CUTTING-EDGE CANCER TREATMENTS (P-1)

Peptide Vaccines -- An alternative to whole cell vaccines?

The creation of a whole-cell vaccine has prompted research on vaccines that are "easier" to create. Whole cell vaccines consist of tumor cells that are taken from the patients body and grown in culture. The tumor cells are then irradiated, engineered to secrete cytokines such as IL-2 or GM-CSF, and injected back into the body. The cytokines signal macrophages, which subsequently ingest the cells and present the processed antigens on their MHC class I receptors. The drawback of the whole cell vaccines are that many tumor types can not be grown in culture and the time involved to culture, which is at least three weeks. (1)

In response to the need for a cell-free alternative, the peptide vaccine was engineered. The peptide vaccine consists of tumor-derived antigens that are normally presented on MHC-class I receptors. Malignant melanomas present small peptides in the MHC I on their surface. These peptides, however, are not immunogenic.(1) Researchers discovered that injection of a polycation adjuvant, like pArg or pLys, along with the peptides resulted in an immune response.(2)

The vaccines were injected into mice before they were injected with tumor cells to test the peptides as possible vaccines and they were injected into mice that already had melanoma. To test the peptides as vaccines, the researchers injected the mice with peptide derived from JAK-1 kinase (frequently found on the MHC I of mice melanoma cells), along with the polycation adjuvant. Tumor cells were then injected. About 25-30% of the mice demonstrated protection against tumor formation, while 50% of mice injected with the whole cell vaccine were protected. In mice with preexisting tumor, vaccination shrunk the tumor 50% of the time, compared with 70% of tumors shrunk in mice injected with the whole cell vaccine.(1)

Peptide vaccines, when injected, are taken up by macrophages. The polycation adjuvant serves not only to increase the immunogenicity of the peptides, but also facilitates the uptake of the peptides into the macrophage. The peptides are then processed and presented on MHC-class I molecules. This same mechanism holds true for whole cell vaccines. So why, if the cells or peptides are introduced by injection, are they presented on MHC I and not MHC II? According to the researchers, "recent investigations have shown that there is considerable plasticity in the processing and compartmentalization of antigens and their peptides...and their presentation on MHC molecules." (1) The petides are also designed to only fit onto MHC I molecules. A cell-mediated response is important because cancer genes are altered self-cells. Therefore an antiboby response would not be appropriate; the problem exists within the cells.

In a human trial, melanoma patients were injected with peptides from the tumor antigen MAGE-3. The tumor shrinkage occured at a low frequency, probably due to the poor immunogenicity of the peptide. (2) Peptide vaccines, while not as effective as whole cell vaccines, are a good alternative for those patients with tumors that can not be cultured. The vaccines also serve a dual purpose: they can prevent tumors and shrink existing ones.

Antiangiogenesis: Can cytokines cause the starvation of cancer cells?

Angiogenesis is the infrequent occurence of vascular endothelial cell division. It occurs after tissue injury and during menstruation to repair damaged vascular endothelium (3).

Tumor cells can not grow more than 1mm3 above the endothelial surface. All endothelial cells are close to vascular endothelium, from which they are nourished. If tumor cells, which are rapidly dividing, reach 1mm3 they are forced to stop growing because they can no longer receive nutrients from the vascular endothelium. The successful tumor, in order to continue growing, must turn on angiogenic factors (3).

The tumor cells undergo mutations at a high rate. Within the tumor mass, many cells are growing and many cells are dying. Cells resistant to death are selected for. P53 is usually mutated in these death resistant cells (p53 regulates the cell cycle and cell death). These death-resistant cells keep dividing and keep mutating until they mutate the gene that regulates angiogenic factors. The mutations cause production of VEGF (vascular endothelial growth factor) and the down-regulation of anti- angiogenic factors (4).

After angiogenic factors are produced the tumor and the cell begin to grow towards each other. The endothelial basement membrane is degraded by proteinases produced in response to the angiogenic factors. The resultant vessels are abnormal in size and shape, but can adequately nourish the tumor cells(4).

The discovery of the regulation of angiogenesis and the isolation of antiangiogenic factors allows science to fight back. Purified antiangiogenic factors can be used to stop the further growth of tumors. TNF-alpha is an antiangiogenic factor that is currently being used to treat limb metastases in melanoma. It can't be used anywhere else because of its toxicity. Researchers are now looking into gene therapy to put the TNF alpha under the control of a promoter active in tumors(4).

Other antiangiogenic cytokines, like IL-2, IL-12, IFN alpha and IFN gamma, are also being researched for their effectiveness. The hope is that the tumor cells will stop growing due to the decrease in vasculature(4).

How do chemoattractant proteins prevent the widespread application of T-cell therapy?

The T-cell's ability to eradicate malignant tumors has been demonstrated in vitro, but use of this technology is hindered by the inability to isolate a sufficient quantity of tumor specific T-cells from a tumor bearing host. A research team consisting of Liamin Peng, Suyu Shu, and John C. Krauss published findings that could make T-cell therapy an effective alternativein treating cancer. Their results suggest that the insufficient quantity of tumor specific T-cells is due to chemoattractants for macrophages. Therefore, suppression of inhibitory chemoattractants in cancer patients could increase the number of tumor specific T-cells(5).

Liamin Peng, Suyu Shu, and John C. Krauss experimented on female mice strain C57B1/6J. The tumor strain they infected the mice with was MCA205 sarcoma 3-methylcholanthiene. They injected half the mice with a control serum and the other half with MCP-1 neutralizing serum. In nine days they harvested the cells, and in vitro, activated the T-cells on a single cell suspension with anti-CD3/IL-2. Next, the mice were injected with MCA205 cells in 1ml of HBSS, and after 18 days they were sacrificed to isolate the pulmonary metastase. The cells were analyzed by immunohistochemical staining, flow cytometry, and ELISA (5).

The results of the experiment provides evidence that the endogenous production of chemoattractant MCP-1 in cancerous cells exerts a negative regulatory influence on the development of tumor sensitized T-cells. The first reason is that when T-cells isolated from the mice infected with MCP-1 were expanded by anti CD3/IL-2, a net expansion of six-fold more than the control occured. Another result supporting this conclusion is, the T-cells from anti-MCP-1 treated mice were more active in eliminating pulmonary mestastases than the control mice. Tumor-sensitized T-cells release IFN-gamma in response to tumor antigen. T-cells from the anti-MCP-1 treated mice secreted twice as much IFN-gamma than the control(5).

MCP-1 has a negative impact on the generation of a therapeutic T-cell response to tumors. The production of MCP-1 inhibits the TH1 cytokine response. Inhibition of cytotoxic T-cell activity prevents the elimination of tumor cells. In the experiment, the mice with inhibited MCP-1 production did not produce IL-4 or IL-10. Both of these cytokines inhibit the TH1 response. Other evidence that MCP-1 inhibits the TH1 response is, when MCP-1 levels are increased, the ability to eliminate tuberculosis is diminished. Tuberculosis, like tumor cells, is most effectively eliminated by cytotoxic T-cells(5).


The increased levels of macrophages in cancer cells correspond to chemoattractant activity. When normal cells encounter antigens, an infiltration of macrophages is usually effectiive in aiding in an effective immune response. For example, fungus inhaled by the lungs is neutralized by an increase of macrophages in the pulmonary tissues. However, in a cancerous situation, cytotoxic T-cells are most effective in neutralizing the antigen. This experiment demonstrates that chemoattractants for macrophages can inhibit the activity of tumor sensitive T-cells. One step in enabling T-cell therapy to become a widespread practice is to inactivate chemoattractants that inhibit tumor specific T-cell activity (5).