The Absence of CCR7 on Donor T Cells Leads to Attenuated Graft-Versus- Host Disease (GVHD) with a Preserved Graft-Versus-Leukemia (GVL) Effect

The development of GVHD depends on the trafficking of donor effector T cells (T_effs) into recipient secondary lymphoid tissue early after hematopoietic stem cell transplantation (HSCT). Chemokine receptor 7 (CCR7) has been shown to be critical for the movement of naïve T cells into lymph nodes and for the organization of T cell and B cell regions in the spleen. Using a murine transplant model, we set out to study the contribution of CCR7 to both GVHD induction and the immunomodulatory properties of regulatory T cells (T_regs) in the HSCT setting.

Methods: C57BL/6 (H-2^b; termed B6) mice served as bone marrow (BM) donors, and B6xDBA/2 F1 (H-2^bxd; termed B6D2) mice functioned as recipients. T_effs and T_regs were obtained from wild-type (WT) B6 or CCR7 knockout (CCR7^−/−) mice extensively backcrossed on a B6 background. For T_eff studies, recipient animals were lethally irradiated to 950 rads on day –1 and administered 3 ×10⁶ T-cell depleted (TCD) BM cells +/− 4×10⁶ splenic T_effs from WT or CCR7^−/− donors on day 0. For GVL studies, 25,000 P815 murine mastocytoma cells were coadministered with the BM/T_eff inoculum on day 0. For T_reg studies, mice received TCD BM +/− 1×10⁶ WT or CCR7−/− T_regs on transplant day 0, with 4×10⁶ WT T_effs dosed on transplant day +2.

Results: WT and CCR7^−/− T_effs generated GVHD responses that were nearly identical during the first 7–10 post transplant. Thereafter, those mice receiving CCR7^−/− T_effs demonstrated a significant attenuation of their GVHD, with 83% surviving long term. In contrast, those animals receiving WT T_effs exhibited relentlessly progressive disease and a 92% mortality rate by day +70 (see figure; P=0.0006 for comparison between WT and CCR7^−/− T_eff groups by the log-rank test). In spite of their relatively impaired capacity to generate lethal GVHD, CCR7^−/− T_effs were found to produce complete donor reconstitution of the CD4⁺ and CD8⁺ T cell compartments by transplant day +40, albeit at a slower rate than that observed with WT T_effs. CCR7^−/− T_effs also demonstrated significant anti-tumor activity in-vivo. B6D2 mice challenged with P815 tumor cells and TCD BM all died with massive splenomegaly and diffuse tumor infiltration of the liver by transplant day +20. Animals receiving WT T_effs at the time of P815 challenge all died of GVHD, with no tumor noted at autopsy. In contrast, 75% of the mice administered P815 cells and CCR7^−/− T_effs survived long term with no signs of malignancy and only mimimal evidence of GVHD clinically. These findings did not appear to be the result of an impaired ability of CCR7^−/− T_effs to respond to alloantigens in-vitro, as proliferative responses of sort-purified CD25^- CD4⁺ and CD8⁺ T cells from WT and CCR7^−/− mice were similar in one-way mixed lymphocyte reactions. Surprisingly, CCR7^−/− T_regs appeared to be capable of protecting against lethal GVHD when administered two days prior to WT T_effs, with 100% of recipients surviving to day +70. Their GVHD scores, however, were somewhat higher than in those mice receiving WT T_regs.

Conclusions: CCR7 is not required for the initial phase of GVHD induction, but appears important for the perpetuation and augmentation of disease 10–14 days post transplantation. The attenuated in-vivo allo-immune responses observed with CCR7^−/− T_effs appear to be independent of any intrinsic deficiency in the proliferative capacity of the T cells themselves. CCR7^−/− T_effs are capable of generating a potent GVL effect in our P815 tumor model. CCR7 does not appear to be required for T_regs to protect against lethal GVHD when they are administered in advance of WT T_effs. At the time of this writing we have successfully generated CCR7^−/− eGFP⁺ mice, and in-vivo trafficking studies are currently planned to elucidate a mechanism for the above findings.

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