A new study suggests that HIV is more virulent when transmitted during sexual intercourse between the penis and the vagina.
- It is estimated that nearly 38 million people are still living with HIV worldwide and that one in six people living with HIV are unaware that they are a carrier.
- In France, 173,000 people are living with HIV.
Infection with the human immunodeficiency virus (HIV), which is responsible for the AIDS disease, is transmissible in certain very specific circumstances: in the event of mother-to-child transmission, during contact with contaminated blood or during sexual intercourse not protected by a condom if there is vaginal, anal or oral penetration.
But not all of these modes of transmission are associated with the same severity of the disease. This is highlighted by a new study published in PLOS Pathogens by researchers at the Indian Institute of Science.
According to its authors, HIV-1 would be more virulent when it is transmitted during vaginal penetration, due to bottlenecks in transmission.
A level of severity that varies according to the mode of transmission
The term “genetic bottleneck” is used to describe the phenomenon that occurs during the primary infection phase, when the HIV strains isolated are genetically uniform. This suggests that only one viral variant is usually transmitted and/or spread in a newly infected individual.
However, in the event of HIV-1 transmission during penile-vaginal intercourse, the bottleneck introduces selection pressures, resulting in a virus better adapted to replicate and infect its host. These new strains then tend to be more virulent.
To better understand the differences in viral fitness related to transmission, particularly in the case of anal and vaginal intercourse, the researchers analyzed the number of helper T cells in HIV-infected people, and in uninfected people by compiling data from 340,000 infected people of various ethnicities and from more than 25 countries. Such a measure of helper T cells is important because reduced numbers of these immune cells early in infection are associated with more virulent strains.
Fewer T cells, indicative of a more virulent strain
By comparing the results obtained in people not infected with HIV, those infected after anal intercourse, and those infected after intercourse between vagina and penis, the authors found that infections transmitted by vaginal intercourse were correlated with a number of lower T cells. This therefore suggests a greater selective pressure at the time of transmission and, therefore, more virulent strains than in transmission between men.
“Transmission bottlenecks are expected to drive the evolution of HIV-1 and influence the design of prevention strategies”, write the authors of the work. According to them, there is no doubt that bottlenecks are indeed affected by the mode of transmission of HIV. “As different risk groups tend to use different predominant modes of transmission, it is possible that the strains of HIV-1 directly affected by the bottlenecks evolved differently in different groups. These findings have implications for our understanding of the pathogenesis, evolution and epidemiology of HIV-1.”
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