Reduced Frequency of Regulatory T Cells in Peripheral Blood Stem Cell Compared to Bone Marrow Transplantations

Peripheral blood stem cell transplantation (PBSCT) is an alternative to bone marrow transplantation (BMT). Although CD4 + CD25 + CD127 lo regulatory T cells (Tregs) have been shown to play important roles in the control of T cell reactivity, the Treg contents of both graft types have not been analyzed comparatively to date. We report herein that Treg frequencies are significantly reduced in PBSC compared to BM transplants. Furthermore, most Tregs from PBSC transplants are CD62L lo , a phenotype reported to have poor suppressor activity. Both granulocyte-colony stimulating factor (G-CSF) administration and leukapheresis were found to contribute to the loss of CD62L + Tregs. Although higher T cell numbers are infused in PBSCT than in BMT, it is possible that the reduced Treg content of PBSC transplants may represent 1 factor contributing to the higher risk of GVHD reported after PBSCT.