Cellular expression of the C5a anaphylatoxin receptor (C5aR): demonstration of C5aR on nonmyeloid cells of the liver and lung.

Abstract

The small-complement C5 activation fragment, C5a, is a potent phlogistic molecule that, on binding to the C5a Receptor (C5aR), mediates contraction of smooth muscle, enhances vascular permeability, and promotes leukocyte functions such as directed chemotaxis, degranulation, mediator release, and production of superoxide anions. Although C5aR expression has traditionally been thought to be limited primarily to myeloid blood cells, including neutrophils, monocytes, macrophages, and eosinophils, we report here that C5aR is expressed by liver and lung cells as well as by cells in several other tissues. By Northern blot analysis, it was determined that mouse liver, baboon liver, human liver, and the human hepatoma-derived cell line HepG2 express a normal size (2.3 kb) C5aR mRNA; in HepG2 cells, the quantity of C5aR mRNA was comparable to that contained in dbcAMP-differentiated U937 cells. HepG2 cells were demonstrated to express the C5aR on their cell surface by flow cytometric and immunofluorescence analyses as well as by 125I-C5a binding assays. The binding data indicated that HepG2 cells express a single class of C5aR with a Kd of 1.18 nM and approximately 28,000 receptors per cell. In vivo expression of C5aR in human liver cells was demonstrated by in situ hybridization and immunohistochemistry analyses. Northern blot analysis of murine and baboon organs shows that, in addition to the liver, other tissues express C5aR mRNA in significant quantities, including the spleen, lung, heart, kidney, and intestine. Moreover, mice treated with LPS show a large increase in C5aR mRNA in all these tissues except the intestine. Immunostaining of human lung tissue demonstrated that bronchial and alveolar epithelial cells, as well as vascular smooth muscle and endothelial cells, also express the C5aR. Collectively, these data indicate that the C5aR is expressed in several different types of cells in liver and lung, and in yet undetermined cell types in spleen, heart, intestine, and kidney. Furthermore, these data suggest that the C5a anaphylatoxin mediates previously unrecognized functions by binding to tissue cells that express the C5aR.