A B S T R A C T By comparing natural immunity to Aspergillus fumigatus (AF) in vivo with the action of human or mouse phagocytes against AF in vitro, we delineated two sequential lines of defense against AF. The first line of defense was formed by macrophages and directed against spores. Macrophages prevented germination and killed spores in vitro and rapidly eradicated conidia in vivo, even in neutropenic and athymic mice. The second was the neutrophilic granulocyte (PMN), which protected against the hyphal form of AF. Human and mouse PMN killed mycelia in vitro. Normal, but not neutropenic mice, stopped hyphal growth, and eradicated mycelia. Either line of defense acting alone protected mice from high challenge doses. Natural immunity collapsed only when both the reticuloendothelial system and PMN were impaired. These findings are in keeping with the clinical observation that high doses of cortisone and neutropenia are the main risk factors for invasive aspergillosis. Cortisone inhibited the conidiacidal activity of mouse macrophages in vivo and of human or mouse mononuclear phagocytes in vitro. Cortisone damaged this first line of defense directly and not through the influence of T lymphocytes or other systems modifying macrophage function as shown in athymic mice and in vitro. In addition, daily high doses of cortisone in mice reduced the mobilization of PMN so that the second line of defense was also impaired. Thus, cortisone can break down natural resistance on its own. Myelosuppression rendered mice susceptible only when the first line of defense was overpowered by high chal-

Nitric oxide synthase (NOS) has received immense interest as an antimicrobial and antitumoral effector system of mononuclear phagocytes from rodents. Because there is increasing doubt that an analogous system exists in human macrophages, NOS was reexamined in these cells. Under tightly controlled conditions, with murine macrophages as positive controls, human macrophages failed to secrete nitric oxide «0.1 #Lmol/106 cells/24 h), even after activation with endotoxin, intcrferon-v, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor-a, bacteria, or proliferating lymphocytes. The discrepancy between murine and human macrophages depended on neither the anatomic source (blood, peritoneum), the agent used for activation, nor the duration of activation. NOS activity was paralleled by metabolization of L-arginine to L-citrulline. Exogenous tetrahydrobiopterin, an essential cofactor of NOS not synthesized by human macrophages, did not support NOS activity in human macrophages. Also, no NOS activity was found in cellular subfractions of human macrophages. It appears that in humans, the inducible high-output NOS is not conserved as an antimicrobial system of macrophages.Nitric oxide (NO) has recently been brought into focus as an antimicrobial and antitumoral effector system ofmononuclear phagocytes with activity against fungi [1,2], bacteria [3,4], parasites [5][6][7][8], and tumor cells [9][10][11][12][13][14][15]. While there is general agreement that phagocytes from mice and rats synthesize abundant NO from L-arginine [I, 7, 16-19], demonstration of high-output NO synthase (NOS) activity in human mononuclear phagocytes remains controversial. On one hand, it has been proposed that the antimicrobial activity of human blood-derived macrophages, seen after prolonged activation against Mycobacterium aviuni-Mycobacteriurn intracellulare [3] or Trypanosoma cruzii [20], depends on NO. On the other hand, human peritoneal, alveolar, and bloodderived macrophages have not been found to secrete substantial amounts of NO [21][22][23][24], even after treatment with endotoxin (lipopolysaccharide, LPS) and interferon-v (IFN-'Y). Also nitrite, a descendant of NO that is unstable under physiologic conditions, was not detectable in supernatants from human macrophages stimulated with LPS and IFN-')" [23][24][25]. Furthermore, alveolar and peritoneal macrophages have not been found to metabolize appreciable amounts of L-arginine, the substrate of NOS [21].Because of the significance attributed to the effector function of NO produced by mouse and rat macrophages [26][27][28]

CD163 mediates the internalization of hemoglobin-haptoglobin (Hb-Hp) complexes by macrophages. Because Hp binding capacity is exhausted during severe hemolysis, an Hp-independent Hb-clearance pathway is presumed to exist. We demonstrate that Hb interacts efficiently with CD163 in the absence of Hp. Not only is Hb internalized into an endosomal compartment by CD163 as a result of active receptordependent endocytosis; it also inhibits the uptake of Hb-Hp complexes, suggesting a common receptor-binding site. Free Hb further induces heme oxygenase mRNA expression in CD163 ؉ HEK293 cells, but not in CD163 ؊ cells. Additional evidence for Hp-independent Hb-CD163 interaction is provided by the demonstration that CD163 mediates the uptake of ␣␣-DBBF crosslinked Hb, a chemically modified Hb that forms minimal Hp complexes. Moreover, certain modifications to Hb, such as polymerization or the attachment of specific functional groups (3 lysyl residues) to the ␤-Cys93 can reduce or enhance this pathway of uptake. In human macrophages, Hp-complex formation critically enhances Hb uptake at low (1 g/mL), but not at high (greater than 100 g/mL), ligand concentrations, lending support for a concentrationdependent biphasic model of macrophage Hb-clearance. These results identify CD163 as a scavenger receptor for native Hb and small-molecular-weight Hb-based blood substitutes after Hp depletion. IntroductionHeme, which is mainly derived from hemoglobin (Hb), is a strong oxidant and has potent pro-inflammatory properties. These properties become apparent if the intricate intra-erythrocytic compartmentalization of heme is compromised after the destruction of erythrocytes. [1][2][3][4] Large quantities of free hemoglobin can be found in the circulation of patients who have undergone transfusion with cell-free hemoglobin products as a blood substitute. 5 Macrophages are the primary scavengers of Hb after systemic hemolysis and during wound healing. These cells also play a key role in the clearance of exogenously administered blood substitutes. 6 CD163 is a member of the cysteine-rich scavenger receptor family and is exclusively expressed by cells of monocyte/ macrophage lineage. 7 Resident tissue macrophages contain the highest levels of CD163, most notably Kupffer cells in the liver and macrophages within the bone marrow and spleen red pulp. [8][9][10] To date, the Hb-haptoglobin (Hp) complex is the only known ligand of CD163, 11,12 and neither Hp alone nor free Hb has been found to display high-affinity binding to the receptor. Because the Hb-Hp complex binds to CD163 with high affinity and the receptor system has a high endocytotic capacity, CD163 is thought to mediate the clearance of Hb-Hp complexes from the blood. 13 Several lines of evidence indicate that CD163 plays a key role in the anti-inflammatory and wound-healing process. First, there is a high level of CD163 expression by macrophages during the down-regulatory phase of inflammatory reactions. 8,14 Second, CD163 expression is strongly induced by glucocorticoids 15,16 and...

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