CCR1 Expression on Donor Leukocytes Is Critical to the Development of Graft Versus Host Disease after Allogeneic SCT.

Acute graft versus host disease (aGVHD) is the major complication of allogeneic stem cell transplantation (allo-SCT) and limits the utility of this treatment strategy. The pathophysiology of aGVHD involves injury to host tissues by inflammatory cytokines and donor-derived cellular effectors, the recruitment of which is likely mediated by chemokine receptor: ligand interactions. CCR1 is expressed on various cell types such as T cells, monocytes and macrophages and binds to a group of CC chemokines that includes RANTES (CCL5) and Mip-1a (CCL3). In light of the previously described role for both donor T cells and monocytes in the development of aGVHD, we tested the hypothesis that CCR1 expression on donor leukocytes contributes to systemic and target organ inflammation that occurs in this setting by using a well established murine SCT model (B6→B6D2F1) and mutant mice deficient in CCR1. Lethally (1100cGy) irradiated B6D2F1 mice received SCT either from syngeneic (B6D2F1) or allogeneic (B6) CCR1+/+ or CCR1−/− donors. The severity of GVHD was assessed after SCT by survival and a clinical scoring system that incorporates changes in weight loss, fur texture, skin integrity, mobility and posture. As expected, syngeneic SCT recipients all survived and where indistinguishable from naïve, untransplanted controls, whereas animals receiving allo-SCT from CCR1+/+ donors developed significant GVHD and only 50% of animals survived by day 35. By contrast, allo-SCT with CCR1−/− donor cells resulted in significantly improved survival (92% vs. 50%) and less severe clinical GVHD (1.0±0.3 vs. 3.0±0.5) compared to allo-CCR1+/+ controls. In addition, serum TNFa levels were significantly decreased by day +7 following CCR1−/− SCT (4.7±2.7 vs. 55.3±14.4 pg/ml) and correlated with a significant reduction in intestinal histopathology both on day 7 (16.3±1.0 vs. 21.5±0.9) and on day 14 (15.8±1.8 vs. 23.8±1.0). GVHD injury to liver and skin was mild at these time points and did not differ between allo groups. Next we investigated the impact of CCR1 expression on allo-specific T cell responses in vivo and found that by day +7 after SCT both splenic T cell expansion (3.7±0.4 vs. 9.6±0.9 x 10⁶ cells) and serum IFNγ levels (4561±559 vs. 10028±681 pg/ml) were significantly lower when CCR1 was absent on donor cells. In summary, we describe a heretofore unknown role for CCR1 on donor leukocytes in the development of aGVHD. The improvement in systemic and target organ disease observed after CCR1−/− SCT may be attributed to 1) alterations in leukocyte recruitment to the intestinal tract resulting in improved intestinal integrity, reduced translocation of endotoxin and decreased TNFa production and 2) modulation of donor T cell responses to host allo-antigens. Experiments are ongoing to determine whether strategies targeting CCR1 signalling can be exploited to prevent GVHD but maintain GVL effects and thereby improve overall survival after allo-SCT.