Immunosorbent Assay Based on Recombinant Hemagglutinin Protein Produced in a High-Efficiency Mammalian Expression System for Surveillance of Measles Immunity

Immunity against measles involves a cellular response and a humoral response (4, 5, 13, 43–45). It is well known that passive antibodies protect an individual against measles (1, 28), and protection was also obtained after passive transfer of cellular immunity, at least in an animal model (38). Monitoring of immunity to measles relies solely on the detection of specific antibodies, but it is not clear to what extent antibody titers reflect protective cellular immunity. Past experience with enzyme-linked immunosorbent assays (ELISAs) based on whole measles virus (MV) suggests that antibodies directed against whole virus are a reliable measure of measles immunity (8, 15, 35, 50). Assays for immunity based on recombinant proteins may offer a number of advantages over whole-virus-based ELISAs. Such advantages include simplified production, improved standardization, and enhanced stability. An inexpensive and simple diagnostic test as an alternative to an MV-infected cell- or whole-virus-based ELISA is also required to monitor measles immunity as part of eradication programs (22). Such assays could rely on the detection of antibodies directed against selected MV proteins (48, 49). Antibodies against the nucleoprotein were found to correlate with total MV antibodies (27). Whether antibodies specific for other proteins also correlate with total MV antibodies and with immunity has not been demonstrated with a panel of human sera. Most functional antibodies are directed against the hemagglutinin (H) protein: they neutralize MV in vitro and provide protection against MV in vivo (9, 10, 14, 17, 20, 21, 32, 46). Therefore, H protein-specific immunoglobulin G (IgG) antibodies are considered to be most important in determining immunity to MV (6). The MV H protein has been expressed in a number of expression systems including baculovirus (40, 47), vaccinia virus (14, 41, 51), canarypox virus (42), adenovirus (3), and other (19) systems. Expression of this glycoprotein in procaryote or lower eucaryote systems should result in glycosylation different from that in MV. Proper glycosylation has been found to be important for the processing, the functional integrity, and the antigenicity of this protein (23–25). For proper posttranslational modification, this glycoprotein should therefore be expressed in mammalian cells. However, in most mammalian systems the yield is low (19). We analyze here whether the H protein expressed in a high-yield mammalian expression system based on the Semliki Forest virus replicon (29, 30) is suitable for monitoring measles immunity. (This work was done by Fabienne Bouche in partial fulfillment of her doctoral thesis.)