Acute graft-versus-host reaction can be aborted by blockade of costimulatory molecules.

Injection of parental lymphocytes into an unirradiated adult F1 host results in an acute GVH reaction characterized by immune deficiency, attack on host lymphohematopoietic tissues, and repopulation with donor-derived cells. All of these events result from the initial activation of donor lymphocytes by host alloantigens. Interaction of pairs of host and donor costimulatory molecules, in particular CD28/CTLA4 and B7-1/B7-2, play a crucial role in this initial activation of donor T cells. We demonstrate here that in vivo treatment of the host with high doses of CTLA4-Ig solely during the initial period of donor alloactivation can completely abort the subsequent development of GVH reaction. Although donor T cells are retained, CTLA4-Ig treatment reduces the initial endogenous cytokine production and arrests the subsequent expansion of donor T cells, the differentiation of anti-host effectors, and the development of severe immune deficiency. This result is consistent with the establishment of host-specific tolerance in the donor population, while maintaining host immune competence.