Immunological tolerance: "forbidden clones" allowed in tetraparental mice.

Tetraparental (allophenic) mice, made chimeric at the eight-cell stage by joining two embryos from histoincompatible parental strains, were examined by a microcytotoxicity test. The results indicate that parental-strain fibroblasts are more effectively destroyed in vitro by lymph node cells from the tetraparental mice than by lymph node cells from the F(1) hybrid or either parental stain. The destruction by tetraparental lymph node cells is indistinguishable from that mediated by lymph node cells from previously immunized allogeneic animals. It can be prevented by serum from the tetraparental mice, but not by sera from the F(1) hybrid or the parental strain animals. The results suggest that a concomitant immunity and serum blocking effect, rather than a central failure of the immune response, may mediate some aspects of normal tolerance.