Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Abstract

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Fig. 1

Genetic manipulations of pigs that are likely to be required to provide lungs for successful clinical xenotransplantation. hCRP = transgenic expression of one or more human complement-regulatory proteins, for example, CD46+/− CD55.

Fig. 2

Schematic representation of CD47-signal regulatory protein-alpha (SIRP-α) interaction in relation to natural expression of SIRP-α on pig pulmonary macrophages. Left: In the organ-source pig, there is a normal inhibitory signal of porcine CD47 (in this example expressed on platelets) that is recognized by porcine SIRP-α. Center: After pig lung xenotransplantation, there might be a lack of recognition of human CD47 by porcine SIRP-α, resulting in phagocytosis of human platelets. (Some binding between SIRP-α and CD47 occurs, but there is a deficiency in signaling that prevents signal transduction, the cause of which is uncertain.) Right: Transgenic expression of human SIRP-α on pig pulmonary macro-phages would result in recognition of human CD47 on human platelets, thus inhibiting phagocytosis.

Fig. 3

Schematic representation of CD47-signal regulatory protein-alpha (SIRP-α) interaction in relation to natural expression of SIRP-α on human macrophages. Left: After transplantation of an unmodified pig lung into a human, the expression of pig CD47 on the endothelial cells of the pulmonary blood vessels will not be recognized by human SIRP-α-expressing macrophages, which will therefore not be inhibited but will become activated; inflammatory cytokines will be produced and graft injury will occur. Right: When a lung from a pig transgenic for human CD47 is transplanted, the human SIRP-α-expressing macrophages will recognize the pig tissues as “self,” and activation will be inhibited; cytokine production and graft injury will not occur.