Epitope mapping of antibodies against prostate-specific antigen with use of peptide libraries.
BACKGROUND
Prostate-specific antigen (PSA) is the most important marker for prostate cancer, but PSA concentrations determined by various assays can differ significantly because of differences in specificity of the antibodies used. To identify epitopes recognized by various monoclonal antibodies (MAbs) to PSA, we have isolated peptides that react with the paratopes of these.


METHODS
Six anti-PSA MAbs representing three major epitope groups were screened with five cyclic phage display peptide libraries. After selection, the peptide sequences were determined by sequencing of the relevant part of viral DNA. Binding of the phage peptides to the MAbs was monitored by immunoassay.


RESULTS
For each MAb, several paratope-binding peptides with distinct sequence motifs were identified, but only approximately 10% showed similarity with the PSA sequence. Some of these correctly predicted the location of the epitopes. By sequential panning of the library with two closely related MAbs, we identified peptides reacting equally with both MAbs. When analyzed against a large panel of PSA MAbs, the peptides generally showed restricted specificity toward the MAb used for selection, but some peptides bound to several related MAbs.


CONCLUSIONS
Most of the cyclic peptides selected with PSA MAbs are specific for the MAb used for selection and do not resemble any sequence on the antigen. Peptides reactive with two MAbs recognizing the same epitope can be obtained by sequential panning. This method can be used to predict the location of some epitopes, but additional methods are needed to confirm the result.
Leinonen
Jari
J
Department of Clinical Chemistry, Helsinki University Central Hospital, Haartmaninkatu 8, FIN-0029 Helsinki, Finland. jari.leinonen@hus.fi
Wu
Ping
P
Stenman
Ulf-Håkan
UH
D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000737:chemistry*; D018604:Epitope Mapping; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019151:Peptide Library; D017430:Prostate-Specific Antigen / Q000276:immunology*
D000911:Antibodies, Monoclonal; D019151:Peptide Library; D017430:Prostate-Specific Antigen