The molecular basis of virus crossreactivity and neutralisation after immunisation with optimised chimeric peptides mimicking a putative helical epitope of the measles virus hemagglutinin protein

The loop comprising aminoacids H236-256, connects two strands of sheet 1 of the propeller-like hemagglutinin (H) protein of the measles virus (MV) and contains a putative active site residue (R-253), a residue implicated in C D46-downregulation (R-243) and the minimal epitope E245L-QL(249) of the neu tralising and protective monoclonal antibody BH129. The objective of this s tudy was to design synthetic peptides which induce neutralising antibodies against this important functional domain. Peptide-design was based on the c olinear synthesis of this sequential B cell epitope (BCE) with different T cell epitopes (TCE). Chimeric constructs were systematically optimised with respect to length and copy number of the BCE and the nature and orientatio n of the TCE. Surprisingly, the induction of MV-crossreactive antibodies di d not correlate with the antigenicity of the peptides. The best MV-crossrea ctive antibodies were obtained with TB oriented constructs containing TCEs of the MV fusion (F) protein and the BCE H236-250 (TB15mer) or H236-255 (TB 20mer). In vitro virus-neutralising sera were obtained solely with the latt er construct. A glycine scan showed that binding to MV depended on a define d pattern of contact residues compatible with the putative alpha helical na ture of this epitope. The contact residues of the neutralising serum (S244E L-QL(249)) differed from those of the non-neutralising serum (S244EL246) bu t no unique differences in the immunoglobulin subclasses were detected. Sur face plasmon resonance measurements detected a higher affinity for the neut ralising serum compared to the TB15mer serum. These results emphasize the n eed of an optimal design of immunogenic peptides which cannot always be gui ded by the antigenicity of the peptide constructs. This study demonstrates that neutralising antibodies can be generated with peptides mimicking this helical epitope, provided that the critical contact residues are recognized with high affinity and underlines the potential of the epitope as an eleme nt of a future subunit vaccine. (C) 1998 Elsevier Science Ltd. All rights r eserved.