A two-phase pathogenesis of graft-versus-host disease in mice

Activation of donor T cells is required for the development of graft-versus-host disease (GVHD), a major complication of bone marrow transplantation. We investigated a murine model of GVHD across major and minor histocompatibility barriers. BALB/c recipients were lethally irradiated and transplanted with 10(7) bone marrow and 5 x 10(6) spleen cells from C57BL/6 donors. There were two separate phases of clinical disease. The first phase was most severe on day 7 after transplant. Weight and condition improved until day 12 and then a second phase of clinical GVHD commenced, which persisted until euthanasia. IL-2 mRNA expression, as a measure of T cell activation, was determined by quantitative PCR. The two phases of clinical GVHD were preceded by two separate peaks of IL-2 mRNA in the spleen. Host MHC class II(+) cells became undetectable by flow cytometry 7 days after transplantation, whereas donor MHC class II(+) cells increased during the first 9 days after transplantation. Removal of donor MHC class II(+) cells from the graft had no effect on the first phase. Possible roles for host and donor antigen-presenting cells (APC) in the two phases of the disease are discussed.