Immunological and genetic requirements for the parental hemopoietic takeover reaction in adult,H-2-incompatible parent-F1 hybrid parabiosis

Parabiosis of adult DBA/2J (H-2 ^d ) mice with adult (DBA/2J× CSH/HeJ)F₁ (H-2 ^d /H-2 ^k ) mice results in survival beyond 100 days in 44% of such pairs, induction ofin situ unresponsiveness to C3H/HeJ skin, and the complete takeover of the erythroid system of the F₁ by parental cells. However, in vitro responsiveness to C3H/HeJ cells remains. Dye exclusion cytotoxicity assays establish the absence ofF ₁ lymphoid cells in the spleens and bone marrow of both partners. The parental takeover of the erythroid system of the F₁ partner requires immune recognition of the hybrid's alloantigens, because this takeover is not seen with tolerant parental cells. PartialH-2 differences (on the C3H background) influence both survival and the takeover reaction when incorporated into parabioses with DBA/2J partners. When only theK andI subregions ofH-2 were targets of the parental response, 58% of parabionts survived, with complete parental hemopoietic takeover. When onlyH-2D was the target, 83% of parabionts survived, with incomplete hemopoietic takeover. Changing the non-H-2 background of the F₁ target did not significantly affect survival or takeover, while substituting a differentH-2 ^d parental strain (BALB/c) eliminated survival altogether.Thus the parabiont takeover reaction encompasses the lymphoid and hemopoietic systems, requires immunorecognition of the target alloantigens, and seems to require a strongH-2 difference for its induction.