Studies on transplantation immunity. IV. Murine natural immunity to lymphoid cells in vivo.

A natural response acting to discriminate between allogeneic and syngeneic lymphocytes in vivo is described for normal mice. Eighteen to 24 h after systemic injections of low doses of 51Cr-labeled cells, less radioactivity is found in the lymph nodes after allogeneic transfer. The spleen but not the liver participates in the response. It can be abolished by neonatal induction of transplantation tolerance, is shown to be immunologically specific, and is resistant to irradiation up to 1000 rd, although a radiosensitive phase occurs during recovery from sublethal irradiation. The response is cell-mediated but depends upon cooperation from a factor present in normal serum. It is thymus-independent: this clearly distinguishes it from the superficially similar immune response acquired by immunization and aligns it with the class of responses to which the hybrid histocompatibility response and natural killer-like phenomena belong. Unlike these, however, it is present at birth and directed primarily against H-2 antigens. It appears to follow the classical transplantation rules in a limited range of strains. Some of the difficulties inherent in this type of study are discussed.