Differential antigenicity of recombinant polyepitope-antigens based on loop- and helix-forming B and T cell epitopes

To investigate a strategy for the design of chimeric antigens based on B ce ll epitopes (BCEs) we have genetically recombined multiple copies of loop- (L) and helix-forming (H) sequential and protective BCEs of the measles vir us hemagglutinin protein (MVH) in a number of high-molecular-weight polyepi tope constructs (24.5-45.5 kDa). The BCE cassettes were combined semi-rando mly together with a promiscuous T cell epitope (TCE; tt830-844) to yield 13 different permutational constructs. When expressed in mammalian cells, all constructs were detectable by Western blot as distinct bands of predicted molecular weight. Flow cytometry with conformation-specific antibodies reve aled the Cys-loop in two [(L4T4)(2) and (L2T2)(4)] and the helix conformati on in one [(H2T2)(4)] of the different permutational constructs. The larger constructs, containing 16 epitope cassettes, seemed more likely to express the BCEs in their native conformation than the 8-mers. In the T cell proli feration assay, constructs with a higher copy number of TCEs, such as (L2T2 )(4), were more antigenic, as long as tandem repeats were separated by spac ers. Since the conformation of even sequential BCEs and the processing of T CEs are both sensitive to their molecular environment it is difficult to pr edict the antigenic properties of polyepitopes. However, with the permutati onal approach we have developed several polyepitope constructs [(L4T4)(2), (L2T2)(4), (H2T2)(4)] based on complex sequential BCEs that are antigenic f or both T and B cells. Several constructs induced sera that reacted with re porter peptides, demonstrating that the sequential nature of the viral epit opes was conserved in the polyepitopes. Although several sera contained ant ibodies directed against amino acids critical for neutralization, only one construct induced antibodies that cross-reacted with the virus. Our results show the difficulty of designing chimeric antigens based on B cell epitope s mimicking their antigenic and immunologic properties even when these are sequential in nature. (C) 2000 Elsevier Science B.V. All rights reserved.