Direct visualization of cytokine-producing recall antigen-specific CD4 memory T cells in healthy individuals and HIV patients

We have used computer-assisted cytokine ELISA spot analysis to measure the frequencies, the type of cytokine, and the amount of cytokine produced by i ndividual recall Ag-specific CD4 memory cells in freshly isolated blood. We studied the memory cells specific for tetanus toroid and purified protein derivative ill 18 healthy individuals and in 22 HIV-infected patients on hi ghly active antiretroviral therapy (HAART). In healthy individuals, the fre quency, cytokine signature, and cytokine production per cell of these memor y cells were stable over time. Although it is presently unclear whether the maintenance of the memory T cell pool depends upon Ag persistence, cross-r eactive Ag stimulation, or cytokine-driven bystander stimulations and expan sions, our data strongly argue for a stable memory cell pool in healthy ind ividuals. In HIV patients, however, the frequency of these memory cells was a function of the viral load. The decreased numbers of functional memory c ells in patients with high viral loads might provide one mechanism behind t he immunodeficient state. Although the cytokine output per cell was unaffec ted in most patients (20 of 24), in some patients (4 of 24) it was >100-fol d reduced, which might provide an additional mechanism to account for the r educed immunocompetence of these patients. The ability to visualize directl y and quantify the cytokine produced by the low frequency memory. cells in freshly isolated blood that have been physiologically stimulated by Ag shou ld aid comprehensive studies of the Ag-specific memory cell pool in vivo, i n health and disease.