Abstract
Hydrophilic lung surfactant proteins have emerged as key immunomodulators which are potent at the recognition and clearance of pulmonary pathogens. Surfactant protein A (SP-A) is a surfactant-associated innate immune molecule, which is known to interact with a variety of pathogens, and display anti-microbial effects. SP-A, being a carbohydrate pattern recognition molecule, has a wide range of innate immune functions against respiratory pathogens, including influenza A virus (IAV). Some pandemic pH1N1 strains resist neutralization by SP-A due to differences in the N-glycosylation of viral hemagglutinin (HA). Here, we provide evidence, for the first time, that a recombinant form of human SP-A (rfhSP-A), composed of α-helical neck and carbohydrate recognition domains, can actually promote the IAV replication, as observed by an upregulation of M1 expression in lung epithelial cell line, A549, when challenged with pH1N1 and H3N2 IAV subtypes. rfhSP-A (10 μg/ml) bound neuraminidase (NA) (∼60 kDa), matrix protein 1 (M1) (∼25 kDa) and M2 (∼17 kDa) in a calcium dependent manner, as revealed by far western blotting, and direct binding ELISA. However, human full length native SP-A downregulated mRNA expression levels of M1 in A549 cells challenged with IAV subtypes. Furthermore, qPCR analysis showed that transcriptional levels of TNF-α, IL-12, IL-6, IFN-α and RANTES were enhanced following rfhSP-A treatment by both IAV subtypes at 6 h post-IAV infection of A549 lung epithelial cells. In the case of full length SP-A treatment, mRNA expression levels of TNF-α and IL-6 were downregulated during the mid-to-late stage of IAV infection of A549 cells. Multiplex cytokine/chemokine array revealed enhanced levels of both IL-6 and TNF-α due to rfhSP-A treatment in the case of both IAV subtypes tested, while no significant effect was seen in the case of IL-12. Enhancement of IAV infection of pH1N1 and H3N2 subtypes by truncated rfhSP-A, concomitant with infection inhibition by full-length SP-A, appears to suggest that a complete SP-A molecule is required for protection against IAV. This is in contrast to a recombinant form of trimeric lectin domains of human SP-D (rfhSP-D) that acts as an entry inhibitor of IAV.
Highlights
The initial response to an infection in a host is the activation of its innate immune system
We have recently shown a recombinant form of human surfactant protein D, containing homotrimeric neck and carbohydrate recognition domain (CRD) region, binds HA and reduces matrix protein 1 (M1) expression in A549 cells challenged with pH1N1 and H3N2 strains ( Al-Ahdal et al, 2018)
Cell binding assay was performed to assess the interaction of recombinant form of human SP-A (rfhSP-A) with A549 lung epithelial cells challenged with pH1N1 and H3N2 (Figure 3)
Summary
The initial response to an infection in a host is the activation of its innate immune system. Understanding how innate immune mechanisms restrict the spread of respiratory infections is crucial to designing therapeutic strategies. Influenza virus has been a major infectious respiratory pathogen, and remains a serious global health concern, resulting in up to half a million respiratory deaths annually (WHO 2018). IAV is responsible for approximately 70% of all influenza related deaths, and has caused highly pathogenic pandemics (WH0 2009). IAV has been reported to exert a greater selective pressure on the host, with a complex pathogenesis characterised by rapid viral replication, and viral distribution within the lungs, while evoking cellular and humoral immunity (Fukuyama et al, 2011). IAV has evolved numerous molecular strategies to avoid and escape the host’s immune responses, and to promote continuous survival within the host, a range of immune responses could potentially target any stage of IAV
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