''Homologous restriction'' in complement lysis: roles of membrane complement regulators.

The complement system is a powerful bactericidal immune defence with the potential to damage self cells. Protection of self is provided by expression on cells of a battery of membrane regulators that inhibit activation of complement. Roles of complement in the rejection of transplanted organs have long been recognized, and are particularly relevant in xenotransplantation, where hyperacute rejection is complement-driven. Inhibiting complement was therefore considered early in the history of xenografting, and the use of membrane complement regulators to this end was proposed more than two decades ago. For each of the membrane regulators in humans, early studies implied a species-specificity of action, inhibiting human complement but not that from other species. The dogma of species-specificity dictated strategies for inhibiting complement in xenografts and drove the creation of donor transgenic pigs expressing human regulators. Here we critically evaluate the evidence for species-specificity in membrane complement regulators from humans and other animals. We challenge the dogma and show that there is considerable cross-species activity for each of the membrane regulators of complement. Acceptance of the fact that species selectivity is not a limitation will open new avenues for protection of the xenograft from complement damage.