Purpose: Allogeneic nonmyeloablative hematopoietic stem cell transplant (NM-HSCT) can result in durable remission of chronic lymphocytic leukemia (CLL). It is thought that the efficacy of NM-HSCT is mediated by recognition of tumor cells by T cells in the donor stem cell graft. We evaluated the development of CTLs specific for CLL after NM-HSCT to determine if their presence correlated with antitumor efficacy.

Experimental Design: Peripheral blood mononuclear cells obtained from 12 transplant recipients at intervals after NM-HSCT were stimulated in vitro with CLL cells. Polyclonal T-cell lines and CD8⁺ T-cell clones were derived from these cultures and evaluated for lysis of donor and recipient target cells including CLL. The presence and specificity of responses was correlated with clinical outcomes.

Results: Eight of the 12 patients achieved remission or a major antitumor response and all 8 developed CD8⁺ and CD4⁺ T cells specific for antigens expressed by CLL. A clonal analysis of the CD8⁺ T-cell response identified T cells specific for multiple minor histocompatibility (H) antigens expressed on CLL in six of the responding patients. A significant fraction of the CD8⁺ T-cell response in some patients was also directed against nonshared tumor-specific antigens. By contrast, CLL-reactive T cells were not detected in the four patients who had persistent CLL after NM-HSCT, despite the development of graft-versus-host disease.

Conclusions: The development of a diverse T-cell response specific for minor H and tumor-associated antigens expressed by CLL predicts an effective graft-versus-leukemia response after NM-HSCT.