  Many cell types display random motility on two-dimensional substrates, but
crawl persistently in a single direction when confined in a microchannel or on
an adhesive micropattern. Does this imply that the motility mechanism of
confined cells is fundamentally different from that of unconfined cells? We
argue that both free- and confined- cell migration may be described by a
generic model of cells as "velocity aligning" active Brownian particles
previously proposed to solve a completely separate problem in collective cell
migration. Our model can be mapped to a diffusive escape over a barrier and
analytically solved to determine the cell's orientation distribution and
repolarization rate. In quasi-one-dimensional confinement, velocity-aligning
cells maintain their direction for times that can be exponentially larger than
their persistence time in the absence of confinement. Our results suggest an
important new connection between single- and collective- cell migration: high
persistence in confined cells corresponds with fast alignment of velocity to
cell-cell forces.
