Pineal grafting and melatonin induce immunocompetence in nude (athymic) mice.

The administration of melatonin to thymocyte-infused nude mice during the evening (dark cycle) of the circadian period restores transplantation immunity permanently, whereas inoculation during the morning cycle is quite ineffective. Transplantation of an intact pineal gland from normal donors into the mediastinal (athymic) tissue of nude mice leads to restoration of the capacity to reject skin grafts. Findings such as these attest to the genesis of transplantation immunity as linked to a thymus-directed establishment of a pineal circadian cyclicity.