Differential IgM response to conformational and linear epitopes of parvovirus B19 VP1 and VP2 structural proteins

The IgM immune response against conformational and linear epitopes of B19 structural proteins VP1 and VP2 was examined in serum samples with a suspect B19 infection to determine the most suitable antigen for use in IgM detection and also to evaluate a possible relationship between the course of B19 infection and the presence of epitope type-specific IgM. The detection of IgM against conformational epitopes was performed by ELISA using undenatured VP1 and VP2 antigens whereas the detection of IgM against linear epitopes was performed by Western blot assays using denatured VP1 and VP2. IgM immune response against VP1 conformational epitopes appeared dominant, being detected in all serum samples positive for specific IgM, whereas IgM against VP2 linear antigen were found less frequently, being identified in less than half of the B19 IgM positive sera. In the examination of the course of infection, IgM against VP1 conformational epitopes appeared in the active phase of B19 infection at the same time and with the same frequency as IgM anti VP2 conformational epitopes and anti linear VP1 epitopes. IgM against VP1 conformational epitopes were seen to be long-lasting because in the recent phase of infection they were still present when other specific IgM were absent. During the active phase of B19 infection, IgM against VP2 linear epitopes were less frequently found than other specific IgM and in the recent phase they underwent a rapid temporal diminution. The data demonstrate that a sensitive B19 IgM test needs to be performed in diagnostic laboratories by ELISA using conformational B19 antigens; Western blot assays can be used only as confirmatory tests using VP1 linear antigens.