Enhancement Of Phagocytosis Research Articles

Domain-specific Interactions between Entactin and Neutrophil Integrins:: G2 DOMAIN LIGATION OF INTEGRIN α3β1 AND E DOMAIN LIGATION OF THE LEUKOCYTE RESPONSE INTEGRIN SIGNAL FOR DIFFERENT RESPONSES

Extracellular matrix proteins activate neutrophils to up-regulate many physiologic functions that are necessary at sites of tissue injury. To elucidate the ligand-receptor interactions that mediate these functions, we examined neutrophil activation by the basement membrane protein, entactin. Entactin is structurally and functionally organized into distinct domains; therefore, we utilized glutathione S-transferase -fusion proteins encompassing its four major domains, G1, G2, E, and G3, to assess interactions between entactin and neutrophil integrin receptors. We show that the E domain, which contains the single RGD sequence of entactin, is sufficient for ligation of the beta3-like integrin, leukocyte response integrin, and signaling for chemotaxis. Moreover, the G2 domain signals for stimulation of Fc receptor-mediated phagocytosis via ligation of alpha3beta1. This receptor-ligand interaction was revealed only after stimulation of neutrophil by immune complexes or phorbol esters. Interestingly, the E domain does not enhance phagocytosis, and the G2 domain is not chemotactic. Furthermore, cleavage of entactin with the matrix metalloproteinase, matrilysin, liberates peptides that retain E domain-mediated chemotaxis and G2 domain-mediated enhancement of phagocytosis. These studies indicate that multiple domains of entactin have the ability to ligate individual integrins expressed by neutrophils and to activate distinct functions.

Met-enkephalin modulates stress-induced alterations of the immune response in mice

Overnight restraint stress of mice decreased ConA-driven lymphocyte proliferation, plaque-forming cell response to sheep red blood cells (SRBC), and NK activity in the spleen, but the phagocytic activity was enhanced. Injection of methionine-enkephalin (MENK), 10 mg/kg, IP, 30 min before restraint, abolished these changes (except for the NK activity) and attenuated the stress-induced elevation of glucocorticoids. However, MENK itself affected the immune responses like stress: It decreased NK activity and the PFC response and enhanced phagocytic activity. Contrary to results with stress, MENK had no effect on cell proliferation. The opioid-receptor antagonist naloxone given before restraint reversed the stress-induced enhancement of phagocytosis and the decrease of T-cell proliferation. Alterations of the immune responses induced by restraint stress seem to be mediated by at least two mechanisms: activation of the hypothalamus-pituitary-adrenal (HPA) axis and the secretion of opioid peptides. MENK injected before stress may interfere with either or both mechanisms. T or B lymphocytes seem to be affected by the activation of the HPA axis, and phagocytes by a direct opioid action, whereas NK cells seem to be under the influence of another control mechanism.

Surfactant protein A stimulates phagocytosis of specific pulmonary pathogens by alveolar macrophages.

Surfactant protein A (SP-A) regulates alveolar macrophage function and has been implicated in the mediation of pulmonary host defense. Our goals were to characterize the interaction of SP-A with various pulmonary pathogens, to investigate the mechanism of SP-A-mediated phagocytosis using an assay that distinguishes bound from internalized bacteria by quenching the fluorescence of extracellular bacteria, and to examine further the interactions of SP-A and the structurally homologous protein complement component 1q (C1q) with alveolar macrophages and peripheral blood monocytes. We found that SP-A binds to and increases the phagocytosis of Haemophilus influenzae, Streptococcus pneumoniae, and Group A Streptococcus; SP-A aggregates only H. influenzae. SP-A neither binds to, aggregates, nor stimulates the phagocytosis of Pseudomonas aeruginosa. We have also found that bronchoalveolar lavage stimulates phagocytosis and that this stimulation is reduced by an anti-SP-A antibody. While the enhancement of phagocytosis by SP-A is inhibited in blood monocytes adhered to C1q-coated surfaces, which presumably clusters the C1q receptor on the basal surface of the cell, alveolar macrophages on C1q-coated slides show no significant change in their response to SP-A. In summary, SP-A stimulates the phagocytosis by alveolar macrophages of specific pulmonary pathogens to which it binds, but aggregation is not required for the effect. Additionally, the role of the C1q receptor in the response to SP-A may differ between monocytes and alveolar macrophages.

1,25-dihydroxyvitamin D3 stimulates phagocytosis but suppresses HLA-DR and CD13 antigen expression in human mononuclear phagocytes.

This study investigated the regulatory activity of 1,25-dihydroxyvitamin D3 (1,25-[OH]2D3) on phagocytic cells obtained from normal human peripheral blood. Flow cytometric analysis enabled identification of two discrete populations of cells, one predominantly monocytes ("monocyte" gate) and one containing primarily lymphoid and other cell types ("lymphoid" gate). The monocyte-associated antigens CD13 and CD33 were highly expressed by cells in this monocyte gate and used to monitor this population. Following 5 days of culture, cells in the monocyte gate manifested high phagocytic activity as determined by ingestion of fluorescent carboxylmicrospheres and exhibited high expression of class II HLA-DR products. 1,25-(OH)2D3 profoundly upregulated phagocytic activity while downregulating HLA-DR antigen expression on the cells in the monocyte gate. Moreover, 1,25-(OH)2D3 also reduced cell surface CD13 expression on the cells with low but not high phagocytic activity in this gate. Proportional activities by the 1,24-(OH)2D3 and 24,25-(OH)2D3 metabolites indicated the regulatory effects are likely mediated by the 1,25-(OH)2D3 receptor (VDR). Prostaglandin E2 (PGE2), a known modulator of monocyte/macrophage activity also markedly inhibited HLA-DR expression while enhancing the phagocytic activity of cells in the monocyte gate. In contrast to 1,25-(OH)2D3, PGE2 clearly upregulated CD13 expression in cells with high phagocyte activity. Since indomethacin, an inhibitor of PGE2 synthesis, failed to reverse the 1,25-(OH)2D3 induced inhibitory effect on HLA-DR expression, this effect is apparently not mediated through endogenous PGE2 synthesis. Based on these findings we speculate that 1,25-(OH)2D3 may be capable of acting as both an upregulating agent during natural immunity via the enhancement of phagocytosis by monocyte/macrophage populations and as a "downregulator" during acquired immune responses via an inhibitory effect on MHC class II antigen expression by professional antigen-presenting cells.

Intracellular bioactivity of macrolides

A brief overview is provided of the bioactivity of macrolides against a range of bacterial species. Topics considered include the cellular pharmacokinetics of uptake and efflux of these drugs and the importance of intra- or extracellular and cytoplasmic or granular location on their activity. Emphasis is placed on the importance of synergy between macrolides and host defenses, with drug accumulation producing modification of cellular function, such as enhancement of phagocytosis, and exocytosis of polymorphonuclear neutrophils. Such interaction may explain the activity of such agents against organisms which normally inhibit fusion of phagolysosomes.

Enhancement of phagocytosis by calcitonin gene-related peptide (CGRP) in cultured mouse peritoneal macrophages.

Calcitonin gene-related peptide (CGRP) is widely distributed in sensory neurons and nerve fibers. To clarify the function of CGRP on the immune system, the effect of CGRP on phagocytosis by peritoneal macrophages was examined by means of flow cytofluorometry. CGRP enhanced phagocytosis of latex beads in a dose-dependent manner. Because the phosphodiesterase inhibitor 3-isobutyl, 1-methylxanthine (IBMX) enhanced the CGRP-induced enhancement of phagocytosis, the enhancement might be mediated by cAMP. In the presence of mannan, the phagocytosis was suppressed and the CGRP-induced enhancement was also blocked, suggesting that mannose receptors on macrophages were involved in mediating the phagocytosis of latex beads, and CGRP enhanced the mannose receptor-mediated phagocytosis. The present results indicate that CGRP can modulate the function of macrophages in nerve terminals of sensory neurons during the development and maintenance of inflammation.

Alteration of macrophage functions by cocaine.

Studies elucidating the effects of cocaine on immune cells and immune function are relatively recent. Although immunomodulation by cocaine has been reported by a number of investigators, it is not consistently associated with immunosuppression. A review sum­marized many of the earlier studies of cocaine on the immune system (1). Another study (2) describes a number of immune abnormalities in mice exposed to cocaine including an enhancement of neutrophil phagocytosis, a reduction of T-cell responses to the mitogen phytohemagglutinin, and cytotoxic activity of immune spleen cells. Other reports have described alteration of spleen cell subsets following injection with cocaine (3,4). A number of reports have described a reduction or alteration in the production of certain cytokines (5–7). In a series of publications, Peterson et al. (8–10) have reported an enhancement of HIV replication in cultures of human monocytes via the induction of transforming growth factor β (TGFβ) and tumor necrosis factor (TNFα). An increase in p24, one of the internal viral proteins, was used to quantify viral replication. A recent study (11) reported that Me culture supernatants from cells exposed to cocaine, modulate mesangial cell proliferation in vitro altering TGFβ, as well as IL-6.KeywordsPlaque SizeMouse Hepatitis VirusMesangial Cell ProliferationViral PlaqueReactive Nitrogen IntermediateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Lung surfactant protein A (SP-A) activates a phosphoinositide/calcium signaling pathway in alveolar macrophages

Lung surfactant protein A (SP-A), the main protein component of lung surfactant which lines the alveoli, strongly enhances serum-independent phagocytosis of bacteria by rat alveolar macrophages. We tested if the effect of SP-A is due to interaction with the macrophages or to opsonization of the bacteria. In phagocytosis assays with fluorescein isothiocyanate labeled bacteria, SP-A had no opsonic effect on Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, but enhanced phagocytosis by acting only on the macrophages. We characterized this activation mechanism. With single cell measurements of fura-2 loaded cells we demonstrate that SP-A raises the intracellular free calcium ion concentration 6 to 8 seconds after addition. This calcium mobilization is dose-dependent in that increased SP-A concentrations lead to a higher percentage of responding cells. Additionally, SP-A leads to a dose-dependent and transient generation of inositol 1,4.5-trisphosphate. Release of intracellular stored calcium by SP-A is a prerequisite for its stimulatory effect on phagocytosis, since SP-A-induced enhancement of phagocytosis can be impaired by prior addition of thapsigargin, a Ca(2+)-ATPase inhibitor that leads to depletion of intracellular calcium stores. We conclude that SP-A activates a phosphoinositide/calcium signaling pathway in alveolar macrophages leading to enhanced serum-independent phagocytosis of bacteria.

Mannose binding protein (MBP) enhances mononuclear phagocyte function via a receptor that contains the 126,000 M r component of the Clq receptor

Mannose-binding protein (MBP), C1q, the recognition component of the classical complement pathway, and pulmonary surfactant protein A (SP-A) are members of a family of molecules containing a collagen-like sequence contiguous with a noncollagen-like sequence, and usually having the properties of a lectin. C1q and SP-A have been shown to enhance monocyte FcR- and CR1-mediated phagocytosis, suggesting that the common structural features of the collagen-like domains may provide a basis forthis immunologically important function. Results presented here demonstrate that MBP also enhanced FcR-mediated phagocytosis by both monocytes and macrophages, and stimulated CR1-mediated phagocytosis in human culture-derived macrophages and in phorbol ester-activated monocytes. Furthermore, a monoclonal antibody that recognizes a 126,000 M r cell surface protein and inhibits C1q-enhanced phagocytosis, inhibited the MBP-mediated enhancement of phagocytosis. Thus, the receptors that mediate the enhancement of phagocytosis by MBP and C1q share at least one critical functional component, the 126,000 M r C1qR p.

Enhancement of phagocytosis in mouse macrophages by pituitary adenylate cyclase activating polypeptide (PACAP) and related peptides

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) and related peptides on phagocytosis of fluorescent latex beads by mouse peritoneal macrophages were examined using flow cytometry (FCM). PACAP38, PACAP27 and vasoactive intestinal peptide (VIP) enhanced phagocytosis in a dose-dependent manner. Relative potencies of related peptides at a concentration of 10−6M were PACAP38 > PACAP27 > VIP > secretin > glucagon > (peptide with NH2-terminal histidine and COOH-terminal methionine amide, in short PHM). PACAP(6-38) was as effective as PACAP38. PACAP(6-27) enhanced phagocytosis more effectively than did PACAP27. PACAP(28-38) slightly enhanced phagocytosis. The present result suggest that PACAP38 is one of the mediators that the nervous system uses to modulate the immune system.

Beta-Endorphin enhances phagocytosis of latex particles in mouse peritoneal macrophages.

The effects of beta-endorphin (beta End) on phagocytosis in peritoneal macrophages were examined by using flow cytometry (FCM). Beta End enhanced phagocytosis in a dose-dependent manner. Leucine-enkephalin (Leu-Enk), methionine-enkephalin (Met-Enk), alpha-endorphin (alpha End), gamma-endorphin (gamma End), alpha End (18-31) and beta End (28-31) had no such activity. Beta End (1-27) and beta End (6-31) enhanced phagocytosis less effectively than beta End did. Naloxone did not inhibit the enhancement of phagocytosis induced by beta End. Unstimulated control phagocytosis was partially suppressed in Ca2(+)-free EGTA-containing solution and even in this solution beta End enhanced phagocytosis. However, the enhancement was suppressed in the solution containing BAPTA-AM. The present study showed that beta End enhanced extracellular Ca2+ ([Ca2+]o)-dependent and -independent phagocytosis and that the enhancement is largely dependent on intracellular Ca2+ ([Ca2+]i). These results support the contention that beta End is one of the mediators that modulates the immune system.

Enhancement of phagocytosis by dynorphin A in mouse peritoneal macrophages

The effects of the opioid peptide dynorphin A (DynA) on phagocytosis in peritoneal macrophages was examined by flow cytometry (FCM). DynA enhanced phagocytosis in a dose-dependent manner. Leucine-enkephalin (Leu-Enk), methionine-enkephalin (Met-Enk), β-neo-endorphin (βNeo-End), DynA(9–17) and DynA(13–17) had no such activity, α -Neo-endorphin (α Neo-End), dynorphin B (DynB), DynA(l–13) and DynA(6–17) enhanced phagocytosis less effectively than DynA. Naloxone did not inhibit the enhancement of phagocytosis induced by DynA. Unstimulated control phagocytosis was partially suppressed in Ca 2+-free EGTA-containing solution and even in this solution DynA enhanced phagocytosis. However, the enhancement by DynA was suppressed in EGTA- and BAPTA-AM-containing Ca 2+-free solution. The present study showed that enhancement of phagocytosis by DynA was independent of extracellular Ca 2+ ([Ca 2+] o) and dependent on intracellular Ca 2+ ([Ca 2+] i). The present results support DynA being one of the mediators from the nervous system that modulates the immune system.

Stimulation of neutrophil phagocytosis ofAspergillus fumigatusconidia by interleukin-8 andN-formylmethionyl-leucylphenylalanine

The effect of the chemoattractants, interleukin-8 (IL-8) and N-formylmethionyl-leucylphenylalanine (fmlp) on neutrophil phagocytosis of serum-opsonised Aspergillus fumigatus conidia were studied. Pre-exposure of neutrophil monolayers to IL-8 (10-100 ng ml-1) or fmlp (10(-6)M) for 20 min increased phagocytosis from 15% to 35%. Furthermore, there was a significant increase in the numbers of conidia ingested by active neutrophils. The chemoattractants did not enhance phagocytosis of conidia when added contemporaneously to neutrophil monolayers. The ability of each chemoattractant to act as a primer for the other was investigated. It was found that fmlp could prime for IL-8 and fmlp enhancement of phagocytosis, but IL-8 could not. An increase from 15% to 55% in phagocytosis was seen at a pre-exposure concentration of 10(-5) M fmlp followed by an incubation period of 30 min in the presence of 20 ng ml-1 IL-8. There was no significant increase in the average number of internalized conidia under these conditions. The response of human neutrophils to priming stimuli may have important implications in the pathogenesis of aspergillosis.

Phagocytosis of microspheres with modified surfaces

A series of hydrogel microspheres having different polystyrene (PSt) contents was prepared by the precipitation polymerization of acrylamide (AAm) and comonomers followed by the seeded polymerization of styrene (St). The resulting AAm-St microspheres had different morphologies and surface properties dependent on the amount of St charged in the seeded polymerization. These AAm-St microspheres were contacted with granulocytes in phosphatebuffered saline (PBS) or plasma at 37 °C to clarify the effect of surface structure of the microspheres and of blood components on phagocytosis. Phagocytosis of the microspheres by granulocytes in PBS was enhanced with increasing PSt content in the microspheres. This suggested that the hydrophobicity of the microspheres is one of the major factors deciding the extent of phagocytosis in PBS. On the other hand, the extent of phagocytosis in plasma depended on the PSt content in the microspheres in an unexpected manner. The microspheres with a small content of PSt were less susceptible to phagocytosis than the microspheres containing no St. Phagocytosis in inactivated serum was slight, regardless of the type of microspheres. These results suggested that complement and the complement-immunogloblin G complex are responsible for the enhancement of phagocytosis of hydrophilic and hydrophobic microspheres, respectively.

Enhancement of lipid phagocytosis by cloned cells derived from rat malignant fibrous histiocytoma under hyperlipemic conditions.

The histogenesis of malignant fibrous histiocytoma (MFH) was studied by observing the influence of hyperlipemic (HL) conditions on the behavior of cloned MT-8 cells with an undifferentiated mesenchymal character and MT-9 cells with histiocytic and fibroblastic features, both derived from a spontaneous rat MFH. The cells were grown in medium containing 10% normal rat serum (CSM) or 10% HL rat serum (HSM) for 1, 3, 8 and 24 h. The cells grown in HSM showed a time-dependent, significant increase in numbers of cytoplasmic lipid droplets per cell and cells positive for acid phosphatase (ACP) and non-specific esterase (NSE) compared with cells grown in CSM. The number of cells positive for two different anti-rat monocyte/macrophage monoclonal antibodies (ED-1 and ED-2) also increased significantly in HSM. This histiocytic nature was confirmed by electron microscopy. Tumors were induced by inoculating MT-8 and MT-9 cells into rats on a standard diet and rats on a HL diet. MT-8 and MT-9 tumor cells in HL rats phagocytized lipid droplets and showed stronger positive reactions for ACP and NSE compared with tumors in control rats. These results imply that xanthomatous cells in MFH may be a phenotype of MFH cells expressing an enhanced histiocytic nature. MFH cells seem to alter their behavior depending on cultural or microenvironmental conditions.

Different signaling pathways for CD18-mediated adhesion and Fc-mediated phagocytosis. Response of neutrophils to LPS.

Abstract The regulation of CD11b/CD18 adhesive and phagocytic functions on human polymorphonuclear leukocytes (PMN) in response to LPS was examined. Adhesion of PMN to surfaces coated with LPS had little or no effect on the cells, but pretreating the LPS-coated surfaces with either diluted serum or LPS-binding protein strongly enhanced their ability to bind C3bi-coated E (EC3bi), a ligand for CD11b/CD18. LPS-binding protein is known to enable responses of cells to LPS by facilitating binding of LPS to CD14. Consistent with this, we found that preformed complexes of LPS with soluble rCD14 stimulated binding of ligand by CD11b/CD18 in a concentration-dependent manner. Known agonists that stimulate CD11b/CD18 binding activity on PMN all cause simultaneous enhancement of Fc-mediated phagocytosis. However, LPS presented in complex with either serum proteins or CD14 failed to stimulate the ingestion of ElgG by PMN. The number of FcRs and their ability to bind ligand were not affected by treatment with LPS, nor were they compromised in their ability to respond to other agonists. These results suggest that LPS generates intracellular signals that alter the ability of CD11b/CD18 to bind ligand, but this alteration is not sufficient to promote phagocytosis of IgG-coated particle. This conclusion was confirmed by showing that PMN treated with LPS and serum produced a lipid with the properties of integrin-modulating factor 1: acetone extracts of these cells stimulated CD11b/CD18 adhesive capacity on PMN. However, the lipid did not enhance Fc-mediated phagocytosis. These studies suggest that CD14 affects CD11b/CD18 function by inducing the synthesis of a lipid such as IMF-1, and that this lipid affects only the binding activity, not the phagocytosis-promoting capacity of CD11b/CD18.

Antibiotics enhance binding of lipid A-specific murine monoclonal antibody E5 to Gram-negative bacteria

Cross-protection by anti-lipid A antibodies may be mediated by contribution to host defence mechanisms such as enhancement of bactericidal activity and phagocytosis. The lipid A-specific murine monoclonal antibody (mAb) E5 was evaluated in vitro for its ability to enhance antibacterial effects in concert with antibiotics in order to investigate underlying mechanisms for the proposed in vivo efficacy. The effect of antibiotic exposure of several E. coli strains on binding by mAb E5 was examined in solid phase enzyme-linked immunoassay (ELISA) and in complement activation and phagocytosis assays. MAb E5 binds scarcely to live E. coli, both encapsulated and unencapsulated, but exposure to subinhibitory concentrations of aztreonam and ceftriaxone led to enhanced binding of mAb E5 compared to mock treated strains. Enhanced binding of mAb E5 to aztreonam-treated E. coli O111:B4 or O7K1 resulted in a modest increase in complement consumption but complement-mediated phagocytosis by human polymorphonuclear leukocytes (PMN) of these complexes was not affected. It was concluded that exposure of E. coli to antibiotics induced specific alterations in the bacteria, resulting in accessbility of epitopes recognized by mAb E5. Enhanced binding was found to support complement-mediated host defense mechanisms and might contribute to better protection in a joint action of antibiotics and antibodies.

A Flow Cytometric Assay Reveals an Enhancement of Phagocytosis by Platelet Activating Factor in Murine Peritoneal Macrophages

Phagocytosis of fluorescein isothiocyanate-labeled latex beads by peritoneal macrophages from thioglycollate-stimulated mice was examined in vitro by means of flow cytometry. This assay revealed that platelet activating factor (PAF), a known inflammatory mediator, enhanced the phagocytosis in a dose-dependent manner. The enhancement was completely suppressed by a PAF antagonist, Y-24180, and partly suppressed by another antagonist, CV-3988. The phagocytosis of both nonstimulated and PAF-stimulated macrophages was suppressed in Ca2+-free ethylene glycol bis(β-aminoethyl ether) N, N′ -tetraacetic acid-containing solution, but the phagocytosis was enhanced by PAF even in this solution. These results suggest that phagocytosis by macrophages is enhanced by PAF in allergic and inflammatory reactions.

Effect of Ceftriaxone on the Phagocytosis and Intracellular Killing ofStaphylococcus aureusby Human Macrophages

The efficacy of an antimicrobial agent in the treatment of infections depends upon the interactions of bacteria, antibiotic and phagocytes. The influence of ceftriaxone on the phagocytic and bactericidal activity of human macrophages in vitro and ex vivo was investigated. At concentrations one-half the minimum inhibitory concentration (1/2 x MIC) the antibiotic caused in vitro a significant enhancement of phagocytosis and a reduction in the survival of intracellular Staphylococcus aureus. The distinction between any effect of ceftriaxone on the staphylococci and the macrophages was made by exposure of each of them to the antibiotic before they were incubated together. The results suggest that ceftriaxone may have a direct positive action on macrophages, possibly by interfering with the cellular membrane functions and hence enhancing engulfment of bacteria. The ex vivo data seem to corroborate this hypothesis.

Cell-surface protein identified on phagocytic cells modulates the C1q-mediated enhancement of phagocytosis.

C1q, a subunit of the first component of the classical C pathway, binds to specific cells of the immune system, triggering a variety of cellular responses. To identify the functional C1qR on phagocytic cells, mAbs were generated by immunization with either C1q-binding proteins isolated from U937 cells or intact U937 cells. Immunoprecipitation followed by Western blot analysis demonstrated that three mAbs, designated R139 (IgG2b), R3 (IgM), and U40.3 (IgG1), recognize the same 100,000 M(r) protein (126,000 M(r) under reducing conditions). These mAbs also co-immunoprecipitate CD43 from detergent extracts of U937, consistent with the possibility that this C1qR is a multi-subunit structure. Two Abs, R3 and R139, but not U40.3, consistently inhibited the enhancement of phagocytosis by monocytes adhered to either C1q or the collagen-like fragment of C1q (C1q-CLF). Interestingly, binding inhibition studies demonstrated that neither R139 nor U40.3 blocked the binding of [125I]C1q-CLF to U937 cells, whereas R3 did inhibit 35 to 45% of the binding of [125I]C1q-CLF to these cells. Thus, the three mAbs recognize distinct epitopes of a 100,000 M(r) polypeptide that is a component of the monocyte C1qR that modulates phagocytosis. All three mAbs recognize the extracellular domain of the molecule on neutrophils, monocytes, and U937 cells, but were not reactive with CEM or RAJI cells, T and B lymphoblastoid cell lines, respectively. Furthermore, none of these three mAbs inhibited or mimicked the C1q-mediated stimulation of superoxide production by neutrophils, suggesting that the C1qR that mediates the enhancement of phagocytosis differs in at least some critical parameter from the C1qR that mediates superoxide generation by the neutrophil.