LETTER

Top Letter References

The recent detection of functional interleukin-2 receptors (IL-2R) on circulating polymorphonuclear neutrophils (PMN) and monocytes has broadened our concept of the role of IL-2 in the inflammatory response. The IL-2R consists of three components, , , and c, that are differentially expressed on a variety of hematopoietic, lymphoid, and some nonlymphoid cell types. This differential expression results in varying affinity of the receptor for IL-2, with the c complex having the highest affinity (10¹¹ M) and the c IL-2R complex having intermediate affinity (10⁹ M). The c IL-2R complex is expressed by neutrophils and monocytes (1). Therefore, the combination of plasma IL-2 concentration and the receptor type determines biological response. This letter reports the results of a study that compared the responses of lymphocytes and neutrophils to IL-2 activation in a whole-blood assay.

Fluorescein isothiocyanate (FITC) was conjugated to Proleukin (aldesleukin) recombinant IL-2 (Chiron Corporation, Emeryville, Calif.). The purified IL-2-FITC conjugate was used to quantitate the percentage of lymphocytes and neutrophils bearing functional IL-2 receptors. Briefly, heparinized whole blood was incubated with various amounts of the IL-2-FITC conjugate for different times at different temperatures. The percentage of lymphocytes or neutrophils that bound the IL-2-FITC conjugate was determined by flow cytometry. Previous experiments demonstrated that FITC-conjugated Proleukin IL-2 and nonconjugated Proleukin IL-2 had similar binding activities.

Data obtained from these assays showed that as the concentration of IL-2-FITC increased, the percentage of neutrophils that bound the conjugate increased and reached saturation more rapidly than lymphocytes (Table 1). These results suggest that the saturable binding of IL-2 by neutrophils may be an early marker of neutrophil activity associated with IL-2-induced toxicity, e.g., vascular leak syndrome. Further studies with humans undergoing Proleukin treatment are under way to verify the clinical significance of monitoring neutrophil binding of IL-2-FITC.