Even though HIV diversifies widely within infected individuals over time, the virus strains that ultimately are passed on through heterosexual transmission often resemble the strain of virus that originally infects the transmitting partner. Learning the characteristics of these preferentially transmitted HIV strains helps advance HIV prevention efforts, particularly with regard to an HIV vaccine.
There is significant increase in population-wide HIV genetic diversity among infected individuals. The amount of genetic change is significantly greater within individuals than across the population as a whole. The genetic diversity of HIV at the population level is limited because only certain strains of the virus within each person are responsible for subsequent sexual transmissions.
The virus strains in the blood of the newly infected partner are more closely related to those found in the blood of the transmitting partner at the earliest available time point than to strains present around the time of transmission. In the heterosexual transmission of HIV, the frequent natural selection of viral strains from early in the infection of the transmitting partner reduces viral diversity at the population level.
Moreover, the newly acquired strain is highly similar or identical to specific variants found in the transmitting partner at both the earliest time point and the time of transmission. These highly transmissible HIV strains from early infection are sustained in the blood at low levels or sequestered in certain cells for transmission at a later time.
The genetic features help HIV bind tightly to a molecule called integrin α4β7. The capacity to bind tightly to α4β7 likely enhances the ability of certain HIV viruses to complete the many steps of sexual transmission and become the “founder” virus that establishes infection in an individual. Certain early-transmitting isolates of HIV can have an affinity for α4β7, is likely that CD4+ T cells with the α4β7 receptor play an important role in the sexual transmission of HIV.
HIV strains found in infected individuals during the early stages of infection have diversified little from the strain that cause infection. Thus, the fact that these early HIV strains somehow are maintained or persist at low levels for transmission later on may have an evolutionary advantage at crossing the genital barrier and causing infection, compared with HIV strains that predominate later in infection.