Cooperative motility emerges in crowds of T cells but not neutrophils

T cells are among the most motile immune cells in the body, and their migration into and within tissues is key to their function. Remarkably, T cells can maintain their motility even in highly crowded environments like the densely packed T cell areas of lymphoid organs, but how they do so remains incompletely understood. Here, we use microfluidic devices and in silico models to study T cells from a crowd dynamics perspective, focusing on a hallmark scenario that has proven instrumental in characterizing other crowded systems such as pedestrians and ants: single-lane traffic. Unexpectedly, T cells in narrow, straight microchannels synchronize their speeds and form stable, motile trains. We show that this behavior can be explained by a preference of T cells to maintain contact with each other after collisions, and an ability of faster T cells to “push” slower ones. We demonstrate that this behavior does not extend to all immune cells; neutrophils in the same settings slow down with increasing cell density. Cooperative motion may benefit T cell motility in difficult and crowded tissue environments, ultimately preventing jams that impair motion in other crowded systems.