Xenotransplantation - Nonhuman Primates in Biomedical Research - page 385

cells. A more recent introduction is pig artery patch xeno-
grafting, which is much simpler than an entire organ
transplant, yet which is susceptible to the same immune
response.

a consumptive coagulopathy in the nonhuman primate
recipient.

The Adaptive Immune Response
If both hyperacute and acute humoral xenograft rejection
are prevented, but immunosuppressive therapy is inade-
quate, there is likely to be a T-cell-dependent elicited
antibody response, resulting in high levels of anti-pig IgG
( Dorling and Lechler, 1998; Mirenda et al., 2005 ). Binding
of these antibodies to the vascular endothelium initiates
histopathological changes indistinguishable from acute
humoral xenograft rejection. Surprisingly, acute cellular
rejection, as seen in the majority of allotransplants, has
virtually never been recorded after pig-to-nonhuman
primate organ xenoTx. This is most likely because the
humoral response overwhelms the cellular response,
though T and B cells may well be seen in the graft.
In pig heart grafts that have survived for more than
approximately 3 months, graft vasculopathy develops
( Figure 16.1 C) ( Kuwaki et al., 2005; Tseng et al., 2005 ), as
seen in human allografts that have survived for many
months or years (chronic rejection). This form of chronic
rejection is as poorly understood in xenoTx as it is in
alloTx.
With the introduction of genetically engineered pigs that
are partially resistant to the primate antibody-mediated
response, it is becoming clear that the coagulation dysfunc-
tion between the two species is playing an increasing role.
Indeed, aberrant coagulation may be a much more important
causative factor in the development of acute humoral xeno-
graft rejection than previously anticipated ( Chen et al.,
2004 ). However, it is likely that it is initiated by immune
factors, such as antibody binding to the vascular endothe-
lium, leading to endothelial activation ( Bach, 1994 ).

PATHOBIOLOGY OF PIG-TO-PRIMATE
ORGAN TRANSPLANTATION
The Innate Immune Response
When the pig organ is unmodified, i.e., not genetically
modified, and no immunosuppressive therapy is given to the
recipient, hyperacute rejection occurs in the majority of
cases ( Figure 16.1 A) ( Rose et al., 1991; Rose and Cooper,
1996, 2000 ). Hyperacute rejection has been defined as
antibody-mediated complement activation, leading to
destruction of the graft within 24 hours; it frequently occurs
within the first hour. It is known to be associated with the
binding of anti-Gal IgM to the Gal epitopes on the pig organ,
with activation of the complement cascade. However, it has
been described after the use of organs from pigs in which the
Gal antigen is not expressed ( 1,3-galactosyltransferase

gene-knockout [GTKO] pigs), and therefore can be initiated
by binding of anti-nonGal antibodies.
If hyperacute rejection is prevented, either by a genetic
manipulation of the pig, or by immunoadsorption of anti-
pig antibodies from the recipient, acute humoral xenograft
rejection (sometimes known as acute vascular rejection or
delayed xenograft rejection), usually develops within days.
This is again usually associated with antibody binding and
complement activation, but there is probably a greater role
of innate immune cells, such as neutrophils and macro-
phages, but the end result is largely the same ( Ezzelarab
2009 ).

a

Coagulation Dysfunction
In both these phenomena, it is increasingly clear that
abnormal coagulation within the vessels of the graft plays
a significant role, and this may well be associated with
molecular incompatibilities between the pig and primate
with regard to the coagulation systems ( Bach et al., 1994;
Robson et al., 2000; Chen and Dorling, 2005 ). For
example, pig tissue factor pathway inhibitor does not
successfully inhibit primate factor Xa, pig thrombomodulin
does not catalyze primate protein C, and pig von Wille-
brand factor is associated with excessive primate platelet
aggregation.
Even if early rejection is avoided, these incompatibili-
ties can result in the development of a thrombotic micro-
angiopathy, in which the vasculature of the organ is steadily
occluded by thrombus, resulting in ischemic necrosis of
the tissues ( Figure 16.1 B) ( Houser et al., 2004 ). The
consumption of clotting factors in this process can lead to

THE PIG-TO-NONHUMAN PRIMATE
MODEL
The pig-to-nonhuman primate model is very demanding
and requires considerable resources in both personnel
and facilities. In the majority of cases, a tether system is
used to enable indwelling intravascular catheters to be
available throughout the course of the experiment for
monitoring blood pressure, blood draws, and drug infusion
(e.g., inotropic or immunosuppressive agents) ( Figure 16.2 )
( Cooper et al., 1994 ).

Intravascular Catheter Placement and
Fluid/Drug Administration
At operation, under full inhalational anesthesia, Tygon
catheters are placed in the carotid artery and jugular veins