British Journal of Haematology , 2001, 112, 1031±1040 Intravenous immunoglobulin preparations induce mild activation of neutrophils in vivo via triggering of macrophages ± studies in a rat model Jessica L. Teeling, 1 Wim K. Bleeker, 1 Gemma M. M. Rigter, 1 Nico van Rooijen, 2 Taco W. Kuijpers 1,3 and C. Erik Hack 1,4 1 Central Laboratory of The Netherlands' Red Cross Blood Transfusion Service and Laboratory of Experimental and Clinical Immunology, Academic Medical Centre, University of Amsterdam, 2 Department of Cell Biology, Faculty of Medicine, Amsterdam, 3 Department of Paediatrics, Academical Medical Centre, University of Amsterdam, and 4 Department of Clinical Chemistry, Academical Hospital 0 Vrije Universiteit', Amsterdam, The Netherlands. Received 13 August 2000; accepted for publication 27 November 2000 Summary. Despite widespread use in various immune disorders, the in vivo mechanisms of action of intravenous immunoglobulin (IVIG) preparations are not well known. We previously reported that human neutrophils degranu- late after incubation with IVIG in vitro as a result of interaction with FcgRII. The purpose of this study was to determine whether IVIG might stimulate neutrophils in vivo. Anaesthetized rats received a bolus intravenous injection of IVIG preparations, containing either high (aged IVIG) or low (fresh IVIG) amounts or IgG dimers at a dose of 250 mg/kg. Administration of aged IVIG induced neutrophil activation in vivo, whereas no effect was observed after infusion of fresh IVIG. Histological examination of lung tissue demonstrated mild influx of neutrophils into the pulmonary tissue after aged IVIG administration, though gross damage did not occur. Macrophage-depleted rats no longer showed activa- tion of neutrophils after infusion of aged IVIG, suggesting that neutrophils become activated via an indirect macro- phage dependent way. We conclude that IVIG induces a mild activation of neutrophils in vivo via triggering of macro- phages depending on the amount of IgG dimers. For this reason, IVIG preparations with a high content of dimers may not always be as harmless as generally believed and may be responsible for some of the side-effects observed during IVIG infusions. Keywords: IVIG, neutrophils, macrophages, rats, L-selectin, CD11b. Although intravenous immunoglobulin (IVIG) preparations were initially introduced as replacement therapy in primary antibody deficiency disorders, they were found to have beneficial effects in patients with autoimmune thrombo- cytopenic purpura and other autoimmune diseases (Dwyer, 1992; Wolf, 1996). Nowadays, IVIG is used in a broad range of autoimmune and systemic inflammatory disorders. Despite its widespread use, the precise mechanisms of action of IVIG are still largely unknown. Different mechanisms have been proposed, such as Fcg-receptor blockade, inhibi- tion of complement deposition, neutralization of super- antigens, neutralization of cytokines and manipulation of the idiotypic network (Imbach et al, 1981; Sultan et al, 1984; Aukrust et al, 1994; 1997; Mouton et al, 1994; Andersson et al, 1996; Basta, 1996; Skanse Ân-Saphir et al, 1997). We previously reported that IVIG interacts with neutro- phils in vitro through binding of IgG dimers and polymers to their Fcg-receptors (Teeling et al, 1998). In particular, binding of IVIG to FcgRIIa was shown to induce activation and, subsequently, degranulation of neutrophils. Neutrophils are endowed with potent mechanisms to kill microbes and thus contribute to the host's defence against microbicidal infections. However, they have also long been recognized as important effectors of the adverse effects of inflammatory reactions because their excessive accumula- tion in tissues may contribute to injury of the host as a result of release of a variety of toxic products. The lungs are particularly susceptible to such insults and constitute a primary site of damage induced by host-defence mechan- isms (Chignard & Renesto, 1995; Kubo et al, 1998). Hence, interaction of IVIG with neutrophils in vivo may, among others, contribute to the development of clinical (side) effects of IVIG. Because IVIG is able to directly activate neutrophils q 2001 Blackwell Science Ltd 1031 Correspondence: J. L. Teeling, Pathophysiology of plasma proteins, Central Laboratory of The Netherlands' Red Cross Blood Transfusion Service, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands. E-mail: J_Teeling@clb.nl