Introduction The identification of potential breast cancer stem cells is of importance as the characteristics of stem cells suggest that they are resistant to conventional forms of therapy. Several techniques have been proposed to isolate or enrich for tumorigenic breast cancer stem cells, including (a) culture of cells in non-adherent non-differentiating conditions to form mammospheres and (b) sorting of the cells by their surface phenotype (expression of CD24 and CD44).

MUC1 is a highly attractive immunotherapeutic target owing to increased expression, altered glycosylation, and loss of polarity in >80% of human cancers. To exploit this, we have constructed a panel of chimeric Ag receptors (CAR) that bind selectively to tumor-associated MUC1. Two parameters proved crucial in optimizing the CAR ectodomain. First, we observed that the binding of CAR-grafted T cells to anchored MUC1 is subject to steric hindrance, independent of glycosylation status. This was overcome by insertion of the flexible and elongated hinge found in immunoglobulins of the IgD isotype. Second, CAR function was highly dependent upon strong binding capacity across a broad range of tumor-associated MUC1 glycoforms. This was realized by using an Ab-derived single-chain variable fragment (scFv) cloned from the HMFG2 hybridoma. To optimize CAR signaling, tripartite endodomains were constructed. Ultimately, this iterative design process yielded a potent receptor termed HOX that contains a fused CD28/OX40/CD3ζ endodomain. HOX-expressing T cells proliferate vigorously upon repeated encounter with soluble or membrane-associated MUC1, mediate production of proinflammatory cytokines (IFN-γ and IL-17), and elicit brisk killing of MUC1+ tumor cells. To test function in vivo, a tumor xenograft model was derived using MDA-MB-435 cells engineered to coexpress MUC1 and luciferase. Mice bearing an established tumor were treated i.p. with a single dose of engineered T cells. Compared with control mice, this treatment resulted in a significant delay in tumor growth as measured by serial bioluminescence imaging. Together, these data demonstrate for the first time that the near-ubiquitous MUC1 tumor Ag can be targeted using CAR-grafted T cells.

The PLU-1 gene is expressed at the level of message in breast cancers and breast cancer cell lines and shows restricted expression in normal adult tissues with the exception of testis. The predicted protein sequence contains several domains, including the PLU domain, which is shared by other proteins involved in transcription and/or development. We have developed a polyclonal antiserum to a C-terminal fragment of the PLU-1 protein, which shows little homology to other family members. Immunohistochemic analysis with the antiserum ␣-PLU-1C confirmed the nuclear localisation of PLU-1. ␣-PLU-1C also reacted with the mouse homologue of PLU-1 (mPlu-1) but not with the closest family member, RBP2. Using Western blot analysis, PLU-1 was shown to be well expressed in breast cancers and breast cancer cell lines, while it was not detected in a range of normal adult tissues. Our results suggest that the PLU-1 protein may belong to the class of testis/cancer antigens. © 2002 Wiley-Liss, Inc. Key words: breast cancer: PLU-1 protein; testis/cancer antigenThe PLU-1 gene was identified as being downregulated in a human mammary epithelial cell line (MTSV1-7) transfected with HER2/c-erbB2, 1 after treatment with the 4D5/HERCEPTIN antibody, which inhibits c-erbB2 signaling. 2 However, in situ hybridisation and Northern blot analyses showed that PLU-1 mRNA is expressed in most breast cancers and breast cancer cell lines, regardless of the level of c-erbB2. 3 However, Northern blot analysis of total mRNA from normal human adult tissues showed high levels of expression in testis and detectable levels in placenta, but levels were undetectable in the other tissues examined.Following our own report on the identification and characterisation of PLU-1, other investigators reported on the cloning of cDNAs of splice and transcriptional variants of the PLU-1 gene. These were referred to as RBP2-H1 4 and RBBP2H1A, 5 respectively. The nomenclature of the splice and transcriptional variants of PLU-1 reflects the high homology between the predicted protein sequence of PLU-1 and the RBP2 6 in conserved domains, particularly in the novel PLU domain. 3,7 PLU-1 binds to a conserved consensus sequence in 2 transcriptions factors through the novel PLU domain. 8 Additionally, PLU-1 was a potent transcriptional repressor and may be involved in gene regulation in breast cancer. We also defined the sequence of mPlu-1 cDNA, which shows at the amino acid level an overall homology with human PLU-1 of 94% and almost 100% identity within the conserved domains. 9 The very conserved sequence of PLU-1 from human to mouse in combination with an almost identical expression pattern in adult tissues and breast cancers in both species suggests a conserved function in breast cancer.The later studies by Vogt et al. 4 and Kashuba et al. 5 on the RNA variants of PLU-1 examined mRNA expression using polyAϩ RNA and probes that would pick up all 3 transcripts. Expression was reported to be less restricted in normal tissues than we observed using total RNA. However, as we repo...