The role of TCF-1+CD8+ exhausted progenitors and TCF-1 in graft-versus-host responses.

In allogeneic hematopoietic transplantation, donor αβ T cells attack recipient tissues, causing graft versus host disease (GVHD). A longstanding question has been how GVHD is maintained despite T cell exhaustion from chronic alloantigen stimulation. In other exhaustion models, CD8 responses are sustained by CD39loTim-3loToxhiTCF-1hi precursor exhausted T cells (TPEX). Here we characterize CD8+ TPEX in the B6(H-2b)→129(H-2b) GVHD model wherein responses against the minor histocompatibility antigen H60 can be tracked using MHCI-tetramers (TetH60). Early after transplant, TetH60+ CD8 cells were uniformly PD-1hiToxhi, whereas TetH60- cells also had PD-1loToxlo cells, indicative of more diverse antigen experiences. Among TetH60+ and TetH60- populations were CD39loTCF-1hi cells. Upon competitive retransplantation, TetH60+CD39loTCF-1hi cells outcompeted TetH60+CD39hiTCF-1lo cells and underwent self-renewal, whereas CD39hiTCF-1lo cells did not yield TCF-1hi cells. To test the role of TCF-1, we studied CD8 cells lacking long TCF-1 isoforms (p45-/-). P45-/- cells were outcompeted by WT cells when transplanted into 129 recipients, though they expanded similarly in syngeneic recipients. In the B6→C3H.SW(H-2b) model, p45-/- CD8 cells caused less weight loss than did WT CD8 cells; however, histopathologic GVHD was similar in both groups. P45-/- and WT CD8 cells also had similar graft versus leukemia activity. These results highlight the complex biology of TCF-1 in supporting alloreactive T cell function.