Molecule of the Month: Hantavirus

A hantavirus outbreak linked to a cruise ship has recently put the spotlight on this zoonotic virus. Globally, it is estimated that tens of thousands of hantavirus infections occur each year. People most commonly become infected by inhaling dust contaminated with the urine, feces, or saliva of infected rodents. Although hantaviruses typically do not cause noticeable illness in the rodents that carry them, in humans infection can trigger serious disease. Different hantaviruses are associated with different forms of illness depending on where they are found in the world. New World hantaviruses, which are found in the Americas, can cause hantavirus pulmonary syndrome, a severe respiratory disease that can rapidly become fatal. In Europe and Asia, Old World hantaviruses are more commonly associated with hemorrhagic fever with renal syndrome, a disease that affects blood vessels and kidney function. In the few documented cases where person-to-person transmission are thought to have occurred, infections were linked to a specific New World hantavirus known as the Andes virus, and appeared to require close personal contact between individuals.

Hantaviruses are membrane-enveloped RNA viruses that vary in shape and size. Although hantaviruses often appear roughly spherical under the microscope, some particles are elongated or tubular. The viral genome consists of three segments of single-stranded RNA, referred to as the small (S), medium (M), and large (L) segments. Each segment encodes different viral components essential for infection and replication. The small segment encodes the nucleocapsid protein, which binds and protects the viral RNA. The medium segment encodes two envelope glycoproteins, called Gn and Gc, which protrude from the viral surface. The large segment encodes an RNA-dependent RNA polymerase, the enzyme responsible for transcribing and copying the viral genome.

The surface of the viral membrane is covered with Gn/Gc protein complexes that assemble into square-shaped tetrameric complexes. These tile-like complexes organize into a characteristic square lattice across the viral surface, illustrated on the right (pdb_00009p3x and pdb_00006zjm). Inside the virus, the tails of the glycoproteins interact with nucleocapsid proteins and viral RNA that form rod-shaped ribonucleoprotein complexes that package the genome.

To infect a cell, hantavirus surface spikes bind to proteins on the surface of target cells, including cells that line blood vessels and the respiratory tract. Once attached and internalized into cells, the viral spikes undergo a dramatic structural rearrangement that allows the viral membrane to fuse with the host cell membrane. This fusion event releases the viral genome into the cell, initiating infection. As the immune system responds to infection, the body produces different antibodies that recognize the virus. Some of these antibodies are “neutralizing antibodies,” meaning that they can block the virus from replicating and spreading. Researchers have isolated neutralizing antibodies from individuals who have recovered from hantavirus infections. While most of these antibodies recognize only a single hantavirus, scientists have also discovered broadly neutralizing antibodies capable of blocking multiple hantaviruses.

One such antibody, called ADI-42898, was isolated from a patient infected with Puumala virus, an Old World hantavirus found primarily in Europe and Russia. Structural studies revealed that the antibody binds at the interface between the Gn and Gc proteins, locking the viral spikes into a prefusion state and preventing the virus from entering cells. Importantly, ADI-42898 was shown to protect against a wide range of both Old World and New World hantaviruses. Researchers later engineered an optimized version of the antibody, called ADI-65534, with even broader blocking activity, including improved activity against the Andes virus. Research into how antibodies recognize and block infection can lead to new therapeutics against hantaviruses, for which no FDA-approved treatment or vaccine are currently available.