Design of the TransplantLines biobank and cohort study

TransplantLines is an ongoing, single-centre, observational cohort study conducted at the University Medical Centre Groningen (UMCG), The Netherlands. It includes recipients of all types of (combined) solid organ transplants as well as living and deceased organ donors. All (potential) solid organ transplant recipients and (potential) living donors at the UMCG were invited to participate. Both cross-sectional and longitudinal biomaterials and data are included. The comprehensive inclusion of a substantial number of recipients and donors across all types of solid organs, alongside extensive demographic, clinical, physical, cognitive and psychosocial data, as well as extensive laboratory testing, combined with a broad array of biomaterials, such as 24-h urine samples, hair, nails, faeces, and perioperatively collected tissues, next to whole blood, buffy coat, serum and plasma stored in various forms, makes this study unique. There are several studies embedded in TransplantLines:

Coronary artery calcium (CAC) study (n = 203): observational study to assess vascular calcification using a CT-scan in kidney transplant recipients [7].

Randomized controlled prospEctiVe trial comparing extended-release with immediate-release tacrOlimus; reducing calcineurin inhibitor related toxicity in LUng TransplantatION patients (REVOLUTION) study (n = 149): interventional study to compare Envarsus to Prograft in lung transplant recipients (ClinicalTrials.gov identifier: NCT05001074).

Vitamin K study (n = 39): interventional study to assess the effects of vitamin K supplementation in kidney transplant recipients (EudraCT Number: 2019–004906-88; NTR number: NL7687).

OPen label multicenter randomized trial comparing standard IMmunosuppression with tacrolimus and mycophenolate mofetil with a low exposure tacrolimus regimen In combination with everolimus in de novo kidney transplantation in Elderly patients (OPTIMIZE) study (n = 130): interventional study to examine an alternative immunosuppressive regime for kidney transplant recipients aged ≥ 65 years (ClinicalTrials.gov identifier: NCT03797196).

SEnsory Neuropathy Scores (SENS) study (n = 169): a study assessing sensory neuropathy scores in kidney transplant recipients, validated using quantitative sensory testing and nerve conduction studies (ClinicalTrials.gov identifier: NCT04664426).

Trajectory of Immunosuppression-Caused Tremor In Kidney Transplant Recipients (TICTIK) study (n = 72): this prospective observational study investigates the progression of tremor after kidney transplantation using various tests. Tremor is assessed before transplantation and at different moments after transplantation (ClinicalTrials.gov identifier: NCT06488651).

Study visits

All laboratory test results recorded in the electronic medical patient files of the participants are included in TransplantLines. Generally, transplant recipients undergo frequent follow-up visits during the first year after transplantation, with the frequency gradually decreasing over time. After the first year, stable kidney transplant recipients are typically followed up at different hospitals, but continue to undergo comprehensive laboratory testing at least once a year at the UMCG. Recipients of other organ transplants generally maintain regular follow-up visits at the UMCG. Living kidney donors receive typically follow up at 3 months, 5 years, and 10 years post-donation.

Additionally, participants of TransplantLines underwent study visits that included comprehensive laboratory testing, biobanking and for a subset of the participants also a physical or cognitive assessment. Potential recipients and those < 1 year post-transplant at the time of inclusion in TransplantLines were included in the longitudinal cohort. They received study visits at pre-transplantation, at transplantation, and at 3, 6, 12, 24, and 60 months post-transplantation. Recipients who were ≥ 1 year post-transplant at inclusion were included in the cross-sectional cohort, and underwent one single study visit. Preceding the study visits, participants completed self-administered questionnaires at home. In case of a re-transplantation, follow-up for the previous transplantation ended, and a new transplantation trajectory was started in the longitudinal cohort for the existing participant, with the same specified time points for the study visits after the re-transplantation. Additionally, kidney transplant recipients undergoing graft biopsy for suspected rejection underwent an extra study visit with comprehensive laboratory testing and biobanking. Living donors received study visits at pre-donation, donation, 3 months post-donation, and/or 5 or 10 years post-donation, with questionnaires preceding these visits regarding the donation trajectory. An overview of the study visits in the longitudinal cohort is shown in Fig. 1.

Fig. 1

Study visits in the longitudinal cohort of TransplantLines. Abbreviation: Tx: transplantation

Data collectionClinical and laboratory measurements

For transplant recipients and living donors, demographic and laboratory test results are extracted from their medical records. Transplant- and donor-related data, including the indication for transplantation, donor type, and HLA mismatches, are obtained from the Eurotransplant registry. For deceased donors, data such as demographics, cause of death, donated organs, and medical history are also derived from the Eurotransplant registry. An overview of donor and recipient data is presented in Supplementary Table S1. All laboratory data recorded in the patients’ medical records are included. In addition to routine laboratory measurements for clinical care, extensive testing is conducted during TransplantLines study visits to minimize indication bias (see Table 1).

Table 1 Laboratory measurements performed at the study visits of TransplantLinesStudy visit

Study visits were conducted at the outpatient clinic following a standard operating procedure (SOP) and performed by trained researchers. During each study visit, data were collected on blood pressure, heart rate, temperature, anthropometry, body composition (measured by bioimpedance analysis), muscle strength, lung function, nutritional status, frailty status, advanced glycation end products (AGEs), and medication use, among others. Additionally, participants either underwent additional physical or cognitive-focused assessments. The additional physical assessment included tests for standing balance, 2- or 6-min walking test, 4-m walk, five-times-stand, timed-up-and-go, and the 9-hole peg test, as well as a neurological examination focused on polyneuropathy and tremor. The additional cognitive-focused assessment included a neuropsychological assessment measuring memory, mental speed, attention and executive functioning. Cross-sectional and 12-month post-transplant study visits also included a dermatological assessment. After 2021, physical study visits were discontinued, and data on blood pressure, heart rate, anthropometry, and medication use were extracted from medical records. An overview of the assessments is presented in Fig. 2.

Fig. 2

Overview of the assessments conducted during study visits of TransplantLines. a: measured multiple times. b: participants were randomized to either a physical assessment (motor function, neurological and dermatology examination) or a cognitive assessment (neuropsychological assessment). c: collected until March 2021

Questionnaires

Participants filled out questionnaires prior to each study visit (except for the study visits at transplantation and at 3 months post-transplant). These questionnaires evaluated physical and mental health symptoms, including fatigue and sleep quality. Lifestyle factors such as smoking behaviour, alcohol use, and physical activity were also assessed. Psychological items received significant attention, with measures for coping strategies, cognitive functioning, well-being, and symptoms of anxiety and depression. The questionnaires also addressed social functioning, including work-related aspects like employment and work performance, societal participation, family relationships, and social support. Therapy adherence was evaluated as well. Furthermore, dietary intake was estimated using food frequency questionnaires. An overview of the questionnaires is provided in Table 2.

Table 2 Overview of questionnairesBiomaterials

At each study visit, a wide range of biomaterials is collected, including various blood-derived components such as buffy coat, plasma (citrate, EDTA, and heparin), PAXgene® (blood RNA tube – QIAGEN), serum, and whole blood. Additionally, 24-h urine collections (both non-acidified and acidified), faeces, hair, and nails are collected. At the time of a kidney allograft biopsy, additional blood and urine samples are collected and immediately stored on ice. During kidney transplantations, additional samples are collected from the skin, subcutaneous tissue, perivascular fat, ureter, external iliac artery, renal artery, perirenal fat, and kidney biopsies. Since 2021, the collection of blood stored in citrate, heparin, and PAXgene® (blood RNA tube – QIAGEN), as well as acidified 24-h urine, faeces, hair and nails, has been discontinued. All samples are stored at -80 °C. Additionally, tissue samples from the kidney graft and surrounding tissues, collected for clinical care, are available for research purposes.