The human A33 antigen is a transmembrane glycoprotein and a novel member of the immunoglobulin superfamily

Macromolecule functionalization of disulfide-bonded polymer hydrogel capsules and cancer cell targeting

We present a generic and versatile method for functionalization of disulfide-stabilized PMA hydrogel capsules (HCs) with macromolecules, including a number of specific antibodies to cancer cells. Functionalization was achieved by reversible addition-fragmentation chain transfer (RAFT) polymerization of poly(N-vinyl pyrrolidone) (PVPON), which introduced biorelevant heterotelechelic end groups (thiol and amine) to the polymer chain. The PVPON with heterotelechelic end groups was conjugated to the outermost layer of PMA HCs through the thiol groups and reacted with biotin via the amine groups to generate PMA/PVPON(biotin) HCs. On the basis of the high specific interaction and high affinity between biotin and avidin, and its derivates, such as NeutrAvidin (NAv), we functionalized the PMA HCs with biotinylated antibodies. We demonstrate significantly enhanced cellular binding and internalization of the antibody (Ab)-functionalized capsules compared with control human immunoglobulin (IgG)-functionalized capsules, suggesting these capsules can specifically interact with cells through antibody/antigen recognition. We anticipate that the versatility of the functionalization approach reported in this study will assist in targeted therapeutic delivery applications.

124I-huA33 antibody uptake is driven by A33 antigen concentration in tissues from colorectal cancer patients imaged by immuno-PET

The primary aim of this analysis was to examine the quantitative features of antibody-antigen interactions in tumors and normal tissue after parenteral administration of antitumor antibodies to human patients.

METHODS

Humanized anti-A33 antibody (10 mg) labeled with the positron-emitting radionuclide (124)I ((124)I-huA33) was injected intravenously in 15 patients with colorectal cancer. Clinical PET/CT was performed approximately 1 wk later, followed by a detailed assay of surgically removed tissue specimens including radioactivity counting, autoradiography, immunohistochemistry, and antigen density determination.

RESULTS

PET/CT showed high levels of antibody targeting in tumors and normal bowel. In tissue specimens, the spatial distribution of (124)I-huA33 conformed to that of A33 antigen, and there was a linear relationship between the amount of bound antibody and antigen concentration. Antibody uptake was high in 1- to 2-mm regions of antigen-positive tumor cells (mean, ~0.05 percentage injected dose per gram) and in antigen-positive normal colonic mucosa (mean, ~0.03 percentage injected dose per gram). The estimated binding site occupancy for tumor and normal colon was 20%-50%.

CONCLUSION

The in vivo biodistribution of (124)I-huA33 in human patients 1 wk after antibody administration was determined by A33 antigen expression. Our data imply that the optimal strategy for A33-based radioimmunotherapy of colon cancer will consist of a multistep treatment using a radionuclide with short-range (α- or β-particle) emissions.

Adhesion protein VSIG1 is required for the proper differentiation of glandular gastric epithelia

VSIG1, a cell adhesion protein of the immunoglobulin superfamily, is preferentially expressed in stomach, testis, and certain gastric, esophageal and ovarian cancers. Here, we describe the expression patterns of three alternatively spliced isoforms of mouse Vsig1 during pre- and postnatal development of stomach and potential function of Vsig1 in differentiation of gastric epithelia. We show that isoforms Vsig1A and Vsig1B, which differ in the 3′untranslated region, are expressed in the early stages of stomach development. Immunohistochemical analysis revealed that VSIG1 is restricted to the adherens junction of the glandular epithelium. The shorter transcript Vsig1C is restricted to the testis, encodes an N-terminal truncated protein and is presumably regulated by an internal promoter, which is located upstream of exon 1b. To determine whether the 5′ flanking region of exon 1a specifically targets the expression of Vsig1 to stomach epithelia, we generated and analyzed transgenic mice. The 4.8-kb fragment located upstream of exon 1a was sufficient to direct the expression of the reporter gene to the glandular epithelia of transgenic stomach. To determine the role of VSIG1 during the development of stomach epithelia, an X-linked Vsig1 was inactivated in embryonic stem cells (ESCs). Although Vsig1^−/Y ESCs were only able to generate low coat color chimeric mice, no male chimeras transmitted the targeted allele to their progeny suggesting that the high contribution of Vsig1^−/Y cells leads to the lethality of chimeric embryos. Analysis of chimeric stomachs revealed the differentiation of VSIG1-null cells into squamous epithelia inside the glandular region. These results suggest that VSIG1 is required for the establishment of glandular versus squamous epithelia in the stomach.

(124)I-huA33 antibody PET of colorectal cancer

Humanized A33 (huA33) is a promising monoclonal antibody that recognizes A33 antigen, which is present in more than 95% of colorectal cancers and in normal bowel. In this study, we took advantage of quantitative PET to evaluate (124)I huA33 targeting, biodistribution, and safety in patients with colorectal cancer. We also determined the biodistribution of (124)I-huA33 when a large dose of human intravenous IgG (IVIG) was administered to manipulate the Fc receptor or when (124)I-huA33 was given via hepatic arterial infusion (HAI).

METHODS

We studied 25 patients with primary or metastatic colorectal cancer; 19 patients had surgical exploration or resection. Patients received a median of 343 MBq (44.4-396 MBq) and 10 mg of (124)I-huA33. Nineteen patients received the antibody intravenously and 6 patients via HAI, and 5 patients also received IVIG.

RESULTS

Ten of 12 primary tumors were visualized in 11 patients. The median concentration in primary colon tumors was 0.016% injected dose per gram, compared with 0.004% in normal colon. The PET-based median ratio of hepatic tumor uptake to normal-liver uptake was 3.9 (range, 1.8-22.2). Quantitation using PET, compared with well counting of serum and tissue, showed little difference. Prominent uptake in bowel hindered tumor identification in some patients. Pharmacokinetics showed that patients receiving IVIG had a significantly shorter serum half-time (41.6 ± 14.0 h) than those without (65.2 ± 9.8 h). There were no differences in clearance rates among the intravenous group, IVIG group, and HAI group, nor was there any difference in serum area under the curve, maximum serum concentration, or volume of distribution. Weak titers of human-antihuman antibodies were observed in 6 of 25 patients. No acute side effects or significant toxicities were associated with huA33.

CONCLUSION

Good localization of (124)I-huA33 in colorectal cancer with no significant toxicity has been observed. PET-derived (124)I concentrations agreed well with those obtained by well counting of surgically resected tissue and blood, confirming the quantitative accuracy of (124)I-huA33 PET. The HAI route had no advantage over the intravenous route. No clinically significant changes in blood clearance were induced by IVIG.

SNAIL regulates interleukin-8 expression, stem cell-like activity, and tumorigenicity of human colorectal carcinoma cells

BACKGROUND & AIMS

Some cancer cells have activities that are similar to those of stem cells from normal tissues, and cell dedifferentiation correlates with poor prognosis. Little is known about the mechanisms that regulate the stem cell-like features of cancer cells; we investigated genes associated with stem cell-like features of colorectal cancer (CRC) cells.

METHODS

We isolated colonospheres from primary CRC tissues and cell lines and characterized their gene expression patterns by microarray analysis. We also investigated the biological features of the colonosphere cells.

RESULTS

Expanded CRC colonospheres contained cells that expressed high levels of CD44 and CD166, which are markers of colon cancer stem cells, and had many features of cancer stem cells, including chemoresistance and radioresistance, the ability to initiate tumor formation, and activation of epithelial-mesenchymal transition (EMT). SNAIL, an activator of EMT, was expressed at high levels by CRC colonospheres. Overexpression of Snail in CRC cells induced most properties of colonospheres, including cell dedifferentiation. Two hundred twenty-seven SNAIL-activated genes were up-regulated in colonospheres; gene regulatory networks centered around interleukin (IL)-8 and JUN. Blocking IL-8 expression or activity disrupted SNAIL-induced stem cell-like features of colonospheres. We observed that SNAIL activated the expression of IL8 by direct binding to its E3/E4 E-boxes. In CRC tissues, SNAIL and IL-8 were coexpressed with the stem cell marker CD44 but not with CD133 or CD24.

CONCLUSIONS

In human CRC tissues, SNAIL regulates expression of IL-8 and other genes to induce cancer stem cell activities. Strategies that disrupt this pathway might be developed to block tumor formation by cancer stem cells.

Antibodies in oncology

Monoclonal antibodies (mAbs) have become one of the largest classes of new therapeutic agents approved for use in oncology, and have revolutionised the treatment of many human malignancies. Clinically useful mAbs can function through several different mechanisms, including inhibition of tumour-related signalling, induction of apoptosis, inhibition of angiogenesis, enhancing host immune response against cancer and targeted delivery of payloads (such as toxins, cytotoxic agents or radioisotopes) to the tumour site. The increasing knowledge of key molecules and cellular pathways involved in tumour induction and progression has led to a rise in the proportion of therapeutic mAbs entering clinical trials. These mAbs consist of various conventional or recombinant, murine, humanised, chimeric or fully human and fusion constructs. In this review, we provide an overview of mAbs approved for use in clinical oncology and those currently in clinical development. We also discuss the mechanisms of action of anti-cancer mAbs, as well as the antigen targets recognised by these antibodies.

Radioimmunotherapy of solid tumors: searching for the right target

Radioimmunotherapy of solid tumors remains a challenge despite the tremendous success of ⁹⁰Y ibritumomab tiuxetan (Zevalin) and ¹³¹I Tositumomab (Bexxar) in treating non-Hodgkin's lymphoma. For a variety of reasons, clinical trials of radiolabeled antibodies against solid tumors have not led to responses equivalent to those seen against lymphoma. In contrast, promising responses have been observed with unlabeled antibodies that target solid tumor receptors associated with cellular signaling pathways. These observations suggest that anti-tumor efficacy of the carrier antibody might be critical to achieving clinical responses. Here, we review and compare tumor antigens targeted by radiolabeled antibodies and unlabeled antibodies used in immunotherapy. The review shows that the trend for radiolabeled antibodies under pre-clinical development is to also target antigens associated with signaling pathways that are essential for the growth and survival of the tumor.

The discovery and validation of colorectal cancer biomarkers

Colorectal cancer is currently the third most common malignancy in the world. Patients have excellent prognosis following surgical resection if their tumour is still localized at diagnosis. By contrast, once the tumour has started to metastasize, prognosis is much poorer. Accurate early detection can therefore significantly reduce the mortality from this disease. However, current tests either lack the required sensitivity and selectivity or are costly and invasive. Improved biomarkers, or panels of biomarkers, are therefore urgently required. We have addressed current screening strategies and potential protein biomarkers that have been proposed. The role of both discovery and hypothesis-driven proteomics approaches for biomarker discovery and validation is discussed. Using such approaches we show how multiple reaction monitoring (MRM) can be successfully developed and used for quantitative multiplexed analysis of potential faecal biomarkers.

Exosomes: extracellular organelles important in intercellular communication

In addition to intracellular organelles, eukaryotic cells also contain extracellular organelles that are released, or shed, into the microenvironment. These membranous extracellular organelles include exosomes, shedding microvesicles (SMVs) and apoptotic blebs (ABs), many of which exhibit pleiotropic biological functions. Because extracellular organelle terminology is often confounding, with many preparations reported in the literature being mixtures of extracellular vesicles, there is a growing need to clarify nomenclature and to improve purification strategies in order to discriminate the biochemical and functional activities of these moieties. Exosomes are formed by the inward budding of multivesicular bodies (MVBs) and are released from the cell into the microenvironment following the fusion of MVBs with the plasma membrane (PM). In this review we focus on various strategies for purifying exosomes and discuss their biophysical and biochemical properties. An update on proteomic analysis of exosomes from various cell types and body fluids is provided and host-cell specific proteomic signatures are also discussed. Because the ectodomain of ~42% of exosomal integral membrane proteins are also found in the secretome, these vesicles provide a potential source of serum-based membrane protein biomarkers that are reflective of the host cell. ExoCarta, an exosomal protein and RNA database (http://exocarta.ludwig.edu.au), is described.

A33 antigen-deficient mice have defective colonic mucosal repair

BACKGROUND

A33 antigen is a transmembrane protein expressed predominantly in normal intestinal epithelium and most colon cancers and cell lines. The function of A33 antigen is unclear, but indirect evidence indicates a role in cell adhesion, trafficking, and the gut immune response. The aim of this study was to determine the contribution made by A33 antigen in mediating colonic repair following colitis induction in the A33 antigen-deficient mutant mouse.

METHODS

Colitis was induced by treatment with TNBS/ethanol. A33 antigen-deficient or wildtype mice were sacrificed at 0, 3, 7, and 14 days after colitis induction and morphological damage, mucosal proliferation, and inflammatory cell infiltration were quantified. In a subsequent study, following the induction of colitis mice were monitored for 22 days and morbidity and mortality determined.

RESULTS

Mice lacking A33 antigen expression were compromised in their ability to resolve TNBS-induced damage and exhibited distinct crypt pathology. In A33 antigen-deficient mice morphological damage remained unresolved at 14 days postcolitis induction. Increases in colonic cell proliferation were delayed in A33 antigen-deficient mice, and the rate of crypt fission was increased after TNBS treatment. Commensurate with these observations, polymorphonuclear cell infiltration was suppressed in the absence of A33 antigen. Mortality following colitis induction was 20% higher in A33 antigen-deficient mice than in wildtype controls.

CONCLUSIONS

Mice deficient in A33 antigen expression show impaired resolution of hapten-induced mucosal damage, leading to increased mortality, associated with impaired epithelial cell proliferation and a suppressed adaptive immune response. This study suggests a contribution for A33 antigen in the colonic healing response following mucosal damage.

KLF4-dependent, PPARgamma-induced expression of GPA33 in colon cancer cell lines

The glycoprotein A33 (GPA33) is a colon cancer antigen. Phase I trials with 131I and 125I monoclonal antibody A33 in colon carcinoma patients showed excellent localization to colorectal cancer and some evidence of tumor response. Using DNA microarrays, we have identified the GPA33 gene as a target of PPARgamma in HT29-Cl.16E colon cancer cells. Treatment of HT29-Cl.16E, Caco2, SW1116 and LS174T colon cancer cells with the PPARgamma agonist GW7845 induced a 2- to 6-fold increase in GPA33 mRNA as determined by real-time PCR. This induction was also found in HT29-Cl.16E cells treated with rosiglitazone and ciglitazone and was prevented by cotreatment with the PPARgamma antagonist GW9662, indicating that this regulation was PPARgamma dependent. No canonical PPAR responsive element was found in the GPA33 promoter. We therefore analyzed the expression of transcription factors involved in GPA33 expression. CDXl, CDX2 and KLF5 expression was not modified by PPARgamma activation. By contrast, a significant increase in KLF4 was seen, both at mRNA and protein levels. Furthermore, chromatin immunoprecipitation studies demonstrated that an increased amount of KLF4 protein was bound to the GPA33 promoter in cells treated with rosiglitazone. Finally, downregulation of KLF4 expression by siRNA reduced rosiglitazone-induced GPA33 expression. This indicates that PPARgamma activation induces KLF4 expression, which in turn increases GPA33 expression. We also demonstrate that PPARgamma activation leads to increased (p21WAF1/Cip1 and keratin 19) or decreased (cyclin D1) expression of known KLF4 targets, suggesting that KLF4 is a nodal player in a network of PPARgamma-regulated genes.

Follow-up of a major linkage peak on chromosome 1 reveals suggestive QTLs associated with essential hypertension: GenNet study

Essential hypertension is a major cardiovascular risk factor and a large proportion of this risk is genetic. Identification of genomic regions consistently associated with hypertension has been difficult in association studies to date as this requires large sample sizes.We previously published a large genome-wide linkage scan in Americans of African (AA) and European (EA) descent in the GenNet Network of the Family Blood Pressure Program (FBPP). A highly significant linkage peak was identified on chr1q spanning a region of 100 cM. In this study, we genotyped 1569 SNPs under this linkage peak in 2379 individuals to identify whether common genetic variants were associated with blood pressure (BP) at this locus.Our analysis, using two different family-based association tests, provides suggestive evidence (P< or =2 x 10(-5)) for a collection of single nucleotide polymorphisms (SNPs) associated with BP. In EAs, using diastolic BP as a quantitative phenotype, three variants located in or near the GPA33, CD247, and F5 genes, emerge as our top hits; for systolic BP, variants in GPA33, CD247, and REN are our best findings. No variant in AAs came close to suggestive evidence after multiple-test corrections (P> or =8 x 10(-5)). In summary, we show that systematic follow-up of a linkage signal can help discover candidate variants for essential hypertension that require a follow-up in yet larger samples. The failure to identify common variants is either because of low statistical power or the existence of rare coding variants in specific families or both, which require additional studies to clarify.

Proteomics analysis of A33 immunoaffinity-purified exosomes released from the human colon tumor cell line LIM1215 reveals a tissue-specific protein signature

Exosomes are 40-100-nm-diameter nanovesicles of endocytic origin that are released from diverse cell types. To better understand the biological role of exosomes and to avoid confounding data arising from proteinaceous contaminants, it is important to work with highly purified material. Here, we describe an immunoaffinity capture method using the colon epithelial cell-specific A33 antibody to purify colorectal cancer cell (LIM1215)-derived exosomes. LC-MS/MS revealed 394 unique exosomal proteins of which 112 proteins (28%) contained signal peptides and a significant enrichment of proteins containing coiled coil, RAS, and MIRO domains. A comparative protein profiling analysis of LIM1215-, murine mast cell-, and human urine-derived exosomes revealed a subset of proteins common to all exosomes such as endosomal sorting complex required for transport (ESCRT) proteins, tetraspanins, signaling, trafficking, and cytoskeletal proteins. A conspicuous finding of this comparative analysis was the presence of host cell-specific (LIM1215 exosome) proteins such as A33, cadherin-17, carcinoembryonic antigen, epithelial cell surface antigen (EpCAM), proliferating cell nuclear antigen, epidermal growth factor receptor, mucin 13, misshapen-like kinase 1, keratin 18, mitogen-activated protein kinase 4, claudins (1, 3, and 7), centrosomal protein 55 kDa, and ephrin-B1 and -B2. Furthermore, we report the presence of the enzyme phospholipid scramblase implicated in transbilayer lipid distribution membrane remodeling. The LIM1215-specific exosomal proteins identified in this study may provide insights into colon cancer biology and potential diagnostic biomarkers.

Exosomes: proteomic insights and diagnostic potential

Exosomes are 40-100-nm diameter membrane vesicles of endocytic origin that are released by most cell types upon fusion of multivesicular bodies with the plasma membrane, presumably as a vehicle for cell-free intercellular communication. While early studies focused on their secretion from diverse cell types in vitro, exosomes have now been identified in body fluids such as urine, amniotic fluid, malignant ascites, bronchoalveolar lavage fluid, synovial fluid, breast milk, saliva and blood. Exosomes have pleiotropic biological functions, including immune response, antigen presentation, intracellular communication and the transfer of RNA and proteins. While they have also been implicated in the transport and propagation of infectious cargo, such as prions, and retroviruses, including HIV, suggesting a role in pathological situations, recent studies suggest that the presence of such infectious cargo may be artefacts of exosome-purification strategies. Improvements in mass spectrometry-based proteomic tools, both hardware and software, coupled with improved purification schemes for exosomes, has allowed more in-depth proteome analyses, contributing immensely to our understanding of the molecular composition of exosomes. Proteomic cataloguing of exosomes from diverse cell types has revealed a common set of membrane and cytosolic proteins, suggesting the evolutionary importance of these membrane particles. Additionally, exosomes express an array of proteins that reflect the originating host cell. Recent findings that exosomes contain inactive forms of both mRNA and microRNA that can be transferred to another cell and be functional in that new environment, have initiated many microRNA profiling studies of exosomes circulating in blood. These studies highlight the potential of exosomal microRNA profiles for use as diagnostic biomarkers of disease through a noninvasive blood test. The exacerbated release of exosomes in tumor cells, as evidenced by their increased levels in blood during the late stage of a disease and their overexpression of certain tumor cell biomarkers, suggests an important role of exosomes in diagnosis and biomarker studies. The aim of this article is to provide a brief overview of exosomes, including methods used to isolate and characterize exosomes. New advances in proteomic methods, and both mass spectrometry hardware and informatics tools will be covered briefly.

Clearance kinetics and external dosimetry of 131I-labeled murine and humanized monoclonal antibody A33 in patients with colon cancer: radiation safety implications

The monoclonal antibody (mAb) A33 detects a membrane antigen that is expressed on greater than 95% of metastatic human colorectal cancers. Previous studies have shown excellent tumor-targeting of (131)I-labeled murine and humanized forms of the mAb. A retrospective analysis of whole-body clearance in the murine form was performed for comparison to the humanized form. Serial whole-body dose rate measurements were obtained for 55 treatments on 30 patients participating in phase I/II dose escalation studies of therapeutic (131)I-murine A33 mAb. Whole-body retention fractions over time were derived. Each treatment was fit with exponential curves to determine the effective half-lives and corresponding clearance fractions. There was a large variability in the calculated mono-exponential clearance effective half-life time, with a mean value of 36.5 h +/- 8.5 h. A bi-exponential fit of all combined data shows that 60% of the administered dose rapidly clears with a biological half-time of 23.9 h and 40% clears with a slower biological half-time of 101.2 h. The whole-body clearance proved to be more rapid in the murine form when compared with recent studies on the humanized form of radiolabeled A33 mAb. The variability in whole-body clearance reinforces the need for patient-specific tracer dosimetry for clinical care and radiation safety precautions. In addition, the slower clearance of the humanized form of the A33 mAb requires longer term radiation safety precautions than the earlier murine form. As other monoclonal antibodies progress from murine to humanized forms, radiopharmacokinetics should be evaluated for clinical and radiation safety implications.

A novel A33 promoter-based conditionally replicative adenovirus suppresses tumor growth and eradicates hepatic metastases in human colon cancer models

PURPOSE

A33 antigen is a membrane-bound protein expressed in intestinal epithelium that is overexpressed in 95% of primary and metastatic colorectal carcinomas but is absent in most epithelial tissues and tumor types. We hypothesized that A33 promoter might be useful in the design of a conditionally replicative adenovirus for the treatment of colorectal cancer (CRC).

EXPERIMENTAL DESIGN

We cloned an A33 promoter fragment (A33Pr) that extends from -105 to +307 bp. Using luciferase activity as a reporter gene, we showed that A33Pr was active in CRC cell lines. We next constructed a conditionally replicative adenovirus named AV22EL where E1A was placed under the control of A33Pr. The tumor-specific oncolytic effect of AV22EL was investigated both in vitro and in vivo.

RESULTS

AV22EL induced specific in vitro lysis of human CRC cell lines that expressed A33 and have negligible lytic capacity on cells that lacked or had minimal A33 expression, including normal human colonic cells. In vivo, a marked reduction of tumor growth and increased long-term survival rates were observed in nude mice xenografted with s.c. CRC tumors. Combination with 5-fluorouracil induced an additive effect in vitro with no toxic effects in vivo. Remarkably, AV22EL completely eliminated established hepatic metastases in >90% of mice and restored hepatic function according to biochemical parameters. Its systemic administration induced E1A expression only in the hepatic metastasis but not in normal organs.

CONCLUSIONS

These data show that AV22EL is a stringently regulated and potent oncolytic agent for the treatment of CRC.

Monoclonal antibody CC188 binds a carbohydrate epitope expressed on the surface of both colorectal cancer stem cells and their differentiated progeny

PURPOSE

Recently, cancer stem cells (CSC), undifferentiated cancer progenitor cells, have been successfully isolated from colorectal tumors. Targeting both CSCs and differentiated, rapidly proliferating tumor cells with therapeutic drugs provides a focused strategy to treat cancer. In this study, we isolated the monoclonal antibody (mAb) CC188 and characterized the epitope recognized by mAb CC188, which is useful for developing biological reagents that target CSCs.

EXPERIMENTAL DESIGN

We used a hybridoma technique to generate mAbs and an immunomagnetic method to isolate colon CSCs. We characterized mAb CC188 binding epitope and examined the epitope distribution in normal and tumor tissues, particularly in CSCs using tissue arrays and immunofluorescence staining method. We also evaluated the effect of mAb CC188 on invasiveness of NSY tumor cells.

RESULTS

mAb CC188 was generated and 98.9% (187 of 189 cases) of colon cancer were positively stained by mAb CC188. "+", "++," and "+++" staining were 25.9%, 28.6%, and 43.4%, respectively. The mAb CC188 binding epitope was identified as a carbohydrate, which was expressed on the surface of colon CSCs (CD133+), differentiated colon cancer cells (CD133-), and cells from various types of epithelial tumors. In contrast, the expression of the carbohydrate epitope was low in normal prostate muscle and pancreatic acinar cells, as well as in some normal epithelial cells of the breast duct, cervix, and skin. A functional study indicated that mAb CC188 suppressed the invasiveness of NSY tumor cells.

CONCLUSION

mAb CC188 selectively targets a carbohydrate epitope expressed on cancer cells, providing a viable method for specific tumor imaging and targeted therapy.

A33 antigen displays persistent surface expression

The A33 antigen is a cell surface glycoprotein of the small intestine and colonic epithelium with homology to tight junction-associated proteins of the immunoglobulin superfamily, including CAR and JAM. Its restricted tissue localization and high level of expression have led to its use as a target in colon cancer immunotherapy. Although the antigen is also present in normal intestine, radiolabeled antibodies against A33 are selectively retained by tumors in the gut as well as in metastatic lesions for as long as 6 weeks. Accordingly, we have studied the trafficking and kinetic properties of the antigen to determine its promise in two-step, pretargeted therapies. The localization, mobility, and persistence of the antigen were investigated, and this work has demonstrated that the antigen is both highly immobile and extremely persistent-retaining its surface localization for a turnover halflife of greater than 2 days. In order to explain these unusual properties, we explored the possibility that A33 is a component of the tight junction. The simple property of surface persistence, described here, may contribute to the prolonged retention of the clinically administered antibodies, and their uncommon ability to penetrate solid tumors.

Surface expression of gpA33 is dependent on culture density and cell-cycle phase and is regulated by intracellular traffic rather than gene transcription

The cell-surface marker, gpA33, a new member of the immunoglobulin superfamily, is expressed by gastrointestinal cells and by 95% of colon cancers. It has become a promising target of immunologic therapy strategies, but its biologic function and potential role in tumorigenesis are unknown. In this study, we have investigated the expression of gpA33 on the mRNA and cell-surface protein levels by quantitative reverse transcriptase polymerase chain reaction and flow cytometry, respectively, in response to cell density in the culture and to cell-cycle arrest in the G1, S, or G2/M phases. As internalization of the surface protein had previously been reported, we also investigated the binding and intracellular migration of an anti-gpA33 fluobody with green fluorescent protein (A33scFv::GFP) by laser confocal microscopy. Contrary to intuition, we found that gpA33 surface expression and mRNA levels do only partly correlate under the conditions tested. Dependent on cell density in culture, gpA33 surface expression peaked at the point of confluence. Dependent on cell-cycle phase, it peaked in the G2/M phase but was lowest in the S phase, whereas mRNA levels were highest in S, but almost absent in G1. Laser confocal microscopy clearly demonstrated the intracellular uptake of A33scFv::GFP and showed the migration of microvesicles over time. These findings are, in part, concordant with the putative role of gpA33 as an adhesion molecule. However, intracellular traffic and recycling to the cell surface appear to play a major role in its function and to have an influence on its surface density superseding translational regulation.

Exosomes with immune modulatory features are present in human breast milk

Breast milk is a complex liquid with immune-competent cells and soluble proteins that provide immunity to the infant and affect the maturation of the infant's immune system. Exosomes are nanovesicles (30-100 nm) with an endosome-derived limiting membrane secreted by a diverse range of cell types. Because exosomes carry immunorelevant structures, they are suggested to participate in directing the immune response. We hypothesized that human breast milk contain exosomes, which may be important for the development of the infant's immune system. We isolated vesicles from the human colostrum and mature breast milk by ultracentrifugations and/or immuno-isolation on paramagnetic beads. We found that the vesicles displayed a typical exosome-like size and morphology as analyzed by electron microscopy. Furthermore, they floated at a density between 1.10 and 1.18 g/ml in a sucrose gradient, corresponding to the known density of exosomes. In addition, MHC classes I and II, CD63, CD81, and CD86 were detected on the vesicles by flow cytometry. Western blot and mass spectrometry further confirmed the presence of several exosome-associated molecules. Functional analysis revealed that the vesicle preparation inhibited anti-CD3-induced IL-2 and IFN-gamma production from allogeneic and autologous PBMC. In addition, an increased number of Foxp3(+)CD4(+)CD25(+) T regulatory cells were observed in PBMC incubated with milk vesicle preparations. We conclude that human breast milk contains exosomes with the capacity to influence immune responses.

Influence of size, surface, cell line, and kinetic properties on the specific binding of A33 antigen-targeted multilayered particles and capsules to colorectal cancer cells

There has been increased interest in the use of polymer capsules formed by the layer-by-layer (LbL) technique as therapeutic carriers to cancer cells due to their versatility and ease of surface modification. We have investigated the influence of size, surface properties, cell line, and kinetic parameters such as dosage (particle concentration) and incubation time on the specific binding of humanized A33 monoclonal antibody (huA33 mAb)-coated LbL particles and capsules to colorectal cancer cells. HuA33 mAb binds to the A33 antigen present on almost all colorectal cancer cells and has demonstrated great promise in clinical trials as an immunotherapeutic agent for cancer therapy. Flow cytometry experiments showed the cell binding specificity of huA33 mAb-coated particles to be size-dependent, with the optimal size for enhanced selectivity at approximately 500 nm. The specific binding was improved by increasing the dosage of particles incubated with the cells. The level of specific versus nonspecific binding was compared for particles terminated with various polyelectrolytes to examine the surface dependency of antibody attachment and subsequent cell binding ability. The specific binding of huA33 mAb-coated particles is also reported for two colorectal cancer cell lines, with an enhanced binding ratio between 4 and 10 obtained for the huA33 mAb-functionalized particles. This investigation aims to improve the level of specific targeting of LbL particles, which is important in targeted drug and gene delivery applications.

T84-intestinal epithelial exosomes bear MHC class II/peptide complexes potentiating antigen presentation by dendritic cells

BACKGROUND & AIMS

Intestinal epithelial cells release antigen-presenting vesicles (exosomes) bearing major histocompatibility complex class II/peptide complexes stimulating specific immune responses in vivo. To characterize further the role of human epithelial exosomes in antigen presentation, their capacity to load antigenic peptides, bind immune target cells, and induce T-cell activation was analyzed in vitro.

METHODS

The capacity of exosomes derived from the HLA-DR4-expressing, intestinal epithelial cell line T84 to load the HLA-DR4-specific peptide (3)H-HSA 64-76 and to activate a HLA-DR4-restricted T-cell hybridoma was tested in the presence or absence of human monocyte-derived dendritic cells (DCs). Interaction of fluorescein isothiocyanate-labeled exosomes with T cells and DCs was analyzed by flow cytometry and confocal microscopy.

RESULTS

T84-derived exosomes, enriched in CD9, CD81, CD82, and A33 antigen, were capable of binding specifically human serum albumin (HSA) 64-76 peptide on HLA-DR4 molecules and of interacting preferentially with DCs. HSA-loaded exosomes were unable to activate the T-cell hybridoma directly but induced a productive T-cell activation through DCs. When HSA peptide was bound to exosomal HLA-DR4 molecules instead of in a soluble form, the threshold of peptide presentation by DCs was markedly decreased (x10(-3)).

CONCLUSIONS

Exosomes released by intestinal epithelial cells bear exogenous peptides complexed to major histocompatibility complex class II molecules and interact preferentially with DCs, strongly potentiating peptide presentation to T cells. Epithelial exosomes constitute a powerful link between luminal antigens and local immune cells by mediating the transfer of tiny amounts of luminal antigenic information and facilitating immune surveillance at mucosal surfaces.

In vitro and in vivo characterization of 177Lu-huA33: a radioimmunoconjugate against colorectal cancer

INTRODUCTION

The humanized monoclonal antibody A33 (huA33) is a potential targeting agent against colorectal carcinoma since the A33 antigen is highly and homogenously expressed in >95% of all colorectal cancers, both primary tumors and metastases. The aim of this study was to determine the biodistribution and tumor-targeting ability of (177)Lu-labeled huA33.

METHODS

huA33 was labeled with the beta-emitting therapeutic nuclide (177)Lu using the chelator CHX-A"-DTPA, and the properties of the (177)Lu-CHX-A"-huA33 ((177)Lu-huA33) conjugate was determined both in vitro and in vivo in a biodistribution study in nude mice xenografted with colorectal SW1222 tumor cells.

RESULTS

The (177)Lu-huA33 conjugate bound specifically to colorectal cancer cells in vitro (with a K(D) value of 2.3+/-0.3 nM, determined by a saturation assay) and in vivo. The tumor uptake of (177)Lu-huA33 was very high, peaking at 134+/-21%ID/g 72 h postinjection (pi). Normal tissue uptake was low; radioactivity concentration in blood (which had the second highest radioactivity concentration) was lower than in tumor at all time points studied (8 h to 10 days). The tumor-to-blood ratio increased with time, reaching 70+/-30, 10 days pi. Throughout the study, the uptake of (177)Lu in bone (known to accumulate free (177)Lu) was low, and the fraction of protein-bound (177)Lu in plasma samples was high (95% to 99%). This indicates high stability of the (177)Lu-huA33 conjugate in vivo.

CONCLUSION

The (177)Lu-huA33 conjugate shows a very favorable biodistribution, with an impressively high tumor uptake and high tumor-to-organ ratios, indicating that the conjugate may be suitable for radioimmunotherapy of colorectal cancer.

The Coxsackie and Adenovirus Receptor Binds Microtubules and Plays a Role in Cell Migration

The Coxsackie and adenovirus receptor (CAR), a cell adhesion molecule of the immunoglobulin superfamily, inhibits cell growth of a variety of tumors. The cytoplasmic domain of CAR has been implicated in decreased invasion and intracerebral growth of human U87 glioma cells. Using affinity binding, we identified tubulin as an interaction partner for the cytoplasmic domain of CAR. The interaction was specific; CAR and tubulin co-immunoprecipitated in cells expressing endogenous CAR and partially co-localized in situ. The binding of CAR to tubulin heterodimers and to microtubules was direct, with dissociation constants of approximately 1 mum for tubulin and approximately 32 nm for in vitro assembled microtubules. Whereas CAR-expressing U87 glioma cells had decreased migration in a chemotactic assay in Boyden chambers as compared with control cells, an effect that depended on the presence of the cytoplasmic domain of CAR, the difference was abrogated at low, non-cytotoxic doses of the taxane paclitaxel, a microtubule-stabilizing agent. These results indicate that CAR may affect cell migration through its interaction with microtubules.

A phase I radioimmunolocalization trial of humanized monoclonal antibody huA33 in patients with gastric carcinoma

In order to determine the in vivo characteristics of huA33, an open label dose escalation biopsy-based phase I clinical trial and radioimmunolocalization study were conducted with a complement determinant region-grafted humanized monoclonal antibody against the A33 antigen in patients with gastric carcinoma. Thirteen patients were entered onto one of four dose levels (1.0, 2.0, 5.0 or 10.0 mg/m(2)). Patients with locally advanced (UICC-TNM [International Union Against Cancer-tumor, node, metastasis] stage over 2 but resectable at clinical diagnosis) gastric carcinoma received a single infusion of (131)I-huA33 1 week prior to surgery. Adverse events were monitored, and imaging studies with gamma camera plus ex vivo scintigraphy of the resected specimen, biodistribution study by dosimetry analysis of the biopsied and resected tissues, and immunohistochemical analysis were carried out and evaluated. No dose-limiting toxicity was observed during the trial. Therefore, the maximum tolerated dose was not reached. Although cancer tissues with + intensity and <25% extent by immunostaining in biopsied frozen sections did not show positive imaging or postoperative dosimetry findings, cancers with ++ or +++ intensity or wide (>25%) extent by frozen and paraffin sections in the biopsied specimen showed positive ex vivo tumor images and positive antigen expression in resected gastric cancer specimens, and the biodistribution analysis showed tumor uptake of (131)I-huA33. In conclusion, humanized monoclonal antibody huA33 demonstrated selective localization to gastric cancer that expressed A33 antigen strongly. These excellent targeting characteristics of huA33 indicate potential for targeted therapy of advanced gastric cancer that is refractory to cytotoxic therapy, and could also be exploitable for curatively resected early gastric cancer in an adjuvant setting.

Targeted radionuclide therapy for solid tumors: an overview

Although radioimmunotherapy (RIT) has been effective in non-Hodgkin's lymphoma (NHL) as a single agent, solid tumors have shown less clinically significant therapeutic response to RIT alone. The clinical impact of RIT or other forms of targeted radionuclide therapy for solid tumors depends on the development of a high therapeutic index (TI) for the tumor vs. normal tissue effect, and the implementation of RIT as part of synergistic combined modality therapy (CMRIT). Preclinical and clinical studies have provided a wealth of information, and new prototypes or paradigms have shed light on future possibilities in many instances. Evidence suggests that combination and sequencing of RIT in CMRIT appropriately can provide effective treatment for many solid tumors. Vascular targets provide RIT enhancement opportunities and nanoparticles may prove to be effective carriers for RIT combined with intracellular drug delivery or alternating magnetic frequency (AMF) induced thermal tumor necrosis. The sequence and timing of combined modality treatments will be of critical importance to achieve synergy for therapy while minimizing toxicity. Fortunately, the radionuclide used for RIT also provides a signal useful for nondestructive quantitation of the influence of sequence and timing of CMRIT on events in animals and patients. This can be readily accomplished clinically using quantitative high-resolution imaging (e.g., positron emission tomography [PET]).

Frizzled-7 dictates three-dimensional organization of colorectal cancer cell carcinoids

Progression of colorectal cancer (CRC) involves spatial and temporal occurrences of epithelial-mesenchymal transition (EMT), whereby tumour cells acquire a more invasive and metastatic phenotype. Subsequently, the disseminated mesenchymal tumour cells must undergo a reverse transition (mesenchymal-epithelial transition, MET) at the site of metastases, as most metastases recapitulate the pathology of their corresponding primary tumours. Importantly, initiation of tumour growth at the secondary site is the rate-limiting step in metastasis. However, investigation of this dynamic reversible EMT and MET that underpins CRC morphogenesis has been hindered by a lack of suitable in vitro models. To this end, we have established a unique in vitro model of CRC morphogenesis, which we term LIM1863-Mph (morphogenetic). LIM1863-Mph cells spontaneously undergo cyclic transitions between two-dimensional monolayer (migratory, mesenchymal) and three-dimensional sphere (carcinoid, epithelial) states. Using RNAi, we demonstrate that FZD7 is necessary for MET of the monolayer cells as loss of FZD7 results in the persistence of a mesenchymal state (increased SNAI2/decreased E-cadherin). Moreover, FZD7 is also required for migration of the LIM1863-Mph monolayer cells. During development, FZD7 orchestrates either migratory or epithelialization events depending on the context. Our findings strongly implicate similar functional diversity for FZD7 during CRC morphogenesis.

The coxsackie- and adenovirus receptor (CAR) is an in vivo marker for epithelial tight junctions, with a potential role in regulating permeability and tissue homeostasis

The coxsackie- and adenovirus receptor (CAR) is a transmembrane protein belonging to the immunoglobulin superfamily. The function of CAR as a virus receptor has been extensively analyzed, while its physiological role and expression pattern in adult tissues have remained less clear. CAR associates with epithelial tight junctions in vitro and mediates cell-cell adhesion. Using a set of affinity-purified antibodies, we show that CAR is predominantly expressed in epithelial cells lining the body cavities in adult mice, where it specifically co-localizes with the tight junction components ZO-1 and occludin. Notably, CAR could not be detected in endothelial cells of the vasculature, including brain capillaries. CAR expression correlated positively with the maturity of tight junctions and inversely with permeability. With a few exceptions, the two known CAR isoforms were co-expressed in most epithelial cells analyzed. A CAR mutant lacking the intracellular tail over-expressed in transgenic mice was diffusely localized over the plasma membrane, showing the importance of this domain for correct subcellular localization in vivo. We conclude that CAR is localized to epithelial tight junctions in vivo where it may play a role in the regulation of epithelial permeability and tissue homeostasis.

Antibody-guided radiation therapy of cancer

Radioimmunotherapy (RIT) using radiolabeled monoclonal antibodies (MAbs) directed against tumor-associated antigens has evolved from an appealing concept to one of the standard treatment options for patients with non-Hodgkin's lymphoma (NHL). Inefficient localization of radiolabeled MAbs to nonhematological cancers due to various tumor-related factors, however, has refrained RIT from outgrowing the experimental stage in solid tumors. Still, small volume or minimal residual disease has been recognized as a potentially suitable target for radiolabeled antibodies. Several strategies are being explored aimed at improving the targeting of radiolabeled MAbs to solid tumors thus improving their therapeutic efficacy. In this review, a historical overview of the application of RIT is given and various aspects of the application of radiolabeled MAbs as anti-cancer agents are discussed. Finally, the clinical results of RIT of NHL, colorectal cancer, ovarian cancer, breast cancer, and renal cell cancer are reviewed.

In vitro characterization of 211 At-labeled antibody A33--a potential therapeutic agent against metastatic colorectal carcinoma

The humanized antibody A33 binds to the A33 antigen, expressed in 95% of primary and metastatic colorectal carcinomas. The restricted pattern of expression in normal tissue makes this antigen a possible target for radioimmunotherapy of colorectal micrometastases. In this study, the A33 antibody was labeled with the therapeutic nuclide (211)At using N-succinimidyl para-(tri-methylstannyl)benzoate (SPMB). The in vitro characteristics of the (211)At-benzoate-A33 conjugate ((211)At-A33) were investigated and found to be similar to those of (125)I-benzoate-A33 ((125)I-A33) in different assays. Both conjugates bound with high affinity to SW1222 cells (K(d) = 1.7 +/- 0.2 nM, and 1.8 +/- 0.1 nM for (211)At-A33 and (125)I-A33, respectively), and both showed good intracellular retention (70% of the radioactivity was still cell associated after 20 hours). The cytotoxic effect of (211)At-A33 was also confirmed. After incubation with (211)At-A33, SW1222 cells had a survival of approximately 0.3% when exposed to some 150 decays per cell (DPC). The cytotoxic effect was found to be dose-dependent, as cells exposed to only 56 DPC had a survival of approximately 5%. The (211)At-A33 conjugate shows promise as a potential radioimmunotherapy agent for treatment of micrometastases originating from colorectal carcinoma.

EpCAM and gpA33 are markers of Barrett's metaplasia

AIMS

To characterise a specific and sensitive marker of Barrett's metaplasia (BM).

METHODS

Cases of normal oesophageal squamous mucosa (11 fresh endoscopic biopsies and 10 formalin fixed, paraffin embedded tissue blocks), BM (11 biopsies and 11 tissue blocks), and normal gastric body mucosa (five biopsies and five tissue blocks) were analysed using reverse transcriptase PCR, Western blotting, and immunohistochemistry for EpCAM, and reverse transcriptase PCR for gpA33.

RESULTS

Strong EpCAM mRNA expression was detected in all the BM cases, in contrast to weak expression in all the normal gastric mucosal samples and no expression in any of the normal oesophageal mucosal samples tested. Strong gpA33 mRNA expression was detected in all the BM cases, in contrast to weak expression in a quarter of the normal gastric mucosal samples and no expression in any of the normal oesophageal mucosal samples tested. Western blotting showed EpCAM protein expression in all the BM cases and in none of the normal gastric or oesophageal mucosal samples tested. Immunohistochemistry showed strong EpCAM protein expression in BM and complete absence of expression in normal oesophageal squamous epithelium. Scattered EpCAM expressing cells were found in the gland bases of normal gastric body mucosa.

CONCLUSIONS

EpCAM protein and gpA33 mRNA expressions are specific and sensitive markers of BM.

The antibody repertoire in evolution: chance, selection, and continuity

All jawed vertebrates contain the genetic elements essential for the function of the adaptive/combinatorial immune response, have diverse sets of natural antibodies resulting from segmental gene recombination, express comparable functional repertoires and can produce specific antibodies following appropriate immunization. Profound variability occurs in the third hypervariable (CDR3) segments of light and heavy chains even within antibodies of the same ostensible specificity. Germline VH and VL elements, as well as the joining (J) segments are highly conserved among the distinct vertebrate species. Conservation is particularly noted among the VH3-like sequences of all jawed vertebrates in the FR2 and FR3 segments, as well as in the FGXGT(R or K)L J-segment characteristic of light chains and TCRs and the WGXGT(uncharged)VT JH segments. Human VH3-53 and Vlambda6 family orthologs may be present over the entire range of vertebrates. Models of the three-dimensional structures of shark VH/VL combining sites indicate similarity in framework structure and comparable CDR usage to those of man. Although carcharhine shark VH regions show greater than 50% identity to the human VH germline prototype, searches of lower deuterostome and invertebrate databases fail to detect molecules with significant relatedness. Overall, antibodies of jawed vertebrates show tremendous individual diversity, but are constructed incorporating design features that arose with the evolutionary emergence of the jawed vertebrates and have been conserved through at least 450 million years of evolutionary time.

A phase I trial of humanized monoclonal antibody A33 in patients with colorectal carcinoma: biodistribution, pharmacokinetics, and quantitative tumor uptake

PURPOSE

To determine the in vivo characteristics of huA33, a CDR-grafted humanized antibody against the A33 antigen, we have conducted an open-label, dose escalation, biopsy-based phase I trial of huA33 in patients with colorectal carcinoma.

EXPERIMENTAL DESIGN

Patients with colorectal carcinoma were infused with [131I]huA33 (400 MBq: 10 mCi) and [125I]huA33 (40 MBq: 1 mCi) 1 week before surgery. There were four huA33 dose levels (0.25, 1.0, 5.0, and 10 mg/m2). Adverse events, pharmacokinetics, biodistribution, tumor biopsies, and immune responses to huA33 were evaluated.

RESULTS

There were 12 patients entered into the trial (6 males and 6 females; age range, 39-66 years). No dose-limiting toxicity was observed. The biodistribution of huA33 showed excellent uptake of [131I]huA33 in metastatic colorectal carcinoma. Pharmacokinetic analysis showed no significant difference in terminal half-life (T1/2beta) between dose levels (mean +/- SD, 86.92 +/- 22.12 hours). Modeling of colon uptake of huA33 showed a T1/2 of elimination of 32.4 +/- 8.1 hours. Quantitative tumor uptake ranged from 2.1 x 10(-3) to 11.1 x 10(-3) %ID/g, and tumor/normal tissue and tumor/serum ratios reached as high as 16.3:1 and 4.5:1, respectively. Biosensor analysis detected low-level human anti-human antibody responses in four patients following huA33 infusion.

CONCLUSIONS

huA33 shows selective and rapid localization to colorectal carcinoma in vivo and penetrates to the center of large necrotic tumors, and colon elimination half-life of huA33 is equivalent to basal colonocyte turnover. The excellent targeting characteristics of this humanized antibody indicate potential for the targeted therapy of metastatic colorectal cancer in future trials.

Identification of adipocyte adhesion molecule (ACAM), a novel CTX gene family, implicated in adipocyte maturation and development of obesity

Few cell adhesion molecules have been reported to be expressed in mature adipocytes, and the significance of cell adhesion process in adipocyte biology is also unknown. In the present study, we identified ACAM (adipocyte adhesion molecule), a novel homologue of the CTX (cortical thymocyte marker in Xenopus) gene family. ACAM cDNA was isolated during PCR-based cDNA subtraction, and its mRNA was shown to be up-regulated in WATs (white adipose tissues) of OLETF (Otsuka Long-Evans Tokushima fatty) rats, an animal model for Type II diabetes and obesity. ACAM, 372 amino acids in total, has a signal peptide, V-type (variable) and C2-type (constant) Ig domains, a single transmembrane segment and a cytoplasmic tail. The amino acid sequence in rat is highly homologous to mouse (94%) and human (87%). ACAM mRNA was predominantly expressed in WATs in OLETF rats, and increased with the development of obesity until 30 weeks of age, which is when the peak of body mass is reached. Western blot analysis revealed that ACAM protein, approx. 45 kDa, was associated with plasma membrane fractions of mature adipocytes isolated from mesenteric and subdermal adipose deposits of OLETF rats. Up-regulation of ACAM mRNAs in obesity was also shown in WATs of genetically obese db/db mice, diet-induced obese ICR mice and human obese subjects. In primary cultured mouse and human adipocytes, ACAM mRNA expression was progressively up-regulated during differentiation. Several stably transfected Chinese-hamster ovary K1 cell lines were established, and the quantification of ACAM mRNA and cell aggregation assay revealed that the degree of homophilic aggregation correlated well with ACAM mRNA expression. In summary, ACAM may be the critical adhesion molecule in adipocyte differentiation and development of obesity.

Phenotypic and functional characterization of intestinal epithelial exosomes

Intestinal epithelial cells (IEC) are located at a strategic position between the external environment and the most extended lymphoid tissue in the body. Besides their central role in the absorption of nutrients, IEC also provide antigenic information to the immune system and are involved in the balance tolerance/allergy to food antigens. Like professional antigen presenting cells, IEC have been shown to secrete 30- to 90-nm diameter vesicles named exosomes, in a polarized way, either from their apical or basolateral side. These vesicles carry molecules involved in adhesion and antigen presentation, comprising major histocompatibility complex (MHC) class I and class II molecules, tetraspan proteins, CD26/dipeptidyl-peptidase IV, and A33 antigen, a molecule essentially restricted to the intestinal epithelium. Invariant chain, transferrin receptor, and Na-K-ATPase are not expressed on epithelial exosomes. In vivo, in mice, epithelial exosomes carrying MHC/ovalbumin peptide complexes induce specific immune responses when injected intraperitoneally. A33 antigen, an Ig-like molecule highly specific for intestinal epithelial cells and enriched in epithelial exosomes, is found at the surface of cells entering mesenteric lymph nodes suggesting exosome migration from the epithelial layer to the gut associated lymphoid system. Taken together, intestinal epithelial exosomes released at the basolateral surface of enterocytes could be antigen-carrying structures constituting a link between luminal antigens and the local immune system and acting as sensors of the antigenic information present in the intestinal lumen.

Phase I Trial of 131I-huA33 in Patients with Advanced Colorectal Carcinoma

PURPOSE

Humanized monoclonal antibody A33 (huA33) targets the A33 antigen which is expressed on 95% of colorectal cancers. A previous study has shown excellent tumor-targeting of iodine-131 labeled huA33 (131I-huA33). Therefore, we did a phase I dose escalation trial of 131I-huA33 radioimmunotherapy.

EXPERIMENTAL DESIGNS

Fifteen patients with pretreated metastatic colorectal carcinoma each received two i.v. doses of 131I-huA33. The first was an outpatient trace-labeled "scout" dose for biodistribution assessment, followed by a second "therapy" dose. Three patients were treated at 20, 30, and 40 mCi/m2 dose levels, and six patients at 50 mCi/m2 to define the maximum tolerated dose.

RESULTS

Hematologic toxicity was 131I dose-dependent, with one episode of grade 4 neutropenia and two episodes of grade 3 thrombocytopenia observed at 50 mCi/m2. The maximum tolerated dose was determined to be 40 mCi/m2. There were no acute infusion-related adverse events, and gastrointestinal toxicity was not observed despite uptake of 131I-huA33 in bowel. Seven patients developed pruritus or rash, which was not related to 131I dose. There was excellent tumor-targeting of 131I-huA33 shown in all patients. The serum T1/2beta of 131I-huA33 was (mean +/- SD) 135.2 +/- 46.9 hours. The mean absorbed tumor dose was 6.49 +/- 2.47 Gy/GBq. Four patients developed human anti-human antibodies. At restaging, 4 patients had stable disease, whereas 11 patients had progressive disease.

CONCLUSION

Radioimmunotherapy using 131I-huA33 shows promise in targeting colorectal tumors, and is deliverable at a maximum tolerated dose of 40 mCi/m2. Further studies of 131I-huA33 in combination with chemotherapy are planned.

Radioimmunotherapy and colorectal cancer

BACKGROUND

Despite the success of radioimmunotherapy (RIT) using radiolabelled monoclonal antibodies (Mabs) directed against tumour-associated antigens in the treatment of non-Hodgkin's lymphoma, therapeutic success in solid tumours has been modest. In the past decade, a dozen Mabs have been investigated clinically for their potential usefulness in RIT of colorectal cancer.

METHODS

The application of radiolabelled Mabs for the treatment of solid cancers is discussed, and clinical trials investigating RIT for colorectal cancer listed in the Medline and Embase databases are reviewed.

RESULTS

Uptake of radiolabelled Mabs in tumour and, consequently, the therapeutic efficacy of RIT is inversely correlated with tumour size. The bone marrow is the most important dose-limiting organ. Twenty-three phase I/II studies were found that investigated the feasibility and efficacy of RIT using five radionuclides and 15 Mabs against carcinoembryonic antigen, tumour-associated glycoprotein 72, epithelial cellular adhesion molecule, A33 or colon-specific antigen p, mainly in patients with advanced colorectal cancer. A few responses were recorded but no particular antibody construct seemed superior.

CONCLUSION

RIT might be an effective adjuvant treatment modality in colorectal cancer. Future studies should focus on its application in patients with small-volume or minimal residual disease.

Protein aggregation during overexpression limited by peptide extensions with large net negative charge

Folding of the human coxsackie and adenovirus receptor immunoglobulin (Ig) variable-type domain (CAR D1) during overexpression in the Escherichia coli cytoplasm was shown previously to be partially rescued by fusion to a 22-residue C-terminal peptide. Here, peptide sequence features required for solubilization and folding of CAR D1 and similar Ig variable-type domains from two other human membrane proteins were investigated. Peptide extensions with net negative charge > -6 fully solubilized CAR D1, and approximately half of the peptide-solubilized protein was correctly folded. The Ig variable-type domains from human A33 antigen and myelin P-zero proteins were only partially solubilized by peptide extensions with net charge of -12, however, and only the solubilized P-zero domain appeared to fold correctly whereas the A33 domain formed soluble microaggregates of misfolded protein. Our results suggest a model where the large net charge of peptide extensions increases electrostatic repulsion between nascent polypeptides. The resulting decrease in aggregation rate can enable some polypeptides to fold spontaneously into their native protein conformations. Analysis of the solubility and folding status of sets of structurally homologous proteins, such as the Ig variable-type domains described here, during overexpression could provide insights into how amino acid and gene sequences influence the efficiency of spontaneous protein folding.

Isolation of human prostate cancer cell reactive antibodies using phage display technology

Here we describe a phage display strategy for the selection of rabbit monoclonal antibodies that recognize cell surface tumor-associated antigens expressed in prostate cancer. Two immune rabbit/human chimeric Fab libraries were displayed on phage and used to search for tumor-associated antigens by panning on DU145 human prostate cancer cells. For this, we developed a novel whole-cell panning protocol with two negative selection steps designed to remove antibodies reacting with common antigens. After three rounds of subtractive panning, a majority of clones bound to DU145 cells as detected by flow cytometry. Among these, we identified several clones that bound selectively to DU145 cells but not to primary human prostate epithelial cell line PrEC. In summary, our work demonstrates the potential of immune rabbit antibody libraries for target discovery in general and the identification of cell surface tumor-associated antigens in particular.

The Coxsackie-Adenovirus Receptor—A New Receptor in the Immunoglobulin Family Involved in Cell Adhesion

The physiological and cell biological aspects of the Coxsackie-Adenovirus Receptor (CAR) is discussed in this review. The receptor obviously recognizes the group C adenoviruses in vivo, but also fibers from other groups except group B in vitro. The latter viruses seem to utilize a different receptor. The receptor accumulates at, or close to, the tight junction in polarized epithelial cells and probably functions as a cell-cell adhesion molecule. The cytoplasmic tail of the receptor is not required for virus attachment and uptake. Although there is a correlation between CAR and uptake of adenoviruses in several human tumor cells, evidence of an absolute requirement for integrins has not been forthcoming. The implication of these findings for adenovirus gene therapy is discussed.

Intestinal epithelial exosomes carry MHC class II/peptides able to inform the immune system in mice

BACKGROUND

Intestinal epithelial cells secrete exosome-like vesicles. The aim of this study was to characterise murine intestinal epithelial exosomes and to analyse their capacity to inform the immune system in vivo in mice.

METHODS

Epithelial exosomes were obtained from the murine epithelial cell line MODE K incubated in the presence or absence of interferon gamma (IFN-gamma) together with pepsin/trypsin ovalbumin hydrolysate (hOVA) to mimic luminal digestion. Exosomes isolated from MODE K conditioned media (EXO-hOVA and EXO-hOVA-IFN) were characterised by western blot, peptide mapping, and mass spectrometry. They were injected intraperitoneally to C3H/HeN mice to test their immunocompetence.

RESULTS

MODE K epithelial exosomes displayed major histocompatibility complex (MHC) class I and class II (upregulated by IFN-gamma) molecules and tetraspan proteins (CD9, CD81, CD82) potentially involved in the binding to target cells. A33 antigen, an Ig-like molecule highly specific for intestinal epithelial cells, was enriched in exosomes and was also found in mice mesenteric lymph nodes, suggesting exosome migration towards the gut associated lymphoid tissues. Intraperitoneal injection of EXO-hOVA or EXO-hOVA-IFN did not induce humoral or cellular tolerance to OVA in mice. In contrast, exosomes obtained after incubation with IFN-gamma (EXO-hOVA-IFN), bearing abundant MHC class II/OVA complexes, induced a specific humoral immune response.

CONCLUSIONS

Epithelial exosomes are antigen presenting vesicles bearing MHC class II/peptide complexes that prime for an immunogenic rather than tolerogenic response in the context of a systemic challenge. In the intestine, both the mucosal microenvironment and local effector cells are probably key players in determining the outcome of the immune response to exosome derived epitopes.

Colon cancer: prevalence, screening, gene expression and mutation, and risk factors and assessment

Colon cancer detection at an early stage and identifying susceptible individuals can result in reduced mortality from this prevalent cancer. Genetic events leading to the development of this cancer involve a multistage progression of adenoma polyps to invasive metastatic carcinomas. Currently, there is no satisfactory screening method that is highly specific, sensitive, or reliable. Dietary patterns associated with the greatest increase in colon cancer risk are the ones that typify a diet rich in fat and calories, and low in vegetable, fruits, and fibers. Genetic susceptibility to environmental carcinogenesis must be factored into the risk assessment for this cancer. Many genes have been shown to be associated with increased expression and mutations in colorectal cancer patients. These genes have been reviewed; it is hoped that by carefully selecting a number of them, a molecular approach that is suitable for arriving at a tumorigenic expression index is developed, which will reliably detect this cancer at an early stage (i.e., before it metastasizes), especially in exfoliated samples (e.g., stool and blood), so that appropriate intervention strategies can be implemented. Illustrated herein is the utility of employing real-time reverse transcriptase polymerase chain reaction (RT-PCR) to quantitatively measure gene expression, and develop an index that is specific for this cancer, which if perfected may result in a reliable and sensitive screening technique for colorectal cancer detection.

CLMP, a novel member of the CTX family and a new component of epithelial tight junctions

The CTX family is a growing group of type I transmembrane proteins within the immunoglobulin superfamily (IgSF). They localize to junctional complexes between endothelial and epithelial cells and seem to participate in cell-cell adhesion and transmigration of leukocytes. Here, we report the identification of a new member of the CTX family. This protein, which was designated CLMP (coxsackie- and adenovirus receptor-like membrane protein), is composed of 373 amino acids including an extracellular part containing a V- and a C2-type domain, a transmembrane region and a cytoplasmic tail. CLMP mRNA was detected in a variety of both human and mouse tissues and cell lines. The protein migrated with an Mr of around 48 on SDS-PAGE and was predominantly expressed in epithelial cells within different tissues. In cultured epithelial cells, CLMP was detected in areas of cell-cell contacts. When exogenously expressed in polarized MDCK cells, CLMP was restricted to the subapical area of the lateral cell surface, where it co-localized with the tight junction markers ZO-1 and occludin. Also endogenous CLMP showed association with tight junctions, as analyzed in polarized human CACO-2 cells. This suggested a role for CLMP in cell-cell adhesion and indeed, overexpressed CLMP induced aggregation of non-polarized CHO cells. Furthermore, CLMP-expressing MDCK cells showed significantly increased transepithelial resistance, indicating a role for CLMP in junctional barrier function. Thus, we conclude that CLMP is a novel cell-cell adhesion molecule and a new component of epithelial tight junctions. We also suggest, based on phylogenetic studies, that CLMP, CAR, ESAM, and BT-IgSF form a new group of proteins within the CTX family.

The JAM family of junctional adhesion molecules

Junctional adhesion molecules are a family of glycoproteins characterised by two immunoglobulin folds (VH- and C2-type) in the extracellular domain. Junctional adhesion molecule proteins localise to intercellular junctions of polarised endothelial and epithelial cells but can also be expressed on circulating leukocytes and platelets. In addition, they bind several ligands, in both a homophilic and heterophilic manner, and associate with several cytoplasmic partners. All these features represent the likely determinants for the role of junctional adhesion molecule proteins in processes as diverse as junction assembly, leukocyte transmigration and platelet activation.

A molecule bearing an immunoglobulin-like V region of the CTX subfamily in amphioxus

An expressed sequence tag with significant similarity to a vertebrate T-cell receptor (Tcr) sequence was found in a cDNA library prepared from the posterior part of the adult amphioxus, Branchiostoma lanceolatum ( Brla). Characterization of the corresponding cDNA clone revealed the presence of an open reading frame encoding a 351 amino acid residue-long polypeptide. The putative protein, tentatively designated Brla-VDB for "V-domain bearing", appears to consist of two domains. The N-terminal domain begins with a putative leader peptide followed by a sequence resembling the V domain of the CTX protein originally found to be expressed in the cortical thymocytes of the clawed frog Xenopus. The C-terminal domain of the VDB protein does not show significant sequence similarity to any entry in the databases and contains five hydrophobic segments separated by short intervening hydrophilic stretches. It may therefore belong to a protein that crosses the plasma membrane five times. These findings support the notion that V domains resembling those found in Tcrs evolved in nonvertebrates before the emergence of the adaptive immune system and may have participated in functions not involved directly in immunity.

Transcriptional regulation of A33 antigen expression by gut-enriched Krüppel-like factor

A33 antigen is a membrane-bound protein that is expressed only in intestinal epithelium and in most human colon cancers. Thus, A33 antigen has been explored as a potential therapeutic target for the treatment of colon cancers. However, little is known about the mechanism responsible for the tissue-specific pattern of its expression. In this report, we demonstrate that gut-enriched Krüppel-like factor (GKLF) binds to the promoter region of A33 antigen gene in colonic carcinoma cells and that mutations in the GKLF binding sequence in this region lead to diminished expression of A33 antigen. In addition, the expression of GKLF is linked to the expression of A33 antigen and blocking the expression of GKLF leads to the abolishment of A33 antigen expression. These results suggest that GKLF is a critical regulator in inducing the expression of A33 antigen in intestinal epithelium. While it has been suggested that GKLF is a regulator in inducing cell growth arrest and differentiation of the intestine, our observation that A33 antigen gene is a downstream target for GKLF suggests a more complex and diverse role for GKLF in the gut.

Adenovirus interaction with its cellular receptor CAR

Representative adenoviruses from four of the five major virus subgroups have been shown to interact with the 46-kDa coxsackievirus and adenovirus receptor (CAR) that is widely expressed on many human cell types, suggesting that the ability to bind CAR may be a conserved feature of many of the approximately 50 known adenovirus serotypes. Receptor binding is a function of the distal 'knob' domain of the trimeric viral fiber protein. Here we review recent structural characterizations of knob, CAR and knob-CAR complexes, and we discuss how knob architecture may have evolved to accommodate opposing selective pressures to vary antigenic structure while conserving receptor binding specificity. In contrast to the hypervariability of the solvent-exposed surface of knob, the CAR receptor was found to be non-polymorphic.

Advancing role of radiolabeled antibodies in the therapy of cancer

This review focuses on the use of radiolabeled antibodies in the therapy of cancer, termed radioimmunotherapy (RAIT). Basic problems concerned with the choice of antibody, radionuclide, and physiology of the tumor and host are discussed, followed by a review of the pertinent clinical publications of various radioantibody constructs in the treatment of hematopoietic and solid tumors of diverse histopathology, grade, and stage, and in different clinical settings. Factors such as dose rate delivered, tumor size, and radiosensitivity play a major role in determining therapeutic response, while target-to-nontarget ratios and, particularly, circulating radioactivity to the bone marrow determine the principal dose-limiting toxicities. RAIT appears to be gaining a place in the therapy of hematopoietic neoplasms, such as non-Hodgkin's lymphoma: several agents are advancing in clinical trials toward registration, and one has recently been approved by the FDA. Although RAIT of solid tumors has shown less progress, use of pretargeting strategies, such as an affinity-enhancement system consisting of bispecific antibodies separating targeting from delivery of the radiotherapeutic, appears to enhance tumor-to-nontumor ratios, and may increase radiation doses to tumors more selectively than directly labeled antibodies.

Analysis of the regulation of the A33 antigen gene reveals intestine-specific mechanisms of gene expression

The A33 antigen is a transmembrane protein expressed almost exclusively by intestinal epithelial cells. The level of its expression is robust and uniform throughout the rostrocaudal axis of the human and mouse intestines. In the colon, strong expression is found in the basolateral membranes of both the proliferating cells in the lower regions of the crypts and the differentiating cells in the upper regions of crypts. Similarly, in the small intestine, the protein is highly expressed by all the epithelial cells in the crypts and by the differentiated cells migrating over the villi. Thus, the A33 antigen has emerged as a definitive marker for all intestinal epithelial cells, irrespective of cell lineage and differentiation status. To understand the molecular mechanisms mediating this rare tissue-specific expression pattern, we undertook a comprehensive analysis of the 5'-regulatory region of the human A33 antigen gene. This allowed us to point to positive cis-regulatory elements incorporating consensus Krüppel-like factor and caudal-related homeobox (CDX)-binding sites, located just upstream from the human A33 antigen transcription start site, as being important for the intestine-specific expression pattern of this gene. Further analysis provided evidence that the A33 antigen gene may be one of only a few target genes to be described thus far for the intestine-specific homeobox transcription factor, CDX1. Taken together, our data lead us to propose that the activity of CDX1 is pivotal in mediating the exquisite, intestine-specific expression pattern of the A33 antigen gene.