Functional adaptation of intrathymically differentiating B cells for central T cell tolerance

胸腺基质细胞表现出许多独特的细胞生物学和分子特征，这些特征似乎已经进化到支持T细胞分化的特定方面。胸腺皮质上皮细胞使用独特的蛋白酶来产生正常阳性选择所需的MHC I类或ii类结合肽。胸腺髓质上皮细胞表达自身免疫调节因子（Aire）基因，从而异位表达外周组织抗原以诱导中枢耐受。某些迁移的树突状细胞亚群可通过携带外周抗原进入胸腺进一步增加中枢耐受范围。除了这些被深入研究的基质成分外，胸腺还含有其他研究较少的基质细胞类型，如B细胞。有趣的是，大多数胸腺B细胞似乎是通过胸腺内B淋巴生成而产生的，而不是骨髓中分化的血源性B细胞迁移的结果。一些报道表明，胸腺B细胞的抗原呈递可导致T细胞克隆性缺失。然而，尚不清楚胸腺B细胞是否仅仅代表了早期T细胞祖细胞发育可塑性的附带现象，还是反映了功能特化的胸腺B细胞谱系的真正生理需求。为了支持后一种观点，我们的初步工作揭示了胸腺B细胞和bm来源的主流B细胞之间的几个显着差异。因此，胸腺内分化的B细胞，与BM或次级淋巴器官中的B细胞相比，表达高水平的MHC II类和共刺激分子。然而，最令人惊讶的是，我们发现胸腺中有相当一部分B细胞表达Aire-mRNA和-蛋白，而BM或脾脏中没有。在此基础上，我们假设胸腺B淋巴生成不只是简单地复制BM中主流B细胞的分化，而是导致胸腺B细胞对T细胞耐受的关键功能相关的独特分子和表型特征的出现。在这项工作中，我们希望(i)解决胸腺B细胞的空气表达和其他耐受性特征是否以及如何通过微环境特异性外部线索进行协调，（ii）表征胸腺B细胞与骨髓源性B细胞相比的空气依赖和空气不依赖的分子和表型特征，以及（iii）研究胸腺B细胞的空气依赖和空气不依赖特征如何与CD4 T细胞耐受性相关。

Thymic stromal cells display a number of unique cell-biological and molecular features that seem to have evolved to support specific aspects of T cell differentiation. Cortical thymic epithelial cells use unique proteases for the generation of MHC class I- or II-bound peptides that are required for normal positive selection. Medullary thymic epithelial cells express the Autoimmune regulator (Aire) gene and thereby ectopically express peripheral tissue antigens for central tolerance induction. Certain migratory dendritic cell subsets may further increase the scope of central tolerance by carrying peripheral antigens into the thymus. Besides these intensely studied stromal components, the thymus also harbors other, less well investigated stromal cell types such as B cells. Interestingly, rather than being a result of immigration of blood-derived B cells that have differentiated in the bone marrow (BM), the majority of thymic B cells instead seem to arise through intrathymic B lymphopoiesis. Some reports have indicated that antigen presentation by thymic B cells can lead to clonal deletion of T cells. Nevertheless, it is unclear whether thymic B cells merely represents an epiphenomenon resulting from the developmental plasticity of early T cell progenitors or instead reflect a true physiological requirement of a functionally specialized thymic B cell lineage. In support of the latter idea, our preliminary work has revealed several remarkable differences between thymic B cells and BM-derived mainstream B cells. Thus, intrathymically differentiating B cells, in contrast to their counterparts in the BM or secondary lymphoid organs, express high levels of MHC class II and co-stimulatory molecules. Most surprisingly, however, we found that a substantial fraction of B cells in the thymus, but not in the BM or the spleen, express Aire-mRNA and -protein. On this basis, we hypothesize that thymic B lymphopoiesis does not simply duplicate mainstream B cell differentiation in the BM, but leads to the emergence of distinct molecular and phenotypic characteristics related to a critical function of thymic B cells for T cell tolerance. In the proposed work, we want to (i) address whether and how Aire-expression and other tolerogenic features of thymic B cells are orchestrated through microenvironment-specific extrinsic cues, (ii) characterize the Aire-dependent and Aire-independent molecular and phenotypic signature of thymic B cells as compared to bone marrow-derived B cells and (iii) investigate how Aire-dependent and Aire-independent features of thymic B cells relate to CD4 T cell tolerance.