STIMULATION OF EARLY EOSINOPHIL PROGENITORS BY A HEAT-STABLE ALVEOLARMACROPHAGE PRODUCT FROM OVALBUMIN-SENSITIZED AND NONSENSITIZED GUINEA-PIGS

Background Alveolar macrophages (AM) may participate in brochopulmonar y hyperreactivity by secreting cytokines that recruit mature eosinophi ls, or induce eosinophil production from recruited circulating progeni tors. Objective To define whether AM products can contribute to lung e osinophil production in immunized guinea pigs (GP), by analysing the e ffect of AM culture supernatants (AM-SN) on in vitro eosinophilopoiesi s. Methods Liquid and semi-solid bone marrow (BM) cultures were seeded with SN from 95% pure AM exposed to LPS. Results AM-SN increased very significantly the long-term viability, cell proliferation and eosinop hil production in liquid culture and supported formation of eosinophil -bearing mixed colonies, by acting on progenitors depleted of mature e osinophils. The effect on eosinophil production was not duplicated by natural or recombinant sources of GM-CSF (which nevertheless supported GM colony formation by GP BM), not by rhIL-8 (which was active on GP cells) and was not due to residual LPS. FPLC separation of active AM S N yielded a peak of apparent m.w. 43 kDa, active on both liquid and se mi-solid cultures. The active moiety was heat- and trypsin-resistant, Neutralizing monoclonal antibodies to hGM-CSF, mGM-CSF, hIL-3 and mIL- 3 failed to deplete the activity in AM-SN. Ovalbumin immunization indu ced its production by AM even without LPS challenge. Conclusions The l ack of T lymphocytes among factor-producing AM, the properties of the active material, the inability of GM-CSF to reproduce these effects, a nd the failure of MoAbs to GM-CSF and to IL-3 to neutralize the activi ty indicate it is not due to the major eosinopoietic factors GM-CSF, I L-3 or IL-5.