Synthetic antibodies
Three substitutions helped the antibody become effective against three groups of ebolaviruses
Vera Mukhina, N+1
Working with human antibodies of a patient who survived Ebola, American researchers discovered and described in detail the mechanism of their interaction with the site of the viral envelope. This site is recognized not only in the Zaire Ebola virus, but also in the related Bundibugio virus, which expands the possibilities of using the future vaccine. Replacing only three amino acids in the original antibody significantly increased its effectiveness and made it also suitable for fighting the Sudanese virus. The results are published in the journal Nature Structural and Molecular Biology (West et al., Structural basis of broad ebolavirus neutralization by a human survivor antibody).
Ebolaviruses cause massive epidemics in Africa, and the mortality rate from infection can reach 90 percent. They have appeared before, but special attention to this disease appeared after its outbreak in 2013-2016, when a single mutation of a surface protein made the virus particularly aggressive and led to numerous deaths. This happened with the Zaire virus subtype, but other subtypes may mutate in a similar way during future epidemics. Therefore, the development of a vaccine that would be suitable for all subtypes at once is an important measure to combat Ebola.
The most dangerous strains are grouped into three groups, named after the place of first detection: "classic" Zaire Ebola, Sudanese and Bundibugio. The symptoms of all these diseases are similar – fever, diarrhea, hemorrhoids. But, despite the same origin and similarity of these viruses, there is no common vaccine for them. The basis of the already developed vaccines against Zaire Ebola is the use of glycoprotein, a component of the cell wall of the virus. Unfortunately, using it as a target for other strains has so far turned out poorly due to the large difference between amino acid sequences.
During assembly, the glycoprotein precursor is cut into two parts, GP1 and GP2, which are held together by a disulfide bridge. GP1 is quite variable, and GP2 is conservative, but hidden under the surface of the virus. Brandin West and his colleagues described new antibodies ADI-15946 of a person who had been ill with Zaire Ebola a few years ago and looked at which parts of the protein it interacts with. To do this, they determined the crystal structure of this complex.
The crystal structure of the interaction of antibody sites (orange) and the Ebola virus envelope protein (green). Drawings from the article by West et al.
It turned out that the antibody binds to a conservative GP2 site in the hydrophobic pocket of GP1, usually closed by a section of the glycan cap. When the virus penetrates, this cap is cut off by the host's proteasomes, so that the antibody is able to interact both intracellularly and (worse due to the fact that the cap interferes with it) with the extracellular form of the protein.
The researchers compared the binding of the antibody with a full-fledged glycoprotein and without a piece of cap, and it turned out that the binding efficiency of the latter is 10,000 times higher. In previous studies, other FVM09 antibodies were found that do not interfere with pathogens, but they are able to "remove" the glycan cap, which is not very important for the virus. The joint work of these two antibodies also increased the effectiveness of neutralization.
The b–antibody (orange) and a section of the glycan cap (green) compete for the same place in the protein pocket, where GP2 is indicated in light green and GP1 is indicated in dark green;d, e – To test this, the researchers tried modifying the pocket, making it uncomfortable for the glycan cap (W291R) or simply removing it (CL).
In these cases, the binding and neutralization of the protein by the antibody turned out to be higher than in the case of a full-fledged protein (WT).
The heavy chains of the variable site of the ADI-15946 antibody bind to the conserved lysine residue (K510) at the GP2 site, and mutation at this site leads to a loss of antibody efficacy. In general, it worked well against the Zaire strain and Bundibugio, but was ineffective against the Sudanese virus. To understand why this happens, the researchers compared the surfaces that the antibody comes into contact with in all viruses. In the immediate vicinity of the key lysine was asparagine (Asn506), which in the Sudanese strain was replaced by arginine and with which the antibody site also interacts. It turned out that only three mutations, one on the heavy chain and two on the light ones, allowed the antibody to effectively fight the Sudanese virus.
In addition to the three listed types of ebolaviruses, two more are known that do not cause serious epidemics in humans. Recently, scientists have found a sixth type, for the first time finding it not in a sick person, but in a carrier – a bat.
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