Measles virus-specific functional antibody responses and viremia during acute measles.

Antibody titers measured in functional and immunofluorescent assays were compared with proportions of peripheral blood mononuclear cells infected with measles virus in 8 adults with measles. In addition, a syncytium inhibition assay (SIA) for measuring neutralizing antibody using low-passage virus was compared with a standard plaque neutralization test (PNT). Antibody-dependent cellular cytotoxicity (ADCC) antibody rose later but attained higher titer than neutralizing, antibody-dependent complement-mediated lysis, IgM, or IgG antibodies. When titer changes between specimens from each patient obtained on different days were compared, only ADCC (r = .81, P = .026) and IgM (r = .81, P = .027) antibodies correlated with reductions in viremia. SIA and PNT correlated well (r = .93, P < .001). ADCC may be an important defense against measles. The delay in ADCC antibody relative to other antibodies is unique among viruses studied. The SIA is a useful alternative to the PNT for measuring measles neutralizing antibody.