The origins of HIV
If a new disease appears, the cause must also be new. If the new disease is infectious, as is acquired immunodeficiency syndrome (AIDS), the causative agent must have existed previously under one of several possible circumstances. First, the etiologic agent may represent a more virulent mutant variant or recombinant of an organism that was either previously infecting the same population without causing disease or had a distinctly different profile of disease pathology. Second, the organism may have been introduced from a relatively isolated population of people who had developed resistance to the lethality of the agent. Third, the organism may have been introduced to humans from another species. This third explanation seems apparent for the human immunodeficiency virus type 2 (HIV-2) and is increasingly likely for HIV-1. Attempts to understand how HIVs and the related simian immunodeficiency viruses (SIVs) evolved are occasionally viewed as insensitive or irrelevant. Yet, understanding how human or nonhuman hosts evolved to resist the lethal effects of HIV or related viruses may provide valuable clues for the development of effective vaccines and drugs.
AIDS was first recognized as a new distinct clinical entity in 1981. The first cases were recognized because of unusual clustering of diseases such as Kaposi’s sarcoma and Pneumocystis carinii pneumonia in young homosexual men. Although such syndromes had occasionally been observed in distinct subgroups of the population (eg, older men of Mediterranean origin in the case of Kaposi’s sarcoma, severely impaired cancer patients in the case of Pcarinii pneumonia,) the occurrences of these diseases in previously healthy young people was unprecedented. Because most of the first cases of this newly defined clinical syndrome to be described involved homosexual men, it seemed logical at first that the cause of this syndrome could be related to a lifestyle habit unique to that population. In the 1960s and 1970s, the revolution in sexual permissiveness brought with it an enhanced societal acceptance of homosexuality. The development of commercial bathhouses and other outlets for homosexual contact increased the incidence of promiscuity in self-selected segments of the male homosexual population. Such factors as frequent exposure to sperm, rectal exposure to sperm, and amyl or butyl nitrate “poppers,” which were used to enhance sexual performance, were considered potential causes of AIDS. Yet, although it was apparent that AIDs was a new disease, most homosexual lifestyle habits had changed only in a relative sense.
AIDS cases were soon reported in other populations including intravenous drug users and hemophiliacs. Although these groups were not necessarily exposed to amyl or butyl nitrate or to frequent contact with sperm, it was argued that they, like male homosexuals, may have been exposed to frequent immuno-stimulatory doses of foreign proteins and tissue antigens. Hemophiliacs used clotting factor preparations, which were prepared from the pooled blood of huge numbers of donors, and intravenous drug users often used needles contaminated with small amounts of blood from previous users, thereby increasing their exposure to foreign tissue antigens. Even independent of clinical AIDS, asymptomatic hemophiliacs and intravenous drug users were often found to have inverted ratios of helper to suppressor T lymphocytes, as did AIDs patients and a proportion of asymptomatic homosexual men. However, for patients not exhibiting the new syndrome, the distorted T-cell ratios were more often a result of an increase in the number of T-suppressor cells, as opposed to the absolute decrease in T-helper cells seen in AIDS patients with progressing disease. The increase in T-suppressor cells is presumably a result of frequent antigenic stimulation. Three new categories of AIDS patients were soon observed: blood transfusion recipients, adults from Central Africa, and infants born to mothers who themselves had AIDS or were intravenous drug users. The patients with transfusion-associated cases were found to have received donations from an AIDS patient at least 3 years before symptoms developed.