Dynamic Changes of Torquetenovirus (TTV) Load in Chinese Renal Transplant Recipients

The goal of this observational study is to learn about dynamic changes of Torquetenovirus (TTV) load in Chinese renal transplant recipients. The main questions it aims to answer are: * Is there correlation between TTV load and rejection? * Is there correlation between TTV load and infection? * Can changes in the TTV load of kidney transplant recipients predict rejection or infection? Participants will: * receive 13 follow-up visits within 1 year after kidney transplantation * provide 2 ml of whole blood for TTV load testing and other related testing at each follow-up * provide 10 ml of whole blood for dd-cfDNA testing at four follow-ups (1, 3, 6 and 12 months after transplantation) * provide 1 ml of serum for donor-specific antibody testing at three follow-ups (1, 6 and 12 months after transplantation)