Human activated T cells adhere to synovial fibroblast-like cells in vitro. The present study was conducted to investigate the consequences of T cell-synovial fibroblast interactions with regard to induction of adhesion molecules and proinflammatory cytokines. A sensitive Western blot technique, polymerase chain reaction (PCR) amplification, and fluorescence-activated cell sorter (FACS) analysis were used to analyze the induction of VCAM-1 and ICAM-1 expression in T cell-synovial fibroblast cocultures. VCAM-1 and ICAM-1 expression could be induced in synovial fibroblast-like cells by 2 h. PCR amplification showed that both forms of VCAM-1 mRNA are found after the interaction of synovial fibroblasts with T cells. Up-regulation of VCAM-1 and ICAM-1 was confined to synovial fibroblasts; T cells did not express VCAM-1 or increased ICAM-1. In contrast to the T cell-synoviocyte interaction, the interaction between T cells and dermal fibroblasts resulted in the up-regulation of ICAM-1 but not VCAM-1, suggesting tissue-specific regulation of VCAM-1. The T cell-synovial fibroblast interaction also resulted in increased levels of tumor necrosis factor (TNF), interferon-gamma, and interleukin-6 in coculture supernatant. Of the neutralizing antibodies used against these cytokines, only anti-TNF could significantly inhibit VCAM-1 and ICAM-1 expression. When T cells were separated from synoviocytes by a chamber that allowed medium exchange but no cell contact, VCAM-1 and ICAM-1 failed to be up-regulated and cytokine accumulation in cocultures was drastically reduced. Our results demonstrate mutual cell activation of T cells and synoviocytes upon cell contact as shown by the release of T cell- and synoviocyte-specific cytokines and suggest a cell contact-mediated and T cell-initiated mechanism for the chronic accumulation and retention of mononuclear cells via VCAM-1/ICAM-1 by synovial fibroblasts in the rheumatoid synovium.

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The neurotransmitter, serotonin (5-hydroxytryptamine, 5HT), has been shown to affect function of cells of the immune system. More recently, specific 5HT receptors have been identified and partially characterized on Jurkat cells. Results presented here characterize the receptor on Jurkat cells as the 5HT1a receptor subtype and show that mitogen-activated but not resting human T cells also express the 5HT1a receptor subtype. Analysis of mRNA in Jurkat cells and activated and resting T cells by PCR or by Northern analysis revealed the presence of 5HT1a receptor. Pharmacologic analysis of this receptor demonstrated that the receptors on Jurkat cells and activated T cells are similar to each other and that they resemble the 5HT1a receptor found in the brain. Analysis of the second messenger pathways activated by the 5HT1a receptor show that ligand-binding to the Jurkat cell 5HT1a receptor results in elevation of intracellular inositol phosphates and Ca2+ and that ligand binding to the 5HT1a receptor on activated T cells modulated intracellular levels of cAMP.

Serotonin (5-hydroxytryptamine) (5HT) a neurotransmitter and vasoactive amine, is a major storage product of platelets that are released at sites of inflammation. Several different subtypes of serotonin receptors have been defined. 5HT receptors have been divided into three major families based on molecular, biochemical, and pharmacologic properties. Binding of serotonin to the 5HT1 family results in inhibition of adenylate cyclase whereas binding to the 5HT2 family results in stimulation of phosphatidylinositol turnover and mobilization of intracellular Ca2+. 5HT has been shown to have effects on lymphoid cells. The question of whether human T lymphocytes express receptors for 5HT and transduce signals through 5HT receptors has not been adequately addressed. As a model system, Jurkat cells (a transformed human T lymphocyte line) were examined to determine if they expressed 5HT receptors and whether 5HT stimulated an increase in inositol phosphates or affected adenylate cyclase activity. The results show that Jurkat cells bind 5HT with an average dissociation constant of 90 nM and that 5HT stimulates an increase in inositol phosphate and intracellular Ca2+ levels. These results link the 5HT receptor on Jurkat cells to the 5HT2 family; however, studies with 5HT receptor agonists and antagonists failed to clearly classify the 5HT receptor on Jurkat cells as a known member of the 5HT2 family.