A synthetic parvovirus B19 capsid protein can replace viral antigen in antibody-capture enzyme immunoassays

To establish a renewable source of parvovirus B19 antigens for diagnostic tests, gene sequences for the viral capsid proteins, VP1 and VP2, were cloned into baculovirus expression vectors and the recombinant viruses used to infect Sf9 insect cells. Cell lysates examined by immunoblotting demonstrated reactive proteins corresponding to the expected sizes of native VP1 (83 kDa) and VP2 (58 kDa). The VP2 protein was produced efficiently in quantity and self-assembled into empty capsids as shown by density equilibration in a CsCl step gradient. The VP2 protein was purified and used as an antigen in antibody-capture enzyme immunoassays for the detection of B19 IgG and IgM antibodies. Compared to a standard antibody-capture EIA based on whole viral antigen, the VP2-EIA gave a sensitivity of 100% and specificity of 97% in detection of B19 IgM in 138 patients suspected of B19 infection. No IgM-positive specimens were missed. IgG detection yielded a sensitivity of 100% and specificity of 96% in the same population. Recombinant VP2 capsid proteins expressed in baculovirus-infected insect cells can substitute for serum-derived B19 virus in standard antibody-capture EIA for the detection of B19 IgG and IgM with comparable results.