Modulation of Fc gamma receptors on the human macrophage cell line U-937

Management of immune thrombocytopenic purpura in children: potential role of novel agents

The treatment of immune thrombocytopenic purpura (ITP) in children is controversial, requiring individualized assessment of the patient and consideration of treatment options. If the platelet count is >10 000/μL and the patient is asymptomatic, a 'watch and wait' strategy is appropriate since most children with ITP will recover completely without pharmacotherapy. If therapy is indicated because of bleeding or a platelet count <10 000/μL, then treatment with glucocorticoids, intravenous immunoglobulin (IVIg), or anti-D are possible initial choices. Glucocorticoid treatment is the least expensive and is our usual first choice of therapy. Its use assumes that the blood counts and blood film have been evaluated to ensure the absence of evidence of alternative diagnoses, such as thrombotic thrombocytopenic purpura or incipient acute leukemia. IVIg is expensive and often causes severe headache, nausea and vomiting, and requires hospitalization at our institution. Anti-D therapy is also expensive and can only be used in patients who are Rhesus D positive. These therapies, even if only transiently effective, can be repeated if necessary. Children usually recover from newly diagnosed ITP, with or without multiple courses of medical therapy. If the disease becomes 'persistent' with severe thrombocytopenia and/or bleeding, and is no longer responsive to the three first-line therapies, the next approach includes the use of thrombopoietin receptor agonists or rituximab. When the disease persists for more than 1 year, it is considered chronic, and, if symptomatic, it may become necessary to consider third-line therapies, including splenectomy, alternative immunosuppressive agents, or combination or investigative chemoimmunotherapy. This review considers the indications, mechanism of action, and effectiveness of the traditional and novel treatment options for patients with ITP.

Functional integrity of intravenous immunoglobulin following irradiation with a virucidal dose of gamma radiation

Although intravenous immunoglobulins (IVIG) and other plasma therapeutics have had a relatively good safety record, improved methods for viral clearance are constantly being evaluated and incorporated into new manufacturing processes. Gamma irradiation has been used routinely to assure sterility of healthcare products and medical devices, but it has not been applied successfully as a viral inactivation method for biologics. We examine whether virucidal doses of gamma irradiation (50 kGy) can be delivered to a manufacturing intermediate form of IVIG, a fractionated plasma paste, with negligible effect on structural and functional integrity of purified IgG product. Immunoglobulins from paste were examined for radiation-induced damage by SDS-PAGE and ELISAs utilizing viral antigens specific for rubella, CMV and mumps. Fc domain integrity was assessed by immunoblotting, quantitatively comparing the binding of irradiated and non-irradiated materials to cell surface Fcgamma receptors, and by employing quantitative RT-PCR to study the kinetics of accumulation of mRNA for the immune modulatory cytokines IL-1alpha, IL-1beta, IL-4, IL-8, IFNgamma, and TNFalpha. The results demonstrate that Fab and Fc domains of IVIG remain essentially intact and functional after gamma irradiation to virucidal doses, suggesting that this method could be used to enhance the safety of IVIG products.

Antibody-dependent enhancement of virus infection and disease

In general, virus-specific antibodies are considered antiviral and play an important role in the control of virus infections in a number of ways. However, in some instances, the presence of specific antibodies can be beneficial to the virus. This activity is known as antibody-dependent enhancement (ADE) of virus infection. The ADE of virus infection is a phenomenon in which virus-specific antibodies enhance the entry of virus, and in some cases the replication of virus, into monocytes/macrophages and granulocytic cells through interaction with Fc and/or complement receptors. This phenomenon has been reported in vitro and in vivo for viruses representing numerous families and genera of public health and veterinary importance. These viruses share some common features such as preferential replication in macrophages, ability to establish persistence, and antigenic diversity. For some viruses, ADE of infection has become a great concern to disease control by vaccination. Consequently, numerous approaches have been made to the development of vaccines with minimum or no risk for ADE. Identification of viral epitopes associated with ADE or neutralization is important for this purpose. In addition, clear understanding of the cellular events after virus entry through ADE has become crucial for developing efficient intervention. However, the mechanisms of ADE still remain to be better understood.

Modulation of the transcriptional rate of Fc gamma receptor mRNA in human mononuclear phagocytes

Human monocytes and macrophages bear three classes of cell surface receptors for the Fc portion of IgG (Fc gamma RI, Fc gamma RII, and Fc gamma RIII). These receptors mediate phagocytosis and other effector functions and are important in the pathophysiology of hematologic disease, inflammation, and host defense. We have previously shown that interferon-gamma (IFN-gamma) and dexamethasone modulate total Fc gamma RII mRNA levels as well as Fc gamma RI and Fc gamma RII protein expression on monocytes and the monocyte-like cell line U937. In this study, we investigated the modulation of Fc gamma RI mRNA. Additionally, we utilized mRNA stability and nuclear run-on assays to study the mechanism of the modulation of Fc gamma receptor transcripts in the monocyte/macrophage cell line U937. In U937 cells, IFN-gamma increased Fc gamma RI mRNA levels 7.5-fold. Treatment with dexamethasone reduced Fc gamma RI mRNA levels to 0.6-fold of baseline and inhibited by 20-60% the increase in mRNA observed after treatment of the U937 cells with IFN-gamma. On monocytes, treatment with IFN-gamma increased monocyte Fc gamma RI mRNA 6.7-fold. Cotreatment of the IFN-gamma-stimulated monocytes with dexamethasone resulted in a 160% further increase in Fc gamma RI expression. Fc gamma RI and Fc gamma RII mRNA half-lives were then determined in U937 cells by incubation with dexamethasone and/or IFN-gamma for 16 hr and then arresting mRNA transcription with actinomycin-D (10 micrograms/ml). The mRNA half-lives in untreated U937 cells were 3.3 +/- 0.3 hr (Fc gamma RI) and 3.1 +/- 0.3 hr (Fc gamma RII). For either Fc gamma RI and Fc gamma RII, the effect of dexamethasone and/or IFN-gamma on mRNA half-life was not significant (P > 0.5). We also performed nuclear run-on experiments with U937 cells which indicated that IFN-gamma increased the transcription of Fc gamma RI 4.2-fold and Fc gamma RII 1.7-fold. Our data suggest that these changes in Fc gamma RI and Fc gamma RII protein are likely due, at least in part, to increases in mRNA levels secondary to alteration in gene transcription.

Dexamethasone up-regulates mannose receptor activity by increasing mRNA levels

The macrophage mannose receptor is highly susceptible to modulation by a variety of inflammatory and anti-inflammatory agents. Previous studies have demonstrated that mannose receptor activity is dramatically enhanced in rat bone marrow macrophages following treatment with dexamethasone. In the present study we have investigated potential mechanisms that might be involved in this up-regulation. Uptake of ligands by the mannose receptor was increased 2.5-fold in a dose- and time-dependent manner. Maximal stimulation was seen following treatment of macrophages with 0.1-1.0 microgram/ml of dexamethasone for 24-48 h. Dexamethasone treatment increased both the number of cell surface binding sites and total cellular binding activity to 250% of control levels. In addition, total receptor protein as measured by immunoprecipitation was increased 2.5-fold. Neither the maturation rate nor the turnover rate of the protein was altered by dexamethasone treatment. Using an oligonucleotide probe derived from sequence data from the cloned human receptor cDNA, we investigated the effect of dexamethasone on the expression of mannose receptor mRNA. Following incubation with dexamethasone for 12-24 h, the level of mRNA was significantly increased. These results demonstrate that dexamethasone treatment of rat bone marrow macrophages induces synthesis of new receptor protein through an increase in the level of mannose receptor mRNA.

Granulocyte Fc gamma receptor recognition of cell bound and aggregated IgG: effect of gamma-interferon

Granulocyte Fc gamma receptors are important components in the recognition of IgG-coated cells and immune complexes. Two proteins have been identified on resting human granulocytes which function as Fc gamma receptors, Fc gamma RII (CD32) and Fc gamma RIII (CD16). A third protein, Fc gamma RI (CD64), is not constitutively expressed on resting granulocytes, but can be induced by activation with gamma-interferon. We examined the role of these three Fc gamma receptors on human granulocytes in the binding of both IgG-sensitized erythrocytes and soluble oligomeric IgG. In these studies we employed anti-Fc gamma receptor antibodies which complete for the Fc gamma RII and Fc gamma RIII ligand binding sites. Preincubation of granulocytes with saturating concentrations of high-affinity anti-Fc gamma RII monoclonal antibody did not alter the recognition of IgG sensitized human cells by granulocytes. Furthermore, ligand binding studies demonstrated that anti-Fc gamma RII antibody altered neither the number nor the affinity of granulocyte binding sites for human trimeric IgG. In contrast, Fab anti-Fc gamma RIII inhibited the binding of both IgG (anti-D) sensitized human RBCs and IgG sensitized sheep RBCs. Similarly, a reduction in the expression of Fc gamma RIII by treatment with phosphatidyl-inositol specific phospholipase C reduced PMN recognition of IgG-sensitized cells. Also, anti-Fc gamma RIII decreased the number of granulocyte binding sites for human IgG trimer without a change in receptor affinity. Fc gamma RI, which was induced by gamma-IFN, increased granulocyte recognition of both IgG sensitized RBCs and IgG trimer. These data suggest that Fc gamma RIII is the primary Fc gamma receptor on granulocytes which recognizes IgG sensitized RBCs and low molecular weight complexes of IgG. With gamma-interferon activated granulocytes, Fc gamma RI appears to enhance this recognition process.

The immunobiology of human Fc gamma receptors on hematopoietic cells and tissue macrophages

The ability to identify and remove potentially dangerous foreign organisms and proteins is critical to the survival of a species. In humans, IgG antibodies serve to recognize such foreign antigens and to facilitate their removal. The latter process involves several hematopoietic cells, including those of the macrophage phagocytic system, which express Fc gamma receptors. These receptors specifically recognize the Fc domain of IgG antibody. On the surface of hematopoietic cells these Fc gamma receptors mediate cell attachment and stimulate several signal transduction events, including those which lead to phagocytosis. Moreover, their perturbation can lead to the generation and release of mediators and enzymes involved in inflammation and in the pathogenesis of autoimmune diseases.

A five day course of high dose corticosteroids has no effect on the clearance of soluble aggregates of human immunoglobulin G in healthy volunteers

It is thought that glucocorticosteroids impair the clearance of immune complexes by the mononuclear phagocyte system (MPS). We studied the effect of a five day course of prednisone (1 mg/kg body weight per day) on MPS function in 10 healthy volunteers, using soluble radiolabeled aggregates of human immunoglobulin G as a probe. MPS function was assessed before steroid treatment, and again 24 hours after the last dose of prednisone. Elimination kinetics and the uptake of the immunoglobulin aggregates by liver and spleen did not change after prednisone treatment. This suggests that, in contrast to general belief, high doses of glucocorticosteroids have no direct, acute effect on MPS function.

Impaired function of macrophage Fc gamma receptors in end-stage renal disease

Infection is a frequent complication in patients undergoing hemodialysis for end-stage renal disease and is the primary cause of mortality among such patients. Macrophages are important in host defense against infection largely because their Fc gamma receptors recognize antibody-coated bacteria. We therefore studied macrophage Fc gamma-receptor function in vivo and in vitro in 56 patients with end-stage renal disease who were on hemodialysis and in 20 healthy volunteers. The clearance of IgG-coated (sensitized) autologous red cells was decreased in 53 patients. The inhibition of clearance in the 56 patients was 52 +/- 3 percent at 1 hour, 41 +/- 5 percent at 1 1/2 hours, and 29 +/- 5 percent at 2 hours (P less than 0.001). The clearance of unsensitized erythrocytes and heat-altered autologous erythrocytes was normal. The impairment of clearance was not correlated with age, sex, nutritional status, HLA haplotype, or the presence of circulating immune complexes. The recognition of these IgG-sensitized red cells in vitro by Fc gamma RI (an Fc gamma-receptor protein that binds monomeric IgG) on blood monocytes from the patients was also significantly decreased (P less than 0.001) but was partially improved by hemodialysis. Nine patients had severe infections during a two-year follow-up period. The clearance of IgG-coated cells in these patients (half-time, 12.9 +/- 1.7 hours) was significantly impaired, as compared with that in the 47 patients without severe infections (half-time, 4.4 +/- 1.8 hours; P less than 0.001). We conclude that macrophage Fc gamma-receptor function is impaired in patients with end-stage renal disease who are undergoing hemodialysis, and that this impairment probably contributes to the observed immunodepression and high prevalence of infection among such patients.

Modulation of human mononuclear phagocyte Fc gamma RII mRNA and protein

Human monocytes and macrophages express three different classes of cell surface receptors for the Fc portion of IgG, Fc gamma RI (CD64), Gc gamma RII (CD32), and Fc gamma RIII (CD16). We utilized a cDNA probe for Fc gamma RII to examine the modulation of Fc gamma RII mRNA by dexamethasone, a synthetic glucocorticoid, and interferon-gamma. We also determined the changes in the expression of both Fc gamma RI and Fc gamma RII protein following treatment with these agents by flow cytometry. In studies performed with the monocyte-like cell line. U937, Northern blot analysis revealed that cells treated with interferon-gamma showed a 2.5-fold increase in Fc gamma RII mRNA levels that was maximal at 14 hr and declined to 1.4-fold over baseline by 48 hr of incubation. Treatment of U937 cells with dexamethasone did not significantly change the level of Fc gamma RII transcripts, but was able to inhibit by up to 50% the increase seen following interferon-gamma treatment. The expression of Fc gamma RII protein on U937 cells was increased 56-72% after 16-24 hr of interferon-gamma treatment, but was only 18% over baseline after 48 hr of incubation. Treatment with dexamethasone caused a small, but significant, decrease in Fc gamma RII protein, and inhibited by 20-60% the induction of Fc gamma RII by interferon-gamma. The modulation by dexamethasone and interferon-gamma of Fc gamma RI protein expression on U937 cells was markedly different from that of Fc gamma RII in both magnitude and kinetics. Interferon-gamma treatment increased Fc gamma RI expression by 240% at 16 hr, and Fc gamma RI remained elevated through 48 hr. Treatment with dexamethasone decreased Fc gamma RI expression by 39%, and also inhibited by 40% the increase induced by interferon-gamma. In contrast to the findings with U937 cells, dexamethasone and/or interferon-gamma treatment had no significant effect on Fc gamma RII mRNA levels or protein expression in monocytes. However, interferon-gamma treatment increased Fc gamma RI expression on monocytes, and this increase was further augmented by treatment with dexamethasone. These data indicate that the modulation of Fc gamma RII on U937 cells is at least in part due to changes in steady state levels of Fc gamma RII mRNA. The difference between the magnitude of the changes in Fc gamma RII mRNA and protein suggests that some translational or post-translational control is involved in regulating the expression of Fc gamma RII.(ABSTRACT TRUNCATED AT 400 WORDS)

Fc gamma receptor recognition of IgG ligand by human monocytes and macrophages

We examined the binding characteristics of human monocytes and macrophages with the IgG ligands, human monomeric IgG and a small human IgG aggregate, trimeric IgG. Our purpose was to utilize fresh monocytes, in vitro cultured monocytes, and alveolar macrophages in direct and indirect binding experiments. Freshly isolated monocytes expressed only a single binding site for IgG monomer and IgG trimer. In contrast, in vitro cultured monocytes, gamma-interferon-treated monocytes, and freshly isolated alveolar macrophages expressed a single binding site for IgG monomer and, in addition, a high and low affinity binding site for IgG trimer. The high affinity binding site for IgG trimer (Kd approximately equal to 1 nM) appeared identical to the binding site for IgG monomer. The low affinity binding site for IgG trimer (Kd = 50 to 250 nM) appeared to be due to Fc gamma RII, because antibody to Fc gamma RII inhibited its expression. Since Fc gamma RII, in contrast to Fc gamma RI, does not bind monomeric IgG, the data suggest that this low affinity receptor for trimeric IgG, Fc gamma RII, can bind low molecular weight circulating immune complexes at concentrations 10- to 100-fold lower than Fc gamma RI. Thus, these studies suggest that at 37 degrees C, macrophage Fc gamma RII may play a functional role in the recognition of small molecular weight immune complexes.