T-cell reconstitution and mixed chimerism after HCT. (A) The donor graft contains HSCs and progenitors that engraft in the marrow (purple) and mature T cells that have been tolerized to the donor and are adoptively transferred (lavender). Over the first 1 to 3 months post-HCT, transferred mature T cells proliferate extrathymically, and donor HSCs seed the thymus. Beginning at 4 to 6 months post-HCT, newly generated donor-derived naïve T cells emerge from the thymus and populate the periphery. (B) In states of full donor chimerism (left panel), marrow HSCs, thymocytes, and peripheral T cells are all donor derived (purple). Rebound of recipient T cells (blue) may occur despite full donor chimerism in HSCs and the development of donor T cells from the thymus (center panel). Mixed chimerism in T cells may also arise if there is mixed chimerism in HSCs, resulting in mixed chimerism in thymocytes and in naïve T cells emerging from the thymus (right panel).