Die Rolle der Intramembranprotease SPPL2a und ihrem Substrat CD74 in der Entwicklung und Funktion dendritischer Zellen

In summary, SPPL2a–deficiency affects DC development and function in a CD74–dependent manner with important consequences on their ability to clear mycobacterial infections. The altered surface abundance of PRR receptors and the disturbed cytokine response observed in SPPL2a-/- BMDCs possibly contributes to MSMD in SPPL2a–deficient patients. Reductions of the anti–inflammatory cytokine IL–10 together with enhanced IL–1beta responses could lead to enhanced inflammatory responses with increased tissue damage, thereby creating an environment supporting the spreading of mycobacteria. Reduced release of IL–12 family members by SPPL2a-/- BMDCs could result in impaired induction of T helper cell type 1 responses, which are required for clearance of mycobacteria. Thus, the results from this thesis reveal that MSMD in SPPL2a–deficient humans is not not only caused by the reduced frequency of cDC2 population, but also by a functional deficit of the remaining DCs.Zusammenfassend lässt sich sagen, dass eine SPPL2a–Defizienz abhängig von CD74 die Entwicklung und Funktion dendritischer Zellen beeinflusst, mit Auswirkungen auf ihre Fähigkeit, mykobakterielle Infektionen zu erkennen und zu beseitigen. Die veränderte Oberflächenhäufigkeit der PRR–Rezeptoren und der gestörten Zytokinreaktion, die in SPPL2a-/- BMDCs beobachtet wurden, tragen möglicherweise zur MSMD bei SPPL2a–defizienten Patienten bei. Verminderte Freisetzung des entzündungshemmenden Zytokins IL–10 zusammen mit vermehrter IL–1beta–Antwort könnte zu stärkeren Entzündungsreaktionen mit erhöhter Gewebeschädigung führen, wodurch eine Umgebung geschaffen wird, die die Verbreitung von Mykobakterien begüunstigt. Reduzierte Freisetzung von IL–12–Familienmitgliedern durch SPPL2a-/- BMDCs können zu einer beeinträchtigten Induktion von Typ1–T–Helferzell–Antworten führen, die für die Beseitigung von Mykobakterien erforderlich sind. So zeigen die Ergebnisse dieser Arbeit, dass MSMD in SPPL2a-defizienten Patienten nicht nur durch die reduzierte Häufigkeit der cDC2–Population verursacht, sondern auch durch ein funktionelles Defizit der verbleibenden DCs beeinflusst werden kann

Atherogenic LOX-1 signaling is controlled by SPPL2-mediated intramembrane proteolysis

The lectin-like oxidized LDL receptor 1 (LOX-1) is a key player in the development of atherosclerosis. LOX-1 promotes endothelial activation and dysfunction by mediating uptake of oxidized LDL and inducing pro-atherogenic signaling. However, little is known about modulators of LOX-1-mediated responses. Here, we show that the function of LOX-1 is controlled proteolytically. Ectodomain shedding by the metalloprotease ADAM10 and lysosomal degradation generate membrane-bound N-terminal fragments (NTFs), which we identified as novel substrates of the intramembrane proteases signal peptide peptidase-like 2a and b (SPPL2a/b). SPPL2a/b control cellular LOX-1 NTF levels which, following self-association via their transmembrane domain, can activate MAP kinases in a ligand-independent manner. This leads to an up-regulation of several pro-atherogenic and pro-fibrotic targets including ICAM-1 and the connective tissue growth factor CTGF. Consequently, SPPL2a/b-deficient mice, which accumulate LOX-1 NTFs, develop larger and more advanced atherosclerotic plaques than controls. This identifies intramembrane proteolysis by SPPL2a/b as a novel atheroprotective mechanism via negative regulation of LOX-1 signaling

Phagosomal signalling of the C-type lectin receptor Dectin-1 is terminated by intramembrane proteolysis

Dectin-1 is a critical component of the innate sensing repertoire which is involved in pattern based recognition of fungal pathogens. Here the authors show that intramembrane proteolysis is involved in the regulation of the antifungal host response by termination of the phagosomal signalling of Dectin-1. Sensing of pathogens by pattern recognition receptors (PRR) is critical to initiate protective host defence reactions. However, activation of the immune system has to be carefully titrated to avoid tissue damage necessitating mechanisms to control and terminate PRR signalling. Dectin-1 is a PRR for fungal beta-glucans on immune cells that is rapidly internalised after ligand-binding. Here, we demonstrate that pathogen recognition by the Dectin-1a isoform results in the formation of a stable receptor fragment devoid of the ligand binding domain. This fragment persists in phagosomal membranes and contributes to signal transduction which is terminated by the intramembrane proteases Signal Peptide Peptidase-like (SPPL) 2a and 2b. Consequently, immune cells lacking SPPL2b demonstrate increased anti-fungal ROS production, killing capacity and cytokine responses. The identified mechanism allows to uncouple the PRR signalling response from delivery of the pathogen to degradative compartments and identifies intramembrane proteases as part of a regulatory circuit to control anti-fungal immune responses