Influence of Host Genetic Variation on Susceptibility to HIV Type 1 Infection
As a prelude to the discussion of host genetic determinants of susceptibility to HIV‐1 infection, we briefly enumerate the nongenetic factors involved and the major issues that arise in designing and analyzing research on genetic variation. Subsequent sections emphasize the importance of evaluating genetically mediated predisposition to infection in the context of genetic influences on clinical responses in individuals who are already infected. Similarities and differences between these 2 types of effects may help distinguish between those factors more involved in initial viral penetration, those more involved in long‐term host adaptation to established infection, and those equally important to both processes.
Nongenetic Influences on Transmission and Acquisition
Biological factors.Viral and host biological characteristics are strong predictors of both transmission and acquisition of HIV‐1. The infectivity of HIV‐1 in a potential “donor” may differ from that in another because of differences in virus subtypes and the set of virus quasi species that are more or less well adapted to the biological individuality of that donor. The envelope glycoprotein of HIV‐1 is one major factor governing its in vitro tropism for certain host cell types, and differences in such properties as size, shape, and net charge of certain envelope motifs could alter the capacity of the virus to penetrate host cells. The duration of infection and the effectiveness of the immune response in the donor further dictate qualitative and quantitative characteristics of virus replication and the capacity to spread to susceptible “recipients.” In general, virus load during the acute and latter stages of infection is higher than during the long middle (“latent”) period, and higher donor virus load at those stages presumably increases the likelihood of transmission to a naive host. Another critical factor is the condition of immune activation in both donors and recipients, especially in areas of close contact; the absence of circumcision, ulceration, and other causes of mucosal disruption presumably increases access to or activates cells targeted by HIV‐1. The age of the recipient may be a surrogate for maturation or senescence in the host cellular immune processes involved in defending against viral penetration and integration. Infection with herpes simplex viruses and, perhaps, other viral and bacterial agents may facilitate propagation of HIV‐1 by activating cells or otherwise promoting virus replication and shedding. Undoubtedly, other recipient immune defense mechanisms that can modulate the risk of infection are still undiscovered.