Immunoglobulin Fc binding activity is associated with the mouse hepatitis virus E2 peplomer protein

Antigenic variation among murine coronaviruses is associated primarily with the surface peplomer protein E2 (180,000 Da). E2 is responsible for attachment of the virus to the host cell, MHV-induced cell fusion, and eliciting neutralizing antibody. We report here the molecular mimicry between E2 and Fc gamma receptor (Fc gamma R). Molecular mimicry between E2 and Fc gamma R may allow the escape of virus-infected cells from destruction by immunological mechanisms. Rabbit IgG, monoclonal rat IgG1 and IgG2b, monoclonal mouse IgG2a and IgG2b, and the rat anti-mouse Fc gamma R monoclonal antibody 2.4G2 immunoprecipitated from MHV-JHM-infected cells a polypeptide with a molecular mass identical to that immunoprecipitated by anti-E2 antibodies. F(ab')2 fragments of rabbit IgG did not immunoprecipitate any proteins from MHV-infected cells. All of these antibodies did not immunoprecipitate any proteins from uninfected cells. The anti-mouse Fc gamma R monoclonal antibody 2.4G2 immunoprecipitated from MHV-JHM-, MHV-3-, or MHV-A59-infected L-2 cells and 17CL-1 cells, or MHV-JHM-infected cultures of neonatal BALB/c brain cells, a protein with a molecular weight identical to that of MHV-JHM E2. The anti-Fc gamma R monoclonal antibody did not immunoprecipitate any proteins from uninfected cells. Furthermore, the 2.4G2 monoclonal antibody (mab), unrelated rat and mouse monoclonal antibodies, and a goat antiserum against E2, but not normal goat serum, immunoprecipitated a 75,000- to 77,000-Da molecule from uninfected WEHI-3 cells, a Fc gamma R bearing cell line. Several lines of evidence demonstrated that the protein immunoprecipitated by the anti-Fc gamma R mab from MHV-JHM-infected cells is the E2 glycoprotein: (1) Partial proteolytic maps obtained by Staphylococcus aureus V-8 protease treatment of the 180,000-Da proteins immunoprecipitated by the anti Fc gamma R mab and the anti-E2 mab were identical. (2) Sequential immunoprecipitation experiments from MHV-JHM-infected cells revealed that the same polypeptide chain was recognized by the anti-E2 mab and by the anti-Fc gamma R mab 2.4G2, (3) Actinomycin D did not influence the induction and expression of the 180,000-Da polypeptide chain that was immunoprecipitated by the anti-Fc gamma R mab, demonstrating that this protein is of viral origin.