Antigen-specific, MHC-unrestricted T cells

Human MUC1 Mucin: A Multifaceted Glycoprotein

Human MUC1 mucin, a membrane-bound glycoprotein, is a major component of the ductal cell surface of normal glandular cells. MUC1 is overexpressed and aberrantly glycosylated in carcinoma cells. The role MUC1 plays in cancer progression represents two sides of one coin: on the one hand, loss of polarity and overexpression of MUC1 in cancer cells interferes with cell adhesion and shields the tumor cell from immune recognition by the cellular arm of the immune system, thus favoring metastases; on the other hand, MUC1, in essence a self-antigen, is displaced and altered in malignancy and induces immune responses. Tumor-associated MUC1 has short carbohydrate sidechains and exposed epitopes on its peptide core; it gains access to the circulation and comes into contact with the immune system provoking humoral and cellular immune responses. Natural antibodies to MUC1 present in the circulation of cancer patients may be beneficial to the patient by restricting tumor growth and dissemination: early stage breast cancer patients with a humoral response to MUC1 have a better disease-specific survival. Several MUC1 peptide vaccines, differing in vectors, carrier proteins and adjuvants, have been tested in phase I clinical trials. They are capable of inducing predominantly humoral responses to the antigen, but evidence that these immune responses may be effective against the tumor in humans is still scarce.

Intradermal vaccination of MUC1 transgenic mice with MUC1/IL-18 plasmid DNA suppresses experimental pulmonary metastases

MUC1 (mucin 1) is a transmembrane glycoprotein normally expressed on epithelia of the pancreas, breast, prostate, colon, and lung. However, this self-antigen is over-expressed and aberrantly glycosylated in adenocarcinomas, thereby making it a potential target for immunotherapy. Toward this goal, DNA plasmids encoding human MUC1 (pMUC1) and mouse interleukin-18 (pmuIL-18) were developed, and previous work demonstrated pMUC1/pmuIL18 vaccination protected MUC1 transgenic mice (MUC1.Tg) from subcutaneous tumor challenge. This report shows that pMUC1/pmuIL-18 is effective in preventing and treating pulmonary metastases in MUC1.Tg mice. Vaccination with pMUC1 or pmuIL-18 alone was insufficient to elicit measurable anti-tumor effects. However, co-administration of pMUC1 with pmuIL-18 reduced the incidence of lung tumors and prolonged survival. Furthermore, pMUC1/pmuIL-18 immunization protected mice from challenge with MUC1+ tumors, but not from MUC1- tumors, indicating that the anti-tumor effect is antigen-specific. More importantly, pMUC1/pmuIL-18 was effective in treating established tumors. Finally, in vivo antibody-mediated lymphocyte depletion and neutralization of interferon gamma (IFNgamma) revealed that CD8+ T cells and IFNgamma mediate the anti-tumor immunity. Collectively, these results demonstrate that pMUC1/pmuIL-18 breaks tolerance to MUC1, and induces antigen-specific immunity with protective and therapeutic benefit. This suggests that pMUC1/pmuIL-18 DNA vaccination may provide clinical benefit for patients with MUC1+ tumors.

Intercellular and Intracellular Events Following the MHC-Unrestricted TCR Recognition of a Tumor-Specific Peptide Epitope on the Epithelial Antigen MUC1

We examined the functional and molecular parameters involved in direct TCR recognition of a tumor-specific peptide epitope on the tumor Ag MUC1. This peptide epitope is tandemly repeated and recognized on the native molecule rather than processed and bound to the MHC. Even though the TCR was not MHC restricted, intercellular interactions found to facilitate this recognition included intercellular adhesion molecule-1/LFA-1, LFA-3/CD2, and class I/CD8. Intracellular parameters of MHC-unrestricted CTL activation were examined to compare the recognition of the MUC1 epitope presented on synthetic microspheres, with the recognition of the native epitope in the context of other molecules on the target cells. The epitope on microspheres induced a transient influx of Ca2+ that was not accompanied by detectable tyrosine phosphorylation of the ζ-associated protein ZAP-70, whereas recognition of MUC1 epitopes on tumor cells caused a sustained Ca2+ influx and ZAP-70 phosphorylation. The transient influx of Ca2+ was not sufficient to cause translocation of the nuclear factor of activated T cells (NF-AT) into the nucleus or CTL proliferation. In contrast, recognition of the MUC1 epitope on tumor cells resulted in full activation of the CTL, nuclear translocation of NF-AT, and proliferation. MHC-unrestricted TCR triggering, therefore, involves similar intercellular and intracellular events that participate in the conventional, MHC-restricted Ag recognition. Direct recognition of the MUC1 peptide epitope by the TCR in the absence of presentation by the MHC induces a partial signal that is completed by further interactions of other receptor/ligand pairs on the surface of the CTL and their target cells.

Differential glycosylation of MUC1 in tumors and transfected epithelial and lymphoblastoid cell lines

The membrane-bound mucin-like protein MUC1 with a specified number of tandem repeats has been expressed by transfection of the cDNAs in both the epithelial cell lines MDCK and LLC-PK1, and human lymphoblastoid cell lines T2 and C1R. The structure and glycosylation states of the MUC1 in these four lines were compared with that of the endogenous MUC1 found in the human pancreatic (HPAF) and breast (BT-20) tumor cell lines using flow cytometry and Western blot analysis with anti-MUC1 antibodies, which are either sensitive or insensitive to the glycosylation state of the tandem repeat, and pretreatment of cells with phenyl-alpha-galactosaminide, an inhibitor of mucin sialylation. A similar analysis of MUC1 expression in transfected normal and O-glycosylation defective CHO cells reveals that the addition of galactose to the core oligosaccharide structure is apparently responsible for the anomalous difference in M(r), between the mature and propeptide forms of the MUC1. Both the tumor cells and the transfected lymphoblastoid cells consistently express significant steady state levels of both the heavily glycosylated mature forms and the poorly glycosylated propeptide forms of the MUC1, whereas MUC1 is found predominantly as the mature extensively glycosylated species in the transfected epithelial cells. Immunofluoresence microscopy of cross sections of the polarized epithelial cells grown on culture filter inserts reveals that the MUC1 is clearly present at the apical surface of the cells, consistent with its expression in normal tissues. Thus, the successful expression of the MUC1 by transfection of either lymphoblastoid cells or epithelial cells yields model systems both for studying the natural structure/function relationships of the protein domains within the MUC1 molecule and for further elucidating the previously reported MHC-independent T-cell recognition of the MUC1.

Intramuscular immunisation with MUC1 cDNA can protect C57 mice challenged with MUC1-expressing syngeneic mouse tumour cells

Much interest is currently being shown in immunotherapy as a treatment for cancer since several tumour-associated antigens have been identified and the genes encoding them cloned. One such molecule is the tumour-associated human MUC1 gene product. In this report we describe tumour rejection studies in a C57B1 murine model system with syngeneic MUC1-expressing tumour cells designed to examine the efficacy of MUC1 cDNA as an immunogen. Intra-muscular immunisation with 100 microgram MUC1 cDNA 3 times at 3-weekly intervals resulted in tumour protection in approximately 80% of mice. Tumour protection was dose-dependent, with 50-100 microgram being the most effective dose. Both humoral and cell-mediated MUC1-specific immune responses were detected. Anti-MUC1 antibodies were detected after immunisation with DNA alone, indicating that the injected DNA was expressed. Humoral immune responses did not correlate with tumour rejection. Tumour challenge with syngeneic tumour cells expressing MUC1 appeared to be a pre-requisite for the generation of MUC1-specific cytotoxic T lymphocytes.

Induction of cellular immunity in chimpanzees to human tumor-associated antigen mucin by vaccination with MUC-1 cDNA-transfected Epstein-Barr virus-immortalized autologous B cells

Aberrant glycosylation of the mucin molecule (encoded by the gene MUC-1) on human epithelial cell tumors leads to the exposure of tumor-associated epitopes recognized by patients' antibodies and cytotoxic T cells. Consequently, these epitopes could be considered targets for immunotherapy. We designed a cellular vaccine, employing, instead of tumor cells, autologous Epstein-Barr virus (EBV)-immortalized B cells as carriers of tumor-associated mucin, to take advantage of their costimulatory molecules for T-cell activation. The vaccine was tested in chimpanzees because of the identity of the human and chimpanzee MUC-1 tandem repeat sequence. EBV-immortalized B cells derived from two chimpanzees were transfected with MUC-1 cDNA, treated with glycosylation inhibitor phenyl-N-acetyl-alpha-D-galactosaminide to expose tumor-associated epitopes, irradiated, and injected subcutaneously four times at 3-week intervals. One vaccine preparation also contained cells transduced with the interleukin 2 (IL-2) cDNA and producing low levels of IL-2. Already after the first injection we found in the peripheral blood measurable frequency of cytotoxic T-cell precursors specific for underglycosylated mucin. The highest frequency observed was after the last boost, in the lymph node draining the vaccination site. Delayed-type hypersensitivity reaction to the injected immunogens was also induced, whereas no appearance of mucin-specific antibodies was seen. Long-term observation of the animals yielded no signs of adverse effects of this immunization. Autologous antigen-presenting cells, like EBV-immortalized B cells, expressing tumor-associated antigens are potentially useful immunogens for induction of cellular anti-tumor responses in vivo.

Structure of a tumor associated antigen containing a tandemly repeated immunodominant epitope

Human mucins are T or S glycosylated tandem repeat proteins. In breast cancer, mucins become under or unglycosylated. Two-dimensional nuclear magnetic resonance experiments are performed on chemically synthesized mucin tandem repeat polypeptides, (PDTRPAPGST-APPAHGVTSA)n the unglycosylated form for n=1,3 where (APDTR) constitutes the antigenic sites for the antibodies isolated form the tumors in the breast cancer patients. These studies demonstrate how the tandem repeats assemble in space giving rise to the overall tertiary structure, and the local structure and presentation of the antigenic site(APDTR) at the junction of two neighboring repeats. The NMR data reveal repeating knob-like structures connected by extended spacers. The knobs protrude away from the long-axis of Muc-1 and the predominant antigenic site (APDTR) forms the accessible tip of the knob. Multiple tandem repeats enhance the rigidity and presentation of the knob-like structures.

MUC-1 epithelial tumor mucin-based immunity and cancer vaccines

Many obstacles still stand in the way to eliciting an effective immune response against cancer, even though several antigens and antigenic peptides have been identified as potential tumor targets. All of them, including the MUC-1 mucin, share the caveat of being normal cellular proteins. Unlike all the others, however, MUC-1 expressed on tumors can still be considered a truly tumor-specific antigen. Its expression on normal cells is hidden from the immune system, and its aberrant glycosylation on tumors creates new epitopes recognized by the immune system. Moreover, all other tumor targets identified so far are MHC-restricted peptides that can only be recognized by patients who carry a specific HLA type, or on tumors which continue to express particular HLA alleles. MUC-1 is powerfully different. Recognized as a native molecule independent of MHC, it is a universal immunogen and a universal target, and if made effectively immunogenic, it would be expected to elicit immune responses in all patients, and against numerous MUC-1 expressing human tumors. It may, in fact, be the extraordinary solution to an extraordinary problem of cancer immunity and immunotherapy.

Specific and effective T-cell recognition of cells transfected with a truncated human mucin cDNA

Epithelial cell mucin has been characterized as a tumor-specific antigen in patients with pancreatic and breast cancer. Mucins are high molecular weight glycoproteins consisting of a heavily glycosylated tandemly repeating 20-amino acid sequence. Aberrant glycosylation of mucins on carcinomatous epithelial cells leads to the exposure of novel core epitopes that are recognized by cytotoxic T lymphocytes (CTLs). We previously reported the establishment of mucin-specific CTL clones that recognize mucin expressed on the surface of EBV-immortalized B cells transfected with the mucin cDNA (MUC1). This recognition was characterized as major histocompatibility complex (MHC)-unrestricted, because of the multivalent nature of mucin. The transfectants had to be incubated with an inhibitor of O-linked glycosylation, phenyl-N-acetyl-alpha-galactosaminide (phenyl-GalNAc) in order to unmask the tandem repeat core epitope recognized by CTLs. In the present study, we examined whether mucin molecules with fewer tandem repeats are capable of MHC-unrestricted recognition by mucin-specific CTL clones. A mucin cDNA expression vector expressing a "truncated" mucin molecule that contains only two tandem repeats was constructed. We found that mucin-specific CTL clones recognize the "truncated" mucin on allogeneic target cells, showing that recognition in this case was MHC-unrestricted as well. In addition, CTL clones lysed "truncated" mucin transfectants significantly better than full-length mucin transfectants treated with phenyl-GalNAc, and controls. The "truncated" construct may represent an effective means of immunizing patients with breast and pancreatic cancer, enabling them to mount a strong and efficient immune response against mucin-bearing tumor cells.