Immunosorbent assay based on recombinant hemagglutinin protein produced in a high-efficiency mammalian expression system for surveillance of measles immunity.

Recombinant hemagglutinin (H) protein of the measles virus (MV) was produced in mammalian cells with a high-yield expression system based on the Semliki Forest virus replicon. Crude membrane preparations of H protein-transfected BHK-21 cells were used to coat microtiter plates to measure specific immunoglobulin G antibodies in 228 serologically defined serum samples mainly from measles late-convalescent adults. The titers by the enzyme-linked immunosorbent assay for the H protein (H-ELISA) closely correlated with neutralization test (NT) titers (R2 = 0.66), hemagglutination inhibition test (HI) titers (R2 = 0.64), with the titers from a certified commercial ELISA based on whole MV-infected cells (MV-ELISA; R2 = 0.45). The correlations described above were better than those of the commercial MV-ELISA titers with the NT (R2 = 0.52) or HI (R2 = 0.48) titers. By using the 2nd International Standard for anti-measles serum, the detection level of the assay corresponds to 215 mIU/ml for undiluted serum, which corresponds to the estimated threshold for protective immunity. The specificity, accuracy, and positive predictive value were, in general, better for the H-ELISA than for a commercial MV-ELISA, independent of whether HI, NT, or HI and NT were used as "gold standards." In contrast, the H-ELISA proved to be slightly less sensitive than the MV-ELISA (sensitivities, 98.6 versus 99.5%, respectively; P was not significant). The assays did not differ significantly in the number of serum samples with positive HI and NT results (n = 212) which measured false negative (H-ELISA, 2 of 212 [0.94%]; MV-ELISA, 1 of 212 [0.47%]), but the H-ELISA detected significantly more measles-susceptible individuals than the MV-ELISA (10 of 11 versus 3 of 11, respectively; P < 0.05) among the individuals whose sera had negative HI and NT results. Our data demonstrate that the H-protein preparation that we describe could be a cost-effective alternative to current whole-virus-based ELISAs for surveillance for immunity to measles and that such an assay could be more efficient in detecting susceptibility to measles. Furthermore, unlike whole MV-based antigens, H-protein would also be suitable for use in the development of a simple field test for the diagnosis of measles.