Stimulation of early eosinophil progenitors by a heat stable alveolar macrophage product from ovalbumin-sensitized and non-sensitized guinea pigs

Background Alveolar macrophages (AM) may participate in brochopulmonary hyperreactivity by secreting cytokines that recruit mature eosinophils, or induce eosinophil production from recruited circulating progenitors. Objective To define whether AM products can contribute to lung eosinophil production in immunized guinea pigs (GP), by analysing the effect of AM culture supernatants (AM-SN) on in vitro eosinophilopoiesis. 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 eosinophil production in liquid culture and supported formation of eosinophil-bearing mixed colonies, by acting on progenitors depleted of mature eosinophils. 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 SN yielded a peak of apparent m.w. 43 kDa, active on both liquid and semi-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 induced its production by AM even without LPS challenge. Conclusions The lack of T lymphocytes among factor-producing AM, the properties of the active material, the inability of GM-CSF to reproduce these effects, and the failure of MoAbs to GM-CSF and to IL-3 to neutralize the activity indicate it is not due to the major eosinopoietic factors GM-CSF, IL-3 or IL-5.