Ontogeny of priming of cytotoxic T cells to minor alloantigens: the development of direct priming precedes that of cross-priming.

Cytotoxic T lymphocytes of heterozygous adult mice primed in vivo with minor alloantigens on cells of one parental H-2 genotype can be boosted in vitro to respond to minor alloantigens on cells of both the immunizing parental H-2 genotype (direct priming) and of the H-2 genotype of the other parent (cross-priming). We studied the ontogeny of this phenomenon and show that during early postnatal life the development of direct priming precedes that of cross-priming. The delayed maturation of cross-primed responses parallels the known development time sequence of functional antigen-presenting cells and provides evidence for the explanation of cross-priming in terms of antigen processing.