Host Environment Dictates the Outcome Following Transfer of Graft-Versus-Host Reactive Effector/Memory T Cells.

Donor leucocyte infusions (DLI) given to established mixed chimeras (MC) can eliminate normal and malignant hematopoietic cells without causing graft-versus-host disease (GVHD). Identical DLI transferred immediately following lethal irradiation lead to severe GVHD. In contrast to freshly irradiated mice developing GVHD, established MC receiving DLI show delayed kinetics of effector/memory T cell (T_E/M) expansion and a distinct distribution with no accumulation of cells in the gut. Lethal irradiation is associated with tissue injury, altered chemokine expression and cytokine dysregulation, and resolution of these changes is important in limiting GVHD following delayed DLI. In an attempt to define the respective roles of ‘intrinsic’ T cell versus ‘extrinsic’ environmental factors in shaping the character of the GVH response elicited, T_E/M developing following GVHD induction were transferred to established MC. Conversely, T_E/M developing following delayed DLI to MC were transferred to lethally-irradiated 2° recipients. In the first series of experiments, we transferred 1 x 10⁷ C57BL/6 (B6) splenocytes and 5 x 10⁶ B6 T-cell depleted bone marrow (TCD BM) to lethally-irradiated BALB/c recipients. On day 4 following BMT, when GVH-reactive T cell activation and expansion was maximal, 3 x 10⁶ nylon wool-passaged T_E/M derived from recipient spleens (equivalent to 1 recipient spleen to 1 secondary recipient) were transferred to established B6 → BALB/c MC or to lethally irradiated BALB/c 2° recipients. The latter mice received B6 TCD BM to prevent death from aplasia. Secondary established MC recipients were resistant to GVHD whereas the lethally-irradiated 2° recipients developed severe, lethal GVHD (median survival time-MST- mixed chimeras >100d versus irradiated mice 52d, p<0.0001). Histological studies confirmed that 4/5 lethally-irradiated 2° recipients developed colitis versus 0/4 MC. T_E/M derived from GVHD mice did not increase donor chimerism in 2° MC recipients. In reciprocal experiments, we transferred 2.5 x 10⁷ B6 CD45.1 splenocytes to established B6 CD45.2 → B6D2F1 MC. This number of splenocytes represents the dose required to convert 100% of MC to full donor chimerism without induction of lethal GVHD. On day 12 post-DLI, when GVH-reactive T-cell expansion was maximal, a cohort of recipient mice was sacrificed and splenocytes harvested. On the basis of 1 recipient spleen to 1 secondary recipient, 2.7 x 10⁷ nylon wool-passaged lymphocytes were transferred to 2° freshly-irradiated B6D2F1 recipients together with B6 TCD BM. In parallel, we depleted host-derived cells by sorting for DLI-derived CD45.1+ lymphocytes and transferred these cells on the basis of 1 recipient to 1 2° recipient (7 x 10⁶ lymphocytes) to a second cohort of lethally-irradiated recipients. After a lag-period of about 20 days, 2° recipients of non-fractionated cells lost >10% weight and developed clinical signs of mild GVHD, but then showed almost complete recovery by day 60 post-BMT. In sharp contrast, after a similar lag period, 2° recipients of CD45.1+-selected lymphocytes developed severe lethal GVHD (MST CD45.1+ cells 55d vs >100d non-fractionated cells, p<0.0005). In conclusion, these findings suggest that the host environment strongly influences the capacity of T_E/M to induce GVHD. Furthermore, host-derived cells pre-existing in or developing following delayed DLI, reduce the potential of T_E/M to induce GVHD.