Human blood monocytes, but not alveolar macrophages, reveal increased CD11b/CD18 expression and adhesion properties upon receptor-dependent activation.

The beta 2 integrin receptor CD11b/CD18 mediates adhesion to the endothelial lining as well as to extracellular matrix components. The present study was undertaken to investigate peripheral human blood monocytes (BMs) and alveolar macrophages (AMs) with respect to quantitative levels of CD11b/CD18 and adhesion properties in relation to the state of activation. BMs and AMs were recruited from healthy subjects. Quantitative analysis of the surface expression of CD11b/CD18 by flow cytometric technique and adhesion properties to albumin-coated surfaces were performed both on resting and N-formyl-methionyl-leucyl-phenylalanine (fMLP)-activated cells. Receptor independent stimuli (phorbol-12-myristate-13-acetate (PMA) and ionomycin) were used in additional experiments. Intracellular stored CD11b/CD18 was evaluated by flow cytometry and immunofluorescence microscopy. The surface expression of CD11b/CD18 on resting BMs increased fivefold (p < 0.01) upon fMLP activation. On resting AMs, the surface expression of CD11b/CD18 was significantly higher (p < 0.01) compared to resting BMs but did not increase further upon activation with fMLP, PMA or ionomycin. In contrast to BMs, no evidence for an additional intracellular pool of CD11b/CD18 was found in AMs. The adherence of resting BMs did not significantly differ from the adherence of resting AMs. After fMLP activation, the adherence of BMs, but not AMs, increased significantly (p < 0.05). Our results indicate that in vivo differentiation of human blood monocytes into alveolar macrophages implies reduced responsiveness to fMLP in terms of CD11b/CD18 upregulation and adhesion properties, and that the lack of upregulation of CD11b/CD18 on alveolar macrophages presumably depends on the absence of an additional intracellular pool.