Quantum dot 800-conjugated anti-Tn IgM 2154F12A4 murine monoclonal antibody

Excerpt

The Tn-antigen is a tumor-associated carbohydrate epitope (N-acetyl-galactosamine (GalNAc)-O-Ser/Thr (GalNAca-O-Ser/Thr)) (1-3). The anti-Tn 2154F12A4 murine monoclonal antibody (mAb) and its quantum dot (Qdot) 800 conjugate (Qdot 800-anti-Tn 2154F12A4 mAb) were developed by Danussi et al. for in vivo imaging and immunotherapy of Tn-expressing tumors (4).

Qdots are nanometer-sized crystals (typically 2–10 nm in diameter) (5-7). In contrast to organic fluorophores, Qdots have a continuous absorption spectrum for wavelengths shorter than the wavelengths of fluorescence emission. The emission spectra are quite narrow and symmetric. Many different colors can be excited with one wavelength of excitation, and the colors can be spectrally well resolved. Qdots exhibit good quantum yields (40–90%) and high extinction coefficients, and they are 20 times as bright as and 100 times as stable against photobleaching as conventional fluorophores. Qdots can be coated with hydrophilic materials and functionalized with antibodies, peptides, nucleic acids, and various compounds. Because of these unique photophysical properties, Qdots have been intensively investigated as biosensors and labels in biological imaging (5-7). However, most Qdots are toxic (8). The most widely used and studied Qdots consist of a core of cadmium selenide or telluride because their quantum confinement regions span the entire optical spectrum. Cadmium ions are able to bind to thiol groups on critical molecules in the mitochondria to cause significant cell death. Qdots may also damage DNA and disrupt normal cell activity from factors such as the surface coatings themselves (8).

Altered glycosylation on the cell surface is a hallmark of malignant transformation and tumor progression. Incomplete synthesis of the carbohydrate chains and precursor accumulation result in loss of the normal carbohydrate antigens and high expression of the tumor-associated carbohydrate antigens (9-11). Lewis Y, TF, Globo H, GM2, polysialic acid, sialyl Lewis A, Tn, and sialyl Tn are some of the antigens investigated intensively as diagnostic markers or as vaccine antigens (10-13). The Tn antigen was first reported as a tumor-associated antigen nearly 40 years ago (14). It is composed of a single GalNAc glycan residue attached via an α-linkage to either the serine (Ser) or the threonine (Thr) of a polypeptide chain (11, 13). In normal tissues, the Tn antigen is generally masked by covalently bound terminal carbohydrate moieties, but in tumors it is unmasked because of defective O-glycosylation. Accordingly, the Tn antigen is rarely expressed in normal tissues, but it is widely expressed in human carcinomas or hematological cancers. It has been reported that the Tn antigen is expressed in 70–90% of breast, colon, lung, bladder, cervical, ovarian, stomach, and prostate tumors (1, 3, 9). The expression levels of the Tn antigen are closely associated with tumor aggressiveness and poor survival of patients (10, 12). In addition, the Tn antigen is recognized by the human immune system as a novel epitope, provoking immune responses in patients. There is a strong correlation among the expression of the Tn antigen, the development of the spontaneous antibodies against Tn, and the prognosis of patients with carcinomas. Clinical trials are under way to deliberately provoke or enhance human immune responses by injecting patients with synthetic peptide antigens bearing Tn structure (3, 10, 15-17).

The Tn antigen has attracted significant interest as a target for tumor diagnosis and immunotherapy. A number of anti-Tn IgG and IgM antibodies have been generated and investigated for their imaging feasibilities and anti-tumor activities (2, 18-25). The results are generally inconsistent. There are still some issues to be resolved, such as immunogenicity, reduced effectiveness in vivo, and cross-reactivity against type-A blood antigen. Danussi et al. developed an anti-Tn IgM mAb and further tested its imaging feasibility and antitumor activity (4).