Preferential pairing of T-B specificities in the same antigen: the concept of directional help

The Relative Positioning of B and T Cell Epitopes Drives Immunodominance

Humoral immunity is crucial for protection against invading pathogens. Broadly neutralizing antibodies (bnAbs) provide sterilizing immunity by targeting conserved regions of viral variants and represent the goal of most vaccination approaches. While antibodies can be selected to bind virtually any region of a given antigen, the consistent induction of bnAbs in the context of influenza and HIV has represented a major roadblock. Many possible explanations have been considered; however, none of the arguments proposed to date seem to fully recapitulate the observed counter-selection for broadly protective antibodies. Antibodies can influence antigen presentation by enhancing the processing of CD4 epitopes adjacent to the binding region while suppressing the overlapping ones. We analyze the relative positioning of dominant B and T cell epitopes in published antigens that elicit strong and poor humoral responses. In strong immunogenic antigens, regions bound by immunodominant antibodies are frequently adjacent to CD4 epitopes, potentially boosting their presentation. Conversely, poorly immunogenic regions targeted by bnAbs in HIV and influenza overlap with clusters of dominant CD4 epitopes, potentially conferring an intrinsic disadvantage for bnAb-bearing B cells in germinal centers. Here, we propose the theory of immunodominance relativity, according to which the relative positioning of immunodominant B and CD4 epitopes within a given antigen drives immunodominance. Thus, we suggest that the relative positioning of B-T epitopes may be one additional mechanism that cooperates with other previously described processes to influence immunodominance. If demonstrated, this theory can improve the current understanding of immunodominance, provide a novel explanation for HIV and influenza escape from humoral responses, and pave the way for a new rational design of universal vaccines.

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.

Increased proteolysis of diphtheria toxin by human monocytes after heat shock: a subsidiary role for heat-shock protein 70 in antigen processing

The expression of heat-shock proteins (hsp) increases after exposure to various stresses including elevated temperatures, oxidative injury, infection and inflammation. As molecular chaperones, hsp have been shown to participate in antigen processing and presentation, in part through increasing the stability and expression of major histocompatibility complex molecules. Heat shock selectively increases human T-cell responses to processed antigen, but does not affect T-cell proliferation induced by non-processed antigens. Here, we have analysed the mechanisms by which stress such as heat shock, and the ensuing hsp over-expression affect the processing of diphtheria toxin (DT) in peripheral blood monocytes. We found that heat shock increased DT proteolysis in endosomes and lysosomes while the activities of the cathepsins B and D, classically involved in DT proteolysis, were decreased. These effects correlated with the heat-shock-mediated increase in hsp 70 expression observed in endosomes and lysosomes. Actinomycin D or blocking anti-hsp 70 antibodies abolished the heat-shock-mediated increase in DT proteolysis. These data indicate that the increased expression of hsp 70 constitutes a subsidiary mechanism that facilitates antigen proteolysis in stressed cells. Confirming these data, presentation by formaldehyde-fixed cells of DT proteolysates that were obtained with endosomes and lysosomes from heat-shocked peripheral blood monocytes showed higher stimulation of T cells than those generated with endosomes and lysosomes from control peripheral blood monocytes.

Human autoantibodies modulate the T cell epitope repertoire but fail to unmask a pathogenic cryptic epitope

Abs can tune the responses of Ag-specific T cells by influencing the nature of the epitope repertoire displayed by APCs. We explored the interaction between human self-reactive T cells and human monoclonal autoantibodies from combinatorial Ig-gene libraries derived from autoimmune thyroiditis patients and specific for the main autoantigen thyroid peroxidase (TPO). All human mAbs extensively influenced the T cell epitope repertoire recognized by different TPO-specific T cell clones. The action of the human mAbs was complex, because sometimes the same Ab suppressed or enhanced the epitopes recognized by the 10 different TPO-specific T cell clones. The human mAbs could modulate the epitope repertoire when TPO was added exogenously and when expressed constitutively on the surface of APCs. However, they could not unmask an immunodominant cryptic TPO epitope. In this study, we show that human autoantibodies influence the activity of self-reactive T cells and prove their relevance in concealing or exposing epitopes recognized by self-reactive T cells. However, our results further stress the biological significance of the immunodominant cryptic epitope we have defined and its potential importance in the evolution of autoimmunity.

Antibody modulation of antigen presentation: positive and negative effects on presentation of the tetanus toxin antigen via the murine B cell isoform of FcgammaRII

Tetanus toxin has been a valuable model antigen to study the MHC class II-restricted antigen processing pathway and is also frequently used to provide T helper determinants in vaccine formulations. To date most basic studies on the processing of this antigen have utilized human T and B cell clones. As a first step towards extending studies on this antigen into the murine system we have generated a panel of T cell clones and mAb in H-2(b) and H-2(d) mice. We investigated the presentation of tetanus toxin C fragment (TTCF) by the murine B cell lines LB27.4 (H-2(dxb)), A20 (H-2(d)) and IIA1.6 (H-2(d)) and the extent to which this could be modulated by the addition of mAb. One mAb, 10G5, induced strikingly enhanced presentation of T cell determinants located in the N-terminal region of TTCF while other antibodies inhibited presentation of these and other epitopes. The enhancing effects of the 10G5 antibody were blocked by the anti-FcR antibody 2.4G2 and were not observed in the FcR-negative IIA1.6 cell line. Interestingly, both FcgammaRIIB1 and FcgammaRIIB2 isoforms of FcgammaRII were able to restore antibody enhanced presentation in IIA1.6 cells but only if the cytoplasmic tails were intact. These results show that the B cell isoform of FcgammaRII (FcgammaRIIB1) can mediate capture and presentation of some antigen/antibody complexes and might play a role in BCR-independent antigen presentation in vivo.

Modulation by epitope-specific antibodies of class II MHC-restricted presentation of the tetanus toxin antigen

Above a certain affinity the dissociation rate of monovalent antigen from antibody becomes slower than the time taken for antigen capture, endocytosis and processing by professional antigen presenting cells. Thus, when high affinity antibodies drive antigen uptake, either directly via B-cell membrane immunoglobulin or indirectly via Fc receptors, the substrate for processing may frequently be an antigen/antibody complex. Here we review studies using the tetanus toxin antigen which show that bound antibodies can dramatically affect proteolytic processing, dependent on the epitope specificity and multiplicity of antibodies bound. Certain antibodies protect or 'footprint' specific domains of the antigen during processing in B-cell clones resulting in modulation of loading of class II MHC-restricted T-cell epitopes. Processing and class II MHC loading of some T-cell epitopes within the footprinted region was hindered, as might be expected, but, surprisingly, presentation of other T-cell epitopes was boosted considerably. These studies show that protein/protein complexes can be processed in an unpredictable fashion by antigen presenting cells and indicate a possible mechanism whereby cryptic T-cell epitopes might be revealed in autoimmune disease.

T cell responses in EAMG-susceptible and non-susceptible mouse strains after immunization with overlapping peptides encompassing the extracellular part of Torpedo californica acetylcholine receptor alpha chain. Implication to role in myasthenia gravis of autoimmune T-cell responses against receptor degradation products

To study the role in myasthenia gravis (MG) of peptides resulting from acetylcholine receptor (AChR) degradation, we examined the ability of AChR peptides to induce T cell responses that are capable of cross-reacting with intact AChR. The studies were carried out in an experimental autoimmune MG (EAMG)-susceptible mouse strain [C57BL/6 (B6)] as well as in two non-susceptible strains [B6.C-H-2bm12 (bm12) and C3H/He]. A set of overlapping peptides encompassing the extracellular part (residues 1-210) of the alpha-chain of Torpedo californica (t) AChR were used, individually or in equimolar mixtures, as immunogens. In B6, immunization with peptides alpha45-60, alpha111-126, alpha146-162 and alpha182-198 gave T cells that responded in vitro to the correlate immunizing peptide. Only the T cells against the latter three peptides cross-reacted with tAChR. Peptide alpha146-162 exhibited the highest in vitro reaction with the immunizing peptide and cross-reaction with tAChR. T cells obtained by immunization of B6 with an equimolar mixture of the peptides responded in vitro to peptides alpha111-126, alpha146-162 and alpha182-198 and cross-reacted very strongly with tAChR. In bm12 and C3H/He, a number of peptides evoked, when used individually as immunogens, strong or moderate T cell responses that recognized in vitro the correlate immunizing peptide but cross-reacted poorly with tAChR. Immunization with the mixture of the peptides gave T cells that recognized several peptides in each strain butdid not cross-react with alpha146-162 or tAChR. The results indicate that the ability to recognize alpha146-162 or AChR by T cells against peptides resulting from receptor degradation can account for the susceptibility to, and aggravation of, MG in B6.

Antibodies and T cells against synthetic peptides of the C-terminal domain (Hc) of botulinum neurotoxin type A and their cross-reaction with Hc

Seventeen peptides containing T cell and/or antibody (Ab) epitopes previously localized on Hc of botulinum neurotoxin type A were used in SJL and BALB/c mice as immunogens either individually or as an equimolar mixture of groups that contained epitopes of T cells, Abs or both, to determine their abilities to generate T cells and/or Abs that recognize intact Hc. In SJL, peptide 897-915 which included both T cell and Ab epitopes, elicited Abs that cross-reacted very strongly with Hc. In BALB/c, peptides 869-887, 883-901, 981-999 and 1275-1296 which contained Ab epitopes generated Abs that cross-reacted strongly with Hc. A mixture of peptides that contained T cell and Ab epitopes was effective in both strains in eliciting T cells and Abs that cross-reacted with Hc. This mixture form gave a quicker rise (after two injections) in cross-reactive (with Hc) Ab titer as compared to other peptide mixtures or the individual peptides, and sustained in BALB/c a high Ab titer upon further booster injections. Some of the regions that elicited crossreactive immunity to Hc have sequence similarity to other clostridial toxins, suggesting that one or more of these synthetic peptides might provide cross-protection against those toxins.

Intersite helper function of T cells specific for a protein epitope that is not recognized by antibodies

Humoral responses to a protein require T-B cell communication for B cell activation by T cells. Previous studies from this laboratory have mapped the T and B cell recognition sites (epitopes) on sperm-whale myoglobin (Mb) and several other proteins. It was found that, five of six regions on Mb recognized by T cells are also recognized by B cells (i.e. antibodies). There is, however, one region (E6) residing within Mb residues 61-77, that is recognized only by T cells and to which no antibody (Ab) responses are detectable. To investigate the function of this exclusive T cell epitope, we established, from E6-primed BALB/c mice, an E6-specific T cell line (T(e6)) which comprised Th2-type cells. These T cells provided help in vitro to B cells from Mb-primed BALB/c mice and activated them to produce anti-Mb Abs of the IgM (58.2%) and IgG (41.8%) isotypes. The helper activity of T(e6) cells was dependent on the concentration of the challenging Ag (intact Mb or peptide E6) in culture. Action of soluble factors released from E6-activated T(e6) cells on B(mb) cells led to low production of anti-Mb Abs, suggesting that activation of the B cells was more dependent on their contact with T cells. Mapping of the epitope recognition of the anti-Mb Abs produced in vitro by B(mb) cells on activation by T(e6) revealed that this activation was not general to all antigenic regions recognized by anti-Mb Abs in BALB/c mice. E6-specific T cells caused in vitro activation and differentiation of B(mb) cells into plasma cells that secreted anti-Mb Abs directed, in decreasing order, against the following Mb regions: E4 (107-120) > E3 (87 - 100) > E1 (10 - 22). Little or no Ab responses could be detected against peptides E2 (50 - 62), E5 (141 - 153) and E6 (61 - 77). With B cells of peptide-primed BALB/c mice, T(e6) cells activated strongly E4-, E3- or E1, and only very slightly E2- or E6-, primed B cells to secrete Abs against the correlate peptide, but failed completely to activate E5-primed B cells. The results show that a protein T cell epitope, to which no Abs are detectable, plays an active role in B cell responses against other epitopes within the same protein.

Predominance of H-2d- and H-2k-restricted T-cell epitopes in the highly repetitive Plasmodium falciparum antigen Pf332

Genetic restriction of immune responses to malaria antigens is an important issue for a better comprehension of malaria immunity as well as for development of subunit vaccines. To experimentally define the major histocompatibility complex restriction of immune responses to the highly repetitive Plasmodium falciparum high-molecular-weight antigen Pf332, H-2-congenic mice were immunized with EB200, a recombinant fragment of Pf332 consisting of degenerate repeat motifs. Strong B- and T-cell responses were elicited in H-2d and H-2k mice whereas responses in H-2b, H-2q and H-2s mice were of lower magnitude. The T-cell specificity elicited by EB200 was defined by in vitro proliferative responses to a panel of overlapping peptides spanning EB200. Dominant epitopes were identified for H-2d and H-2k mice, respectively, and an additional epitope was recognized by all five mouse strains. Selected EB200-derived peptides were further investigated for their ability to elicit T-cell help when injected as multiple antigen peptides. Defined H-2d- and H-2k-restricted T-cell epitopes generated high antibody levels in the respective mouse strains, as did several peptides lacking defined epitopes indicating the presence of additional H-2d- and H-2k-restricted, cryptic or subdominant T-cell epitopes in EB200. The biased H-2 restriction pattern of T-cell epitopes in Pf332 and, as previously reported, in structurally related repeats in the malaria antigens Pf11.1 and Pf155/RESA may be explained by a shared motif for H-2d and H-2k class II-restricted T-cell epitopes, as revealed by alignment of these sequences.

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.

Modulation of antigen processing by bound antibodies can boost or suppress class II major histocompatibility complex presentation of different T cell determinants

Bound antibodies can modulate antigen processing but it is not clear to what extent this affects antigen presentation. Here we show that presentation of T cell determinants in tetanus toxin can be either enhanced or suppressed as a direct consequence of antibody modulation of antigen processing in human B lymphoblastoid cells. Remarkably, a single bound antibody or its Fab fragment can simultaneously enhance the presentation of one T cell determinant by more than 10-fold while strongly suppressing the presentation of a different T cell determinant. Biochemical analysis demonstrates that both the suppressed and boosted determinants fall within an extended domain of antigen stabilized or "footprinted" by this antibody during proteolysis. These results demonstrate that bound antibodies can modulate the capture of peptides by class II major histocompatibility complex (MHC), thus manipulating the T cell response towards or away from particular determinants. Altered processing of protein-protein complexes leading to enhanced loading of class II MHC and substantially lowered threshold for T cell activation suggests a novel mechanism that might reveal "cryptic" self determinants.

Neonatal tolerance to an immunodominant T cell reactivity does not confer resistance to EAMG induction in Lewis rats

The overall goal of this study was to determine, during induction of experimental autoimmune myasthenia gravis (EAMG) in Lewis rats, the relative importance of acetylcholine receptor (AChR)-reactive helper T cells associated with one particular immunodominant fine specificity. Thus, experiments presented below were designed to evaluate the immunopathological role played by helper T cells with reactivity against the AChR alpha subunit region associated with amino acid residues 100-116 (i.e., alpha 100-116); in particular, the relationship between T cell reactivity with this specificity and disease induction was assessed. In order to examine the importance of this T cell reactivity, Lewis rat neonates were made T cell tolerant to a synthetic peptide alpha 100-116 and subsequently evaluated for anti-AChR antibody production and resulting neuromuscular dysfunction. Results indicated that although T cell reactivity against the alpha 100-116 peptide could be effectively removed from the Lewis T cell repertoire, tolerized Lewis rats immunized with AChR could undergo an active anti-AChR antibody response that produced symptoms of EAMG. Thus, other AChR T cell reactivities appeared capable of providing adequate help to B cells leading to production of anti-AChR antibodies with pathogenic potential.

Fine specificity of immune responses to epitopic sequences in synthetic peptides containing B and T epitopes from the conserved Plasmodium falciparum blood-stage antigens

Immunisation with two chemically synthesised, linear, multiple epitope peptides (MEP) containing B and T cell epitopes from two conserved blood-stage antigens of the human malaria parasite, Plasmodium falciparum, induced high levels of circulating antibodies without the use of a carrier protein. Immunisation of BALB/c mice with MEP constructs (P1 and P2) induced antibodies against the various epitope sequences included in their structures, although the immune response was focused more towards the N terminal and the middle portion of the peptides. In vitro T cell proliferation assays indicated that only one of the two Th epitopes included in P1 and P2 are functional. Both P1 and P2, based on P. falciparum sequences, cross-reacted with sera from P. yoelii-infected mice. Immunisation with P1 in CFA, but not with P2, provided partial protection to BALB/c mice against P. yoelii challenge infection. Peptide P1 was highly immunogenic in alum also, and a somewhat higher level of protection was observed as compared to CFA immunisation. We found that immunisation with P1 induced antibody responses in different strains of mice, although to different extents. These results suggest that linear, multiple epitope peptides may offer attractive alternatives as subunit vaccine candidate molecules, but at the same time highlight the fact that the design principles are far from being clear and have yet to be worked out.

Antigenicity and immunogenicity of P30-derived peptides in experimental models of toxoplasmosis

P30, also referred to as SAG-1, is now recognized as a major Toxoplasma gondii antigen potentially important for both diagnosis and immunoprophylaxis of toxoplasmosis. By using predictive algorithms, five synthetic peptides (48-67, 82-102, 213-230, 238-256 and 279-285) derived from P30, were investigated for B- and T-cell determinants in mouse and rat experimental models. Antibody recognition appeared more broadly distributed along the P30 sequence, whereas T-cell recognition was mainly targeted on the 238-256 peptide. In the absence of any carrier protein, this peptide induced a B- and T-cell immune response independent of the route of immunization (oral route or subcutaneous injection). This peptide (238-256) induced multiple antibody isotypes. In contrast with the 238-256 peptide, the 48-67 peptide, either free or in the form of a multiple antigenic peptide (MAP) construct or the 279-295 peptide, elicited antibodies associated with a TH2 response. This study reports for the first time the analysis of the antigenic and immunogenic properties of P30-derived peptides and are potentially useful for vaccinal strategies incorporating the P30 Toxoplasma gondii antigen.

Analysis of the antigenic profile of measles virus haemagglutinin in mice and humans using overlapping synthetic peptides

In this study, a panel of 55 synthetic peptides representing 92.2% of the haemagglutinin (H) glycoprotein of measles virus (MV) were used to study the antigenic profile of the H molecule of anti-MV antibodies raised in mice and late convalescent human sera. In addition the immunogenicity of these peptides was tested in two mouse strains. Mouse anti-MV antibodies had different fine specificity of binding to the peptides depending on the mouse strain. Thus in BALB/c (H-2d) mice, anti-MV antibodies recognised six peptides representing residues 103-117; 123-137; 242-255; 293-307 and 463-477. In TO (H-2s) mice, anti-MV antibodies recognised peptides representing residues 49-72 and 463-477. When the immunogenicity of the peptides was tested, 29 were immunogenic in BALB/c mice and 34 were immunogenic in TO mice. Several of the anti-peptide antisera were found to cross-react with MV, depending on the solid phase assay system used but none were able to inhibit virus infectivity in vitro. The reactivity of a panel of late convalescent human sera with the peptides was heterogeneous and the extent of the binding to the peptides was related to the titre of anti-MV. However, human sera recognized certain peptides more frequently than others, in particular peptides at the carboxyl-terminus.

Immunoregulation in fish. I: Intramolecular-induced suppression of antibody responses to haptenated protein antigens studied in Atlantic salmon (Salmo salar L)

We report here evidence for intramolecular-induced suppression of the in vivo antibody response in fish, using a panel of T-dependent hapten-carrier antigens. Atlantic salmon were immunized intraperitoneally with protein antigens (Limulus polyphemus hemocyanin, chicken gamma globulin, and Aeromonas salmonicida A-layer protein) given in their native form or haptenated with either 4-hydroxy-3-iodo-5-nitrophenyl-acetic acid (NIP), 2,4,6-trinitrophenyl-acetic acid (TNP), or fluorescein-5-iso-thiocyanate (FITC). The salmon immune system responds to these hapten-carrier antigens by eliciting high anti-hapten titers whereas the antibody titers against protein determinants were suppressed 87-99%, determined by ELISA. NIP also induced suppression of the anti-FITC response when NIP and FITC were intramolecularly conjugated to Limulus polyphemus hemocyanin (LPH). The suppression was found to be independent of haptenation ratios and time after immunization. The possibility that haptenation interferes with or blocks the protein determinants is not likely because antisera raised against native LPH recognize LPH-specific epitopes even on heavily NIP-substituted LPH. Although the mechanism behind intramolecular-induced suppression is poorly understood, even in mammals, this study demonstrates that intramolecular-induced suppression may be one means by which antibody responses in fish are regulated. The possible impact of antigen-induced suppression on immune responses against vaccine antigens in fish is discussed.

Fusion proteins with heterologous T helper epitopes. Recombinant E. coli heat-stable enterotoxin proteins

Fusion proteins containing specific B cell and T cell epitopes were used to examine how the intramolecular arrangement of T and B cell epitopes within a chimeric protein influences antigen-specific B cell antibody responses as well as specific T cell activation. Chimeric proteins, containing single or multiple copies of the Th epitope ovalbumin 323-339 (ova) linked at different positions to STa, the heat-stable enterotoxin of E. coli, were compared with respect to their ability to induce STa-specific antibody production and to induce ova-specific T cell activation. Chimeric proteins induced ova-dependent antibody production against STa at the amino terminal end, irrespective of the positioning of ova. Multiple tandem copies of ova in any position led to increased levels of antibody production against this epitope. In contrast, T cell help for antibody production against a second B cell epitope at the carboxy terminus of the fusion proteins was more effective after insertion of multiple copies of ova in a distal than in an adjacent position. A fusion protein, containing four copies of ova effectively elicited T cell help for antibody production against both examined B cell determinants, showing that activated Th cells recognizing a single epitope could simultaneously provide help for distinct sets of B cells specific for widely separated epitopes within a protein. T cell recognition of ova in all chimeric peptides, independently of its position, following the same pattern of genetic restriction (i.e. immunodominant in H-2d and nonimmunogenic in H-2k) as in the native ovalbumin molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Examination of characteristics that may distinguish disease-causing from benign AChR-reactive antibodies in experimental autoimmune myasthenia gravis

In summary, the strategies of the experimentation described above were designed to address the confusion resulting from observations concerning the lack of correlation between antibody titers and disease severity in MG patients. Lessons learned from these studies of EAMG suggest that if the proportion of the total expressed/produced anti-AChR antibody repertoire with disease-causing potential differs from patient-to-patient with MG, then assessment of the total antibody titer becomes meaningless unless a particular patient produces disease-causing reactivities that make up a major portion of the total titer. Not only may disease severity depend on the titer of a small subset of disease-causing antibody(s) reactive with a particular conformation-dependent AChR region, but may also depend on the relative contribution of additional subsets of antibody with functionally irrelevant or potentially protective activity. The key to exploiting the existence of antibody subsets with differing disease-causing potential will be to create probes that would allow the easy monitoring of the relevant reactivities. For instance, carefully selected anti-idiotypic antibodies (such as the 11E10 monoclonal antibody described above) may be of great value when specifically capable of recognizing idiotypes that are selectively associated with disease-causing anti-AChR antibodies and under-represented on antibodies lacking disease-causing capability. If, in addition, characteristics of helper T cells are identified that allow more accurate prediction of D+ Id production, exciting opportunities would become available to more directly evaluate disease mechanisms and to develop more highly efficacious immunotherapeutic strategies.

Immunogenicity of viral B- and T-cell epitopes expressed in recombinant bacterial proteins

Foreign polypeptides can be expressed as genetic inserts in several permissive sites of MalE and LamB, two Escherichia coli envelope proteins. Several viral B and T-cell epitopes have been inserted in these proteins and we analyzed the role of the molecular environment on the immunogenicity of the foreign epitopes. These studies demonstrated that the antigenicity and immunogenicity of B-cell epitopes depend on their site of insertion in the carrier protein. Using bacteria expressing B-cell epitopes either at the cell surface or in the periplasm, it was also shown that the cellular location of a foreign B-cell epitope expressed by recombinant bacteria determines its T-cell dependent or independent characteristics. Analysis of in vivo immunogenicity of purified LamB or MalE hybrid proteins expressing two different T-cell epitopes established that the immunogenicity of recombinant T-cell epitopes may be strongly affected by both the insertion site and inserted adjacent residues. The in vitro analysis of specific T-cell hybridoma response to hybrid MalE proteins also showed that the molecular context of a T-cell determinant alters the diversity of its T-cell recognition.

Analysis of antibody response to the measles virus using synthetic peptides of the fusion protein. Evidence of non-random pairing of T and B cell epitopes

The measles virus induces a life-long immune response associated with antibodies specific for the fusion protein. To map the linear immunodominant recognition sites of the fusion (F) protein of the measles virus, we have reacted a complete set of 108 overlapping pentadecapeptides with purified IgG obtained from donor sera with elevated anti-measles titers. The antibodies recognized about 20% of the peptides and generated a characteristic binding pattern, defining about 6 or 7 distinctive regions (31-75; 111-145; 151-165; 191-215; 271-320; 421-440; 481-530) which include the major hydrophobic segment (111-145) of the intersubunit region and the C-terminal Cys-cluster region. The binding sites were located in close proximity of the few experimentally defined T cell epitopes. This pairing of T and B cell epitopes was corroborated by computer-assisted T cell prediction. The significance of a non-random association of T and B cell epitopes for processing and presentation is discussed. It is speculated that in long-term immunity against measles (F protein), B cells of the same sIg specificity play an important role both as antigen presenting cells and as antibody producing cells. In contrast to human sera from late convalescent donors, mouse and rabbit MV antisera with high neutralizing titers as well as neutralizing MV-F specific monoclonal antibodies did not react with the peptides.

Analysis of the neutralizing antibody response to the measles virus using synthetic peptides of the haemagglutinin protein

Infection or immunization with measles virus induces a protective immune reaction including neutralizing antibodies against the haemagglutinin and fusion protein. The reactivity of the polyclonal IgG response of sera obtained from late convalescent donors was studied, using overlapping 15mer peptides covering the complete sequence of the measles virus haemagglutinin. Most sera reacted with a similar set of peptides generating a characteristic binding pattern. The reactive peptides correspond to a region mediating cell hemolysis (aa310-325), to regions which serve as targets to neutralizing antibodies and to a putative transmembrane region (aa35-58). The latter region contains also a human T-cell epitope providing evidence of a non-random association of T- and B-cell epitopes. We also immunized different strains of mice and rabbits with measles virus. In contrast to the human sera, animal sera with strong neutralizing activities did not react with any of the H-protein peptides. The mostly weak reactivities with the linear sequences contrast with the strong neutralizing activities of the human or animal antibodies, suggesting that these primarily recognize the fusion protein or conformational epitopes of the haemagglutinin protein.

Co-dominant and reciprocal T-helper cell activity of epitopic sequences and formation of junctional B-cell determinants in synthetic T:B chimeric immunogens

The identification of defined T-helper (Th) cell determinants, particularly those recognized in the context of several MHC or HLA haplotypes, and their use to provide effective carrier help to short synthetic constructs representing a B-cell epitope have made it feasible to synthesize putatively potent immunogens. However, a number of crucial questions regarding immunogenicity of epitopic sequences need to be addressed before an optimally effective synthetic vaccine can be designed. The present study deals with the hybrid constructs incorporating a known B-cell epitope from the merozoite surface protein-1 (MSP-1) of a human malarial parasite, Plasmodium falciparum, and the promiscuous Th-cell epitope from tetanus toxin or from the circumsporozoite protein of P. falciparum. Here, we provide data which suggest that B- and T-cell determinants present in a hybrid construct could, in fact, provide reciprocal helper activity for antibody production; that antibodies to a Th-cell epitope may not necessarily block its helper function; and that junctional B-cell epitopes may be formed. All this may influence, in an unpredictable manner, the quality of protective immune response sought to be generated using the chimeric immunogens, with important implications for vaccine design.

Comprehensive delineation of antigenic and immunogenic properties of peptides derived from the nef HIV-1 regulatory protein

The Human Immunodeficiency Virus (HIV-1) nef regulatory protein, a protein involved in AIDS pathology, was used as a model to investigate and analyze B- and T-cell epitopes. In this paper, we describe the potential structural basis of antigenic and immunogenic reactivity of synthetic peptides derived from the macromolecular antigen. The relationship between B- and T-cell determinants in the context of regulatory mechanisms involved in immune recognition, while integrating recent data concerning MHC presentation. As a result of the recent progress in the field of peptide recognition and presentation, the potential of the peptide approach for constructing successful synthetic vaccines needs to be continuously re-evaluated.

A synthetic peptide to the E glycoprotein of Murray Valley encephalitis virus defines multiple virus-reactive T- and B-cell epitopes

Synthetic peptides from the envelope glycoprotein sequence of Murray Valley encephalitis (MVE) virus were previously evaluated in various strains of mice for both the induction of antibody and the in vitro proliferation of peptide-primed T-helper (Th) cells. MVE peptide 6 (amino acids 230 to 251) elicited reciprocal Th- and B-cell reactivity with native MVE virus after primary inoculation of C57BL/6 mice. In this study, we prepared overlapping subunit peptides of MVE peptide 6 and evaluated their immunogenicity. Analysis of these peptides delineated at least two B-cell epitopes that induced antibody reactive with MVE and other Japanese encephalitis serocomplex viruses. This antibody at low titer neutralized MVE virus. Genetic restriction of the antibody response to various T-cell elements within peptide 6 was observed in C3H, BALB/c, C57BL/6, and B10 congenic mice. One element demonstrable after primary immunization, located in the carboxy terminus, associated only with major histocompatibility complex class II IAb and IAbiEk glycoproteins. Functional stimulation with the peptides in association with IAkIEk and IAdIEd molecules was observed only after in vivo secondary stimulation. Peptide 6-1 (amino acids 230 to 241) was nonimmunogenic but could be recognized by Th cells from peptide 6-immunized mice. Further association of peptide 6 with the IAkIEk and IAdIEd subregions was demonstrated by the finding that T cells from MVE peptide 6-inoculated C3H and BALB/c mice primed for an antibody response to MVE virus. These results suggest that the peptide 6 sequence, which is relatively conserved among a number of flaviviruses, should be given consideration when synthetic immunogens for vaccine purposes are designed.

T and B cell responses to chimeric proteins containing heterologous T helper epitopes inserted at different positions

The ability of a T helper (Th) epitope to induce help for B cells recognizing different determinants within a multideterminant antigen was investigated. Chimeric fusion proteins, containing inserts of single or multiple copies of the Th epitope ovalbumin 323-339 (ova) at two different positions, were compared with respect to their ability to induce specific antibody production and ova-specific T cell activation. The antibody responses against B cell determinants at the amino and carboxy terminus, respectively was differently influenced by the molecular positioning of the inserted Th determinant. All ova-containing fusion proteins induced antibody production against the B cell determinant at the amino terminal end irrespective of the positioning of ova. In addition, multiple copies of ova in any position led to increased levels of antibody production against this epitope. In contrast, T cell help for antibody production against the determinant at the carboxy terminus was more effective after insertion of multiple copies of ova in a distal than in an adjacent position. Furthermore a fusion protein, containing four copies of ova effectively elicited T cell help for high levels of antibody production against both examined B cell determinants, showing that activated Th cells recognizing a single epitope could simultaneously provide help for distinct sets of B cells specific for widely separated epitopes within a protein. Immunodominant T cell recognition of ova in all chimeric peptides, independently of its position, was demonstrated by lymph node cell (LNC) proliferation of primed BALB/c mice. The level of ova-specific T cell proliferation was similar, irrespective of which chimeric peptide that had been used for priming, and thus did not reveal any differences in T cell priming efficiencies related to the number of ova copies in the fusion proteins. However, when the peptides were presented to a ova-specific T cell line by A20 B lymphoma cells, a close correlation between IL-2 production by the clonal T cells and the number of ova epitopes in the chimeric peptides was observed. Thus, increased cytokine production by ova-specific T cells may be important for the increased level of in vivo antibody production observed in response to multiple copies of ova in the chimeric antigens.

Cellular and humoral immune responses to rubella virus structural proteins E1, E2, and C

Better understanding of cell-mediated immune responses to rubella virus would provide the basis for the development of safe and effective vaccines against rubella and would aid in analysis of the pathophysiology of congenital rubella syndrome. We have expressed individual rubella virus structural proteins, E1, E2 and C, via vaccinia virus recombinants. Using the expressed recombinant proteins as antigens, we were able to demonstrate antigen-specific lymphocyte proliferative responses in control individuals and individuals with congenital rubella syndrome. Among the two human groups studied, E1 glycoprotein proved to be a better immunogen than E2 or C. For the control individuals, significant differences in proliferative responses to the structural proteins E1, E2, and C were observed. These differences were not significant in individuals with congenital rubella syndrome. In parallel to the lymphoproliferative responses, immunoglobulin G responses were also found directed mainly to the E1 glycoprotein. These results suggest that E1 may be the most important rubella virus antigen to study in determining the domains required for constructing subunit vaccines against rubella.

Construction of synthetic immunogens: use of T- and B-cell epitopes of CS and RESA proteins of Plasmodium falciparum

An invariant T-helper epitope of the sequence ENDIEKKICKMEKCSSVFNV (residue no. 376-395) from the circumsporozoite (CS) protein was coupled chemically with the repeat sequences, namely (EENV)2, EENVEHDA and DDEHVEEPTVA, of ring-infected erythrocyte surface antigen (RESA) protein of Plasmodium falciparum. The CS sequence was tested for helper and proliferative activity in five inbred strains of mice of different haplotypes. The CS peptide showed dose-dependent lymphocyte proliferative response in all the strains tested. On the other hand, no proliferative response was observed with the dimers of the three RESA repeat sequences. The antibody levels in these strains immunized with RESA-CS hybrid structures showed high titres and a booster effect during subsequent immunization. Such a phenomenon was not observed with RESA peptides alone. The above CS sequence could be an ideal T-helper epitope which can be linked to hydrophilic B-cell epitopes of the RESA sequence to overcome major histocompatibility complex restriction in the host.

Priming of anti-human immunodeficiency virus (HIV) CD8+ cytotoxic T cells in vivo by carrier-free HIV synthetic peptides

The generation of antiviral cytotoxic T lymphocytes (CTLs) is a critical component of the immune response to viral infections. A safe and nontoxic vaccine for AIDS would optimally use a carrier-free synthetic peptide immunogen containing only components of HIV necessary for induction of protective immune responses. We report that hybrid synthetic peptides containing either a HIV envelope gp120 T-cell determinant (T1) or the envelope gp41 fusion domain (F) N-terminal to HIV CTL determinants are capable of priming murine CD8+, major histocompatibility complex class I-restricted anti-HIV CTLs in vivo. These data demonstrate that carrier-free, nonderivatized synthetic peptides can be used in vivo to induce anti-HIV CTL responses.

Antibodies raised against peptide fragments of bovine alpha s1-casein cross-react with the native protein, but recognize sites distinct from the determinants on the protein

Bovine alpha s1-casein (alpha s1-CN) and its peptides 61-110 and 91-110, which contain both T and B cell determinants on alpha s1-CN and can elicit peptide-native protein cross-reactive antibodies, were selected as model antigens to study whether or not the immune response to the peptides is similar to that to the corresponding regions of the native protein, because they both have a similar disordered conformation in solution. Both alpha s1-CN- and peptide 61-110-primed T cells responded to peptides 61-80 and 91-100, but not to peptides 76-95 and 101-110. In addition, T cells immunized with peptide 91-110 were also stimulated by peptide 91-100, but not by peptide 101-110. These results suggest that the location of the T cell determinant was almost the same in alpha s1-CN and its peptides. On the contrary, antibodies raised against alpha s1-CN bound to peptides 76-95 and 91-100, but not to peptides 61-80 nor 101-110, while anti-peptide 61-110 antibodies preferentially reacted with peptides 61-80 and 101-110, and anti-peptide 91-110 antibodies also bound to peptide 101-110 but not to peptide 91-100. These results indicate that the B cell epitopes were not similar between alpha s1-CN and its peptides. This difference may have arisen because the antigen-B cell or T-B interactions required for the development of a specific antibody response occurred in a different manner between alpha s1-CN and its peptides. These findings may be useful for basic studies on immunology, and could also be applied to the design of new peptide vaccines.

Relative positioning of the T cell and B cell determinants on an immunogenic peptide: its influence on antibody response

A tryptic peptide of bovine beta-casein (amino acid residues 184-202) was used as a model antigen to investigate how the relative position of the (helper) T cell and B cell determinants on a protein antigen influences the antibody response. Immunization with the peptide elicited a considerably higher anti-peptide response in the C3H/He strain than in the C57BL/6 strain, despite the fact that the C57BL/6 T cells showed higher reactivity than the C3H/He T cells. The T cell and B cell determinants of the peptide were identified in these two strains. Each strain recognized a single B cell determinant and a single T cell determinant. In the C3H/He strain, the T and B cell determinants were located apart from one another, while the T and B cell determinants of the C57BL/6 strain were located in a region close each other. The results suggest that the level of an antibody response depends on the topological relationship of the T cell and B cell determinants on the antigen molecule.

Synthetic immunogens constructed from T-cell and B-cell stimulating peptides (T:B chimeras): preferential stimulation of unique T- and B-cell specificities is influenced by immunogen configuration

In our effort to develop synthetic immunogens as vaccines, we have focused on the combination of a known T-cell stimulating peptide with putative B-cell stimulating peptide epitopes derived from the sequences of respiratory syncytial (RS) virus proteins. The T-cell stimulating peptide consists of residues 45 through 60 of the 1A protein of RS virus, and it also contains an overlapping antibody binding (B-cell) site. Herein, we have combined the 1A T-cell stimulating peptide with a putative B-cell peptide epitope derived from the viral G glycoprotein using linear synthesis or using chemical crosslinking. The chimeric immunogens were compared to each other and to free peptides for their T- and B-cell stimulating properties. Both chimeras had potent T-cell stimulating and antibody-inducing activity. However, T-cells primed to free peptide differentially recognized the two chimeras and immunization with the chimeras primed T-cells with different specificity. Most strikingly, the two chimeras had opposite antibody-inducing properties: The chimera constructed by linear synthesis overwhelmingly elicited antibody directed against the G peptide, whereas the chimera constructed by chemical crosslinking overwhelmingly elicited antibody directed against the 1A peptide. Competition blocking studies revealed that the chimeras adopted different configurations in solution. The resulting antibody response, and hence the B-cell clone elicited, was consistent with the antibody accessibility of the individual peptide epitope.

Class II-restricted T-cell clones to a synthetic peptide of influenza virus hemagglutinin differ in their fine specificities and in the ability to respond to virus

Fifteen T-cell clones were derived from BALB/c or DBA/2 mice immunized with a synthetic peptide corresponding to the C-terminal 24 residues (residues 305 to 328) of the HA1 chain of H3 subtype influenza virus hemagglutinin. All of the clones proliferated when the peptide was presented in association with I-Ed. By using shorter homologs, it was shown that the T-cell response was focused predominantly on the region at the N-terminal end of the peptide encompassed by residues 306 to 319. Individual clones recognizing this region differed in their absolute requirements for residues at the extremities of the site and also in their patterns of efficiency of recognition of shorter homologs. One particular clone defined another site of T-cell recognition within residues 314 to 328. The response of the clones to peptide analogs identified certain residues within the sites that were critical for recognition, with the substitution Gln-311----Ser having a differential effect on clones responding to the N-terminal site. Only one of the clones responded well to influenza virus itself. This clone also required relatively low concentrations of the parent peptide for optimum stimulation and was suppressed by higher concentrations. The data demonstrate striking heterogeneity in the T-cell response even to a short synthetic peptide, with different T-cell clones recognizing slightly different but overlapping areas of the molecule.

Development and application of synthetic peptides as vaccines

Vaccination against bacterial and viral diseases has been one of the major achievements in medicine and immunology since the beginning of this century. Extensive vaccination programs have been able to control or, in the case of smallpox, virtually wipe out some of the most dangerous infectious diseases e.g. poliomyelitis, measles, whooping cough, diphtheria and tetanus. However, as this success has been limited mainly to the developed, affluent countries, infectious diseases still remain the worlds largest health problem. Furthermore, vaccines against human parasites are non-existent. Recent advances in immunology and molecular biology including recombinant DNA technology have provided the basis for new approaches to vaccine development.

RNA virus evolution and the control of viral disease

RNA viruses and other RNA genetic elements must be viewed as organized distributions of sequences termed quasi-species. This means that the viral genome is statistically defined but individually indeterminate. Stable distributions may be maintained for extremely long time periods under conditions of population equilibrium. Perturbation of equilibrium results in rapid distribution shifts. This genomic organization has many implications for viral pathogenesis and disease control. This review has emphasized the problem of selection of viral mutants resistant to antiviral drugs and the current difficulties encountered in the design of novel synthetic vaccines. Possible strategies for antiviral therapy and vaccine development have been discussed.