The role of ICOS and retinoic acid in Mercury-Induced Autoimmunity

Abstract

Metal-induced autoimmunity is an experimental model of environmentally induced autoimmune syndrome. Subtoxic doses of heavy metals administered to genetically susceptible mice resulted in the production of highly specific IgG antinucleolar antibodies (ANoA) accompanied by lymphoproliferation and serum increases in IgG1 and IgE. In this study, the induction of tolerance to mercuric chloride (HgCl2)-induced autoimmunity by pre-exposure to the low-dose mercury was reported. The ultimate mechanisms through which the immune system obtains the tolerance to a low dose of heavy metals remain unknown. The previous experiment showed that CD4+CD25+ regulatory T cells (Tregs) contributed to the maintenance of immunological self-tolerance and to the prevention of autoimmune diseases. The tolerized mice had a higher percentage of Tregs and ICOS+ regulatory T cells than the nontolerized mice. ICOS (Inducible T-cell COStimulator) is a costimulatory receptor homologous to CD28 and CTLA-4. The expression of ICOS occurs on activated T cells and is dependent upon TCR and CD28 signals. The anti-ICOS blockade restored the ability of tolerized mice to produce elevated amounts of IgG1, IgE and anti-nucleolar antibodies. The ICOS expression on Tregs and Teff cells increased after the mercury challenge. Mice that received anti-ICOS had a low percentage of Tregs and showed an increased production of several cytokines. Taken together, these results suggested that Tregs maintained the immune tolerance in response to chemical challenges. The ICOS pathway is important for the differentiation of Tregs and blocking this pathway could prevent peripheral tolerance to the low dose mercury. The results also showed the splenocytes from the tolerized group produced a higher amount of IL-10 than the nontolerized group. This promoted us to study the role of IL-10 in tolerance induction. To validate the role of Interleukin-10 in tolerance maintenance, the tolerized mice were treated with blocking anti-IL-10 and anti-IL-10 receptor mAb. Those tolerized mice treated with IL-10 blocking antibodies produced a higher amount of serum IgG1, IgE and anti-nucleolar antibodies compared with the group treated with control antibodies. This suggested IL-10 is critical in maintaining the peripheral tolerance in a mercury treated mouse model. All Trans retinoic acid (ATRA) is the metabolically active derivative of vitamin A and functions as potent regulator of gene expression. Vitamin A and retinoic acid play vital roles in the homeostatic control of the immune system because vitamin A-deficient individuals are incapable of controlling bacterial, viral, and protozoan diseases. In this study, the role of ATRA was reported in the first time in mercuric chloride (HgCl2)-induced autoimmunity by feeding the ASW mice ATRA. The results showed that retinoid acid exacerbated the mercury-induced autoimmunity. To study whether retinoic acid plays a role in low dose mercury induced tolerance, three groups of tolerized ASW mice were treated with either ATRA, mercury or both. The results showed retinoic acid could break the low-dose mercury-induced tolerance. The splenocytes from the group that received both mercury and ATRA treatment produced much more IL-2 and IFN-γ. This group had the lowest percentage of IL-10+ cells. The group that received ATRA had a lower percentage of early apoptosis cells. To further study the mechanism, a PCR array that included 84 genes, involved in T cell and B cell activation, was used to study four groups of mice treated with mercury, ATRA, both or neither. 10 candidate genes were selected and analyzed by Real-Time PCR to validate the PCR array results. Further study is needed to characterize the expression and the role of molecules that are upregulated by mercury and ATRA.