MHC-positive intracellular structures in dendritic cells as well as B cells. We focus on the generation of multivesicular bodies, where Class II MHC products acquire antigenic peptide, on the endosomal transport of peptide-loaded Class II MHC to the cell surface and on the importance of Class II MHC localization in membrane microdomains.     

Function of macrophages in genetic control of immune responsiveness.

Macrophages serve an essential but poorly understood role in the cellular and molecular events that underlie immune competence. Antigenic proteins are now known to bind initially to macrophages prior to their recognition by T lymphocytes. Antigen uptake by macrophages is a metabolism-dependent event that results in an association of the antigen or a fragment thereof with a product of genes linked to the major histocompatibility complex of the species. For recognition of this associative form of antigen and self to result in cell proliferation, a direct physical interaction of antigen-bearing macrophage and lumphocyte must occur. Soluble forms of the altered antigen complexed to self may, however, function in nonproliferative T cell activation phenomenon. Using antigens of defined structure, it is possible to derive data which indicate that genetic control of immune responsiveness resides at the level of the antigen-presenting cell, thus indicating that these latter cells have profound discriminatory influences on host immune competence.     

An induced fit hypothesis for antigen recognition by T lymphocytes: a role for specific antigen retention structures on antigen-presenting cells

The nature of T lymphocyte recognition of foreign antigens is not known, despite recent advances in elucidating the cellular structures that may be involved in the specific interactions. The central difficulty in this process is that T cells respond to foreign antigen only in the context of major histocompatibility complex (MHC) antigens expressed by another antigen-presenting cell. In addition, T cells that interact with class II MHC antigens do not bind foreign protein antigens in their native form, but seem to recognize only proteolytic peptide fragments as the relevant antigen. The simplest explanation for these observations is that the class II MHC antigens themselves bind antigenic peptides to form the appropriate determinant that interacts with the antigen-specific T cell receptor. However, to date no such antigenic complex has been found with MHC antigens despite rigorous attempts at their demonstration. One alternative explanation described here is that there is no preexisting foreign antigen-MHC antigen complex prior to interaction with T cells, and it is the T cells that cause the two moieties to become associated for recognition by a single antigen-specific T cell receptor. Central to this mechanism is that foreign antigenic peptides must be associated with specific antigen retention structures (SARS) expressed by antigen-presenting cells which retain and protect the peptide on the cell surface. These SARS, upon interaction with T cell membrane moieties, would subsequently associate with MHC antigens. A hypothesis to describe this mechanism is developed to account for published observations of antigen processing by antigen-presenting cells and T cell antigen recognition, and makes several predictions that are experimentally testable. This mechanism is also generally applicable to other cellular interactions in which soluble peptide mediators may become associated with surface components of one cell type, and this newly formed complex is in turn recognized by a receptor on a second cell type to deliver functional signals.     

The role of B7 costimulation in T-cell immunity.

CD4+ T cells are considered to be the major controlling element of the adaptive immune response. They recognize foreign peptides by interaction of the T cell receptor (TCR) with peptide complexed to major histocompatibility complex (MHC) class II molecules on the surface of antigen presenting cells (APC). Once activated, CD4+ T cells orchestrate the various phases of the immune response. They are responsible for the production of numerous cytokines, which activate specific immune effector cell populations including B cells, eosinophils, mast cells and macrophages. Not surprisingly, the activation of CD4+ T cells needs to be tightly regulated and is subject to finely tuned control mechanisms. The requirement for a second or 'costimulatory' signal, in addition to the antigenic signal, provides a key element for the exquisite control of T cell activation. One of the major signalling pathways responsible for delivery of this costimulatory signal is induced by interaction of CD28 on T cells with B7 molecules found only on APC. The present review outlines our current understanding of the physiological role of B7 costimulatory signals in regulating CD4+ T cell responses.     

Antibody responses to glycolipid-borne carbohydrates require CD4+ T cells but not CD1 or NKT cells

Naturally occurring anti-carbohydrate antibodies play a major role in both the innate and adaptive immune responses. To elicit an anti-carbohydrate immune response, glycoproteins can be processed to glycopeptides and presented by the classical antigen-presenting molecules, major histocompatibility complex (MHC) Class I and II. In contrast, much less is known about the mechanism(s) for anti-carbohydrate responses to glycolipids, although it is generally considered that the CD1 family of cell surface proteins presents glycolipids to T cells or natural killer T (NKT) cells. Using model carbohydrate systems (isogloboside 3 and B blood group antigen), we examined the anti-carbohydrate response on glycolipids using both antibody neutralisation and knockout mouse-based experiments. These studies showed that CD4(+) T cells were required to generate antibodies to the carbohydrates expressed on glycolipids, and unexpectedly, these antibody responses were CD1d and NKT cell independent. They also did not require peptide help. These data provide new insight into glycolipid antigen recognition by the immune system and indicate the existence of a previously unrecognised population of glycolipid antigen-specific, CD1-independent, CD4(+) T cells.     

Toxoplasma gondii and MHC-restricted antigen presentation: on degradation, transport and modulation

Resistance against Toxoplasma gondii, an obligate intracellular protozoan parasite surrounded by a parasitophorous vacuolar membrane, is mediated by the cellular arm of the immune system, namely CD8+ and CD4+ T cells. Thus, priming and activation of these cells by presentation of antigenic peptides in the context of major histocompatibility complex class I and class II molecules have to take place. This is despite the fact that the vacuolar membrane avoids fusion with the endocytic compartment and acts like a molecular sieve, restricting passive diffusion of larger molecules. This raises several cell biological and immunological questions which will be discussed in this review in the context of our current knowledge about major histocompatibility complex-restricted antigen presentation in other systems: (1) By which pathways are parasite-derived antigens presented to T cells? (2) Has the parasite evolved mechanisms to interfere with major histocompatibility complex-restricted antigen presentation in order to avoid immune recognition? (3) To what extent and by which mechanism is antigenic material, originating from the parasite, able to pass through the vacuolar membrane into the cytosol of the infected cell and is it then accessible to the antigen presentation machinery of the infected cell? (4) What are the actual antigen-presenting cells which prime specific T cells in lymphoid organs? An understanding of these mechanisms will not only provide new insights into the pathogenesis of Toxoplasma gondii and possibly other intravacuolar parasites, but will also improve vaccination strategies.     

Arsonate-specific murine T cell clones. IV. Properties of I-E- and I-A-restricted clones

The T cell antigen L-tyrosine-p-azobenzenearsonate is unique in being a simple determinant that can be presented in the context of both I-A and I-E. I-E-restricted T cell clones derived from B10.A(5R) mice were found to fall into three groups: Type I clones recognized antigen only in the context of syngeneic apcs, Type II clones recognized antigen with the same highly specific major histocompatibility complex restriction but in addition proliferated in response to allogeneic stimuli; Type III clones were "degenerate" in their major histocompatibility complex-restricted recognition of antigen and proliferated when antigen-presenting cells bearing Eb beta Ek alpha (syngeneic), Ek beta Ek alpha, or Ed beta Ed alpha were used. These observations allow some conclusions to be drawn about sites on the I-E molecule that may be functionally significant in the presentation of this antigen. By using the B cell hybridoma LK35.2 as target cells, some of these T cell clones act as cytotoxic cells in the Class II-restricted manner predicted from the results of proliferative assays. Class II-restricted cytotoxicity can therefore be controlled by both I-A and I-E mouse Ir gene loci.     

Polysaccharide processing and presentation by the MHCII pathway

The adaptive immune system functions through the combined action of antigen-presenting cells (APCs) and T cells. Specifically, class I major histocompatibility complex antigen presentation to CD8(+) T cells is limited to proteosome-generated peptides from intracellular pathogens while the class II (MHCII) endocytic pathway presents only proteolytic peptides from extracellular pathogens to CD4(+) T cells. Carbohydrates have been thought to stimulate immune responses independently of T cells; however, zwitterionic polysaccharides (ZPSs) from the capsules of some bacteria can activate CD4(+) T cells. Here we show that ZPSs are processed to low molecular weight carbohydrates by a nitric oxide-mediated mechanism and presented to T cells through the MHCII endocytic pathway. Furthermore, these carbohydrates bind to MHCII inside APCs for presentation to T cells. Our observations begin to elucidate the mechanisms by which some carbohydrates induce important immunologic responses through T cell activation, suggesting a fundamental shift in the MHCII presentation paradigm.     

Functional HLA-DM on the surface of B cells and immature dendritic cells

HLA-DM (DM) plays a critical role in antigen presentation through major histocompatibility complex (MHC) class II molecules. DM functions as a molecular chaperone by keeping class II molecules competent for antigenic peptide loading and serves as an editor by favoring presentation of high-stability peptides. Until now, DM has been thought to exert these activities only in late endosomal/lysosomal compartments of antigen-presenting cells. Here we show that a subset of DM resides at the cell surface of B cells and immature dendritic cells. Surface DM engages in complexes with putatively empty class II molecules and controls presentation of those antigens that rely on loading on the cell surface or in early endosomal recycling compartments. For example, epitopes derived from myelin basic protein that are implicated in the autoimmune disease multiple sclerosis are down-modulated by DM, but are presented in the absence of DM. Thus, this novel concept of functional DM on the surface may be relevant to both protective immune responses and autoimmunity.     

The cell biology of T-dependent B cell activation

The requirement that CD4+ helper T cells recognize antigen in association with class II Major Histocompatibility Complex (MHC) encoded molecules constrains T cells to activation through intercellular interaction. The cell biology of the interactions between CD4+ T cells and antigen-presenting cells includes multipoint intermolecular interactions that probably involve aggregation of both polymorphic and monomorphic T cell surface molecules. Such aggregations have been shown in vitro to markedly enhance and, in some cases, induce T cell activation. The production of T-derived lymphokines that have been implicated in B cell activation is dependent on the T cell receptor for antigen and its associated CD3 signalling complex. T-dependent help for B cell activation is therefore similarly MHC-restricted and involves T-B intercellular interaction. Recent reports that describe antigen-independent B cell activation through coculture with T cells activated by anti-T-cell receptor or anti-CD3 antibodies suggest that cellular interaction with T cells, independent of antigen presentation or lymphokine secretion, induces or triggers B cells to become responsive to T-derived lymphokines, and that this may be an integral component of the physiological, antigen- and MHC-restricted T-dependent B cell activation that leads to antibody production.     

Proteasome-independent major histocompatibility complex class I cross-presentation mediated by papaya mosaic virus-like particles leads to expansion of specific human T cells

The development of versatile vaccine platforms is a priority that is recognized by health authorities worldwide; such platforms should induce both arms of the immune system, the humoral and cytotoxic-T-lymphocyte responses. In this study, we have established that a vaccine platform based on the coat protein of papaya mosaic virus (PapMV CP), previously shown to induce a humoral response, can induce major histocompatibility complex (MHC) class I cross-presentation of HLA-A*0201 epitopes from gp100, a melanoma antigen, and from influenza virus M1 matrix protein. PapMV proteins were able to assemble into stable virus-like particles (VLPs) in a crystalline and repetitive structure. When we pulsed HLA-A*0201+ antigen-presenting cells (APCs) with the recombinant PapMV FLU or gp100, we noted that antigen-specific CD8+ T cells were highly reactive to these APCs, demonstrating that the epitope from the VLPs were processed and loaded on the MHC class I complex. APCs were preincubated with two different proteasome inhibitors, which did not affect the efficiency of peptide presentation on MHC class I. Classical presentation from an endogenous antigen was abolished in the same conditions. Clearly, antigen presentation mediated by the PapMV system was proteasome independent. Finally, PapMV-pulsed APCs had the capacity to expand highly avid antigen-specific T cells against the influenza virus M1 HLA-A*0201 epitope when cocultured with autologous peripheral blood mononuclear cells. This study demonstrates the potential of PapMV for MHC class I cross-presentation and for the expansion of human antigen-specific T cells. It makes VLPs from PapMV CP a very attractive platform to trigger cellular responses for vaccine development against chronic infectious diseases and cancers.     

Signal transduction in T helper cells: CD4 coreceptors exert complex regulatory effects on T cell activation and function

The immune system provides a highly sophisticated surveillance mechanism to detect diverse antigens and to protect the host organism from invading pathogens and altered cells (e.g., virus-infected and tumor cells). Adaptive immune responses depend on the recognition of antigen by specific antigen receptors that are expressed on the surface of T and B lymphocytes. Helper T cells provide regulatory functions and direct the adaptive immune system to respond appropriately to a particular antigen (i.e., cytotoxic T cell responses against viral infections and tumor cells, humoral responses against extracellular bacteria and parasitic worms). Helper T cells express CD4 coreceptors, which recognize conserved domains on proteins expressed by the class II major histocompatibility complex, the same proteins that present antigen to the T cell receptor. Recent progress in T cell biology has identified multiple regulatory functions of CD4 during thymocyte development and antigen stimulation of mature T helper cells. Signaling pathways induced by engagement of CD4 independently of T cell receptor signaling mediate these regulatory functions. In this review, we discuss the regulation of T cell signaling and emphasize the functional consequences of proper and improper CD4 coreceptor signaling.