Modification of PCV-2 virulence by substitution of the genogroup motif of the capsid protein

Allemandou, Aude; Grasland, Béatrice; Hernandez-Nignol, Anne-Cécile; Kéranflec'h, André; Cariolet, Roland; Jestin, André

The hypothesis is that the PCV-2b capsid protein might less interact with the PCV-2b Rep compared to the PCV-2a Cap protein with the PCV-2a Rep. In this case, the PCV-2a Cap protein might lead to the most effective control of replication. For PCV-2b, the Cap protein with less interaction with the Rep could result to enhanced replication and accumulation of doublestranded DNA, like for the Geminiviruses [37] . In the same way, the introduction of the PCV-2a motif in the PCV-2b backbone (PCV-2b/motif 2a clone) induced a decrease of viral DNA in organs compared to the PCV-2b and PCV-2a groups. This suggests that the motif specific of genogroup may be involved in the control of replication. However, on the other hand, the introduction of PCV-2b motif in the PCV-2a backbone (PCV-2a/motif 2b group) led to a decrease of genome copies in organs compared to the PCV-2a but also to a decrease of the produced infectious particles. These latter results showed that the genogroup motif could be involved in the replication regulation but not solely. In this study, the PCV-2a/motif 2b mutant presented an attenuation of virulence compared to its parent the PCV-2a. The swap of the genogroup motif i.e. four amino acids mutated in PCV-2a, might have generated a defective virus. But the opposite with the introduction of 4 mutations in the PCV-2b capsid protein did not cause the same effect. It has been shown for PCV-2a that only two amino acid mutations in the capsid protein induced virulence attenuation [32] . Consequently, we can assume that the capsid protein of PCV-2 does not support a lot of variation. The genogroup motif is an important determinant for characteristics and virulence of isolates. The results of the two mutants of this study demonstrated that the capsid background of this motif is also strongly implicated in these mechanisms. Comparison of the capsid protein sequence of the two parental strains and of the two mutants. Differences in amino acids between the two parental isolates are indicated in bold and underlined. The genogroup motif is highlighted in grey. Asterisk indicated the six amino acid difference between two PCV-2 isolates showing difference in virulence [31] and the dot the two mutations responsible for attenuation by Fenaux et al. [31] .