A precipitating antigen, rho, was first detected in the blood of persons with rubella and in rubella virus-infected cell culture fluids (1). Partially purified antigens from both sources were examined and shown to have similar properties, although antigen from serum sedimented more heterogeneously, with estimated coefficients from 15 to 21 S, while that from culture fluids sedimented in the 11–14 S region. In each case, antigen was located in the ß-1 zone after electrophoresis in agarose, and at a density of 1.305 g/ml after centrifugation in CsCl. Stability characteristics were typical of protein antigens.

Immunofluorescent microscopy revealed that rubella virus induced the appearance of rho antigen scattered throughout the cytoplasm of infected cells. When cells containing antigen were exposed for 24 h to 5 µg/ml actinomycin D rho was no longer detectable, indicating the probable cellular origin of the antigen. Also, titers in medium of infected cultures showed a reduction after actinomycin treatment, but levels of the virus-specified antigen, iota, were relatively unaffected.

Rho appears to be a protein common to man and many animals. In vitro, it was induced by rubella virus and by adenovirus. In vivo, rho titers were shown to be elevated after rubella virus infection and, to a lesser extent, after infection with certain other viruses. High titers were also demonstrated in women late in pregnancy and in patients with rheumatoid arthritis. In man and the chimpanzee, the appearance and decline of rho in the blood after rubella virus infection were temporally similar to the patterns of CRP, although rho seemed to be a more sensitive indicator of infection.

The data presented indicate that rho is a newly recognized acute phase protein inducible by certain virus infections and by other unidentified stimuli present prominently in pregnancy and rheumatoid arthritis.

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