Am. J. Respir. Cell Mol. Biol., Vol 17, No. 4, 10 1997, 462-470.
Nonimmune phagocytosis of liposomes by rat alveolar macrophages is enhanced by vitronectin and is vitronectin-receptor mediated
DG Perry, P Wisniowski, GL Daugherty, J Downing and WJ Martin 2nd
Department of Medicine and School of Allied Health Sciences, Indiana University School of Medicine, Indianapolis 46202, USA.
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.
