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

Measles Virus Hemagglutinin epitopes immunogenic in natural infection and vaccination are targeted by broad or genotype-specific neutralizing monoclonal antibodies

Measles virus (MV) remains a leading cause of vaccine-preventable deaths in children. Protection against MV is associated with neutralizing antibodies that preferentially recognize the viral hemagglutinin (MV-H), and to a lesser extent, the fusion protein (MV-F). Although MV is serologically monotypic, 24 genotypes have been identified. Here we report three neutralization epitopes conserved in the more prevalent circulating MV genotypes, two located in the MV-H receptor binding site (RBS) (antigenic site III) and a third in MV-H/MV-F interphase (antigenic site Ia) which are essential for MV multiplication. In contrast, two MV-H neutralization epitopes, showed a genotype-specific neutralization escape due to a single amino acid change, that we mapped in the "noose" antigenic site, or an enhanced neutralization epitope (antigenic site IIa). The monoclonal antibody (mAb) neutralization potency correlated with its binding affinity and was mainly driven by kinetic dissociation rate (k_off). We developed an immunoassay for mAb binding to MV-H in its native hetero-oligomeric structure with MV-F on the surface of a MV productive steady-state persistently infected (p.i.) human cell lines, and a competitive-binding assay with serum from individuals with past infection by different MV genotypes. Binding assays revealed that a broad neutralization epitope, in RBS antigenic site, a genotype specific neutralization epitopes, in noose and IIa sites, were immunogenic in natural infection and vaccination and may elicit long-lasting humoral immunity that might contribute to explain MV immunogenic stability. These results support the design of improved measles vaccines, broad-spectrum prophylactic or therapeutic antibodies and MV-used in oncolytic therapies.

Measles virus hemagglutinin epitopes are potential hotspots for crossreactions with immunodeficiency-related proteins

AIMS

Measles virus (MV) infection induces a protective immunity that is accompanied by a transient pathologic suppression of the immune system. This immunologic paradox remains unexplained in spite of the numerous hypotheses that have been advanced (i.e., cytokine production, soluble immunosuppressive factor, cell cycle block, signaling lymphocyte activation molecule receptor and MV infection of dendritic cells, among others).

METHODS

Searching for molecular link(s) between MV infection and host immunodeficiency, this study used the Immune Epitope DataBase to analyze the peptide sharing between the antigenic MV hemagglutinin (H) protein and human proteins associated with immunodeficiency.

RESULTS

It was found that the majority of MVH derived epitopes share several exact pentapeptide sequences with numerous human proteins involved in immune functions and immunodeficiency, such as B- and T-cell antigens, and complement components.

CONCLUSION

The data suggest that crossreactivity might contribute to our understanding of the link between MV immunogenicity and MV-induced immunosuppression, and highlight peptides unique to MV as a basis for developing effective and safe anti-MV vaccines.

Bioinformatic and immunological analysis reveals lack of support for measles virus related mimicry in Crohn's disease

BACKGROUND

A link between measles virus and Crohn's disease (CD) has been postulated. We assessed through bioinformatic and immunological approaches whether measles is implicated in CD induction, through molecular mimicry.

METHODS

The BLAST2p program was used to identify amino acid sequence similarities between five measles virus and 56 intestinal proteins. Antibody responses to measles/human mimics were tested by an in-house ELISA using serum samples from 50 patients with CD, 50 with ulcerative colitis (UC), and 38 matched healthy controls (HCs).

RESULTS

We identified 15 sets of significant (>70%) local amino acid homologies from two measles antigens, hemagglutinin-neuraminidase and fusion-glycoprotein, and ten human intestinal proteins. Reactivity to at least one measles 15-meric mimicking peptide was present in 27 out of 50 (54%) of patients with CD, 24 out of 50 (48%) with UC (CD versus UC, p = 0.68), and 13 out of 38 (34.2%) HCs (CD versus HC, p = 0.08). Double reactivity to at least one measles/human pair was present in four out of 50 (8%) patients with CD, three out of 50 (6%) with UC (p = 0.99), and in three out of 38 (7.9%) HCs (p >0.05 for all). Titration experiments yielded different extinction curves for anti-measles and anti-human intestinal double-reactive antibodies. Epitope prediction algorithms and three-dimensional modeling provided bioinformatic confirmation for the observed antigenicity of the main measles virus epitopic regions.

CONCLUSIONS

Measles sequences mimicking intestinal proteins are frequent targets of antibody responses in patients with CD, but this reactivity lacks disease specificity and does not initiate cross-reactive responses to intestinal mimics. We conclude that there is no involvement of measles/human molecular mimicry in the etiopathogenesis of CD.

Neutralization capacity of measles virus H protein specific IgG determines the balance between antibody-enhanced infectivity and protection in microglial cells

Neutralizing antibodies directed against measles virus (MV) surface glycoproteins prevent viral attachment and entry through the natural receptors. H protein specific IgG can enhance MV infectivity in macrophages via Fcγ receptor (FcγR)-dependent mechanism. H-specific IgM, anti-F antibodies and complement cascade activation are protective against antibody-mediated enhancement of MV infection. However, protective role of anti-H IgG against antibody-enhanced infection is not well understood. Here we designed a set of experiments to test the protective effect of H-specific IgG against FcγR-mediated infection in microglial cells. Microglial cells are also potential target of the antibody-mediated enhancement and spread of MV infection in the central nervous system. A partially neutralizing IgG monoclonal antibody (MAb) CL55, specific for MV H protein, at 10 μg/ml enhanced MV infection in mouse microglial cells by 13-14-fold. Infection-enhancing antibody concentrations induced large multinucleated syncytia formation 48-72 h post-inoculation. We generated anti-H IgG MAb 20H6 with a strong neutralization capacity >1:80,000 at 1mg/ml concentration in MV plaque-reduction neutralization assay. In contrast to the partially protective MAb CL55, enhancement of MV infectivity by MAb 20H6 required dilutions below the 1:120 serum titer considered protective against measles infection in humans. At a concentration of 10 μg/ml MAb 20H6 exhibited a dominant protective effect and prevented MAb CL55-mediated enhancement of MV infection and virus-mediated fusion. These results indicate that neutralization capacity of the H-specific IgG determines the balance between antibody enhancement and protection against MV infection in microglial cells.

Pathogen evolution and disease emergence in carnivores

Emerging infectious diseases constitute some of the most pressing problems for both human and domestic animal health, and biodiversity conservation. Currently it is not clear whether the removal of past constraints on geographical distribution and transmission possibilities for pathogens alone are sufficient to give rise to novel host-pathogen combinations, or whether pathogen evolution is also generally required for establishment in novel hosts. Canine distemper virus (CDV) is a morbillivirus that is prevalent in the world dog population and poses an important conservation threat to a diverse range of carnivores. We performed an extensive phylogenetic and molecular evolution analysis on complete sequences of all CDV genes to assess the role of selection and recombination in shaping viral genetic diversity and driving the emergence of CDV in non-dog hosts. We tested the specific hypothesis that molecular adaptation at known receptor-binding sites of the haemagglutinin gene is associated with independent instances of the spread of CDV to novel non-dog hosts in the wild. This hypothesis was upheld, providing compelling evidence that repeated evolution at known functional sites (in this case residues 530 and 549 of the haemagglutinin molecule) is associated with multiple independent occurrences of disease emergence in a range of novel host species.

Genetic variability of measles virus in acute and persistent infections

RNA viruses have high nucleotide substitution rates, and therefore the potential to mutate rapidly. In the case of vaccine preventable RNA viruses, this may potentially lead to emergence of vaccine escape mutants. The WHO has targeted measles virus (MV) for elimination in many regions, and its genetic variability is monitored to estimate appearance of such mutants. Phylogenetic analysis of partial N or H genes of 230 MV strains circulating in the UK over a 10-year period was performed. Substitution rates in three outbreaks were determined to be 3.9 x 10(-3) to 6.7 x 10(-3) per nucleotide per annum. This is an order of magnitude higher than previously reported for circulating MV. Analysis of virus detected sporadically in the UK between 1992 and 2000 lead to a slightly higher substitution rate of 7.8 x 10(-3) per site per year. Additionally, genetic variability of persistent MV, isolated from subacute sclerosing panencephalitis (SSPE) patients, was investigated and appeared more stable than circulating viruses. Profiles of nucleotide changes in acute and persistent virus were compared. In acute virus, 33% of all mutation events occurred from A-to-G, which contrasts the predominant U-to-C mutations found in persistent infections. Mutations do not seem to be driven by positive selection and no association with known biological functions could be found. We conclude that substitution rates in circulating virus may be higher than in persistent, hypermutated virus and that the high substitution rate of MV may allow evolution of escape. Diversity of circulating strains should be closely monitored in the future.

Neutralizing B cell response in measles

Co-evolving mechanisms of immune clearance and of immune suppression are among the hallmarks of measles. B cells are major targets cells of measles virus (MV) infection. Virus interactions with B cells result both in immune suppression and a vigorous antibody response. Although antibodies fully protect against (re)infection, their importance during the disease and in the presence of a potent cellular response is less well understood. Specific serum IgM appears with onset of rash and confirms clinical diagnosis. After isotype switching, IgG1 develops and confers life-long protection. The most abundant antibodies are specific for the nucleoprotein, but neutralizing and protective antibodies are solely directed against the two surface glycoproteins, the hemagglutinin and the fusion protein. Major neutralizing epitopes have been mapped mainly on the hemagglutinin protein with monoclonal antibodies, producing an increasingly comprehensive map of functional domains.

Immune and artificial selection in the haemagglutinin (H) glycoprotein of measles virus

We present a maximum likelihood (ML) analysis of the selection pressures that have shaped the evolution of the large (L) protein and the haemagglutinin (H) glycoprotein of measles virus (MV). A number of amino acid sites that have potentially been subject to adaptive evolution were identified in the H protein using sequences from every known genotype of MV. All but one of these putative positively selected sites reside within the ectodomain of the H protein, where they often show an association with positions of potential B-cell epitopes and sites known to interact with the CD46 receptor. This suggests that MV may be under pressure from the immune system, albeit relatively weakly, to alter sites within epitopes and hence evade the humoral immune response. The positive selection identified at amino acid 546 was shown to correlate with the passage history of MV isolates in Vero cells. We reveal that Vero cell passaging has the potential to introduce an artificial signal of adaptive evolution through selection for changes that increase affinity for the CD46 receptor.

Differential antigenicity of recombinant polyepitope-antigens based on loop- and helix-forming B and T cell epitopes

To investigate a strategy for the design of chimeric antigens based on B cell epitopes (BCEs) we have genetically recombined multiple copies of loop- (L) and helix-forming (H) sequential and protective BCEs of the measles virus hemagglutinin protein (MVH) in a number of high-molecular-weight polyepitope constructs (24.5-45.5 kDa). The BCE cassettes were combined semi-randomly together with a promiscuous T cell epitope (TCE; tt830-844) to yield 13 different permutational constructs. When expressed in mammalian cells, all constructs were detectable by Western blot as distinct bands of predicted molecular weight. Flow cytometry with conformation-specific antibodies revealed the Cys-loop in two [(L(4)T(4))(2) and (L(2)T(2))(4)] and the helix conformation in one [(H(2)T(2))(4)] of the different permutational constructs. The larger constructs, containing 16 epitope cassettes, seemed more likely to express the BCEs in their native conformation than the 8-mers. In the T cell proliferation assay, constructs with a higher copy number of TCEs, such as (L(2)T(2))(4), were more antigenic, as long as tandem repeats were separated by spacers. Since the conformation of even sequential BCEs and the processing of TCEs are both sensitive to their molecular environment it is difficult to predict the antigenic properties of polyepitopes. However, with the permutational approach we have developed several polyepitope constructs [(L(4)T(4))(2), (L(2)T(2))(4), (H(2)T(2))(4)] based on complex sequential BCEs that are antigenic for both T and B cells. Several constructs induced sera that reacted with reporter peptides, demonstrating that the sequential nature of the viral epitopes was conserved in the polyepitopes. Although several sera contained antibodies directed against amino acids critical for neutralization, only one construct induced antibodies that cross-reacted with the virus. Our results show the difficulty of designing chimeric antigens based on B cell epitopes mimicking their antigenic and immunologic properties even when these are sequential in nature.

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.