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.

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.

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.

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.