H2-O Mediated Antigen Focusing Modulates B Cell Immunity

描述（由申请人提供）：MHC II类分子在内体区室中装载有其肽货物。该过程由II类分子HLA-DM（DM;小鼠中的H2-M）催化。HLA-DO（DO;小鼠中的H2-O），一种MHC编码的II类分子，在转运至内体隔室期间和之后与DM缔合。有趣的是，与II类加工途径的所有其他组分不同，DO表达仅限于B细胞和胸腺上皮细胞。现在清楚的是，DO改变DM的肽装载活性。不太清楚的是DO的净效应; DO可以促进或抑制II类肽加载，这取决于实验系统。DO/H2-O对B细胞的限制性表达使我们假设H2-O主要对通过B细胞受体（BCR）内化的抗原（Ag）发挥其作用。我们的初步数据显示，BCR连接导致H2-O从H2-M快速解离，并且游离H2-M与内化的BCR也显著区室化。将Ag呈递的关键组分集中在一个隔室中清楚地表明了H2-O仅在BCR与Ag的BCR接合后显著增强II类呈递的机制。这种“Ag聚焦”机制似乎对于T依赖性B细胞免疫应答，特别是对于表达低亲和力BCR的B细胞至关重要。这些概念将在本申请中直接检验。 在目标1中，我们将1）确认并扩展我们的研究，检查BCR连接细胞中H2-O与H2-M的解离; 2）鉴定H2-O和H2-M运输的亚细胞区室; 3）检查H2-O解离对II类呈递的影响。在目标2中，我们将确定H2-O在体内T细胞依赖性免疫应答中的作用。将检查H2-O表达对T细胞依赖性、抗原特异性抗体应答和生发中心形成的影响。我们还将研究H2-O对体细胞超突变、亲和力成熟和B细胞阳性选择的影响。拟议的研究将阐明H2-O在体内II类抗原加工途径中的作用，在免疫应答过程中。这些研究与肿瘤免疫、自身免疫和疫苗开发相关。

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.