Donor CD8⁺ T Cells Mediate GVL without GVHD in Recipients Conditioned with Anti-CD3 mAb.

Donor CD8⁺ T cells play a critical role in mediating graft versus leukemia (GVL), but also induce graft versus host disease (GVHD) in recipients conditioned with total body irradiation (TBI). Here, we report that injections of donor C57BL/6 (H-2^b) or FVB/N (H-2^q) CD8⁺ T with bone marrow cells induced chimerism and eliminated BCL1 leukemia/lymphoma cells without GVHD in anti-CD3-conditioned BALB/c (H-2^d) recipients. In contrast, the same dose of donor CD8⁺ T and marrow cells induced lethal GVHD in TBI-conditioned recipients. In addition, the anti-CD3-conditioned long-term complete chimeras without prior exposure to host-type BCL1 cells also eliminated the tumors when being challenged with BCL1 cells 120 days after HCT. This is in contrast to the report that long-term complete chimeras induced with delayed donor lymphocyte infusion lost GVL activity. Using in vivo and ex vivo bioluminescent imaging, we observed that donor CD8⁺ T cells expanded rapidly and infiltrated GVHD target tissues in TBI-conditioned recipients, but donor CD8⁺ T cell expansion in anti-CD3-conditioned recipients was confined to lympho-hematological tissues. This confinement was associated with lack of up-regulated expression of α4β7 integrin and chemokine receptors (i.e. CXCR3) on donor CD8⁺ T cells. In addition, host-reactive donor CD8⁺ T cells in anti-CD3-conditioned recipients were only partially deleted, and the residual cells were rendered heterogeneous: some unresponsive/anergic, some Tc2, some Foxp3⁺ suppressive cells, and some effector cells. The whole population of residual donor CD8⁺ T cells from anti-CD3-conditioned recipients mediated GVL without GVHD in TBI-conditioned secondary recipients. These results indicate that anti-CD3-conditioning separates GVL from GVHD via confining donor CD8⁺ T cell expansion to host lympho-hematological tissues as well as tolerization of the residual donor CD8⁺ T cells, in which the residual host-reactive effector cells mediate persistent GVL, and the regulatory CD8⁺ T cells prevent them from damaging host tissues.