  Collections of cells exhibit coherent migration during morphogenesis, cancer
metastasis, and wound healing. In many cases, bigger clusters split, smaller
sub-clusters collide and reassemble, and gaps continually emerge. The
connections between cell-level adhesion and cluster-level dynamics, as well as
the resulting consequences for cluster properties such as migration velocity,
remain poorly understood. Here we investigate collective migration of one- and
two-dimensional cell clusters that collectively track chemical gradients using
a mechanism based on contact inhibition of locomotion. We develop both a
minimal description based on the lattice gas model of statistical physics, and
a more realistic framework based on the cellular Potts model which captures
cell shape changes and cluster rearrangement. In both cases, we find that cells
have an optimal adhesion strength that maximizes cluster migration speed. The
optimum negotiates a tradeoff between maintaining cell-cell contact and
maintaining cluster fluidity, and we identify maximal variability in the
cluster aspect ratio as a revealing signature. Our results suggest a collective
benefit for intermediate cell-cell adhesion.
