Expression Of Major Histocompatibility Complex Class II Molecules On Lungs In Pulmonary Fibrosis

Pulmonary fibrosis (PF) is the final common pathway of a diverse group of lung disorders known as a devastating disease. It's characterized by epithelial injury, the formation of distinctive fibroblast/myofibroblast foci, and excessive extracellular matrix accumulation. The mechanisms by which lung fibrosis develops are not fully ascertained. Recent studies proved that abnormality of immune function might play an important role in the development of PF. Major histocompatibility complex (MHC) class II molecules play a critical role in the induction and regulation of adaptive immune responses through presentation of processed antigen to CD4+T lymphocytes, and they contribute to the maintenance of self-tolerance, as well as to the breakdown of tolerance in autoimmune diseases. The class II transactivator CIITA is the key regulator for expression of MHC class II molecules. The highly regulated expression pattern of CIITA ultimately dictates the cell type specificity, induction and level of MHC class II expression. Human leukocyte antigen (HLA)-DR is the classical MHC class II antigen. MHC class II molecules, when aberrantly expressed on epithelial cells, result in the activation of autoreactive lymphocytes and play a primary role in the initiation of autoimmune disease. It has been proved that MHC class II molecules were involved in the development of hepatic and renal fibrosis in rats. In the liver, it presumes that antigen presentation by MHC class II molecules induces differentiation of B cells and acts as an accelerator of immunoglobulin production, and high immunoglobulinemia and deposition of immunoglobulin in the liver. And in the kidney, it is said that MHC class II expressing cells presents antigens to unprimed T cells, which determines Th1/Th2 lymphocyte polarization in turn. But to date, the role of MHC class II molecules in the pathogenesis of pulmonary fibrosis has not been reported.Objective: (1) To explore the expression levels of MHC class II molecules and its regulator gene CIITA on bleomycin-induced pulmonary fibrosis in rats. (2) To study the expression of HLA-DR molecules on lungs in patients with idiopathic pulmonary fibrosis.Methods: (1) Twenty male Wistar rats were randomly divided into control group, fibrosis group. The rats were either treated with a single intratracheal bleomycin injection (fibrosis group) or normal saline injection (control group). Four to six rats in each group were sacrificed 7 and 28 days after intratracheal instillation. Histological changes in the lungs were evaluated by hematoxylin-eosin and masson stain, and scored. Contents of hydroxyproline in lung tissues were detected. (2) The expression of MHC class II molecules in lung tissues was evaluated, and the percentage of MHC class II positive cells was measured. The amounts of total CIITA and type I, III and IV CIITA and TGF-β1 mRNA of lung tissues were measured by real-time PCR using Taqman probe. (3) Levels of IFN-γand IL-4 in bronchoalveolar lavage fluid (BALF) of rats were measured by enzyme-linked immunosorbent assay (ELISA). (4) HLA-DR antigens in lung specimens from 10 cases of idiopathic pulmonary fibrosis (IPF) were detected using immunohistochemical technology.Results: (1) The percentage of MHC class II positive cells in lung tissues increased significantly in fibrosis group compared with that of control group on the 7th day and 28th day(P<0.05, P<0.01, respectively); In fibrosis group, the percentage on day 28 was higher than that on day 7 (P<0.05); (2) Compared with control group on the 7th day, total CIITA mRNA increased 170.4%(P<0.05), type I CIITA mRNA increased 258.8%(P<0.05), while type IV CIITA mRNA decreased 87.2%(P<0.01); On the 28th day, total CIITA mRNA increased 98.6%(P<0.05), type I CIITA mRNA increased 137.1%(P<0.05), type IV CIITA mRNA still decreased, but there was no significant difference (P>0.05); In fibrosis group, type IV CIITA mRNA was 667.3%(P<0.01)higher on day 28 than that on day 7; (3) The amount of TGF-β1 mRNA increased significantly in fibrosis group compared with that of control group on day 7, 28(P<0.01, respectively); (4) The concentration of IFN-γin BALF of fibrosis group was significantly lower than that of control group on day 7, 28(P<0.01, respectively), yet the IL-4 of fibrosis group was significantly higher than that of control group(P<0.01, respectively); (5) The HLA-DR antigens were abnoamally expressed on hyperplastic bronchio-alveolar epithelial cells in IPF, but not in normal control lung tissues. The accumulated positive scores of HLA-DR was 27 in the IPF group and 2 in control group (Z=-3.002, P=0.001).Conclusion: (1) MHCII/CIITA system of lung tissues was involved in the bleomycin-induced rat pulmonary fibrosis. It might participate in the pathogenesis of lung fibrosis by regulating the Th1/Th2 balance in the lung. (2) Inappropriate MHC class II molecules expression was present in bronchio-alveolar epithelium in pulmonary fibrosis. Epithelial cells expressing class II MHC molecules might be better recognized by autoreactive CD4+T cells and serve as better targets for MHC class II mediated immunic injury, which might lead to repeated lung injury and abnormal repair and remodeling. This might lead to the formation of lung fibrosis and further progression.