Multiple components contribute to ability of saliva to inhibit influenza viruses

INTRODUCTION

Saliva is a potentially important barrier against respiratory viral infection but its mechanism of action is not well studied.

METHODS

We tested the antiviral activities of whole saliva, specific salivary gland secretions, and purified salivary proteins against strains of influenza A virus (IAV) in vitro.

RESULTS

Whole saliva or parotid or submandibular/sublingual secretions from healthy donors inhibited IAV based on hemagglutination inhibition and neutralization assays. This differs from human immunodeficiency virus (HIV), for which only submandibular/sublingual secretions are reported to be inhibitory. Among purified salivary proteins, MUC5B, scavenger receptor cysteine-rich glycoprotein 340 (salivary gp-340), histatins, and human neutrophil defensins (HNPs) inhibited IAV at the concentrations present in whole saliva. In contrast, some abundant salivary proteins (acidic proline-rich proteins and amylase) had no activity, nor did several other less abundant salivary proteins with known activity against HIV (e.g. thrombospondin or serum leukocyte protease inhibitor). Whole saliva and MUC5B did not inhibit neuraminidase activity of IAV and viral neutralizing and aggregating activity of MUC5B was potentiated by the neuraminidase inhibitor oseltamivir. Hence, MUC5B inhibits IAV by presenting a sialic acid ligand for the viral hemagglutinin. The mechanism of action of histatins requires further study.

CONCLUSIONS

These findings indicate that saliva represents an important initial barrier to IAV infection and underline the complexity of host defense activity of oral secretions. Of interest, antiviral activity of saliva against IAV and HIV differs in terms of specific glandular secretions and proteins that are inhibitory.

Inhibition of influenza viral neuraminidase activity by collectins

The collectins, lung surfactant proteins A and D (SP-A and SP-D), contribute to innate host defense against influenza A virus (IAV) in vivo. Although collectins bind to the viral hemagglutinin (HA) and inhibit early stages of viral infection in vitro, they also bind to the neuraminidase (NA) and inhibit NA activity. We used a variety of NA functional assays, viral strains and recombinant (mutant or wild type) collectins to characterize the mechanism of NA inhibition. NA inhibition by SP-D correlates with binding of its carbohydrate recognition domain (CRD) to oligomannose oligosaccharides on the viral hemagglutinin (HA). The effects of SP-D are additive with oseltamivir, consistent with differences in mechanism of action. NA inhibition was observed using fetuin or MDCK cells as a substrate, but not in assays using a soluble sialic acid analogue. Collectin multimerization and CRD binding properties are key determinants for NA inhibition. SP-D had greater NA inhibitory activity than mannose-binding lectin, which in turn had greater activity than SP-A. The markedly greater NA inhibitory activity of SP-D compared to SP-A may partly account for the finding that deletion of the SP-D gene in mice has a greater effect on viral replication in vivo.

Proposal for genetic characterisation of wild-type mumps strains: Preliminary standardisation of the nomenclature

Though mumps virus (MuV) is a monotypic virus, genetic variation between strains has been described. Viruses have been placed into genotypes designated A-L based on the nucleotide sequence of the small hydrophobic (SH) gene, which is the most variable gene in the mumps genome. Molecular characterisation of MuV is an important component of mumps surveillance because it can help identify the transmission pathways of the virus as well as distinguish between wild-type and vaccine strains. Here, we propose a standardized nomenclature and an analysis protocol for the genetic characterisation of mumps strains to facilitate expansion of molecular epidemiological studies. In addition to assigning standard reference strains for the recognized genotypes of MuV, a convention is proposed for naming for strains and criteria to designate a new genotype.

Molecular characterisation of two mumps virus genotypes circulating during an epidemic in Lithuania from 1998 to 2000

An epidemic of mumps in Lithuania started in December 1998 and continued until May 2000. The total registered number of cases was about 11.000 of a total of 3,7 million inhabitants in Lithuania (29,7 cases/10,000). Virus-containing samples were collected from 80 patients treated at the hospital of Kaunas from October 1999 until the end of the epidemic. Out of the 80 patients with parotitis, meningitis was observed in 11 patients and orchitis in 22 of 69 male patients. Twenty-seven virus strains were genotyped by nucleotide sequencing of the small hydrophobic (SH) protein gene, and the 57 amino acid sequences of the gene were deduced. Twenty-five virus strains belonged to the C genotype and two were of the D genotype. By phylogenetic analysis the virus strains causing meningitis grouped in a separate cluster, designated C1, within the C genotype. Another group of ten of the 25 genotype C strains exhibited an amino acid triplet at amino acid positions 28 to 30 of the protein, consisting of valine, alanine and serine, instead of the previously recognised valine, valine and serine combination of genotype C. The amino acid alanine at position 29 was found in combination with the amino acid serine at position 48. This variant was designated C2 and it was associated with parotitis. The amino acid alanine at position 29 and serine in position 48 of the C2 genotype may constitute a marker of low neurovirulence compared to other genotype C strains.

Mumps virus neutralizing antibodies do not protect against reinfection with a heterologous mumps virus genotype

In April 1999, a previously healthy 22-year-old woman was taken ill with fever and bilateral swelling of the parotid glands. A chronic course of disease extending from April to December was found with swelling of the parotid glands, fatigue, low grade fever, episodes of tachycardia and nightswetting. Mumps virus RNA of genotype A character based on the SH (small hydrophobic) protein gene classification was demonstrated in three serum samples collected during the course of clinical disease. Different criteria for reinfection were fulfilled including demonstration of IgG antibodies by ELISA in a preinfection serum sample. The preinfection serum sample of the patient was able to efficiently neutralize the infectivity of a heterologous genotype D strain but was unable to neutralize the homologous genotype A virus. The findings in the present study may offer an explanation of a mechanism behind previously observed vaccine failures and the occurrence of reinfection with heterologous mumps virus strains.

Antigenic and genetic characterization of the fusion (F) protein of mumps virus strains

Twelve strains of mumps virus, belonging to the A, C and D genotypes of the small hydrophobic (SH) protein gene, were investigated by nucleotide sequencing of the fusion protein gene. The nucleotide sequences and deduced amino acid sequences were aligned and compared with previously reported sequences of the gene. In addition an antigenic comparison between the F protein of different strains of the A, C and D genotypes was performed with ten monoclonal antibodies directed against the F protein of genotype A. Phylogenetic analysis of the coding region of the F gene showed the expected clustering of the different genotypes, as previously determined from the SH protein gene. Comparison of the 538 long amino acid sequence of the protein showed that only a small number of amino acids differed between the viral strains. The A genotype differed from B, C and D whereas the latter showed fewer consistent amino acid differences between themselves. Nine of the ten monoclonal antibodies reacted with the C and D genotypes and one failed to react with these genotypes. It is concluded that the structure and antigenicity of the F protein is well conserved both intra- and intergenotypically over long periods of time.

Characterization of three co-circulating genotypes of the small hydrophobic protein gene of mumps virus

Eighteen virus isolates and 22 serum samples collected between 1971 and 1997 from patients with mumps were genotyped by PCR with specific primer pairs for the A, C and D genotypes of the small hydrophobic (SH) protein gene. All serum samples were subjected to nucleotide sequence analysis of the gene, and the deduced 57-amino-acid sequences were aligned with previously published sequences from the USA, Canada, Portugal, the UK, France, Germany, Switzerland, Denmark, Sweden, Russia, China and Japan. The existence of six genotypes of the SH protein gene, named A to F, was confirmed. In the Stockholm area, co-circulation of genotypes A, C and D at different times was found. There was a striking difference in genotype between the virus isolates and the serum samples. The 18 virus isolates represented genotypes C and D, whereas the 22 serum samples contained genotype A. In most cases, the amino acid sequences of the 22 genotype A specimens were identical to the previously described SBL-1 strain of genotype A. Genotypes C and D were always associated with meningitis, and in some cases parotitis, whereas infection with genotype A most often resulted in parotitis and seldom in meningitis.