A New TCR Transgenic Model of GVHD Reveals That, Independent of Repertoire, Effector Memory T Cells Are Severely Limited, and Central Memory T Cells Somewhat Limited, in Their Ability to Cause GVHD.

Abstract 233

Graft versus host disease (GVHD) remains a major cause of morbidity and mortality in allogeneic stem cell transplantation (alloSCT). In murine models of alloSCT, effector memory (EM) T cells engraft, respond to antigen, and mediate graft versus leukemia, but do not cause GVHD. There are three potential explanations for EM T cells' inability to cause GVHD. First, unlike naïve T cells, EM T cells fail to traffic to lymph nodes and Peyer patches, areas which may be important for initiation of GVHD. Second, a more-restricted T cell receptor repertoire in the EM T cell pool may lead to a reduced ability to recognize alloantigens. Third, it is possible that EM T cells do not possess the necessary effector mechanisms or are incapable of the proliferation or survival required for induction of GVHD. We recently reported that the inability to migrate to lymph nodes and Peyer patches is not responsible for the inability of EM T cells to cause GVHD (Anderson et al, Blood. 2008). To date, the role of repertoire has been difficult to test because in existing models of GVHD the disease causing T cells are undefined. Furthermore, whether central memory (CM) CD4 T cells are also incapable of causing GVHD remains unclear, in part because it has been difficult to isolate pure populations of polyclonal CM CD4 T cells. In order to concurrently address the role of repertoire and determine if CM CD4 T cells can cause GVHD, we developed a novel T cell receptor transgenic GVHD model. In this model naïve CD4+ TS1 T cells on a RAG-deficient background, which recognize an epitope of influenza hemagglutinin (HA), are transferred, along with syngeneic bone marrow, into irradiated transgenic recipients that express HA in all tissues (HA104 mice). Within a week post transplant, HA104 recipients of naïve TS1 cells developed a GVHD-like condition characterized by weight loss, visible wasting, and pathology of the skin, colon, and liver. An advantage of this model is that the disease causing T cells are defined, enabling us to determine if naïve and memory T cells of identical specificity have inherent differences in their ability to cause GVHD. We generated memory TS1 cells using in vitro stimulation followed by transfer into RAG−/− mice, according to Farber and colleagues (Ahmadzadeh et al. PNAS 2002). After 2 to 3 months, pure populations of CD62L+ CM TS1 cells and CD62L- EM TS1 cells were isolated by FACS. Upon transfer into irradiated HA104 recipients, EM TS1 cells initially did not cause disease symptoms, however, 30 days post transplant, EM TS1 recipients developed mild weight loss. These results indicate that repertoire differences are not responsible for the inability of EM T cells to cause GVHD. Interestingly, CM TS1 cells caused more weight loss than EM T cells, though not as much as that caused by naïve TS1 cells. These findings indicate that, independent of repertoire, CM T cells are also inherently limited in their ability to cause GVHD, though they are not as disabled in this respect as EM T cells. A major issue in GVHD work has been the inability to track, quantify and characterize the actual alloreactive GVHD-inducing T cells. With a TCR transgenic model, this is now possible and we are currently exploiting this feature to determine the fate of naïve, EM, and CM T cells after transfer. Initial experiments demonstrated that, in comparison to naïve and CM TS1 cell recipients, the secondary lymph nodes of EM TS1 cell recipients contained fewer TS1 cells 60 days post transplant, suggesting that in the context of GVHD, EM cells are inherently limited in their ability to expand or survive. We are currently tracking naïve, EM and CM TS1 cells throughout the course of a GVHD experiment, and assessing how, when, and where the fates of these cell types diverge. Results from these ongoing experiments will be presented.