Neutralization of influenza virus by low concentrations of hemagglutinin-specific polymeric immunoglobulin A inhibits viral fusion activity, but activation of the ribonucleoprotein is also inhibited

Immunoglobulin A response to SARS-CoV-2 infection and immunity

The novel coronavirus disease (COVID-19) and its infamous "Variants" of the etiological agent termed Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) has proven to be a global health concern. The three antibodies, IgA, IgM, and IgG, perform their dedicated role as main workhorses of the host adaptive immune system in virus neutralization. Immunoglobulin-A (IgA), also known as "Mucosal Immunoglobulin", has been under keen interest throughout the viral infection cycle. Its importance lies because IgA is predominant mucosal antibody and SARS family viruses primarily infect the mucosal surfaces of human respiratory tract. Therefore, IgA can be considered a diagnostic and prognostic marker and an active infection biomarker for SARS CoV-2 infection. Along with molecular analyses, serological tests, including IgA detection tests, are gaining ground in application as an early detectable marker and as a minimally invasive detection strategy. In the current review, it was emphasized the role of IgA response in diagnosis, host defense strategies, treatment, and prevention of SARS-CoV-2 infection. The data analysis was performed through almost 100 published peer-reviewed research reports and comprehended the importance of IgA in antiviral immunity against SARS-CoV-2 and other related respiratory viruses. Taken together, it is concluded that secretory IgA- Abs can serve as a promising detection tool for respiratory viral diagnosis and treatment parallel to IgG-based therapeutics and diagnostics. Vaccine candidates that target and trigger mucosal immune response may also be employed in future dimensions of research against other respiratory viruses.

When secretion turns into excretion - the different roles of IgA

IgA deficiency is the commonest immunodeficiency affecting up to 1 in 700 individuals. The effects of IgA deficiency are difficult to see in many individuals, are mild in many fewer and severe in fewer still. While monovalent IgA is found in serum, dimeric IgA is secreted through mucosal surfaces where it helps to maintain epithelial homeostasis. Studies with knockout mice have taught us that there are subtle inflammatory consequences of removing secretory IgA (sIgA), and the best explanation for these changes can be related by the loss of the 'excretory' immune system. The excretion of antigens is a logical process in regulating the immune system, given the long half-life of complement fixing antibodies. But the function of IgA as an immune or inflammation regulator may go beyond antigen removal.

How to assess the effectiveness of nasal influenza vaccines? Role and measurement of sIgA in mucosal secretions

Secretory IgAs (sIgA) constitute the principal isotype of antibodies present in nasal and mucosal secretions. They are secreted by plasma cells adjacent to the mucosal epithelial cells, the site where infection occurs, and are the main humoral mediator of mucosal immunity. Mucosally delivered vaccines, such as live attenuated influenza vaccine (LAIV), are able to mimic natural infection without causing disease or virus transmission and mainly elicit a local immune response. The measurement of sIgA concentrations in nasal swab/wash and saliva samples is therefore a valuable tool for evaluating their role in the effectiveness of such vaccines. Here, we describe two standardized assays (enzyme‐linked immunosorbent assay and microneutralization) available for the quantification of sIgA and discuss the advantages and limitations of their use.

Colocalization of intracellular specific IgA (icIgA) with influenza virus in patients' nasopharyngeal aspirate cells

Inhibition of viral replication by icIgA antibodies has only been observed with in vitro studies using epithelial cell lines in transwell cultures. This effect appears to involve an interaction between polymeric immunoglobulin A (pIgA) and viral particles within an intracellular compartment, since IgA is transported across polarized cells. Polyclonal guinea pig antisera against purified influenza A virus and mouse antisera prepared against Influenza A/H3N2 hemagglutinin (HA₀) cleavage loop peptides, were used in confocal fluorescence microscopy to show specific staining of wild-type influenza H1N1 and H3N2 viruses in clinical specimens. The HA₀ cleavage loop peptides used for intranasal immunization of mice were designed and synthesized from specific conserved regions of influenza A/H1N1 & A/H3N2 viruses. Anti-human secretory IgA antibodies were used to show co-localisation of influenza A virus and icIgA. The results showed specific immunofluorescent staining of influenza A/H3N2 (X31) (HA₀ uncleaved)-infected MDCK cells and the presence of icIgA in respiratory exudate cells of infected patients. Both results confirm specific co-localisation and suggest interaction between influenza A virus and icIgA in patients' respiratory exudate cells. Importantly, antisera to the mouse anti-HA₀ cleavage site were specific for wild-type virus in clinical specimens, indicating that the conserved region of HA₀ was present in the uncleaved form. Similar staining and colocalization patterns between icIgA and virus were observed with polyclonal guinea pig antisera against influenza A virus. These are the first observations of co-localization of influenza A virus and intracellular IgA in clinical specimens. Role of icIgA: This report shows the co-localization of influenza A virus HA₀ and icIgA antibodies in respiratory exudate cells of patients who were culture and viral RNA positive, suggesting that icIgA directed against the conserved HA₀ site may have a privileged and unique opportunity to act on immature virus and thus prevent HA₀ cleavage, maturation and subsequent cycles of viral replication. The precise mechanism by which icIgA mediates intracellular viral neutralization remains to be fully elucidated.

SIGNIFICANCE

The above findings in clinical specimens would contribute strongly to our understanding of the mechanisms and kinetics of icIgA neutralization in relation to viral entry and early replication steps of mucosal viral infections. A rapid, objective and sensitive assay - by ex vivo enumeration of respiratory epithelial cells that have co-localized influenza virus and icIgA - would contribute to further mucosal immunity studies and inform the design of more effective vaccines against influenza and other viral infections transmitted via the mucosal route e.g. respiratory syncytial virus, rotavirus.

Neutralization of Virus Infectivity by Antibodies: Old Problems in New Perspectives

Neutralizing antibodies (NAbs) can be both sufficient and necessary for protection against viral infections, although they sometimes act in concert with cellular immunity. Successful vaccines against viruses induce NAbs but vaccine candidates against some major viral pathogens, including HIV-1, have failed to induce potent and effective such responses. Theories of how antibodies neutralize virus infectivity have been formulated and experimentally tested since the 1930s; and controversies about the mechanistic and quantitative bases for neutralization have continually arisen. Soluble versions of native oligomeric viral proteins that mimic the functional targets of neutralizing antibodies now allow the measurement of the relevant affinities of NAbs. Thereby the neutralizing occupancies on virions can be estimated and related to the potency of the NAbs. Furthermore, the kinetics and stoichiometry of NAb binding can be compared with neutralizing efficacy. Recently, the fundamental discovery that the intracellular factor TRIM21 determines the degree of neutralization of adenovirus has provided new mechanistic and quantitative insights. Since TRIM21 resides in the cytoplasm, it would not affect the neutralization of enveloped viruses, but its range of activity against naked viruses will be important to uncover. These developments bring together the old problems of virus neutralization-mechanism, stoichiometry, kinetics, and efficacy-from surprising new angles.

Secretory IgA in saliva can be a useful stress marker

To evaluate secretory immunoglobulin A (slgA) in saliva as an immunological stress marker, we reviewed the literature on slgA and its variation caused by psychosocial factors. Among the studies on the effect of academic stress on slgA secretion, we could distinguish two kinds of stress effects: the immediate stress effect which increases slgA secretion immediately after stress, and the delayed stress effect which decreases slgA secretion several days after stress. On the basis of production and secretion mechanisms of slgA, we also speculated on possible mechanisms that underlie the variations of slgA caused by stress. Eventually, we concluded diat slgA in saliva can be a useful stress marker if we analyze the delayed stress effect on slgA separately from the immediate stress effect on slgA.

Comparison of the cross-reactive anti-influenza neutralizing activity of polymeric and monomeric IgA monoclonal antibodies

Here we examined whether polymeric IgA (pIgA) and monomeric IgA (mIgA) antibodies differ in their ability to neutralize drift viruses within the same subtype. We used an IgA monoclonal antibody (mAb; H1-21) against influenza virus strain A/Hiroshima/52/2005 (A/Hiroshima; H3N2). The mAb was obtained after immunizing mice mucosally with a split-virion (SV) vaccine. The mAb contained both mIgA and pIgA forms. It reacted with the homologous virus and cross-reacted with drift viruses A/New York/55/2004 (H3N2) and A/Wyoming/3/2003 (H3N2) in hemagglutinin-inhibition (HI) and neutralizing Ab assays. The mAb also cross-reacted with A/Panama/2007/99 (H3N2) in an ELISA. We separated the mAb into pIgA and mIgA fractions by gel filtration, and then tested them for neutralizing Ab activity. The neutralizing activity for the A/Hiroshima/52/2005, A/New York/55/2004, and A/Wyoming/3/2003 viruses was lower for the mIgA than the pIgA fraction. However, the neutralizing efficiency for drift variants relative to that for the homotype did not differ between pIgA and mIgA, and pIgA only neutralized variants that could also be neutralized by mIgA. These results suggest that the polymerization of IgA enhances its antiviral immune responses, but does not increase the number of influenza virus strains neutralized by the IgA.

Evasion of influenza A viruses from innate and adaptive immune responses

The influenza A virus is one of the leading causes of respiratory tract infections in humans. Upon infection with an influenza A virus, both innate and adaptive immune responses are induced. Here we discuss various strategies used by influenza A viruses to evade innate immune responses and recognition by components of the humoral and cellular immune response, which consequently may result in reduced clearing of the virus and virus-infected cells. Finally, we discuss how the current knowledge about immune evasion can be used to improve influenza A vaccination strategies.

Pandemic H1N1 influenza virus causes a stronger inflammatory response than seasonal H1N1 influenza virus in ferrets

A 2009 H1N1 influenza virus pandemic, which had its origin in swine, caused severe illness and mortality in humans. Inflammatory responses may be responsible for pathogenesis caused by infection with influenza viruses. To better understand the pathogenic mechanism, clinical signs and inflammatory responses in ferrets infected with the pandemic H1N1 were compared with those caused by seasonal H1N1 influenza virus. Ferrets infected with the 2009 pandemic H1N1 virus displayed higher body temperatures, greater reduction in body weight, and higher viral titers in the tracheae and lungs. Levels of inflammatory cytokines, including interleukin-6, interferon-alpha, and tumor necrosis factor-alpha, were higher in the lungs of ferrets infected with the 2009 pandemic H1N1. The data support the idea that increased pathogenesis caused by the 2009 pandemic H1N1 influenza virus may have been partially mediated by a higher induction of pro-inflammatory cytokines in the lungs of affected humans or animals.

Intranasal delivery of an IgA monoclonal antibody effective against sublethal H5N1 influenza virus infection in mice

Highly pathogenic avian H5N1 influenza viruses are endemic in poultry in Asia and pose a pandemic threat to humans. Since the deployment of vaccines against a pandemic strain may take several months, adequate antiviral alternatives are needed to minimize the effects and the spread of the disease. Passive immunotherapy is regarded as a viable alternative. Here, we show the development of an IgA monoclonal antibody (DPJY01 MAb) specific to H5 hemagglutinin. The DPJY01 MAb showed a broad hemagglutination inhibition (HI) profile against Asian H5N1 viruses of clades 0, 1.0, 2.1, 2.2, and 2.3 and also against H5 wild bird influenza viruses of the North American and Eurasian lineages. DPJY01 MAb displayed also high neutralization activity in vitro and in vivo. In mice, DPJY01 MAb provided protection via a single dose administered intranasally before or after inoculation with a sublethal dose of H5N1 viruses of clades 1.0 and 2.2. Pretreatment with 50 mg of DPJY01 MAb kg of body weight at either 24, 48, or 72 h before highly pathogenic H5N1 virus (A/Vietnam/1203/2004 [H5N1]) inoculation resulted in complete protection. Treatment with 50 mg/kg at either at 24, 48, or 72 h after H5N1 inoculation provided 100%, 80%, and 60% protection, respectively. These studies highlight the potential use of DPJY01 MAb as an intranasal antiviral treatment for H5N1 influenza virus infections.

Neutralization of animal virus infectivity by antibody

Neutralization is the ability of antibody to bind to and inactivate virus infectivity under defined conditions in vitro. Most neutralizing antibodies also protect animals in vivo, but protection is more complex as it also involves interaction of antibody with cells and molecules of the innate immune system. Neutralization by antibody can be mediated by a number of different mechanisms: by aggregation of virions, destabilization of the virion structure, inhibition of virion attachment to target cells, inhibition of the fusion of the virion lipid membrane with the membrane of the host cell, inhibition of the entry of the genome of non-enveloped viruses into the cell cytoplasm, inhibition of a function of the virion core through a signal transduced by an antibody, transcytosing IgA, and binding to nascent virions to block their budding or release from the cell surface. The mechanism of neutralization is determined by the properties of both a virion epitope and the antibody that reacts with it. Further, since a virus has at least several unique epitopes sited in different locations on the virion, and since the paratope and other properties of the reacting antibody can vary, this means that a virus can be neutralized by several different mechanisms. Understanding the processes of neutralization informs the creation of modern vaccines, and gives valuable insights into virus-cell interactions.

Biological Functions of IgA

Immunoglobulin A (IgA) is the most enigmatic of immunoglobulins. It is by far the most abundant of human Igs, being present in the blood plasma at concentrations approximating 2–3mg/mL, as well as the dominant isotype in most secretions where its output amounts to some 5–8g/day in adults. Furthermore, its evolutionary origins appear to precede the synapsid– diapsid divergence in tetrapod phylogeny (>300 million years ago) because it is present in both mammals and birds and therefore possibly also in reptiles (reviewed in Peppard et al., 2005); an IgA-like molecule has now been identified in a lizard (Deza et al., 2007).

Structural insights into antibody-mediated mucosal immunity

The mucosal regions of the body are responsible for defense against environmental pathogens. Particularly in the lumen of the gut, antibody-mediated immune responses are critical for preventing invasion by pathogens. In this chapter, we review structural studies that have illuminated various aspects of mucosal immunity. Crystal structures of IgA1-Fc and IgA-binding fragments of the polymeric immunoglobulin receptor and Fc alphaRI, combined with models of intact IgA and IgM from solution scattering studies, reveal potential mechanisms for immune exclusion and induction of inflammatory responses. Other recent structures yield insights into bacterial mechanisms for evasion of the host immune response.

Total viral genome copies and virus-Ig complexes after infection with influenza virus in the nasal secretions of immunized mice

The kinetics of infectious virus (p.f.u.), total virus and virus-Ig complex formation following influenza A/PR8 (H1N1) viral infection was examined in the nasal secretions of naive mice and mice immunized with A/PR8, A/Yamagata (H1N1), A/Guizhou (H3N2) and B/Ibaraki influenza viruses. The total number of virus particles and the number within virus-Ig complexes, captured in advance using an anti-mouse Ig-coated plate, were determined on the basis of viral genome copy number using quantitative RT-PCR. The kinetics of infectious and total virus particle formation, the latter of which increased by 10(3)-10(4)-fold above infectious virus numbers, showed that virus elimination from the nasal area was earlier in A/PR8, A/Yamagata and A/Guizhou-X virus-immunized mice, in decreasing order, compared with naive mice. Early virus elimination correlated with the level of A/PR8 virus-reactive antibodies in immunized mice. Virus elimination coincided with the appearance of virus-Ig complexes shortly after infection. This result suggested that antibodies led to the formation of immune complexes in a dose-dependent manner together with a reduction in number of infectious virus particles. The fact that a large number of virus particles was observed in immune complexes for a wide range antibody levels made it difficult to detect slight differences in virus number within the immune complexes, depending on antibody level. These results suggested that the formation of virus-Ig complexes in virus-immunized mice shortly after infection is involved in early virus elimination, which is determined by the strength of protective immunity against challenge viruses.

Intranasal Cold-Adapted Influenza Virus Vaccine Combined with Inactivated Influenza Virus Vaccines

Although influenza vaccine delivery strategies have improved coverage rates to unprecedented levels nationally among persons aged 65 years and older, influenza remains one of the greatest vaccine-preventable threats to public health among elderly in the US. A new, intranasal live attenuated influenza vaccine (LAIV) was recently approved by the US FDA for use in persons aged 5-49 years, which excludes the elderly population. Limitations of immune response to inactivated influenza vaccine (IAIV) and effectiveness of current influenza vaccination strategies among the elderly suggest that a combined approach using LAIV and/or the IAIV in various permutations might benefit this group. We explore characteristics of the LAIV, data regarding its utility in protecting against influenza in the elderly, and challenges and opportunities regarding potential combined inactivated/live attenuated vaccination strategies for the elderly. Although LAIV appears to hold promise either alone or in combination with IAIV, large well conducted randomised trials are necessary to define further the role of LAIV in preventing influenza morbidity and mortality among the elderly. We also suggest that innovative vaccine coverage strategies designed to optimise prevention and control of influenza and minimise viral transmission in the community must accompany, in parallel, the acquisition of clinical trials data to best combat morbidity and mortality from influenza.

Multiple functions of immunoglobulin A in mucosal defense against viruses: an in vitro measles virus model

Three defense functions of immunoglobulin A (IgA), immune exclusion, intracellular neutralization, and virus excretion, were assessed in a measles virus model using polarized epithelial cells expressing the polymeric immunoglobulin receptor and monoclonal antibodies against the viral H and F envelope proteins and the internal N protein. Anti-H IgA was the most effective antibody at preventing infection via the apical surface, i.e., immune exclusion. This IgA was also the most effective at intraepithelial cell neutralization after infection at the apical surface and endocytosis of IgA at the basolateral surface, although an antibody against the internal N protein was also effective. In the intracellular neutralization experiments, confocal immunofluorescence microscopy showed prominent colocalization of anti-H IgA and H protein inside virus-infected cells, whereas colocalization of anti-F and F protein and of anti-N and N protein was much less, in agreement with the neutralization results. Combinations of IgA anti-H, anti-F, and anti-N showed no synergistic effects in intracellular neutralization. In the immune excretion experiments, virus immune complexes with either anti-H or anti-F IgA placed beneath polarized epithelial cells could be transported to the apical supernatant. Anti-F IgA, which was relatively poor at immune exclusion and intracellular neutralization, was the most robust at virus excretion. Thus, the studies collectively demonstrated three different antiviral functions of IgA in relation to epithelium and also suggested that the particular viral component with which a given IgA antibody reacts is an important determinant of the magnitude of the antiviral effect.

Occupancy and mechanism in antibody-mediated neutralization of animal viruses

Neutralization of virus infectivity by antibodies is an important component of immunity to several virus infections. Here, the immunochemical basis for the action of neutralizing antibodies, and what role their induction of conformational changes in the antigen might play, is reviewed. Theories of the mechanisms by which antibodies neutralize virus infectivity in vitro are also presented. The theoretical and empirical foundation of the hypothesis that viruses are neutralized by a single antibody per virion is critically reviewed. The relationship between antibody occupancy on virions and the mechanism of neutralization is explored. Examples of neutralization mediated through antibody interference with virus attachment and entry are discussed and test implications of refined theories of neutralization by antibody coating of virions are formulated.

Hemagglutinin 1-specific immunoglobulin G and Fab molecules mediate postattachment neutralization of influenza A virus by inhibition of an early fusion event

In standard neutralization (STAN), virus and antibody are reacted together before inoculation of target cells, and inhibition of almost any of the processes concerned in the early interaction of virus and cell, including inhibition of virus attachment to cell receptors, can be the cause of neutralization by a particular monoclonal antibody (MAb). To simplify the interpretation of antibody action, we carried out a study of postattachment neutralization (PAN), where virus is allowed to attach to target cells before neutralizing antibody is introduced. We used influenza virus A/PR/8/34 (H1N1) and monoclonal immunoglobulin G (IgG) molecules and their Fabs specific to antigenic sites Sb (tip), Ca2 (loop), and Cb (hinge) of the hemagglutinin 1 (HA1) protein. All IgGs and Fabs gave PAN, although with reduced efficiency compared with STAN. Thus, bivalent binding of antibody was not essential for PAN. By definition, none of these MAbs gave PAN by inhibiting virus attachment, and they did not elute attached virus from the target cell or inhibit endocytosis of virus. However, virus-cell fusion, as demonstrated by R18 fluorescence dequenching or hemolysis of red blood cells, was inhibited in direct proportion to neutralization and in a dose-dependent manner and was thus likely to be responsible for the observed neutralization. However, to get PAN, it was necessary to inhibit the activation of the prefusion intermediate, the earliest known form on the fusion pathway that is created when virus is incubated at pH 5 and 4 degrees C. PAN antibodies may act by binding HA trimers in contact with the cell and/or trimers in the immediate vicinity of the virus-cell contact point and so inhibit the recruitment of additional receptor-HA complexes.

Two influenza A virus-specific Fabs neutralize by inhibiting virus attachment to target cells, while neutralization by their IgGs is complex and occurs simultaneously through fusion inhibition and attachment inhibition

Mabs H36 (IgG2a) and H37 (IgG3) recognize epitopes in antigenic sites Sb and Ca2, respectively, in the HA1 subunit of influenza virus A/PR/8/34 (H1N1). Their neutralization was complex. Our aim here was to investigate the mechanism of neutralization by the IgGs and their Fabs. In MDCK and BHK cells, both IgGs neutralized primarily by inhibiting virus-cell fusion, although at higher IgG concentrations virus attachment to target cells was also inhibited. In contrast, the Fabs neutralized entirely by inhibiting virus attachment, although a higher concentration of Fab than IgG was required to bring this about. Both H36 and H37 exerted a concentration-dependent spectrum of neutralization activity, with virus-cell fusion inhibition and virus-cell attachment inhibition being the predominant mechanisms at low- and high-antibody concentration, respectively, and both mechanisms occurring simultaneously at intermediate concentrations. However, it may be that attachment inhibition was a secondary event, occurring to virus that had already been neutralized through inhibition of its fusion activity. Neutralization by H36 and H37 Fabs was a simple process. Both inhibited virus attachment but required much higher (>100-fold) molar concentrations for activity than did IgG. The functional affinities of the IgGs were high (0.4-0.6 nM) and differences between these and the affinity of their Fabs (H36, nil; H37, 23-fold) were not sufficient to explain the differences observed in neutralization. Similar neutralization data were obtained in two different cell lines. The dose-response curve for neutralization by H36 F(ab')(2) resembled that for IgG, although eightfold more F(ab')(2) was required for 50% neutralization. Overall, neutralization mechanisms of H36 and H37 antibodies were similar, and thus independent of antigenic site, antibody isotype, and target cell.

Carbohydrate biopolymers enhance antibody responses to mucosally delivered vaccine antigens

We have evaluated the ability of two carbohydrate biopolymers, chitosan and gellan, to enhance antibody responses to subunit influenza virus vaccines delivered to the respiratory tracts of mice. Groups of mice were vaccinated three times intranasally (i.n.) with 10 microg of purified influenza B/Panama virus surface antigens (PSAs), which consist of hemagglutinin (HA) and neuraminidase (NA), either alone or admixed with chitosan or gellan solutions. Separate groups were vaccinated subcutaneously (s.c.) with PSAs adsorbed to Alhydrogel or chitosan or gellan alone i.n. Serum antibody responses were determined by enzyme-linked immunosorbent assay (ELISA) for influenza virus-specific immunoglobulin G (IgG) and by HA inhibition (HAI) and NA inhibition (NAI) assays. The local respiratory immune response was measured by assaying for influenza virus-specific IgA antibody in nasal secretions and by enumerating nasal and pulmonary lymphocytes secreting IgA, IgG, and IgM anti-influenza virus-specific antibodies by enzyme-linked immunospotting (ELISPOT). When administered alone i.n., B/Panama PSA was poorly immunogenic. Parenteral immunization with B/Panama PSA with Alhydrogel elicited high titers of anti-B/Panama antibodies in serum but a very poor respiratory anti-B/Panama IgA response. In contrast, i.n. immunization with PSA plus chitosan stimulated very strong local and systemic anti-B/Panama responses. Gellan also enhanced the local and serum antibody responses to i.n. PSA but not to the same extent as chitosan. The ability of chitosan to augment the immunogenicity of influenza vaccines given i.n. was confirmed using PSA prepared from an influenza A virus (A/Texas H1N1).

Comparative pathogenesis of enteric viral infections of swine

At least 11 enteric viruses belonging to 6 distinct families (Adenoviridae, Astroviridae, Caliciviridae, Coronaviridae, Parvoviridae, and Reoviridae) cause diarrhea in swine mainly during the nursing and immediate post-weaning period. Most infect the small intestinal enterocytes, inducing various degrees of villous atrophy and subsequently a malabsorptive, maldigestive diarrhea. In addition rotaviruses possess an enterotoxin (NSP4) which induces a secretory diarrhea in mice. These viruses have distinct predilections for different vertical (villus/crypt) and horizontal (duodenum, jejunum, ileum, colon) replication sites in the intestine and the diarrhea intensity is often related to the extent of viral replication at these sites. In addition concurrent infections with multiple enteric viruses can produce synergistic or additive effects leading to more extensive villous atrophy throughout the intestine and more severe and prolonged diarrhea. Knowledge of enteric viral replication sites and comparative mechanisms of diarrhea induction may lead to new or improved vaccine strategies or therapeutic approaches for the prevention or treatment of these viral diarrheas.

Quantitation of virus-specific classes of antibodies following immunization of mice with attenuated equine herpesvirus 1 and viral glycoprotein D

The antibody responses of CBA/J mice infected intranasally (i.n.) with either the attenuated KyA strain or the pathogenic RacL11 strain of equine herpesvirus 1 (EHV-1) or immunized with recombinant glycoprotein D (rgD) were investigated using the ELISPOT assay to measure EHV-1-specific antibody-secreting cells (ASC) in the regional lymphoid tissue of the respiratory tract. IgG, IgA, and IgM ASC specific for EHV-1 were detected in the mediastinal lymph nodes (MLN) and lungs 2 weeks after i.n. infection with EHV-1 strain KyA or RacL11, or immunization with heat-killed KyA or rgD. EHV-1-specific ASC were present in the MLN and lungs at 4 and 8 weeks, but declined in frequency by fivefold in the lung at 8 weeks. However, i.n. immunized (2 x 10(6) pfu KyA or 50 microgram rgD/mouse) mice infected at 8 weeks with pathogenic EHV-1 RacL11 resisted challenge and showed eight- and tenfold increases in MLN ASC and lung ASC, respectively, by 3 days after challenge. In contrast to the intranasal route of immunization, intraperitoneal immunization yielded ASC frequencies in the MLN and lungs that were only slightly above those of nonimmunized control mice. These data indicate that immunization with infectious or heat-killed EHV-1 KyA, or rgD, induces significant levels of virus-specific ASC both in the MLN and lungs, a specific memory B-cell response, and long-term protective immunity. The finding that the numbers of ASC induced by the pathogenic strain versus the attenuated strain of EHV-1, which were virtually identical, indicated that the ability to generate a B-cell response is independent of and does not contribute to EHV-1 virulence.

Is there a role for a mucosal influenza vaccine in the elderly?

Influenza infection is an acute respiratory disease with a high morbidity and significant mortality, particularly among the elderly and individuals with chronic diseases. The majority of countries now recommend annual influenza vaccination for all people aged 65 years or older, and for those with high risk conditions. Most commercially available influenza vaccines are administered systemically and while these are effective in children and young adults, efficacy levels in elderly individuals have been reported to be much lower. Mucosal vaccines may offer an improved vaccine strategy for protection of the elderly. As the influenza virus causes a respiratory infection, it is potentially more beneficial to administer a vaccine that will boost protection in the mucosal surfaces of the upper and lower respiratory tract. Mucosal influenza vaccines are aimed at stimulating protective immunity in the respiratory tract via oral or intranasal immunisation. This review examines our present knowledge of mucosal immunity and current strategies for mucosal vaccination. It also stresses that the use of serum antibody levels as a 'surrogate marker' for protection against influenza is potentially misleading; serum antibody, for example, may be a quite inappropriate marker to assess a mucosal vaccine. This marker does not reflect other immune responses to vaccination that are crucial for protection.

The induction of systemic and mucosal immune responses following the subcutaneous immunization of mature adult mice: characterization of the antibodies in mucosal secretions of animals immunized with antigen formulations containing a vitamin D3 adjuvant

Systemic and mucosal immune responses were effectively induced following the subcutaneous administration of Haemophilus influenzae type b oligosaccharide conjugated to diphtheria toxoid vaccine in a formulation containing the active form of vitamin D3. IgA and IgG antibodies with specificity for both the protein and oligosaccharide components of the vaccine were detectable in mucosal secretions following immunization. The IgA and IgG mucosal antibodies were produced locally, and were functional as demonstrated by their diphtheria toxin neutralizing activity. Our data suggests that subcutaneous tissues can effectively serve as effective antigen presenting sites for both mucosal and systemic immune responses to antigens administered in combination with vitamin D3.

Current status of a mucosal vaccine against dental caries

The evidence of a specific bacterial cause of dental caries and of the function of the salivary glands as an effector site of the mucosal immune system has provided a scientific basis for the development of a vaccine against this highly prevalent and costly oral disease. Research efforts towards developing an effective and safe caries vaccine have been facilitated by progress in molecular biology, with the cloning and functional characterization of virulence factors from mutans streptococci, the principal causative agent of dental caries, and advancements in mucosal immunology, including the development of sophisticated antigen delivery systems and adjuvants that stimulate the induction of salivary immunoglobulin A antibody responses. Cell-surface fibrillar proteins, which mediate adherence to the salivary pellicle, and glucosyltransferase enzymes, which synthesize adhesive glucans and allow microbial accumulation, are virulence components of mutans streptococci, and primary candidates for a human caries vaccine. Infants, representing the primary target population for a caries vaccine, become mucosally immunocompetent and secrete salivary immunoglobulin A antibodies during the first weeks after birth, whereas mutans streptococci colonize the tooth surfaces at a discrete time period that extends around 26 months of life. Therefore, immunization when infants are about one year old may establish effective immunity against an ensuing colonization attempts by mutans streptococci. The present review critically evaluates recent progress in this field of dental research and attempts to stress the protective potential as well as limitations of caries immunization.

Oral microbial ecology and the role of salivary immunoglobulin A

In the oral cavity, indigenous bacteria are often associated with two major oral diseases, caries and periodontal diseases. These diseases seem to appear following an imbalance in the oral resident microbiota, leading to the emergence of potentially pathogenic bacteria. To define the process involved in caries and periodontal diseases, it is necessary to understand the ecology of the oral cavity and to identify the factors responsible for the transition of the oral microbiota from a commensal to a pathogenic relationship with the host. The regulatory forces influencing the oral ecosystem can be divided into three major categories: host related, microbe related, and external factors. Among host factors, secretory immunoglobulin A (SIgA) constitutes the main specific immune defense mechanism in saliva and may play an important role in the homeostasis of the oral microbiota. Naturally occurring SIgA antibodies that are reactive against a variety of indigenous bacteria are detectable in saliva. These antibodies may control the oral microbiota by reducing the adherence of bacteria to the oral mucosa and teeth. It is thought that protection against bacterial etiologic agents of caries and periodontal diseases could be conferred by the induction of SIgA antibodies via the stimulation of the mucosal immune system. However, elucidation of the role of the SIgA immune system in controlling the oral indigenous microbiota is a prerequisite for the development of effective vaccines against these diseases. The role of SIgA antibodies in the acquisition and the regulation of the indigenous microbiota is still controversial. Our review discusses the importance of SIgA among the multiple factors that control the oral microbiota. It describes the oral ecosystems, the principal factors that may control the oral microbiota, a basic knowledge of the secretory immune system, the biological functions of SIgA, and, finally, experiments related to the role of SIgA in oral microbial ecology.

In Vitro Comparison of the Biologic Activities of Monoclonal Monomeric IgA, Polymeric IgA, and Secretory IgA

Secretory IgA (S-IgA), a major humoral mediator of mucosal immunity, is a polymeric Ig containing an unusual extra polypeptide, secretory component (SC), added during transcytosis through epithelial cells. Polymeric S-IgA is more effective than monomeric IgA (mIgA) and IgG in neutralizing viruses. It is not known whether this increased efficacy is due solely to the polymeric structure of the molecule or whether SC itself makes S-IgA more efficient; a quantitative in vitro comparison of the biologic activities of S-IgA and pIgA has not been reported. We prepared purified pIgA and mIgA mAbs directed toward the H1 hemagglutinin of PR8 influenza virus and purified monoclonal S-IgA (made from monoclonal pIgA injected into a Lewis rat and collected as S-IgA from bile) and compared their abilities to carry out hemagglutination inhibition (HI) and neutralization of the infectivity of PR8 influenza virus in vitro. The polymeric Igs (pIgA and S-IgA) were 5 times more effective than mIgA in HI and 7 to 10 times more effective than mIgA in virus neutralization. Addition of SC to pIgA did not modify its ability to mediate HI and had only a minimal effect (S-IgA was 1.4 times more effective) on its ability to neutralize influenza virus in vitro. Trypsin preincubation partially abolished mIgA- or pIgA-mediated, but not S-IgA-mediated, viral neutralization. Thus, although S-IgA is more stable functionally than pIgA, the addition of SC does not influence, positively or negatively, the biologic activity associated with the Fab of S-IgA.

Interaction of antigens and antibodies at mucosal surfaces

Infections often involve the mucosal surfaces of the body, which form a boundary with the outside world. This review focuses on immunoglobulin A (IgA) antibodies because IgA is the principal mucosal antibody class. IgA is synthesized by local plasma cells and has a specific polymeric immunoglobulin receptor-mediated transport mechanism for entry into the secretions. By serving as an external barrier capable of inhibiting attachment of microbes to the luminal surface of the mucosal epithelial lining, IgA antibodies form the first line of immune defense. In addition to this traditional mode of extracellular antibody function, recent evidence suggests that IgA antibodies can also function in a nontraditional fashion by neutralizing viruses intracellularly, if a virus is infecting an epithelial cell through which specific IgA antibody is passing on its way to the secretions. IgA antibodies are also envisaged as providing an internal mucosal barrier beneath the mucosal lining. Antigens intercepted by IgA antibodies here can potentially be ferried through the epithelium and thereby excreted. In addition to the polymeric immunoglobulin receptor on mucosal epithelial cells, IgA antibodies can bind to receptors on a variety of leukocytes and have been shown, in some experimental systems, to be capable of activating the alternative complement pathway, making IgA antibodies potential participants in inflammatory reactions. Although the relationship of IgA antibodies to inflammation is not entirely clear, the bias presented is that IgA is basically noninflammatory, perhaps even anti-inflammatory. According to this view, the major role of the Fc portion of IgA antibodies is to transport IgA across mucosal epithelial cells and not, as in the case of the other classes of antibody, to activate secondary phenomena of the kind that contribute to inflammation. Because of IgA's key role as an initial barrier to infection, much current research in mucosal immunology is directed toward developing new vectors and adjuvants that can provide improved approaches to mucosal vaccines. Finally, because of advances in monoclonal antibody technology, topical application of antibodies to mucosal surfaces has significant potential for preventing and treating infections.

A human IgG1 (b12) specific for the CD4 binding site of HIV-1 neutralizes by inhibiting the virus fusion entry process, but b12 Fab neutralizes by inhibiting a postfusion event

The human b12 IgG1, specific for the CD4 binding site of the gp120 of HIV-1, was prepared by recombinant DNA technology. It had a high neutralization rate constant (-3.5 x 10(5) M(-1) sec(-1)), although this is about 10-fold less than the values for the best poliovirus or influenza A virus MAbs. The recombinant b12 Fab neutralized well, with about one-tenth of the activity of b12 IgG. The mechanisms by which b12 IgG1 and its Fab neutralize HIV-1 IIIB on C8166 cells have been investigated. Neither inhibited attachment of virus to the target cell as judged by FACS, immunofluorescence, and ELISA data. This was controlled using MAb F105, another human IgG1, that did neutralize by inhibiting attachment under our conditions. The interactions of b12 IgG- and Fab-neutralized virions with target cells were compared with those of nonneutralized virus using a number of different techniques (fluorescence dequenching of R18-labeled virions, immunofluorescence of virion gp41 and p24 antigens, and acquisition of resistance to removal of virions from the cell by protease). These and the inhibition of HIV-1-mediated cell-cell fusion all demonstrated that b12 IgG neutralized by inhibiting the primary fusion-uncoating mechanism. However, the interactions of b12 Fab-neutralized and nonneutralized virions with C8166 cells were indistinguishable. Thus b12 Fab did not inhibit fusion uncoating, and by inference inhibited a stage of infection that occurs after the entry of the virion core into the cytoplasm. It is therefore possible that b12 IgG kills HIV-1 twice over, by fusion-inhibition and by inhibiting the postentry event proposed for the Fab. The mechanism of neutralization of b12 Fab and of other MAbs that neutralize in a similar way and why b12 Fab and IgG neutralize by different mechanisms are discussed.

Mucosal immunity: an overview and studies of enteric and respiratory coronavirus infections in a swine model of enteric disease

Based on the tenet of a common mucosal immune system, antigenic stimulation at one mucosal site results in the distribution of antigen-specific IgA precursor cells to distant mucosal sites. However, recent studies suggest that functional compartmentalization and limited reciprocity may exist within some components of the common mucosal immune system. Although oral immunization is often very effective in inducing immunity to respiratory pathogens, the converse (respiratory immunization to prevent enteric diseases) may not be as effective. To address this question and to study interactions between the bronchus-associated (BALT) and gut-associated (GALT) lymphoid tissues related to protective immunity, we used as a model two antigenically related porcine coronaviruses which replicate primarily in the intestine (transmissible gastroenteritis virus, TGEV) or respiratory tract (porcine respiratory coronavirus, PRCV). The tissue distribution and magnitude of the antibody secreting cell (ASC) responses (measured by ELISPOT) and cell-mediated immune responses (measured by lymphoproliferative assays, LPA) coincided with the viral tissue tropisms. Immunization via GALT (gut infection with TGEV) elicited high numbers of IgA ASC and high LPA responses in GALT (gut lamina propria, LP or mesenteric lymph nodes, MLN), but lower responses in BALT (bronchial lymph nodes, BLN) and induced complete protection against enteric TGEV challenge. In contrast immunization via BALT (respiratory infection with PRCV) elicited systemic type responses (high numbers of IgG ASC in the BLN), but few ASC and low LPA responses in the gut LP or MLN and induced only partial protection against enteric TGEV challenge. Thus administration of vaccines intranasally may not be optimally effective for inducing intestinal immunity in contrast to the reported efficacy of oral vaccines for inducing respiratory immunity.

Isotype-specific antibody responses to foot-and-mouth disease virus in sera and secretions of "carrier' and "non-carrier' cattle

Isotype-specific antibody responses to foot-and-mouth disease virus (FMDV) were measured in the sera and upper respiratory tract secretions of vaccinated and susceptible cattle challenged with FMDV by direct contact or by intranasal inoculation. A comparison was made between cattle that eliminated FMDV and those that developed and maintained a persistent infection. Serological and mucosal antibody responses were detected in all animals after challenge. IgA and IgM were detected before the development of IgG1 and IgG2 responses. IgM was not detected in vaccinated cattle. Challenge with FMDV elicited a prolonged biphasic secretory antibody response in FMDV "carrier' animals only. The response was detected as FMDV-specific IgA in both mucosal secretions and serum samples, which gained statistical significance (P < 0.05) by 5 weeks after challenge. This observation could represent the basis of a test to differentiate vaccinated and/or recovered convalescent cattle from FMDV "carriers'.

Human immune responses to influenza virus vaccines administered by systemic or mucosal routes

Healthy adult volunteers were immunized by parenteral or oral routes with trivalent inactivated influenza vaccine (A/Chile/1/83 (H1N1), A/Mississippi/1/85 (H3N2), and B/Ann Arbor/1/86), or intranasally with live attenuated, cold-adapted influenza type A/Texas/1/85 (H1N1) reassortant virus. In all volunteers, cells spontaneously secreting IgA, IgG or IgM antibodies specific to influenza virus were detected in peripheral blood on days 6-13 after immunization, and specific IgA, IgG and IgM antibodies to influenza vaccine were measured in sera and external secretions (saliva and nasal lavage). Following systemic immunization, a raise in specific antibodies of all isotypes was observed in sera beginning on day 13. Although small variations in IgA and IgM antibodies in saliva and nasal lavages were detected, antigen-specific IgG significantly increased between days 13 and 27. Intranasal administration of attenuated virus induced IgA and IgG antibodies in serum as well as in secretions. Serum antibodies were not substantially influenced by oral immunization, only a small increase in all isotypes was observed in volunteers' sera 21 days after ingestion of vaccine. However, in secretions, antigen-specific IgA and IgG responses were detected one week after immunization and reached a peak response on day 20. These studies show that different routes of immunization can be effective for the induction of specific antibodies, and support the concept of the common mucosal immune system in humans by demonstrating that the oral or intranasal administration of antigen-induced specific antibodies of IgA isotype in external secretions, preceded by the transient appearance in peripheral blood of specific antibody-producing cells.

Intracellular neutralization of influenza virus by immunoglobulin A anti-hemagglutinin monoclonal antibodies

Traditionally, immunoglobulin A (IgA) was thought to neutralize virus by forming complexes with viral attachment proteins, blocking attachment of virions to host epithelial cells. Recently we have proposed an intracellular action for dimeric IgA, which is actively transported through epithelial cells by the polymeric immunoglobulin receptor (pIgR), in that it may be able to bind to newly synthesized viral proteins within the cell, preventing viral assembly. To this effect, we have previously demonstrated that IgA monoclonal antibodies against Sendai virus, a parainfluenza virus, colocalize with the viral hemagglutinin-neuraminidase protein within infected epithelial cells and reduce intracellular viral titers. Here we determine whether IgA can interact with influenza virus hemagglutinin (HA) protein within epithelial cells. Polarized monolayers of Madin-Darby canine kidney epithelial cells expressing the pIgR were infected on their apical surfaces with influenza virus A/Puerto Rico/8-Mount Sinai. Polymeric IgA anti-HA, but not IgG anti-HA, delivered to the basolateral surface colocalized with HA protein within the cell by immunofluorescence. Compared with those of controls, viral titers were reduced in the supernatants and cell lysates from monolayers treated with anti-HA IgA but not with anti-HA IgG. Furthermore, the addition of anti-IgA antibodies to supernatants did not interfere with the neutralizing activity of IgA placed in the basal chamber, indicating that IgA was acting within the cell and not in the extracellular medium to interrupt viral replication. Thus, these studies provide additional support for the concept that IgA can inhibit replication of microbial pathogens intracellularly.

Neutralization of respiratory syncytial virus after cell attachment

Little is known about the mechanisms of antibody-mediated neutralization of respiratory syncytial virus (RSV) which causes recurrent infections in human despite the virtually universal presence of neutralizing serum antibodies. Human serum neutralization titers showed strong correlation with post-cell-attachment neutralizing titers for both RSV-convalescent sera and control sera but showed less strong correlation with cell-attachment blocking titers. Neutralization was effective for the first 60 min of infection, indicating that immune serum-mediated neutralization of RSV infection largely involves inhibition of early events following cell attachment.

Protective antibodies inhibit reovirus internalization and uncoating by intracellular proteases

We identified in vitro correlates of in vivo protection mediated by nonneutralizing antibodies specific for reovirus capsid proteins. We defined mechanisms of antibody action by analyzing monoclonal antibody (MAb) effects at sequential steps in reovirus serotype 3 strain Dearing (T3D) infection of L cells. Two types of experiments showed that protective MAbs specific for the outer capsid proteins sigma 3 or mu 1 inhibited T3D infection independent of effects on binding. First, MAbs which had no effect on T3D binding inhibited T3D growth. Second, MAb-coated T3D attached to L cells did not replicate as efficiently as T3D without bound antibody. We therefore defined sigma 3-specific MAb effects on postbinding steps in T3D infection. T3D coated with MAb sigma 3-10G10 exhibited prolonged sensitivity to growth inhibition by ammonium chloride. Since ammonium chloride inhibits endosomal acidification and proteolytic processing of the T3D capsid, this suggested that MAbs inhibit early steps in T3D infection. This was confirmed by direct demonstration that several sigma 3-specific MAbs inhibited proteolytic uncoating of virions by fibroblasts. We identified two mechanisms for antibody-mediated inhibition of virion uncoating: (i) inhibition of internalization of T3D-MAb complexes bound to the cell surface, and (ii) inhibition of intracellular proteolysis of the T3D capsid. Studies using a cell-free system confirmed that sigma 3-specific MAbs directly block proteolytic uncoating of the T3D virion. In addition, we found that sigma 3-specific MAbs block (and therefore define) two distinct steps in proteolytic uncoating of the reovirion. We conclude that antibodies which are protective in vivo inhibit postbinding events in reovirus infection of permissive cells. Protective antibodies act by inhibiting internalization and intracellular proteolytic uncoating of the virion. Analysis of postbinding mechanisms of MAb action may identify targets for vaccine development and antiviral therapy.

Characteristics of IgA antibodies against HIV-1 in sera and saliva from HIV-seropositive individuals in different clinical stages

IgA antibodies were analysed in sera and saliva from 40 HIV-1 seropositive individuals. The level of total IgA in serum was elevated according to the progress of the disease. IgA antibodies against p24 and gp160 were detected in the asymptomatic phase of infection. However, they declined in the symptomatic phases in contrast with IgG antibodies. Interestingly, three patients in the symptomatic phase who showed high levels of IgA antibodies were all in relatively good clinical condition. The IgG and IgA antibodies in saliva declined in the symptomatic phase. The level of IgG anti-p24 antibodies in saliva correlated with that in serum, suggesting that IgG anti-p24 antibodies in saliva originated from those in the serum. These results indicate that IgA antibodies are regulated independently from IgG antibodies and that the mucosal immune system is impaired early in the symptomatic phase of HIV infection, which starts with mucosal impairment. Detection of IgA antibodies may be useful for prognosis of the disease in HIV-infected individuals. The results indicate also that treatment for the impaired IgA mucosal immune system should be taken into consideration.