Schneppenheim Janna, Loock Ann-Christine, Hüttl Susann, Schweizer Michaela, Lüllmann-Rauch Renate, Oberg Hans-Heinrich, Arnold Philipp, Lehmann Christian H K, Dudziak Diana, Kabelitz Dieter, Lucius Ralph, Lennon-Duménil Ana-Maria, Saftig Paul, Schröder Bernd
Biochemical Institute, Christian-Albrechts-University of Kiel, D-24118 Kiel, Germany.
Institute of Anatomy, Christian-Albrechts-University of Kiel, D-24118 Kiel, Germany.
J Immunol. 2017 Jul 1;199(1):172-185. doi: 10.4049/jimmunol.1601533. Epub 2017 May 26.
The invariant chain (CD74) mediates assembly and targeting of MHC class II (MHCII) complexes. In endosomes, CD74 undergoes sequential degradation by different proteases, including cathepsin S (CatS) and the intramembrane protease signal peptide peptidase-like 2a (SPPL2a). In their absence, CD74 N-terminal fragments (NTFs) accumulate. In B cells, such an NTF impairs endosomal trafficking and BCR signal transduction. In mice, this leads to a loss of splenic B cells beyond the transitional stage 1. To gain insight into CD74 determinants and the role of MHCII, we compared B cells from , , and SPPL2a-MHCII double-deficient mice. We assessed differentiation of B cells in bone marrow and spleen and analyzed their endosomal morphology, BCR expression, and signal transduction. We demonstrate that MHCII is dispensable for the B cell phenotype of mice, further supporting a CD74-intrinsic effect. Despite significant vacuolization of endosomal compartments similar to B cells, traditional stage 1 B cells show unimpaired degradation of endocytic cargo, have intact BCR signaling, and do not exhibit any relevant defects in maturation. This could indicate that CD74 NTF-induced structural changes of endosomes are not directly involved in these processes. We further found that the block of CD74 degradation in B cells is incomplete, so that NTF levels are significantly lower than in B cells. This suggests a dose dependency and threshold for the CD74 NTF-associated impairment of B cell signaling and maturation. In addition, different functional properties of the longer, MHCII-bound CD74 NTF could contribute to the milder phenotype of B cells.
恒定链（CD74）介导MHC II类（MHCII）复合物的组装和靶向运输。在内体中，CD74会被包括组织蛋白酶S（CatS）和膜内蛋白酶信号肽肽酶样2a（SPPL2a）在内的不同蛋白酶依次降解。在它们缺失的情况下，CD74 N端片段（NTF）会积累。在B细胞中，这样的NTF会损害内体运输和BCR信号转导。在小鼠中，这会导致超过过渡阶段1的脾B细胞缺失。为了深入了解CD74的决定因素以及MHCII的作用，我们比较了来自[具体基因缺失情况未明确]、[具体基因缺失情况未明确]和SPPL2a-MHCII双缺陷小鼠的B细胞。我们评估了骨髓和脾脏中B细胞的分化，并分析了它们的内体形态、BCR表达和信号转导。我们证明MHCII对于[具体基因缺失情况未明确]小鼠的B细胞表型是可有可无的，这进一步支持了CD74内在效应。尽管内体区室有明显的空泡化，类似于[具体基因缺失情况未明确]B细胞，但传统的阶段1 B细胞显示内吞货物的降解未受损，具有完整的BCR信号，并且在成熟过程中没有表现出任何相关缺陷。这可能表明CD74 NTF诱导的内体结构变化不直接参与这些过程。我们进一步发现，[具体基因缺失情况未明确]B细胞中CD74降解的阻断是不完全的，因此NTF水平明显低于[具体基因缺失情况未明确]B细胞。这表明CD74 NTF相关的B细胞信号和成熟受损存在剂量依赖性和阈值。此外，与MHCII结合的较长CD74 NTF的不同功能特性可能导致[具体基因缺失情况未明确]B细胞的表型较轻。
