The model of porcine kidney transplantation provides a unique opportunity to further the field of human transplantation due to similarities in surgical aspects, physiology, biochemistry, and immunology 14.
Depending on the purpose of the experimental study, the model of renal autotransplantation has several advantages compared to the allotransplantation model. Although several groups report good renal graft function after allotransplantation 28, immunosuppression in pigs is challenging, especially in renal transplantation. Preoperative blood sample analyses to ensure compatibility for swine leukocyte antigen (SLA) are feasible, but expensive and impractical 14. Postoperatively, proposed immunosuppressive agents such as tacrolimus and cyclosporine (calcineurin inhibitors, CNI) are administered orally or i.v. 28. Oral administration is impractical, as pigs usually refuse to swallow oral medication. Furthermore, intestinal obstructions might obviate sufficient absorption of immunosuppressive medications and maintenance of therapeutic drug levels. The continuous infusion of CNI's i.v. in active animals is technically demanding. I.v. bolus administration leads to high peak values, which cause toxicity. Thus, for the investigation of new preservation techniques, the model of renal autotransplantation has several advantages. In the representative results of the allotransplantated kidney graft demonstrated above, a delayed and increased peak of creatinine and BUN indicate rejection, which was demonstrated by histological assessment.
The porcine model of autotransplantation has previously been used to investigate new preservation techniques 14,18,29. However, the reported postoperative serum creatinine and BUN values of autotransplanted pigs in a heart-beating scenario vary considerably depending on the experimental system 22,30. The heart-beating donor protocol we present here results in a low postoperative serum creatinine peak of 2.8 mg/dl (± 0.7) and BUN peak of 25.3 mg/dl (± 7.4). These results are comparable with the low peak values presented by Hanto and colleagues 28 and Snoeijs and colleagues 31.
To ensure a successful outcome after renal transplantation in a porcine autotransplantation model, we have identified several key technical factors that minimize the rate of certain complications. The use of histidine-tryptophan-ketoglutarate solution (HTK) reduces the risk of vasospasm due to its lower content of potassium when compared to University of Wisconsin (UW) solution. To further decrease the risk of vasospasm at the point of reperfusion, verapamil can be injected into the renal artery, and papaverine can be administered topically during retrieval and after reperfusion. In addition, a continuous drip of norepinephrine titrated to maintain the systolic blood pressure above 100 mmHg ensures a homogeneous reperfusion. It is useful to maintain this blood pressure at least until the pig is positioned prone. Furthermore, the positioning of the transplanted graft is important to prevent kinking of the newly anastomosed blood vessels. Therefore, it is helpful to resect the contralateral left kidney prior to sewing the anastomoses of the graft to avoid extensive mechanical manipulation. After finishing the ureteral anastomosis, wrapping small intestine around the transplanted graft secures its position after closure of the abdominal wall. Complications such as bowel obstructions due to kinking of the intestine are rarely observed but can lead to severe complications, including ileus, bowel perforation, and death. Overall, accurate surgical technique, attentive anesthesia and close monitoring during follow up ensure good clinical outcome and graft function.
Arterial and venous anastomoses can be performed using different techniques. Orthotopic placement of the graft allows end-to-end anastomoses of the renal artery and vein. In the case of heterotopic transplantation, the graft can be positioned in the contralateral renal fossa for end-to-end anastomoses, onto the iliac vessels, or the distal aorta directly. Heterotopic transplantation with anastomoses to aorta and cava directly in end-to-side technique are preferred in this model as it can reduce the risk of thrombosis and vasospasm 32. Anatomical variations with very early venous bifurcations might lead to the need of sewing two separate venous anastomoses. If the artery or vein are relatively short, the graft can be turned 180° to gain length of the vessels. Ureteral side-to-side anastomosis can achieve good experimental results without complicating strictures or urinary leak.
In general, the porcine model of renal transplantation offers advantages compared to other animal models. As described above, certain similarities exist between the porcine and the human setting, which allows relatively fast translation of new techniques into clinical practice. The technique of transplantation is technically easier compared to rodent models. In addition, by placement of venous catheters, peripheral blood samples can be collected easily and processed for further investigation. The collection of urine allows further assessment of kidney injury and function. To collect urine samples, a percutaneous catheter can be inserted into the urinary bladder. To avoid manipulation by the pig, the distal end should be tunneled subcutaneously to the back of the animal. Another option for urine collection is the use of metabolic cages, which allow prolonged collection periods to estimate the creatinine clearance and concentration of additional biomarkers in the urine. Sonography, CT scans, and MRI images are possible. Donation after circulatory death protocols can be mimicked by applying warm ischemia prior to retrieval. Furthermore, pigs are relatively easy to handle if castrated to limit their aggressive behavior.
Disadvantages include the high costs of animal purchase, housing, surgical and other medical equipment, and manpower. These factors mean that it is not feasible to include large numbers of animals in each study group. Furthermore, compared to rodent models, a limited number of references are available in the literature for pig normative biological data. As an alternative for the assessment of new developed techniques, such as novel preservation methods, other groups have described the normothermic ex vivo reperfusion as an alternative to renal transplantation 33,34. This technique is easier to perform and less expensive. However, standardized kidney graft transplantation provides a model more similar to the clinical practice and allows longer follow up periods. Therefore, it serves for a more realistic graft assessment.
In conclusion, the porcine model of heterotopic renal autotransplantation provides a clinical important scenario to investigate innovative novel approaches for the improvement of kidney graft outcomes. In particular, this protocol features important technical details that will facilitate successful establishment of a renal autotransplantation model and allows the rapid translation of new findings to clinical trials.