The survival of transplanted cells or organs requires the incorporation of the graft into the human blood circulatory system. The major barriers to solid organ xenotransplantation occur at the interface of the human blood supply and the pig organ, where human blood flows through pig blood vessels. A series of well defined reactions occur on the pig endothelial lining cells of blood vessels. These are defined as hyperacute, delayed and cellular rejection.
Revivicor’s in-house expertise in molecular/cell biology, immunology, and animal biotechnology has positioned the company, as the only commercially-viable group worldwide to ultimately deliver a xenograft that works, because we have developed genetic modifications which have the potential to overcome each of the known sequential causes of rejection.
Hyperacute rejection is the first hurdle that has to be overcome; a reaction to the 'foreign' organ by the body's normal immune system. Humans and primates differ from other animals in that they lack an enzyme (alpha 1,3 galactosyltransferase) that places a particular sugar group (Gal) on the branched sugar chains which occur on cell surfaces. Our bodies recognize its presence on grafted pig organs as a signal to attack. Revivicor's has inactivated the gene in pigs which makes the enzyme that attaches this Gal sugar group, producing the worlds first alpha 1,3 galactosyltransferase (Gal) knock-out pigs. Organs from Gal knock-out pigs transplanted into non-human primates did not undergo HAR; thus the initial attack of HAR was overcome by the use of these GE pigs.
While the presence of the foreign Gal sugar is by far the major signal for initiating an attack by the immune system, there are other mediators of immune rejection at play. Revivicor has also added a human gene to the pigs to produce a protein called CD46 that moderates the action of the immune system. This gene addition strategy, combined with Gal knock-out and immune suppression drugs, demonstrated encouraging results of pig hearts in primates, with survival and function for up to 8 months.
Overcoming hyperacute rejection is only the first, but essential, step in Revivicor's comprehensive approach. The transplanted xeno pig organ can still be rejected by delayed xenograft rejection (DXR). DXR results from a loss of the anti-coagulation factors which normally exist on the surface of blood vessels to stop the blood clotting and blocking of these vessels. When a xeno organ or xeno islet cell is transplanted, these protective anti-coagulation factors are lost. Revivicor intends to overcome this problem with the addition of human anti-coagulant genes to the pig which will cause the replacement of the anti-coagulation factors in the donor tissues. These “anticoagulant” transgenic pigs are already being produced and pre-clinical testing is ongoing at Revivicor and with its collaborators.
The final cause of xenograft rejection is long term or cellular rejection, which occurs in both allo (human-to-human) and xeno transplants. Long term rejection is caused by attack by T cells, part of the body's immune defense system which recognizes a specific different "foreign" signal. In allo transplantation long term rejection is controlled by continuous drug therapy. Revivicor's approach is to add genes to pigs which can decrease or stop this T cell response. While the use of immune-suppression drugs will still be required, the aim is to make the GE pig cells and organs sufficiently compatible, such that the amount of drug therapy required for a xeno transplant will be equivalent to that currently used for human-to-human allo transplants. Next generation transgenic pigs expressing T cell inhibitor genes are also currently being produced and tested by Revivicor and its scientific collaborators.
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