Inactivated pepsin inhibits neutrophil activation by Fcgamma-receptor-dependent and independent stimuli.

Pepsin is widely used to produce F(ab')2 fragments of immunoglobulin G (IgG). In many cases, at least part of the pepsin will remain present in the F(ab')2 preparation, albeit in (irreversibly) inactivated form. Here we report on a potent immunomodulatory effect of irreversibly inactivated pepsin on activated human neutrophils. Degranulation, induced by coated IgG or via cytochalasin B/N-formyl-Met-Leu-Phe, was measured by quantifying elastase release, and was found to be inhibited in a dose-dependent manner by inactivated pepsin. Since a number of intravenous immunoglobulin (IVIg) products are also treated by limited digestion with pepsin, we investigated if pepsin would be present in quantities large enough to inhibit neutrophil activation. The amounts of pepsin detected in three different pepsin-treated IVIg products were found to be too low to induce an effect, at least in an in vitro setting.

Inhibition of FcγR-mediated phagocytosis by IVIg is independent of IgG-Fc sialylation and FcγRIIb in human macrophages.

In immune thrombocytopenia and warm autoimmune hemolytic anemia, circulating immunoglobulin G (IgG)-opsonized blood cells are cleared from the circulation by macrophages. Administration of intravenous immunoglobulin (IVIg) can prevent uptake, but the exact working mechanism is not known. The prevailing theory from murine studies, which states that Fc-sialylated IgG alters the balance between activating and inhibitory Fc-gamma receptors (FcγRs) by inducing upregulation of the inhibitory FcγRIIb on effector macrophages, is currently debated. We studied phagocytosis of IgG-opsonized blood cells in a human system, assessing the effect of IVIg and blocking anti-FcγR F(ab')2 fragments on uptake by monocyte-derived macrophages (both M1 and M2 macrophages). Phagocytosis was remarkably sensitive to administration of IVIg, but unexpectedly, recombinant Fc-sialylated IgG or sialic acid-enriched IVIg were equally active as unsialylated IgG fractions in mediating this inhibition, independent of FcγRIIb expression. Instead, IVIg inhibited phagocytosis by direct blockade of FcγRs. IgG fractions enriched for IgG dimers with enhanced avidity for FcγRs showed increased inhibition compared with monomeric IgG fractions. Together, our data demonstrate that inhibition of IgG-mediated phagocytosis in human macrophages by IVIg is dependent on the capacity to directly bind FcγRs but is independent of FcγRIIb or sialylation of the Fc fragment in the human setting.

A prominent lack of IgG1-Fc fucosylation of platelet alloantibodies in pregnancy.

Immunoglobulin G (IgG) formed during pregnancy against human platelet antigens (HPAs) of the fetus mediates fetal or neonatal alloimmune thrombocytopenia (FNAIT). Because antibody titer or isotype does not strictly correlate with disease severity, we investigated by mass spectrometry variations in the glycosylation at Asn297 in the IgG Fc because the composition of this glycan can be highly variable, affecting binding to phagocyte IgG-Fc receptors (FcγR). We found markedly decreased levels of core fucosylation of anti-HPA-1a-specific IgG1 from FNAIT patients (n = 48), but not in total serum IgG1. Antibodies with a low amount of fucose displayed higher binding affinity to FcγRIIIa and FcγRIIIb, but not to FcγRIIa, compared with antibodies with a high amount of Fc fucose. Consequently, these antibodies with a low amount of Fc fucose showed enhanced phagocytosis of platelets using FcγRIIIb(+) polymorphonuclear cells or FcγRIIIa(+) monocytes as effector cells, but not with FcγRIIIa(-) monocytes. In addition, the degree of anti-HPA-1a fucosylation correlated positively with the neonatal platelet counts in FNAIT, and negatively to the clinical disease severity. In contrast to the FNAIT patients, no changes in core fucosylation were observed for anti-HLA antibodies in refractory thrombocytopenia (post platelet transfusion), indicating that the level of fucosylation may be antigen dependent and/or related to the immune milieu defined by pregnancy.

Defects in neutrophil granule mobilization and bactericidal activity in familial hemophagocytic lymphohistiocytosis type 5 (FHL-5) syndrome caused by STXBP2/Munc18-2 mutations.

Familial hemophagocytic lymphohistiocytosis (FHL) is caused by genetic defects in cytotoxic granule components or their fusion machinery, leading to impaired natural killer cell and/or T lymphocyte degranulation and/or cytotoxicity. This may accumulate into a life-threatening condition known as macrophage activation syndrome. STXBP2, also known as MUNC18-2, has recently been identified as the disease-causing gene in FHL type 5 (FHL-5). A role for STXBP2 in neutrophils, and for neutrophils in FHL in general, has not been documented thus far. Here, we report that FHL-5 neutrophils have a profound defect in granule mobilization, resulting in inadequate bacterial killing, in particular, of gram-negative Escherichia coli, but not of Staphylococcus aureus, which rather depends on intact reduced NAD phosphate oxidase activity. This impairment of bacterial killing may contribute to the apparent susceptibility to gastrointestinal tract inflammation in patients with FHL-5.

Dimeric IgG complexes from IVIg are incapable of inducing in vitro neutrophil degranulation or complement activation.

PURPOSE: Intravenous immunoglobulin (IVIg) products contain various amounts of dimeric IgG complexes. Current insights into the possible biological activities of these dimers remain controversial, and both immunemodulating and immune-activating effects have been reported. Here, we analyzed the putative immune-activating effects of dimers isolated from IVIg. METHODS: Dimers isolated from IVIg were purified by high-performance size-exclusion chromatography (HP-SEC) and tested for the ability to induce neutrophil degranulation in vitro. RESULTS: Dimers isolated from IVIg were found to be incapable of inducing in vitro neutrophil degranulation or complement activation, even at concentrations exceeding those expected to be reached upon administration in patients. These results depend on the removal of artefactual activation by using 0.1 micron filtration and the use of poloxamer to prevent adsorption of IgG onto the solid phase. CONCLUSIONS: The data suggest dimeric IgG found in IVIg may bind to Fc-receptors without causing activation.

Preventing adsorption of immunoglobulin G to solid surfaces using poloxamer 407 eliminates artifactual stimulation of neutrophils.

To study the effect of polyclonal intravenous immunoglobulin G (IVIG) on neutrophils in vitro, adsorption of immunoglobulin G (IgG) to solid surfaces has to be prevented, because IgG bound to a solid surface can activate neutrophils through activating FcγRs. In this study we demonstrate that poloxamer 407, a non ionic surfactant, at low concentration (0.05%) prevented the adsorption of high concentrations of IgG (5 mg/ml) better than other blocking agents without interfering with the interaction of IgG with the neutrophils. Poloxamer 407 is therefore a suitable blocking agent to prevent the interaction of immunoglobulin with solid surfaces in cell-based in vitro experiments.