Donor T Cell Gene Expression and GVHD.

GVHD is initiated by donor T cell responses to host alloantigens. However, the occurrence and severity of GVHD are not determined solely by the level of histoincompatibility between donor and recipient. Two MHC-identical subjects will display over 50 minor histocompatibility antigen differences. If histoincompatibility is sufficient for triggering GVHD, the rate of GVHD in MHC-matched recipients of allogeneic hematopoietic cell transplantation (HCT) that receive no immunosuppressive agents should be 100%. Under these conditions, however, GVHD is found in only 50% and 73% of mouse and human recipients, respectively. Histoincompatibility is thus necessary but not sufficient to elicit GVHD. We tested the hypothesis that some donors may be “stronger alloresponders” than others, and consequently more likely to elicit GVHD. To this end, we studied the gene expression profiles of CD4 and CD8 T cells from 50 HCT donors using microarrays and qRT-PCR. We found that gene expression profiling before HCT was able to distinguish those donors whose cells caused GVHD from those whose cells did not. The “dangerous donor” trait (GVHD+ recipient) is under polygenic control and is shaped by the activity of genes that regulate TGF-β signaling and cell proliferation. The donor gene profile defined on day 0 shows strong correlation with that of recipient CD4 and CD8 T cells harvested one year post-AHCT. The latter correlation provides compelling evidence that a significant portion of the differential gene profiles between GVHD+ and GVHD– donors is imprinted at the hematopoietic stem cell level. Moreover, stability of the gene expression profiles over a one-year period suggests that the profiles result from inherited genetic traits as opposed to environmental factors. The gene with the best GVHD-predictive accuracy was SMAD3, a key component of the TGF-β pathway. By testing a cohort of 450 subjects using qRT-PCR, we found that amounts of SMAD3 transcripts varied over a 6-fold range. In mice and humans, SMAD3 is constitutively activated (as evidenced by phosphorylation and accumulation in the nucleus) in many leukocyte subsets. We found in mice that induction of TGF-β signaling in donor T cells is an early event following AHCT and that Smad3-deficient donors trigger more severe GVHD than wild-type littermates. These findings strongly suggest that the donor gene expression profile has a dominant influence on the occurrence of GVHD. In allogeneic HCT, the ability to discriminate strong and weak alloresponders using gene expression profiling could help select low-risk donors and permit tailoring GVHD prophylaxis regimens according to the probability of GVHD occurrence.