Studies on dendritic cells in multiple sclerosis and experimental allergic encephalomyelitis

Abstract: Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) resulting in demyelination, axonal loss and, ultimately, formation of gliotic scar tissue in the brain and spinal cord. MS remains the major disabling neurological illness of young adults of North European origin. The cause of MS is unknown, and there exists no cure for the disease. MS lesions contain activated T cells and macrophages, suggesting that T cellmediated autoimmunity is involved in MS pathogenesis. Immune effectors like T and B cells have been extensively studied in MS, while studies of the initiators of immune responses, like dendritic cells (DCs), are lacking. DCs function not only as antigen-presenting cells, but also as critical controllers of immune responses, especially of T cell responses. DCs capture information concerning foreign pathogens entering from outside the body and transfer signals to the cells of the adaptive immune system. DCs are critical for the induction of primary immune responses as well as for the induction of immunological tolerance, and the polarization of T helper cells (Th) into either Th I or Th2 responses. In the present studies, the MS animal model experimental allergic encephalomyelitis (EAE) was induced in Lewis rats by immunization with myelin basic protein peptide 68-86 (MBP68-86) and complete Freund's adjuvant (CFA). DCs were either prepared from rat spleen adherent mononuclear cells (MNCs) or derived from bone marrow adherent cells by culture. Spontaneous remission of acute EAE was accompanied by changes of DC functions that included suppression of autoreactive T cell proliferation, induction of CD4+ T cell apoptosis and increased production of nitric oxide (NO) and IFN-[gamma]. Subcutaneous re- injection of bone marrow-derived DCs that have been cultured and pulsed with MBP68-86, resulted in tolerance to MBP68-86 + CFA-induced EAE. Tolerance was associated with high inducible NO synthase expression and NO production, which mediated T cell apoptosis, resulting in reduced infiltration of inflammatory cells within the CNS. In MS patients and controls, blood DCs were prepared from adherent MNCs. Human blood DCs were found to be potent stimulators of T cells in allogeneic and, to a lesser extent, in autologous mixed leukocyte reactions (MLR), reflecting specifically stimulatory function of DCs. MS patients had higher blood levels of IFN-[gamma]-, TNF-[alpha]- and IL-6-secreting DCs than healthy controls. The differences for IFN-[gamma]- and TNF-[alpha]-secreting cells were confined to the subgroup of untreated MS patients, and not observed in the subgroup undergoing IFN-[beta] treatment. Blood DCs from MS patients expressed low levels of the co- stimulatory molecule CD86, which enhances Th2 cell differentiation. Correspondingly, production of IFN-[gamma] by blood MNCs was strongly enhanced by DCs derived from MS patients. IFN-[beta] and IL-10 separately inhibited stimulatory effects of DCs in MLR, and had additive effects in suppressing IL-12 production by DCs. Accordingly, DCs pre-treated in vitro with IFN-[beta] and IL-10 significantly suppressed IFN-[gamma] production by blood MNCs. IFN-[beta] also upregulated CD80 and CD86 expression on DCs in vitro. Upon culture in the presence of IFN-[beta], blood DCs differentiated into CD14- CD1a- HLA-DR+ cells which expressed CD123 (IL-3R[alpha]). IFN-[beta]-treated DCs produced predominantly IL-10 but only low levels of IL-12p4O. IFN-[beta]-treated DCs strongly suppressed IFN-[gamma] but enhanced IL-10 production by allogeneic blood MNCs. IFN-[beta]-treated DCs thus represent the DC2 subset of DCs, which secrete high levels of IIL-10 and are IL-12-deficient, providing a permissive environment for Th2 differentiation. In conclusion, DCs reveal heterogeneous functions during acute vs. recovery phase of EAE. During recovery, changes of DC functions included suppression of antigen-specific T cell proliferation and induction of CD4+ cell apoptosis. Bone marrow-derived DC pulsed with encephalitogenic peptide MBP69-86 induced tolerance to EAE, which was antigen-specific. DCs from MS patients showed increased secretion of pro-inflammatory cytokines, downregulated expression of CD86, and enhanced capacity to induce IFN-[gamma] production by MNCs. IFN-[beta]-treated DCs developed into Th2-driving DC2, which may have a potential in the therapy of diseases with an autoimmune background like MS.