Lack of a Requirement for the Inhibitory Fcγ Receptor In IVIG-Mediated Treatment of ITP

Abstract 3338

One of the most quoted articles in publications and at International meetings regarding the mechanism of intravenous immunoglobulin (IVIg) therapy in autoimmune and inflammatory diseases is that by Samuelsson et al., Science 291, 484 (2001). This article deals with the mechanism of effect of IVIg being due to an increase in the expression and function of the inhibitory Fcγ receptor (FcγRIIB) in splenic macrophages. The conclusions in this Science paper are based primarily on the use of an experimental mouse model of immune thrombocytopenia (ITP) and mice made deficient in FcγRIIB (FcγRIIB^-/- knockouts). We have not been able to support these previous findings but instead find the presence of FcγRIIB receptor not necessary for successful treatment of experimental ITP through IVIg treatment. Administration of IVIg to wild-type Balb/c mice previously made thrombocytopenic using an escalating dose of antiplatelet antibody, MWReg30, leads to amelioration of experimental ITP while in untreated mice, platelet counts stay close to nadir. Similar dynamics of the amelioration of ITP were found using splenectomized or FcγRIIB^-/- knockout (Taconic Labs) Balb/c mice. However, as previously published (Blood 15, 558 (2003)), IVIg does not work with FcγRIIB^-/- knockout mice that are on a mixed B6:129S4 background, obtained from the Jackson Labs. Indeed, B6 (129S4-Fcgr2b^tm1Rav/J) FcγRIIB^-/- knockout mice made thrombocytopenic are completely unresponsive to IVIg treatment. However, surprisingly, when using the recommended control wild-type mice for this knockout, the B6.129SF2 mouse, we found that this wild-type FcγRIIB^+/+ mouse also does not respond to IVIg treatment. Confirmation of genotype was done; thus, we suggest that something about the 129S background prevents a response to IVIg therapy and this phenomenon is independent of the FcγRIIB. We have confirmed that wild-type 129S4 mice made thrombocytopenic do not respond to IVIg. We have also examined the B6 FcγRIIB^-/- knockout mice from Taconic Labs which are purported to be fully congenic and these mice also do not respond to IVIg. Because of the lack of response to IVIg of wild-type 129S4 mice, we believe that the Taconic B6 FcγRIIB^-/- knockout mice may, in fact, be mixed background as are the Jackson mice. Thus, we plan to use SNP protocols for determination of congenicity, comparing C57BL/6 and 129S4 strains to Taconic and Jackson Labs B6 FcγRIIB^-/- knockout mice. We expect to show that both B6 FcγRIIB^-/- knockout animals are not fully congenic, being a mixture of both C57BL/6 and 129S4 strains. Previous publications that indicated a lack of response to IVIg using FcγRIIB^-/- knockout mice either did not test the fully congenic Taconic Balb/c knockouts and/or failed to use the most suitable control animals with the nonfully congenic B6 knockouts, using instead, C57BL/6 wild-type mice which respond well to IVIg treatment. In order to avoid misinterpretation of results, all experiments testing the role for FcγRIIB in the mechanism of IVIg should be done using Taconic Balb/c FcγRIIB^-/- knockout mice until congenicity can be established with the B6 knockout animals. These data taken together lead us to the conclusion that the presence of the FcγRIIB receptor is not important for the therapeutic action of IVIg.