Generation of PSA-reactive effector cells after vaccination with a PSA-based vaccine in patients with prostate cancer

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-gamma) 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 x 10(5)/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 microg/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.