Combination Therapy of Allogeneic HSCT and In Vivo Activated iNKT Cells Prevents Acute Gvhd While Augmenting GVL Effect

Adult acute lymphoblastic leukemia (ALL) is a highly aggressive cancer with poor clinical prognosis and high relapse rate. Whereas allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment for highly differentiated hematologic malignancies, allo-HSCT provides only limited benefits for treating ALL with relapses frequent. Additionally, the development of graft-versus-host-disease (GVHD) remains a major concern for allo-HSCT recipients.

Invariant natural killer T (iNKT) cells are a subset of T lymphocytes that express both a semi-invariant T cell receptor and natural killer cell markers. They recognize lipid antigens presented by non-polymorphic CD1d and have been reported to directly target CD1d⁺ solid tumors. Many B-ALL, including the P210 B-ALL cell line, are also CD1d^+. Clinically, increased iNKT cells have simultaneously been correlated with reduced risk of developing acute GVHD and lower rates of malignant relapse.

Because iNKT cells comprise only a tiny fraction of total T cells, we established a protocol to insert an iNKT TCR into murine HSC, thereby generating retrogenic mice with 5-10x expansion of iNKT cells in spleen and liver. Although engineered T cell therapy, including CD19-CART therapy, has proven to be effective in inducing remission in many B-ALL patients, when used as a monotherapy, relapse remains a major obstacle. We therefore tested whether a combination of allo-HSCT and iNKT cell therapy could robustly prevent GVHD while preserving graft-versus-leukemia effects (GVL).

To test whether the addition of iNKT cells into allo-graft was capable of preventing acute GVHD (aGVHD), lethally irradiated BALB/C were given B6 bone marrow (2.5 x 10⁶) and whole splenocytes (2.5 x 10⁶, 5 x 10⁶, or 10 x 10⁶) from either B6 or retrogenic B6-iNKT mice. Mice that received 5 x 10⁶ B6 splenocytes developed lethal aGVHD and became moribund by Day 10. Half of the mice that received 2.5 x 10⁶ B6 splenocytes develop lethal aGVHD and died by Day 20. In contrast, only half of the mice that received 5 x 10⁶ or 10 x 10⁶ B6-iNKT splenocytes developed lethal aGVHD by Day 20, and all recipients of 2.5 x 10⁶ B6 splenocytes remained alive by Day 20, suggesting the addition of iNKT cells into allograft may reduce the severity of aGVHD.

To test whether adding iNKT cells to allograft could enhance GVL without causing GVHD, BALB/C mice with pre-established P210 B-ALL leukemia were given allo-HSCT with or without iNKT cell addition. Initially, our data showed freshly isolated iNKT cells were rapidly cleared following transplantation. We therefore included lethally irradiated P210 loaded with α-Galactosylceramide (α-Galcer) as adjuvant, and were able to see robust expansion of iNKT cells in vitro and in vivo (>20% vs <1% in WT in BM and Liver). Additionally, we found that HSCT recipients that also received iNKT cells were able to fully clear bone marrow residing B-ALL cells by Day8, compared to less than half in control, suggesting an important role of iNKT cells for targeting bone marrow residing B-ALL cells to prevent relapse. Taken together, combination therapy of allogeneic HSCT with infusion of donor-type iNKT cells activated via irradiated α-Galcer-loaded host-type ALL represent a novel approach for preventing GVHD and augmenting GVL effect against ALL.