Molecule of the Month: RSV Fusion Glycoprotein

Vaccines are one of the most powerful tools in the medical community’s fight against viral diseases. Vaccines work by challenging us with pieces of a virus, which stimulate the immune system to build antibodies to recognize these pieces and ultimately prepare us for when the actual virus attacks. Often, however, the success of a vaccine is critically dependent on the choice of the virus molecules that are used. The first vaccines, such as the polio and smallpox vaccines, took a simple approach, using inactivated whole viruses or less dangerous relatives of the virus. Current approaches are more targeted, choosing only the most effective portion of the virus to create the vaccine. Structural biology helps tune these vaccine molecules for maximal effectiveness.

Researchers have been working for many years to create a vaccine to fight infection by RSV (respiratory syncytial virus). RSV causes respiratory tract infections that can be life threatening in infants and in elderly people. Much of this vaccine development focused on the fusion glycoprotein of the virus. This glycoprotein forms spikes on the surface of the virus that allow it to find and enter the cells that it infects. Fusion glycoprotein is an attractive choice for a vaccine since it is readily accessible to antibodies, and by blocking it, we can block attachment of the virus to cells. It is a challenging target, however, because it undergoes enormous structural transitions during the process of infection. As seen in PDB ID 4jhw, the “prefusion” form on the surface of the virus is a compact trimer. Several loops on the upper side (colored bright red here) are readily accessible to antibodies and comprise the major antigenic site. When the virus attaches to the cell, the trimer pops open and inserts into the cellular membrane, bridging the virus to the cell. The whole protein then undergoes a massive rearrangement, ending up in the form seen in PDB ID 3rrr, the “postfusion” form. As seen in the structure, everything is in a different place, and the major antigenic sites are buried in the middle of the protein, mostly inaccessible to antibodies.

The breakthrough in design of an RSV vaccine came when researchers used these structures to engineer a form of the fusion glycoprotein that is glued into the prefusion shape. This engineered protein mimics the shape that is found on infectious viruses, and thus stimulates the immune system to focus on the virus. This idea led to the creation of RSV vaccines (Arexvy and Abrysvo) that have just received approval from the US Food and Drug Administration to protect older people from RSV infection and for use during pregnancy to protect newborn infants from infection. A similar approach was used in the mRNA vaccines that are currently protecting us from infection by SARS-CoV-2, which challenge the immune system with prefusion-stabilized versions of the viral spike protein.

The medical community is using a complementary approach to protect particularly susceptible infants from RSV infection. In these cases, anti-RSV antibodies are administered to provide a front-line barrier against the viruses. These antibodies are similar to the ones that are elicited by the vaccine, binding to the prefusion form of the fusion glycoprotein on the surface of the virus. PDB ID 5udc shows the antibody therapy nirsevimab in action.