Immunization with a prostate cancer xenoantigen elicits a xenoantigen epitope-specific T-cell response

Vaccines encoding xenoantigens, "non-self" proteins that are highly homologous to their autologous counterparts, have been investigated as a means to increase immunogenicity and overcome tolerance to "self" antigens. We have previously shown that DNA vaccines encoding native prostatic acid phosphatase (PAP) were able to elicit PAP-specific T cells in both rats and humans, but required multiple immunization courses. In this study, we investigated in a preclinical model whether immunizations with a DNA vaccine encoding a xenoantigen could elicit a cross-reactive immune response to the native protein, potentially requiring fewer immunizations. Lewis rats were immunized with a DNA vaccine encoding human PAP and splenocytes from immunized rats were screened with a human peptide library containing overlapping, 15-mer PAP-derived peptides using T-cell proliferation and interferon γ (IFNγ) release as measures of the immune response. One dominant PAP-specific, RT1.A(l)-restricted, epitope was identified. Direct immunization with the immunodominant peptide (HP(201-215)) containing this epitope demonstrated that it included a naturally presented MHC Class I epitope recognized by CD8(+) T cells in Lewis rats. However, no cross-reactive immune response was elicited to the corresponding rat peptide despite a difference of only three amino acids. Immunization with DNA vaccines encoding rat PAP (rPAP) in which this foreign dominant epitope was included as well as with DNA vaccines coding for a variant of the xenoantigen from which this epitope was deleted, did not elicit responses to the native antigen. Overall, these results indicate that the immunization with a xenoantigen-coding DNA vaccine can lead to an immune response which potentially favors foreign epitopes and hence limits any cross-reactive response to the native antigen.