Complex formation by human cytomegalovirus glycoproteins M (gpUL100) and N (gpUL73)

The envelope glycoproteins of human cytomegalovirus (HCMV) virions are incompletely characterized. We have analyzed complex formation between glycoprotein M (gM or gpUL100) and a second glycoprotein. gM-homologous proteins are conserved throughout the herpesvirus family and represent type III membrane proteins containing multiple hydrophobic sequences. In extracellular HCMV particles, gM was found to be complexed through disulfide bonds to a second protein with an apparent molecular mass of 50 to 60 kDa. The 50- to 60-kDa protein was found to be derived from reading frame UL73 of HCMV strain AD169. UL73-homologous genes are also conserved within herpesviruses. When transiently expressed by itself, the UL73 gene product consisted of a protein of 18 kDa. However, in the presence of gM, the UL73 gene product was posttranslationally modified to the 50- to 60-kDa species. Thus, gM and the UL73 gene product, which represents the gN homolog of herpesviruses, form a disulfide-linked complex in HCMV virions. Transient expression of gM and gN followed by fluorescence imaging with monoclonal antibodies against either protein demonstrated that complex formation was required for transport of the proteins from the endoplasmic reticulum to the Golgi and trans-Golgi compartments. Finally, we tested the gM-gN complex for reactivity with sera from HCMV-seropositive donors. Whereas most sera failed to react with either gM or gN when expressed alone, 62% of sera were positive for the gM-gN complex. Because a murine monoclonal antibody reactive with gN in the gM-gN complex efficiently neutralizes infectious virus, the gM-gN complex may represent a major antigenic target of antiviral antibody responses.

Identification of the gene coding for rhesus cytomegalovirus glycoprotein B and immunological analysis of the protein

The nucleotide sequence of the gene encoding glycoprotein B (gB) of rhesus cytomegalovirus (RhCMV) was determined and the protein characterized. The open reading frame of gB encoded a protein of 854 amino acids with 60% identity and 75% similarity at the amino acid level to human cytomegalovirus (HCMV) gB. Cysteine residues in the extraluminal part of the protein are perfectly conserved. Out of the 16 potential N-linked glycosylation sites present in HCMV gB, 15 are conserved in RhCMV gB. Immunoblot analyses with antisera detected three bands of 150 kDa, 90-110 kDa and 55 kDa representing the full-length gB as well as the proteolytic cleavage products. Cross-reactivity and cross-neutralization of a number of HCMV gB-specific monoclonal antibodies with RhCMV gB indicated sharing of immunogenic epitopes between the two molecules. The RhCMV gB regions corresponding to antigenic domains AD-1, 2 and 3 of HCMV gB were immunogenic during natural RhCMV infection with the AD-1 region being the immunodominant domain. The data indicate that RhCMV might represent a useful model to investigate pathogenesis and immune surveillance of cytomegaloviruses.

The humoral immune response against human cytomegalovirus is characterized by a delayed synthesis of glycoprotein-specific antibodies

An individual analysis of IgG antibodies against 12 known antigenic domains of human cytomegalovirus (HCMV)-derived structural phospho- and glycoproteins and nonstructural polypeptides was performed. In HCMV-seropositive healthy persons, the separate determination of antibody titers against the various antigens resulted in an antibody profile that was characteristic for each individual. Profiles were qualitatively stable over a period of >4 years. However, quantitative changes were observed in some persons. During primary HCMV infection, a delay of 50-100 days in the appearance of glycoprotein-specific antibodies was observed, whereas immunoglobulins directed against other HCMV-specific antigens were promptly synthesized. In contrast, during reactivation or reinfection, a synchronized production of antibodies was found. Levels of glycoprotein-specific antibodies and detection of viral DNA in peripheral blood inversely correlated. Precursor B cell analyses showed no significant differences between glycoprotein-specific and phosphoprotein-specific B cells.

Comparative analysis of fourteen individual human cytomegalovirus proteins for helper T cell response

The potential of selected proteins of human cytomegalovirus (HCMV) to induce a helper T (Th) cell immune response was investigated in healthy HCMV-seropositive donors. Recombinant derived glycoproteins B (gpUL55), H (gpUL75), integral membrane protein (pUL100), the US6-US11 glycoprotein family (pUS6-US11), the matrix proteins pp65 (ppUL83), pp28 (ppUL99) and the immediate early proteins IE1 (pUL123), IE2 (pUL122) and UL69 (pUL69) were used as stimulating antigens in a lymphocyte proliferation assay. The antigen-specific proliferative response was measured in HCMV-specific T cell lines (phenotype CD4+ CD8-) generated from five donors by stimulation of peripheral blood mononuclear cells with purified HCMV or HCMV-infected fibroblasts. A proliferative T cell response was induced by pp65, gB, gH, IE1, IE2 and UL69, with a dominant response to pp65 in all donors. Three T cell lines responded to gB and gH, respectively. For IE1, IE2 and UL69 a T cell stimulation could be demonstrated in single cell lines generated with lysate of HCMV-infected fibroblasts.

An ELISA using recombinant proteins for the detection of neutralizing antibodies against human cytomegalovirus

Two prokaryotically expressed fusion proteins encompassing amino acids 484-650 (AD-1) and 27-100 (AD-2) of glycoprotein gp58/116 of human cytomegalovirus (HCMV) were purified from E. coli lysates and used in ELISA to determine antibody levels in human sera. The specificity of the test was established by comparison of 116 randomly selected sera with commercially available HCMV-ELISA tests. The recombinant polypeptides were then used for the analysis of antibody titers in 112 human sera and were compared to the capacity to neutralize HCMV. A strong correlation between the neutralization titer and antibody levels against AD-1 and a weaker correlation for AD-2 was observed. Of 29 sera with a high neutralization titer (> 1:128), 96% and 62% were positive for AD-1 and AD-2, respectively, while 44% and 19% were positive in sera with low neutralization titer (< 1:8). Serum pools prepared from human sera selected on the basis of recognition of the recombinant antigens had a 10-fold higher neutralization capacity than pools prepared from sera with a high titer in commercially available HCMV tests. A synchronous increase in neutralization capacity and titer against recombinant antigens was observed in transplant patients.

A continuous sequence of more than 70 amino acids is essential for antibody binding to the dominant antigenic site of glycoprotein gp58 of human cytomegalovirus

Antigenic domain 1 (AD-1) on glycoprotein gp58 of human cytomegalovirus was characterized in detail, using mouse and human monoclonal antibodies as well as human convalescent sera. Series of procaryotically expressed fusion proteins and synthetic peptides of various lengths were used as sources of antigen. Binding of antibodies was found to depend on a continuous sequence of more than 70 amino acids between residues 552 and 635 of gp58. The fine specificities for sequences involved in antibody binding were (i) amino acids 557 to 635 for neutralizing as well as nonneutralizing mouse monoclonal antibodies, (ii) amino acids 552 to 630 for a neutralizing human monoclonal antibody, and (iii) amino acids 557 to 630 for antibodies present in human sera. Experiments involving fragments of AD-1, presented either as procaryotically expressed fusion protein or as synthetic peptides, indicated that the intact structure was required for recognition of AD-1 by antibodies.