Focus formation by rubella virus in rabbit kidney (RK-13) cell cultures and its application to the virus titration. Brief report

Sequence variation and biological activity of rubella virus isolates

Haemagglutination (HA) by rubella virus is mediated by the E1 glycoprotein. Rubella isolates which haemagglutinate with different avidity have been characterised. A significant reduction of HA titre at pH 6.0 was observed in one isolate in which isoleucine is substituted for threonine at rubella E1 residue 280. This residue is located in an epitope (EP1) which we have previously identified and shown to bind HA inhibiting (HA1) monoclonal antibodies. The isolates studied are also distinguishable by plaque size but no sequence variations in the immunogenic region of E1 were identified which might account for this difference. No correlation was observed between infectivity and binding affinity of neutralising monoclonal antibodies for different rubella virus strains.

Use of synthetic peptides to map regions of rubella virus capsid protein recognized by human T lymphocytes

Synthetic peptides (SPs), 18-29 amino acids long, representing selected sequences of rubella virus (RV) capsid (C) protein were used in lymphocyte proliferation assays to identify antigenic regions recognized by T lymphocytes from healthy RV-reactive adults. Four SPs, C(1-29), C(90-114), C(108-134) and C(255-300), stimulated proliferation of peripheral blood mononuclear cells and RV-specific T-cell lines from the same donors. C(1-29V), an SP analogue containing an RA27/3 RV vaccine strain sequence, stimulated higher levels of proliferation in T cells obtained from RV-vaccinated subjects than did the comparable wild-type (M33 strain) RV sequence.

Identification of immunoreactive regions of rubella virus E1 and E2 envelope proteins by using synthetic peptides

Relatively large (16-33 aa) synthetic peptides (SPs) representing defined sequences of rubella virus (RV) E1 and E2 envelope proteins were used in lymphocyte stimulation and enzyme immunoassays to map immunoreactive regions recognized by peripheral blood mononuclear cells (PBMNC) and serum antibodies from healthy RV-seropositive, RV-seronegative, and RV-vaccinated adults. Five distinct immunoreactive regions were identified in RV E1 protein, spanning residues (11-39), (154-179), (199-239), (226-277), and (389-412), which stimulated cellular responses in 29-83% of the subjects tested. Two SPs, E1(213-239) and E1(258-277) containing previously-identified virus neutralizing antibody domains, reacted with serum antibodies and also stimulated lymphoproliferation suggesting that these E1 sequences contain linked or overlapping B-and T-cell antigenic sites. The frequency and magnitude of cellular responses to E2 SPs were somewhat lower. SPs encompassing E2 residues (50-72), (140-199), and (244-263) stimulated lymphocyte responses in 28-64% of the subjects tested, while to a lesser degree, SPs within residues (1-36) were also stimulatory. E2 SPs within the regions (1-36), (151-170), and (244-263) also showed low levels of antibody reactivity with sera from RV-seropositive subjects. E2(244-263) which induced the highest level of response among the E2 SPs tested, was of interest due to previous reports of sequence homology of this RV region with human myelin and its potential immunopathogenic role in demyelinating autoimmune diseases. Identification of these potentially immunodominant regions of RV envelope proteins is an important first step in the rational design of new RV vaccines.

Detection of antibodies to individual proteins of rubella virus

Individual rubella virus structural polypeptides were electroeluted from SDS-polyacrylamide gels. The eluted polypeptides were used, without further purification, as antigens in ELISA assays for the detection of rubella-specific antibodies in patients' sera. This provided a more sensitive detection method than that involving classical serological assays such as HI or VN or that using immunoprecipitation. Antisera against individual viral polypeptides were raised in mice. No haemagglutination inhibition activity was observed in any of these sera and weak virus neutralizing activity was only detected with antiserum to the E1 protein. Antisera to either the E1 or E2(a,b) complex proteins cross-reacted with both the E1 and E2(a,b) complex proteins.