Background: PBMC can be expanded ex vivo into aggressive cytotoxic effector cells (CEC) comprising T, NK and NKT cells. We identified the phenotype, cytotoxicity and mechanisms of killing of these CEC.
Methods: CY- and G-CSF-mobilized PBMC from myeloma patients were placed in Aim-V serum-free medium, IL-2 (50 IU/mL) and OKT-3 (50 ng/mL). Cytotoxicity was evaluated by selectively blocking the TCR, MHC class I or NKG2D receptor.
Results: The CEC expanded three-fold by day 7 and aggressively lysed myeloma cells (41.9%) compared with day 0 (4%; P=0.012). CD8+ CD56+ NKT cells performed the majority of lysis. The CD8+ cells greatly increased NKG2D expression during culture (P=0.005). Cytotoxicity correlated with target NKG2D ligand expression (P=0.0002). Blocking the TCR or MHC class I did not affect cytotoxicity (P>0.22). CD8+ cell-mediated lysis dropped 48% when the NKG2D receptor was blocked. Day 7 CEC aggressively lysed myeloma cells in an MHC- and non-MHC-restricted fashion, through the NKG2D receptor.
Discussion: Because MHC expression is often down-regulated on tumor cells and the NKG2D ligands are generally specific to malignant cells, the adoptive transfer of CEC that kill through different pathways may circumvent tumor-resistant mechanisms and improve outcomes.