Detection of human antibodies using "convergent" combinatorial peptide libraries or "mixotopes" designed from a nonvariable antigen: Application to the EBV viral capsid antigen p18

We have previously described the use of synthetic combinatorial "convergent " libraries, or "mixotopes" as immunogens or as antigens to represent natur ally hypervariable sequences. The success of this approach suggests that su ch a mixture of closely related peptides could, at least in part, convenien tly represent a nonvariable epitope during its multiple interactions with a n antibody population. To address this possibility, we have designed from a nonvariable immunodominant peptide of the EBV-viral capsid antigen of 18 k D (VCAp18) a series of three mixotopes containing from 65,000 to 16 million combinatorial sequences. The reactivity of VCAp18 and its three derived mi xotopes was examined in ELISA towards a collection of 74 human sera from do cumented EBV-negative or EBV-positive donors, and analyzed in terms of sens itivity and specificity. Following the observation that the two least degen erated mixotopes could improve the sensitivity of detection of some sera of low reactivity for VCAp18, we decided to combine each mixotope with the VC Ap18 peptide. In the case of the least degenerated mixotope in combination with VCAp18, sensitivity and specificity for immunoenzymatic EBV-serodiagno sis, were enhanced to 100%. Our results suggest that synthetic "convergent" combinatorial peptide libraries or "mixotopes," designed from nonvariable antigens, could be useful adjuncts to an antigenic single-sequence peptide in immunoenzymatic serodiagnosis.