Direct observations of the kinetics of migrating T-cells suggest active retention by endothelial cells with continual bidirectional migration

Abstract

The kinetics and regulatory mechanisms of T-cell migration through endothelium have not
been fully defined. In experimental filter-based assays in vitro, transmigration of lymphocytes
takes hours, compared to minutes in vivo. We cultured endothelial cell (EC) monolayers on
filters, solid substrates or collagen gels, and treated them with tumour necrosis factor-α (TNF),
interferon-γ (IFN), or both, prior to analysis of lymphocyte migration in the presence or absence
of flow. Peripheral blood lymphocytes (PBL), CD4+ cells or CD8+ cells, took many hours to
migrate through EC-filter constructs for all cytokine treatments. However, direct microscopic
observations of EC-filters which had been mounted in a flow chamber showed that PBL crossed
the endothelial monolayer in minutes and were highly motile in the subendothelial space.
Migration through EC was also observed on clear plastic, with or without flow. After brief
settling without flow, PBL and isolated CD3+ or CD4+ cells all crossed EC in minutes, but the
numbers of migrated cells varied little with time. Close observation revealed that lymphocytes
continuously migrated back and forth across endothelium. Under flow, migration kinetics and
the proportions migrating back and forth were little altered. On collagen gels, PBL again crossed
EC in minutes and migrated back and forth, but showed little penetration of the gel over hours.
In contrast, neutrophils migrated efficiently through EC and into gels. These observations suggest
a novel model for lymphoid migration, in which endothelial cells support migration but retain
lymphocytes (as opposed to neutrophils), and additional signal(s) are required for onward
migration.

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