Antibody-dependent cellular cytotoxicity mediated by lung macrophages: a comparison of two target cells

Mononuclear cell-mediated cytotoxicity may be an important cellular immune function in host lung defense. Prior investigators have shown that lung macrophages participate in cell cytotoxicity which is antibody-dependent (ADCC). We tested the hypothesis that alveolar macrophages share some cell surface receptors for the Fc portion of IgG, i.e., Fc receptors, similar to those found on circulating monocytes in order to function in ADCC. Hence, ADCC mediated by autologous human blood monocytes and lung macrophages was studied by measuring the release of chromium-51 from prelabeled target erythrocytes coated with IgG. Alveolar macrophages were obtained from healthy adult subjects by bronchoalveolar lavage and tested against two different erythrocyte target cells to measure ADCC activity. Our results show significant activity by alveolar macrophages demonstrated against chicken erythrocytes at a target to effector cell ratio of 2:1 or 10:1 and with an antibody concentration of 1:100 or 1:400 (volume per volume, p less than 0.05, Student's test). However, when a peripheral blood monocyte specific target cell (human type B erythrocyte) was utilized, alveolar macrophages were not as capable of significant ADCC activity against these monocyte-specific target cells. The inability of lung macrophages to function in ADCC against other target cells (i.e., human type B erythrocytes) unlike the peripheral blood monocytes suggests that some Fc receptors are not shared. In other words, these different cell types share IgG receptors but differences in activity may be due to some changes in the Fc portions of IgG due to cellular differentiation. The use of these target cells may potentially be useful in functionally discriminating between two types of adherent autologous mononuclear cells (lung macrophages vs. blood monocytes).     

Human FcγRIIA induces anaphylactic and allergic reactions

IgE and IgE receptors (FcεRI) are well-known inducers of allergy. We recently found in mice that active systemic anaphylaxis depends on IgG and IgG receptors (FcγRIIIA and FcγRIV) expressed by neutrophils, rather than on IgE and FcεRI expressed by mast cells and basophils. In humans, neutrophils, mast cells, basophils, and eosinophils do not express FcγRIIIA or FcγRIV, but FcγRIIA. We therefore investigated the possible role of FcγRIIA in allergy by generating novel FcγRIIA-transgenic mice, in which various models of allergic reactions induced by IgG could be studied. In mice, FcγRIIA was sufficient to trigger active and passive anaphylaxis, and airway inflammation in vivo. Blocking FcγRIIA in vivo abolished these reactions. We identified mast cells to be responsible for FcγRIIA-dependent passive cutaneous anaphylaxis, and monocytes/macrophages and neutrophils to be responsible for FcγRIIA-dependent passive systemic anaphylaxis. Supporting these findings, human mast cells, monocytes and neutrophils produced anaphylactogenic mediators after FcγRIIA engagement. IgG and FcγRIIA may therefore contribute to allergic and anaphylactic reactions in humans.     

FcγRIV deletion reveals its central role for IgG2a and IgG2b activity in vivo

Cellular Fcγ receptors are essential for IgG-dependent effector functions in vivo. There is convincing evidence that selective activating Fcγ receptors are responsible for the activity of individual IgG subclasses. Thus, IgG1 activity is absent in FcγRIII-deficient mice, and several studies suggest that the activity of the most potent IgG subclasses, IgG2a and IgG2b, might be dependent on either individual or a combination of activating FcγRs. To study the role of individual activating FcγRs for IgG subclass activity, we generated an FcγRIV-deficient mouse and showed that a variety of IgG2a- and IgG2b-dependent effector functions are impaired in the absence of this activating Fc receptor in models of autoimmunity and antibody-dependent cellular cytotoxicity.     

Lymphokine-activated killer cells from normal and lymphoma subjects are cytotoxic for cells coated with antibody derivatives displaying human Fc gamma

Lymphokine-activated killer (LAK) cells were successfully generated in all cases from blood mononuclear cells obtained from six patients with lymphoma. The LAK cells from three of these patients and from five normal adult donors were tested for their effector abilities in antibody-dependent cellular cytotoxicity (ADCC) against guinea pig leukemic lymphocytes coated with various antiidiotype antibodies. Cells from all the donors behaved similarly. Mouse monoclonal antibodies of IgG1, IgG2a, and IgG2b isotypes invoked no ADCC. However, substantial ADCC was invoked by the chimeric antibody FabFc, in which Fab'gamma from mouse antiidiotype is thioether-bonded to human normal Fc gamma. Similar results were obtained on testing LAK cells from a normal donor against uncultured human lymphoma targets coated with native or chimeric antiidiotype. The ADCC invoked by the mouse-human chimeric antibodies appears to depend on the human Fc gamma they display and not on the univalency of the derivatives used. The findings imply that LAK technology could usefully augment serotherapy that uses antibody derivatives displaying human Fc gamma.     

Neutrophils express the high affinity receptor for IgG (Fc gamma RI, CD64) after in vivo application of recombinant human granulocyte colony-stimulating factor.

Fc receptors are important effector molecules of neutrophilic granulocytes (polymorphonuclear neutrophils [PMN]), connecting phagocytic cells and the specific immune response. Neutrophils from healthy donors express the low-affinity receptors for IgG Fc gamma RII (CD32) and Fc gamma RIII (CD16), but not the high-affinity receptor Fc gamma RI (CD64). The latter has been found on neutrophils from patients with certain bacterial infections and can be induced in vitro after incubation with interferon-gamma. We show here that neutrophils strongly express Fc gamma RI after in vivo application of recombinant human granulocyte colony-stimulating factor (rhG-CSF). PMN from patients receiving rhG-CSF displayed higher cytotoxicity against Daudi lymphoma cells in vitro compared with control patients and with healthy donors. Fab fragments against Fc gamma RII (monoclonal antibody [MoAb] IV.3) inhibited neutrophil-mediated cytotoxicity of healthy donors but not of patients during rhG-CSF therapy. Therefore, expression of Fc receptors by PMN was investigated by flow cytometry and the mean fluorescence intensity (MFI) was compared. After staining with MoAb 32.2 against Fc gamma RL, the median MFI of neutrophils from G-CSF patients (median, 4.78; range, 2.40 to 8.50; n = 5) was significantly higher (P = .002 and P = .001, respectively) than the median MFI of patients not receiving G-CSF (median, 1.23; range, 1.01 to 1.58; n = 6) and the median MFI of healthy donors (median, 1.04; range, 0.67 to 1.12; n = 6). Fc gamma RI disappeared after the discontinuing of the G-CSF injections, but was reinduced during the next treatment cycle with rhG-CSF. The high expression of Fc gamma RI during rhG-CSF therapy correlated with enhanced cytotoxicity. In vitro incubation with rhG-CSF also enhances cytotoxicity, but only minor increments in Fc gamma RI expression were observed. Thus, during in vivo application of rhG-CSF neutrophils acquire an additional potent receptor for mediating tumor cell killing in vitro by induction of the high-affinity receptor for IgG (Fc gamma RI, CD64).     

Monocyte/macrophage Fc gamma RIII, unlike Fc gamma RIII on neutrophils, is not a phosphatidylinositol-linked protein.

The low affinity IgG Fc receptor, Fc gamma RIII, expressed on circulating neutrophils, natural killer (NK) cells, and tissue macrophages, is involved in effector functions such as cytotoxicity and immune complex clearance by these cells. While Fc gamma RIII is reported to be a phosphatidylinositol (PI)-linked, rather than peptide-linked, protein on neutrophils and NK cells, its membrane linkage in macrophages has not been studied. We examined the sensitivity of Fc gamma RIII to cleavage by PI-specific phospholipase C (PI-PLC) in cultured monocytes and alveolar tissue macrophages and report that this receptor is not PI-linked on these cells. We also observed normal levels of Fc gamma RIII on cultured monocytes of a patient with paroxysmal nocturnal hemoglobinuria, a disease in which PI-linked proteins are deficient. The results suggest that Fc gamma RIII occurs solely in a transmembrane form in cells of the monocyte/macrophage lineage. In addition, we studied Fc gamma RIII on a cloned NK cell line and found it to be resistant to the effects of PI-PLC under conditions that cleaved Fc gamma RIII on neutrophils. Taken together, our results provide evidence for a distinct form of Fc gamma RIII that differs from the neutrophil receptor in its structure and, possibly, in its function.     

Cytotoxic activity of rat granulocytes against Mesocestoides corti

Rat eosinophils or neutrophils were purified from peritoneal washings which had been enriched either for eosinophils by infection with the parasite Mesocestoides corti or by intravenous injection with Sephadex G200 particles, or for neutrophils by the intraperitoneal injection of glycogen. Neither eosinophils nor neutrophils attached to or damaged live M. corti parasites in vitro although they did lyse chick erythrocytes in the presence of rat anti-chick red blood cell antibody, with the neutrophils showing the highest level of cytotoxicity and the eosinophils from the infected rats the lowest. Neutrophils gave a specific antibody-dependent cytotoxic response to chick erythrocytes coated with solubilized M. corti extract, a response not seen with eosinophils. The cytotoxicity shown by neutrophils could not be blocked by adding eosinophils or sera obtained from chronically infected rats although it was reduced by incubating the neutrophils with cell-free supernatants obtained from the spleen cells of infected rats following stimulation with solubilized parasite extract in vitro. Eosinophils from infected rats expressed fewer membrane Fc receptors for antibody than did neutrophils or eosinophils from uninfected animals. Incubation of neutrophils and eosinophils from uninfected rats with the immune spleen cell supernatants reduced Fc receptor expression to levels similar to those seen with eosinophils from infected animals. These same supernatants had no effect on the expression of granulocyte complement receptors. It is suggested that infection of rats with M. corti can lead to the production of an antigen-specific suppression capable of impairing the antibody-dependent activity of granulocytes in vitro.     

DNA vaccine for hepatitis B: evidence for immunogenicity in chimpanzees and comparison with other vaccines.

Vaccination of two chimpanzees against hepatitis B virus (HBV) by intramuscular injection of plasmid DNA encoding the major and middle HBV envelope proteins induced group-, subtype- and preS2-specific antibodies. These were initially of IgM isotype, and then they were of IgG (predominantly IgGl) isotype. The chimpanzee injected with 2 mg of DNA attained >100 milli-international units/ml of anti-HBs antibody after one injection and 14,000 milli-international units/ml after four injections. A smaller dose (400 microg) induced lower and transient titers, but a strong anamnestic response occurred 1 year later. Comparison with responses in 23 chimpanzees receiving various antigen-based HBV vaccines suggests that the DNA approach is promising for prophylactic immunization against HBV.     

Surface markers of human eosinophils

Peripheral blood eosinophils from patients with eosinophilia and from healthy subjects were studied for surface immunoglobulins, receptors for the Fc region of IgG, complement receptors, and spontaneous rosette formation with sheep and mouse erythrocytes. Eosinophils were found to have receptors for complement and for aggregated IgG, and to have the same two types of complement receptors as do lymphocytes and monocytes. Immune adherence type receptors were specific for C4 or C3b, while C3d receptors were specific for C3d but unreactive with C4. Eosinophils differed from fully mature neutrophils in that the former had C3d receptors and relatively weak immune adherence (C4 or C3b) receptors, while the later did not have the C3d receptors and had strong immune adherence receptors. Eosinophil phagocytosis of complement-receptor bound erythrocytes was dependent on the presence of IgG in the antibody coating the red blood cells; this requirement for IgG resembled that found in neutrophil phagocytosis. No surface Ig or spontaneous erythrocyte rosette formation was observed with eosinophils.     

Expression of biological effector functions by immunoglobulin G molecules lacking the hinge region

Several biological effector functions mediated by sites on the Fc region of human IgG1 have been studied in two variant IgG1 kappa monoclonal proteins (Dob and Lec) which contain deletions corresponding to the entire hinge region of the heavy chains. Neither Dob nor Lec protein in aggregated form was able to activate the classical complement pathway, and this was shown to be due to an inability to bind the first component of complement (C1). By rosette inhibition assays, Dob and Lec proteins were shown to have no measurable affinity for Fc receptors on human B cells or neutrophils. Dob and Lec proteins had a much reduced affinity for Fc receptors on the murine macrophage-like cell line P388D1 when compared to normal human IgG1. Furthermore, the hinge-deleted proteins were able to compete with murine IgG2b for P388D1 receptors but not with murine IgG2a. In contrast, the binding of Dob and Lec proteins to protein A from Staphylococcus aureus was entirely normal. The functional consequences of the hinge deletion were parallel to those seen when normal IgG1 was reduced and alkylated. It was concluded that the functional impotency of Dob and Lec proteins was related to the close association between the Fab and Fc regions in these molecules and the limited degree of segmental flexibility permitted in the absence of the hinge region. The data also suggest a major role for the C gamma 2 domain (C is the constant region) in mediating effector functions in normal IgG1.     

Mechanisms for adherence of eosinophils to an antibody-coated surface

Eosinophils may act by degranulation after attachment to a surface. As the mechanisms of adherence are not understood, we have investigated the dependency on Fc(IgG) receptors and other mechanisms by studying the adherence of human eosinophils to albumin-Sepharose beads coated with either specific rabbit IgG antibody, F(ab')2 antibody fragments or serum under different conditions. Adherence to Sephadex beads and albumin-coated microtitration plates was also investigated. 50% of the eosinophils adhered spontaneously to all 3 different surfaces not coated with the antibody, whereas only 25% of neutrophils and less than 10% of mononuclear cells adhered. A small but significant increase in adherence to albumin-Sepharose or albumin-coated plastic occurred after addition of the IgG-antibody, but not after addition of F(ab')2 fragments, indicating that the Fc region was responsible for some increase in adherence. Incubation of eosinophils with IgG-Fc fragments prevented the additional antibody-mediated adherence. As Fc receptor-negative eosinophils adhered almost as well as Fc receptor-positive cells, it appears that the Fc receptors are of minor importance and instead, a nonspecific adherence mechanism, possibly unique for the eosinophil, seems to be the most important in eosinophil adherence to antibody-coated surfaces.     

Antigen topography is critical for interaction of IgG2 anti-red-cell antibodies with Fcγ receptors

IgG antibodies to the Rh D polypeptide on red cells are normally IgG1 or IgG3, whereas antibodies produced in response to carbohydrate antigens such as the A and B blood groups are predominantly IgG2. The consequences of this isotype restriction for the immune destruction of red cells were investigated. Human IgG2 anti-D and IgG2 anti-A were isolated by affinity purification from an unusual anti-D serum (DEL) and anti-A sera, respectively. These antibodies were compared with IgG1 and IgG3 monoclonal anti-D in in vitro functional assays of the interaction between IgG-coated red cells (EA-IgG) and cells bearing IgG Fc receptors (FcγR). Dimeric IgG2 anti-D bound efficiently to cell lines transfected with FcγRIIa-H131, an allotypic form of FcγRIIa which readily interacts with IgG2, IgG1 and IgG3. Unexpectedly, however, -D-phenotype red cells coated with IgG2 anti-D did not form rosettes with these cells, whereas EA-IgG2 anti-A and EA-IgG1 and EA-IgG3 anti-D effectively formed rosettes with these transfectants at the same sensitization level (100 000 molecules IgG/red cell). In antibody-dependent cell-mediated cytotoxicity (ADCC) assays, lysis of EA-IgG2 anti-A was mediated via FcγRIIa, whereas lysis of EA-IgG1 and EA-IgG3 anti-D was mediated via FcγRI or FcγRIII; EA-IgG2 anti-D was inactive in all functional assays. These experiments suggest that both IgG subclass and antigen structure affect functional IgG–FcγR interactions. The topography of the Rh D antigen, an integral membrane protein, ensures that anti-D is bound near the lipid bilayer surrounded by the glycocalyx. This may sterically hinder access of FcγRIIa-H131 to the FcγR recognition site on the relatively inflexible IgG2 anti-D, but not to that of IgG1 or IgG3 anti-D. In contrast, IgG2 bound to the A antigen on glycoproteins is not so constrained. The topography of the D and A antigens may thus determine whether functional interactions of red-cell-bound IgG2 anti-D and IgG2 anti-A with cells bearing Fcγ receptors can occur.     

Protection to Fasciola hepatica in the gut mucosa of immune rats is associated with infiltrates of eosinophils,IgG1 and IgG2a antibodies around the parasites

We investigated the immune effector mechanisms that underlie protection against F. hepatica in the gut wall of immune rats, using (immuno)histochemistry. In the lamina propria of immune Wistar rats, four weeks after oral infection, frequencies of IgE-positive cells, eosinophils and mucosal mast cells were significantly increased, compared with naïve rats. These factors represent the traditional effector mechanisms against helminths. No significant differences were detected between the two groups in frequencies of IgM-, IgG2a-, IgG1- and IgA- positive cells, CD4- and CD8-positive cells, NK cells, macrophages, neutrophils or goblet cells. Upon challenge of immune rats with F. hepatica in an ex vivo gut segment, NEJs that migrated through the (sub)mucosa were coated with IgG1 and IgG2a antibodies and surrounded by eosinophils. No IgE or IgA antibodies were detected on the parasites. The onset of these immune effector responses, two h after challenge, was related to the expression of protection. These results suggest that NEJs are killed by an eosinophil-mediated cytotoxic response involving IgG antibodies. These antibodies were not produced in the intestine, but infiltrated the gut upon challenge. The observed immune effector responses were not restricted to the site where the primary infection is located, namely the small intestine, but were also detected in the large intestine. The presence of the protective immune mechanisms in two other rat strains demonstrates the pivotal importance of these responses, irrespective the genetic background of the host.     

小鼠感染后不同时间血清对日本血吸虫童虫细胞介导的细胞毒作用

用抗体依赖、细胞介导的细胞毒试验(ADCC)观察了小鼠感染后不同时间血清在体外杀伤系统中对日本血吸虫童虫的作用规律。未加补体时,嗜酸粒细胞或巨噬细胞介导的童虫杀伤作用于感染后4wk出现明显作用,分别于5—7wk和6—8wk达高峰,然后下降,至11wk时降至4wk时的水平;而未加补体时无明显的中性粒细胞介导的杀伤作用。在补体参与下,这三种细胞均能介导对童虫的杀伤作用,且作用增强。证实ADCC是日本血吸虫获得性抵抗力的一个重要组分,嗜酸粒细胞、巨噬细胞介导的对童虫的细胞毒作用是不依赖补体的,而中性粒细胞介导的作用是依赖补体的。结果对选择免疫预防适宜时间和评价候选疫苗效果有参考意义。     

Bovine immune response to African trypanosomes: specific antibodies to variable surface glycoproteins of Trypanosoma brucei

Cattle were infected with two different clones of Trypanosoma hrurei (MITat 1.2 and ILTat 1.3) and antibody response was followed by radioimmunoassay. In four of the seven animals there were at least two peaks of antibody activity to the infecting clones, with the second peak much higher than the first. Specific antibodies (IgG1 and IgM but not IgG2) were eluted from the immunoabsorbent columns on which the variant surface glycoproteins (VSGs) were coupled. By neutralization of infectivity tests, IgM antibodies from the first peak of antibody activity were more efficient in neutralizing trypanosomes than IgGl but the reverse was true of the antibodies isolated from the second peak. By absorption with multiple variable antigen types isolated during the course of infection, all the IgM and IgG1 in the first 3 weeks of infection were shown to be specific. It is suggested that polyclonal B cell stimulation leading to dysfunction in the control of IgM and IgG production may not be responsible for the high levels of these immunoglobulins in bovine trypanosomiasis.     

小鼠感染后不同时间血清对日本血吸虫童虫细胞介导？..

The tests of antibody-dependent cell-mediated cytotoxicity (ADCC) were performed to observe the kinetics of in vitro killing of Schistosoma japonicum schistosomula mediated by eosinophils, macrophages or neutrophils with infected mouse sera of different duration. The results showed that when complement was not involved, the killing rates of schistosomula mediated by eosinophils or macrophages began to increase significantly at 4 wk post-infection, reached a plateau in 5-7 wk and 6-8 wk respectively, and then declined to the levels of that at 4 wk till 11 wk. In case of neutrophils, there was no significant cytotoxicity detected. When complement was involved, all the three effector cells could mediate cytotoxicity to schistosomula, with the killing rates higher than those without complement at the correspondent time intervals. These indicate that ADCC appears to be an important ingredient in the acquired resistance to S. japonicum, and demonstrate that eosinophil- and macrophage-mediated cytotoxicity to schistosomula is complement-independent, whereas neutrophil-mediated cytotoxicity to schistosomula is complement-dependent. This method may be of some value for choosing optimal time for vaccination and estimating the effectiveness of a candidate vaccine.     

Les récepteurs pour la portion Fc des anticorps,cibles et/ou outils thérapeutiques dans l'allergie

Effector cells of the allergic reaction include a variety of myeloid cells most of which are also the effector cells of innate immunity. Some, like mast cells and basophils, initiate the acute reaction, while others, like eosinophils, neutrophils and monocytes, generate a chronic inflammation which accounts for most allergic symptoms. All these cells express receptors for the Fc portion of antibodies (FcRs) whose engagement by antibodies and allergens modulates, positively and negatively, their many biologic activities. These include activating IgE and IgG receptors (FcεRI and FcγRIIA/C, respectively, in humans) and inhibitory IgG receptors (FcγRIIB). FcRs provide potent therapeutic targets and/or tools in allergic diseases. Well established treatments, such as desensitization and the administration of humanized monoclonal anti-IgE antibodies (Omalizumab), which have demonstrated their therapeutic efficacy, happen to displace the balance between positive and negative signals generated by activating and inhibitory FcRs. New approaches that deliberately act on or use FcRs are being experimentally developed, and prototypic molecules aiming at decreasing mast cell numbers or at inhibiting mast cell activation are being constructed and investigated as for their potential therapeutic use in allergic diseases.     

Immunoglobulin and cytokine profile in murine secondary hydatidosis

We have investigated specific immune responses in BALB/c mice with experimentally induced secondary hydatidosis. Following intraperitoneal inoculation of brood capsules containing Echinococcus granulosus-protoscoleces, the course of the infection was followed for 513 days. The sera of the mice were screened for the presence of a number of cytokines, and for specific antibodies. During the first 129 days of infection, high levels of cytokines TNFα, IL-α, IFNγ, IL-6, andIL-10 and specific IgGl andIgG3 isotypes were detected, as compared to uninfected controls. The levels oflgM and IgG2a were slightly increased following infection, and remained elevated throughout the period of observation. The levels of IL-lα and specific immunoglobulin of all isotypes except IgM and IgG2α, were significantly decreased 103 days post infection fp.i.), whereas TNFα was sharply decreased 129 days p.i. During the period of 129 to 209 days of infection there was an increase in secreted IL-10, and a slow decrease in the levels of IL-6 andlFN-γ. Levels of IgM, IgG, IgGl, and IgG2a plateaud during this period, whereas IgG3 and TNFα showed a peak at day 190 p.i. These data suggest the induction of Th2 antibody-mediated immunity with a parallel expansion of Thl-mediated inflammatory responses as important mechanism of host defence against the metacestode.     

Monoclonal antibody-mediated toxicity in Cryptococcus neoformans infection: mechanism and relationship to antibody isotype.

Antibody reagents represent an alternative for the therapy of human cryptococcosis, and monoclonal antibody 18B7 (IgG1) is a candidate for phase I trial in humans with cryptococcosis. However, antibody administration to mice with established Cryptococcus neoformans infection has been reported to produce acute lethal toxicity (ALT). The present study confirmed this phenomenon and investigated the mechanism of ALT. ALT was associated with hemoconcentration, hypotension, and circulatory collapse; however, toxicity could be prevented by platelet-activating factor inhibitor, rat antibody to Fc receptor, or IgM before IgG1. Significant isotype-specific differences were found in ALT, which can be interpreted as consistent with the hypothesis that there are distinct Fc receptors for murine IgG1 and IgG3. The IgM and IgG3 isotype preference in antibody responses to polysaccharide antigens in mice may translate to a lack of toxicity of antigen-antibody complexes during the course of infections with encapsulated pathogens.     

Human neutrophil Fc gamma receptor distribution and structure.

Fc receptors (FcR) for IgG on human cells were analyzed with two monoclonal antibodies, 3G8 and 4F7. Fab fragments of both hybridoma IgGs inhibited binding to neutrophils of soluble rabbit IgG complexes and sheep erythrocytes (E) coated with rabbit IgG. The number of sites for 3G8 Fab was 135,000 per neutrophil, roughly equivalent to the number of sites for rabbit IgG in immune complexes (112,000 per cell). We did not observe human IgG1 binding sites on neutrophils, although binding of IgG1 to FcR-bearing lines U937 and HL-60 was readily demonstrated. Other cell types bearing 3G8 binding sites were mature chronic myelogenous leukemia cells, eosinophils, 6% of E-rosetting lymphocytes, and 15% of Ig-bearing peripheral lymphocytes. No binding of 3G8 Fab was observed on the FcR-bearing cell lines U937, HL-60 Raji, Daudi, and K562 or on blood monocytes. However, 15% of monocytes cultured for 7 days and 60% of lung macrophages expressed 3G8 antigen. When analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the neutrophil FcR immunoprecipitated with 3G8 or 4F7 Fab-Sepharose displayed a broad band extending from Mr 73,000 to 51,000; in many experiments this band was resolved into two poorly separated components, centered at Mr 66,000 and 53,000. These results show that human neutrophil FcR for IgG is different from that on monocytes, with respect to both antigenic composition and binding of monomeric IgG1.