Human exposure to Pneumocystis carinii is common but, in the absence of acquired or genetic dysfunction of either cellular or humoral immunity, exposure rarely leads to illness. Although alveolar macrophages can degrade P. carinii, macrophage receptors involved in P. carinii recognition have not been clearly defined. Characterization of a predominant surface glycoprotein of the high mannose type led us to investigate the role of the macrophage mannose receptor in this process. We report here that binding and uptake of cultured rat P. carinii by human and rat alveolar macrophages is reduced by 90% in the presence of competitive inhibitors of mannose receptor activity and by adherence of alveolar macrophages to mannan-coated surfaces. Further, only those COS cells transfected with the human macrophage mannose receptor complementary DNA that express surface mannose receptors bind and ingest P. carinii. These studies establish that the macrophage mannose receptor is sufficient for uptake of P. carinii and emphasize the role of the alveolar macrophage in first-line host defence against P. carinii.

SummaryThe macrophage mannose receptor is an integral membrane protein expressed on the surface of tissue macrophages . After ligation ofmannose-rich glycoconjugates or pathogens, the receptor mediates endorytosis and phagocytosis of the bound ligands by macrophages. The cDNAderived primary structure of the mannose receptor predicts a cysteine-rich NH2-terminal domain, followed by a fibronectin type II region. The remainder of the ectodomain is comprised of eight carbohydrate recognition-like domains, followed by a transmembrane region, and a cytoplasmic tail. Transfection of the mannose receptor cDNA into Cos-I cells is necessary for receptor-mediated endorytosis of mannose-rich glycoconjugate as well as phagocytosis of yeasts. Deletion of the cytoplasmic tail results in a mutant receptor that is able to bind but not ingest the ligated pathogens, suggesting that the signal for phagocytosis is contained in the cytoplasmic tail.P lasma clearance and cell uptake experiments (1) indicated the existence ofa receptor that bound glycoproteins bearing high mannose chains. Receptor activity was especially evident in the liver and spleens, although most other tissues were also positive. Subsequent experiments have established that the mannose receptor is expressed on the cell surface of tissue macrophages which reside in a wide anatomical distribution throughout most organs (2) . Although originally defined by its ability to mediate endorytosis ofmannosylated or fucosylated glycoproteins, the mannose receptor's predominant role appears to be in host defense. The receptor engages yeasts (3) and parasites (4) directly resulting in the engulfment of these particles and the release ofbiologically active secretory products like reactive oxygen intermediates (5), arachidonate metabolites (6), neutral proteinases (7), and monokines like IM and tumor necrosis factor (8) . As circulating monocytes do not express the mannose receptor on their cell surface (9), the functional role for the mannose receptor appears to be on tissue macrophages, which in addition to their localization in the reticular endothelial system line the alveolus and form a reticular network beneath epithelial surfaces in the skin, the gastrointestinal tract, kidney, and placenta (10). These sites are the portals of maximum antigen load and underscore the role for the mannose receptor in first line host defense.Recent experiments suggest that a circulating hepatocytederived serum mannose-binding protein is the functional serum equivalent of the tissue macrophage mannose receptor. The human mannose-binding protein (MBP)l is an acute-phase reactant (11) that binds certain viruses (12) and other mannose rich pathogens (13). Mannose-binding proteins after engaging organisms mediate attachment, uptake, and killing of opsonized bacteria by circulating phagocytes that do not express the mannose receptor (13). Underlying structural similarity between the MBP and the mannose receptor, thereby explaining their functional equivalence, was suggested by the discovery that h...

SummaryPhagocytosis has traditionally been viewed as a specialized function of myeloid and monocytic cells. The mannose receptor (MR) is an opsonin-independent phagocytic receptor expressed on tissue macrophages. When human MR cDNA is transfected into Cos cells, these usually nonphagocytic cells express cell surface MR and bind and ingest MR ligands such as zymosan, yeast, and Pneumocystis carinii. Expression of cDNA for Fc~RI (CD64), the high-affinity Fc receptor, in Cos cells confers binding but barely detectable phagocytosis of antibody-opsonized erythrocytes (EA). We report here that chimeric receptors containing the ligand-binding ectodomain of the Fc receptor and the transmembrane and cytoplasmic domains of the MR ingest bound EA very efficiently, whereas chimeras with the Fc receptor ecto-and transmembrane domains and the MR tail, or the Fc receptor ecto-and cytoplasmic domains and the MR transmembrane region, are significantly less phagocytic. All of the chimeric receptors bind ligand with equal avidity, but gain of functional phagocytosis is only conferred by the MR transmembrane and cytoplasmic domains. Endocytosis of monomeric immunoglobulin G by chimeric receptors demonstrates a similar pattern, with optimal uptake by the chimera containing both tail and transmembrane regions from the MR. The chimeric receptors with only the transmembrane or the cytoplasmic domain contributed by the MR were less efficient. Site-directed mutagenesis of the single tyrosine residue in the cytoplasmic tail (which is present in a motif homologous to an endocytosis consensus motif in the LDL receptor cytoplasmic tail [Chen, W.-J., J. L. Goldstein, and M. S. Brown. 1990. J. Biol. Chem. 265:3116]) reduces the efficiency of phagocytosis and endocytosis to a similar extent.