Immunoglobulin secreted by myeloma cells contains a unique antigenic determinant (idiotype [Id]) that may serve as a tumor-specific antigen. Although Id-protein–specific T-cell responses have been reported in patients with myeloma, it is not known whether primary myeloma tumor cells can present naturally processed Id peptides on their surface as a target. We immunized 2 healthy human stem-cell donors with Id proteins from their recipients. T cells from the immunized donors released high levels of T-helper 1–type cytokines in response to stimulation with myeloma cells from their recipients. The T-cell–mediated cytokine response to tumor cells was blocked by a major histocompatibility complex (MHC) class I monoclonal antibody, whereas the response to soluble Id protein was dependent on MHC class II. To investigate whether Id-specific CD8+ T cells can recognize and kill autologous myeloma cells, we generated T cells from peripheral blood mononuclear cells from a third patient with myeloma by means of in vitro stimulation with autologous dendritic cells pulsed with Id protein. Tumor-specific lysis of myeloma cells was demonstrated by the lack of killing of autologous nonmalignant B cells or natural killer–sensitive K562 cells. Lysis of autologous myeloma targets was restricted by MHC class I molecules. These data represent the first report of class I–restricted T-cell recognition of fresh autologous myeloma targets and formally demonstrate that human myeloma cells can serve as targets of an Id-specific T-cell response.

#5046 Introduction: Basal-like breast cancer is associated with high grade, poor prognosis, and younger patient age. Because tumors are estrogen receptor (ER) and HER2 negative, effective targeted treatment for basal breast cancer remains elusive. The lack of efficacy of currently used chemotherapies may reflect their inability to eradicate cancer stem cells (CSC) within a basal breast tumor.
 Chondroitin sulfate proteoglycan 4 (CSPG4), a molecule initially identified in melanoma cells, is a cell surface antigen involved in migration and invasion of tumor cells. The rat homolog of the human CSPG4, named NG2, is expressed by progenitor cells in several types of tissues. Furthermore, NG2 expression is typically maximal when progenitor cells are mitotic and motile. As one important characteristic of cancer stem cells is their ability to migrate and metastasize, and both CSPG4 and NG2 play a role in tumor cell migration, we tested whether CSPG4 is expressed on cells with the CSC phenotype in ER negative/HER2 negative (basal-like) breast cancer, and can thus serve as a target for immunotherapy of breast CSC.
 Methods and Results: The mRNA levels of CSPG4 in ER- and Her2- breast cancer were compared to ER-/Her2+, ER+/Her2-, and ER+/Her2+ breast cancer using a publicly available, clinically annotated breast cancer data set. The gene expression level of CSPG4 was elevated in the ER- and Her2- subgroup when compared to ER-/Her2+ (Mann Whitney, p=0.03), ER+/Her2- (Mann Whitney, p=0.01), and ER+/Her2+ (Mann Whitney, p=0.06). These results suggest that CSPG4 expression is best correlated with the basal phenotype of breast cancer. Flow cytometric analysis showed that CSPG4 is expressed on CD44+, CD24-/lo cells in the human basal breast cancer cell lines, i.e., MDA-MD-231, MDA-MB-435, HS578T and SUM149, but is not detectable on the luminal breast cancer cell lines, i.e., MCF-7, SK-BR-3 and T-47D. Furthermore CSPG4 is expressed on the cell surface of putative breast CSC identified as lineage-negative (CD2,3,10,16,18,31,45,64,140b), CD44+, CD24-/lo cells in pleural effusions of 12 patients with breast cancer. Further, inoculation of flow cytometric sorted CD44+CD24-/low CSPG4+ cells from the MDA-MB-231-luciferease expressing cell line, to the mammary fat pads of immunodeficient mice, led to the development of tumors detected by whole-body bioluminescent reporter imaging 3 weeks after the injection. In contrast, inoculation of CD44+CD24-/low CSPG4- cells, sorted from the same cell line, did not generate tumors. More importantly, CSPG4-specific mAb significantly inhibited post-surgery tumor recurrence and lung metastases in SCID mice transplanted with the human basal breast cancer cell line MDA-MB-435, which has the putative breast CSC phenotype (CD44+, CD24-/lo) in more than 99% of cells.
 Conclusion: These results suggest that the CSPG4 is a potential new immunotherapeutic target for basal-like breast cancer and breast CSC. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5046.