Specificity of IgM and IgM Antibodies after Challenge with Antigenically Related Togaviruses

Rabbits were immunized with a group A or a group B togavirus and challenged 6 weeks later with a related togavirus from a different subgroup. IgG antibodies in antisera cross-reacted broadly at all times in hemagglutination-inhibition tests with group B viruses whereas within group A they were specific for the primary virus until challenge after which they reacted with both primary and challenge viruses. In contrast, IgM antibodies were clearly specific within both groups for the most recent immunizing virus; however, IgM memory was expressed when the challenge and primary viruses belonged to the same antigenic subgroup. The antigenic composition of both groups of togaviruses is discussed in relation to their envelope proteins, and hypotheses are proposed to account for the antibody specificities of the observed immunoglobulin responses.