Antigen-processing organelles from DRB1*1101 and DRB1*1104 B cell lines display a differential degradation activity

The role of MHC- and non-MHC-associated genes in determining the human immune response to malaria antigens

Individual susceptibility to malaria infection, disease and death is influenced by host genotype, parasite virulence and a number of environmental factors including malaria-specific immunity. Immune responses are themselves determined by a combination of host genes and environmental effects. The extent to which host genotype limits the spectrum of possible immune responses may influence the outcome of infection and has consequences for vaccine design. Associations have been observed between human major histocompatibility complex (MHC) genotype and susceptibility to severe malaria, but no similar associations have been observed for mild malarial disease or for specific antibody responses to defined malaria antigens. Epidemiological studies have shown that, in practice, neither T helper cell nor antibody responses to malaria parasites are limited by host MHC genotype, but have revealed that genes lying outside the MHC may influence T cell proliferative responses. These genes have yet to be identified, but possible candidates include T cell receptor (TcR) genes, and genes involved in TcR gene rearrangements. More importantly, perhaps, longitudinal epidemiological studies have shown that the anti-malarial antibody repertoire is selective and becomes fixed in malaria-immune individuals, but is independent of host genotype. These findings suggest that the antibody repertoire may be determined, at least in part, by stochastic events. The first of these is the generation of the T and B cell repertoire, which results from random gene recombinations and somatic mutation and is thus partially independent of germline genes. Secondly, of the profusion of immunogenic peptides which are processed and presented by antigen presenting cells, a few will, by chance, interact with T and B cell surface antigen receptors of particularly high affinity. These T and B cell clones will be selected, will expand and may come to dominate the immune response, preventing the recognition of variant epitopes presented by subsequent infections - a process known as original antigenic sin or clonal imprinting. The immune response of an individual thus reflects the balance between genetic and stochastic effects. This may have important consequences for subunit vaccine development.

Ex vivo monitoring of antigen-specific CD4+ T cells after recall immunization with tetanus toxoid

To monitor antigen-specific CD4+ T cells during a recall immune response to tetanus toxoid (TT), a sequential analysis including ex vivo phenotyping and cytokine flow cytometry, followed by cloning and T-cell-receptor (TCR) spectratyping of cytokine-positive CD4+ T cells, was performed. Grossly, twice as many TT-specific CD4+ T-cell clones, ex vivo derived from the CCR7+/- CD69+ interleukin-2-positive (IL-2+) CD4+ subsets, belonged to the central memory (T(CM); CD62L+ CD27+ CCR7+) compared to the effector memory population (T(EM); CD62L- CD27- CCR7-). After the boost, a predominant expansion of the T(CM) population was observed with more limited variations of the T(EM) population. TCR beta-chain-variable region (BV) spectratyping and sequencing confirmed a large concordance between most frequently expressed BV TCR-CDR3 from ex vivo-sorted CCR7+/- CD69+ IL-2+ CD4+ subsets and BV usage of in vitro-derived TT-specific CD4+ T-cell clones, further demonstrating the highly polyclonal but stable character of the specific recall response to TT. Taken together, ex vivo flow cytometry analysis focused on the CCR7+/- CD69+ IL-2+ CD4+ subsets appears to target the bulk of antigen-specific T cells and to reach an analytical power sufficient to adequately delineate in field trials the profile of the antigen-specific response to vaccine.

Analysis of T-cell receptor beta chain variable gene segment usage in healthy adult responders and nonresponders to recombinant hepatitis B vaccine

One to 10 per cent of healthy adult individuals do not produce protective levels of anti-hepatitis B surface (HBs) antibodies, following a standard vaccination protocol. Lack of an HBs antigen (Ag)-specific T-cell repertoire is amongst the possible defects, which may lead to humoral unresponsiveness and is the main objective of this study. We analysed TcR BV (T-cell receptor beta chain variable) gene usage in T lymphocytes from nine healthy adult responders and six nonresponders to recombinant HB vaccine, before and after booster vaccination. CD4+ and CD8+ T-cell populations were isolated from peripheral blood mononuclear cells by magnetic beads, and the expression of TcR BV genes in each population was investigated by reverse transcription polymerase chain reaction and hybridization with specific probe. When the usage of each TcR BV gene within CD4+ and CD8+ T cells of the responders was compared with that of nonresponders, statistically significant difference (P < 0.01) was noted for BV5S2-3 gene family in CD4+ T cells of nonresponders. Furthermore, individual vaccinees were shown to overexpress several TcR BV genes. To characterize the T-cell repertoire and determine their clonal nature, analysis of CDR3 length polymorphism was performed. Our results show that T-cell response to HBsAg is generally oligoclonal and involves multiple BV families. Furthermore, overexpressed individual TcR BV genes and CDR3 length distributions in response to HBsAg are subject-dependent. In conclusion, our results are not in line with the notion that defective TcR repertoire may be an explanation for unresponsiveness to recombinant HBsAg vaccine.

Major differences in antigen-processing correlate with a single Arg71<-->Lys substitution in HLA-DR molecules predisposing to rheumatoid arthritis and with their selective interactions with 70-kDa heat shock protein chaperones

Several HLA-DR alleles are genetically associated with rheumatoid arthritis. DRB1*0401 predominates in Northern Europe and has a characteristic (70)QKRAA motif. This sequence contacts bound peptides and the TCR. Further interactions have been suggested with additional proteins during Ag loading. We explored the much stronger processing/presentation of full-length recombinant human acetylcholine receptor alpha subunit to a specific T cell clone by APC from DRB1*0401+ than *0408+ donors. Using DR*04 transfectants, we show that this difference results largely from the single Lys71<-->Arg interchange (0401<-->0408), which scarcely affects epitope binding, rather than from any other associated polymorphism. Furthermore, we proved our recombinant polypeptides to contain the Escherichia coli 70-kDa heat shock protein molecule DnaK and its requirement for efficient processing and presentation of the epitope by DRB1*0401+ cells. According to a recent report, 70-kDa heat shock protein chaperones preferentially bind to the QKRAA, rather than the QRRAA, motif. Variations between the shared epitope motifs QKRAA and QRRAA are emphasized by underlining. We propose that such interactions enhance the intracellular epitope loading of *0401 molecules. They may thus broaden immune responses to pathogens and at least partially explain the distinct contributions of DRB1*0401 and other alleles to disease predisposition.

Endosomal proteases and antigen processing

T cells are activated by fragments of antigenic proteins bound to major histocompatibility complex (MHC) molecules and displayed on the cell surface. MHC class II proteins scavenge processed protein antigens from within endosomal compartments. The antigenic peptides are generated within these and other intracellular compartments using the array of proteolytic enzymes normally involved in terminal protein degradation. Antigen-presenting cells use different mechanisms to exploit and control the activity of these enzymes so as to ensure the generation of a wide variety of peptides, while preventing the destruction of antigenic epitopes by excessive proteolysis.

Peptide-MHC complexes assembled following multiple pathways: an opportunity for the design of vaccines and therapeutic molecules

Antigen degradation and peptide loading to major histocompatibility complex class I and class II molecules are described with special emphasis on "noncanonical" pathways. Examples of specific peptide loading for measles proteins are provided. In addition, characterization of defined epitopes presented to T cells can lead to the design of products of special interest in medicine and, in particular, in development of vaccines.

Epitope cleavage by Leishmania endopeptidase(s) limits the efficiency of the exogenous pathway of major histocompatibility complex class I-associated antigen presentation

The activation of CD8+ T cell responses is commonplace during infection with a number of nonviral pathogens. Consequently, there has been much interest in the pathways of presentation of such exogenous antigens for major histocompatibility complex class I-restricted recognition. We had previously shown that Leishmania promastigotes transfected with the ovalbumin (OVA) gene could efficiently target OVA to the parasitophorous vacuole (PV), with subsequent recognition by class II-restricted T cells. We now report the results of studies aimed at evaluating the PV as a route of entry into the exogenous class I pathway. Bone marrow-derived macrophages can present soluble OVA (albeit at high concentrations) to the OVA(257-264)-specific T cell hybridoma 13.13. In contrast, infection with OVA-transfected Leishmania promastigotes failed to result in the stimulation of this hybridoma. This appeared unrelated to variables such as antigen concentration, parasite survival, and macrophage activation status. These results prompted an analysis of the effects of promastigotes on class I peptide binding using RMA-S cells and OVA(257-264). Our data indicate that the major surface protease of Leishmania, gp63, inhibits this interaction by virtue of its endopeptidase activity against the OVA(257-264) peptide. The data suggest that this activity, if maintained within the PV, would result in loss of the OVA(257-264) epitope. Although we can therefore draw no conclusions from these studies regarding the efficiency of the PV as a site of entry of antigen into the exogenous class I pathway, we have identified a further means by which parasites may manipulate the immune repertoire of their host.

Antigenicity of HIV-derived T helper determinants in the context of carrier recombinant proteins: effect on T helper cell repertoire selection

T helper (Th) epitopes can be included in a recombinant protein with B and CTL epitopes to create more effective immunogens. To determine whether the antigenicity of HIV Th epitopes is preserved in this altered molecular context, human Th clones specific for peptides of HIV gp120 and reverse transcriptase p66 were challenged with recombinant proteins carrying the antigenic epitopes in different sites. We found that a given epitope was recognized by a specific T cell clone only when it was inserted in a particular position of the carrier. However, the permissive position was not the same for all epitopes. Enzymatic excision from a nonpermissive context or insertion of a polyserine spacer between the epitope and the carrier restored antigenicity. Nevertheless, antigenicity was not abolished in a synthetic peptide encompassing the epitope and the neighboring residues from the nonpermissive location. These data suggest that, in this case, the primary sequence of the chimeric protein flanking the HIV peptide is not responsible for loss of antigenicity. Furthermore, constructs carrying the epitope in a given position were recognized by peptide-specific Th clones raised from some individuals, but not from others. We show that this is due neither to individual modes of processing nor to the use of distinct major histocompatibility complex MHC class II restriction elements, but rather that it is related to the fine specificity of the clones. To study the effect of epitope context on selection of T cell repertoire in a naive individual, T cell lines were generated in vitro by stimulation with different peptide constructs. This resulted in the induction of diverse clonotypes defined by the pattern of recognition of different constructs, by T cell receptor V beta gene usage and by fine epitope mapping.

Indirect recognition of donor HLA-DR peptides in organ allograft rejection

To determine whether indirect allorecognition is involved in heart allograft rejection T cells obtained from peripheral blood and graft biopsy tissues were expanded in the presence of IL-2 and tested in limiting dilution analysis (LDA) for reactivity to synthetic peptides corresponding to the hypervariable regions of the mismatched HLA-DR antigen(s) of the donor. Serial studies of 32 patients showed that T cell reactivity to donor allopeptides was strongly associated with episodes of acute rejection. The frequency of allopeptide reactive T cells was 10-50-fold higher in the graft than in the periphery indicating that T cells activated via the indirect allorecognition pathway participate actively in acute allograft rejection. In recipients carrying a graft differing by two HLA-DR alleles the response appeared to target only one of the mismatched antigens of the donor. Indirect allorecognition was restricted by a single HLA-DR antigen of the host and directed against one immunodominant peptide of donor HLA-DR protein. However, intermolecular spreading was demonstrated in patients with multiple rejection episodes by showing that they develop allopeptide reactivity against the second HLA-DR antigen. These data imply that early treatment to suppress T cell responses through the indirect pathway of allorecognition, such as tolerance induction to the dominant donor determinant, may be required to prevent amplification and perpetuation of the rejection process.

Antibody response to sheep red blood cells in major histocompatibility (B) complex aneuploid line of chickens

An integral part of the immune response is the production of antibodies specific for different antigenic challenges. Genes of the MHC encode products that regulate immunity. This study utilized the FCT-15 line of chickens, which is aneuploid for the chromosome containing the ribosomal RNA genes (rDNA) and the MHC or B complex to determine whether an antibody response to SRBC would vary as a function of B complex gene dose. Mating of trisomic parents (B15B15B15) animals produced progeny having either a disomic (B15B15), trisomic (B15B15B15), or tetrasomic (B15B15B15B15) B complex dosage. The number of B/rDNA chromosomes, and thus the B complex dosage was determined by feather pulp nucleolar typing of chicks at hatch. A 5% SRBC antigenic challenge, which induces a T cell-dependent antibody response, was injected at 6 wk of age. Samples taken prior to SRBC injection as well as 5, 8, and 12 d postinjection were assayed for total and mercaptoethanol-resistant antibody. Peak antibody titers (log2), day of peak titer and rate of titer decline were calculated using a quadratic equation for each bird. Differences among the three B complex dosages were evaluated by analysis of variance. Antibody titers rose from 5 to 8 d postinjection and declined thereafter without significant differences among the three B complex doses. Calculations from the quadratic equations showed that B complex dose affected neither peak antibody titer nor day of peak titer. However, trisomic and tetrasomic animals had significantly more rapid rates of decline from the maximum titer. In aneuploid chickens, changes in antigen processing, antigen presentation, or persistence of processed antigen may maintain levels of antibody production found in disomic chickens and explain the more rapid decline of titer.

New approaches to specific immunomodulation in transplantation

T cells can recognize foreign MHC antigens by two distinct routes, either directly as intact molecules, or indirectly as processed peptides. Recent evidence strongly suggests that the indirect pathway of allorecognition plays a key role in initiating and sustaining graft rejection. Theoretically, all mismatched HLA alloantigens could generate immunogenic peptides which may be recognized in the context of any of the two self HLA-DR molecules. However, indirect recognition appears to be limited to a single peptide determinant of an allogeneic HLA-DR molecule and restricted by one self HLA-DR molecule. Furthermore, T cells involved in the self-restricted allopeptide recognition express a limited array of T cell receptor variable genes. These findings suggest that selective immune interventions, such as peptide blockade of the self HLA-DR molecule involved in the presentation of the dominant allopeptide, induction of high-zone tolerance or TCR antagonism, may be devised to prevent graft rejection.

Differences in processing of an autoantigen by DR4:Dw4.2 and DR4:Dw14.2 antigen-presenting cells

Variations in antigen processing can influence class II-restricted T cell responses. We now report a highly significant difference (p < 0.001) between the ability of antigen-presenting cells from three HLA-DR4:Dw14.2 (Arg71) and six DR4:Dw4.2 (Lys71) individuals to present recombinant or native acetylcholine receptor antigens to a myasthenia gravis T cell clone. The difference was greatest with longer antigens, and not seen with short synthetic peptides, suggesting that it may result from a difference in antigen processing between the two alleles. The results were not related to the presence of myasthenia gravis or of steroid therapy. They could, however, be of relevance in rheumatoid arthritis where particularly severe disease associates with Dw4.2/Dw14.2 heterozygosity.