The molecular role of the invariant chain/CD74 and its proteolytic degradation in dendritic cell function and anti-mycobacterial immune recognition

不变链（CD 74）在抗原呈递细胞中MHC II类（MHCII）复合物的组装和靶向中起核心作用。在抗原加工区室中，CD 74的管腔结构域通过连续蛋白水解降解，从而释放MHCII。最终的CD 74膜结合的N-末端片段（NTF）通过信号肽肽酶样2a（SPPL 2a）进行膜内蛋白水解，如我们之前所证明的。SPPL 2a-/-小鼠表现出脾B细胞成熟的停滞和树突状细胞（DC）的损失。最近，基于某些DC亚群的耗竭，鉴定了三名具有孟德尔分枝杆菌病易感性（MSMD）的SPPL 2a缺陷型人类患者。在这两种情况下，积累的未切割的CD 74 NTF诱导所描述的表型。然而，由该片段触发的病理生理序列仍然不完全清楚。在鼠SPPL 2a-/- B细胞中，我们发现中心存活途径，特别是B细胞抗原受体的下游，受到干扰。虽然额外的影响可能有所贡献，但我们的研究结果表明，CD 74 NTF诱导的相关受体的错误运输是一种中心机制。在拟议的项目中，我们计划更详细地分析SPPL 2a缺陷对树突状细胞功能的影响。初步结果表明SPPL 2a-/-DC在细胞因子应答中表现出明显的变化，特别是关于抗分枝杆菌免疫识别。通过表征这种功能缺陷，我们希望阐明除了DC减少之外，这如何有助于SPPL 2a缺陷患者和小鼠的MSMD表型。与我们在B细胞中的发现类似，我们注意到，以C型凝集素受体Dectin-1为例的某些质膜蛋白的细胞分布在SPPL 2 a-/-DC中改变，导致细胞表面水平降低。我们假设，参与病原体识别或DC分化的其他受体可能会经历由CD 74 NTF诱导的错误运输。我们希望使用假设驱动和无偏方法的组合来系统地分析这一点，并将确定这在多大程度上解释SPPL 2a-/-DC的功能缺陷。使用Dectin-1作为模型，我们将研究哪些贩运途径主要由CD 74 NTF调节。为了了解NTF在分子水平上的影响，我们进行了IP-MS筛选，并确定了该片段的几个潜在相互作用伙伴，并记录了该片段在膜运输中的作用。在多步骤验证策略中，我们的目标是确认这些假定的相互作用，并分析它们与SPPL 2a-/-DC的细胞表型的功能相关性。由于SPPL 2a代表了在自身免疫中耗尽B细胞和/或树突细胞的有前景的治疗靶标，因此从翻译的角度来看，对本研究提供的分子机制的更好理解也是相关的。

The invariant chain (CD74) plays a central role in assembly and targeting of MHC class II (MHCII) complexes in antigen-presenting cells. In antigen-processing compartments, the luminal domain of CD74 is degraded by sequential proteolysis thereby releasing MHCII. The final CD74 membrane-bound N-terminal fragment (NTF) undergoes intramembrane proteolysis by Signal peptide peptidase-like 2a (SPPL2a) as we demonstrated before. SPPL2a-/- mice exhibit an arrest of splenic B cell maturation and a loss of dendritic cells (DCs). Recently, three SPPL2a-deficient human patients with Mendelian Susceptibility to Mycobacterial Disease (MSMD) based on the depletion of certain DC subsets were identified. In both contexts, the accumulating uncleaved CD74 NTF induces the described phenotypes. However, the pathophysiological sequence triggered by this fragment remains incompletely understood. In murine SPPL2a-/- B cells, we found that central survival pathways, in particular downstream of the B cell antigen receptor, were disturbed. Though additional effects may contribute, our findings indicate that a CD74 NTF-induced mis-trafficking of the involved receptors represents a central mechanism. In the proposed project, we plan to analyse the impact of SPPL2a-deficiency on dendritic cell function in more detail. Prelimininary results indicate that SPPL2a-/- DCs exhibit distinct changes in cytokine responses, in particular with regard to anti-mycobacterial immune recognition. By characterising this functional deficit, we want to elucidate how this contributes to the MSMD phenotype of SPPL2a-deficient patients and mice in addition to the DC reduction. In analogy with our findings in B cells, we noticed that the cellular distribution of certain plasma membrane proteins, exemplified by the C-type lectin receptor Dectin-1, is altered in SPPL2a-/- DCs leading to reduced levels at the cell surface. We hypothesise that further receptors involved in pathogen recognition or DC differentiation may undergo mis-trafficking induced by the CD74 NTF. We want to analyse this systematically using a combination of hypothesis-driven and unbiased approaches and will determine to what extent this accounts for the functional deficits of SPPL2a-/- DCs. Using Dectin-1 as a model, we will investigate which trafficking routes are primarily modulated by the CD74 NTF. To understand effects by the NTF at the molecular level, we have performed an IP-MS screen and identified several potential interaction partners of this fragment with a documented role in membrane trafficking. In a multistep validation strategy, we aim to confirm these putative interactions and to analyse their functional relevance with regard to the cellular phenotypes of SPPL2a-/- DCs. Since SPPL2a represents a promising therapeutic target to deplete B cells and/or dendritic cells in autoimmunity, a better understanding of the molecular mechanism as provided by this study will be relevant also from a translational point of view.