Measles virus-specific antibodies and immunoglobulin M antiglobulin in sera from multiple sclerosis and rheumatoid arthritis patients.

When rheumatoid factor in rheumatoid arthritis and multiple sclerosis sera was titrated by the fluorescent antibody method on measles virus-infected cells, there was a marked and variable drop in titer on acetone-fixed cells as compared with unfixed cells. This was accounted for by the failure of measles virus hemolysin-inhibiting (HLI) antibody of the immunoglobulin G class to bind to acetone-fixed infected cells. It was shown by staining unfixed and acetone-fixed measles virus-infected cells that rheumatoid factor in most rheumatoid arthritis sera combined with measles virus-specific hemagglutinin-inhibiting and HLI antibodies, whereas rheumatoid factor in multiple sclerosis sera combined only with HLI antibody. Rheumatoid factor of similar specificity was also observed in normal sera and occasionally in rheumatoid arthritis sera. Both rheumatoid arthritis and multiple sclerosis sera showed almost identical increases in average titer above normal of measles virus-specific fluorescent staining immunoglobulin G and HLI antibodies.