Urinary and tissue black carbon and their impact on kidney function in transplant recipients

Air pollution, particularly exposure to black carbon (BC) nanoparticles, has been linked to adverse kidney outcomes. Transplant recipients may be especially vulnerable, making investigation of environmental influences on kidney function in this subpopulation essential. Building on recent evidence that BC nanoparticles can translocate into urine and kidney tissue, this study explores the link between internal and external air pollution on kidney function in transplant recipients one year post-transplantation. We included 183 kidney transplant recipients with protocol biopsies and 24-h urine samples between October 2019 and December 2023. BC in tissue and urine was assessed using white-light generation under femtosecond-pulsed illumination. Kidney function biomarkers were evaluated in association with modelled PM_2.5 and BC, proximity to a major road, and tissue and urinary BC using Pearson correlation and linear regression. Each 10 % increase in urinary BC was associated with a 14.24 mg/dL increase in urinary urea (95 %CI: 10.83, 17.64; p < 0.01), a 2.71 % increase in KIM-1 (95 %CI: 1.85, 3.59; p < 0.01), a 4.47 % increase in microalbumin (95 %CI: 0.89, 4.88; p < 0.01), a 4.60 % increase in alpha-1-microglobulin (95 %CI: 2.58, 6.67; p < 0.01), and a 0.02 mg/dL reduction in urinary creatinine clearance (95 %CI: 0.03, -0.01; p < 0.01). Tissue BC was borderline associated with reduced eGFR (0.20 mL/min/1.73 m²; 95 %CI: 0.42, 0.02; p = 0.07). Living farther from a major road was associated with lower urinary KIM-1 (-1.28 %; 95 %CI: 2.16, -0.40; p < 0.01) and reduced urinary urea (-5.00 mg/dL; 95 %CI: 8.84, -1.16; p = 0.01). In kidney transplant patients, urinary BC was significantly associated with biomarkers of glomerular and tubular kidney damage. Additionally, living farther from major roads correlated with improved kidney function, emphasizing the impact of e.g., BC accumulation on post-transplant kidney health.