Impairment of regulatory T cells in myasthenia gravis: studies in an experimental model.

Myasthenia gravis (MG) is an antibody mediated, T cell dependent autoimmune disease characterized by muscle fatigability in which autoantibodies directed to the acetylcholine receptor (AChR) impair neuromuscular transmission. The identification of CD4⁺CD25⁺Foxp3⁺Treg cells as important regulators of tolerance opened a major area of investigation raising the possibility that a dysfunction in the Treg compartment is involved in the etiology and pathogenesis of autoimmune diseases, including MG. In this paper we summarize shortly Treg abnormalities that were reported in MG patients and report on our studies of Treg in experimental autoimmune MG (EAMG). Hopefully these studies would pave the way towards the development of novel Treg-based treatment modalities that will restore self-tolerance in MG and other autoimmune diseases. In our previous studies in EAMG we have shown that Treg cells transferred from healthy rat donors to myasthenic rats suppress EAMG. However, Treg cells from sick animals do not have the same in vivo suppressive activity as those from healthy donors. The objective of the present study was to further characterize quantitative and qualitative alterations in Treg cells of rats with EAMG. We found that the frequency of CD4⁺CD25⁺Foxp3⁺Treg cells within the spleen and PBL was decreased in EAMG rats as compared to naïve and CFA-immunized healthy controls. Treg cells from myasthenic rats were less effective than Treg cells from controls in suppressing the proliferation of CD4⁺T effector cells in response to ConA and of B cells in response to LPS. Moreover, CD4⁺CD25⁺ cells from EAMG rats exhibited an elevated extent of apoptosis and expressed upregulated levels of FAS and of Th17-associated cytokines. Since EAMG is an induced disease, these quantitative and qualitative alterations in Treg cells do not reflect predisposing impairments and seem to be associated with the specific autoimmune response resulting from AChR immunization.

Regulatory T cell-based immunotherapies in experimental autoimmune myasthenia gravis.

Establishment of tolerance in myasthenia gravis (MG) involves regulatory T (T(reg)) cells. Experimental autoimmune MG (EAMG) in rats is a suitable model for assessing the contribution of T(reg) cells to the immunopathology of the disease and for testing novel T(reg) cell-based treatment modalities. We have studied two immunotherapeutic approaches for targeting of T(reg) cells in myasthenia. By one approach we demonstrated that treatment of sick rats by ex vivo-generated exogenous T(reg) cells derived from healthy donors suppressed EAMG. By a different approach, we aimed at affecting the endogenous T(reg)/Th17 cell balance by targeting IL-6, which has a key role in controlling the equilibrium between pathogenic Th17 and suppressive T(reg) cells. We found that treatment of myasthenic rats by neutralizing anti-IL-6 antibodies shifted this equilibrium in favor of T(reg) cells and led to suppression of EAMG. Our results show that T(reg) cells could serve as potential targets in treating MG patients.

Blocking of IL-6 suppresses experimental autoimmune myasthenia gravis.

Suppressive regulatory T cells (Treg) and pathogenic T helper 17 (Th17) cells are two lymphocyte subsets with opposing activities in autoimmune diseases. The proinflammatory cytokine IL-6 is a potent factor in switching immune responses in vivo from the induction of Treg to pathogenic Th17 cells. We studied the Treg and Th17 cell compartments in experimental autoimmune myasthenia gravis (EAMG) and healthy control rats in order to assess whether the equilibrium between Treg and Th17 cells is perturbed in the disease. We found that Th17 cell-related genes are upregulated and Treg-related genes are downregulated in EAMG. The shift in favor of Th17 cells in EAMG could be reversed by antibodies to IL-6. Administration of anti-IL-6 antibodies to myasthenic rats suppressed EAMG when treatment started at the acute or at the chronic phase of disease. Suppression of EAMG by anti-IL-6 antibodies was accompanied by a decrease in the overall rat anti-AChR antibody titer and by a reduced number of B cells as compared with control treatment. Administration of anti-IL-6 antibodies led to down-regulation of several Th17 related genes including IL-17, IL-17R, IL-23R and IL-21 but did not affect the number of Treg cells in the lymph nodes. These data identify IL-6 as an important target for modulation of autoimmune responses.

Involvement of phosphodiesterases in autoimmune diseases.

We have previously shown that several phosphodiesterase (PDE) subtypes are up-regulated in muscles and lymph node cells (LNC) of rats with experimental autoimmune myasthenia gravis (EAMG). In the present study we investigated PDE expression during the course of EAMG and experimental allergic encephalomyelitis (EAE) and found that the up-regulated expression of selected PDE subtypes in both experimental models is correlated with disease severity. In EAMG, PDE expression is correlated also with muscle damage. A similar up-regulation of PDE was also observed in the respective human diseases, MG and multiple sclerosis (MS). Our findings suggest that change in PDE expression levels is a general phenomenon in autoimmune diseases and may also be used as a marker for disease severity.

The upstream open reading frame of the Arabidopsis AtMHX gene has a strong impact on transcript accumulation through the nonsense-mediated mRNA decay pathway.

Approximately 20% of plant genes possess upstream open-reading frames (uORFs). The effect of uORFs on gene expression has mainly been studied at the translational level. Very little is known about the impact of plant uORFs on transcript content through the nonsense-mediated mRNA decay (NMD) pathway, which degrades transcripts bearing premature termination codons (PTCs). Here we examine the impact of the uORF of the Arabidopsis AtMHX gene on transcript accumulation. The suggestion that this uORF exposes transcripts containing it to NMD is supported by (i) the increase in transcript levels upon eliminating the uORF from constructs containing it, (ii) experiments with a modified uORF-peptide, which excluded peptide-specific degradation mechanisms, (iii) the increase in levels of the native AtMHX transcript upon treatment with cycloheximide, which inhibits translation and blocks NMD, and (iv) the sensitivity of transcripts containing the uORF of AtMHX to the presence of introns. We also showed that introns can increase NMD efficiency not only in transcripts having relatively short 3' untranslated regions (UTRs), but also in uORF-containing transcripts. AtMHX transcript levels were almost unaltered in mutants of the NMD factors UPF3 and UPF1. Possible reasons, including the existence of a NMD-compensatory mechanism, are discussed. Interestingly, the levels of UPF3 transcript were higher in upf1 mutants, suggesting a compensatory mechanism that links weak function of the NMD machinery to increased expression of UPF3. Our findings highlight that uORFs, which are abundant in plants, can not only inhibit translation but also strongly affect transcript accumulation.

Suppression of experimental autoimmune myasthenia gravis by inhibiting the signaling between IFN-gamma inducible protein 10 (IP-10) and its receptor CXCR3.

We have previously demonstrated that the chemokine IFN-gamma inducible protein 10 (IP-10) and its receptor CXCR3, are overexpressed in myasthenia gravis (MG) and its animal model experimental autoimmune MG (EAMG). We now studied the potential of modulating rat EAMG by interference in CXCR3/IP-10 signaling. Two different approaches were used: 1) blocking IP-10 by IP-10-specific antibodies and 2) inhibiting the CXCR3 chemokine receptor by a CXCR3 antagonist. Treatment by either of these reagents led to suppression of EAMG suggesting that inhibition of CXCR3/IP-10 signaling can be considered as a potential treatment modality for MG.