Human Langerhans-Type Dendritic Cells Break Tolerance against the Tumor Antigen, WT1, by a Largely IL-15-Dependent Mechanism.

The Wilms’ tumor 1 (WT1) gene encodes a transcription factor essential for cell growth and development, but overexpression in many leukemias, myelodysplastic syndromes, and solid tumors correlates with poor prognosis. The WT1 protein could therefore serve as a therapeutic target, if tolerance could be broken against this self-differentiation, tumor-associated antigen (TAA). Vaccination protocols using peptide-pulsed dendritic cells (DCs) to induce anti-tumor responses present only defined epitopes restricted to certain human leukocyte antigens (HLA). In contrast, vaccination with mRNA-electroporated DCs supports processing and presentation of multiple TAA epitopes tailored to patient-specific HLA. Human Langerhans-type dendritic cells (LCs) derived from CD34⁺ hematopoietic progenitors are superior to other conventional DC subtypes, like monocyte-derived DCs (moDCs), in producing IL-15 to generate MHC-restricted, Ag-specific cytolytic T lymphocytes (CTL). Mature LCs transcribe 3 to 5-fold higher amounts of IL-15R-alpha mRNA and express the highest levels of IL-15R-alpha protein compared with other conventional DCs as measured by real-time RT-PCR, Western blot, and immunofluorescence microscopy (P<0.001 to 0.05, n=3–4 independent expts). Anti-IL-15R-alpha blockade also significantly reduces but does not completely eliminate the capacity of LCs to stimulate CTL. This indicates that LCs have a greater capacity than moDCs for presenting IL-15 in trans to stimulate lytic T cell responses. We therefore compared the ability of LCs and moDCs electroporated with WT1 mRNA to break tolerance against this self-differentiation TAA by generating CTLs against WT1⁺ tumor cells. After a single 7-day round of stimulation at T cell:LC ratio of only 10:1 to 30:1 and in the absence of exogenous IL-15, T cells stimulated by WT-1-electroporated LCs lyse 85–95% of a WT-1⁺ tumor cell line. MoDCs are completely incapable of generating WT-1-specific CTL under comparable conditions and require exogenous IL-15 10ng/ml to generate 70–80% specific lysis of WT-1⁺ targets. Lysis of a WT-1- HLA-A*0201⁺ target cell line or NK cell-sensitive LCL721.221 targets is at background levels (<5%) in all cases. Most importantly, WT-1-electoporated LCs also induce 63% lysis (+/− 11 SEM) of primary WT1⁺ HLA-A*0201⁺ blasts from a patient with AML after 7 days’ coculture at a T cell:LC ratio of 10:1 without exogenous IL-15. These results demonstrate a major role for the IL-15R-alpha-IL-15 complex on LCs in expanding anti-tumor cytolytic T cells and breaking tolerance against self-differentiation Ags like WT1. These data also support the use of IL-15-producing LCs in active immunization or expansion of T cells ex vivo for adoptive immunotherapy.