Characterisation of a chicken anaemia virus variant population that resists neutralisation with a group-specific monoclonal antibody.

A variant population of chicken anaemia virus (CAV), termed P310 2A9-resist, that resists neutralisation by the monoclonal antibody (MAb) 2A9, was selected from Cux-1 virus that had been passaged 310 times (P310) in MDCC-MSB1 cells. Substantially higher concentrations of MAb 2A9 were required to neutralise the selected virus compared to those required to neutralise a low-passage (P13) Cux-1 isolate. Virus neutralisation tests showed that serum from chickens infected with the P310 2A9-resist virus neutralised P13 virus and that serum from chickens infected with P13 virus conversely neutralised the P310 2A9-resist virus. MDCC-MSB1 cells infected with the P310 2A9-resist virus produced no staining with low dilutions (1:100) of Mab 2A9 in an indirect immunofluorescence (IF) test, whereas cells infected with P13 virus reacted positively at high MAb dilutions (1:80,000). Experimental infections of 1-day-old SPF chicks showed that the P310 2A9-resist virus was substantially attenuated. Chimaeric viruses constructed using PCR-amplified regions from the P310 2A9-resist and a pathogenic low-passage cloned Cux-1 isolate showed that the reduced MAb reactivity and attenuation exhibited by the P310 2A9-resist virus were mainly associated with a region encoding the N-terminal half of the 50 kDa capsid protein VP1 and C-terminal regions of VP2 and VP3. The nucleotide sequence of the protein-coding region of the P310 2A9-resist virus is reported and the amino acid sequences of the 3 encoded proteins compared with those of other Cux-1 isolates.