1. ¹The George Washington University Medical Center, Washington, District Columbia, USA
2. ²Central Laboratory for Immunology, St Ivan Rilski Hospital, Sofia, Bulgaria
3. ³Department of Urology, St Ann University Hospital, Sofia, Bulgaria
4. ⁴National Institute of Hematology and Transfusion Medicine, Sofia, Bulgaria

Correspondence: Dr Milcho Mincheff, Tumor Immunology Laboratory, Department of Medicine, The George Washington Medical Center, 2300 Eye Street, NW, Ross Hall 705, Washington, DC 20037, USA. E-mail: mcamsm@gwumc.edu

Received 21 April 2003.

Abstract

The ability of two plasmid DNA vaccines to stimulate lymphocytes from normal human donors and to generate antigen-specific responses is demonstrated. The first vaccine (truncated; tPSMA) encodes for only the extracellular domain of prostate-specific membrane antigen (PSMA). The product, expressed following transfection with this vector, is retained in the cytosol and degraded by the proteasomes. For the "secreted" (sPMSA) vaccine, a signal peptide sequence is added to the expression cassette and the expressed protein is glycosylated and directed to the secretory pathway. Monocyte-derived dendritic cells (DCs) are transiently transfected with either sPSMA or tPSMA plasmids. The DCs are then used to activate autologous lymphocytes in an in vitro model of DNA vaccination. Lymphocytes are boosted following priming with transfected DCs or with peptide-pulsed monocytes. Their reactivity is tested against tumor cells or peptide-pulsed T2 target cells. Both tPSMA DCs and sPSMA DCs generate antigen-specific cytotoxic T-cell responses. The immune response is restricted toward one of the four PSMA-derived epitopes when priming and boosting is performed with sPSMA. In contrast, tPSMA-transfected DCs prime T cells toward several PSMA-derived epitopes. Subsequent repeated boosting with transfected DCs, however, restricts the immune response to a single epitope due to immunodominance.