Constitutive and inflammatory lymphocyte trafficking.

Here we provide a brief overview of lymphocyte trafficking with particular emphasis on the current state of knowledge in the pig. We discuss how the emphasis of research has changed since early studies in the 1960s and outline the current hypothesis of a multistep cascade for lymphocyte migration through specialized endothelia. During the last several years our research has focused mainly on lymphocyte migration in vivo. The inbred Babraham herd of MHC homozygous Large White pigs has allowed study of entry of either labelled (FITC or 51Cr) or unlabelled CD45 allotype-different donor lymphocytes and their subsets into various lymphoid, non-lymphoid and inflammatory tissues. The findings are considered under three different categories. Firstly, constitutive lymphocyte entry via 'high endothelial venules' (HEV-mediated), secondly, non-HEV-mediated lymphocyte homing and thirdly, lymphocyte entry into several models of inflammation with particular reference to the role of E-selectin. These findings demonstrate and underline the complexity and heterogeneity of lymphocyte homing, both at the whole population and subset level and yet, whilst a major step forward, the current hypotheses are perhaps too simple to explain much of this heterogeneity.