Estimating residence times of lymphocytes in ovine lymph nodes

The ability of lymphocytes to recirculate between blood and secondary lymphoid tissues such as lymph nodes (LNs) and spleen is well established. Sheep have been used as an experimental system to study lymphocyte recirculation for decades and multiple studies exist documenting accumulation and loss of intravenously (i.v.) transferred lymphocytes in efferent lymph of various ovine LNs. Yet, surprisingly little work has been done to accurately quantify the dynamics of lymphocyte exit from the LNs and to estimate the average residence times of lymphocytes in ovine LNs. In this work we developed a series of mathematical models based on fundamental principles of lymphocyte recirculation in the body and specifically, on how lymphocytes enter and exit lymph nodes in non-inflammatory (resting) conditions. We fitted these models to data from several independent experiments. Our analysis suggested that in sheep, recirculating lymphocytes spend on average 3 hours in the spleen and 20 hours in skin or gut-draining LNs with a distribution of residence times in LNs following a skewed gamma (lognormal-like) distribution. The latter result is in contrast which recent suggestions that the distribution of residence times of naive T cells in murine LNs is exponential and that lymphocyte residence times depend on the LN type (e.g., gut- vs. skin-draining). Our mathematical models also suggested an explanation for a puzzling observation of the long-term persistence of i.v. transferred lymphocytes in the efferent lymph of the prescapular LN (pLN); the model predicted that this is a natural consequence of long-term persistence of the transferred lymphocytes in circulation. We also found that lymphocytes isolated from the skin-draining pLN have a two-fold increased entry rate into the pLN as opposed to the mesenteric (gut-draining) LN (mLN). Likewise, lymphocytes from mLN had a three-fold increased entry rate into the mLN as opposed to entry rate into pLN. Importantly, these cannulation data could not be explained by preferential retention of cells in LNs of origin. Taken together, our work illustrates the power of mathematical modeling in describing the kinetics of lymphocyte migration in sheep and provides quantitative estimates of lymphocyte residence times in ovine LNs.