HIV particles (yellow) accumulate on the surface of infected cells (purple). Color scanning electron microscope image. Stephane Fremont, Jeremy Dufloo, Arnaud Echard, Timothee Bruel, Olivier Schwartz, Jean-Marc Panaud, Institut Pasteur
A team from the Pasteur Institute, CNRS, The Vaccine Institute (VRI) and the Sorbonne University of Paris has discovered the new function of anti-HIV-1 antibodies by applying cutting-edge microscopy techniques to virus culture in vitro. Scientists have found that certain antibodies known to effectively target the HIV-1 envelope (Env) protein prevent infected cells from releasing viral particles, thereby preventing transmission of the virus. Antibodies are y-shaped, allowing them to attach themselves to or directly to infected cells and virus particles. This chain of antibodies and virus particles prevents the virus from spreading. These findings suggest that these powerful antibodies exhibit different antiviral activities in addition to neutralization. The study is published in Nature Communications February 2, 2022.
Broadly neutralizing antibodies (bNAb) targeting viral envelope (Env) proteins have great potential for the treatment of HIV-1. They were first discovered in rare patients whose serum suppressed multiple HIV strains. These antibodies exhibit a variety of antiviral activities. In addition to neutralizing the virus, that is, preventing it from infecting new cells, they also kill infected cells. Therefore, they are called pluripotent molecules. It is necessary to fully understand the range of these antiviral activities in order to use existing antibodies more effectively or to refine selection criteria for new antibodies. In addition, further study of the versatility of anti-HIV-1 antibodies could help us better understand the role that antibodies play in dealing with other viral infections.
Initially, the team from the Pasteur Institute, CNRS, VRI and Sorbonne tried to determine whether antibodies could prevent infected cells from producing virus particles. To do this, they cultured CD4 T cells (the natural target of HIV) in vitro with various antibodies for 24 hours. They then measured the number of virus particles produced by the cells in the culture medium as well as the number remaining in the cells. As a result of these experiments, the scientists were able to show that certain antibodies increased the number of viruses in cells but reduced the number of viruses in the culture medium. This intriguing finding led them to believe that certain antibodies block the release of virus particles rather than preventing their production.
To test the theory, scientists used various microscope techniques to watch cells produce virus particles. They initially examined the cells with a fluorescence microscope, a technique used to distinguish viral proteins. This allowed them to show that infected cells accumulate large amounts of mature viral proteins. This finding suggests that intact virus particles accumulate in cells. To determine the exact location of the virus particles, the scientists then used a scanning electron microscope to look at the surface of the infected cells. “Using this approach, we observed that these antibodies (bNAb) promote the accumulation of virus particles on the cell surface, forming clusters and highly atypical structures (see figure),” commented Timothee Bruel,
Next, the scientists combined transmission electron microscopy with immunogold markers. This allowed them to show that the antibody inserts itself between the virus particle and the infected cell, forming chain-like clusters. Subsequent experiments with mutated antibodies showed that the y-shape of the antibody formed this cluster structure. Their arms are capable of attaching two or one viruses to an infected cell membrane, and their attachment sites are strong enough to trigger this phenomenon.
“We have shown that only the most powerful antibodies bind virus particles to the surface of infected cells. The captured virus particles can no longer infect new cells, “concludes Olivier Schwartz, co-author of the study and head of the Division of Viruses and Immunity at the Pasteur Institute.
This work revealed novel antiviral activity that broadly neutralizes anti-HIV-1 antibodies. It deepens our understanding of how these antibodies work and explains their efficacy in clinical trials. Scientists are now studying antibodies against other viruses, including SARS-COV-2, to see if they also inhibit viral transmission by this mechanism.