CD4+FoxP3+ regulatory T cells (Tregs) play a central role in the maintenance of immune tolerance after allogeneic hematopoietic stem cell transplantation. We recently reported that daily administration of low-dose IL-2 induces selective expansion of functional Treg and clinical improvement of chronic graft-versus-host disease (GVHD). To define the mechanisms of action of IL-2 therapy we examined the immunologic effects of this treatment on homeostasis of CD4 T cell subsets after transplant. We first demonstrate that chronic GVHD is characterized by constitutive phosphorylation of Stat5 in conventional CD4 T cells (Tcon) associated with elevated levels of IL-7 and IL-15 and relative functional deficiency of IL-2. IL-2 therapy resulted in the selective increase of Stat5 phosphorylation in Treg and decrease of pStat5 in Tcon. Over an eight-week period, IL-2 therapy induced a series of changes in Treg homeostasis, including increased proliferation, increased thymic export and enhanced resistance to apoptosis. Low-dose IL-2 had minimal effects on Tcon. These findings define the mechanisms whereby low-dose IL-2 therapy restores the homeostasis of CD4 T cell subsets and promotes the re-establishment of immune tolerance.

CD40L has a well-established role in enhancing the immunostimulatory capacity of normal and malignant B cells, but a formulation suitable for clinical use has not been widely available. Like other TNF family members, in vivo and in vitro activity of CD40L requires a homotrimeric configuration, and growing evidence suggests that bioactivity depends on higher-order clustering of CD40. We generated a novel formulation of human recombinant CD40L (CD40L-Tri) in which the CD40L extracellular domain and a trimerization motif are connected by a long flexible peptide linker. We demonstrate that CD40L-Tri significantly expands normal CD19+ B cells by over 20- to 30-fold over 14 days and induces B cells to become highly immunostimulatory antigen-presenting cells (APCs). Consistent with these results, CD40L-Tri-activated B cells could effectively stimulate antigen-specific T responses (against the influenza M1 peptide) from normal volunteers. In addition, CD40L-Tri could induce malignant B cells to become effective APCs, such that tumor-directed immune responses could be probed. Together, our studies demonstrate the potent immune-stimulatory effects of CD40L-Tri on B cells that enable their expansion of antigen-specific human T cells. The potent bioactivity of CD40L-Tri is related to its ability to self-multimerize, which may be facilitated by its long peptide linker.Electronic supplementary materialThe online version of this article (doi:10.1007/s00262-012-1331-4) contains supplementary material, which is available to authorized users.

FTY720 is a novel immunosuppressant that improves the outcomes after solid organ and bone marrow transplantation (BMT) due to the sequestration of T cells into LN. We tested the hypothesis that the sequestration of donor T cells in LN by FTY720 would enhance their interaction with host APC, thus causing a greater degree of activationinduced apoptosis of alloreactive T cells, and thereby resulting in a reduction of graftvs.-host disease (GVHD). The short-term administration of FTY720 improved the recipient survival after allogeneic BMT. FTY720 treatment facilitated a rapid contraction of the donor T cell pool in association with an increased degree of apoptosis of donor T cells. The donor T cell reactivity to host alloantigens was diminished in host's LN and adoptive transfer of donor T cells isolated from LN of FTY720-treated recipients of allogeneic BMT induced less severe GVHD in secondary recipients than the transfer from controls. Caspase-dependent apoptosis was involved in this mechanism because FTY720-induced protection was abrogated when a pan-caspase inhibitor was administered. These findings thus demonstrate the presence of a novel mechanism by which FTY720 modulates the allogeneic T cell responses: namely, by the induction of activation-induced apoptosis of alloreactive T cells in LN.

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