Adenosine deaminase 2 (ADA2) plays a critical role in immune regulation, particularly in monocyte differentiation and activation. Unlike ADA1, which primarily functions intracellularly, ADA2 can be secreted or trafficked to lysosomes. Deficiency in ADA2 (DADA2) leads to systemic vasculitis marked by elevated TNF-α levels, excessive pro-inflammatory cytokine production, and impaired differentiation of monocytes into anti-inflammatory M2 macrophages. Research demonstrates that intracellular ADA2 localizes within endolysosomes of macrophages, and its reduction correlates with heightened TNF-α secretion. This suggests ADA2 modulates lysosomal adenosine levels, influencing inflammatory pathways. In pneumonia patients, elevated ADA2 levels in bronchoalveolar lavage (BAL) align with increased pro-inflammatory cytokines, while cord blood exhibits low ADA2, fostering an immunosuppressive milieu. Secreted ADA2 binds apoptotic cells, reducing extracellular adenosine and activating immune responses, highlighting its dual intra- and extracellular roles in inflammation.

Monocyte subsets exhibit differential ADA2 expression. CD16⁺ monocytes, key TNF-α producers, show lower intracellular ADA2 compared to classical CD16⁻ subsets. This disparity mirrors findings in DADA2 patients, where ADA2 deficiency leads to unregulated TNF-α release. GM-CSF-differentiated macrophages, associated with pro-inflammatory M1 polarization, express less ADA2 than M-CSF-driven M2 macrophages. CpG oligonucleotides (ODNs) enhance ADA2 retention in lysosomes, potentially stabilizing the enzyme and dampening TNF-α production. Such dynamics underscore ADA2’s role in balancing immune activation. Clinical data reveal ADA2 as a biomarker in pneumonia: BAL levels rise during acute phases and decline post-treatment, correlating with cytokine profiles. This positions ADA2 as a prognostic tool for monitoring disease progression and therapeutic efficacy.

Mechanistically, ADA2 regulates lysosomal adenosine, which may influence DNA methylation and gene expression. In DADA2, impaired adenosine clearance in lysosomes disrupts epigenetic regulation, amplifying inflammatory signals. ADA2 also binds apoptotic cells via interactions with DNA and proteoglycans, suggesting a role in resolving inflammation by clearing immunomodulatory adenosine. Experiments with THP-1 cells show adenosine-induced apoptosis, mitigated by ADA2’s enzymatic activity. These findings propose ADA2 as a safeguard against cytotoxic adenosine accumulation, linking its intracellular activity to cell survival and immune homeostasis.

The study highlights growth factors like GM-CSF and M-CSF as regulators of ADA2 expression and trafficking. GM-CSF reduces ADA2 levels, promoting M1 polarization and TNF-α secretion, while M-CSF sustains ADA2, favoring anti-inflammatory M2 phenotypes. Mutations in ADA2 disrupt protein stability and trafficking, explaining phenotypic variability in DADA2. Enzyme replacement therapy shows limited efficacy, as extracellular ADA2 fails to curb TNF-α in DADA2 monocytes, emphasizing the importance of intracellular ADA2. Targeting lysosomal ADA2 activity emerges as a potential strategy to modulate macrophage polarization and inflammation.

Clinical implications extend beyond DADA2. Elevated ADA2 in tuberculosis and HIV underscores its role in chronic infections, while low cord blood levels align with neonatal immune tolerance. The enzyme’s dual function—lysosomal adenosine regulation and extracellular immune activation—positions it as a multifaceted therapeutic target. Future research could explore ADA2’s interaction with nucleic acids and its impact on Toll-like receptor signaling, offering insights into autoimmune and inflammatory disorders. Overall, ADA2’s intracellular and extracellular dynamics reveal a complex interplay between adenosine metabolism, immune cell activation, and disease pathogenesis, paving the way for novel interventions in inflammation and cancer.
DOI: 10.1007/s11684-024-1110-6