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| Resistance to HIV |
Despite the lethality of HIV/AIDS, susceptibility to HIV infection and progression to AIDS is rather variable. There are individuals who have been exposed to HIV multiple times but who either remain uninfected or if they are infected, progress more slowly to full-blown AIDS. Recent studies have shown that some of the variation in HIV resistance has a genetic component.
HIV operates by subverting the immune system; therefore, it is logical that differences in the immune system may play a role in the genetic variation of resistance to HIV. Indeed, some HIV-resistant individuals possess different chemokine receptors than HIV-susceptible individuals. What's a chemokine receptor? First, let's discuss chemokines.
Chemokines are molecular signals released by cells of the immune system that stimulate white blood cells to move to inflamed tissues. They are metaphoric "cries for help." The chemokines bind to receptors located on the white blood cells. Macrophages - those white blood cells that engulf foreign particles and are an early stage of defense - possess the chemokine receptor that is encoded by the gene CCR5. By subverting the normal function of this chemokine receptor, HIV is able to gain entry into macrophages. (See the HIV and AIDS unit.)
Individuals that have lower expressions of this protein due to variants of the CCR5 gene have an increased resistance to HIV; their macrophages are metaphorically more cautious about the signals they respond to. The most obvious case of a "more cautious" CCR5 variant is the allele that has a deletion of thirty-two nucleotides. Individuals who are heterozygous for this variant, CCR5-delta32, have substantially increased resistance to HIV infection; if infected, progress to full-blown AIDS is much slower than normal. Individuals that are homozygous for CCR5-delta32 are virtually completely resistant to HIV. In European populations about twenty percent of individuals are heterozygotes, and one percent are homozygotes in some populations. In contrast, the allele is rare in the Asian populations and virtually absent in the African populations.
Why is this deletion variant present in some populations in such high frequencies? HIV is, at most, a couple centuries old and, more likely, less than a hundred years old. That isn't sufficient time for natural selection to increase the frequency of a rare allele, such as is observed in the European populations. Furthermore, the selection pressures caused by HIV should be much higher in Africa than in Europe. It is also probable that the decreased receptivity to chemokines would be somewhat costly. Some biologists have suggested that the deletion allele could be a vestige of plague resistance. It may have led to increased survival during the Black Plague of the fourteenth century in Europe, and has had an unintended -- but welcome -- consequence of HIV resistance. The increased frequency of the variant in Europe would be consistent with that scenario.
The environment, and in particular, disease has continued to exert strong pressures on human populations. Generally, we are unable to directly observe changes in species because these changes occur in time scales that exceed human lifespans. Yet, we may be able to detect small changes in allele frequencies that have occurred in populations due to epidemics.