The median period of time between infection with HIV and the onset of clinically apparent disease is approximately 10 years in western countries, according to prospective studies of homosexual men in which dates of seroconversion are known (Lemp et al., 1990; Pantaleo et al., 1993a; Hessol et al., 1994) (Figure 4). Similar estimates of asymptomatic periods have been made for HIV-infected blood-transfusion recipients, injection drug users and adult hemophiliacs (reviewed in Alcabes et al., 1993a).

HIV disease, however, is not uniformly expressed in all individuals. A small proportion of persons infected with the virus develop AIDS and die within months following primary infection, while approximately 5 percent of HIV-infected individuals exhibit no signs of disease progression even after 12 or more years (Pantaleo et al., 1995a; Cao et al., 1995). Host factors such as age or genetic differences among individuals, the level of virulence of the individual strain of virus, as well as influences such as co-infection with other microbes may determine the rate and severity of HIV disease expression in different people (Fauci, 1993a; Pantaleo et al., 1993a). Such variables have been termed "clinical illness promotion factors" or co-factors and appear to influence the onset of clinical disease among those infected with any pathogen (Evans, 1982). Most people infected with hepatitis B, for example, show no symptoms or only jaundice and clear their infection, while others suffer disease ranging from chronic liver inflammation to cirrhosis and hepatocellular carcinoma (Robinson, 1990). Co-factors probably also determine why some smokers develop lung cancer, while others do not.

As disease progresses, increasing amounts of infectious virus, viral antigens and HIV-specific nucleic acids in the body correlate with a worsening clinical course (Allain et al., 1987; Nicholson et al., 1989; Ho et al., 1989; Schnittman et al., 1989, 1990a, 1991; Mathez et al., 1990; Genesca et al., 1990; Hufert et al., 1991; Saag et al., 1991; Aoki-Sei et al., 1992; Yerly et al., 1992; Bagnarelli et al., 1992; Ferre et al., 1992; Michael et al., 1992; Pantaleo et al., 1993b; Gupta et al., 1993; Connor et al., 1993; Saksela et al., 1994; Dickover et al., 1994; Daar et al., 1995; Furtado et al., 1995).

Cross-sectional studies in adults and children have shown that levels of infectious HIV or proviral DNA in the blood are substantially higher in patients with AIDS than in asymptomatic patients (Ho et al., 1989; Coombs et al., 1989; Saag et al., 1991; Srugo et al., 1991; Michael et al., 1992; Aoki-Sei et al., 1992). In both blood and lymph tissues from HIV-infected individuals, researchers at the National Institutes of Health found viral burden and replication to be substantially higher in patients with AIDS than in early-stage patients (Pantaleo et al., 1993b). This group also found deterioration of the architecture and microenvironment of the lymphoid tissue to a greater extent in late-stage patients than in asymptomatic individuals. The dissolution of the follicular dendritic cell network of the lymph node germinal center and the progressive loss of antigen-presenting capacity are likely critical factors that contribute to the immune deficiency seen in individuals with AIDS (Pantaleo et al., 1993b).

More recently, the same group studied 15 long-term non-progressors, defined as individuals infected for more than seven years (usually more than 10 years) who received no antiretroviral therapy and showed no decline in CD4+ T cells. They found that viral burden and viral replication in the peripheral blood and in lymph nodes, measured by DNA and RNA PCR, respectively, were at least 10 times lower than in 18 HIV-infected individuals whose disease progression was more typical. In addition, the lymph node architecture in long-term non-progressors remained intact (Pantaleo et al., 1995a).

Longitudinal studies also have quantified viral burden and replication in the blood and their relationship to disease progression (Schnittman et al., 1990a; Connor et al., 1993; Saksela et al., 1994; Daar et al., 1995; Furtado et al., 1995). In a study of asymptomatic HIV-infected individuals who ultimately developed rapidly progressive disease, the number of CD4+ T cells in which HIV DNA could be found increased over time, whereas this did not occur in patients with stable disease (Schnittman et al., 1990a). Using serial blood samples from HIV-infected individuals who had a precipitous drop in CD4+ T cells followed by a rapid progression to AIDS, other groups found a significant increase in the levels of HIV DNA concurrent with or prior to CD4+ T cell decline (Connor et al., 1993; Daar et al., 1995). Increased expression of HIV mRNA in peripheral blood mononuclear cells has also been shown to precede clinically defined progression of disease (Saksela et al., 1994).

In the longitudinal Multicenter AIDS Cohort Study (MACS), homosexual and bisexual men for whom the time of seroconversion had been documented had increasing levels of both plasma HIV RNA and intracellular RNA as disease progressed and had CD4+ T cell numbers that declined (Gupta et al., 1993; Mellors et al., 1995). Men who remained asymptomatic with stable CD4+ T cell numbers maintained extremely low levels of viral RNA. These findings suggest that plasma HIV RNA levels are a strong, CD4-independent predictor of rapid progression to AIDS. Another longitudinal study found that increasing plasma RNA levels were highly predictive of the development of zidovudine (AZT) resistance and death in patients on long-term therapy with that drug (Vahey et al., 1994).

Other evidence suggests that changes in viral load due to changes in therapy can predict clinical benefit in patients. It was recently found that the amount of HIV RNA in the peripheral blood decreased in patients who switched to didanosine (ddI) after taking AZT and increased in patients who continued to take AZT (NTIS, 1994; Welles et al., 1995). Decreases in HIV RNA were associated with fewer progressions to new, previously undiagnosed AIDS-defining diseases or death. This study provided the first evidence that a therapy-induced reduction of HIV viral load is associated with clinical outcome. Similarly, studies of blood samples collected serially from HIV-infected patients found that a decrease in HIV RNA copy number in the first months following treatment with AZT strongly correlated with improved clinical outcome (O'Brien et al., 1994; Jurriaans et al., 1995).

The emergence of HIV variants that are more cytopathic and replicate in a wider range of susceptible cells in vitro has also been shown to correlate with disease progression in HIV-infected individuals (Fenyo et al., 1988; Tersmette et al., 1988, 1989a,b; Richman and Bozzette, 1994; Connor et al., 1993, Connor and Ho, 1994a,b). Similar results have been seen in vivo with macaques infected with molecularly cloned SIV (Kodama et al., 1993). It has also been reported that HIV isolates from patients who progress to AIDS have a higher rate of replication compared with HIV isolates from individuals who remain asymptomatic (Fenyo et al., 1988; Tersmette et al., 1989a), and that rapidly replicating variants of HIV emerge during the asymptomatic stage of infection prior to disease progression (Tersmette et al., 1989b; Connor and Ho, 1994b).