The question driving our research is how the immune system generates resistance to infection without causing excessive inflammation. Innate immunity’s pattern recognition receptors trigger rapid and robust anti-microbial responses. In the last years my group has intensively studied how the cord factor of the mycobacterial cell wall is sensed by the C-type lectin receptor Mincle. We have characterized the signaling leading to activation of APC and directing the development of adaptive Th1/Th17 responses. In ongoing work, we are now addressing macrophage reprogramming by the cord factor as a mycobacterial evasion strategy. The risk of collateral damage creates a need for endogenous molecular brakes. We focus on macrophages because of their dual role as sensors of infection and innate immune effector cells. In studies on the mechanism of macrophage deactivation by IL-10, we have identified several molecular players in attenuating and specifying the responses to pattern recognition and cytokine receptor signaling (e.g. Socs3, Dusp1 and other MAPK phosphatases). Insights into the mechanisms of activation and deactivation may be harnessed for immune modulation: more robust innate immune activation for improved vaccination, or attenuation of pathological acute or chronic inflammatory responses.
Vaccination with recombinant subunit vaccines is a safe and attractive strategy but requires the use of adjuvants that activate innate immune cells and drive T cell responses. The glycolipids Trehalose-dibehenate (TDB) and Trehalose-dimycolate (TDM), the cord factor of mycobacteria, are potent adjuvants that direct protective Th1 and Th17 immunity when combined with recombinant antigens. We have identified the Syk-Card9 pathway as critical for activation of APC in vitro and for the adjuvanticity of TDB and TDM in vivo (Werninghaus 2009 J Exp Med). We furthermore showed that the C-type lectin receptor Mincle is binding TDB and the cord factor and drives APC activation and adjuvanticity (Schoenen 2010 J Immunol). Ongoing work is directed towards the identification of the intracellular signaling components involved and their contribution to the generation of Th1/Th17 responses in vivo and has revealed a role for IL-1R/Myd88 signaling in vivo (Desel 2013 PLoS One). Given the attractiveness of the TDB adjuvant for use in humans, we have also started to investigate the response of human APC to these glycolipids (Ostrop 2015 J Immunol).
The cord factor is also an important virulence factor of pathogenic mycobacteria. TDM delays phagosomal maturation and impairs IFNg-induced gene expression to create an intracellular niche for mycobacterial replication. In ongoing work, we address the question which role the Mincle-Syk pathway plays in the host-pathogen interaction and the immune evasion strategies of mycobacteria (Patin 2016 Innate Immunity).
The crucial role of Toll-like receptors (TLR) in innate immunity is well established and activation of MAPK and NFkB constitute the canonical signaling pathways driving gene expression in response to microbial stimuli. We have employed genome-wide expression analysis of innate immune cells to identify gene signatures for different pattern recognition receptors, adapter proteins and transcription factors. We have investigated the phosphoproteome of TLR-activated macrophages (in collaboration with the lab of M. Mann, MPI for Biochemistry, Martinsried). This work has revealed numerous, previously unknown phosphoproteins with a role in TLR signaling, identified the footprints of kinase activation and suggests a role for phosphorylated transcription factors in control of inflammatory gene expression in macrophages (Weintz 2010 Mol Syst Biol). Cytoskeletal and motor proteins were strongly enriched in the TLR-regulated phosphoproteome, suggesting that LPS and other PAMPs remodel cytoskeleton-associated macrophage functions. Indeed, we identified a contribution of the class myosin Myo1e in the regulation of cell surface expression of MHC-II molecules and the capacity of macrophages to elicit MHC-II-restricted CD4 T cell triggering (Wenzel 2015 Eur J Immunol).
Based on extensive transcriptome analysis of the macrophage response to the cord factor, we have searched for induced transcriptional regulators and identified a pivotal role for C/EBPb in Mincle expression and downstream gene expression, whereas HIF1a was selectively required for upregulation of iNOS (Schoenen 2014 J Immunol). In addition to the genetic analysis of cord factor recognition and signaling, we have now begun to study kinase activation downstream of Mincle biochemically and aim to identify the key phosphorylation events in ongoing work.
IL-10 is an essential endogenous regulator of the immune system and deactivates macrophages via the transcription factor Stat3. We have in the past generated transgenic mice expressing IL-10 constitutively in macrophages, allowing us to study the consequences of increased production of IL-10 in infection (Lang 2002 J Immunol). The question which IL-10 induced genes are critical mediators of macrophage deactivation is still unresolved. We have identified IL-10 induced genes by microarray studies and tested their functional role (Lang 2003 Nat Immunol; Hammer 2006 J Exp Med). Ongoing work is directed at the mechanisms and consequences of IL-10-induced macrophage reprogramming in inflammation and models of chronic infection like tuberculosis (Schreiber 2009 J Immunol) and Coxiella burnetii infection (collaboration with PD Dr. Anja Lührmann).
The Mitogen-activated protein kinases (MAPK) p38, ERK and JNK are a key signaling module for rapid transcriptional responses. In innate immune cells, they contribute differentially to the production of cytokines and chemokines. We have identified the MAPK phosphatase Dusp1 (also known as MKP-1) as critical regulator of inflammatory responses to TLR signals. Our investigation of its function in models of infectious diseases show that the increased inflammatory response in Dusp1-/- mice does not benefit the host but leads to increased immunopathology. Dusp1 belongs to the family of Dual specificity phosphatases, which show substrate selectivity for different MAPK and regulated expression. This led us to propose that Dusp phosphatases specify innate immune responses by controlling the relative strength of MAPK activation. We are specifically interested in the immunological function of Dusp9 and Dusp16, two family members expressed in macrophages and dendritic cells. Dusp16-deficient mice die perinatally and their macrophages selectively overproduce IL-12 upon TLR-triggering (Niedzielska 2014 J Biol Chem). Dusp9 is selectively expressed in plasmacytoid DC and can regulate TLR9-induced activation of ERK and IFNb expression (Niedzielska 2015). These studies in the mouse model will explore the potential of the Dusp MAPK phosphatases as targets for immune modulation strategies.