Human Thymus Exports Naive CD8 T Cells That Can Home to Nonlymphoid Tissues

Functionally naive CD8 T cells in peripheral blood from adult humans can be fully described by their CD45RA bright CCR7 + CD62L + cell surface phenotype. Cord blood lymphocytes, from healthy newborns, are homogenously functionally naive. Accordingly, the majority of cord blood CD8 T cells express the same pattern of cell surface molecules. Unexpectedly, however, a significant fraction of cord blood CD8 T cells express neither CCR7 nor CD62L. Yet these cells remain functionally naive as they contain high levels of TCR excision circles, have long telomeres, display highly polyclonal TCRs, and do not exhibit immediate effector functions. In addition, these CD8 T cells already represent a significant fraction of the mature naive CD8 single-positive thymocyte repertoire and may selectively express the cutaneous lymphocyte Ag. We suggest that CD8 single-positive thymocytes comprise two pools of naive precursors that exhibit distinct homing properties. Once seeded in the periphery, naive CCR7 + CD62L + CD8 T cells patrol secondary lymphoid organs, whereas naive CCR7 − CD62L − CD8 T cells selectively migrate to peripheral tissues such as skin.