number of total, vaccine-binding, or recombinant HA-binding IgM-secreting cells
present in PBMC samples. ELISpot plates (Millipore) were coated overnight at
4°C with QIV (diluted 1:100), tetanus/diphtheria vaccine (diluted 1:50),
and 10 µg/mL anti-human Ig (Jackson ImmunoResearch) and 3 µg/mL
recombinant HA proteins. Plates were blocked the following morning for 90 min at
37°C with RPMI 1640 supplemented with 10% FBS. Dilutions of washed PBMCs
were incubated for 18 h in RPMI supplemented with 10% FBS. After washing the
plates, secreted antibodies were detected with anti-human IgM-biotin
(Invitrogen, 1:2000) and streptavidin-HRP (Jackson ImmunoResearch), and plates
were developed with AEC substrate (Sigma). Total, vaccine-binding, or
recombinant HA-binding IgG and IgA-secreting cells were detected using IgG/IgA
double -color ELISpot Kits (Cellular Technologies, Ltd.) according to the
manufacturer’s instructions. ELISpot plates were analyzed using an
ELISpot counter (Cellular Technologies Ltd.).