The role of surfactant protein B in influenza virus infection

Abstract

Influenza virus is the most common virus causing respiratory tract infection leading to hospitalization and death. Severe illness is more common in young children, elderly, pregnant women, and individuals with obesity or underlying diseases. However, many patients with severe influenza do not have any of these risk factors.

Pulmonary surfactant is important in maintaining alveolar stability. In addition, surfactant protein A (SP-A) and surfactant protein D (SP-D) are known to inhibit influenza virus replication. Genetic polymorphisms of surfactant protein genes have been associated with respiratory tract infections. The objective of this study is to find out whether surfactant protein is important in the clinical severity of influenza virus infection. Firstly, genetic polymorphisms in the surfactant protein genes were compared between severe and mild cases of influenza A (H1N1)pdm09 and A(H3N2) infection. Secondly, antiviral effect of surfactant protein B (SP-B) was investigated.

Twelve single nucleotide polymorphisms (SNPs)of surfactant protein genes were compared between influenza A patients suffering from severe and mild illness. In the first cohort of 42 severe and 42 mild cases of A(H1N1)pdm09 infection, a significant association was found between the C allele of SNP rs1130866 (located in the surfactant protein B gene SFTPB)with severe infection (odds ratio [OR] =3.37, p = 0.0048). In the second cohort of 296 patients, both univariate and multivariate analyses confirmed C/C genotype of rs1130866 to be an independent risk factor for severe A(H1N1)pdm09 infection (OR=2.087, p = 0.023). Moreover, C/C genotype was overrepresented in hospitalized A(H1N1)pdm09 patients when compared to the general Han Chinese population (OR=3.232, p = 0.00000056). On the other hand, there was no significant association between rs1130866 genotype and disease severity of A(H3N2) infection, but the C/C genotype was significantly overrepresented in hospitalized A(H3N2) patients when compared with the general Han Chinese population (OR=1.521, p = 0.020). Collectively, our findings suggested that the SFTPB polymorphism rs1130866-C was associated with severe influenza.

Sincers1130866 is a functional SNP affecting the secretion and maturation of SP-B, individuals with SFTPB polymorphism rs1130866-C may have less mature SP-B secreted into alveolar surface. Therefore, we hypothesized that SP-B is an antiviral peptide and low levels of SP-Bin the alveoli of individuals with rs1130866-C may predispose them to severe influenza. Virus yield reduction assay and plaque reduction assay showed that SP-B inhibited viral replication of A(H1N1)pdm09 and A(H7N9). Time-of-addition assay showed that the inhibition of viral replication did not occur when SP-B was added 3 hours after virus inoculation. Immunofluorescence assay showed that viral nucleoprotein expression at 6 hours after virus inoculation was inhibited by SP-B. Therefore, SP-B could inhibit viral replication, and the inhibition likely occurred during the early stages of viral lifecycle.

Our study showed that SFTPB gene polymorphism rs1130866-C was associated with severe influenza and SP-B had direct antiviral activity in vitro. Further studies are needed to verify the antiviral activity of SP-B in vivo, and whether exogenous SP-B can act as an adjunctive therapy in the treatment of severe influenza.