| Summary: | NKG2D is a unique immunoreceptor of key significance to immune surveillance of tumor cells, and it represents an attractive candidate in the development of both molecular and cell-based immunotherapies. NKG2D is a stimulatory receptor expressed by human natural killer (NK) cells, cytokine induced killer/lymphokine activated killer (CIK/LAK) cells, γδ T cells and CD8+ &agr;β T cells. The NKG2D receptor plays a pivotal role in both innate and adaptive immunity, where it stimulates the cytotoxic function of (NK) cells and CD8 + T cells upon recognition of a diverse array of MHC-related ligands. In turn, these ligands are specifically induced under pathological conditions.Current research has established a prominent role for the NKG2D receptor in the modulation of tumor immunity by both NK cells and T cells. We discovered a novel genetic modifier that limits the effectiveness of NK/T cell immunotherapy in human cancer patients. Our objective of this study was to investigate how these genetic modifiers affect NK receptor-ligand interactions and ultimately tumor cell recognition, immunity, and immune evasion. We characterized an alternate splice variant of human NKG2D that encodes a truncated receptor lacking the ligand-binding ectodomain. This truncated NKG2D variant (NKG2DTR) does not activate cytotoxicity in a ligand-dependent manner, and its enforced expression inhibits killing mediated by the full-length NKG2D isoform (NKG2DFL). Enforced expression of NKG2DTR resulted in the retention of NKG2DFL in intracellular compartments. The relative abundance of NKG2DTR transcripts varies among PBMC and activated LAK/CIK cells and NK cells of diverse donors and inversely correlates with the killing capacity of expressing cells. Furthermore, specific shRNA-mediated knockdown of the endogenous NKG2D TR isoform in human CIK cells and NK cells enhances endogenous NKG2D FL-mediated cytotoxicity. Co-immunoprecipitation studies revealed that NKG2DTR pairs with DAP10 and heterodimerizes with NKG2D FL, suggesting that its dominant negative impact on cytotoxicity is due to the formation of heterodimeric NKG2DTR/FL receptor complexes incapable of ligand binding. Thus, competitive interference via an alternatively spliced NKG2D variant constitutes a novel mechanism for regulation of NKG2D-mediated signaling and cytotoxicity in LAK/CIK cells and NK cells.
|
|---|
