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

• Applicant: Professor Dr. Bernd Schröder
• Subject Area: Immunology ; Cell Biology
• Term: from 2014 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 251390220

Final Report Abstract

The invariant chain (CD74) facilitates assembly and targeting of MHC class II (MHCII) complexes in antigen-presenting cells. After having delivered MHCII to antigen-processing compartments, CD74 is degraded by sequential proteolysis. The residual membrane-bound N-terminal fragment (NTF) is cleared by the intramembrane protease 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). Furthermore, SPPL2a-deficient human patients develop Mendelian Susceptibility to Mycobacterial Disease (MSMD) which has been linked to the depletion of certain DC subsets. In this project, we comprehensively characterised cytokine responses of SPPL2a-deficient DCs upon sensing of mycobacteria. These were specifically altered with an increased release of certain pro- and reduced secretion of select anti-inflammatory cytokines significantly shifting the balance. Therefore, our work has identified an additional mechanism of MSMD pathogenesis. In addition to DC depletion caused by SPPL2a deficiency, the altered cytokine profiles of the remaining DCs will contribute to non-protective anti-mycobacterial immune responses with increased inflammation. Mechanistically, alteration of mycobacteria-induced cytokine responses could be linked with the pattern recognition receptors Dectin-1 and TLR4. These functional phenotypes, like the DC depletion and B cell phenotype of SPPL2a-deficient mice, are induced by the accumulating uncleaved CD74 NTF. In some cases, mis-trafficking of critical membrane receptors, like Dectin-1 in DCs and the B cell antigen receptor in B cells, is involved. So far, the molecular events triggered by the CD74 NTF leading to the respective phenotypes remain incompletely understood. Therefore, a major part of this project was to validate potential CD74 interacting proteins identified by a screen performed beforehand. In a multistep strategy, we probed these interactions in co-immunoprecipitation experiments ranging from overexpression to endogenous conditions. These experiments scrutinised specifically one of the candidates for which an interaction with CD74 was reproducibly observed in different experimental systems. This protein has a documented role in phosphoinositide (PIP) metabolism and by this means in influencing endosomal membrane trafficking. Further work was focused to analyse this validated CD74 interactor in detail and to analyse the functional relevance with regard to the cellular phenotypes of SPPL2a-/- DCs and also B cells. These experiments have identified alterations in PIP metabolism in SPPL2a-deficient DCs which provide support that the newly identified CD74 interactor is involved in the development of CD74 NTF-mediated phenotypes associated with SPPL2a deficiency. How this newly discovered mechanism is involved in shaping the endocytic pathway of wild type cells will be addressed in follow-up studies.

Publications

• Physiological functions of SPP/SPPL intramembrane proteases. Cellular and Molecular Life Sciences, 77(15), 2959-2979.
  Mentrup, Torben; Cabrera-Cabrera, Florencia; Fluhrer, Regina & Schröder, Bernd
  
• Deficiency of the Intramembrane Protease SPPL2a Alters Antimycobacterial Cytokine Responses of Dendritic Cells. The Journal of Immunology, 206(1), 164-180.
  Gradtke, Ann-Christine; Mentrup, Torben; Lehmann, Christian H. K.; Cabrera-Cabrera, Florencia; Desel, Christine; Okakpu, Darian; Assmann, Maike; Dalpke, Alexander; Schaible, Ulrich E.; Dudziak, Diana & Schröder, Bernd
  
• Phagosomal signalling of the C-type lectin receptor Dectin-1 is terminated by intramembrane proteolysis. Nature Communications, 13(1).
  Mentrup, Torben; Stumpff-Niggemann, Anna Yamina; Leinung, Nadja; Schlosser, Christine; Schubert, Katja; Wehner, Rebekka; Tunger, Antje; Schatz, Valentin; Neubert, Patrick; Gradtke, Ann-Christine; Wolf, Janina; Rose-John, Stefan; Saftig, Paul; Dalpke, Alexander; Jantsch, Jonathan; Schmitz, Marc; Fluhrer, Regina; Jacobsen, Ilse D. & Schröder, Bernd
  
• Signal peptide peptidase-like 2 proteases: Regulatory switches or proteasome of the membrane?. Biochimica et Biophysica Acta (BBA) - Molecular Cell Research, 1869(1), 119163.
  Mentrup, Torben & Schröder, Bernd
  
• The intramembrane proteases SPPL2a and SPPL2b regulate the homeostasis of selected SNARE proteins. The FEBS Journal, 290(9), 2320-2337.
  Ballin, Moritz; Griep, Wolfram; Patel, Mehul; Karl, Martin; Mentrup, Torben; Rivera‐Monroy, Jhon; Foo, Brian; Schwappach, Blanche & Schröder, Bernd
  
• Dynamic association of the intramembrane proteases SPPL2a/b and their substrates with tetraspanin-enriched microdomains. iScience, 26(10), 107819.
  Leinung, Nadja; Mentrup, Torben; Patel, Mehul; Gallagher, Tom & Schröder, Bernd
  
• Genome-wide detection of human intronic AG-gain variants located between splicing branchpoints and canonical splice acceptor sites. Proceedings of the National Academy of Sciences, 120(46).
  Zhang, Peng; Chaldebas, Matthieu; Ogishi, Masato; Al Qureshah, Fahd; Ponsin, Khoren; Feng, Yi; Rinchai, Darawan; Milisavljevic, Baptiste; Han, Ji Eun; Moncada-Vélez, Marcela; Keles, Sevgi; Schröder, Bernd; Stenson, Peter D.; Cooper, David N.; Cobat, Aurélie; Boisson, Bertrand; Zhang, Qian; Boisson-Dupuis, Stéphanie; Abel, Laurent & Casanova, Jean-Laurent