Lymphocyte Interactions with Endothelial Cells

The endothelial lining of the vasculature forms the interface of the vessel wall and peripheral tissues with the blood. Therefore, circulating lymphocytes must contact endothelial cells as they home to a site of immune inflammation. Morphologically, both lymphocytes and endothelial cells appear “activated” during this encounter, but the details and mechanisms of the activation processes are not fully understood. We have used cultured human endothelial cells (HEC) and peripheral blood mononuclear cells (PBMC) to study potential interactions between these cell types. In bulk coculture, HEC activate allogeneic T cells, leading PBMC to proliferate. In contrast, dermal fibroblasts or vascular smooth muscle cells do not induce allogeneic PBMC proliferation. The HEC modify the response of PBMC to exogenous stimulation; inclusion of HEC in a culture of PBMC stimulated by phytohemagglutinin leads to increased PBMC proliferation and up to ten-fold or greater enhancement of interleukin 2 production. The phenotypes in the proliferating PBMC are altered by the presence of HEC. Prior to coculture with PBMC, HEC do not express class II major histocompatibility complex (MHC) antigens, the major stimulus of PBMC proliferation to allogeneic PBMC. However, within 1–2 days of coculture, the HEC uniformly express class II molecules. Since PBMC proliferation begins on day 4 or 5, the induced HEC class II molecules could be involved in the T cell activation. Immune interferon (IFN-7), a mediator secreted by activated T cells, mimics the effect of coculture by inducing class II molecules. All three class II loci (DR, DP, DQ) are activated, leading to de novo appearance of mRNA and surface antigen expression. The IFN-7 also induces quantitatively comparable expression of class II molecules on dermal fibroblasts or smooth muscle cells, but such cells still fail to stimulate allogeneic PBMC, even in the presence of exogenous interleukin 1. It appears unlikely that this “defect” resides in tissue-specific structural differences of the class II molecules, since IFN-γ-treated dermal fibroblasts are lysed by class-II-specific cytolytic T lymphocyte clones and activate cloned helper T cells. Coculture of HEC with PBMC or treatment with IFN-γ causes cultured HEC to undergo morphological changes. Specifically, IFN-γ-treated HEC become plump and elongated, overlap, rearrange their actin filaments, and lose their matrix-associated fibronectin. These morphological changes are not seen with IFN-γ-treated dermal fibroblasts, nor can they be induced in HEC cultures by nonimmune interferons (IFN-α,s). Tumor necrosis factor (TNF), a product of activated monocytes, causes IFN-γ-like morphological changes in HEC cultures. Low concentrations of TNF and IFN-γ act synergistically, whereas higher concentrations of these mediators in combination produce unique effects. Murine monoclonal antibody H4/18 detects an antigen induced on HEC by coculture with PBMC. In contrast to the modulation of MHC antigens, which plateau after several days and persist, the antigen recognized by H4/18 peaks in several hours and disappears. This transient induction can be mimicked by TNF and interleukin 1, but not IFN--y. and parallels several othcr transicntly expressed HEC properties such as procoagulant activity and leukocyte adhesion. These studies demonstrate that the interaction of lymphocytes and endothelium in culiure alters the behavior and phenotype of both cells and suggest that lymphokines, acting separatcly and in combination. may mediale many of these interactions.