Regulation of lymphocyte proliferation by eosinophils via chymotrypsin‐like protease activity and adhesion molecule interaction

We investigated the regulatory mechanisms responsible for release of eosinophil cationic protein (ECP) from eosinophils activated by platelet‐activating factor (PAF) and monitored intra‐cellular pH (pH i ) changes using a pH‐sensitive fluorescent probe. We also explored the mechanisms by which eosinophils suppress T‐lymphocyte proliferation induced by phytohaemagglutinin (PHA). In these experiments, a separated culture to investigate the ECP‐mediated pathway and a coculture to identify the adhesion molecules involved in eosinophil‐lymphocyte interactions were employed. Chymostatin (1×10 −6   M ) inhibited ECP release by about 50% via stimulation by PAF or recombinant interleukin 5(rIL‐5) plus IgG. PAF (1×10 −7   M ) raised eosinophil pH i from 6.9 to 7.3 within 20 s and pretreatment of these cells with chymostatin (1×10 −6   M ), but not with leupeptin or E64‐d, completely prevented this increase. Calcium ionophore A23187 (1×10 −7   M ) induced ECP release and raised pH i to within a range similar to that of PAF, however, chymostatin had no effect on either. Chymostatin reversed ECP‐mediated suppression of PHA‐induced T‐lymphocyte proliferation in separated cultures, but not in cocultures. In coculture, eosinophils exhibited the same level of suppression of both CD4 + and CD8 + T‐cell proliferation in response to PHA. Monoclonal antibodies against CD11a, CD18 and CD54, but not CD11b, restored eosinophil suppression of T‐lymphocyte proliferation which was chymostatin‐resistant in coculture. Eosinophils were unable to suppress the proliferative response to lymphocytes to anti‐CD3 stimulation. In conclusion, chymostatin specifically inhibited both the eosinophil pH i increase and ECP release induced by PAF. Eosinophils regulate PHA‐induced T‐lymphocyte proliferation via the ECP‐mediation associated with chymotrypsin‐like protease activity. These cells also control interactions with lymphocyte between adhesion molecules, CD11a, CD18 and CD54.British Journal of Pharmacology (2000) 130 , 1539–1546; doi: 10.1038/sj.bjp.0703473