Clinical trials using monoclonal antibodies (mAb) against cell-surface markers have yielded encouraging therapeutic results in several cancer types. Generally, however, anticancer antibodies are only efficient against a subpopulation of cancers, and there is a strong need for identification of novel targets and human antibodies against them. We have isolated single-chain human mAbs from a large naïve antibody phage display library by panning on a single-cell suspension of freshly isolated live cancer cells from a human breast cancer specimen, and these antibodies were shown to specifically recognize cancer-associated cell-surface proteins. One of the isolated human antibody fragments, Ab39, recognizes a cellsurface antigen expressed on a subpopulation of cancer cell lines of different origins. Immunohistochemical analysis of a large panel of cancerous and normal tissues showed that Ab39 bound strongly to several cancers, including 45% breast carcinomas, 35% lung cancers, and 86% melanomas, but showed no or weak binding to normal tissues. A yeast twohybrid screen of a large human testis cDNA library identified the glucose-regulated protein of 78 kDa (GRP78) as the antigen recognized by Ab39. The interaction was confirmed by colocalization studies and antibody competition experiments that also mapped the epitope recognized by Ab39 to the COOH terminus of GRP78. The expression of GRP78 on the surface of cancer cells, but not normal cells, makes it an attractive target for cancer therapies including mAb-based immunotherapy. Our results suggest that the human antibody Ab39 may be a useful starting point for further genetic optimization that could render it a useful diagnostic and therapeutic reagent for a variety of cancers. [Cancer Res 2007;67(19):9507-17]

Identification of the cell surface proteome and comparison of their expression between cells with different phenotypic characteristics is crucial to the discovery of novel cancer drug targets as well as elucidating the basic biologic processes of cancer. However, cell surface proteomics are complex and technologically challenging, and no ideal method is currently available. Here, we describe a strategy that allows scanning of the entire cell surface and identification of molecules that exhibit altered expression between two cell types. Concurrently, this method gives rise to valuable reagents for further characterization of the identified proteins. The strategy is based on subtractive immunization of mice, and we used the two isogenic cell lines, NM-2C5 and M-4A4, derived from the MDA-MB-435 cancer cell line, as a model system. Although the two cell lines are equally tumorigenic, only M-4A4 has metastatic capabilities. Our results yielded a large panel of monoclonal antibodies (mAbs) that recognized cell surface markers preferentially or exclusively expressed on metastatic vs nonmetastatic cancer cells. Four mAbs and their corresponding antigens were further characterized. Importantly, analysis on an extended panel of breast cancer cell lines demonstrated that the four mAbs bound preferentially to cell lines known to be metastatic in vivo, suggesting that these markers have general applications. Immunohistochemical analysis showed that mAb 11E6 reacted preferentially with neuroendocrine tumors while exhibiting no or very weak reactivity with normal tissues. mAb 15C7 stained a variety of cancers as well as some normal lymphoid organs and was subsequently identified to react with HLA-DR-beta. A third mAb, 31D7, that also specifically recognized HLA-DR-beta was capable of inhibiting the growth of MZ2 melanoma cells in vitro. Further, we found that the reduced expression of HLA-DR antigens in nonmetastatic cells of this model was not regulated by class II transactivator, but by posttranscriptional mechanisms. The study demonstrates the advantage of using the exquisitely discriminating recognition system of the immune system itself to scan the cell surface proteome for differentially expressed proteins. The subtractive immunization strategy should be broadly applicable as a quantitative and comparative proteomic approach for screening the cell surface and also allow generation of mAbs to study these cell surface antigens in more detail.

Several fatty acids and lysolipids have been shown earlier to increase the permeability of membranes of artificial liposomes, thereby increasing the release of drugs such as doxorubicin (Dox) contained within them. Free fatty acids can also inhibit cancer cell growth in vitro, and it has been suggested that this inhibition results from increased membrane permeability. Clearly, therefore, increased membrane permeability could be used in the design of liposomes for targeted drug delivery. For example, as free fatty acids and lysolipids are released upon phospholipase degradation of the liposome, the liposome could deliver membrane permeability enhancers in addition to the drug to increase the targeted anticancer effect. In this study, we examined the effect on Dox uptake in the breast cancer cell lines MDA-MB-231, MCF7, and MCF7-MDR when incubated with a large panel of different free fatty acids and lysolipids. Dox uptake was quantified by flow cytometry and fluorescence microscopy. We observed no increased Dox uptake in any of the breast cancer cell lines, suggesting that growth inhibitory effects observed earlier subsequent to the addition of free fatty acids to cancer cells are not caused by increased cell membrane permeability.

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