Interactions of T cells and fibroblasts appear to be important in the development of fibrosis, for example, the restrictive lung disease that follows the inflammatory process of alveolitis in scleroderma (systemic sclerosis, SSc). The intermolecular interactions mediating fibroblast activation are not well characterized. CD154 (CD40 ligand) is an activation-induced T-cell surface molecule which counter-receptor is CD40 expressed on target cells, including fibroblasts. We have found CD154 expression on a number of activated CD8+ T cell clones derived from bronchoalveolar lavage (BAL) fluids from SSc patients. To begin investigating the potential role of CD154-CD40 interactions in fibroblast activation, we co-cultured CD154+ Jurkat D1.1 cells or CD154- Jurkat E6-1 cells with fibroblast lines derived from dermal biopsies or BAL fluids from SSc patients and control donors. Collagen α 2(I) mRNA expression in fibroblasts was measured by RT-PCR, with ribosomal protein S9 as an internal standard. Total soluble collagen was measured in co-culture supernatants, using Sircol Biocolor assay. In fibroblasts co-cultured for 6 h with CD154+ cells, but not CD154- cells, normalized collagen mRNA expression and total soluble collagen production were 2 times higher than in fibroblasts cultured alone. Intracellular fluorescent staining did not detect IL-4, IL-10, IFNγ, or CD95 ligand expression in either D1.1 or E6-1 cells. These data suggest that CD154-CD40 interactions may enhance collagen production in fibroblasts. As this process continues uncontrolled, it may lead to the development of fibrosis.