Although some evidence suggests injection drug use can cause certain immunologic abnormalities, such as reduction in natural killer (NK) cell activity (reviewed in Kreek, 1990), the specific immune deficit that leads to AIDS--a progressive reduction of CD4+ T cells resulting in persistent CD4+ T lymphocytopenia--is rare in HIV-seronegative injection drug users in the absence of other immunosuppressive conditions (Des Jarlais et al., 1993; Weiss et al., 1992).

In a survey of 229 HIV-seronegative injection drug users in New York City, mean CD4+ T cell counts of the group were consistently over 1000/mm3 (Des Jarlais et al., 1993). Only two individuals had two CD4+ T cell measurements of fewer than 300/mm3, one of whom died with cardiac disease and non-Hodgkin's lymphoma listed as the cause of death. In a study of 180 HIV-seronegative injection drug users in New Jersey, the participants' average CD4+ T cell count was 1169/mm3 (Weiss et al., 1992). Two of these individuals, both with generalized lymphocytopenia, had CD4+ T cell counts less than 300/mm3.

In the MACS, median CD4+ T cell counts of 63 HIV-seronegative injection drug users rose from 1061/mm3 to 1124/mm3 in a 15 to 21 month follow-up period (Margolick et al., 1992). In a cross-sectional study, 11 HIV-seronegative, long-term heroin addicts had mean CD4+ T cell counts of 1500/mm3, while 11 healthy controls had CD4+ T cell counts of 820 cells/mm3 (Novick et al., 1989).

Recent data also refute the notion that a certain lifetime dosage of injection drugs is sufficient to cause AIDS in HIV-seronegative individuals. In a Dutch study, investigators compared 86 HIV-seronegative individuals who had been injecting drugs for a mean of 7.6 years with 70 HIV-seropositive people who had injected drugs for a mean of 9.1 years. Upon enrollment in 1989, CD4+ T cell counts were 914/mm3 in the HIV-seronegative group, and 395/mm3 in the seropositive group. By 1994, there were 25 deaths attributable to AIDS-defining conditions in the seropositive group; among HIV-seronegative individuals, eight deaths occurred, none due to AIDS-defining diseases (Cohen, 1994a).

Excess mortality among HIV-infected injection drug users as compared to HIV-seronegative users has also been observed by other investigators. In a prospective Italian study of 2,431 injection drug users enrolled in drug treatment programs from 1985 to 1991, HIV-seropositive individuals were 4.5 times more likely to die than HIV-seronegative subjects (Zaccarelli et al., 1994). No deaths due to AIDS-defining conditions were seen among 1,661 HIV-seronegative individuals, 41 of whom died of other conditions, predominantly overdose, liver disease and accidents. Among 770 individuals who were HIV-seropositive at study entry or who seroconverted during the study period, 89 died of AIDS-related conditions and 52 of other conditions.

In HIV-seropositive individuals, a number of investigators have found no statistical association between injection drug use and decline of CD4+ T cell counts (Galli et al., 1989, 1991; Schoenbaum et al., 1989; Margolick et al., 1992, 1994; Montella et al., 1992; Alcabes et al., 1993b, 1994; Galai et al., 1995), nor a difference in disease progression between active versus former users of injection drugs (Weber et al., 1990; Galli et al., 1991; Montella et al., 1992; Italian Seroconversion Study, 1992).

Taken together, these studies suggest that any negative effects of injection drugs on CD4+ T cell levels are limited and may explain why many investigators have found that HIV-seropositive injection drug users have rates of disease progression that are similar to other HIV-infected individuals (Rezza et al., 1990; Montella et al., 1992; Galli et al., 1989; Selwyn et al., 1992; Munoz et al., 1992; Italian Seroconversion Study, 1992; MAP Workshop, 1993; Pezzotti et al., 1992; Margolick et al., 1992, 1994; Alcabes, 1993b, 1994; Galai et al., 1995).