Visualizing the Rapid and Dynamic Elimination of Allogeneic T Cells in Secondary Lymphoid Organs

Allogeneic organ transplants are rejected by the recipient immune system within several days or weeks. However, the rejection process of allogeneic T (allo-T) cells is poorly understood. In this study, using fluorescence-based monitoring and two-photon live imaging in mouse adoptive transfer system, we visualized the fate of allo-T cells in the in vivo environment and showed rapid elimination in secondary lymphoid organs (SLOs). Although i.v. transferred allo-T cells efficiently entered host SLOs, including lymph nodes and the spleen, ∼70% of the cells had disappeared within 24 h. At early time points, allo-T cells robustly migrated in the T cell area, whereas after 8 h, the numbers of arrested cells and cell fragments were dramatically elevated. Apoptotic breakdown of allo-T cells released a large amount of cell debris, which was efficiently phagocytosed and cleared by CD8⁺ dendritic cells. Rapid elimination of allo-T cells was also observed in nu/nu recipients. Depletion of NK cells abrogated allo-T cell reduction only in a specific combination of donor and recipient genetic backgrounds. In addition, F₁ hybrid transfer experiments showed that allo-T cell killing was independent of the missing-self signature typically recognized by NK cells. These suggest the presence of a unique and previously uncharacterized modality of allorecognition by the host immune system. Taken together, our findings reveal an extremely efficient and dynamic process of allogeneic lymphocyte elimination in SLOs, which could not be recapitulated in vitro and is distinct from the rejection of solid organ and bone marrow transplants.