IFN-γ Regulation of the Type IV Class II Transactivator Promoter in Astrocytes

The transcriptional activation of class II MHC genes requires the class II transactivator (CIITA) protein, a regulator that is essential for both constitutive and IFN-γ-inducible class II MHC expression. The CIITA gene is controlled by multiple independent promoters; two promoters direct constitutive expression, while another, the type IV CIITA promoter, mediates IFN-γ-induced expression. We investigated the molecular regulation of IFN-γ-induced type IV CIITA promoter activity in astrocytes. IFN-γ inducibility of the type IV CIITA promoter is dependent on three cis-acting elements contained within a 154-bp fragment of the promoter; the proximal IFN-γ activation sequence (GAS) element, the E box, and the proximal IFN regulatory factor (IRF) element. Two IFN-γ-activated transcription factors, STAT-1α and IRF-1, bind the proximal GAS and IRF elements, respectively. The E box binds upstream stimulating factor-1 (USF-1), a constitutively expressed transcription factor. Furthermore, STAT-1α binding to the proximal GAS element is dependent on the binding of USF-1 to the adjacent E box. Functionally, the proximal IRF element is essential for IFN-γ induction of type IV CIITA promoter activity, while the proximal GAS and E box elements contribute to the IFN-γ inducibility of this promoter. In astrocytes, TNF-α enhances IFN-γ-induced class II MHC transcription. Our results demonstrate that TNF-α does not enhance IFN-γ-induced transcriptional activation of the type IV CIITA promoter, indicating that the enhancing effect of TNF-α is mediated downstream of CIITA transcription. These results define the molecular basis of IFN-γ activation of the type IV CIITA promoter in astrocytes.

CD28 Costimulation Is Crucial for the Development of Spontaneous Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is a severe central nervous system disease. Experimental autoimmune encephalomyelitis (EAE) mimics MS in mice. We report that spontaneous development of EAE in RAG-1-deficient mice transgenic for a myelin basic protein (MBP)-specific TCR (TgMBP+/RAG-1−/−) requires expression of the T cell costimulatory molecule CD28. Surprisingly, T cells from CD28−/−TgMBP+/RAG-1−/− mice proliferate and produce IL-2 in response to MBP1–17 peptide in vitro, excluding clonal anergy as the mechanism of CD28-regulated pathogenesis. Proliferation of autoaggressive T cells was dependent on the concentration of the MBP peptide, as was the development of MBP-induced EAE in CD28-deficient PL/J mice. These results provide the first genetic evidence that CD28 costimulation is crucial for MBP-specific T cell activation in vivo and the initiation of spontaneous EAE.

Astrocytes Express Elements of the Class II Endocytic Pathway and Process Central Nervous System Autoantigen for Presentation to Encephalitogenic T Cells

Astrocytes are nonprofessional APCs that may participate in Ag presentation and activation of pathogenic CD4+ T cells involved in central nervous system (CNS) inflammatory diseases. Using immortalized pure astrocytes as a complement to the study of primary astrocytes, we investigated whether these astrocytes express elements involved in the class II endocytic pathway and if they are capable of processing native myelin basic protein (MBP), a step that could be necessary for initiating or perpetuating T cell recognition of this self-Ag in vivo. Upon IFN-γ-stimulation, primary and immortalized astrocytes up-regulate class II transactivator (CIITA), invariant chain (Ii) (p31 and p41), H-2Ma, and H-2Mb. Analysis of CIITA cDNA sequences demonstrated that CIITA transcription in astrocytes is directed by a promoter (type IV) that mediates IFN-γ-inducible CIITA expression and encodes a CIITA protein that differs in its N-terminal sequence from CIITA reported in professional APC. Comparing live and fixed APC for Ag presentation, we show that Ag processing by APC is required for presentation of native MBP to autopathogenic T cells specific for the major MBP epitope, Ac1-11. We have observed that primary astrocytes and some, but not all, astrocyte lines in the absence of contaminating microglia are capable of processing and presenting native MBP, suggesting that there may be heterogeneity. Our study provides definitive evidence that astrocytes are capable of processing CNS autoantigen, indicating that astrocytes have potential for processing and presentation of CNS autoantigen to proinflammatory T cells in CNS autoimmune disease.