Transitioning of renal transplant pathology from allograft to xenograft and tissue engineering pathology: Are we prepared?

Table 1 Classification and diagnostic criteria used in pig-to-primate solid organ transplantation pathology

Hyperacute rejection	Acute humoral xenograft rejection	Acute cellular xenograft rejection
Time period
Immediately after reperfusion of the graft (typically within 24 h)	Later after reperfusion (after 24 h)	After 3 d
Immediate graft function
No (no urine since reperfusion)	Yes, urine formation initially	Yes, urine formation initially
Histopathologic features
Massive hemorrhage; Immunoglobulin and fibrin deposition; Complement (C5b-9) deposition; Presence of neutrophils; Thrombosis, ±	Hemorrhage present; Immunoglobulin and fibrin deposition; Complement (C5b-9) deposition; Presence of neutrophils; Lymphocytes may be present; Necrosis and transmural infiltration by neutrophils in blood vessels can be present; Apoptosis may be present; Thrombosis present	No hemorrhage; Immunoglobulin and fibrin deposition, rare; Complement, ±; Presence of mononuclear (lymphoid) cells associated with tissue destruction (e.g., tubulitis); No thrombosis