Autologous stem cell transplant for myeloma improves response rates, but the immunologic mechanisms contributing to this improvement are unknown. Our results indicate that this beneficial effect may be due to a subpopulation of CD8+T cells that express an NK cell activating receptor, called NKG2D. NKG2D is present on some CD8+ T cells that mediate TCR-independent and non-MHC restricted tumor cell killing. Three NKG2D ligands (MICA, ULBP1, and ULBP3) are expressed on patients’ myeloma cells. Our data indicate that NKG2D expression can be up-regulated on some CD8+T cells making these cells highly effective at killing myeloma cells. We previously developed an ex vivo expansion method that enriches for NKG2D+CD8+T cells using mobilized blood progenitor cells (Cytotherapy 2008). These ex vivo expanded NKG2D+CD8+ T cells aggressively lysed myeloma cells and blocking the NKG2D receptor significantly inhibited this killing (p<0.0009). Due to these intriguing laboratory results, we conducted a phase II trial using adoptive cellular immunotherapy following autologous transplant, using ex vivo expanded cells enriched for NKG2D+CD8+ T cells. Myeloma patients received high-dose melphalan (200mg/m2) followed by an autologous transplant. Low dose IL-2 (6×105 IU/m2/d × 20 days) and GM-CSF (250 μg/m2/d) were administered following transplant. The ex vivo expanded cells (1 × 109 CD3+ T cells per infusion) were administered at weeks 1, 2, 3, and 8 following transplant. Nineteen of twenty-three patients are evaluable. Median engraftment of neutrophils was 13 days (range: 12–16 days) and 16 days for platelets (range: 12–26 days). All nineteen patients completed the full course of post-transplant IL-2. There were no treatment-related deaths. Due to the required CD3+T cell number in each of the ex vivo expanded cell infusions, fifteen patients received the ex vivo expanded cells (4 infusions n = 9; 3 infusions n = 3; 2 infusions n = 2; 1 infusion n = 1). Transplant-related adverse effects (> Grade 3) included nausea/vomiting (n=2), fever (n=7), elevated AST/ALT (n=2), anorexia (n=2), pneumonia (n=2), enteritis (n=1), diarrhea (n=2), pulmonary embolism (n=1), or typhlitis (n=2).There was an increased number and function of NKG2D+CD8+T cells circulating in vivo post-transplant. At 1 month post-transplant, there was an increase in the number of NKG2D+CD8+T cells (p < 0.0004), CD3+CD8+ T cells (p < 0.027), CD8+CD56+T cells (p < 0.0086), and NKG2D+CD56+ T cells (p < 0.0036). The circulating NKG2D+CD8+T cells recognized and lysed autologous myeloma cells (p<0.005) and lysis was significantly inhibited by blocking NKG2D (p<0.0014). NKG2D ligand expression on patients’ myeloma cells strongly correlated with % cell lysis. These results suggest that NKG2D+CD8+ T cells recognize and kill autologous myeloma cells in an NKG2D-dependent manner. Since myeloma cells down regulate MHC-I, NKG2D+CD8+T cells’ MHC-I unrestricted killing of myeloma cells may improve outcomes in transplanted myeloma patients. Ongoing experiments will identify the kinetics of NKG2D+CD8+ T cells following transplant and the molecular mechanisms of cytotoxicity.
No relevant conflicts of interest to declare.
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