Inclusion Criteria:  Adult (age ≥ 18 and ≤75) patients suffering from BKV infection/viremia following kidney transplantation  BKV viremia defined as positive (≥ 250 copies/mL) BK qPCR AND Presence of evidence of invasive BKV infection (BK Nephropathy)  AND ONE OF THE FOLLOWING CRITERIA:  New, persistent and/or worsening BKV-related symptoms, signs and/or markers of end organ compromise despite being on lower immunosuppressive medication, OR  Experiencing adverse effects of lower immunosuppressive medications (e.g., dnDSA, biopsy proven rejection)  Original donor ≥ 18 years if available, BKV IgG positive, eligible and capable of undergoing a single standard 2 blood volume leukapheresis or donation of one unit of whole blood OR If original donor is not available, BKV IgG negative OR ineligible, then a BKV IgG positive fully or partially matched (at least 2/6 HLA-A, -B, -DRB1) family donor will be used Written informed consent given by patient Written informed consent given by donor Eligible Donor  Exclusion Criteria:  Non-kidney organ transplant recipient Patient with acute rejection of the kidney allograft at time of T-cell transfer Patient receiving steroids (>0.5 mg/kg body weight (BW) prednisone equivalent) at the time of T-cell transfer Patient treated with Thymoglobulin (ATG), Alemtuzumab or T-cell immunosuppressive monoclonal antibodies within 28 days Patients with extra renal tissue invasive BKV infection ( biopsy proven) Concomitant enrollment in another clinical trial interfering with endpoints of this study Any medical condition which could compromise participation in the study according to the investigator's assessment Known HIV infection Female patient who is pregnant or breast-feeding, or adult of reproductive potential not willing to use an effective method of birth control during study treatment Note: Women of childbearing potential must have a negative urine pregnancy test at study entry. Patients unwilling or unable to comply with the protocol or unable to give informed consent  Donor selection priority: The original donor will be the first choice as source of T cells. If the original donor is not available for donation of peripheral mononuclear cells or does not meet all donor eligibility criteria, alternative related donors will be selected, with preference for fully matched related donors over those with partial HLA matching (≥ 2/6 HLA loci).  All donors must be ≥ 18 years old, available, BK IgG positive, eligible and capable of undergoing a single standard 2 blood volume leukapheresis.  If original kidney transplant donor is not available, BK IgG negative or ineligible, a BK IgG positive fully or partially matched (≥ 2/6 HLA loci) family donor will be used.  Related donors must be at least partially HLA compatible, matching with recipient in at least 2/6 HLA loci (HLA-A, HLA-B and HLA-DRB1 will be considered for this). Donors must be BK IgG seropositive. Donor must meet the criteria for donor selection defined in the Standard Operating Policies and Procedures (SOP B2.001) of the UWHC Hematopoietic Stem Cell Transplant Program.