Caspase recruitment domain–containing protein 9 (CARD9) is a critical signaling adaptor that functions downstream of several C-type lectin receptors, playing a vital role in host immune responses against fungal pathogens. CARD9 deficiency is an autosomal recessive primary immunodeficiency that leads to severe, life-threatening fungal infections. Specifically, dematiaceous fungal infections associated with CARD9 deficiency are clinically intractable and carry high mortality rates, yet their precise immunopathological mechanisms have not been fully elucidated.

Recently, the team led by Prof. Fan Bai from the Biomedical Pioneering Innovation Center (BIOPIC) at Peking University, in collaboration with Prof. Xiaowen Wang’s team from Peking University First Hospital and Dr. Wenyan Wang’s team from the School of Basic Medical Sciences at Tsinghua University, published a research article titled "CARD9-dependent macrophage plasticity regulates effective fungal clearance" in The Journal of Clinical Investigation. This study utilized single-cell RNA sequencing to analyze the immune landscape of skin lesions from patients and murine models of subcutaneous phaeohyphomycosis, discovering that CARD9 deficiency significantly promotes the differentiation of TREM2^high macrophages. This process impairs their antifungal effector functions and exacerbates exhaustion-like Th1 cell phenotypes, thereby damaging both innate and adaptive antifungal immunity. The research further demonstrates that targeting TREM2 can partially reverse these immune abnormalities and delay disease progression, offering a new potential therapeutic strategy for CARD9-related dematiaceous fungal infections.

The research team established a subcutaneous infection model using Phialophora verrucosa in both WT and Card9 knockout (Card9^-/-) mice, finding that Card9-deficient mice exhibited a significantly higher susceptibility to infection. Local single-cell transcriptomic analysis revealed that macrophages are the predominant immune cell population expressing CARD9 in skin lesions, comprising 61.69% of Card9⁺ cells, suggesting they play a pivotal role in the development of the disease and serving as the core focus of this study.

Further investigation confirmed that the TREM2^high macrophage subset is significantly enriched in the skin lesions of both Card9-deficient mice and human patients with dematiaceous fungal infections. Functionally, these macrophages exhibit impaired fungicidal capacity and decreased production of reactive oxygen species (ROS), leading to compromised innate antifungal immunity.

Additionally, TREM2^high macrophages interact closely with Th1 cells through multiple immunosuppressive signaling pathways, promoting the development of an "exhaustion-like" phenotype in Th1 cells. This further weakens the adaptive antifungal immune response.

At the molecular level, the study revealed that upon stimulation with dematiaceous fungi, the activation of the NF-κB signaling pathway was restricted in CARD9-deficient macrophages, whereas the activity of the PI3K/AKT/GSK3β/CREB pathway was significantly enhanced, subsequently upregulating the expression and activity of the transcription factor C/EBPβ. C/EBPβ directly binds to the Trem2 promoter region to promote Trem2 transcription, thereby driving the shift of macrophages toward an anti-inflammatory phenotype and impacting their effector functions.

In terms of therapeutic exploration, the team discovered that a blocking antibody against TREM2 significantly reduced the proportion of TREM2^high macrophages in Card9-deficient mice and alleviated T-cell exhaustion. This intervention effectively slowed disease progression and reduced the local fungal burden. Furthermore, using siRNA to knock down TREM2 expression in vitro enhanced the fungal-killing ability and ROS generation of Card9-deficient macrophages, further supporting the feasibility of TREM2 as a potential therapeutic target.

Finally, the research team highlighted potential therapeutic directions, demonstrating that TREM2-blocking antibodies significantly reduce TREM2^high macrophages and mitigate T-cell exhaustion in Card9^-/- mice, effectively alleviating the disease course. The knockdown of TREM2 via siRNA in vitro also improved the fungicidal functions of CARD9-deficient macrophages, suggesting that TREM2 is a promising target for immunotherapy in CARD9-related dematiaceous fungal infections.

In summary, this study systematically elucidates the key mechanism by which CARD9 influences antifungal immunity by regulating macrophage plasticity, revealing the pathological role and therapeutic value of TREM2^high macrophages. This work not only deepens the understanding of the immunopathology of CARD9 deficiency but also provides a theoretical basis for future targeted immunomodulatory strategies, expanding the application of immunotherapy in infectious diseases.

Dr. Lu Zhang (Peking University First Hospital), Zhichun Tang (BIOPIC, Peking University), Dr. Yi Zhang, and Dr. Wenjie Liu are the co-first authors of this study. Prof. Xiaowen Wang, Prof. Fan Bai, and Dr. Wenyan Wang serve as the co-corresponding authors. This research received significant support from Prof. Ruoyu Li (Peking University First Hospital) and Prof. Yang-Xin Fu (Tsinghua University), with additional contributions from researchers including Haitao Jiang, Li Yu, Kexin Lei, and Yubo Ma.

Paper Link: https://www.jci.org/articles/view/188827