Hepatitis A virus mutant spectra under the selective pressure of monoclonal antibodies: codon usage constraints limit capsid variability.

Severe structural constraints in the hepatitis A virus (HAV) capsid have been suggested as the reason for the lack of emergence of new serotypes in spite of the occurrence of complex distributions of mutants or quasispecies. Analysis of the HAV mutant spectra under immune pressure by the monoclonal antibodies (MAbs) K34C8 (immunodominant site) and H7C27 (glycophorin binding site) has revealed different evolutionary dynamics. Populations composed of complex ensembles of mutants with very low fitness or single dominant mutants with high fitness permit the acquisition of resistance to each of the MAbs, respectively. Deletion mutants were detected as components of the mutant spectra: up to 61 residues, with an average of 19, and up to 83 residues, with an average of 45, in VP3 and VP1 proteins, respectively. A clear negative selection of those replacements affecting the residues encoded by rare codons of the capsid surface has been detected through the present quasispecies analysis, confirming a certain beneficial role of such clusters. Since these clusters are located near or at the epitope regions, the need to maintain such clusters might prevent the emergence of new serotypes.