Human eosinophils express messenger RNA encoding RANTES and store and release biologically active RANTES protein.

Eosinophils synthesize and store various cytokines with potential autocrine activity. We hypothesized that eosinophils synthesize and store RANTES, a CC-chemokine with potent eosinophil chemotactic activity. Expression of RANTES mRNA in highly purified eosinophil populations was detected by reverse transcription followed by polymerase chain reaction analysis. In situ hybridization (ISH) with 35S-labeled RANTES-specific riboprobes showed that 6.8-10% of peripheral blood eosinophils obtained from atopic subjects expressed RANTES mRNA, increasing to 25% after incubation (16 h) with interferon (IFN)-gamma, but not ionomycin in vitro. Peripheral blood eosinophils also showed specific immunoreactivity with an anti-RANTES monoclonal antibody, consistent with translation of the mRNA. By enzyme-linked immunosorbent assay, blood eosinophils were shown to contain a median of 7300 pg (range 5200-8800) RANTES per 10(6) cells, of which a mean of 24% was released into culture supernatants after stimulation of the cells with serum-coated particles in vitro. These culture supernatants exhibited eosinophil chemotactic activity which was inhibited (mean 68%) by a specific anti-RANTES antibody. Sequential immunocytochemistry and ISH on biopsies obtained from allergen-induced late-phase cutaneous reactions showed that 55-75% of the infiltrating RANTES mRNA+ cells were EG2+ eosinophils. Allergen, but not diluent challenge, was also associated with a time-dependent increase in the number of cells showing RANTES immunoreactivity. Of these cells, 55% were identified as eosinophils by morphological criteria. Thus, human eosinophils have the capacity to synthesize, store and secrete physiologically relevant quantities of RANTES, and may therefore be an important source of this chemokine in allergic inflammation.