Site-specific and Global Glycan Analysis of the Human Cytomegalovirus Pentameric Glycoprotein by High Resolution Mass Spectrometry

The Human Cytomegalovirus (HCMV) gH/gL/UL128/UL130/UL131A complex (Pentamer) is among the candidates for the development of a vaccine against HCMV; indeed, it brought to more immunogenic formulations which were able to protect the fetus against vertical transmission in pregnant women. Pentamer-specific antibodies have shown to be hundredfold more potent than gB or gH/gL antibodies, endowed with strong neutralizing activity. The often inadequate antibody immune response elicited by glycoprotein antigen has been, in part, attributed to the abundance of glycosylation on the protein. Here, we determine the composite glycan population of each of the N-linked and O-linked glycosylation sites present on the Pentamer by multienzymatic proteolysis and mass spectrometry; furthermore, total N-glycans profiles of Pentamer expressed in different cell systems have been compared exploiting the glycan fluorescent labeling and the immunogenicity of Pentamer with different glycosylation patterns has been investigated. Our analysis reveals the presence of underprocessed oligomannose-type glycans on some glycosylation sequons, which are probably derived as a result of sterically reduced accessibility to glycan processing enzymes. Furthermore, the antigen glycosylation pattern affects the elicitation of neutralizing antibodies in mice.