NONIMMUNE PHAGOCYTOSIS OF LIPOSOMES BY RAT ALVEOLAR MACROPHAGES IS ENHANCED BY VITRONECTIN AND IS VITRONECTIN-RECEPTOR MEDIATED

Pulmonary alveolar macrophages (AMs) engulf diverse materials. The mec hanisms allowing AMs to recognize, bind, and phagocytose these materia ls are poorly understood. To test the hypothesis that the adhesive gly coprotein vitronectin (Vn) acts as a nonimmune opsonin, we studied AM- Vn binding and AM phagocytosis of fluorescent liposomes under the foll owing conditions: (I) pretreatment of AMs with Vn, followed by incubat ion of AMs with liposomes containing increased amounts of Vn; (2) inhi bition of phagocytosis by gly-arg-gly-asp-ser (RGD) and gly-pen-gly-ar g-gly-asp-ser-pro-cys-ala (GPen); and (3) antibody blockade of the alp ha(nu)beta(3) vitronectin receptor (VnR). Pretreatment of AMs with 0.1 , 1, and 2 mu M Vn progressively enhanced AM-Vn binding from 23,622 +/ - 3,328 cpm to 40,847 +/- 6,530 cpm, 57,149 +/- 2,789 cpm, and 124,852 +/- 42,930 cpm, respectively (P<0.05). AM pretreatment also increased phagocytosis of Vn-enriched liposomes, but not empty liposomes (20.7 +/- 0.4 liposomes/cell versus 11.5 +/- 0.5 liposomes/cell, P<0.05). Mo reover, increased concentrations of Vn in liposomes progressively incr eased phagocytic activity (3.7 +/- 0.3, 6.5 +/- 0.2, 11.5 +/- 0.5, and 16.5 +/- 0.6 liposomes/cell with 0.01, 0.1, and 1 mu M Vn, respective ly, P<0.05). RGD inhibited Vn-enhanced phagocytosis (8.1 +/- 0.4 lipos omes/cell to 3.4 +/- 0.2, 2.4 +/- 0.4, and 2.2 +/- 0.2 liposomes/cell with 0.02, 0.2, and 2 mh? RGD, respectively, P< 0.05), as did GPen (4. 7 +/- 0.8 liposomes/cell versus control = 10.9 +/- 1.5 liposomes/cell, P<0.05) and anti-VnR antibody (3.3 +/- 0.4 liposomes/cell versus cont rol = 8.9 +/- 1.7 liposomes/cell, P<0.05). We conclude that AMs employ Vn as a nonimmune opsonin to enhance the efficiency of phagocytosis.