The proportions of six immunoglobulin isotypes (IgA, IgM, IgG1, IgG2, IgG3, and IgG4) in rubella antibody responses were quantified in 40 serum samples (20 patients). The first sample from each patient was taken during the first days of the illness, and the second sample 10 +/- 1 days later. A tenfold average increase in antibody concentration was observed between the first and the second sample. IgM was the predominant isotype in the first sample (average, 73% of all antibodies), followed by IgG1 (19%). IgA and IgG3 antibodies were detected in only a few of the first samples, and IgG2 or IgG4 in none. In the second samples IgG1 was the predominant antibody isotype (average, 59%). Next came IgM (23%), followed by IgA (8%) and IgG3 (3%). No IgG2 or IgG4 antibodies were detected. Although the proportion of IgM antibodies was lower in the second than in the first samples, their concentration increased in all patients (the average factor was 7). The kinetics of the IgA response was irregular. In some patients there was a strong (up to 90-fold) increase in IgA antibodies, but in two patients a small drop was detected. The kappa- to lambda-chain ratio of rubella antibodies appears to be close to the expected 2:1. It decreased in some patients during the 10 days and increased in others.

SUMMARYSemliki Forest, Sindbis and rubella viruses can fuse erythrocytes from several different species. Large fusion vesicles consisting of tens to hundreds of red blood cells were seen under optimal conditions. For the haemagglutination and cell fusion to occur the adsorption of virus and further incubation had to be carried out at pH 5"8. Haemagglutination took place over a wide temperature range (o to 4o °C) whereas fusion required temperatures between 37 and 42 °C. Haemolysis of red blood cells induced by togaviruses also required initial incubation at pH 5"8 to enable attachment of the virus to occur after which the pH of the buffer could be raised to neutrality without inhibiting the haemolysis. The amount of togaviruses and Sendal virus required to fuse red blood cells was about the same [I haemagglutinating unit (HAU)/ml] but different ionic conditions were required for fusion.

Two tests were introduced recently for assessment of the avidity of rubella immunoglobulin antibodies. In the quantitative test--avidity-enzyme linked immunosorbent assay (ELISA)--IgG antibodies obtained from individuals shortly after primary infection with rubella virus are distinguished from those with past immunity by their antigen-elution characteristics. This method uses agents that disrupt hydrophobic bonds in proteins [Kamoun PP (1988): Denaturation of globular proteins by urea: Breakdown of hydrophobic bonds? Trends in Biological Sciences 13:424-425.]. In the semiquantitative, presumptive test--haemolysis typing--the low-avidity rubella-IgG antibodies are distinguished from the high-avidity antibodies by the quality of their haemolytic zones in a radial haemolysis test. In the present study, both tests were applied to sera taken before and after vaccination with two different strains (Cendehill or RA 27/3) of live attenuated rubella virus. It was found that after vaccination of previously nonimmune subjects, IgG synthesized during the first 2 months had a very low avidity; IgG avidity increased dramatically during the subsequent 4 months and less markedly between 6 and 12 months after vaccination. On the contrary, the initially high IgG avidity of previous immune vaccinees remained at an elevated level postvaccination. These results provide a basis for identification of recent primary rubella virus infections, or vaccination reactions, by the avidity of specific IgG and also for their separation from rubella reinfections.

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Sera from 30 infants with suspected chlamydial pneumonitis were studied by enzyme immunoassay (EIA) with three antigens: reticulate bodies (RB), purified major outer membrane protein ( MOMP ) of Chlamydia trachomatis strain L2, and purified lipopolysaccharide from Re mutants of Salmonella (Re LPS), which shows complete cross-reaction with chlamydial glycolipid. The immunofluorescence test (I/RB IFAT), which detected IgM antibodies (titer of greater than or equal to 1:64) in 16 patients whose clinical picture was consistent with chlamydial pneumonitis, was the standard method. EIA measured IgM antibodies to the purified antigens but not to RB; 15 sera were positive with the MOMP antigen and two with the Re LPS antigen. High-titered IgG antibodies were detected by I/RB IFAT in 15 and by MOMP EIA in 13 of the 30 sera. By the RB EIA, 17 sera were positive. The MOMP EIA was thus as sensitive and specific as the I/RB IFAT. Because the EIA can be automated, it would make possible the screening of all children younger than six months of age with respiratory-tract symptoms and IgM antibodies to Chlamydia.