A team of French researchers has succeeded in filming HIV infecting a healthy cell. UNAIDS spoke to Morgane Bomsel, Research Team Director at the French National Center for Scientific Research (CNRS), about the feat.
What motivated you to film HIV transmission?
Morgane Bomsel: HIV transmission has not been studied much and we had no precise idea of the exact sequence of events leading to HIV infection of genital fluids during sexual intercourse. Neither did we know how immune cells are infected and what the consequences are. The vast majority of new HIV infections are acquired via the genital and rectal mucosa; however, the outer layer, the epithelium, of those tissues varies and affects how HIV enters the body.
What were the challenges?
MB: The challenges involved building an experimental model that mimicked genital mucosa infected by genital fluids suitable for live imaging. We reconstructed in vitro human male urethral mucosa based on human cells, the surface of which had been engineered to be red, and an infected white blood cell (a T lymphocyte, the main infectious element in sexual fluids) that was engineered to be fluorescent green and in turn would produce fluorescent green HIV infectious particles.
We had to render the system fluorescent to be able to visualize it and track HIV entry in the mucosa by live fluorescent scanning. Finally, we had to devise a system to allow the microscope lens to visualize the contact between the cells. All of this, of course, was done in an extremely secure setting and all of us were wearing two pairs of gloves and a hat, a coat, glasses and a mask.
When did you know you had a breakthrough?
MB: Our eureka moment was when we captured on film the spillage of a string of viruses, like a gun showering bullets. This lasts for a couple of hours and then, as if the infected cell has lost interest, it detaches itself and moves on.
Please walk us through the video
MB: The HIV-infected cells are labelled in green and produce fluorescent viruses that appear as green dots.
What we see is the HIV-infected cell attaching itself closely to the outer layer, the epithelium, of healthy reconstructed cells of a genital tract mucosal lining.
White blood cells of the immune system, macrophages, that usually engulf foreign substances, debris or cancer cells are seen engulfing the red particles slightly moving next to the blue macrophage nucleus.
The HIV-infected cell approaches the surface of the mucosa and places itself gently on the surface. Owing to, or induced by, contact, the infected cell recruits preformed viruses towards the cell contact (the intense yellow green patches) and then starts to spit those preformed viruses as full infectious viruses that appear as green dots.
These green viruses penetrate the outer layer of the tissue by a process called transcytosis—a type of transcellular transportation. The viruses enter the cell and exit, still infectious, at the other side of the epithelial barrier. As a result, HIV penetrates the types of white blood cells responsible for detecting, engulfing and destroying foreign substances and infects them. Once inside the nucleus, the virus inserts itself in the genetic material, the DNA, and the blood cells that are meant to protect the body start to produce viruses.
Interestingly enough, the video showed that the production of viruses does not last very long. After three weeks, the infected white blood cells become dormant and a reservoir of white blood cells is formed.
What makes HIV particularly tricky to cure?
MB: Attempts to cure HIV have been very difficult because of the dormant infected white blood cells. Those cells are hard for the immune system to find and kill, and for the scientist to study. Antiretroviral medicines prevent the virus from spreading throughout the body and the immune system targets cells that are actively transcribing viral DNA. But because of the reservoir, these cells become a problem if a patient stops taking antiretroviral therapy. They can slowly awaken, allowing the virus to replicate freely.