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 mechanisms allowing AMs to recognize, bind, and phagocytose these materials are poorly understood. To test the hypothesis that the adhesive glycoprotein vitronectin (Vn) acts as a nonimmune opsonin, we studied AM-Vn binding and AM phagocytosis of fluorescent liposomes under the following conditions: (1) pretreatment of AMs with Vn, followed by incubation of AMs with liposomes containing increased amounts of Vn; (2) inhibition of phagocytosis by gly-arg-gly-asp-ser (RGD) and gly-pen-gly-arg-gly-asp-ser-pro-cys-ala (GPen); and (3) antibody blockade of the alpha(v)beta3 vitronectin receptor (VnR). Pretreatment of AMs with 0.1, 1, and 2 microM 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). Moreover, increased concentrations of Vn in liposomes progressively increased 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 microM Vn, respectively, P < 0.05). RGD inhibited Vn-enhanced phagocytosis (8.1 +/- 0.4 liposomes/cell to 3.4 +/- 0.2, 2.4 +/- 0.4, and 2.2 +/- 0.2 liposomes/cell with 0.02, 0.2, and 2 mM 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 control = 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.
Perry
D G
DG
Department of Medicine and School of Allied Health Sciences, Indiana University School of Medicine, Indianapolis 46202, USA.
Wisniowski
P
P
Daugherty
G L
GL
Downing
J
J
Martin
W J
WJ
D000818:Animals; D005260:Female; D008081:Liposomes; D016676:Macrophages, Alveolar / Q000502:physiology*; D009895:Opsonin Proteins; D010587:Phagocytosis; D011869:Radioligand Assay; D051381:Rats; D017207:Rats, Sprague-Dawley; D019030:Receptors, Vitronectin / Q000378:metabolism*; D019096:Vitronectin / Q000378:metabolism*
D008081:Liposomes; D009895:Opsonin Proteins; D019030:Receptors, Vitronectin; D019096:Vitronectin