2022 Volume 158 Pages 110-118
In order to clarify human mucosal and systemic immunity against influenza viral infection, we measured the anti-influenza viral-specific secretory IgA (s-IgA) titers in the nasal secretions and IgG titers in the serum of adults. We used a safe and easy method for collecting nasal washings, the so-called nasal spray and aspiration (NSA) method, developed by us. The antiviral nasal s-IgA to total IgA ratio and serum IgG titers in 155 healthy adults and the changes in their levels at one month after subcutaneous vaccination were examined by an ELISA method in 2006. In addition, the induction and maintenance of antiviral nasal s-IgA and serum IgG responses throughout the first year after influenza infection were measured in 16 adults in 2007–2009. About 70% of the 155 healthy adults had nasal antiviral s-IgA against the A(H1N1) and A(H3N2) subtype and B type influenza viruses, and almost all had serum antiviral IgG prior to the vaccination. The mean serum antiviral IgG titers increased significantly after vaccination in the subjects with low pre-vaccination IgG titers, but not in those with high pre-vaccination IgG titers. On the other hand, no significant increase of the mean nasal antiviral IgA titer was found after the vaccination, irrespective of the pre-vaccine immune status. The antiviral s-IgA titers in the nasal washings in the patients with influenza virus infection were significantly lower at the onset of illness than that in healthy adults. While the serum antiviral IgG titers and HI titers varied widely, the HI titers were over 40 in 63% of patients at the onset of illness. Both the nasal s-IgA and serum IgG titers increased significantly at a later phase after the onset of illness, reaching their peak titers within day 21. The induced nasal s-IgA titers were maintained for 5 months, and returned toward the initial levels by 300 days after the onset of illness. Currently available subcutaneous influenza vaccines induce systemic immunity, with the appearance of antiviral IgG in the serum, in adults. However, subcutaneous vaccination did not elicit mucosal immunity (antiviral secretory IgA in the nasopharynx). Our findings also suggest that subjects with low levels of nasal antiviral secretory IgA are at an elevated risk for influenza infection.
