Deficiency of activating Fcγ-receptors reduces hepatic clearance and deposition of IC and increases CIC levels in mercury-induced autoimmunity

Abstract

Background: Inorganic mercury (Hg) induces a T-cell dependent, systemic autoimmune condition (HgIA) where activating Fcγ-receptors (FcγRs) are important for the induction. In this study we examined the influence of activating FcγRs on circulating levels and organ localization of immune complexes (IC) in HgIA.

Methods and principal findings: Mercury treated BALB/c wt mice showed a significant but modest increase of circulating IC (CIC) from day 12 until day 18 and day 35 for IgG2a- and IgG1- CIC, respectively. Mercury-treated mice lacking the trans-membrane γ-chain of activating FcγRs (FcRγ(-/-)) had significantly higher CIC levels of both IgG1-CIC and IgG2a-CIC than wt mice during the treatment course. The hepatic uptake of preformed CIC was significantly more efficient in wt mice compared to FcγR(-/-) mice, but also development of extrahepatic tissue IC deposits was delayed in FcRγ(-/-) mice. After 35 days of Hg treatment the proportion of immune deposits, as well as the amounts was significantly reduced in vessel FcRγ(-/-) mice compared to wt mice.

Conclusions: We conclude that mice lacking functional activating FcγRs respond to Hg with increased levels and altered quality of CIC compared with wt mice. Lack of functional activating FcγRs delayed the elimination of CIC, but also significantly reduced extrahepatic tissue localization of CIC.

Figure 1. Development of circulating IgG1 immune complexes and tissue IgG1 immune complex deposits.

Development of circulating IgG1-containing immune complexes and tissue IgG1 immune complex deposits during 26–35 days of treatment of female BALB/c wild type and FcRγ^−/− mice with 15 mg/L HgCl₂ in the drinking water or drinking water without any addition of Hg (controls). (A) PEG-precipitated circulating immune complexes containing IgG1 antibodies. The bars denote mean ± SD. ** p<0.01 and *** p<0.001 (Mann-Whitney's test). (B) Renal mesangial IgG1 and (C) splenic vessel wall IgG1 deposits. Each symbol represents a single mouse. * p<0.05 (Mann-Whitney's test).

Figure 2. Development of circulating IgG2a immune complexes.

Development of PEG-precipitated circulating IgG2a-containing immune complexes during 26–35 days of treatment of female BALB/c wild type and FcRγ^−/− mice with 15 mg/L HgCl₂ in the drinking water or drinking water without any addition of Hg (controls). The bars denote mean ± SD. * p<0.05, ** p<0.01 and *** p<0.001 (Mann-Whitney's test).

Figure 3. Uptake of circulating immune complexes in the liver.

Uptake of preformed circulating HSA/DNP-IgG immune complexes in the liver of female BALB/c wild type and FcRγ^−/− mice following 15–17 days of treatment with 15 mg/L HgCl₂ in the drinking water or drinking water without any addition of Hg (controls). The bars denote mean ± SD. * p<0.05 (Mann-Whitney's test).

Figure 4. Tissue IgG1 immune complex deposits.

Direct immunofluorescence using FITC-conjugated anti-IgG1 antibodies on cryostate sections from female BALB/c wild type (A, C) and FcRγ^−/− (B, D) mice treated with 15 mg/l HgCl₂ in the drinking water for 5 weeks. Heavy granular staining in splenic vessel walls (A) and renal mesangium (C) in wild type mice, but only slight deposits in the corresponding tissues of FcRγ^−/− mice (B, D).