Dok3–protein phosphatase 1 interaction attenuates Card9 signaling and neutrophil-dependent antifungal immunity


From left: Dr Huo Jian Xin, Dr Susana Kim, Prof Lam Kong Peng, Dr Loh Jia Tong, Dr Xu Shengli

Authors

Jia Tong Loh1, Shengli Xu1,2, Jian Xin Huo1, Susana Soo-Yeon Kim1, Yue Wang3,4 and Kong-Peng Lam1,5,6

1 Bioprocessing Technology Institute, Agency for Science, Technology and Research, Singapore
2 Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
3 Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
4 Department of Biochemistry and 5 Department of Microbiology and Immunology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
6 School of Biological Sciences, College of Science, Nanyang Technological University, Singapore

Published in Journal of Clinical Investigation 2019 129(8): 2717-2729 (Online Version)


Abstract

Invasive fungal infection is a serious health threat with high morbidity and mortality. The risk of infection is especially high among the immunocompromised, including patients in intensive care unit and people undergoing cancer chemotherapy. However, treatment of Candida infection remains challenging. Current anti-fungal drugs only demonstrate partial success in improving prognosis, and the rapid emergence of drug resistance among Candida species is a growing problem. As such, there is a pressing need to develop novel anti-fungal therapies to improve clinical outcomes. Understanding the mechanistic interaction between host immune cells and fungal pathogens holds the key for uncovering novel immune-based treatments to combat candidiasis.

Here, we report that Downstream of kinase (Dok) 3 adaptor negatively regulates anti-fungal immunity in neutrophils. Our data revealed that loss of Dok3 enhances various anti-fungal effector functions of neutrophils including phagocytosis, netosis and production of pro-inflammatory cytokines, thereby increasing neutrophilic fungicidal activity. Biochemically, Dok3 recruited Protein Phosphatase 1 (PP1) to de-phosphorylate Card9, an essential player in innate anti-fungal defense, to dampen downstream NF-kB and JNK activation and immune responses. As such, loss of Dok3 enhances the innate anti-fungal immune responses and protects mice from morbidity and mortality induced by systemic C. albicans infection. Since Dok3 suppresses Card9 signaling, disrupting Dok3-Card9 interaction or inhibiting PP1 activity represents therapeutic opportunities to develop drugs to combat candidaemia. Such immune-based approach could act as an adjunctive therapy to current anti-fungal drugs for further improvement of outcomes in Candida-infected patients.


Figure 1. Proposed model of Dok3 regulation of Card9 during anti-fungal immune signaling.

In resting neutrophils, Dok3 recruits PP1 to maintain Card9 in its de-phosphorylated and inactive state. Upon Candida infection, Dok3 is phosphorylated and Dok3-PP1 complex dissociates from Card9, and Dok3 is also subsequently degraded. This enables Card9 to be phosphorylated by PKCd, thereby activating downstream NF-kB and JNK signaling, leading to fungal clearance.