We sought to develop a better understanding of the T cells involved in the human allogeneic immune response, in order to eventually engineer a donor graft with reduced GVHD-mediating potential, without ablating general immune competence. Prior studies reported that all the activated CD4+ T cells responding to a specific antigen challenge reside within the CD4high population expressing high levels of membrane CD4. We identified a new population of activated CD8+ T cells that developed during an in vitro allogeneic immune response, along with the allo-activated CD4high T cell population. Analogous to activated CD4+ T cells, this new T cell population was distinguished by up-regulated CD8 (and CD38) expression (CD8highCD38+). In accordance with Martins et al. (Blood 2004, 104:3429), we found that the depletion of the CD4highCD38+ population resulted in reduced 2o response to the original 2nd party stimulators. In contrast, depletion of the CD8highCD38+ population resulted in an increased 2o response to 2nd party cells, with no change in the response to 3rd party or CMV antigens. Elevated numbers of CD8highCD38+ T cells potently reduced the 1o and 2o responses to 2nd party, but not to 3rd party cells or CMV antigens. The complementary, non-activated CD8normalCD38 T cell population had no inhibitory effect. Importantly, we found that

  • CD8highCD38+ T cells mediated both a specific cytotoxic response (that could be inhibited by the pan-caspase inhibitor, Z-VAD), and a specific suppressive response toward the original 2nd party stimulators (that was not affected by Z-VAD), and

  • within this CD8highCD38+ population, there was a subpopulation of cytotoxic T cells (perforin+LAMP1+CD56+CD11b+CD11c+) and a subpopulation of non-cytotoxic T cells.

Furthermore, we found that although CD8highCD38+ T cells differentially expressed CD28, both CD8highCD38+CD28 and CD8highCD38+CD28 T cells mediated a cytotoxic as well as a suppressor T cell response toward the original 2nd party cells (different from the published suppressive function of CD8+CD28 T cells observed by Liu et al, Int Immunol 1998, 10:775). Upon separation of cytotoxic CD8highCD38+ T cells from suppressor CD8highCD38+ T cells, we will explore the GVHD potential of these 2 novel activated CD8high T cell subpopulations, in a sensitive in vivo xenograft model for GVHD using NOD/SCID/IL2Rγnull immunodeficient mice.

Disclosure:Financial Information: The Johns Hopkins University holds patents on CD34 monoclonal antibodies and inventions related to stem cells. Dr. Civin is entitled to a share of the sales royalty received by the University under licensing agreements between the University, Becton Dickinson Corporation and Baxter HealthCare Corporation. The terms of this arrangement are being managed by the Johns Hopkins University in accordance with its conflict of interest policies.