Activation of granulocyte cytotoxic function by purified mouse colony-stimulating factors

Highly purified mouse colony-stimulating factors (CSF) were tested for their effect on neutrophil cytotoxic function in a homologous antibody-dependent cell-mediated cytotoxicity (ADCC) assay in which TNP-coupled mouse thymoma cells coated with mouse anti-TNP antibodies were used as targets, and purified normal mouse bone marrow neutrophils or induced peritoneal neutrophils were used as effector cells. Biochemically pure granulocyte-macrophage (GM)- and granulocyte (G)-CSF enhanced the cytotoxic activity of neutrophils obtained from both sources, allowing them to kill target cells at low antibody concentrations. Furthermore, GM- and G-CSF showed an additive effect, suggesting either the presence of separate receptors for GM- and G-CSF or of separate subsets of neutrophils. Induced peritoneal neutrophils showed a higher level of basal cytotoxic activity than did bone marrow neutrophils, suggesting neutrophil activation in vivo, but both reached similar levels of cytotoxicity upon maximal stimulation with CSF. In addition, CSF was found to be cross-reactive between mouse and human species in their enhancement of neutrophil cytotoxicity. By testing purified mouse CSF on human neutrophils, it could be shown that G-CSF and GM-CSF are functionally distinct molecules, because only G-CSF enhanced ADCC by human neutrophils. These experiments show that the purified factors that control the production of neutrophils by progenitor cells in vitro also activate differentiated neutrophils to carry out their cytotoxic activity in a more effective manner.     

Modulation of granulocyte colony-stimulating factor receptors on murine peritoneal exudate macrophages by tumor necrosis factor-alpha.

Modulation of granulocyte CSF (G-CSF) receptors on murine peritoneal exudate macrophages (PEM) by various cytokines was investigated. At 4 degrees C, 125I-G-CSF receptor binding on PEM reached a plateau after 6 h and was specifically competed by unlabeled human rG-CSF but not by other cytokines, including human rG-CSF-1, murine recombinant granulocyte-macrophage CSF, murine rIFN-gamma, human rIL-1 beta, and murine rTNF-alpha. 125I-G-CSF bound to PEM has a half-life of 30 min at 37 degrees C. Preincubation of PEM with murine rTNF, murine recombinant granulocyte-macrophage CSF, CSF-1, or G-CSF for 30 min at 37 degrees C resulted in partial reduction of 125I-G-CSF binding capacity, whereas IL-1 or IFN-gamma did not inhibit G-CSF binding. Further studies indicated that reduction of G-CSF binding caused by TNF was a dose- and time-dependent process and did not require FCS. The reduction was transient, and receptor binding was recovered by incubation at 37 degrees C for 8 h. The recovery of G-CSF binding was inhibited in the presence of cycloheximide. In addition, G-CSF binding studies suggested that the TNF-induced decrease in G-CSF binding to PEM was probably due to a reduction in receptor number rather than receptor affinity. Modulation of G-CSFR by TNF was also observed on nonelicited macrophages from various strains of mice. Our results demonstrate a physiologic response of G-CSFR on macrophages that is modulated by TNF. This phenomenon may play an important, as yet unknown, role in the macrophage inflammatory response.     

Hierarchical down-modulation of hemopoietic growth factor receptors

Granulocytes and macrophages can be produced in vitro when progenitor cells from mouse bone marrow are stimulated by any of four distinct colony stimulating factors, Multi-CSF (IL-3), GM-CSF, G-CSF, and M-CSF (CSF-1). At 0 degrees C the four CSFs do not cross-compete for binding to bone marrow cells, indicating that each has a specific cell surface receptor. However, at 21 degrees C or 37 degrees C, Multi-CSF inhibits binding of the other three CSFs and GM-CSF inhibits binding of G-CSF and M-CSF. Rather than competing directly for receptor binding, the binding of Multi-CSF, GM-CSF, or G-CSF to their own receptor induces the down-modulation (and thus activation) of other CSF receptors at 37 degrees C. The pattern and potency of down-modulation activity exhibited by each type of CSF parallels the pattern and potency of its biological activity. We propose a model in which the biological interactions of the four CSFs are explained by their ability to down-modulate and activate lineage-specific receptors.     

Granulocyte colony-stimulating factor modulation of cytokine receptors on murine bone marrow cells. In vivo and in vitro studies.

Human recombinant granulocyte CSF (G-CSF) modulation of cytokine receptors on murine bone marrow cells (BMC) in vivo and in vitro was investigated. In vivo, G-CSF reduced 125I-G-CSF binding to BMC by greater than 95% within 30 min, with return to base line after 48 h. Human rCSF-1 binding was reduced greater than 85% after 30 min and failed to recover even after 48 h. Murine rTNF-alpha or recombinant granulocyte/macrophage CSF binding was not significantly altered. However, human rIL-1 alpha binding increased greater than 1.5-fold after 3 h, was elevated greater than 5-fold between 6 and 12 h, and declined to base line after 48 h. In vitro, G-CSF induced a greater than 1.5-fold increase in IL-1 binding to BMC after 8 h, suggesting that up-modulation of IL-1 binding in vivo required G-CSF and other influences. Further studies indicated that BMC responded to glucocorticoids and G-CSF with a synergistic increase of IL-1 binding. This synergistic IL-1R modulation was a time- and dose-dependent process and was inhibited by cycloheximide or actinomycin D in a dose-dependent manner. Binding studies further revealed that the synergistic stimulation of IL-1R expression on BMC was probably due to increased receptor number, rather than increased receptor affinity. In addition, this phenomenon was also observed in other hematopoietic cells. Our results demonstrated that G-CSF was capable of stimulating IL-1R expression on BMC both in vivo and in vitro and G-CSF in combination with glucocorticoids synergistically up-modulated IL-1 binding to BMC in vitro. Inasmuch as IL-1 induces the secretion of G-CSF and glucocorticoids in vivo, this synergistic induction may play an important, as yet unknown, role in the inflammatory cascade.     

Granulocyte colony-stimulating factor promotes adhesion of neutrophils

Granulocyte colony stimulating factor(G-CSF) is well known for its ability to drive the maturation andmobilization of neutrophils. G-CSF also appears to have the potentialto activate functions of mature neutrophils, influencing recruitment atsites of inflammation and tissue injury. We investigated the ability ofG-CSF to stimulate adhesion of isolated blood neutrophils. G-CSFinduced significant adherence to intercellular adhesion molecule(ICAM)-1 that was both macrophage antigen-1 (Mac-1) and leukocytefunction-associated antigen-1 dependent. The kinetics ofG-CSF-stimulated adhesion to ICAM-1 peaked at 11 min without detectablesurface upregulation of Mac-1. This was in marked contrast tochemokines, in which peak activation of adhesion is seen within 1 minof stimulation. In contrast to chemokine-induced adhesion, G-CSFstimulation was not inhibited by pertussis toxin. G-CSF also augmentedthe attachment of neutrophils to activated human umbilical veinendothelial cells (HUVEC) through specific effects on neutrophils,because HUVEC appear to lack functional G-CSF receptors.

     

Interleukin 1 alpha and interleukin 1 beta bind to the same receptor on T cells

Pure, E. coli-derived recombinant murine interleukin 1 alpha (IL 1 alpha) was labeled with 125I and used for receptor binding studies. The 125I-IL 1 binds to murine EL-4 thymoma cells in a specific and saturable manner. Scatchard plot analysis for binding studies carried out at 4 degrees C reveals a single type of high affinity binding site with an apparent dissociation constant of approximately 2.6 X 10(-10) M and the presence of approximately 1200 binding sites per cell. The rate of association of the 125I-IL 1 with EL-4 cells is slow, requiring more than 3 h to reach apparent steady state at 4 degrees C. Cell-bound 125I-IL 1 cannot be dissociated from EL-4 cells upon removal of unbound 125I-IL 1 and incubation of the cells at 4 degrees C in the presence or absence of unlabeled IL 1. Unlabeled recombinant murine IL 1 competes for 125I-IL 1 binding in a dose-dependent manner, whereas interferon-alpha A, interleukin 2 (IL 2), epidermal growth factor, and nerve growth factor have no effect. The 125I-IL 1 binding site is sensitive to trypsin, suggesting that it is localized on the cell surface. We have also examined the ability of purified recombinant human IL 1 alpha and IL 1 beta to compete for binding of the radiolabeled murine IL 1 to its receptor and to stimulate IL 2 production by EL-4 cells. Previous reports have shown that human IL 1 alpha is approximately 60% homologous in amino acid sequence with murine IL 1, but that human IL 1 beta is only about 25% homologous with either murine IL 1 or human IL 1 alpha. Despite these marked differences, however, we report here that both human IL 1 proteins are able to recognize the same binding site as mouse IL 1. In addition, murine as well as both human IL 1 proteins stimulate IL 2 production by EL-4 cells.     

Type I transforming growth factor-beta receptors on neutrophils mediate chemotaxis to transforming growth factor-beta.

Participation of human polymorphonuclear neutrophils in the inflammatory response is mediated, in part, by soluble factors such as chemotactic peptides and cytokines. Although the cytokine, transforming growth factor beta (TGF-beta), has been shown to recruit monocytes and promote the inflammatory process, its effects on neutrophils are unknown. In this investigation, [125I]TGF-beta 1 affinity binding studies were employed to show that neutrophils express TGF-beta receptors (350 +/- 20 receptors/cell), which exhibit high affinity for the ligand (dissociation constant, 50 pM). Affinity cross-linking studies identified the receptors to be primarily of the type I class. In contrast to the receptors on monocytes, neutrophil TGF-beta receptors were not down-regulated by exposure to specific inflammatory mediators. Additional studies examined whether exposure of neutrophils to TGF-beta could enhance specific functions, as occurs with monocytes. TGF-beta was shown to cause directed migration of neutrophils at femtomolar concentrations, thus it is the most potent neutrophil chemotactic factor yet identified. Neutrophil production of reactive oxygen intermediates was not stimulated by TGF-beta, nor did TGF-beta enhance or depress subsequent PMA- or FMLP-stimulated superoxide production. However, the stable expression of neutrophil TGF-beta receptors, and the capacity of this cytokine to stimulate neutrophil chemotaxis, suggest that the pro-inflammatory effects of TGF-beta are mediated by neutrophils in addition to monocytes.     

Binding of 125I-labeled granulocyte colony-stimulating factor to normal murine hemopoietic cells

The binding of granulocyte colony-stimulating factor (G-CSF) to murine bone marrow cells was investigated using a radioiodinated derivative of high specific radioactivity which retained full biological activity. The binding was time- and temperature-dependent, saturable and highly specific. The apparent dissociation constant for the reaction was 60-80 pM at 37 degrees C and 90-110 pM at 4 degrees C, similar to that found for the binding of G-CSF to murine leukemic cells (WEHI-3B D+) and significantly higher than the concentration of G-CSF required to stimulate colony formation in vitro. Autoradiographic analysis confirmed the specificity of binding since granulocytic cells were labeled but lymphocytes, erythroid cells and eosinophils were not. Blast cells and monocytic cells were partially labeled, the latter at low levels. In the neutrophilic granulocyte series, grain counts increased with cell maturity, polymorphs being the most heavily labeled but all cells showed considerable heterogeneity in the degree of labeling. Combination of Scatchard analysis of binding with autoradiographic data indicated that mature granulocytes from murine bone marrow exhibited 50-500 G-CSF receptors per cell.     

Oncostatin M is a differentiation factor for myeloid leukemia cells.

Oncostatin M (OSM) is a 28-kDa glycoprotein produced by stimulated macrophages and T lymphocytes that inhibits the proliferation of a number of different cell lines derived from solid tumors. Analysis of both amino acid sequence and gene structure has demonstrated that OSM is a member of a cytokine family that includes leukemia inhibitory factor (LIF), IL-6, and granulocyte colony-stimulating factor (G-CSF). We demonstrate that, like LIF, IL-6 and G-CSF, OSM can induce the differentiation of the myeloblastic M1 murine leukemia cells into macrophage-like cells. The morphologic and functional changes induced by OSM are more similar to those observed with LIF and IL-6 than those induced with G-CSF. OSM can also induce the differentiation of the histiocytic U937 human leukemia cells in the presence of granulocyte-macrophage CSF, a property shared with LIF and IL-6. In murine M1 cells, binding of labeled OSM is completely inhibited by excess LIF or OSM, reflecting the binding of OSM to the high affinity form of the murine LIF receptor. In contrast, the binding of labeled OSM to human U937 leukemia cells is inhibited by OSM, but the inhibition by LIF is significantly less. These results suggest that, in human leukemia cells, OSM may act through the LIF receptor and an OSM-specific receptor. The existence of an OSM-specific receptor was confirmed by both growth inhibition and competition binding assays on A375 human melanoma cells. The growth of human A375 cells was inhibited by OSM and IL-6 but not LIF or G-CSF. Neither LIF, G-CSF, nor IL-6 could compete with the binding of labeled OSM to A375 cells.     

Human articular cartilage and chondrocytes produce hemopoietic colony-stimulating factors in culture in response to IL-1.

The hemopoietic CSF, granulocyte-macrophage CSF (GM-CSF) and granulocyte CSF (G-CSF), are cytokines that mediate the clonal proliferation and differentiation of progenitor cells into mature macrophages and/or granulocytes. We have employed an all-human cell culture system, specific ELISA for GM-CSF and G-CSF, and Northern analysis to investigate whether chondrocytes are a potential source of CSF in rheumatoid disease. We report that human rIL-1 stimulated in a dose-dependent manner the production of GM-CSF and G-CSF by human articular cartilage and chondrocyte monolayers in organ and cell culture, respectively. Increased levels of the CSF Ag were detected after 2 to 8 h stimulation with IL-1, and the optimum dose of IL-1 was 10 to 100 U/ml (0.06 to 0.6 nM IL-1 alpha; 0.02 to 0.2 nM IL-1 beta); neither CSF was detectable in nonstimulated cultures nor in IL-1-stimulated cultures treated with actinomycin D or cycloheximide, indicating the requirement for de novo RNA and protein synthesis. The IL-1-mediated increase in GM-CSF could also be inhibited by the corticosteroid, dexamethasone, but not by the cyclo-oxygenase inhibitor, indomethacin. Although having little effect when tested alone, TNF-alpha and lymphotoxin (TNF-beta) could synergize with IL-1 for the production of GM-CSF. Basic fibroblast growth factor, platelet-derived growth factor, and IFN-alpha and IFN-gamma each had no effect on GM-CSF levels. Results obtained by Northern analysis of chondrocyte total RNA reflected those found for the CSF Ag, namely that CSF mRNA levels were elevated in response to IL-1, but not TNF, and that there was synergy between these two cytokines. We propose that chondrocyte CSF production in response to IL-1, and the concurrent destruction of cartilage by IL-1, could provide a mechanism for the chronic nature of rheumatoid disease.     

Presence of IL-1 receptors on human and murine neutrophils. Relevance to IL-1-mediated effects in inflammation

Inflammatory responses are characterized by the infiltration of polymorphonuclear neutrophils (PMN) at the involved site. IL-1 may have an important role in mediating this response, but whether IL-1 acts directly on PMN is controversial. In this study, we examined PMN for the presence of IL-1R and determined the effect of IL-1 on PMN migration in vivo. Thioglycollate, proteose-peptone, or IL-1 elicited peritoneal exudate cells were found to bind 125I-IL-1 alpha in a specific and saturable manner. This binding was localized to the PMN in the exudate. Scatchard plot analysis indicates the presence of approximately 1700 receptors per PMN and an apparent dissociation constant of 3.0 x 10(-10) M. Binding sites for 125I-IL-1 alpha were also found on human PMN prepared from peripheral blood. There are approximately 900 receptors per cell on human PMN with a dissociation constant similar to that observed for elicited murine PMN. Binding of 125I-IL-1 alpha to the mouse and human PMN is inhibited by both recombinant human IL-1 alpha and IL-1 beta, indicating that both IL-1 proteins bind to the same receptor on these cells. Human PMN were able to internalize radioiodinated IL-1. We conclude that PMN possess receptors for IL-1 and that these binding sites may be important in mediating IL-1 effects on granulocytes that are involved in the inflammatory response.     

Granulocyte colony-stimulating factor and differentiation-induction in myeloid leukemic cells

The granulocyte colony-stimulating factor (G-CSF) belongs to a family of hemopoietic growth factors regulating the production of granulocytes and macrophages. Murine G-CSF stimulates the proliferation and differentiation of precursors of neutrophilic granulocytes and is also able to stimulate the functional activities of mature neutrophils. Among the hemopoietic growth factors, G-CSF has an outstanding capacity to induce terminal differentiation and suppression of self-renewal in myeloid leukemic cells. Murine and human G-CSF's show complete biological cross-reactivity across species and bind equally well to G-CSF receptors of either species. Specific receptors for G-CSF exist on all normal neutrophilic cells and have not been lost in the generation of primary human myeloid leukemias. This data indicates that G-CSF may be a useful reagent in the treatment of myeloid leukemia, in hemopoietic regeneration and in increasing resistance against infections.     

Tyrosine 462 of the membrane-proximal F'-G' loop of murine Mpl is not essential for high-affinity binding of thrombopoietin

The ligand binding site of Mpl, the thrombopoietin (Tpo) receptor, has not been determined. Tyr(462)of murine Mpl corresponds to Tyr(421)of the common beta chain of the human IL-3, IL-5 and GM-CSF receptors. Tyr(421)has been identified as essential for high-affinity ligand binding. To determine whether Tyr(462)is similarly required for Tpo binding, wild-type murine Mpl (Mpl-WT) or mutant receptors containing an alanine (Y462A) or lysine (Y462K) in place of Tyr(462)were expressed in BaF3 cells. In proliferation studies, the Y462A mutation had no effect on Tpo-induced growth. In contrast, the Y462K mutation led to an attenuated proliferative response to Tpo. In single-point binding studies, both Mpl-WT and Y462A cells were able to bind [(125)I]Tpo in a specific manner. In contrast, there was a marked reduction in binding of [(125)I]Tpo by Y462K cells. Mpl-WT cells bound Tpo with a K(d)of approximately 330 pM, while Y462A cells bound Tpo with a K(d)of approximately 268 pM. The binding affinity of Y462K cells was below that quantifiable by Scatchard analysis. This study suggests that unlike the corresponding Tyr(421)of the common human beta chain, Tyr(462)of murine Mpl is not required for high-affinity ligand binding, although it may be located in proximity to the ligand binding site.     

Identification of distinct receptors for two hemopoietic growth factors (granulocyte colony-stimulating factor and multipotential colony-stimulating factor) by chemical cross-linking

Granulocyte colony-stimulating factor (G-CSF) and multipotential colony-stimulating factor (multi-CSF or interleukin 3) are two members of a family of hemopoietic growth and differentiation factors. Using biologically active radioiodinated derivatives and chemical cross-linking (predominantly with the homobifunctional reagent disuccinimidyl suberate) followed by gel electrophoresis and autoradiography, receptors for these two factors have been identified. The G-CSF receptor was identified as a single subunit protein of Mr approximately 150,000 while two molecular species able to specifically cross-link to 125I-multi-CSF were identified of Mr approximately 75,000 and 60,000. For both CSFs specificity of formation of cross-linked species was demonstrated by showing that the homologous unlabeled CSF (but not other CSFs) competed for formation of the complexes with the appropriate dose-response relation, by showing that saturation occurred over the appropriate range of 125I-CSF concentration and by showing that the cellular specificity of CSF binding paralleled that for cross-linked complex formation. The formation of cross-linked complexes was dependent on the concentration and type of chemical cross-linker, especially for cross-linking of 125I-multi-CSF. Based on a number of criteria it is suggested that the two species cross-linked to 125I-multi-CSF do not represent receptors of different affinity but, rather, two noncovalently associated subunits of a receptor complex.     

Specific binding of radioiodinated granulocyte-macrophage colony-stimulating factor to hemopoietic cells

The hemopoietic growth factor granulocyte-macrophage colony-stimulating factor, GM-CSF, specifically controls the production of granulocytes and macrophages. This report describes the binding of biologically-active 125I-labeled murine GM-CSF to a range of hemopoietic cells. Specific binding was restricted to murine cells and neither rat nor human bone marrow cells appeared to have surface receptors for 125I-labeled GM-CSF. 125I-Labeled GM-CSF only appeared to bind specifically to cells in the myelomonocytic lineage. The binding of 125I-labeled GM-CSF to both bone marrow cells and WEHI-3B(D+) was rapid (50% maximum binding was attained within 5 min at both 20 degrees C and 37 degrees C). Unlabeled GM-CSF was the only polypeptide hormone which completely inhibited the binding of 125I-labeled GM-CSF to bone marrow cells, however, multi-CSF (also called IL-3) and G-CSF partially reduced the binding of 125I-labeled GM-CSF to bone marrow cells. Interestingly, the binding of 125I-labeled GM-CSF to a myelomonocytic cell line, WEHI-3B(D+), was inhibited by unlabeled GM-CSF but not by multi-CSF or G-CSF. Scatchard analysis of the binding of 125I-labeled GM-CSF to WEHI-3B(D+) cells, bone marrow cells and peritoneal neutrophils indicated that there were two classes of binding sites: one of high affinity (Kd1 = 20 pM) and one of low affinity (Kd2 = 0.8-1.2 nM). Multi-CSF only inhibited the binding of 125I-labeled GM-CSF to the high affinity receptor on bone marrow cells: this inhibition appeared to be a result of down regulation or modification of the GM-CSF receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta2 integrin-dependent phosphorylation of protein-tyrosine kinase Pyk2 stimulated by tumor necrosis factor alpha and fMLP in human neutrophils adherent to fibrinogen.

Tumor necrosis factor alpha and fMLP can activate a broad range of cellular functions in neutrophils adherent to biological surfaces. These functions are mediated by integrins and involve the activation of tyrosine kinases. Here, we report that Pyk2, a member of the focal adhesion kinase family, was present in human neutrophils and was rapidly phosphorylated and activated following tumor necrosis factor alpha and fMLP stimulation in an adhesion-dependent manner. Tyrosine phosphorylation of Pyk2 was attenuated by beta2 integrin blocking with specific antibodies. The tyrosine phosphorylation of Pyk2 was downstream of protein kinases Lyn, Syk and protein kinase C and cytoskeletal organization. The activation of Pyk2 may play a role in adhesion/cytoskeleton-associated neutrophils function.     

Identification of platelet-activating factor receptors in P388D1 murine macrophages

Platelet-activating factor (PAF) binding and metabolism by eight murine and human cell lines was analyzed. Only the murine P388D1 macrophage line had specific, high affinity PAF binding sites. PAF binding reached saturation within 10 min at room temperature and was irreversible. Minimal PAF metabolism was observed at the time binding saturation was achieved. Scatchard analysis of PAF binding revealed a single class of PAF receptors (7872 +/- 1310/cell) which had a dissociation constant of 0.08 +/- 0.01 nM (mean +/- SEM, eta = 6). The dissociation constant was confirmed independently by quantifying the kinetics of initial specific PAF binding. PAF binding was stereospecific, required an sn-2 acetyl substituent, and was inhibited by structurally diverse PAF antagonists including kadsurenone, BN 52021, triazolam, and CV3988. The fact that the receptors are functionally active was shown by the observation that 1 to 100 pM PAF increased free intracellular calcium in P388D1 cells in a dose-related manner. These studies demonstrate that P388D1 macrophages have functional PAF receptors whose affinity and structural specificities are similar to PAF receptors in other cells. The availability of a stable cell line that binds but does not metabolize PAF will greatly facilitate studies of the PAF receptor.     

The effects of cytokines, platelet activating factor, and arachidonate metabolites on C3B receptor (CR1, CD35) expression and phagocytosis by neutrophils

The cytokines tumor necrosis factor alpha (TNF alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), and interleukin 1 (IL 1) all caused an upregulation of C3b receptors (CR1) on neutrophils that ranged from around 76% (G-CSF and IL 1) to 93% (TNF alpha and GM-CSF) of the upregulation obtained by pretreatment of the neutrophils with the chemotactic peptide FMLP. However, only TNF alpha and G-CSF caused a significant increase in phagocytosis of opsonized microspheres. Platelet derived growth factor, interleukin 2, and transforming growth factor beta had no effect on either of these parameters. The mediators platelet activating factor (PAF) and leukotriene B4 (LTB4) both caused a large upregulation of CR1 (93% and 80%, respectively, of the FMLP-mediated value); however, only PAF caused a significant enhancement of phagocytosis by the neutrophils. Prostaglandin E2 and thromboxane B2 had no effect on these parameters. Considerable individual variation was observed among some of the untreated and mediator-treated neutrophil preparations regarding CR1 expression and phagocytosis. The upregulation of CR1 and associated increase in phagocytic capacity of neutrophils caused by certain cytokines and other mediators may be important in host defense. Also the lack of enhancement of phagocytosis accompanying an upregulation of CR1 is unusual and may have important implications regarding the cellular mechanisms of phagocytosis by neutrophils.