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

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.

• DFG Programme: Research Grants