The so-called refractory state, one sequela of acute graft-versus-host disease, has been studied in adult (CB x MHA)F1 hybrid Syrian hamsters inoculated with sublethal numbers of MHA-anti-CB lymphoid cells. Intracutaneous challenge of these animals with 200 million MHA-anti-CB lymphoid cells after the acute syndrome subsided failed to evoke epidermal necrolysis, whereas a similar challenge administered to normal F1 recipients invariably resulted in lethal epidermolysis. Moreover, the gradual attrition of lymphatic tissues in these hosts and their fading capacity to display adequately immune lymphocyte transfer reactions in the skin coincided with increasing evidence of host refractoriness, suggesting a causal interrelationship. It was possible to circumvent refractoriness by challenging these animals intracutaneously with MHA-anti-CB cells if: (a) the hosts had been lethally irradiated and reconstituted with F1 hematopoietic cells, or (b) the intracutaneous inocula contained admixed F1 lymphoid cells. This evidence provides additional support for the hypothesis that in GVH disease donor lymphocytes attack primarily host lymphoid cells bearing offending homologous antigens. The GVH process can continue so long as these lymphocyte-bound antigens persist within the host, and will abate only as the aggregate host lymphatic mass is effectively destroyed (hamsters) or its antigenic determinants are masked by isoantibodies (rats, mice, man?). At this point, insufficient target tissues remain for rechallenge to incite significant recrudescence of the disease.

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