Identification and Functional Characterization of a Novel Fc Gamma-Binding Glycoprotein in Rhesus Cytomegalovirus.

ABSTRACT: Receptors recognizing the Fc part of immunoglobulin G (Fc?Rs) are key determinants in antibody-mediated immune responses. Members of the Herpesviridae interfere with this immune regulatory network by expressing viral Fc?Rs (vFc?Rs). Human cytomegalovirus (HCMV) encodes four distinct vFc?Rs that differ with respect to their IgG subtype specificity and their impact on antibody-mediated immune function in vitro The impact of vFc?Rs on HCMV pathogenesis and immunomodulation in vivo is not known. The closest evolutionary animal model of HCMV is rhesus CMV (RhCMV) infection of rhesus macaques. To enable the characterization of vFc?R function in this model, we studied IgG binding by RhCMV. We show that lysates of RhCMV-infected cells contain an IgG-binding protein of 30?kDa encoded by the gene Rh05 that is a predicted type I glycoprotein belonging to the RL11 gene family. Upon deletion of Rh05, IgG-Fc binding by RhCMV strain 68-1 is lost, whereas ectopic expression of Rh05 results in IgG binding to transfected cells consistent with Rh05 being a vFc?R. Using a set of reporter cell lines stably expressing human and rhesus Fc?Rs, we further demonstrate that Rh05 antagonizes host Fc?R activation. Compared to Rh05-intact RhCMV, RhCMV?Rh05 showed an increased activation of host Fc?R upon exposure of infected cells to IgG from RhCMV-seropositive animals, suggesting that Rh05 protects infected cells from opsonization and IgG-dependent activation of host Fc?Rs. However, antagonizing host Fc?R activation by Rh05 was not required for the establishment and maintenance of infection of RhCMV, even in a seropositive host, as shown by the induction of T cell responses to heterologous antigens expressed by RhCMV lacking the gene region encoding Rh05. In contrast to viral evasion of natural killer cells or T cell recognition, the evasion of antibody-mediated effects does not seem to be absolutely required for infection or reinfection. The identification of the first vFc?R that efficiently antagonizes host Fc?R activation in the RhCMV genome will thus permit more detailed studies of this immunomodulatory mechanism in promoting viral dissemination in the presence of natural or vaccine-induced humoral immunity.IMPORTANCE Rhesus cytomegalovirus (RhCMV) offers a unique model for studying human cytomegalovirus (HCMV) pathogenesis and vaccine development. RhCMV infection of nonhuman primates greatly broadened the understanding of mechanisms by which CMVs evade or reprogram T cell and natural killer cell responses in vivo However, the role of humoral immunity and viral modulation of anti-CMV antibodies has not been studied in this model. There is evidence from in vitro studies that HCMVs can evade humoral immunity. By gene mapping and with the help of a novel cell-based reporter assay system we characterized the first RhCMV encoded IgG-Fc? binding glycoprotein as a potent antagonist of rhesus Fc?R activation. We further demonstrate that, unlike evasion of T cell immunity, this viral Fc? receptor is not required to overcome anti-CMV immunity to establish secondary infections. These findings enable more detailed studies of the in vivo consequences of CMV evasion from IgG responses in nonhuman primate models.

SUBMITTER: Kolb P 

PROVIDER: S-EPMC6364020 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature