Perforin and Fas Ligand Are Required for the Regulation of Alloreactive CD8⁺ T Cells.

We evaluated the role of Fas ligand and perforin, the major T cell-mediated cytotoxic pathways that regulate T cell homeostasis, in a CD8⁺ T cell mediated model of graft-versus-host disease (GVHD) where donor and recipients differ at a single MHC class I antigen (B6 → bm1). Lethally irradiated (11Gy) bm1 mice were transplanted with T cell depleted BM and CD8⁺ T cells from either wild type (wt) or cytotoxic double deficient (cdd, deficient in both pathways) B6 donors. We hypothesized that cdd CD8⁺ T cells would be unable to mediate significant GVHD. Contrary to our hypothesis, recipients of cdd donor CD8⁺ T cells demonstrated significantly greater histopathologic damage from GVHD and increased serum levels of IFN-gamma and TNF-alpha compared to controls (Table 1). In order to understand this increase, we evaluated the in vivo expansion of donor T cells in these recipients as well as in BMT recipients of allogeneic CD8⁺ T cells from FasL deficient (gld) and perforin deficient (pfp^−/−) donors. CD8⁺ wt T cells expanded until at day 10 after BMT, followed by a rapid decline. By contrast, cdd CD8⁺ T cells expanded continuously up to day 30 after BMT, peaking at almost one hundred times the number of wt T cells. gld T cells showed kinetics similar to wt T cells, whereas the pfp^−/− T cells showed a significantly greater peak on day 10 but a similar contraction by day 30. Percentages of annexin⁺ cdd donor CD8⁺ T cells were significantly reduced compared to the other groups. Persistence of host antigen presenting cells did not account for the unrestrained expansion of cdd donor T cells because host dendritic cells were not detected in either the spleen, BM or gut of recipients of cdd CD8⁺ T cells on day 6 after BMT. In addition, alloantigen expression on epithelial target cells did not enhance GVHD because B6 donor cdd T cells induced equivalently lethal GVHD in [bm1 → B6] and [bm1 → bm1] chimeras (MST of 30 days and 27 days, respectively). We conclude that both perforin and Fas ligand pathways are required for alloreactive CD8⁺ T cell populations to contract after their initial expansion during a GVH reaction and that the absence of both these pathways results in donor CD8⁺ T cell unrestricted expansion and more severe GVHD.

Table 1