H2-O Mediated Antigen Focusing Modulates B Cell Immunity

描述（由申请人提供）：MHC II类分子在内体隔室中装载肽货物。这一过程是由类分子HLA-DM（小鼠为DM； H2-M）催化的。HLA-DO （DO；小鼠中的H2-O）是一种mhc编码的l类分子，在转运到内体室期间和之后都与DM相关。有趣的是，与II类加工途径的所有其他成分不同，DO的表达仅限于B细胞和胸腺上皮细胞。现在很清楚，DO改变了DM的肽负载活性。不太清楚的是DO的净效应；根据实验系统的不同，DO可以促进或抑制II类肽的负载。DO/H2-O在B细胞中的限制性表达使我们假设H2-O主要作用于通过B细胞受体（BCR）内化的抗原（Ags）。我们的初步数据表明，BCR连接导致H2-O与H2-M的快速解离，并且游离H2-M与内化BCR的显著区分开。将银呈递的关键成分集中在一个隔室中，清楚地表明，只有在BCR与银结合后，H2-O才能显著增强II类呈递。这种“Ag聚焦”机制似乎对t依赖性B细胞免疫应答至关重要，特别是对表达低亲和力BCR的B细胞。这些概念将在本应用程序中直接讨论。

DESCRIPTION (provided by applicant): MHC class II molecules are loaded with their peptide cargo in endosomal compartments. This process is catalyzed by the class ll-like molecule, HLA-DM (DM; H2-M in mice). HLA-DO (DO; H2-O in mice), an MHC-encoded, class ll-like molecule, associates with DM both during and after transport to endosomal compartments. Intriguingly, unlike all other components of the class II processing pathway, DO expression is restricted to B cells and thymic epithelial cells. It is now clear that DO modifies the peptide loading activity of DM. Less clear is the net effect of DO; DO can promote or inhibit class II peptide loading depending on the experimental system. The restricted expression of DO/H2-O to B cells has led us to hypothesize that H2-O exerts its affects predominantly on antigens (Ags) internalized via the B cell receptor (BCR). Our preliminary data shows that BCR ligation causes rapid dissociation of H2-O from H2-M and, also a dramatic compartmentalization of free H2-M with the internalized BCR. Focusing the critical components of Ag presentation within one compartment clearly suggests a mechanism by which H2-O substantially enhances class II presentation only after the BCR engagement of the BCR with Ag. Such an "Ag focusing" mechanism would appear to be critical for T-dependent B cell immune responses, particularly for B cells expressing low affinity BCR. These concepts will be directly examined in this application. In aim 1 we will 1) confirm and extend our studies examining H2-O dissociation from H2-M in BCR-ligated cells; 2) identify the subcellular compartments to which H2-O and H2-M traffic; and 3) examine the impact that H2-O dissociation has on class II presentation. In aim 2, we will determine the role H2-O plays in T cell-dependent immune response in vivo. The effect of H2-O expression on T cell-dependent, antigen specific antibody responses and germinal center formation will be examined. We will also study the impact of H2-O on somatic hypermutation, affinity maturation and B cell positive selection. The proposed studies will elucidate the role of H2-O in the class II antigen-processing pathway in vivo, during immune responses. These studies are relevant for tumor immunity, autoimmunity and vaccine development.