Monoclonal antibodies specific for the G1 glycoprotein of La Crosse virus that react with other California serogroup viruses

The Adaptive Immune Response against Bunyavirales

The Bunyavirales order includes at least fourteen families with diverse but related viruses, which are transmitted to vertebrate hosts by arthropod or rodent vectors. These viruses are responsible for an increasing number of outbreaks worldwide and represent a threat to public health. Infection in humans can be asymptomatic, or it may present with a range of conditions from a mild, febrile illness to severe hemorrhagic syndromes and/or neurological complications. There is a need to develop safe and effective vaccines, a process requiring better understanding of the adaptive immune responses involved during infection. This review highlights the most recent findings regarding T cell and antibody responses to the five Bunyavirales families with known human pathogens (Peribunyaviridae, Phenuiviridae, Hantaviridae, Nairoviridae, and Arenaviridae). Future studies that define and characterize mechanistic correlates of protection against Bunyavirales infections or disease will help inform the development of effective vaccines.

Development and evaluation of an indirect enzyme-linked immunosorbent assay for serological detection of Schmallenberg virus antibodies in ruminants using whole virus antigen

Background

In late 2011, a new Orthobunyavirus of the Simbu serogroup named Schmallenberg virus (SBV) emerged in continental Europe. The virus is transmitted by hematophagous arthropods, with the Culicoides species as, so far known, main vectors. Infection with the virus can cause clinical signs in adult ruminants including diarrhea, fever and reduced milk production. Transplacental infection of the developing fetus can lead to malformations of varying severity. To assess seroprevalence of SBV in Sweden an indirect enzyme-linked immunosorbent assay (ELISA) was established in connection with the surveys. Here, we describe the development and evaluation of the indirect ELISA, based on whole virus as the coating antigen and a monoclonal antibody for the detection of antibodies to SBV in ruminant sera. The evaluation includes comparison between the in-house ELISA, virus neutralization test and an indirect commercial ELISA.

Results

The optimal working dilutions of antigens and conjugate were estimated with checkerboard titrations. Comparative studies, including ROC analyses, were used for the selection of an optimal cut-off (S/P value = sample value as percentage of positive control value). With an estimated S/P value of 15% the whole virus ELISA showed a specificity of 100% and a sensitivity of 99.19% compared to virus neutralization test (VNT) and with a good consistency as shown in reproducibility and variability experiments. Furthermore, the comparison of our whole virus indirect ELISA to an indirect ELISA with a SBV nucleoprotein antigen, demonstrated a higher sensitivity of our test.

Conclusion

The indirect whole virus ELISA described in this paper is a readily available test for serological analysis of SBV antibodies. Since this in-house ELISA demonstrates a specificity and sensitivity comparable to virus neutralization test and also shows a higher sensitivity compared to commercially available indirect ELISA, it is a useful alternative for surveillance and screening purposes of SBV.

Validation of a commercially available indirect ELISA using a nucleocapside recombinant protein for detection of Schmallenberg virus antibodies

A newly developed Enzym Like Immuno Sorbant Assay (ELISA) based on the recombinant nucleocapsid protein (N) of Schmallenberg virus (SBV) was evaluated and validated for the detection of SBV-specific IgG antibodies in ruminant sera by three European Reference Laboratories. Validation data sets derived from sheep, goat and bovine sera collected in France and Germany (n = 1515) in 2011 and 2012 were categorized according to the results of a virus neutralization test (VNT) or an indirect immuno-flurorescence assay (IFA). The specificity was evaluated with 1364 sera from sheep, goat and bovine collected in France and Belgium before 2009. Overall agreement between VNT and ELISA was 98.9% and 98.3% between VNT and IFA, indicating a very good concordance between the different techniques. Although cross-reactions with other Orthobunyavirus from the Simbu serogroup viruses might occur, it is a highly sensitive, specific and robust ELISA-test validated to detect anti-SBV antibodies. This test can be applied for SBV sero-diagnostics and disease-surveillance studies in ruminant species in Europe.

Analysis of the medium (M) segment sequence of Guaroa virus and its comparison to other orthobunyaviruses

Guaroa virus (GROV), a segmented virus in the genus Orthobunyavirus, has been linked to the Bunyamwera serogroup (BUN) through cross-reactivity in complement fixation assays of S segment-encoded nucleocapsid protein determinants, and also to the California serogroup (CAL) through cross-reactivity in neutralization assays of M segment-encoded glycoprotein determinants. Phylogenetic analysis of the S-segment sequence supported a closer relationship to the BUN serogroup for this segment and it was hypothesized that the serological reaction may indicate genome-segment reassortment. Here, cloning and sequencing of the GROV M segment are reported. Sequence analysis indicates an organization similar to that of other orthobunyaviruses, with genes in the order GN-NSm-Gc, and mature proteins generated by protease cleavage at one, and by signalase at possibly three, sites. A potential role of motifs that are more similar to CAL than to BUN virus sequences with respect to the serological reaction is discussed. No discernable evidence for reassortment was identified.

Identification and localization of conserved antigenic epitopes on the G2 proteins of California serogroup Bunyaviruses

California (CAL) serogroup Bunyaviruses are significant agents of arboviral encephalitis in humans. They are maintained and transmitted in nature by mosquitoes to preferred vertebrate amplifying hosts. The G2 envelope glycoprotein of La Crosse virus (LAC) was proposed by Ludwig et al. to be a determinant for virus attachment to mosquito midgut cells. Monoclonal antibodies to G2 neutralize the infectivity of pronase-treated virus for mosquito cells. We determined the location of antigenic sites on the LAC G2. We showed that antigenic areas present on the LAC G2 protein are conserved among viruses in the California encephalitis and Melao subgroups of the CAL serogroup, but not in trivatattus virus, nor within the BUN serogroup. A comparison of the G2 exodomain amino acid sequences of eight CAL and three BUN viruses with monoclonal antibodies (MAb) binding data predicted the possible location of the antigenic sites. We used in vitro mutagenesis of the LAC G2 gene to construct a set of G2 genes with replacement sequences in the coding regions for the suspected MAb binding sites. The native and mutated proteins were expressed in Hela cells and the ability of MAbs to bind to the expressed proteins was tested. Four discontinuous amino acid sequences, conserved among eight CAL serogroup viruses, were identified as contributing to two conformational binding domains for neutralizing LAC G2 MAbs.

DNA-based vaccine against La Crosse virus: protective immune response mediated by neutralizing antibodies and CD4+ T cells

La Crosse virus (LACV)-mediated encephalitis is the most frequently reported arboviral disease in the United States, but to date no vaccine against this virus is available. We have established a new animal model, genetically targeted mice lacking a functional interferon type I receptor (IFNAR-1). These mice show an age-independent susceptibility to LACV and develop an acute encephalitis within 6 days of infection, thereby allowing the evaluation of vaccines against LACV. Taking advantage of this knockout mouse model, we have assessed the feasibility of DNA vaccination against this viral disease. Plasmid DNAs, encoding either the virus surface glycoproteins G1 and G2 or the internal nucleocapsid protein N, were used to immunize IFNAR-1-deficient mice. Mice vaccinated with DNA encoding the glycoproteins G1 and G2 produced neutralizing antibodies and exhibited a high degree of protection against challenge with high doses of LACV. Depletion of CD4+ T cells in mice vaccinated with DNA encoding G1/G2 reduced their capacity to control the infection. Virus titration and immunohistological analysis revealed that the protected mice showed no evidence of LACV particles in the brain. This indicates that the vaccine-induced immune response efficiently blocked viral spreading from the primary replication site to the brain. In contrast, immunization with DNA encoding protein N yielded only a partial protective effect that can be attributed to the cellular immune response. Taken together, this study shows that DNA vaccines can be designed to efficiently induce a protective immune response based on neutralizing antibodies and CD4+ T cells.

Antigenic diversity of Akabane virus detected by monoclonal antibodies

The antigenic properties of 21 Japanese field isolates and two Australian strains of Akabane (AKA) virus (Simbu serogroup, bunyavirus) isolated from 1959 to 1990 were compared by enzyme-linked immunosorbent assay (ELISA), plaque-reduction neutralization (PRNT) and hemagglutination inhibition (HI) tests using monoclonal antibodies (Mabs) to the OBE-1 strain of AKA virus. Sixteen Mabs were established by fusing P3X63Ag8U1 mouse myeloma cells and spleen cells from BALB/c mice immunized with the OBE-1 strain. Of the 16 clones, 13 produced immunoglobulin (Ig) which precipitated glycoprotein G1 and three produced Ig which precipitated nucleoprotein (N). Twelve out of 13 Mabs had both NT and HI activities to not only the homologous OBE-1 strain but also the other isolates. By the competitive binding assay, at least five antigenic regions for G1, and two for N were defined. Some of the anti-G1 Mabs which reacted to the same antigenic region had unique reactivity while anti-N Mabs recognizing the same epitope reacted with almost the same degree to all of the isolates. Finally, nine epitopes of the G1 protein in five different antigenic regions have been identified. There was no striking correlation between isolation date and place of the isolates and their reactivity to Mabs. A most interesting result is that three isolates collected in the same place over a three week period had different reactivity patterns detected by ELISA, showing great antigenic variation of the virus. AKA virus may be a single gene pool consisting of different genotypes in the field.

A second envelope glycoprotein mediates neutralization of a pestivirus, hog cholera virus

Several monoclonal antibodies (MAbs) raised against hog cholera virus (HCV) reacted with the HCV structural glycoprotein gp44/48 and neutralized the virus. The presence of HCV gp44/48 on the viral surface was directly demonstrated by immunogold electron microscopy. Eight anti-HCV gp44/48 MAbs were tested by immunoperoxidase assay against a panel of pestivirus strains. Each MAb showed a distinct pattern of reactivity with HCV strains. It is suggested that the MAbs are well suited for epidemiological investigations of HCV outbreaks.

Monoclonal antibody characterization of Jamestown Canyon (California serogroup) virus topotypes isolated in Canada

Jamestown Canyon (JC) virus of the California (CAL) serogroup has been isolated in 12 American states and 6 Canadian provinces. A study was undertaken to produce monoclonal antibodies (MAbs) to JC virus and to use these MAbs to assay for possible heterogeneity among naturally occurring JC topotypes in Canada. MAbs were produced to the prototype strain of JC virus using BALB/c mice. Twenty-seven secreting MAbs were obtained and three of these MAbs were propagated and studied. All three MAbs, M1 (IgG1), M2 (IgG2b), and M3 (IgG2a), were reactive by immunofluorescent antibody assay against JC-infected vero cells and by ELISA against JC antigen. MAb M2 reacted with all members of the Melao complex, MAb M1 reacted only with Keystone virus, while MAb M3 exhibited no reactivity with other CAL serogroup viruses. Only MAb M3 possessed neutralization and hemagglutination inhibition activities against JC virus. The MAbs were also tested by ELISA and for neutralizing activity against 13 JC topotypes isolated in 5 provinces from Newfoundland to Saskatchewan. ELISA confirmed closer identity of the Canadian topotypes to JC as opposed to the closely related South River virus. The MAbs verified all Canadian topotypes to be JC virus but revealed different patterns of reactivity between these topotypes and prototype JC virus.

Effects of specific monoclonal antibodies on La Crosse virus neutralization: aggregation, inactivation by Fab fragments, and inhibition of attachment to baby hamster kidney cells

At high concentrations, several monoclonal antibodies to the G1 glycoprotein of La Crosse (LAC) virus aggregated the virus. To determine whether this accounted for the neutralization, the monoclonal antibodies were digested to make Fab fragments. With one exception, each monovalent antibody neutralized LAC virus to the same extent that bivalent antibody did, although higher concentrations were needed. Fab fragments of synergistic pairs of antibodies also exhibited enhanced binding in a competition binding assay but did not increase neutralization. To determine specific mechanisms for neutralization, the effects of polyclonal or monoclonal antibodies on virus attachment were examined. Polyclonal antibody to LAC virus reduced virus attachment by only 68% although it neutralized 99.99% of the virus. When virus was preincubated with a neutralizing monoclonal antibody to each of seven antigenic regions on G1, only antibody to one region reduced attachment of virus by as much as 92%. Antibodies to two regions that neutralize virus by 90-98% only inhibited attachment by 9 and 13%, respectively. The other antibodies showed intermediate degrees of neutralization and inhibition of attachment. Pairs of antibodies previously shown to be synergistic in neutralizing activity did not inhibit attachment any more than the single antibodies did.