NaCl and urea modulate CD8+ T cell survival, renal accumulation, and response to BK virus.

BK virus nephropathy is a severe, graft-threatening complication of kidney transplantation that requires an effective T cell response. It typically emerges in the kidney medulla. Elevated osmolyte concentrations that dynamically respond to loop diuretic therapy characterize this environment. Here, BK viremia development in kidney graft recipients negatively correlated with loop diuretic therapy. The association remained significant in multivariable and propensity score-matched analyses. Kidney function was better preserved and CD8+ T cell abundance higher in loop diuretic-exposed allografts. CD8+ T cell densities in healthy human and murine kidney medulla were lower than in cortex and increased upon loop diuretic therapy in mice. As a potential underlying mechanism, kidney medullary NaCl and urea concentrations decreased primary human CD8+ T cell numbers in vitro by induction of cell death and limitation of proliferation, respectively. Both osmolytes downregulated interferon-related gene expression. NaCl induced p53-dependent apoptosis and upregulated Na+-transporter SLC38A2, which promoted caspase-3 activation. Both decreased T cell response and cytokine secretion in response to viral peptide and allogeneic tubular epithelial cell killing, components of anti-BK virus response in the kidney allograft. Our results propose osmolyte-mediated mitigation of CD8+ T cell function as a what we believe to be novel mechanism that impairs immune response to BK virus, the therapeutic potential of which is testable.