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The cytokine hormone prolactin (PRL) has been shown previously to modulate native cellular responses and maturation of antigen-presenting cells. Here we have addressed its effect on the antigen-specific response of cytotoxic T lymphocytes (CTL). CTL were generated from HLA-A2 lymphocytes after three rounds of stimulation with autologous dendritic cells loaded with HLA-A2-restricted carcinoembrionic antigen (CEA) Cap-1 (YLSGANLNL) peptide. Selected cultures were expanded on cytokine-supplemented feeder-layers, enriched for CD8+ lymphocytes and analysed for PRL-receptor (PRL-R) expression and PRL responsiveness. Resting CD8+ lymphocytes were negative for PRL-R, whereas antigen-activated CD8+ lymphocytes derived from long-term cultures were highly positive. Results of a 51Cr release assay showed CTL killing of CEA-loaded, but not unloaded, T2 cell line and the CEA-positive gastric carcinoma cell line KATO, but not of the CEA-negative T leukaemia cell line Jurkat. Interferon (IFN)-γ release, evaluated in an ELISPOT assay against CEA-loaded T2, was enhanced (P < 0·05) by concentrations of PRL (12–25 ng/ml) very close to the physiological levels (6–20 ng/ml), but was decreased (P < 0·05) by high concentrations (200 ng/ml). Pre-incubation of the stimulators with the anti-MHC class I MoAb W6·32 induced a 40–60% decrease of the PRL-boosted IFN-γ release, thus proving the MHC restriction of the lymphocyte response. Cytotoxicity against CEA-loaded T2 and KATO cell lines was also increased by 12–25 ng (P < 0·05) and decreased (P < 0·05) by 200 ng PRL. Pre-incubation of CTL with an antibody specific for the PRL-R almost completely abrogated this effect.