Convalescent phase non-A, non-B (NANB) human and chimpanzee sera were utilized in a solid-phase radioimmunoassay (RIA) in an attempt to identify a specific NANB
antigen in human plasma, plasma-derived pellets and
NP-40 disrupted pellets proven to transmit NANB
infection to chimpanzees. RIA reactivity was noted in 5 of 8 pedigreed NANB infectious plasmas, but did not appear to be virus-specific. The RIA reactive fraction: (i) had a density of 1.27 to 1.32 gm per cm3 in
cesium chloride; (ii) showed immunoreactivity corresponding to normal
IgM, but peak RIA activity coincided with an aberrant
IgM having a peak sedimentation coefficient of 56 S, and (iii) eluted at higher pH (5.6 to 5.0) than normal
IgM on chromatofocusing columns. The reactant shared properties with
rheumatoid factor in that it was absorbed by aggregated
IgG and was unstable to
2-mercaptoethanol, but it did not react in standard agglutination tests for
rheumatoid factor. The reactant was detected in both the acute and chronic phase of NANB
infections in chimpanzee and man. By reacting with
IgG in presumptive NANB convalescent sera, this aberrant
rheumatoid factor could simulate a NANB
antigen and represent a cause of false-positive reactions in putative NANB assays.