JBT 1001 is a vaccine used for therapy of prostate cancer (CA), which consists of recombinant prostate‐specific antigen (PSA) with lipid A formulated in liposomes. Patients with prostate CA were vaccinated with JBT 1001 emulsified in mineral oil (n = 5) or with the vaccine in combination with granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) administered locally at the site of vaccination (n = 5). Frequency of PSA‐reactive T cells was measured in peripheral blood mononuclear cells (PBMC) before and after immunization, using an interferon‐gamma (IFN‐γ) enzyme‐linked immunospot (ELISPOT) assay with autologous dendritic cells (DC) as antigen‐presenting cells. The hypothesis tested was that PSA‐based vaccines induce T cell responses to human PSA.

In order to expand precursor cells, in vitro sensitization (IVS) was performed. Microcultures of peripheral blood lymphocytes (PBL) (1 × 10⁵/well) in medium supplemented with interleukin‐2 (IL‐2) (10 IU/ml) and interleukin‐7 (IL‐7) (10 ng/ml) were stimulated twice (day 0 and day 7) with monocyte‐derived autologous DC, generated by culture with interleukin‐4 (IL‐4) and GM‐CSF and pulsed with PSA (10 μg/ml) at an effector to stimulator ratio of 10:1. ELISPOT assays were performed on day 14 of culture. In addition, PBMC were separated on immunobeads into CD4⁺ and CD8⁺ subsets for ELISPOT assays performed without IVS.

Two patients had PSA‐reactive responses before vaccination (frequency range, 1/700–1/4,400). After vaccination, 8/10 patients had measurable PSA‐reactive T‐cell frequencies, ranging from 1/200–1/1900, using IVS. In contrast, without IVS, but after immunoselection to enrich in CD8⁺ and CD4⁺ T cells, only 2/10 patients had detectable PSA‐reactive T cells after vaccination, at a frequency ranging from 1/2,600–1/4,000.

Vaccination with PSA formulated into liposomes induced T‐cell responses in 8/10 patients with prostate carcinoma. The frequency of PSA‐reactive precursor T cells was relatively low in the blood of these patients, and IVS, leading to amplification of the precursor cells prior to ELISPOT, was necessary for quantification of the PSA‐responding T cells. Cellular responses to PSA were predominantly mediated by CD4⁺ T lymphocytes. Prostate 43:88–100, 2000. © 2000 Wiley‐Liss, Inc.