Enhanced Immune Reconstitution and Function by CD4+CD25+ Regulatory T Cells Following Allogeneic Hematopoietic Cell Transplantation.

Regulatory T cells (Treg), defined phenotypically by CD4+CD25+ expression, reduce the incidence and severity of acute graft-versus-host disease (GvHD) in murine models of major-MHC mismatched hematopoietic cell transplantation (HCT), presumably by dampening the proliferation and activation of mature effector T cells. It is unclear whether the effect of Treg on effector T cells is a selective or nonselective process or if Treg regulate the process of intrathymic and peripheral T cell maturation and selection following HCT, particularly given the intrinsic link of GvHD and immune reconstitution. The current study assessed the impact of Treg on the quantitative and functional lymphoid reconstitution in a murine model of major-MHC mismatched HCT. Treg (5x10⁵) from the spleen and lymph nodes of FVB/N (H2q) mice were co-transplanted into lethally irradiated Balb/c (H2d) host along with wild-type FVB/N T-cell depleted bone marrow (TCD-BM) cells (5x10⁶) and splenocytes (1.25x10⁶) (Tcon), the latter to induce GvHD. Chimerism studies were performed on day 14 and 40 post-transplantation to measure the level of donor immune reconstitution. At both time points, total lymphoid reconstitution was delayed in the GvHD control group and enhanced in the recipients transplanted with Treg (p-values=0.0005 on day 14, <0.0001 on day 40). T cell reconstitution, particularly CD4+ cells, was enhanced in the Treg group at both time points and reached statistical significance on day 40 (p-value=0.003). The number of donor natural killer cells was particularly enhanced on day 14 in the Treg recipients (p-value=0.0003). B-cell reconstitution, as measured by percentage of CD19+ cells, was minimal for all groups at day 14, but was enhanced at day 40 in recipient animals that received Treg (p-value=0.006). The T cell repertoire assessed by V-beta TCR screening with FACS analysis showed a polyclonal distribution. To determine if the improved and diverse lymphoid reconstitution is associated with increased immune function, mice were challenged with murine CMV (5x10e5 pfu/mouse) intraperitoneally at day 14 post-transplantation. Two weeks after infection, 66% of animals which received Treg in addition to Tcon, and 11% of animals which received Tcon alone were alive (p-value=0.05). Uninfected mice in the respective groups served as controls to separate the effect of CMV infection and GvHD on survival. Compared to infected animals, no deaths were observed in the respective uninfected groups at this time point (Tcon alone, p-value=0.0004; Treg+Tcon, p-value=0.21). In both infected and control uninfected animals, Treg treated animals had no evidence of significant clinical GvHD while animals that received Tcon alone had severe GvHD. These findings indicate that Treg enhance both the quantitative and functional recovery of the lymphoid cell populations while providing protection against GvHD.