Systemic inflammation in xenograft recipients (SIXR)
Authors: Ezzelarab Mohamed B. , Cooper David K. C. 
Published: February 2013

Dysregulation of the coagulation system commonly develops in pig xenograft recipients, and remains an obstacle to successful pig organ xenotransplantation. Uncontrolled activation of coagulation leads to consumptive coagulopathy (CC) in the recipients, and thrombotic microangiopathy (TM) in the grafts. T cell‐directed immunosuppression successfully prevents the adaptive immune response to pig antigens after xenotransplantation and prolongs survival of organ xenografts. In some reports, T cell‐directed immunosuppression was able to delay (or prevent) the development of CC and/or TM.

Recent reports have confirmed that inflammation can lead to activation of the coagulation system. Additionally, pro‐coagulant proteins, e.g. thrombin, are considered as pro‐inflammatory factors. In fact, an amplification loop is suggested to exist between inflammation and coagulation, leading to escalation of each other. Our in vitro data indicate that thrombin activation of pig endothelial cells is associated with upregulation of human T cell responses, suggesting that control of activation of coagulation and prevention of thrombin activation may facilitate the regulation of immune responses to xenografts in vivo.

We hypothesize that a state of systemic inflammation develops after pig organ xenotransplantation, which is generated by both adaptive and innate immune responses. Even if T cell‐directed immunosuppression can control activation of coagulation induced by adaptive immune responses, pro‐inflammatory signals induced by the innate immune system can still promote activation of coagulation.

We studied two models of xenotransplantation of different antigen loads; (i) pig aortic patch xenotransplantation, i.e. low antigen load, and (ii) pig organ (heart and kidney) xenotransplantation, i.e. high antigen load. We evaluated activation of coagulation, development of a T cell‐dependent immune response, and production of innate and adaptive pro‐inflammatory factors.

In recipients of a low antigen load xenograft, effective prevention of the adaptive immune response by T cell‐directed immunosuppression (i.e. suppression of T cell proliferation in response to pig antigens and prevention of elicited antibody production) was associated with reduced thrombin activation. However, there was (i) upregulation of C‐reactive protein (CRP) and (ii) fibrinogen levels, (iii) increased IL‐6 production in the circulation, and (iv) an increase in the absolute number of innate immune cells (monocytes and neutrophils). Furthermore, (v) monocytes and dendritic cells showed significant upregulation of tissue factor expression, and aggregation with platelets after transplantation.

In recipients with a high antigen load xenograft, short‐term organ survival was associated with high levels of CRP and IL‐6 early after transplantation. In long‐term organ survival, high levels of CRP and IL‐6 preceded the development of CC. There was intense CRP deposition in kidney xenografts (more than in heart xenografts) suggesting a stronger innate immune response. Additionally, CRP‐positive cells were detected in native lungs, suggesting an innate systemic inflammatory response.

In conclusion, efficient blockade of the T cell‐dependent adaptive immune response in xenograft recipients is associated with systemic upregulation of inflammatory markers. Systemic inflammation in xenograft recipients (SIXR) is associated with upregulation of tissue factor expression on innate immune cells and their aggregation with platelets. As inflammation is known to break tolerance after transplantation, understanding the underlying mechanisms and regulation of SIXR may be necessary to achieve long‐term survival of organ xenografts (and T cell tolerance to pig antigens). Also, further genetic modifications of donor pigs to express anti‐inflammatory proteins may be essential.