Lymph node topology dictates T cell migration behavior

Supplemental ReSultS periodic behavior of simulated t cells Velocity fluctuations of our in silico T cells are comparable to those of real T cells (Fig. S2, A and B, top panels). Furthermore, Fourier and autocorrelation analyses of these brief time series (64 data points) result in a similar periodicity (Fig. S2, A and B, bottom two panels). In our model, periodic velocity fluctuations result from pausing due to random obstacles. We also studied T cell velocity fluctuations over long time periods and found no evidence of periodic behavior (Fig. S3 A). Hence, the large velocity fluctuations make it appear that there is a characteristic period when one analyzes brief time series. The analysis of long time series makes it clear that there is in fact no such characteristic period in the velocity fluctuations of our simulations. To see how easily stops caused by a true internal clock would be picked up by such an analysis, we modified the model and let the directional propensity of T cells be briefly turned off exactly every 2 min. In that case, the signal is not always picked up if one analyzes brief time series (Fig. S3 B). In contrast, periodic pauses are now convincingly shown in Fourier and autocorrelation analyses of long time series (Fig. S3 C). These findings indicate that the analysis of experimental velocity data performed thus far is insufficient to establish whether T cells have an internal clock that regulates velocity fluctuations over time. Furthermore, for this type of analysis, three-dimensional instead of two-dimensional, lateral velocities are required. Otherwise, the periodicity found in each cell differs markedly from that found with three-dimensional velocities (Fig. S4). Therefore, we performed new 2PM experiments with the aim to follow velocity fluctuations of individual T cells for long periods (see Results in main text). Figure S1. effect of Rn density on t cell displacement and velocity fluctuations. Mean displacement plot (A) and the standard deviation of T cell velocities (B) for different rod numbers. Note that the average velocity of T cells is approximately the same in these simulations, which we achieved by adjusting μ max