Selection of variants of rhinoviruses

Specificity of human rhinovirus 2A(pro) is determined by combined spatial properties of four cleavage site residues

The 2A proteinase (2A(pro)) of human rhinoviruses cleaves the virally encoded polyprotein between the C terminus of VP1 and its own N terminus. Poor understanding of the 2A(pro) substrate specificity of this enzyme has hampered progress in developing inhibitors that may serve as antiviral agents. We show here that the 2A(pro) of human rhinovirus (HRV) 1A and 2 (rhinoviruses from genetic group A) cannot self-process at the HRV14 (a genetic group B rhinovirus) cleavage site. When the amino acids in the cleavage site of HRV2 2A(pro) (Ile-Ile-Thr-Thr-Ala*Gly-Pro-Ser-Asp) were singly or doubly replaced with the corresponding HRV14 residues (Asp-Ile-Lys-Ser-Tyr*Gly-Leu-Gly-Pro) at positions from P3 to P2', HRV1A and HRV2 2A(pro) cleavage took place at WT levels. However, when three or more positions of the HRV1A or 2 2A(pro) were substituted (e.g. at P2, P1 and P2'), cleavage in vitro was essentially eliminated. Introduction of the full HRV14 cleavage site into a full-length clone of the HRV1A and transfection of HeLa cells with a transcribed RNA did not give rise to viable virus. In contrast, revertant viruses bearing cysteine at the P1 position or proline at P2' were obtained when an RNA bearing the three inhibitory amino acids was transfected. Reversions in the enzyme affecting substrate specificity were not found in any of the in vivo experiments. Modelling of oligopeptide substrates onto the structure of HRV2 2A(pro) revealed no appreciable differences in residues of HRV2 and HRV14 in the respective substrate binding sites, suggesting that the overall shape of the substrate is important in determining binding efficiency.

The time course of the humoral immune response to rhinovirus infection

The specific humoral immune response of 17 volunteers to infection with human rhinovirus type 2 (HRV-2) has been measured both by neutralization and by ELISA. Six volunteers who had HRV-2-specific antibodies in either serum or nasal secretions before HRV-2 inoculation were resistant to infection and illness. Of the remaining 11 volunteers who had little pre-existing HRV-2-specific antibody, one was immune but 10 became infected and displayed increases in HRV-2-specific antibodies. These antibodies first increased 1-2 weeks after infection and reached a maximum at 5 weeks. All six resistant volunteers who had high pre-existing antibody and eight of the volunteers who became infected maintained their HRV-2-specific antibody for at least 1 year. At this time they were protected against reinfection. Two volunteers showed decreases in HRV-2-specific antibodies from either serum or nasal secretions. They became infected but not ill after HRV-2 inoculation 1 year later.

Detection of serotypic variants in viral preparations: sensitivity of a procedure for selection

The efficiency of an in vitro method (the breakthrough neutralization procedure) for selecting serotypic variants from preparations of human rhinovirus (HRV) 17 and a temperature sensitive strain (Ts-1) of HRV-2 was examined. Viruses were plaqued in the presence of homologous polyclonal antisera, and plaques which escaped neutralization were isolated. For control purposes, isolates were obtained by plaquing in the absence of antisera. Any clone that consistently (minimum of 2 tests) yielded a 4-fold or greater difference in serum neutralization titer compared to the parent virus when tested with antiserum to the parent, was considered a serotypic variant. The efficiency of the breakthrough neutralization procedure was calculated using varying mixtures of 2 related rhinovirus serotypes. Using this method, the ability to isolate serotypic subpopulations from a mixture was enhanced at least 7000-fold. This procedure allows the rapid isolation of variants which differ from the parent virus by as little as 4-fold in a serum neutralization test. Hence, viral preparations used to prepare reference grade reagents such as seed stocks and polyclonal and monoclonal antisera, can be tested for specificity prior to use.

Cross relationships among 37 rhinoviruses demonstrated by virus neutralization with potent monotypic rabbit antisera

Antisera produced in rabbits to 37 rhinovirus (RV) types have been examined for antibody to the 36 corresponding heterologous types. Reciprocal neutralization was noted between RV types 2 and 49 and RV types 13 and 41. Five additional monotypic rhinoviral rabbit antisera neutralized one heterologous rhinovirus. Neutralizing antibody titers against heterologous RV serotypes were similar to those shown in the RV 1A, 1B and RV 9, 32 reciprocal cross-reactions, and would not be expected to result in false identification of serotypes. Comparison of neutralizing antibody titers and neutralization rate constants in serial serum specimens of immunized rabbits showed that antibody response to the immunizing RV type and the cross-reacting type followed a similar time pattern.