Impaired thymic function is independently associated with an increased risk of posttransplant infections in kidney recipients.

Kidney transplant recipients are highly susceptible to infections due to chronic immunosuppression. Signal joint T cell receptor excision circles (sjTRECs), markers of thymic output, have emerged as potential indicators of immune competence, but their clinical relevance in transplantation remains unclear. In this single-center retrospective study, sjTREC levels were measured at transplantation in 733 kidney recipients and categorized into 4 strata. Posttransplant infections were assessed using Cox models adjusted for demographic, clinical, and immunologic variables. During follow-up, 508 patients (69.3%) developed an infection after a mean time of 1.17 ± 1.61 years. The 3-year cumulative incidence increased among sjTREC strata, from 52.4% to 86.7% (P < .0001). In multivariable analysis, older recipient age (hazard ratio [HR], 1.01; 95% confidence interval [CI], 1.00-1.02; P = .007), thymoglobulin induction (HR, 1.35; 95% CI, 1.13-1.62; P = .001), and lower sjTREC levels (log-transformed; HR, 0.73; 95% CI, 0.63-0.86; P < .001) were independently associated with infection. Sensitivity analyses confirmed consistent results across pathogen-specific models and after accounting for death as a competing event. A nonsignificant age interaction (P = .084) suggested possible attenuation in young recipients. Lower sjTREC levels at transplantation are independently associated with higher posttransplant infection risk, supporting their use for immune risk stratification in kidney transplantation.