Host Immune Status Determines the Effects of Therapeutic Interleukin-2 Administration: Enhancement of GVL or Induction of Tolerance?

Interleukin-2 (IL-2) has a central role in immune tolerance thorough maintaining the homeostasis of CD4⁺CD25⁺Foxp3⁺ regulatory T cell (Treg). We recently reported that administration of low-dose IL-2 could preferentially enhance Treg in vivo and suppress clinical manifestations of chronic GVHD after allogeneic hematopoietic stem cell transplantation (NEJM2011). On the other hand, IL-2 is also necessary for the development of cytotoxic T cell function and has been used for the systemic immune therapy to amplify anti-tumor immunity. Thus, IL-2 administration after transplantation can induce the bipolar effects, namely, enhancement of Graft-versus-Leukemia effect (GVL effect) or prevention of Graft-versus-Host Disease (GVHD). For the appropriate therapeutic use of IL-2 for different purpose, we need to elucidate the factors to predict the effect of IL-2, however, the determinants except for the IL-2 dosage has not been characterized. To address this issue, we explored the impact of the immunological reconstitution after transplant on the effect of IL-2 therapy. First, we examined the in vivo reactivity to exogenous IL-2 in different lymphocyte subsets; CD8+ T cells, CD4+ conventional T cells (Tcon) and CD4+ regulatory T cells (Treg). We purified CD62L+ naïve lymphocytes and CD62L- lymphocytes from B6 spleen by cell-sorting and labeled with CFSE, adaptively transferred cells into irradiated B6 host and subcutaneously administrated 5000 IU of IL-2 or control vehicle from day1 to 5. At day6, spleen cells were harvested and the in vivo proliferation of each lymphocyte were evaluated by CFSE dilution assay. Proliferation of Tcons was just limited but both CD8 T cells and Tregs showed vigorous proliferations in response to IL-2. As expected, IL-2 induced the proliferation of CD62L- activated/memory CD8 T cells more than CD62L+ naïve CD8 T cells. In contrast, oppositely to CD8+ subsets, IL-2 induced the proliferation of CD62L+ naïve Tregs more than CD62L- Tregs, suggesting naïve Treg might be “primed” naturally and be ready to respond to IL-2 without antigenic TCR stimulation. Next, we examined the balance between naïve and memory phenotype in each lymphocyte subsets after allogeneic HSCT using murine BMT model. Lethally irradiated B6D2F1 mice were transplanted with 5x10⁶ spleen cells from the CD45.2 B6 mice together with 5x10⁶ TCD-BM from CD45.1 B6 donors. The balance between graft-derived cells (CD45.2) and BM-derived cells (CD45.1) in CD8+ T cell, CD4+ Tcon and Treg were monitored separately at day7, 14, 21,28 and 35. In the early phase of transplant, graft-derived cells showing CD62- were predominant, peaked at day21 and thereafter decreased in each subset. After day28, BM-derived cells generally emerged in each subset. Using this model, we compared the effects of IL-2 administration in the early phase (Day5-12) and in the late phase (Day35-42). Interestingly, IL-2 administration of daily 5000 IU of IL-2 in the early phase resulted in the dominant expansion of circulating CD62-CD44+ effector/memory CD8 T cell without Treg increase (CD8+T; 162 cells vs 75 cells/uL, P<0.05: Treg; 0.42 cells vs 0.48 cells/uL, NS). In contrast, the same dose of IL-2 administration in the late phase resulted in the vigorous increase of CD62+ Treg without CD8 T cell increase (CD8+T; 205 cells vs 185 cells/uL, NS: Treg; 57.1 cells vs 21.0 cells/uL, P<0.05). GVL experiments using 2.5x104 host-type P815 leukemia cells, that were uniformly lethal to the recipients of syngeneic BMT by day 15 after BMT, showed that IL-2 injection from day5 delayed the leukemia relapse (P<0.05), indicating IL-2 enhanced CD8+ CTL might mediated anti-tumor immunity. In conclusion, these data suggests that not only the dose of IL-2 but also host immune status at IL-2 administration is a critical factor to determine the in vivo effect of IL-2 therapy. From the point of view, the adjusted dose and timing of IL-2 infusion based on the detailed immune monitoring might enable to optimize and personalize this cytokine therapy. Our findings provide important information for developing therapeutic strategies to modulate immune cells in vivo and promote well-balanced immune recovery after transplantation.