Appendix: Additional information about avian influenza A(H5N1) neuraminidase inhibition antibodies in healthy adults after exposure to influenza A(H1N1)pdm09. Click here to view.(476K, pdf) Acknowledgments We thank Christopher J. Middle East, Africa, and North America, HMN-176 as previously observed in 2005 (clade 2.2 virus) and in HMN-176 2014C2015 (clade 2.3.4.4c virus) ((black-face spoonbill). Similarly, 93.75% (30/32) persons possessed cross-reactive NAI titers against H6N1 virus (GMT 26.50), and 40.6% (13/32) possessed NAI titers >1:40 (Appendix Figure 1). However, cross-reactivity did not extend to N4 protein of an avian influenza A(H6N4) virus isolated from wild bird surveillance (A/environment/Hong Kong/HKU_MPT_2022; Env/HK/22) originated from (northern pintail) (Appendix Physique 1). Overall, we observed high correlations between NAI titers against pH1N1 and H6N1 viruses (Spearman ?=?0.875; p<0.001) and between H5N1 and H6N1 viruses (Spearman ?=?0.874; p<0.001). Open in a separate window Physique Age-stratified HAI and NAI antibody responses to influenza A(H1N1)pdm09 (California/09) and A(H5N1) (Spoonbill/HK/22) viruses in serum samples collected from healthy adults in 2020 and 2009, Hong Kong, China. A, B) Results for serum samples of 63 healthy adults collected in 2020. C, D) Results for serum samples of 50 healthy adults collected in 2009 2009. The assay detection limit was 1:10, and samples with antibody below the detection limit were assigned an arbitrary antibody titer of 5, which is used to calculate geometric mean titer. The HAI and NAI titers across different age groups were compared using Kruskal-Wallis test and Dunns multiple comparison test. HAI, hemagglutination inhibition; NAI, neuraminidase inhibition. We examined NAI titers to the homologous virus and cross-reaction to H5N1 virus in archival ferret antiserum against seasonal A(H1N1) viruses and pH1N1 (Table 1). Ferret antiserum against H1N1 viruses circulating during 1977C2007 showed no cross-reactive NAI response to H5N1 but had NAI titers at 1:320C1:1280 against the homologous viruses. Ferret antiserum against pH1N1 virus showed a homologous NAI titer of 1 1:2,560 and cross-reactive NAI titers to H5N1 at 1:320 to 1 1:640. Table 1 NAI Lamp3 antibody titers detected in postinfection ferret antiserum against influenza A(H1N1), A(H1N1)pdm09, and A(H5N1) viruses, Hong Kong, China
A/USSR/90/1977320<10<10A/Chile/01/1983320<10<10A/Singapore/06/1986320<10<10A/Texas/36/1991320<10<10A/Brisbane/59/20071,280<10<10A/California/04/20092,5602,560320C640 Open in a separate window *NAI antibody response was decided using recombinant A(H6N1) viruses carrying the N1 proteins derived from A(H1N1), A(H1N1)pdm09, or A(H5N1) viruses that were generated as described previously (10). Although we used recombinant H6N1 viruses in ELLA assays to avoid interference of hemagglutinin-reactive antibodies, we might not completely prevent the interference of cross-reactive antibodies that bind to the stalk of H6 protein and inhibit neuraminidase activity through stearic hindrance. NAI, neuraminidase inhibition. ?One ferret antiserum against each of the influenza A(H1N1) viruses and 6 ferret antiserum against the influenza A(H1N1)pdm09 were used. To further confirm whether exposure to pH1N1 virus contributed to the cross-reactive NAI antibodies, we used serum samples of 50 healthy blood donors (17C55 years of age) collected in July 2009, before pH1N1 had become widespread in Hong Kong (11). HAI antibodies to pH1N1 were detected in 22% (11/50) of the samples; HMN-176 GMT was low, at 7.07 (Figure, panel C). We detected NAI antibodies to pH1N1 in 40% (20/50) of healthy blood donors; GMT was low, at 8.24. NAI antibodies to H5N1 were detected in 40% (22/50) of donors; GMT was 8.35 (Figure, panel D). Most participants (16/20) with detectable NAI antibodies to pH1N1 also had NAI antibodies to H5N1. Overall, the cross-reactive NAI titers detected in 2009 2009 (Physique, panel HMN-176 D) were lower than the those detected in 2020 (Physique, panel B). This result suggests that previous exposure to pH1N1 virus is the potential source of cross-reactive NAI antibodies to H5N1 virus. The NA protein of Spoonbill/HK/22 differed from the pH1N1 (California/09) NA by.