The historical postulate: Is it the basis, at the level of the system, for self-nonself discrimination?

ABSTRACT

Burnet envisaged the early presence of self-antigens in development, or 'the history' of an animal, ablates the animal's ability to immunologically respond against them. Lederberg added the idea that the continuous presence of self-antigens is required to maintain tolerance throughout life. We refer to Lederberg's proposal as 'The Historical Postulate'. The mechanism of central tolerance, as now understood, is consistent with The Historical Postulate. Some observations, reflecting peripheral tolerance, appear inconsistent with this postulate. For example, some foreign peripheral tissues, grafted onto an animal before the immune system develops, can be rejected as the immune system matures. The original two-signal model of lymphocyte activation was proposed in part because it accounted for peripheral tolerance in a manner consistent with The Historical Postulate. We proposed that lymphocyte activation required antigen-mediated lymphocyte cooperation, whereas antigen would inactivate lymphocytes when insufficient in number to achieve activation. We argue here that the exceptions to The Historical Postulate can be explained by the two-signal model of lymphocyte activation they reflect the existence of greater numbers of lymphocytes specific for these antigens than for natural peripheral antigens, and so are outside the physiological limits important in selecting through evolution this mechanism of peripheral tolerance. We argue that a consideration of whether The Historical Postulate is valid is important, even if only valid within certain understandable limits. The currently popular DAMP model of CD4 T cell activation is, strictly speaking and in a manner we discuss, in violation of this postulate.