Lymphopenia Promotes the Development of Graft-Versus-Host Disease upon Donor Leukocyte Infusion.

In murine models of hematopoeitic cell transplantation (HCT), a graft-versus-leukemia (GVL) effect can be effectively separated from graft-versus-host disease (GVHD) by giving delayed donor-leukocyte infusions (DLI) to established mixed hematopoietic chimeras (MC) (Mapara, Transplantation 2003). In these models, the absence of conditioning-induced inflammation at the time of DLI prevents trafficking to GVHD target tissues, limiting the GVH response to the lymphohematopoeitic system (Chakraverty, J Exp Med 2006). While clinical trials have shown that the same outcome is possible in humans, some patients develop severe GVHD following delayed DLI (Dey, Blood Biol Marrow Transplant 2003). In contrast to mice, who recover T cell numbers quickly after HCT through de novo thymopoiesis, humans remain lymphopenic for many months. The current study investigates the role of recipient lymphopenia in the development of GVHD following delayed DLI to MC. Allogeneic (B10.A) or syngeneic (B6) lymphocytes (30 × 10⁶ whole splenocytes) and bone marrow cells (10 × 10⁶) were administered to unconditioned RAG–1 knockout (KO) B6 recipients or to established MC, and animals were followed for development of GVHD. By day 7 after injection, the RAG KO recipients of allogeneic lymphocytes (allogeneic group) had lost significantly more weight (p<0.001) and had significantly higher clinical GVHD scores (p<0.0001) than RAG KO recipients of syngeneic lymphocytes (syngeneic group) or MC recipients of allogeneic lymphocytes (MC group). Histologic evaluation on day 7 showed significantly increased GVHD in liver and colon of the allogeneic group. Chimerism analysis on day 7 showed the presence of an ongoing lymphohematopoietic GVH response (LGVHR) in both the allogeneic and MC groups, though the response was slower in the MC group. When followed over time, clinical GVHD in the allogeneic group resolved by day 56, weight differences disappeared by day 65, but mild histologic GVHD persisted as late as day 239. Importantly, the same dose of allogeneic T cells given to freshly conditioned animals results in rapid, lethal GVHD, whereas these unconditioned, lymphopenic animals developed only a relatively mild, largely self-limited GVHD. These results demonstrate that in the absence of conditioning-induced inflammation, lymphopenic recipients of allogeneic T cells do develop mild GVHD, consistent with the possibility that recipient lymphopenia contributes to the GVHD seen in human MC recipients of DLI. The mechanisms by which lymphopenia promotes GVHD in non-conditioned MC recipients of DLI require further investigation, but may include the absence of regulatory T cells, the potential for lymphopenia-induced proliferation of T cells, and the presence of infection-induced inflammation.