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However, the specific immunologic profile that typifies AIDS--a progressive reduction of CD4+ T cells resulting in persistent CD4+ T lymphocytopenia and profound deficits in cellular immunity--is extraordinarily rare in the absence of HIV infection or other known causes of immunosuppression. This was recently demonstrated in several surveys that sought to determine the frequency of idiopathic CD4+ T-cell lymphocytopenia (ICL), which is characterized by CD4+ T cell counts lower than 300 cells per cubic millimeter (mm3) of blood in the absence of HIV antibodies or conditions or therapies associated with depressed levels of CD4+ T cells (reviewed in Fauci, 1993b; Laurence, 1993).

In a CDC survey, only 47 (.02 percent) of 230,179 individuals diagnosed with AIDS were both HIV-seronegative and had persistently low CD4+ T cell counts (<300/MM3) IN THE ABSENCE OF CONDITIONS OR THERAPIES ASSOCIATED WITH IMMUNOSUPPRESSION (SMITH ET AL., 1993).

In the MACS, 22,643 CD4+ T cell determinations in 2,713 HIV-seronegative homosexual men revealed only one individual with a CD4+ T cell count persistently lower than 300 cells/mm3, and this individual was receiving immunosuppressive therapy (Vermund et al., 1993a). A similar review of another cohort of homosexual and bisexual men found no case of persistently lowered CD4+ T cell counts among 756 HIV-seronegative men who had no other cause of immunosuppression (Smith et al., 1993). Analogous results were reported from the San Francisco Men's Health Study, a population-based cohort recruited in 1984. Among 206 HIV-seronegative heterosexual and 526 HIV-seronegative homosexual or bisexual men, only one had consistently low CD4+ T cell counts (Sheppard et al., 1993). This individual also had low CD8+ T cell counts, suggesting that he had general lymphopenia rather than a selective loss of CD4+ T cells. No AIDS-defining clinical condition was observed among these HIV-seronegative men.

Studies of blood donors, recipients of blood and blood products, and household and sexual contacts of transfusion recipients also suggest that persistently low CD4+ T cell counts are extremely rare in the absence of HIV infection (Aledort et al., 1993; Busch et al., 1994). Longitudinal studies of injection-drug users have demonstrated that unexplained CD4+ T lymphocytopenia is almost never seen among HIV-seronegative individuals in this population, despite a high risk of exposure to hepatitis B, cytomegalovirus and other blood-borne pathogens (Des Jarlais et al., 1993; Weiss et al., 1992).