An early cellular event in the development of psoriatic lesions is the infiltration of the target tissue by macrophages and activated T lymphocytes (8-10). Lesional psoriatic skin contains activated memory T lymphocytes ( 10-15 ) whose production of mRNA for lymphokines such as IL-2, y-IFN, and TNF-a is elevated relative to normal or uninvolved psoriatic epidermis (16). That the T cell activation and soluble lymphokine production plays a crucial role in the maintenance of epidermal hyperplasia in the psoriatic lesion is indicated by the beneficial clinical effect of such immunosuppressive agents as cyclosporin A, FK 506, anti-CD3, and anti-CD4 in the treatment of psoriasis (17)(18)(19)(20)(21)(22). Psoriatic keratinocytes do appear to have been modulated by T cell lymphokines in vivo, because they abnormally express class II MHC and IP-IO molecules uniquely induced on keratinocytes by the T cell product y-IFN (23-27). Indeed, T cells producing y-IFN have been cloned from psoriatic lesions (28) and they appear to be able to induce keratinocyte class II MHC and intracellular adhesion molecule expression (29) + 50 U/ml rIL-2 (Collaborative Biomedical Products) media in 24-well plates and then stimulated with PHA (lIg/ml) and allogeneic MNCs (0.5 x 106/well). After 48 h the MNCs were removed from the cultures by Ficoll-Hypaque density gradient centrifugation. 10-12 d were required for the T cells to reach resting state after PHA plus MNC activation, at which time they were then plated on fibronectin and anti-CD3-coated culture dishes in serum-free and IL-2-free AIM V media. Before cell plating the culture plates were coated initially with anti-CD3 (1 Mzg/ml) overnight 40C, washed with PBS (Gibco Laboratories) and then coated with fibronectin (Sigma Chemical Co.) (20 Yg/ ml) overnight 40C and washed with PBS, as previously described (36).Supernatants were collected 48 h after plating, sterile filtered, and stored at -70TC. Lymphokine production by activated lesional psoriatic T cells was determined by using ELISA kits for GM-CSF, IL-3, IL-4 (R&D Systems, Inc., Minneapolis, MN), y-interferon (Genzyme Corp., Boston, MA) and TNF-a (PharMingen, San Diego, CA).Clonal growth assay. Epidermal cell suspensions were prepared from normal and uninvolved psoriatic keratomes as described above. Cells were plated into 6-well culture plates (1 x 106 cells/ml, 2 ml/ well) in KBM (Gibco Laboratories) media supplemented with I1%FBS.Activated T cell supernatants and AIM V media, as control, were diluted at 1:10 with KBM and added at the time of culture initiation to the cultures (200 pl/well). The media was changed on every third day, each time supplemented with activated T cell supernatant or AIM V media as described. After 2 wk plates were fixed with 1% formalin and stained with Rhodamine B (Sigma Chemical Co.) (37).Short-term epidermal proliferation assay. Freshly isolated psoriatic uninvolved and normal epidermal cells were plated in 6-well culture plates at 1 x 106 cells/ml, 2 ml/well, and cultured for 72 h in the presence of act...