Intravenous immunoglobulins (IVIgs) are a therapeutic preparation of pooled normal polyspecific human IgGs obtained from large numbers of healthy donors. The preparation contains antibodies to nonself-antigens (microbial antigens), antibodies to self-antigens (natural autoantibodies), and antibodies that recognize other antibodies, also known as antiidiotypic antibodies [1,2].
In order to explain the therapeutic benefit conferred by IVIg in a large number of heterogeneous disorders, several mutually nonexclusive mechanisms of action have been proposed, in the contexts of primary immune deficiency (PID), autoimmunity, and inflammation. In PID, it is generally believed that IVIg is beneficial, as it contains a broad spectrum of antibody specificities against bacterial, viral, parasitic, and mycoplasma antigens that are capable both of opsonization and neutralization of microbes and toxins. In light of some of our recent results on the effect of IVIg on dendritic cells and B lymphocytes, we ask whether, apart from a passive effect, IVIg in PIDs could have a role that goes beyond a mere transfer of neutralizing antibodies?
In the context of autoimmunity and inflammation, several mechanisms of action for IVIg have been proposed since the early 1980s, from the initial suggestion of Fc receptor saturation, to idiotypy, anticomplement effects, helper T-cell (Th) Th1/Th2 balance modulation, accelerated catabolism of pathogenic autoantibodies through saturation of the Fc neonatal receptor (FcRn), induction of inhibitory FcgammaRIIB receptors, and effects on dendritic cells and thymic-derived CD25+ CD4+ T regulatory cells (Tregs) .
PROPOSED INTRAVENOUS IMMUNOGLOBULIN MECHANISMS OF ACTION
Following the initial use of IVIg in idiopathic thrombocytopenic purpura by Paul Imbach, it was suggested that IVIg Fc fragment-mediated competitive blockade of FcgammaR inhibits binding of opsonized antigens [4,5]. In the mid-1980s, Kazatchkine et al.  demonstrated that in vitro, IVIg inhibited anti-Factor VIII autoantibodies from patients with autoimmune haemophilia. This effect was attributed to the presence in IVIg of antiidiotypic antibodies against anti-Factor VIII autoantibodies . Subsequently, several other groups demonstrated the existence of antiidiotypic antibodies against a wide range of pathogenic autoantibodies .
A proposed anti-inflammatory mechanism of action for IVIg was an anticomplement scavenging effect. Indeed, binding of IVIg with complement inhibits the generation of the C5b-9 membrane attack complex and subsequent complement-mediated tissue damage, such as that which occurs in muscle microvasculature during dermatomyositis . In the course of the 1990s, it was thought that inflammation and autoimmunity could be explained by Th1/Th2 balance modulation. Some studies pointed to the fact that IVIg could influence the equilibrium from proinflammatory Th1 phenotype to anti-inflammatory Th2 activity . Thus, in experimental autoimmune encephalomyelitis, IVIg inhibited a predominantly Th1 type of cellular immune response. There was a significant reduction in the amount of IL-2 and IFN-γ released by lymph node cells of IVIg-treated rats . Later studies by Jeffrey Ravetch presented the Fc moiety of immunoglobulins as the key element involved in IVIg mechanism of action. Indeed, the molecular basis for the anti-inflammatory effect of IVIg was shown to depend on the expression of inhibitory Fc receptor FcgammaRIIB on macrophages. In a murine model of immune thrombocytopenia, IVIg administration induced surface expression of FcgammaRIIB on splenic macrophages, thus conferring protection . More recent studies demonstrated that the anti-inflammatory activity of IgG depends on sialylation of the N-linked glycan of the IgG Fc fragment and implicates a novel Th2 pathway. Efforts are currently being made to exploit this property to engineer more potent IVIg replacement [12,13]. However, the presence of sialylated Fc is not a prerequisite for protection in certain animal models of autoimmune disease [14,15].
DENDRITIC CELLS: LINK BETWEEN INNATE AND ADAPTIVE IMMUNITY
Dendritic cells are specialized antigen presenting cells, which form a link between innate and adaptive immunity. Mature, activated dendritic cells trigger an immune response via a cascade of events that include inflammatory cytokine production and induction of effector T-cells. Several reports have highlighted the critical role of dendritic cells in immune tolerance via anti-inflammatory cytokines, anergy, or deletion/elimination of autoreactive T-cells, and induction of Tregs [16,17]. The equilibrium between activation and tolerance is controlled by a diversity of molecular signals, and the disruption of this equilibrium leads to immune dysfunction.
Depending on the disorders, distinct doses of IVIg are applied. In autoimmune and inflammatory diseases, high-dose therapeutic IVIg ranges from 1–2 g/kg, equivalent to 25–35 mg/ml plasma (0.15–0.21 mmol/l). These doses of IVIg are capable of inhibiting dendritic cells differentiation, maturation, and function [3,18]. Thus, tolerance is imposed due to the inhibition of dendritic cells function. In contrast, in primary and secondary immunodeficiencies, IVIg is used to maintain a normal level of circulating IgG. Inhibition of dendritic cells function is not a desired effect in these patients. Replacement is generally administered at doses between 300 and 500 mg/kg, corresponding to approximately 12 mg/ml plasma (0.072 mmol/l) immediately after administration. Distinct mechanisms of action could therefore be at the root of these differences in dose.
INTRAVENOUS IMMUNOGLOBULIN REPLACEMENT IN PRIMARY IMMUNE DEFICIENCIES
Primary immune deficiencies in which IVIg use has been studied include X-linked agammaglobulinaemia (XLA) and common variable immunodeficiency (CVID). X-linked agammaglobulinaemia is a disease consecutive to mutations in the Bruton tyrosine kinase (btk) gene, characterized by a paucity of circulating B-cells and a marked reduction in circulating natural antibodies. Mutations in btk appear to have no effect on myeloid cell and T-cell functions . CVID is a clinically and molecularly heterogeneous syndrome characterized by hypogammaglobulinaemia and recurrent bacterial infections .
Intravenous immunoglobulin effect on dendritic cells
A comparative analysis of differentiation markers on dendritic cells from healthy donors and XLA patients showed that CD1a, CD83, CD80, and CD86 levels were significantly lower in XLA patients compared with healthy individuals, suggesting a functional defect in dendritic cells from patients with PIDs. In vitro, signalling by 10 mg/ml IVIg restored normal phenotypes of dendritic cells from XLA and CVID patients (in terms of CD83, HLA-DR, CD11c, CD86, CD80, and CD40). Therefore, IVIg stimulates dendritic cells at physiological doses. The studies showed that natural antibodies in IVIg induced the secretion of tolerogenic cytokines (IL-10) [20–23]. Of note, this effect could also be due to lower dendritic cells activation in patients without antibodies.
Intravenous immunoglobulin effect on B-cells
In hypogammaglobulinaemic CVID patients, the inability of B-cells to produce immunoglobulins may be due to an intrinsic B-cell defect or to a defect in T-cell signalling to B-cells. As such, defects in CD40L or inducible co-stimulator signalling, or in interleukin secretion from T-cells may trigger a functional anergy in CVID patient B-cells. We examined whether IVIg can compensate for defective T-cell signalling. The CD40L–CD40 interaction is essential for normal B-cell function, and because anti-CD40 IgG have been described in IVIg, we hypothesized that natural anti-CD40 IgG are capable of restoring B-cell proliferation and immunoglobulin secretion, independent of T-cell help. We demonstrated that IVIg induces B-cell proliferation and IgM synthesis in CVID patients treated with ‘replacement’ IVIg dose (rather than high-dose) . Therefore, through effects on B-cells, ‘replacement therapy’ in PID may actively restore immune functions, in addition to its passive role in preventing recurrent infections.
In conclusion, data on IVIg used at different doses depending on the disorder are pointing to a dynamic equilibrium between activation and tolerance operating at the level of dendritic cells. It would be interesting to investigate this equilibrium in other immune deficiencies, and to study other cell populations involved in innate and adaptive immunity, to determine whether IVIg has additional effect(s) beyond simple substitution of pathogen-neutralizing antibodies, and most importantly, to assess how in-vitro data can be translated in-vivo to optimize patient treatment.