There is a shortage of cadaver donor organs for transplantation, and many patients remain indefinitely on dialysis or even die waiting for other organ transplants. A significant increase in cadaver donors for allografts is not anticipated. Xenotransplantation is a potential solution, but the rejection response is more difficult to thwart. Clinical attempts with concordant (non-human primates) donors have had transient success, but large scale application would necessitate the use of discordant species. Natural (NA) and antibodies complement (C) lead to hyperacute rejection (HAR), but this can be prevented by NA and C depletion. Nevertheless, long-term function is difficult to achieve. We propose to study the cellular rejection response to the pig hearts in rats in whom HAR was prevented by NA and C depletion. The window between prevention or delay of HAR rejection will be used to identify the immune cell subsets that infiltrate the xenograft. We will then use this information to then target specific immunosuppressive strategies against the subsets. We also will determine the kinetics by which immunoglobulin antibodies reappear by adoptive transfers of various subsets. This work should allow the relationship between the cellular and the humoral responses in rejection of a discordant xenograft to be established, and may lead to development of specific immunosuppressive strategies to prevent xenograft rejection.