The Role of Leukemia Derived B7-H1 in Tumor-T-Cell Interactions in Humans.

Objective: Expression of the B7-homologue B7-H1 (PD1-Ligand) has been proposed to enable tumor cells to evade immune surveillance. Recently, B7-H1 on murine leukemia cells was reported to mediate resistance to cytolytic T-cell destruction. In this study we investigated the expression and functional role of the B7-homologue B7-H1 in human leukemia.

Patients and Methods: Leukemia cells from 20 patients and 9 human leukemia cell lines were investigated for B7-H1 expression by flow cytometry. Functional relevance of B7-H1 for tumor-immune interactions was assessed by coculture experiments using purified, alloreactive CD4 and CD8 T-cells in the presence of a neutralizing anti-B7-H1 antibody.

Results: Significant B7-H1 expression levels on leukemia cells were detected in 13 of 20 patients and in 8 of 9 cell lines. In contrast to various other tumor entities and the data reported from a murine leukemia system we did not observe any significant inhibitory effect of leukemia-derived B7-H1 on CD4 and CD8 cytokine production (IFN-g, IL-2) or expression of T-cell activation markers (ICOS, CD69). In the presence of a neutralizing B7-H1 antibody (mAb 5H1) no significant changes in T cell IFN-g or IL-2 production were observed.

Conclusions: Our data demonstrate that leukemia-derived B7-H1 seems to have no direct influence on T-cell activation and cytokine production in humans. Further experiments are warranted to delineate factors and characterize yet unidentified B7-H1 receptor(s) that determine inhibitory and stimulatory functions of B7-H1 in human leukemia.