Cell interactions between histoincompatible T and B lymphocytes. The H-2 gene complex determines successful physiologic lymphocyte interactions

The show and tell of cross-presentation

Cross-presentation is the culmination of complex subcellular processes that allow the processing of exogenous proteins and the presentation of resultant peptides on major histocompatibility class I (MHC-I) molecules to CD8 T cells. Dendritic cells (DCs) are a cell type that uniquely specializes in cross-presentation, mainly in the context of viral or non-viral infection and cancer. DCs have an extensive network of endovesicular pathways that orchestrate the biogenesis of an ideal cross-presentation compartment where processed antigen, MHC-I molecules, and the MHC-I peptide loading machinery all meet. As a central conveyor of information to CD8 T cells, cross-presentation allows cross-priming of T cells which carry out robust adaptive immune responses for tumor and viral clearance. Cross-presentation can be canonical or noncanonical depending on the functional status of the transporter associated with antigen processing (TAP), which in turn influences the vesicular route of MHC-I delivery to internalized antigen and the cross-presented repertoire of peptides. Because TAP is a central node in MHC-I presentation, it is targeted by immune evasive viruses and cancers. Thus, understanding the differences between canonical and noncanonical cross-presentation may inform new therapeutic avenues against cancer and infectious disease. Defects in cross-presentation on a cellular and genetic level lead to immune-related disease progression, recurrent infection, and cancer progression. In this chapter, we review the process of cross-presentation beginning with the DC subsets that conduct cross-presentation, the signals that regulate cross-presentation, the vesicular trafficking pathways that orchestrate cross-presentation, the modes of cross-presentation, and ending with disease contexts where cross-presentation plays a role.

A Disquisition on MHC Restriction and T Cell Recognition in Five Acts

The seminal discovery in the early 1970s, credited to Peter Doherty and Rolf Zinkernagel, of major histocompatibility complex (MHC) restriction exhibited by cytotoxic T cells represented a major conceptual advance in understanding antigen recognition by conventional T cells. This advance also led to other major new insights into the ontogeny and immunobiology of T cells and catalyzed a renaissance in viral immunology. In this commentary in honor of Peter Doherty, I offer five brief reflections on different aspects of the phenomenon of MHC restriction and the process by which it was discovered and explained. In the first of these sections, I offer a reinterpretation of MHC restriction that reframes the constraints on self-MHC recognition in terms of the probabilities of recognizing a given nominal antigen peptide in the context of an MHC molecule that is nonself on the basis of differing in amino acid sequence from the self-restriction element at one or more positions. Subsequent sections address: (i) the ways in which general ideas, developed subsequent to the discovery of MHC restriction, about the intricacies of antigen recognition by antibodies apply to T cell receptors binding to MHC/peptide complexes; (ii) how to reconcile the existence of MHC restriction with the impressive magnitude of T cell responses to nonself MHC antigens; (iii) the possible relevance to MHC restriction and immune system function of ideas from mathematical logic that relate to the consequences of self-reference; and (iv) the implications for the philosophy of science of MHC restriction and the processes of its discovery and acceptance within the immunology research community.

In silico analysis suggests less effective MHC-II presentation of SARS-CoV-2 RBM peptides: Implication for neutralizing antibody responses

SARS-CoV-2 antibodies develop within two weeks of infection, but wane relatively rapidly post-infection, raising concerns about whether antibody responses will provide protection upon re-exposure. Here we revisit T-B cooperation as a prerequisite for effective and durable neutralizing antibody responses centered on a mutationally constrained RBM B cell epitope. T-B cooperation requires co-processing of B and T cell epitopes by the same B cell and is subject to MHC-II restriction. We evaluated MHC-II constraints relevant to the neutralizing antibody response to a mutationally-constrained B cell epitope in the receptor binding motif (RBM) of the spike protein. Examining common MHC-II alleles, we found that peptides surrounding this key B cell epitope are predicted to bind poorly, suggesting a lack MHC-II support in T-B cooperation, impacting generation of high-potency neutralizing antibodies in the general population. Additionally, we found that multiple microbial peptides had potential for RBM cross-reactivity, supporting previous exposures as a possible source of T cell memory.

MHC-II constrains the natural neutralizing antibody response to the SARS-CoV-2 spike RBM in humans

SARS-CoV-2 antibodies develop within two weeks of infection, but wane relatively rapidly post-infection, raising concerns about whether antibody responses will provide protection upon re-exposure. Here we revisit T-B cooperation as a prerequisite for effective and durable neutralizing antibody responses centered on a mutationally constrained RBM B cell epitope. T-B cooperation requires co-processing of B and T cell epitopes by the same B cell and is subject to MHC-II restriction. We evaluated MHC-II constraints relevant to the neutralizing antibody response to a mutationally-constrained B cell epitope in the receptor binding motif (RBM) of the spike protein. Examining common MHC-II alleles, we found that peptides surrounding this key B cell epitope are predicted to bind poorly, suggesting a lack MHC-II support in T-B cooperation, impacting generation of high-potency neutralizing antibodies in the general population. Additionally, we found that multiple microbial peptides had potential for RBM cross-reactivity, supporting previous exposures as a possible source of T cell memory.

Defective Influenza A Virus RNA Products Mediate MAVS-Dependent Upregulation of Human Leukocyte Antigen Class I Proteins

Human leukocyte antigens (HLAs) are cell surface proteins that regulate innate and adaptive immune responses to viral infection by engaging with receptors on immune cells. Many viruses have evolved ways to evade host immune responses by modulating HLA expression and/or processing. Here, we provide evidence that aberrant RNA products of influenza virus genome replication can trigger retinoic acid-inducible gene I (RIG-I)/mitochondrial antiviral signaling (MAVS)-dependent remodeling of the cell surface, increasing surface presentation of HLA proteins known to inhibit the activation of an immune cell known as a natural killer (NK) cell. While this HLA upregulation would seem to be advantageous to the virus, it is kept in check by the viral nonstructural 1 (NS1) protein, which limits RIG-I activation and interferon production by the infected cell.

Influenza A virus (IAV) increases the presentation of class I human leukocyte antigen (HLA) proteins that limit antiviral responses mediated by natural killer (NK) cells, but molecular mechanisms for these processes have not yet been fully elucidated. We observed that infection with A/Fort Monmouth/1/1947(H1N1) IAV significantly increased the presentation of HLA-B, -C, and -E on lung epithelial cells. Virus entry was not sufficient to induce HLA upregulation because UV-inactivated virus had no effect. Aberrant internally deleted viral RNAs (vRNAs) known as mini viral RNAs (mvRNAs) and defective interfering RNAs (DI RNAs) expressed from an IAV minireplicon were sufficient for inducing HLA upregulation. These defective RNAs bind to retinoic acid-inducible gene I (RIG-I) and initiate mitochondrial antiviral signaling (MAVS) protein-dependent antiviral interferon (IFN) responses. Indeed, MAVS was required for HLA upregulation in response to IAV infection or ectopic mvRNA/DI RNA expression. The effect was partially due to paracrine signaling, as we observed that IAV infection or mvRNA/DI RNA-expression stimulated production of IFN-β and IFN-λ1 and conditioned media from these cells elicited a modest increase in HLA surface levels in naive epithelial cells. HLA upregulation in response to aberrant viral RNAs could be prevented by the Janus kinase (JAK) inhibitor ruxolitinib. While HLA upregulation would seem to be advantageous to the virus, it is kept in check by the viral nonstructural 1 (NS1) protein; we determined that NS1 limits cell-intrinsic and paracrine mechanisms of HLA upregulation. Taken together, our findings indicate that aberrant IAV RNAs stimulate HLA presentation, which may aid viral evasion of innate immunity.

IMPORTANCE Human leukocyte antigens (HLAs) are cell surface proteins that regulate innate and adaptive immune responses to viral infection by engaging with receptors on immune cells. Many viruses have evolved ways to evade host immune responses by modulating HLA expression and/or processing. Here, we provide evidence that aberrant RNA products of influenza virus genome replication can trigger retinoic acid-inducible gene I (RIG-I)/mitochondrial antiviral signaling (MAVS)-dependent remodeling of the cell surface, increasing surface presentation of HLA proteins known to inhibit the activation of an immune cell known as a natural killer (NK) cell. While this HLA upregulation would seem to be advantageous to the virus, it is kept in check by the viral nonstructural 1 (NS1) protein, which limits RIG-I activation and interferon production by the infected cell.

Obsessive-Compulsive Behavior Isn't Necessarily a Bad Thing

It is difficult to believe that in about 1960 practically nothing was known about the thymus and some of its products, T cells bearing αβ receptors for antigen. Thus I was lucky to join the field of T cell biology almost at its beginning, when knowledge about the cells was just getting off the ground and there was so much to discover. This article describes findings about these cells made by others and myself that led us all from ignorance, via complete confusion, to our current state of knowledge. I believe I was fortunate to practice science in very supportive institutions and with very collaborative colleagues in two countries that both encourage independent research by independent scientists, while simultaneously ignoring or somehow being able to avoid some of the difficulties of being a woman in what was, at the time, a male-dominated profession.

The role of B cell antigen presentation in the initiation of CD4+ T cell response

B cells have been known for their ability to present antigens to T cells for almost 40 years. However, the precise roles of B cell antigen presentation in various immune responses are not completely understood. The term "professional" antigen-presenting cells (APCs) was proposed to distinguish APCs that are required for initiating the immune responses from those use antigen presentation to enhance their own effector functions. Unlike dendritic cells, which are defined as professional APCs for their well-established functions in activating naive T cells, B cells have been shown in the past to mostly present antigens to activated CD4+ T cells mainly to seek help from T helper cells. However, recent evidence suggested that B cells can act as professional APCs under infectious conditions or conditions mimicking viral infections. B cell antigen receptors (BCRs) and the innate receptor Toll-like receptors are activated synergistically in response to pathogens or virus-like particles, under which conditions B cells are not only potent but also the predominant APCs to turn naive CD4+ T cells into T follicular helper cells. The discovery of B cells as professional APCs to initiate CD4+ T cell response provides a new insight for both autoimmune diseases and vaccine development.

Viruses Teaching Immunology: Role of LCMV Model and Human Viral Infections in Immunological Discoveries

Virology has played an essential role in deciphering many immunological phenomena, thus shaping our current understanding of the immune system. Animal models of viral infection and human viral infections were both important tools for immunological discoveries. This review discusses two immunological breakthroughs originally identified with the help of the lymphocytic choriomeningitis virus (LCMV) model; immunological restriction by major histocompatibility complex and immunotherapy using checkpoint blockade. In addition, we discuss related discoveries such as development of tetramers, viral escape mutation, and the phenomenon of T-cell exhaustion.

Thymoproteasome and peptidic self

Positive selection of T cells in the thymus is induced by low-affinity TCR recognition of self-peptide-MHC complexes expressed by cortical thymic epithelial cells (cTECs). cTECs express a specialized type of proteasomes, the thymoproteasome, which generates a unique spectrum of MHC class I-associated peptides and plays a critical role in thymic positive selection of CD8⁺ T cells. However, it remains unclear how the thymoproteasome contributes to the thymic positive selection. More than 30 years ago, the "peptidic self" hypothesis proposed that TCRs recognize MHC-presented peptides only, without interacting with MHC molecules, which turned out to be incorrect. Interestingly, however, by implying that a set of MHC-associated peptides forms immunological self, this hypothesis also predicted that positive selection in the thymus is the primary immune response to "foreign epitope" peptides during T cell development. The thymoproteasome-dependent unique self-peptides may create those foreign epitope peptides displayed in the thymus for positive selection of T cells.

Remembrance of immunology past: conversations with Herman Eisen

Herman Eisen and Sondra Schlesinger spent several days together in September 2007 in Woods Hole, Massachusetts, talking about immunology, focusing on his remembrances of the field over the more than 60 years of his involvement. This article is an abridged version of those discussions (the full version is available on the Annual Reviews website). It is both an oral history and a written memory of some important but selected areas of immunology.

T lymphocytes responding to Mls-locus antigens are Lyt-1+, 2- and I-A restricted

We have investigated primary and secondary responses of mouse splenic T cells to strong mixed lymphocyte stimulating antigens controlled by the Mls locus using MHC-identical mixtures of cells. Our studies show that strong primary Mls-locus specific responses involve recognition of self I-A antigens, since BUdR and light suicide or F1 into parent radiation bone-marrow chimeras both demonstrate a preference of unprimed F1 T cells to respond to Mls-locus antigens associated with one parent's MHC antigens. Furthermore, conventional anti-I-A antisera and monoclonal anti-I-A antibody both inhibit Mls-locus responses in an MHC-specific manner. Finally, as is typical of T cells responding to I-A antigens or to nominal antigens associated with self I-A, Mls-locus responses are mediated by Lyt-1+, 2 cells. One striking finding in these studies was the very high frequency of cells capable of responding to Mls-locus antigens, the highest being 1/300 splenic T cells. This plus evidence for recruitment during primary Mls-locus responses may account for reports of a lack of I-A restriction in secondary anti-Mls locus responses to strong Mls-locus antigens, a finding with which we concur. The possibility that these secondary responses between noncongenic strains of mice may be directed at other genetic loci is also discussed. These experiments leave open the question of the biological role of the Mls-locus and of the very large number of T cells reactive to it.

Mechanisms of genetically-based resistance to malaria

Malaria remains one of the most prevalent parasitoses worldwide. About 350 to 500 million febrile episodes are observed yearly in African children alone and more than 1 million people die because of malaria each year. Multiple factors have hampered the effective control of this disease, some of which include the complex biology of the Plasmodium parasites, their high polymorphism and their increasingly high resistance to antimalarial drugs, mainly in endemic regions. The ancient interaction between malarial parasites and humans has led to the fixation in the population of several inherited alterations conferring protection against malaria. Some of the mechanisms underlying protection against this disease are described in this review for hemoglobin-inherited disorders (thalassemia, sickle-cell trait, HbC and HbE), erythrocyte polymorphisms (ovalocytosis and Duffy blood group), enzymopathies (G6PD deficiency and PK deficiency) and immunogenetic variants (HLA alleles, complement receptor 1, NOS2, tumor necrosis factor-α promoter and chromosome 5q31-q33 polymorphisms).

Exposure to Nicotine during Gestation Interferes with the Colonization of Fetal Bone Marrow by Hematopoietic Stem/Progenitor Cells

Environmental factors, including cigarette smoke components, can cross the placental barrier and accumulate in amniotic fluid and fetal tissue, and, therefore, interfere with the normal course of ontogenesis. Although cigarette smoke contains numerous compounds, the most adverse effects on mammalian tissues have been associated with nicotine. The aim of this study was to investigate the effect of intrauterine exposure to nicotine on hematopoiesis during fetal development and postpartum. Intrauterine exposure of mice to nicotine resulted in a more than two-fold reduction of the delayed- type hypersensitivity (DTH) response and a 2.5-fold decrease in the number of plaque forming cell (PFC) in offspring after 1 month of postnatal life, and correlated with low counts of mature lymphocytes and lymphoid progenitors in hematopoietic tissues. Neonates exposed to nicotine during gestation showed a significant decrease in the number of bone marrow hematopoietic progenitors, as measured by colony-forming unit (CFU) and long-term culture initiating cell (LTC-IC) assays, and decreased concentration of interleukin-6 (IL-6) in their serum. Analysis of the fetal bone marrow (E15) obtained from nicotine-exposed fetuses demonstrated a lower number of hematopoietic progenitors, whereas their number in the fetal liver was not significantly changed. Our data provide evidence that by targeting the nicotinic acetylcholine receptor (nAChR) nicotine interferes with the fetal development of the hematopoietic system. Inferior colonization of the fetal bone marrow by hematopoietic stem/progenitor cells (HSPC) subsequently results in an imbalance of mature blood and immune cell production after birth.

Structural basis of plasticity in T cell receptor recognition of a self peptide-MHC antigen

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide-major histocompatibility complex (pMHC) antigen H-2Kb-dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR beta chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC alpha helices suggests a generalized orientation that is mediated by the Valpha domain of the TCR and rationalizes how TCRs can effectively "scan" different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.

T-cell receptors: feeling out the complex

Recent crystallographic studies show that the T-cell receptor has a largely immunoglobulin-like structure and binds to MHC-peptide complexes through loops from paired Valpha and Vbeta domains that focus on the central amino acids of the MHC-bound peptide, and to bacterial superantigens via peripheral aspects of the Vbeta domain.

Polyreactive antigen-binding B cells in the peripheral circulation are IgD+ and B7-

Polyreactive antibodies are naturally occurring antibodies, primarily of the IgM isotype, that are capable of reacting with a wide variety of different self and non-self antigens. Previously, we reported that a B cell capable of making polyreactive antibody has Ig receptors on its surface that can bind different antigens. The present investigation was initiated to characterize these polyreactive antigen-binding B cells further. A panel of fluorescein isothiocyanate-labeled antigens (insulin, IgG Fc fragment or beta-galactosidase) served as probes to select polyreactive antigen-binding B cells by cell sorting. Our experiment revealed that these polyreactive antigen-binding B cells were mainly of the IgD isotype. They expressed high levels of CD40 and major histocompatibility complex class II molecules, but little or no B7-1, B7-2, or Fas. In contrast to the binding of antigens to monoreactive receptors (usually high affinity), the binding of antigens to polyreactive receptors (usually moderate or low affinity) did not up-regulate the expression of B7-1 or B7-2. Antigens that bound to polyreactive receptors, however, were internalized and degraded, although not as efficiently as antigens that bound to monoreactive receptors. Despite the ability of these B7- cells to process antigens, they were not able to activate T cells in a mixed leukocyte reaction. It is concluded that polyreactive antigen-binding B cells have properties that are consistent with the ability to induce immunological tolerance.

Structure of the complex between human T-cell receptor, viral peptide and HLA-A2

Recognition by a T-cell antigen receptor (TCR) of peptide complexed with a major histocompatibility complex (MHC) molecule occurs through variable loops in the TCR structure which bury almost all the available peptide and a much larger area of the MHC molecule. The TCR fits diagonally across the MHC peptide-binding site in a surface feature common to all class I and class II MHC molecules, providing evidence that the nature of binding is general. A broadly applicable binding mode has implications for the mechanism of repertoire selection and the magnitude of alloreactions.

Inhibition of the CD40-CD40ligand pathway prevents murine membranous glomerulonephritis

Several forms of glomerulonephritis are induced by antibodies against self or foreign antigens. Normal B lymphocyte antibody production requires T cell costimulatory signals provided in part by T cell surface expression of gp39/CD40ligand (CD40L) that engages the B cell receptor CD40 and induces B cell differentiation and immunoglobulin class switching. We assessed the effect of disrupting the CD40L-CD40 costimulatory pathway, using a CD40-Ig fusion protein, on the development of membranous glomerulonephritis (MGN) in the mouse. MGN is induced by mouse antibodies that recognize and bind to exogenously administered rabbit anti-mouse renal tubular brush border (RbAMBB) IgG immobilized in the glomerular capillary wall. MGN did not occur in nude mice, showing the need of the T cell function. C57Bl/10 mice immunized with RbAMBB and treated with CD40-Ig fusion protein displayed a delayed autologous response and absence of MGN lesions, while control fusion proteins failed to prevent the development of the disease. These observations provide evidence that disruption of the CD40-CD40L costimulatory pathway can prevent the development of MGN by suppressing T cell-dependent antibody production.

Intrinsic defect in B cells of patients with hyper-immunoglobulin M syndrome

We challenge the theory that the CD40-CD40 ligand is the only explanation for X-linked immunodeficiency in patients with hyper-immunoglobulin M (IgM) syndrome (HIGM1), and we demonstrate an intrinsic defect in the patients' B cells. Patients with HIGM1 have a defective CD40 ligand on their activated T-helper cells; therefore, they cannot receive signals for isotype switching when the cells are activated by T cell-dependent antigens. We activated mononuclear cells from three patients with HIGM1 and from three healthy blood donors with T cell-independent mitogens and studied their proliferative responses and Ig secretion. Normal murine plasma membrane fragments were implanted into peripheral blood mononuclear cells, and the cells were activated with Staphylococcus aureus Cowan I, pokeweed mitogen, and lipopolysaccharide. This implantation significantly augmented the proliferative responses to the mitogens in two patients. However, it augmented IGM secretion in response to B-cell mitogens in only one patient. No IgG or IgA response could be detected in the implanted mononuclear cells that originated from patients with HIGM1, unlike implanted mononuclear cells from healthy donors, which responded by IgM, IgG, and IgA antibody secretion following their stimulation with B-cell mitogens. The data suggest that the B cells of patients with HIGM1 possess an additional defect which prevents Ig isotype switching in response to T cell-independent mitogens. This defect is not located in the membrane receptors or within the membrane enzymes.

Humoral immune responses in CD40 ligand-deficient mice

Individuals with X-linked hyper-IgM syndrome fail to express functional CD40 ligand (CD40L) and, as a consequence, are incapable of mounting protective antibody responses to opportunistic bacterial infections. To address the role of CD40L in humoral immunity, we created, through homologous recombination, mice deficient in CD40L expression. These mice exhibited no gross developmental deficiencies or health abnormalities and contained normal percentages of B and T cell subpopulations. CD40L-deficient mice did display selective deficiencies in humoral immunity; basal serum isotype levels were significantly lower than observed in normal mice, and IgE was undetectable. Furthermore, the CD40L-deficient mice failed to mount secondary antigen-specific responses to immunization with a thymus-dependent antigen, trinitrophenol-conjugated keyhole limpet hemocyanin (TNP-KLH). By contrast, the CD40L-deficient mice produced antigen-specific antibody of all isotypes except IgE in response to the thymus-independent antigen, DNP-Ficoll. These results underscore the requirement of CD40L for T cell-dependent antibody responses. Moreover, Ig class switching to isotypes other than IgE can occur in vivo in the absence of CD40L, supporting the notion that alternative B cell signaling pathways regulate responses to thymus-independent antigens.

A speculative view of immune recognition

The speculation that immunologically reactive haptens must be those attached to carriers' immunodominant epitopes suggests a clearer mechanism by which the mysterious hapten-carrier phenomena are generated. This review focuses on the molecular biological nature of immune recognition of hapten-protein antigens both by the T-cell and the B-cell. T and B lymphocytes specifically recognize one determinant of the same antigen molecule in two different ways and in different circumstances. The B-cell recognizes an antigen by the preliminary antigen receptors on the cell's surface, at the time it is still intact, interiorizes it and presents the processed antigenic peptide after an antigen processing procedure. In contrast, the T-cell recognizes a hidden antigenic determinant, together with portions of the MHC on the presenting cell. The immune memory is mainly directed to the hidden internal determinant of an antigen. Some aspects of the clonal selection theory of antibody formation are also discussed at the modern molecular level.

Membrane-incorporated surrogate receptors increase antigen presentation by resting B cells, but not by LPS-activated blasts

Palmitate-conjugated monoclonal antibodies specific to ovalbumin were inserted into the cell membrane of normal resting B cells and LPS-activated blasts. These two decorated B cells were tested for their ability to act as antigen-presenting cells for ovalbumin-specific I-Ad-restricted T-cell hybridomas. It was found that the antibody-decorated resting B cells presented antigen more efficiently than non-decorated controls. However, no increment was observed when decorated LPS blasts were compared with non-decorated blasts. This is explained by the fact that the inserted antibodies quickly disappeared from the cell membrane of LPS blasts, while they were retained for a long period in the membrane of resting B cells.

Identification of a novel surface protein on activated CD4+ T cells that induces contact-dependent B cell differentiation (help)

CD4+ T lymphocytes provide contact-dependent stimuli to B cells that are critical for the generation of specific antibody responses in a process termed T helper function. The surface structures on activated CD4+ T cells that mediate this function are not fully known. We previously reported the isolation of a functionally unique subclone of the Jurkat leukemic T cell line (D1.1) that constitutively expressed contact-dependent helper effector function. To identify T cell surface molecules that mediate contact-dependent T helper function, a monoclonal antibody (mAb), designated 5c8, was generated that inhibits D1.1-mediated B cell activation and immunoprecipitates a novel 30-kD protein structure from surface-iodinated D1.1 cells. Normal CD4+ T cells express 5c8 antigen (Ag) transiently 5-6 h after activation by phorbol myristate acetate and phytohemagglutinin with maximal expression 5-6 h after activation and absence of expression by 24 h. In contrast, neither resting nor activated CD8+ T cells express 5c8 Ag. In functional studies, mAb 5c8 inhibits the ability of fixed, activated CD4+ T cells to induce B cell surface CD23 expression. In addition, mAb 5c8 inhibits the ability of CD4+ T cells to direct terminal B cell differentiation driven by pokeweed mitogen. Taken together, these data suggest that 5c8 Ag is a novel, activation-induced surface T cell protein that is involved in mediating a contact-dependent element of the helper effector function of CD4+ T lymphocytes.

Membrane-incorporated immunoglobulin receptors increase the antigen-presenting ability of B cells

Monoclonal antibodies specific for ovalbumin were conjugated to palmitate and inserted into the membrane of normal spleen B cells. Their presence in the membrane, as well as their ability to bind ovalbumin, was established by immunofluorescence. The so called anti-ovalbumin-'decorated' B cells were tested for their ability to act as antigen-presenting cells for ovalbumin-specific I-Ad-restricted T-cell hybridomas. It was found that the antibody-decorated B cells presented antigen more efficiently than non-decorated B cells.

T and B cell tolerance and responses to viral antigens in transgenic mice: implications for the pathogenesis of autoimmune versus immunopathological disease

Experiments with transgenic mice illustrate clonal elimination of T cells specific for antigens expressed appropriately in the thymus, but presence of inducible T cells when the antigen presented on class I MHC antigens is expressed exclusively on non-lymphohemopoietic cells in the periphery (pancreatic beta islet cells). TCR-transgenic LCMV-carrier mice expressing LCMV in the thymus exhibit clonal elimination at the early CD4+8+ thymocyte stage, causing CTL unresponsiveness in these mice. In contrast, studies with RIP LCMV-GP-transgenic mice (expressing GP in pancreatic beta cells) and with TCR-RIP LCMV-GP double-transgenic mice show that CTL reactivity is normal. These experiments argue against so-called peripheral anergy of class I MHC antigen-restricted cytotoxic T cells as a general mechanism of peripheral immunological tolerance to self. They reveal that self epitopes that are genetically self and presented by class I antigens may not be considered immunologically self if expressed solely extrathymically, despite the fact that they are antigenic and can be recognized by induced effector T cells. Genetic self that is presented on cells which can induce neither tolerance nor an immune response is immunologically dealt with as foreign and therefore may be called nonimmunological self. Appropriate presentation of the same epitope on antigen-presenting cells promptly induces effector T cells and causes disease; such disease should not be called autoimmune because it is an immunopathological T-cell mediated disease, comparable to an unfavorably balanced immunopathological T-cell response to a virus. Mechanisms that control autoantibody responses were studied in mice expressing a viral transgene. Such mice generate neutralizing antiviral autoantibody responses only when the transgenic viral antigen is linked to a foreign T-helper determinant. These findings, therefore, document differences in levels of T- vs B-cell tolerance (so-called split tolerance) under a given expression level of a "self" antigen. They illustrate how unresponsiveness of B cells to produce T-independent IgM is dose-dependent and that IgG autoantibodies are triggered by introducing foreign T-helper determinants that can be recognised in a linked fashion. This model suggests that, while T-cell tolerance to tolerogenic self in the thymus is solid, B-cell tolerance in general is not. From the point of view of autoantibody responses these T-helper cells may also be called immunopathological; i.e., these T-helper cells are specific for foreign epitopes that, via linked recognition, trigger truly autoimmune B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen processing and presentation by small and large B cells

We have investigated the ability of different cells from non-immunized mice of the BALB/c strain to present antigen to two ovalbumin-specific I-Ad-restricted T hybridomas. Lipopolysaccharide-activated B-cell blasts were found to be the most efficient antigen-presenting cells. Purified small and dense splenic B cells also stimulated the hybridomas, although not to the same extent as the activated blasts, but comparable to non-fractionated spleen cells. Glutaraldehyde-treated B cells failed to present antigen, whereas F(ab')2 anti-mouse IgM-treated B cells exhibited markedly increased ability to present antigen. Using flow cytometry, we further purified the resting B cells by sorting the small lymphocytes to ensure that the ability of these cells to activate the hybridomas was not due to contamination with large non-resting B cells. The sorted small B cells retained the ability of antigen presentation. Their resting state was confirmed by the fact that they did not incorporate [3H]-thymidine as shown by autoradiographic analysis.

Human autologous tumor-specific T cells in malignant melanoma

T cell lines and clones with autologous tumor-specific activity have been developed in malignant melanoma by stimulating peripheral blood lymphocytes (PBL), lymph node lymphocytes or tumor-infiltrating lymphocytes (TIL) with autologous melanoma cells in the presence of recombinant interleukin 2 (rIL2). T-cell lines and clones have been developed with specific cytotoxicity and/or proliferative responses for autologous melanoma targets but not for allogeneic melanoma tumor cells, autologous normal cells or natural killer (NK)-sensitive targets. The concentration of rIL2 is critical for the generation of autologous tumor-specific T-cell lines, with low rIL2 concentrations (up to 800 IU/ml) facilitating the growth of T-cell lines with tumor-specific activity. The alpha beta T-cell receptor (TCR) and the CD3 antigen are involved in specific cytotoxicity and/or proliferative responses of these T-cell lines and clones. An oligoclonal pattern of beta-chain TCR gene rearrangements was observed on T-cell lines and clones with autologous tumor-specific cytotoxicity, suggesting that they are comprised of T cells that have undergone a clonal expansion in response to particular antigen. Autologous tumor-specific cytotoxic T cells are HLA-restricted and recognize on the melanoma tumor cells HLA Class I or possibly Class II antigens plus a tumor-specific determinant. TIL from patients with metastatic melanoma have unique characteristics in comparison with PBL and lymph node lymphocytes and they appear to contain substantial proportions of T cells that have been locally sensitized to autologous tumor cells. Single stimulation of TIL with autologous tumor cells in the presence of rIL2 is sufficient for the generation of T cell lines with autologous tumor-specific activity, whereas, multiple stimulation of PBL and lymph node lymphocytes was required to achieve the same purpose. TIL-derived T cell lines have been expanded in rIL2 in vitro by at least 1,500-fold without losing their activity. Approximately, 40% of the patients exhibited complete or partial responses to adoptive immunotherapy with melanoma TIL and rIL2.

Intracellular events associated with inhibition of B cell activation by monoclonal antibodies to HLA class II antigens

We have investigated several aspects of the inhibitory effects of monoclonal antibodies (mAb) directed against MHC class II antigens in B cell activation/proliferation, using a panel of mAb specifically reactive with antigens encoded by HLA class II loci (DP, DQ, DR). All mAb except the anti-DP mAb inhibited significantly anti-mu plus B cell growth factor-induced DNA synthesis. Only one mAb, however, which was reactive with gene products of all three class II loci (DP, DQ, DR) inhibited anti-mu-induced DNA synthesis as well as c-myc mRNA expression. In addition, the same mAb inhibited the early events induced by anti-mu stimulation alone, including phosphatidylinositol turnover and elevation of [Ca2+]i. In contrast to previous findings in the murine system, none of the anti-MHC class II mAb used in this study increased the cAMP levels.

Developmental biology of T cell receptors

T cell receptors are the antigen-recognizing elements found on the effector cells of the immune system. Two isotypes have been discovered, TCR-gamma delta and TCR-alpha beta, which appear in that order during ontogeny. The maturation of prothymocytes that colonize the thymic rudiment at defined gestational stages occurs principally within the thymus, although some evidence for extrathymic maturation also exists. The maturation process includes the rearrangement and expression of the T cell receptor genes. Determination of these mechanisms, the lineages of the cells, and the subsequent thymic selection that results in self-tolerance is the central problem in developmental immunology and is important for the understanding of autoimmune diseases.

Self versus non-self carrier in primary and secondary anti-TNP B cell response: I. Specificity of help

Humoral anti-hapten responses are supposed to require carrier-specific help. Yet, "TNP-specific" help can be activated with TNP coupled to syngeneic lymphocytes. To further clarify the role of hapten-specific vs. carrier-specific helper T cells (TH) in the humoral anti-TNP response, BALB/c mice were immunized with TNP coupled to cellular or soluble self or non-self carriers, and we analyzed primary and secondary B cell responses as well as the apparent specificity of help. TNP bound to syngeneic red blood cells (sRBC), syngeneic albumin (sA) and syngeneic polyclonal or monoclonal immunoglobulin (sIg/smIg) and TNP coupled to non-self carriers initiated equivalent primary anti-TNP responses. On the other hand, a secondary anti-TNP response was only obtained with heterologous carriers or with smIg. Even with heterologous carriers or with smIg, the magnitude of the secondary anti-TNP response exceeded only slightly the amplitudes of a primary anti-TNP response. Furthermore, if animals were challenged shortly after priming, they appeared un/-hyporesponsive. In vitro analysis revealed that in the primary as well as in the secondary anti-hapten response, TNP-specific TH were involved, i.e., the primary response to sIg, sA and sRBC was exclusively due to TNP-specific help, the response after priming with TNP-smIg or TNP-heterologous carrier was due to the additive effects of carrier- and TNP-specific help. Since "carrier-specific" help with smIg was independent of the antibody specificity as well as of the Ig class, we suppose that smIg activated idiotype-specific TH, which functioned like "carrier" TH. Two mechanisms were responsible for the low magnitude of the secondary anti-hapten response: competition for carrier-specific help (using heterologous carriers), and antibody (AB)-dependent suppression.

Alpha, beta and gamma T-cell receptor genes: rearrangements correlate with haematological phenotype in T cell leukaemias

We have studied the arrangement of the alpha, beta and gamma T cell receptor (TCR) genes in 27 patients with T cell lymphoproliferative disorders. Nine patients had acute lymphoblastic leukaemia (T-ALL), nine patients had prolymphocytic leukaemia (PLL), six patients presented with a T-CLL/T-lymphocytosis syndrome, two patients had Sezary syndrome (SS) and one patient had HTLV-I positive T-cell leukaemia/lymphoma (ATLL). alpha TCR gene rearrangement could be demonstrated by the use of three available probes in only one case. By contrast, both beta and gamma TCR gene rearrangement could be demonstrated by Southern blot analysis of DNA samples digested with appropriate restriction enzymes in the majority of cases. In general, when rearrangements were present they involved both alleles. The proportion of rearranged chromosomes was lower in T-ALL than in other forms of T-cell leukaemia and it was lower in cases with the CD4-/CD8+ phenotype than in those with a CD4+/CD8- phenotype. In three out of 34 cases of B-cell leukaemia the TCR beta-gene but not the TCR gamma-gene was rearranged, just as in two out of 26 cases of T-cell leukaemia the immunoglobulin (Ig) heavy chain but not the light chain genes were rearranged. These data suggest that development of the machinery required for gene rearrangement may precede commitment to B or T cell lineage. The use of this technique is especially useful for the classification of cases of ALL in which the cells are negative with respect to most current phenotypic markers and in cases of T cell lymphocytosis in which the finding of a gene rearrangement identifies a monoclonal cell population.