Fig. 1. Astrocytes expressing nef secrete a factor that is chemotactic for monocytes. A chemotaxis assay was performed using THP-1 cells. Cell culture supernatants from 16-hour incubations of astrocytic U251MG-parental (P) or stably transfected U251MG-Nef_Bru (clone 2/8.2, 4/4.1 and 4/4.2) or U251MG-Nef_TH, or with the vector carrying the neomycin resistance gene (-pNeo) were placed in the lower part of the chemotaxis unit. Recombinant CCL2/MCP-1 (25 ng/ml) was used as a positive control stimulus. Monocytic THP-1 cells were added into the upper part and allowed to migrate for 2 hours into the lower part. Subsequently, cell numbers in the lower part were counted and chemotaxis indexes were calculated as described in the Material and Methods. Data represent mean ± s.e.m. from five independent experiments; *, P<0.05.