Salivary Blockade Protects the Lower Respiratory Tract of Mice from Lethal Influenza Virus Infection

Abstract

It is possible to model the progression of influenza virus from the upper respiratory tract to the lower respiratory tract in the mouse using viral inoculum delivered in a restricted manner to the nose. In this model, infection with the A/Udorn/ 307/72 (Udorn) strain of virus results ultimately in high viral titers in both the trachea and lungs. In contrast, the A/Puerto Rico/8/34 (PR8) strain causes an infection that is almost entirely limited to the nasal passages. The factors that govern the progression of virus down the respiratory tract are not well understood. Here, we show that, while PR8 virus grows to high titers in the nose, an inhibitor present in the saliva blocks further progression of infection to the trachea and lungs and renders an otherwise lethal dose of virus completely asymptomatic. In vitro, the salivary inhibitor was capable of potent neutralization of PR8 virus and an additional 20 strains of type A virus and two type B strains that were tested. The exceptions were Udorn virus and the closely related H3N2 strains A/Port Chalmers/1/73 and A/Victoria/3/75. Characterization of the salivary inhibitor showed it to be independent of sialic acid and other carbohydrates for its function. This and other biochemical properties, together with its virus strain specificity and in vivo function, indicate that the mouse salivary inhibitor is a previously undescribed innate inhibitory molecule that may have evolved to provide pulmonary protection of the species from fatal influenza virus infection. IMPORTANCE Influenza A virus occasionally jumps from aquatic birds, its natural host, into mammals to cause outbreaks of varying severity, including pandemics in humans. Despite the laboratory mouse being used as a model to study influenza virus pathogenesis, natural outbreaks of influenza have not been reported in the species. Here, we shed light on one mechanism that might allow mice to be protected from influenza in the wild. We show that virus deposited in the mouse upper respiratory tract will not progress to the lower respiratory tract due to the presence of a potent inhibitor of the virus in saliva. Containing inhibitor-sensitive virus to the upper respiratory tract renders an otherwise lethal infection subclinical. This knowledge sheds light on how natural inhibitors may have evolved to improve survival in this species.KEYWORDS influenza, innate inhibitors I nfluenza A virus (IAV) is a respiratory pathogen of global significance, responsible for 250,000 to 500,000 deaths annually. Innate immunity is critical to the host in the early stages of IAV infection, restricting the escalation of viral loads prior to the induction of virus-clearing responses mediated by the adaptive immune system. As part of the innate immune system, a variety of soluble molecules are present in respiratory secretions to provide an initial barrier to infection. They include collectins, pentraxins,

An infant mouse model of influenza virus transmission demonstrates the role of virus-specific shedding, humoral immunity, and sialidase expression by colonizingStreptococcus pneumoniae

“…Given the remarkable capacity of infant mice to support IAV transmission among littermates (25), we sought to validate and optimize the infant mouse as a potential new model to study IAV transmission. Restricted URT infection of infant C57BL/6J pups in a litter (index) was performed with low volume intranasal (IN) inoculum (3μl) using IAV strain A/X-31 (H3N2) (24, 29). Intralitter transmission was assessed in littermates (contact/recipient) by measuring virus from retrograde tracheal lavages at 4-5 days post-infection (p.i.)…”

Mouse Saliva Inhibits Transit of Influenza Virus to the Lower Respiratory Tract by Efficiently Blocking Influenza Virus Neuraminidase Activity

“…In the accompanying paper (11), we show that the RDE-resistant salivary inhibitor is responsible for preventing the progression of PR8 virus down the respiratory tree, while the RDE-sensitive inhibitor of Udorn virus did not have this capacity. Here, we investigated whether the presence of the PR8 NA was responsible for halting virus progression to the lungs.…”

“…In the accompanying paper (11), we show that following total respiratory tract (TRT) delivery (i.e., when the majority of the inoculum bypasses the oropharynx) PR8 virus replicates to high titers in the nose, trachea, and lungs and is lethal to mice at doses as low as 100 PFU. However, when the inoculum is restricted to the nose, doses as high as 10 4.5 PFU are nonlethal and infectious virus is rarely recovered from the lungs.…”

Quantifying between-Host Transmission in Influenza Virus Infections

“…Mice were the first animals used to study how the host influences influenza infections (Schulman and Kilbourne 1963), revealing that infection is highly dependent on both the strain of the virus and the strain of the mouse model (Schulman and Kilbourne 1963;Lowen et al 2006;Haller et al 2015;Ivinson et al 2017). Most studies have found that H3N2 strains can transmit when mice are in direct contact with each other, whereas H1N1 strains do not transmit efficiently unless adapted to do so (Lowen et al 2006;Edenborough et al 2012).…”

“…Most studies have found that H3N2 strains can transmit when mice are in direct contact with each other, whereas H1N1 strains do not transmit efficiently unless adapted to do so (Lowen et al 2006;Edenborough et al 2012). Currently, establishing standard laboratory procedures is underway to produce mice that are more effective transmission models of influenza (Edenborough et al 2012;Ivinson et al 2017).…”