In the DBA/2→unirradiated (C57BL/6 × DBA/2)F1 (BDF1) chronic graft-versus-host disease (cGVHD) model, anti-CD137 mAb is highly effective in inhibiting cGVHD by deleting donor CD4⁺ T cells which are required for breaking host B-cell tolerance (4). In a more clinically relevant cGVHD model, anti-CD137 mAb reverses skin fibrosis, ulceration, and alopecia, a dominant feature of cGVHD, ultimately improving a general health condition (5). The reversal is associated with increased apoptosis of donor CD4⁺ T cells. The Fas death pathway is required for activation-induced cell death (AICD) of alloreactive CD4⁺ T cells.

In this study, we hypothesized that anti-CD137 mAb might induce AICD of alloreactive CD8⁺ T cells as well as CD4⁺ T cells if they received strong allostimulation. We chose the C57BL/6→unirradiated BDF1 acute GVHD (aGVHD) model as a model system to test this hypothesis, since strong alloimmunity for donor CD4⁺ and CD8⁺ T cells occurs in this disease model. BDF1 mice received C57BL/6 T cells and anti-CD137 mAb (3H3) immediately after the cell transfer. FACS analysis showed that there was a marked increase in apoptosis of both splenic CD4⁺ and CD8⁺ T cells in anti-CD137-treated mice 5 days after the cell transfer (Fig. 1A). A majority of donor CD4⁺ and CD8⁺ T cells expressed low levels of CD62L in both control Ig- and anti-CD137-treated mice (Fig. 1B), indicating that AICD caused their apoptosis following injection of anti-CD137 mAb, as seen previously (4). At this time point, a higher percent of donor CD4⁺T cells underwent activation and apoptosis following administration of anti-CD137 mAb, as compared with donor CD8⁺ T cells (Fig. 1). This result may indicate that donor CD4⁺ T cells had a more rapid kinetics for their activation and subsequent AICD in response to anti-CD137 mAb.

A long-term observation demonstrated that control Ig-treated mice experienced severe loss of body weight and their mortality rate was high (70%), whereas anti-CD137-treated mice maintained normal body weight and stayed healthy until the termination of experiments (Fig. 2). FACS analysis for splenocytes showed that administration of anti-CD137 mAb prevented severe lymphodepletion in the spleen, a parameter for aGVHD (Table I). Anti-CD137-mediated inhibition of aGVHD was due to the deletion of donor CD4⁺ and CD8⁺ T cells and a subsequent failure of donor cell engraftment (Table I).

In two preclinical models of cGVHD, now it is clear that agonistic anti-CD137 mAb has the ability to delete pathogenic alloreactive CD4⁺ T cells and autoreactive B cells. However, there is evidence showing that agonistic anti-CD137 mAb can delete antigen-specific CD8⁺ T cells as well as CD4⁺ T cells in vivo(6,7). Earlier treatment with agonistic anti-CD137 mAb maintains elevated levels of TNF-α in lymphocytic choriomeningitis virus (LCMV)-infected mice, leading to Fas expression on activated CD8⁺T cells and this in turn results in Fas-mediated apoptosis (6). Even though Fas-mediated death signal is not sufficient to delete LCMV antigen-specific CD8⁺ T cells, STAT3 activation by signaling through CD137 in dendritic cells is required for their complete AICD (7). In this study, we showed that agonistic anti-CD137 mAb could completely delete not only donor CD4⁺ T cells but also CD8⁺ T cells in the C57BL/6→BDF1 aGVHD model. As seen in cGVHD (4,5) it is likely that engagement of CD137 provides strong costimulatory signaling leading to AICD for donor CD4⁺and CD8⁺ T cells that receive sustained allostimulation during the evolution of aGVHD. It remains to be elucidated whether other host hematopoietic and/or nonhematopoietic cells are needed for AICD of donor T cells by agonistic anti-CD137 mAb in GVHD.