Syndecan-2 and -4 expressed on activated primary human CD4+ lymphocytes can regulate T cell activation

Syndecans bind to cell adhesion molecules, growth factors and cytokines, and can act as coreceptors, and in this way modulate leukocyte cell function. Here, expression of the syndecans on primary human CD4 T cells was examined. Cell stimulation dramatically increased the amount of syndecan-4, and in a lower extent that of syndecan-2. Expression of syndecan-2 and -4 show different induction kinetics. Whereas syndecan-4 expression is fast and significant, that of syndecan-2 is more delayed and short-lived decreasing its mRNA expression at day 4. Both CD45RA+ naive and CD45RA− memory CD4 T cells express syndecan-2 and -4 upon activation. When incubated with human peripheral blood lymphocytes in a mixed leukocyte reaction, anti-syndecan-4 but not anti-syndecan-2 antibodies, decreased T cell proliferation. However, cross-linking of cell-bound syndecan-2 or syndecan-4 via immobilized antibodies blocked proliferation and decreased TNF production of T cells in the presence of optimal levels of anti-CD3. These findings suggest that syndecan-2 and -4 act as inhibitors of T cell activation. We also investigated the role that MAPK signalling pathways play in control of syndecan expression in T cells. We show that production of syndecan-2 but not syndecan-4 requires signaling via p38 MAP kinase α/β in T CD4 cells. As mechanisms that confer syndecan-2 expression are unknown, we analyse the chromatin hypersensitivity of syndecan-2 promoter proximal region in Jurkat T cells and endothelial cells. The analysis reveals a chromatin accessible site in the +3.5 kb intronic region, concomitant with a region showing high evolutionary conservation. We isolate and analyse 5′-flanking regions of human syndecan-2 gene, by transfection assays. The +3.5 kb hypersensitive site in the intronic region demonstrates basal promoter activity in Jurkat. This study provides evidence for the up-regulation of syndecan-2 and -4 in human primary CD4 T cells during in vitro activation and suggest an inhibitory role for these syndecans in CD4 T cells.