CD4+CD25+ Regulatory T Cells Enhance Immune Reconstitution Following Allogeneic Hematopoietic Cell Transplantation by Protecting Thymic and Lymphoid Compartments from Graft-Versus-Host Disease Damage without Impacting T Cell Repertoire Development.

Regulatory T cells (Treg) protect from acute graft-versus-host disease (GvHD) in murine models of major-MHC mismatched hematopoietic cell transplantation (HCT) presumably by dampening the proliferation 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. We previously showed that Treg improved the quantitative and functional lymphoid reconstitution in a murine model of HCT. In the current study, we hypothesize that Treg prevent thymic and lymphoid damage from GvHD, leading to enhanced lymphoid reconstitution. Lethally-irradiated adult thymectomized Balb/c (H2^d) recipients were transplanted with wild-type FVB (H2^q) T cell depleted bone marrow (TCD-BM) cells and CD4+/CD8+ cells (Tcon), the latter to induce GvHD, with or without donor Treg given at a 1:1 dose ratio with Tcon. At day 30, when all groups had reached full donor chimerism, transplant recipients were challenged with murine CMV (5×10⁵ pfu/mouse) intraperitoneally. At day 90, survival for thymectomized groups with TCD-BM alone, with Tcon, or with Tcon+Treg was 78%, 0%, and 45%, respectively, compared to 100%, 0%, and 86% survival in their respective euthymic infected counterparts (p<0.05 for thymectomized vs euthymic Treg groups). Elispot for Interferon-γ showed CMV-specific donor responses in all infected groups. CMV viral titers in the liver and kidney 2 weeks after infection was lower in recipients that received Treg compared to animals that received Tcon alone. Compared to euthymic transplant controls, thymectomized animals had higher viral titers in the liver, lungs, and kidneys in all groups. Uninfected thymectomized mice in the respective groups served as controls to separate the effect of CMV infection and GvHD on survival. All animals, infected or uninfected, that received Treg had no evidence of clinical GvHD while animals that received Tcon alone had significant GvHD. In euthymic recipients, gross and histologic examination confirmed the general preservation of thymic integrity and architecture in animals that received Treg compared with smaller involuted thymuses partially replaced by adipose tissue in animals that received Tcon alone. T cell repertoire assessed by V-beta TCR screening with FACS analysis showed a polyclonal distribution in animals with or without Treg. Spectratyping at day 30 post-transplantation showed that Treg had no significant impact on the TCR repertoire diversity in animals which received Tcon. Based on survival of a subset of infected thymectomized animals that received Treg, we evaluated the impact of Treg on secondary lymphoid organs following HCT. Animals without transferred Treg had significant splenic and lymph node fibrosis and hypoplasia with a reduction in T cell numbers due to GvHD. Our findings indicate that Treg indirectly enhance immune reconstitution by protecting the thymic and secondary lymphoid compartments from GvHD damage, allowing the generation and peripheral expansion of lymphoid cells without impacting the diversity of T cell repertoire.