Expression And Function Of CD49a In The Liver Immune System

Integrins are a large family of heterodimeric adhesion receptors, which are composed of noncovalently associated α and β chains. Intergins bind counterreceptors on adjacent cells or extracellular matrix proteins and play an important role in inflammation, innate and adaptive immunity.CD49a is α chain of integrin α1β1, the major receptor for type IV collagen in the extracellular matrix. CD49a is extensively expressed on mesenchymal cells and epithelial cells. Cytokines upregulated the surface expression of CD49a and activate that cellular functions relevant to the inflammation by "outside-in" signaling. Moreover, CD49a is also expressed on NK cells, NKT cells, T cells and macrophage after their activation.The liver is an important metabolic organ and is critical in protein, carbohydrate and lipid metabolism. Portal venous blood and hepatic arterial blood flow into the liver, pass through the sinusoid and leave the liver via inferior vein. Mount of immune cells exist in the liver sinusoid. Type IV collagen is the main component of the basement membrane in the liver sinusoid and may affect functions of hepatic immune cells in many aspects. CD49a, the receptor for type IV collagen, has been reported to be tightly correlated with migration, activation, proliferation and apoptosis of immune cells. Thus our study included two parts. In the first part, we investigated the role of CD49a in the T-cell immune response using the muring hepatitis model induced by Con A. The second part focused on the mechanism underlying the retention of CD49a+DX5-NK cells in the liver. The major results are described as following:Ⅰ:The role of CD49a in T-cell mediated acute hepatitis1. CD49a deficiency ameliorated liver injury caused by Con AWe injected mice with lethal dose and non-lethal dose of Con A respectively. After lethal dose of Con A treatment, all wild-type mice died while most of CD49a deficient mice survived, indicating CD49a improved the rate of survival significantly. Compared to the wild-type mice, CD49a deficient mice displayed lower serum ALT levels and less liver necrosis after non-lethal dose of Con A injection.2. CD49a deficiency impaired the production of inflammatory cytokinesWe compared the serum level of cytokines after Con A injection and found the inflammatory cytokines such as IFN-y and IL-17A were much lower in CD49a deficient mice. In vitro stimulation of liver mononuclear cells with Con A for24hours showed IFN-γ and IL-17A secreted by CD49a deficient liver mononuclear cells were much lower than those secreted by wild-type liver mononuclear cells.3. CD49a deficiency did not influence the recruitment of T cellsAs the main source for IFN-y and IL-17A during Con A-induced hepatitis are iNKT cells and CD4+T cells, and previous studies suggested that CD49a deficiency impaired T cell migration to the inflamed sites, we then investigated the recruitment of iNKT and CD4+T cells in the liver of CD49a deficient mice after Con A injection. We found no significant difference in the numbers of iNKT cells and CD4+T cells in the liver of CD49a deficient mice and wild-type mice. Thus CD49a deficicency did not influence the recruitment of T cells.4. CD49a is critical for the inflammatory cytokine production by iNKT anc CD4+T cellsWe detected the expression of CD49a on hepatic iNKT and CD4+T cells. We found CD49a is positive for iNKT cells both before and after Con A injection. By contrast, CD4+T cells seldom express CD49a in rest, but upregulated CD49a after Con A injection. The expression of CD49a on both iNKT and CD49a after Con A injection suggested CD49a maybe involved in the development of Con A-induced acute liver failure. Consistently, we found IFN-y and IL-17A derived from CD49a deficient iNKT cells and CD4+T cells are much less than those derived from wild-type iNKT cells and CD4+T cells. Thus CD49a enhanced inflammatory cytokine release by iNKT and CD4+T cells in Con A-induced acute liver failure.5. CD49a blockade antibody is therapeutic in Con A-induced acute liver failureTo further confirm the pro-inflammatory role of CD49a in Con A-induced acute liver failure, we pretreated mice with CD49a blockade mice to abrogate CD49a signal. We found CD49a blockade treatment attenuated Con A-induced acute liver failure, reflected in lower serum ALT levels and less liver necrosis. We also found lower serum levels of IFN-y and IL-17Ain the CD49a blockade group.Conclusion Ⅰ:CD49a prompt Con A-induced acute liver failure by enhancing the inflammatory cytokine production by iNKT and CD4+T cells but not by regulating the migration of iNKT cells and CD4+T cells. Our study unveils a yet unrecognized role of CD49a in acute liver failure which will shed new light on the mechanism of T-cell mediated immune response in acute liver failure. Further, the expression pattern and role of CD49a on iNKT cells have not been demonstrated before. Here, we describe CD49a expression by hepatic iNKT cells and CD4+T cells in different patterns and investigate the impact of CD49a on the function of iNKT cells and CD4+T cells. Our findings suggest CD49a is correlated with IFN-y and IL-17A production by T cells and thus may be a potential therapeutic target for acute liver failure.II. The mechanism underlying the retention of CD49a+DX5-NK cells in the liver1. CD49a+DX5-NK cells are liver-specific NK cellsAbout half of the hepatic NK cells are DX5-. The percentage of DX5-NK cells in the liver is much higher than that in the spleen and bone marrow. Further, the proliferation of DX5-NK cells in the liver is not as active as those in the bone marrow.2. CD49a+DX5-NK cells are retained in the liver sinusoidCD49a+DX5-NK cells did not exist either in the afferent or in the efferent blood. However, we detected the existence of DX5-CD49a+NK cells in the liver sinusoid blood. Immunofluorescence assay showed DX5-NK cells in the liver adhered to liver sinusoid endothelial cells. The numbers of hepatic DX5-NK decreased after depletion of kuffer cells.3. CD49a+DX5-NK cells exist in the liver at very early phase of individual developmentWe detected the composition of NK cells in the liver of mice at different ages and found the majority of NK cells in the livers from fetal mice and neonatal mice were CD49a+DX5-.4. CD49a+DX5-NK cells adhere to type IV collagenImmunofluorescence assay showed CD49a+DX5-NK cells in the liver adhered to type IV collagen in vivo. However, interaction between CD49a and type IV collagen is not the determinant for the retention of CD49a+DX5-NK cells as CD49a deficient mice had normal percentage of DX5-NK cells in the liver of CD49a deficient mice.Conclusion Ⅱ:Hepatic CD49a+DX5-NK cells are retained in the liver sinusoid and maybe derived from local precusors in the liver. CD49a+DX5-NK cells adhere to the liver sinusoid endothelial cells and their retention depend on Kupffer cells. As CD49a+DX5-NK cells also adhere to type IV collagen in the liver, their retention in the liver may be the combinational results of multiple elements.