Two different allelic polymorphisms among the isoforms of human Fcy receptors have been defined: the low-responder (LR)-high-responder (HR) polymorphism of huFcyRIIA expressed on both PMN and monocytes and the NA1-NA2 polymorphism of the neutrophil FcyRIII (huFcyRIIIB). To address the issues of whether the LR-HR polymorphism has a significant impact on Fc'yR-mediated functions in human blood cells and whether any differences in LR-HR might be related to higher Fc'yR-mediated phagocytosis in NAI donors, we examined Fc'yR-specific binding and internalization by donors homozygous for the two huFc-yRIIA alleles. PMN from LR homozygotes showed consistently higher levels of internalization of erythrocytes opsonized with pooled human IgG (E-hIgG). The absence of an LR-HR phagocytic difference with erythrocytes opsonized with either anti-FcyRIIA MAb IV3 or rabbit IgG, as opposed to E-hIgG, suggested that the Fc piece of the opsonin might be important for this LR-HR difference. Accordingly, we studied HR and LR homozygotes with human IgG subclass-specific probes. Both PMN (independent of huFctyRIIIB phenotype) and monocytes from LR donors bound and internalized erythrocytes coated with human IgG2 (EhIgG2) efficiently, whereas phagocytes from HR donors did so poorly. E-hIgG2 internalization was completely abrogated by blockade of the ligand binding site of huFc'yRIIA with IV3 Fab, indicating that huFc'yRIIA is essential for the handling of hIgG2 and that the mechanism of the LR-HR phagocytic difference is at the level of ligand binding to huFcyRIIA. In contrast, the difference in internalization of E-hIgG between NA1 and NA2 homozygous donors was independent of the huFc'yRIIA phenotype and did not manifest differences in ligand binding. Thus, the two known allelic polymorphisms of human Fc'yR have distinct and independent mechanisms for altering receptor function, which may influence host defense and immune complex handling. (J. Clin. Invest. 1992.

Introduction (9). In contrast to these systems, signaling through the T cell receptor upregulates the avidity of CD2 for CD58 (LFA-1) through inside-out signaling requiring specific domains of the CD2 cytoplasmic tail (10,11

Human polymorphonuclear leukocytes (PMN) express two classes of Fc gamma R: Fc gamma RII the 42-kDa receptor with a traditional membrane spanning domain and cytoplasmic tail and Fc gamma RIIIPMN the 50- to 80-kDa receptor with a glycosyl-phatidylinositol membrane anchor expressed on PMN. To explore the capacity of Fc gamma RIIIPMN to generate intracellular signals, we have analyzed the ability of Fab and F(ab')2 anti-Fc gamma R mAb to induce actin filament assembly, a prerequisite for motile behaviors. Multivalent ligation of Fc gamma RIIIPMN, independent of Fc gamma RII, results in an increase in F-actin content that is [Ca2+]i dependent. Multivalent ligation of Fc gamma RII also initiates actin polymerization but uses a [Ca2+]i-independent initial pathway. In addition to providing a mechanism for Fc gamma RIIIPMN triggered effector functions, the increase in F-actin and [Ca2+]i generated by Fc gamma RIIIPMN ligation also serves as a "priming" signal to modify PMN responses to other stimuli. Experiments using erythrocytes specifically coated with anti-Fc gamma RII Fab demonstrate that cross-linking of Fc gamma RIIIPMN with anti-Fc gamma RIII F(ab')2 enhances phagocytosis mediated by Fc gamma RII. Thus, Fc gamma RIIIPMN, a glycosyl-phosphatidylinositol anchored protein, may contribute directly to an intracellular program of actin assembly that may trigger and prime neutrophil effector functions.

Using monoclonal anti-adenosine A1 receptor antibodies that bind the A1 receptor ligand binding site, we demonstrate that A1 receptors are expressed on cultured monocytes and rheumatoid synovial fluid mononuclear phagocytes. This finding is associated with the acquisition of reactivity with selective adenosine A1 receptor agonists and is temporally coordinated with the induction of adenosine A2 receptors on cultured monocytes. In a rapid, concentration-dependent fashion, these two distinct adenosine receptors modulate Fc gamma receptor-mediated phagocytosis, a response critical to the pathogenesis of immune complex diseases. Occupancy of A1 receptors by N6-cyclopentyladenosine (an A1-specific adenosine analogue) or mAb AA1 (an anti-A1 mAb) results in a potent stimulation that is blocked by adenosine receptor antagonists. This A1 receptor-induced enhancement of Fc gamma receptor-mediated phagocytosis is a consequence of preferential augmentation of Fc gamma RI function, suggesting distinct mechanisms for receptor-effector coupling of Fc gamma receptor families. In contrast, ligation of A2 receptors by A2-specific agonists decreases Fc gamma receptor-mediated phagocytosis in cultured monocytes. The opposing effects of adenosine A1 and A2 receptors allow for a concentration-dependent feed-back loop that responds more rapidly than effects elicited by other endogenous modulators. Low concentrations of adenosine are proinflammatory providing enhanced Fc gamma receptor function via A1 receptors, whereas higher concentrations that can occur with tissue damage are anti-inflammatory providing inhibition via A2 receptors. This rapid and potent modulation of Fc gamma receptor-mediated function suggests that adenosine is an important local regulator of the inflammatory response.