HSM inhibits IAV cleavage of the 4MU-Neu5Ac reporter substrate. (A) Cleavage of 4MU-Neu5Ac reporter substrate by A/Aichi/2/68(H3N2) was tested in the presence of HSM from two donors, PSM (10 nmol Sia), or PBS buffer. HSM from both donors inhibited the cleavage of 4MU-Ne5Ac. In contrast PSM did not inhibit cleavage of 4MU-Neu5Ac, similar to the buffer control. (B) Cleavage of 0.1 nmol 4MU-Neu5Ac by three IAV strains was tested in the presence of HSM, PSM (4.7 pmol Sia), or PBS buffer in triplicates. All virus strains were inhibited by HSM but not by PSM. Bars represent standard error, *P<0.05, **P<0.01 (two-tailed T-Test, n=3).