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.

Suggestive evidence that the "blocking antibodies" of tumor-bearing individuals may be antigen--antibody complexes.

Sera from mice carrying progressively growing sarcomas induced by Moloney virus or methylcholanthrene can block the cytotoxic effect of lymphocytes immune to the tumor-specific antigens of the respective neoplasms. The blocking effect can be specifically removed by absorbing sera with the respective types of tumor cells, and it can be recovered from these cells by elution at low pH. If the low pH is maintained, it is possible to separate out a low and a high molecular weight fraction from the eluates.