Background: Lymphocyte activation, associated with vaccination or infection, can be measured by positron emission tomography (PET). We investigated the ability of PET to detect and measure magnitude of lymph-node activation among asymptomatic HIV-1-infected individuals. Methods: Initially we assessed PET response in eight HIV-1-uninfected individuals who had received licensed killed influenza vaccine. In an urban teaching hospital, we recruited 12 patients recently infected with HIV-1 (<18 months since seroconversion) and 11 chronic long-term HIV-1 patients who had stable viraemia by RT-PCR (non-progressors). After injection with fluorine-18-labelled fluorodeoxyglucose, patients underwent PET. We correlated summed PET signal from nodes with viral load by linear regression on log-transformed values. Findings: Node activation was more localised after vaccination than after HIV-1 infection. In early and chronic HIV-1 disease, node activation was greater in cervical and axillary than in inguinal and iliac chains (p<0·0001), and summed PET signal correlated with viraemia across a 4 log range (r2=0·98, p<0·0001). Non-progressors had small numbers of persistently active nodes, most of which were surgically accessible. Interpretation: The anatomical restriction we noted may reflect microenvironmental niche selection, and tight correlation of PET signal with viraemia suggests target-cell activation determines steady-state viral replication.
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