Identification of inflections in T-cell counts among HIV-1-infected individuals and relationship with progression to clinical AIDS

Studies of circulating T (CD3⁺) lymphocytes have shown that on a population basis T-cell numbers remain stable for many years after HIV-1 infection (blind T-cell homeostasis), but decline rapidly beginning approximately 1.5-2.5 years before the onset of clinical AIDS. We derived a general method for defining the loss of homeostasis on the individual level and for determining the prevalence of homeostasis loss according to HIV status and the occurrence of AIDS in more than 5,000 men enrolled in the Multicenter AIDS Cohort Study. We used a segmented regression model for log₁₀ CD3⁺ cell counts that included separate T-cell trajectories before and after a time (the T-cell inflection point) where the loss of T-cell homeostasis was most likely to have occurred. The average slope of CD3⁺ lymphocyte counts before the inflection point was close to zero for HIV- and HIV+ men, consistent with blind T-cell homeostasis. After the inflection point, the HIV+ individuals who developed AIDS generally showed a dramatic decline in CD3⁺ cell counts relative to HIV- men and HIV+ men not developing AIDS. A CD3⁺ cell decline of greater than 10 percent per year was present in 77% of HIV+ men developing AIDS but in only 23% of HIV+ men with no onset of AIDS. Our findings at the individual level support the blind T-cell homeostasis hypothesis and provide strong evidence that the loss of homeostasis is an important mechanism in the pathogenesis of the severe immunodeficiency that characterizes the late stages of HIV infection.