CCR1 Is Required for Treg-Mediated Protection From Lethal Acute Graft-Versus-Host Disease.

Graft-versus-host disease (GVHD) is a major complication following allogeneic bone marrow transplantation (BMT). Despite advances in understanding the etiology of GVHD it remains a formidable obstacle to the widespread application of BMT. A number of studies have demonstrated that T regulatory (T_reg) cells represent a potential therapy for GVHD as T_regs have been shown to inhibit GVHD while preserving the beneficial graft-versus-leukemia (GVL) effect. Numerous groups, including our own, have demonstrated the importance of T cell migration in the pathology of GVHD. Following conditioning, donor T cells migrate to secondary lymphoid tissues. Once activated in the lymphatics, T cells migrate to GVHD target organs including; the skin, liver, lung and the gastrointestinal (GI) tract in response to the local production of chemokines. Disruption of chemokine-chemokine receptor interactions has been demonstrated to affect the pathology of GVHD. Previously, we have shown that T_regs lacking the chemokine receptor CCR5, which binds CCL3, CCL4, and CCL5, do not protect animals from lethal GVHD as well as WT T_regs, due to their impaired migration to the liver and lung. Thus, a greater understanding of the function of chemokine receptors on T_regs is important in deciphering how T_regs function and whether targeting these cells to lymphoid tissue or GVHD target organs would be preferable for treating patients in clinical trials.

We utilized a parent into F1 haploidentical model to assess the role of CCR1 in T_reg-mediated protection from GVHD. Here we demonstrate T_regs lacking CCR1, another receptor for CCL3 and CCL5, were unable to protect animals against lethal acute GVHD. While 67% of B6D2 recipients given 1×10⁶ WT Tregs supplemented with 5×10⁶ WT T cells and 3×10⁶ B6 T cell-depleted BM cells survived, only 15% of the recipients given CCR1^−/− Tregs survived (p < 0.03; Fisher's exact test). B6D2 recipient mice given WT T_regs had significantly reduced clinical scores for GVHD compared to B6D2 recipients of CCR1^−/− Tregs (p <0.05) with elevated GVHD scores starting on day 28 post-transplant. Histopathology revealed significantly worse pathology in the liver (p < 0.03) and colon (p < 0.05) of CCR1^−/− T_reg recipients vs. WT T_reg recipients. In vitro analysis demonstrated that CCR1^−/− T_regs were capable of suppressing T cell responses to allo-antigen equally as well as WT T_regs, and CCR1^−/− T_regs attained a normal activation phenotype. Interestingly, preliminary experiments suggested that CCR1^−/− T_regs migrated to and/or expanded in GVHD target organs to a similar extent as WT T_regs.

Treg expression of CCR1 is required for the inhibition of GVHD. T_regs lacking CCR1 led to significantly more tissue destruction in the liver and colon, two predominant sites of GVHD pathology. Of interest, the migration of CCR1^−/− T_regs to GVHD target organs and secondary lymphoid tissues did not appear to be compromised suggesting that CCR1 may be required for the function of T_regsin vivo.

No relevant conflicts of interest to declare.