Cytotoxicity Is Not Required for the Anti-Leukemia Efficacy of Human H-Y-Specific CD4+ T Lymphocytes In Vivo.

Minor histocompatibility antigens (mHag) play a major role in the graft-versus-leukemia (GvL) effect following HLA-matched allogeneic hemopoietic cell transplantation (allo-HCT). Clinically, the GvL effect coincides with the emergence of mHag-specific CD8+ cytotoxic T lymphocytes (CTL). Experimentally, targeting a single mHag with human CD8+ CTL has a major anti-leukemia effect in NOD/scid mice. Altogether, these observations suggest that mHag-specific cytotoxicity by CD8+ T cells is an important component of the GvL effect. In contrast, little is known on the contribution of mHag-specific CD4+ T cells. Female-to-male allo-HCT is characterized by a low rate of leukemia relapse, indicating that H-Y-encoded mHag are potent leukemia-regression antigens. Earlier, we described a DRB3*0301-restricted H-Y mHag epitope inducing CD4+ helper T-cell responses in H-Y-mismatched HLA-matched allo-HCT. Aim: The aim of this study is to elucidate the role of mHag-specific human CD4+ T lymphocytes on the GvL effect.

The ALL-CM leukemia cell line, derived from a male (i.e. H-Y+) HLA-A0201+, DRB30301+ patient, reproducibly engrafts in NOD/scid mice after administration of 10×106 cells. Both an HLA-A0201-restricted H-Y-specific CD8+ CTL clone and the DRB30301-restricted H-Y-specific CD4+ helper T-cell clone that we earlier described were used to investigate the anti-leukemia efficacy of CD8+ and CD4+ T cells in NOD/scid mice.

In vitro, the CD8+ H-Y specific CTL clone was highly cytotoxic against the ALL-CM leukemia. The H-Y specific CD4+ helper T-cell clone did not lyse the leukemia, but produced IFN-γ upon recognition. Infusion of the H-Y-specific CD8+ CTL clone (25×106 cells/mouse) 3 days after ALL-CM leukemia challenge significantly delayed leukemia progression by 3 weeks compared to a CMV-specific CD8+ CTL control clone (p<0,001). Despite no measurable in vitro cytotoxicity, the H-Y-specific CD4+ helper T-cell clone (25×106 cells/mouse) delayed leukemia progression by 2 weeks compared to a leukemia non-reactive HLA-DR1-specific CD4+ helper T-cell control clone (p<0,001). In vitro co-incubation of the H-Y-specific CD4+ helper T-cell clone did not influence leukemia proliferation but induced up-regulation of MHC-class I and II, CD80, CD86 and CD40. In vitro, pre-incubation of leukemia cells with the H-Y-specific CD4+ helper T-cell clone irradiated did not improve the in vivo anti-leukemia efficacy of the H-Y-specific CD8+ CTL clone. Co-infusion of the H-Y specific CD4+ helper T-cell clone did not augment the in vivo persistence of the H-Y-specific CD8+ CTL T-cell clone. Nevertheless, the co-infusion resulted in a delay in leukemia progression of approximately 5 weeks, suggesting an additive, non overlapping anti-leukemia mechanism.

Minor Hag-specific human CD4+ T lymphocytes may contribute to the GvL effect through a direct, non cytotoxic mechanism, which could be additive to that of CD8+ CTL. The nature of this non cytotoxic GvL effect is currently under investigation. A.B. and L.H. equally contributed to this study.

No relevant conflicts of interest to declare.