Figure 1.

Effect of SL-401 on pDCs, MM cells and pDC-induced MM cell growth. (a) pDC (green) and MM (blue) cells were stained with anti-IL-3 Rα-PE antibody or matched isotype control and analyzed by flow cytometry. (Number of patient samples =6). Left panel: A representative histogram shows IL-3Rα/CD123 expression on pDCs (green) and MM cells (blue). Right panel: Frequency of IL-3 Rα/CD123 expression on patient pDCs and tumor cells. The data is presented as mean fluorescence intensity (MFI) of IL-3Rα/CD123 expression on pDCs (green closed circles) and MM cells (blue closed circles) isolated from patient BM samples (n =6). MFI is shown for each cell type (P<0.0101 for pDCs versus MM cells) (b) Schematic of SL-401 construction: SL-401 is a targeted therapy directed to IL-3R consisting of IL-3 fused to a truncated DT payload. The IL-3 domain of SL-401, which replaces the binding domain of DT, targets SL-401 to cells that express the IL-3R. (c) Purified pDCs and MM cells from patients (n =9) BM were treated with indicated concentrations of SL-401 for 48 h, and analyzed for apoptosis (IC₅₀ for pDCs: 0.83 ng/ml =13.67 picomolar) (mean ±s.d., P<0.005; n =4). (d) MM.1 S cells (5 × 10⁴ cells per 200 μl), MM.1 R (5 × 10⁴ cells per 200 μl), and pDCs (1 × 10⁴ cells per 200 μl) were cultured either alone or together (1:5 pDC:MM ratio) for 72 h in the presence or absence of indicated concentrations of SL-401, and DNA synthesis was measured by ³H-TdR uptake (mean ±s.d., n =3). (e) Patient (n = 9) MM cells were cultured with or without autologous pDCs for 72 h in the presence or absence of indicated concentrations of SL-401, and DNA synthesis was measured by ³H-TdR uptake (mean ±s.d. of triplicate cultures; P<0.004 for all samples). (f) Purified indicated cell types from MM patient (n =5) and total PBMCs from normal donors (n =4) were treated with DMSO vehicle or SL-401 for 72 h and analyzed for viability (mean ±s.d. of quadruplicate cultures). Anti-pDC activity of SL-401 served as a positive control.