Early Treatment with CD4+CD25+ Regulatory T Cells Provides Prolonged Suppressive Effects Which Control Evolving but Not Established Graft-Versus-Host-Disease.

CD4+CD25+ Regulatory T cells (Treg) reduce the incidence and severity of acute graft-versus-host disease (GvHD) in murine models of allogeneic bone marrow transplantation (BMT) when given at the time of transplantation at a 1:1 ratio with donor effector T cells (Tcon). In our previous study with Treg trafficking, bioluminescene imaging (BLI) indicated a persistence of signal consistent with a prolonged survival of Treg in vivo following allogeneic BMT. In the current study, we evaluated the duration of Treg suppression and the impact of Treg on evolving and established GvHD. Lethally irradiated Balb/c (H2d) hosts received 5x10⁶ T-cell depleted bone marrow (TCD-BM) cells from wild-type FVB mice (H2q) on day 0. On day 2, 3x10⁶ splenocytes, or 1x10⁶ purified CD4+/CD8+ T cells (Tcon) from luciferase+ transgenic mice (FVB) were infused to induce GvHD. Treg, purified from wt-FVB mice, in a 1:1 dose ratio with Tcon, were infused either on day 0, 2, 9, or 23 post-transplantation. Bioluminescence imaging was used to localize and quantitate the proliferation of Tcon in the absence or presence of Treg. Signal intensity, measured by photons/second/mouse, was significantly decreased in animals which received Treg at day 0, 2, or 9 (p-value < 0.05). More importantly, the greatest reduction in signal intensity occurred when Treg were given prior to the induction of GvHD by Tcon. This reduction was associated with a significantly lower clinical score for GvHD. Studies in which Treg are given up to 10 days prior to the addition of Tcon show similar findings. At day 23, when clinical GvHD was fully established in mice which received Tcon, the addition of Treg did not alter the increasing BLI signal level or the clinical course such that all animals died of GvHD (p-value=0.38). Lymphoid reconstitution was not affected by the addition of Treg prior to the induction of GvHD. In dose titration studies whereby Treg are given two days prior to the induction of GvHD, a 10-fold dose reduction in Treg was sufficient to significantly reduce Tcon proliferation and suppress clinical GvHD. We next assessed the duration of Treg suppressive effect by inducing GvHD on day 7, 14, 19 with luc+ Tcon following the infusion of Treg on day 0 of allogeneic BMT. Treg provided protection from the Tcon challenge at all 3 time points, leading to improved survival (p-value < 0.05). We conclude that Treg provide prolonged protection due to their ability to proliferate in vivo in an allogeneic setting. The capacity of Treg to proliferate in vivo permits a significant reduction in the number of Treg needed for adoptive transfer to induce a clinical response, a practical finding given the rarity of Treg. In addition, we conclude that Treg suppress the early proliferation of Tcon, allowing them to prevent and control evolving but not established GvHD.