Influenza viruses can move across the surface of host
cells while
interacting with their glycocalyx. This motility may assist in finding
or forming locations for cell entry and thereby promote cellular uptake.
Because the binding to and cleavage of cell surface receptors forms
the driving force for the process, the surface-bound motility of influenza
is expected to be dependent on the receptor density. Surface gradients
with gradually varying receptor densities are thus a valuable tool
to study binding and motility processes of influenza and can function
as a mimic for local receptor density variations at the glycocalyx
that may steer the directionality of a virus particle in finding the
proper site of uptake. We have tracked individual influenza virus
particles moving over surfaces with receptor density gradients. We
analyzed the extracted virus tracks first at a general level to verify
neuraminidase activity and subsequently with increasing detail to
quantify the receptor density-dependent behavior on the level of individual
virus particles. While a directional bias was not observed, most likely
due to limitations of the steepness of the surface gradient, the surface
mobility and the probability of sticking were found to be significantly
dependent on receptor density. A combination of high surface mobility
and high dissociation probability of influenza was observed at low
receptor densities, while the opposite occurred at higher receptor
densities. These properties result in an effective mechanism for finding
high-receptor density patches, which are believed to be a key feature
of potential locations for cell entry.