*Purpose: The main challenge of immunosuppressive therapy after solid organ transplantation is to find the optimal dose regimen, and prevent allograft rejection as well as opportunistic infection. Torque teno virus (TTV; ubiquitous and non-pathogenic) has been proposed as a marker of functional immunity in immunocompromised patients. We investigate whether TTV loads predict the risk of common viral infection and allograft rejection in kidney transplantation (KTx) recipients.

*Methods: In a retrospective cohort of 389 KTx recipients, individual TTV loads in were measured by qPCR in consecutive plasma samples during one year follow-up. The endpoints were allograft rejection, BK polyomavirus (BKPyV) viremia and cytomegalovirus (CMV) viremia. Repeated measures and survival data were analysed in a joint model.

*Results: During follow-up TTV was detected in 100% of KTx recipients. The median viral load increased to 10⁷ genome copies/ml 3 months after KTx. Rejection, BKPyV viremia and CMV viremia occurred in 23%, 27% and 17% of the patients, respectively. With every 10-fold TTV load-increase, the risk of rejection decreased considerably (HR: 0.74, CI 95%: 0.71-0.76), while the risk of BKPyV and CMV viremia remained the same (HR: 1.03, CI 95%: 1.03-1.04 and HR: 1.01, CI 95%: 1.01-1.01).

*Conclusions: In conclusion, TTV load kinetics predict allograft rejection in KTx recipients, but not the BKPyV and CMV infection. The potential use of TTV load levels as a guide for optimal immunosuppressive drug dosage to prevent allograft rejection deserves further validation.

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