MadSci Network: Cell Biology
Query:

Re: If you injected a disease, into a tumor would T cells attack?

Date: Tue Jun 30 11:07:52 2009
Posted By: Sarah Earley, Postdoctoral Fellow
Area of science: Cell Biology
ID: 1238183435.Cb
Message:

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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