NKG2D Signaling in Podocyte DNA Damage Drives Cardiorenal Axis and CKD Progression: A Therapeutic Strategy and Prognostic Biomarker

We elucidated the mechanism by which DNA damage induces NKG2D ligand expression, activating NKG2D+CD8+T cells, thereby exacerbating proteinuria and kidney damage, using mice with podocyte-specific expression of I-PpoI, a homing endonuclease inducing non-mutagenic DNA double-strand breaks (DSBs) (Cell rep 2023). However, systemic effects of the NKG2D+CD8+T cell activation have not been elucidated. In this study, we aim to determine the therapeutic effect of NKG2D signal blockade on both CKD progression and cardiac function in clinically relevant kidney disease models, such as diabetes and nephrotic syndrome. Furthermore, we aim to establish non-invasive predictive biomarkers to select patients who would benefit from this therapeutic strategy, thereby facilitating its clinical application.

The NKG2D neutralizing antibody (CX5, 200 μg twice weekly, ip) was administered to db/db mice, STZ mice, and mice with ADR nephropathy to assess therapeutic efficacy. Systemic effects were investigated by assessing cardiac function (LVEF/FS) in I-PpoI mice and evaluating changes after CX5 administration. In addition, we studied 25 renal biopsy patients to correlate glomerular NKG2D ligand MICA levels with soluble MICA in serum (sMICA) and renal pathology. A prognostic analysis was conducted on 40 CKD outpatients to assess sMICA as a non-invasive marker.

In db/db mice, WT-1+γH2AX+ cells were significantly increased in the renal tissue, suggesting the accumulation of podocyte DNA DSBs in this model. When the CX5 was administered to db/db mice, it suppressed proteinuria and alleviated glomerulosclerosis. Furthermore, the CX5 reduced albuminuria in both STZ mice and ADM nephropathy mice, suggesting the broad efficacy of NKG2D signal blockade for CKD involving podocyte injury. The NKG2D+CD44+CD8+ T cell fraction was increased in the peripheral blood of I-PpoI mice, suggesting that NKG2D signaling activated by podocyte DNA damage may systemically engage T cell activation. Evaluation of cardiac function in I-PpoI mice and db/db mice revealed a decrease in LVEF and FS. CX5 administration in I-PpoI mice and db/db mice significantly improved LVEF and FS, suggesting that NKG2D signaling is involved in the cardiorenal axis originating from podocyte DNA damage. Analysis of 25 renal biopsy patients revealed that serum sMICA significantly correlated with glomerular MICA expression and renal pathology (T cell infiltration, fibrosis, and tubular atrophy), suggesting sMICA reflects podocyte DNA damage and inflammation. Furthermore, in 40 CKD outpatients, sMICA concentration negatively correlated with the eGFR decline rate (a correlation strengthened by including the protease ADAM10), supporting sMICA's utility as a non-invasive biomarker for patient selection and prognosis.

NKG2D signal inhibition is promising to inhibit the progression of diverse kidney diseases involving podocyte injury and may thus represent a comprehensive strategy to treat both renal and cardiac diseases by disrupting the cardio-renal axis stemming from podocyte DNA damage. Specifically, it's expected to be a promising therapeutic approach against the worsening of cardiorenal complications in patients with diabetes. Furthermore, the soluble NKG2D ligand, sMICA, is effective as a non-invasive biomarker that reflects podocyte-derived immunogenicity and predicts renal prognosis.