MadSci Network: Cell Biology |
Hi Kaitlyn, Good question! This is a key question in a field of study called tumor immunology. Your teacher is right that tumor cells are derived from “self” cells, but sometimes tumor cells differ from normal cells enough that the immune system recognizes them as aberrant, and responds. This is because cancer results from numerous genetic mutations, which make them different from normal cells. Often, however, the immune system does not or cannot respond effectively to tumors. Theoretically, modifying tumor cells to make them more obvious to the immune system would promote tumor rejection by the immune system. This is a strategy that some scientists are using to fight cancer. I will try to explain the general mechanism by which T cells recognize infected or transformed cells, in hopes of addressing your specific question. All cells in the body express major histocompatibility complex (MHC) class I molecules, which are receptors on the cell surface that bind to small pieces of protein called peptides. If a cell is normal and uninfected by pathogens, the peptides displayed by the MHC molecules are “self” peptides. T lymphocytes (T cells) recognize the peptide-bound MHC molecules as self and leave the cell alone (usually—autoimmune diseases are an exception). Unlike MHC class I, MHC class II molecules are expressed primarily on certain white blood cells called antigen presenting cells, but the concept is the same in that MHC class II molecules also present peptide to T cells. MHC class I presents peptide to cytotoxic T cells, whereas MHC class II presents peptide to helper T cells. Injecting a foreign gene (DNA) into a tumor might result in a T cell response if introduction of the gene leads to expression of foreign protein that is processed into peptide and loaded onto MHC molecules, which then present the foreign peptide to T cells. Many events must occur successfully for this scenario to be effective, though. Biological systems are complex and not always predictable based on our current knowledge, so what should work does not always end up working. Sometimes cancer cells present aberrant proteins naturally. When cancer cells present peptides derived from mutant proteins, these peptides are called tumor antigens. T cells recognize these tumor antigens as foreign, and attack the cell. Because most cancer cells do not express MHC class II molecules, the T cell response is typically only by cytotoxic T cells. Having both the cytotoxic and helper T cell responses provides a stronger immune response to cancer than the cytotoxic T cell response alone, and scientists have shown that for some types of cancer, modifying cancer cells to express MHC class II molecules (which might then present tumor antigens to helper T cells) can lead to tumor rejection. Promoting the T cell response to tumors is just one type of immunotherapy. Other immunotherapies include the use of monoclonal antibodies that bind to receptors on the surface of cancer cells, factors secreted by cells, and vaccines that promote an immune response to tumors. Interestingly, introduction of a bacterium called BCG is sometimes used to treat bladder cancer. You can find more information about this in the "Other Uses" section of the following site: http://en.wikipedia.org/wiki/BCG_vaccine Tumor immunology is complicated, and considerably more research must be carried out to determine how the immune system responds to tumors. This information will allow scientists to design therapies that make the immune response to tumors more effective. Perhaps you will be one of these scientists! For more information about this topic, visit the National Cancer Institute website (www.nci.gov) and search for “immunotherapy.” I hope this helps answer your question! Sarah Earley Postdoctoral Fellow Albany Medical College
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