How T cells see antigen

γδ T cells recognize the insulin B:9-23 peptide antigen when it is dimerized through thiol oxidation

The insulin peptide B:9-23 is a natural antigen in the non-obese diabetic (NOD) mouse model of type 1 diabetes (T1D). In addition to αβ T cells and B cells, γδ T cells recognize the peptide and infiltrate the pancreatic islets where the peptide is produced within β cells. The peptide contains a cysteine in position 19 (Cys19), which is required for the γδ but not the αβ T cell response, and a tyrosine in position 16 (Tyr16), which is required for both. A peptide-specific mAb, tested along with the T cells, required neither of the two amino acids to bind the B:9-23 peptide. We found that γδ T cells require Cys19 because they recognize the peptide antigen in an oxidized state, in which the Cys19 thiols of two peptide molecules form a disulfide bond, creating a soluble homo-dimer. In contrast, αβ T cells recognize the peptide antigen as a reduced monomer, in complex with the MHCII molecule I-A(g7). Unlike the unstructured monomeric B:9-23 peptide, the γδ-stimulatory homo-dimer adopts a distinct secondary structure in solution, which differs from the secondary structure of the corresponding portion of the native insulin molecule. Tyr16 is required for this adopted structure of the dimerized insulin peptide as well as for the γδ response to it. This observation is consistent with the notion that γδ T cell recognition depends on the secondary structure of the dimerized insulin B:9-23 antigen.

Comparison of the immunogenicity of wasp venom peptides with or without carbohydrate moieties

Three synthetic vespulakinin analogues either with or without carbohydrate moieties and mastoparan B isolated from Vespa basalis venom were investigated for their immunogenic activity and solution conformation. Mice immunized with these wasp venom peptides, with the exception of (Gal alpha)Thr3, (Gal alpha)Thr4-vespulakinin 1, showed positive antibody responses. However, the response elicited by mastoparan B was much higher than those induced by vespulakinin analogues. The class of antibody induced by these peptides was identified as an IgG1 isotype with kappa-light chain, suggesting stimulation of a T-cell-dependent immune response by these peptides. According to the circular dichroism spectra of these peptides, the structures of the vespulakinin analogues in solution were largely unordered, while mastoparan B exhibited a conformation rich in alpha-helices. The presence of carbohydrate moieties and the rather random structure in vespulakinins may interfere with T-cell recognition of the peptides, leading to lower immune responses.

Induction of human cytotoxic T lymphocytes specific for prostate-specific antigen

Prostate-specific antigen (PSA), a tissue-specific protein expressed by most adenocarcinomas of the prostate, might be a useful target for T-cell-mediated immunotherapy of prostate cancers. The current study examined whether it is possible to elicit human cytotoxic T lymphocytes (CTL) with specificity for PSA. A synthetic nonamer peptide, corresponding to residues 146-154 of PSA and containing a canonical HLA-A2-binding motif, was shown to stabilize the expression of HLA-A2 on the T2 antigen-processing mutant cell line. Repeated in vitro stimulation of peripheral blood lymphocytes from a normal HLA-A2+ donor induced CTL with specificity for the PSA 146-154 peptide. The peptide-induced CTL expressed the CD4- CD8+ cell surface phenotype and were restricted by HLA-A2. A large portion of patients with prostate cancer express the HLA-A2 phenotype, implying that many prostate cancers might be targeted by HLA-A2-restricted CTL with specificity for the PSA 146-154 epitope.

Lateral dynamics of major histocompatibility complex class II molecules bound with agonist peptide or altered peptide ligands

We examined the lateral diffusion of I-Ad on A20 cells following the binding of ovalbumin-derived peptides. The peptides were OVA323-339 and OVA325-335 and a related peptide OVA325-335s substituted H331Q. Only OVA323-339 and OVA325-335 were effectively presented by A20 cells to DO-11.10/S4.4 T cells as assessed by IL-2 production. Fluorescence photobleaching recovery (FPR) measurements showed anti-I-Ad to have a lateral diffusion coefficient on untreated A20 cells of 1.8 +/- 1.0 x 10(-10) cm2 s-1 at 25 degrees C with fluorescence recovery after photobleaching greater than 50%. After 24 h incubation of A20 cells with OVA323-339 or OVA325-335, a subpopulation of A20 cells appeared that were approximately half the size of untreated A20 cells. Culture of A20 with OVA325-355s did not stimulate DO-11.10 cells or induce a size change in A20 cells. Class II molecules were laterally immobile on these small cells with fluorescence recoveries after photobleaching of less than 20%. The relative number of small cells in the A20 cell population was correlated with the immunogenicity of the peptides. These results suggest that immobilization of surface I-Ad may be an important event in antigen presentation.

Aging and haemopoiesis. Implications for treatment with haemopoietic growth factors

Senescence of the lympho-haemopoietic system is associated with an increased incidence of neoplasia, autoimmune diseases and infections. Myelosuppression, either in the context of cancer chemotherapy or in the face of severe infections, commonly manifests as pancytopenia, and has an adverse impact on the prognosis of the elderly cancer patient by increasing infection and bleeding-related morbidity. The physiological basis of this blunted haemopoietic response is unclear, and has been ascribed to age-related deficits in marrow progenitor cell numbers, changes in the marrow microenvironment, decreased production of regulatory growth factors, or a combination of these mechanisms. These age-related deficits tend to be subtle and are only of clinical importance either when present cumulatively or under conditions of extreme haemopoietic stress. Furthermore, some of these deficits can be circumvented with the use of haemopoietic growth factors (HGFs). Thus, the availability in the clinic of various HGFs has had a tremendous impact on the care of the elderly cancer patient. The HGFs currently approved for use are: granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor and epoetin-alpha (recombinant human erythropoietin). However, we still need to better elucidate age-related changes in the early stages of haemopoiesis. The question of haemopoietic exhaustion, particularly under prolonged growth factor stimulation, is real and still unanswered.

The immune system victorious: selective preservation of self

How the body successfully distinguishes its own tissue cells from those that are foreign and genetically nonidentical to it has been a focus of much research. Clonal deletion maintains that immune system cells with the potential to injure self constituents are eliminated during development, thereby neutralizing their capacity to induce self injury. Selected self-reactive maturing T cell clones undergo deletion in the thymus. A two-step selection process affects immature T cells that enter the thymus. Positive selection makes certain that all surviving cells are able to identify major histocompatibility complex (MHC) proteins present on all body cells. These MHC proteins interact with antigens and present them to T lymphocytes. Negative selection is essential for self-tolerance. It eliminates potentially injurious self-reactive T cells by placing them in contact with a mixture of self antigens in the thymus. Clonal anergy might act together with clonal deletion to maintain self tolerance. Self-reactive T cells in the blood of healthy subjects could represent cells whose affinities for antigen are too weak to initiate an immunologic disease. The fate of T cells reacting to a specific antigen has been traced in transgenic mice. Class I MHC molecules present peptides manufactured within the cell, whereas class II MHC molecules present peptides from extracellular proteins. Interaction of a T cell receptor with its homologous antigen associated with MHC molecules leads to proliferation of that T cell in the presence of costimulatory signals. Investigations elucidating the role of T cell receptors, MHC molecules and antigen peptides in self-nonself discrimination are discussed. The article concludes with an introductory summary of the remaining articles in the issue that address selected topics in self-nonself discrimination.

T-cell immunity to the joining region of p210BCR-ABL protein

The hallmark of chronic myelogenous leukemia is the translocation of the human c-abl protooncogene (ABL) from chromosome 9 to the specific breakpoint cluster region (bcr) of the BCR gene on chromosome 22. The t(9;22)(q34;q11) translocation results in the formation of a BCR-ABL fusion gene that encodes a 210-kDa chimeric protein with abnormal tyrosine kinase activity. The ABL and BCR genes are expressed by normal cells and thus the encoded proteins are presumably nonimmunogenic. However, the joining-region segment of the p210BCR-ABL chimeric protein is composed of unique sequences of ABL amino acids joined to BCR amino acids that are expressed only by malignant cells. The current study demonstrates that the joining region of BCR-ABL protein is immunogenic to murine T cells. Immunization of mice with synthetic peptides corresponding to the joining region elicited peptide-specific, CD4+, class II major histocompatibility complex-restricted T cells. The BCR-ABL peptide-specific T cells recognized only the combined sequence of BCR-ABL amino acids and not BCR or ABL amino acid sequences alone. Importantly, the BCR-ABL peptide-specific T cells could recognize and proliferate in response to p210BCR-ABL protein. The response of peptide-specific T cells to protein demonstrated that p210BCR-ABL can be processed by antigen-presenting cells so that the joining segment is bound to class II major histocompatibility complex molecules in a configuration similar to that of the immunizing peptide and in a concentration high enough to stimulate the antigen-specific T-cell receptor. Thus, BCR-ABL protein represents a potential tumor-specific antigen related to the transforming event and shared by many individuals with chronic myelogenous leukemia.

Peripheral blood T lymphocytes in patients with monoclonal gammopathies: expanded subsets as depicted by capacity to bind to autologous monoclonal immunoglobulins or reactivity with anti-V gene-restricted antibodies

The presence of T cells binding F(ab')2 fragments of the idiotypic immunoglobulin was examined by immunofluorescence in peripheral blood of patients with monoclonal gammopathy. In 3 out of 11 tested patients, 1-15% idiotype-binding T cells of either CD4 or CD8 phenotype were found. In 1 patient both a CD4+ and a CD8+ idiotype-binding T-cell fraction were present. In 1 patient the idiotype-binding T cells also reacted with a mAb directed against the variable parts of the TCR alpha or beta chains, further indicating a clonal origin at the alpha/beta level. 3 patients had an expanded predominant T-cell receptor V gene usage based on the reactivity with the limited panel of TCR mAb, but these "clonal" T cells did not bind the idiotype. The study supports the existence of idiotype-specific T cells in peripheral blood of patients with monoclonal gammopathy. Such cells might have a regulatory role on the monoclonal B-cell clone and may be an important target for immunotherapy.

Early evolution of MHC polymorphism

There is unwarranted satisfaction with the view that MHC polymorphism evolved because there was a selective advantage in having a variety of MHC proteins to bind a variety of peptide subsets for presentation to T cells. While this may, in part, explain its maintainance, polymorphism may have evolved initially to reject foreign virus "grafts". The possession of similar membranes promotes aggregation between "like" cells, but it also promotes aggregation between the cells and viruses which retain membrane components of their previous host. The selection pressure afforded by hostile virus "grafts" would favour cells which developed polymorphic membrane components (since "like" will not aggregate with "not-like"). This polymorphism would have evolved before the appearance of multicellular organisms. Thus, the evolution of modern immune systems would have been imposed upon pre-existing polymorphic systems. A path this evolution may have taken involves the development of mechanisms for intracellular distinction between self and not-self.

Kinetics of negative and positive selection in the thymus

Recent experiments show that CD4+8+ thymocytes represent the critical stage in T cell development at which the specificity of randomly generated alpha beta T cell receptors is screened. These cells are deleted when the receptor binds to the MHC molecule plus specific peptide presented by bone marrow derived cells but are rescued from cell death and induced to mature if the receptor binds to the MHC molecule on thymic epithelium in the absence of the specific peptide. Different tolerogens delete CD4+8+ thymocytes earlier or later during their lifespan and negative selection can occur prior to positive selection. The specificity of the alpha beta T cell receptor for either class I or class II thymic MHC molecules determines the CD4-8+ and CD4+8- phenotype of mature T cells.

Low allergenicity of clonidine impedes studies of sensitization mechanisms in guinea pig models

During clinical trials, a clonidine transdermal device has been found to induce clonidine-specific allergic contact dermatitis in up to 25% of patients during a treatment period of 1 year. Using 3 different guinea pig strains, development was attempted of an experimental guinea pig model that would allow for in-depth studies into the mechanism of sensitization, and a possible role of transdermal device components. Transient low-level clonidine allergy could be obtained only in a minority of animals, with severe sensitization procedures departing from epicutaneous applications, combined with intradermal (adjuvant) FCA injections. Sensitization was not potentiated by additional booster procedures, including cyclophosphamide pretreatment, nor any of the putative cofactors (UV-treatments, C. parvum or acetaldehyde involvement) studied. These results suggest that the persistent skin contacts in man, with transdermal devices for sustained drug delivery, generate unique conditions favouring the development of allergic contact dermatitis, which are difficult to mimic in experimental animal models. Thus, clinical allergy may develop even to extremely weak sensitizing drugs that can be safely used orally, and escape most currently available predictive contact allergy animal models. Clinical studies remain unavoidable for studying factors that may reduce sensitization rates to more acceptable levels.

Autoimmune diseases: the failure of self tolerance

The ability to discriminate between self and nonself antigens is vital to the functioning of the immune system as a specific defense against invading microorganisms. Failure of the immune system to "tolerate" self tissues can result in pathological autoimmune states leading to debilitating illness and sometimes death. The induction of autoimmunity involves genetic and environmental factors that have focused the attention of researchers on the trimolecular complex formed by major histocompatibility complex molecules, antigen, and T cell receptors. Detailed molecular characterization of these components points to potential strategies for disease intervention.