Coexpression of IL-2 receptor p55 and p75 by circulating blood lymphocytes.

By using high-sensitivity fluorescence and flow cytometry, it is possible to show that 30-40% of lymphocytes from PBL express the p55 chain of the IL-2 receptor, whereas the p75 chain is expressed at low concentrations on most lymphocytes without in vitro activation. The availability of a second fluorochrome capable of high sensitivity allows simultaneous analysis of p55 and p75, albeit with some sacrifice in sensitivity. Two-color analysis shows that a small proportion of cells (1-6%) coexpress measurable concentrations of both chains of the IL-2 receptor, and three-color studies show that these cells are predominantly CD4-positive T cells and express the CD45R0 isoform of the leucocyte-common antigen, i.e., have the phenotype of activated helper T cells. These cells may be a useful indicator of immune activation.