Adenosine deaminase 2 (ADA2) in immune cell development and activation

Adenosine deaminase 2 (ADA2) is an extracellular enzyme and growth factor secreted by myeloid lineage cells. Biallelic loss-of-function variants in the ADA2 gene cause ‘Deficiency of adenosine deaminase 2’ (DADA2), a rare, autoinflammatory disease that typically presents in childhood. DADA2 has broad manifestations including early-onset strokes, immunodeficiency, and hematologic abnormalities. Clinical and experimental evidence infer regulatory role(s) for ADA2 in human immune cell development and activation. However, the in vivo functions of ADA2 and their dependence on enzymatic activity (deaminating adenosine to inosine), growth factor activity, or both remain debated. Additionally, there is uncertainty regarding the impact of a variant’s location within the ADA2 gene on protein function and, consequently, the clinical heterogeneity observed in DADA2. Finally, the potential association between elevated ADA2 enzyme activity and immune-mediated inflammatory conditions is an unexplored area of clinical interest. To begin to address these knowledge gaps, I comparatively analyzed ADA2 variants associated with distinct DADA2 phenotypes in an in vitro expression system. I evaluated ADA2 expression during ex vivo differentiation of CD34⁺ cord blood-derived hematopoietic stem cells to monocytes and granulocytes. Additionally, I quantified ADA2 enzyme activity in clinical samples from individuals with inflammatory syndromes and during immune reconstitution post-hematopoietic stem cell transplant (HSCT). Results from these investigations provide new evidence for an ADA2 variant-specific secretion defect associated with constitutive expression of Type I interferons, and for elevated ADA2 enzyme activity in plasma during immune reconstitution and the onset of chronic graft-versus-host disease in patients who are post-HSCT. ADA2 is expressed during CD34⁺ hematopoietic cell differentiation to monocytes and, to a lesser extent, granulocytes, and may have roles in hematopoiesis beyond enzyme activity. These findings support exploration of how monocyte-derived ADA2 influences immune cell development, whether elevated ADA2 activity reflects pro-inflammatory immune cell activation, and how ADA2 variants drive autoinflammation in patients with DADA2.