Oct 08, 2009 (National Institutes of Health Documents and
Publications/ContentWorks via COMTEX) -- U.S. Department of Health and Human
Services NATIONAL INSTITUTES OF HEALTH NIH News National Cancer Institute (NCI)
www.nci.nih.gov For Immediate Release: Wednesday, October 7, 2009
PROSTATE TUMORS CAN CHANGE THE FUNCTION OF IMMUNE CELLS IN MICE
Researchers have discovered that prostate tumors in mice can cause immune cells
known as CD8+ T cells to change their function from cells that have antitumor
activity to cells that suppress immune responses. This finding, by researchers
at the National Cancer Institute (NCI), part of the National Institutes of
Health, has important implications for the design of immune-based therapies for
cancer. The new study, available online, appears in the Oct. 15, 2009, issue of
the Journal of Immunology.
"The conversion of CD8+ T cells into suppressor cells may be one of the
mechanisms by which tumors restrict the immune system's ability to control tumor
growth," said Arthur A. Hurwitz, Ph.D., head of the Tumor Immunity and Tolerance
Group at NCI's Center for Cancer Research. "Studying this process in mice may
help explain why some cancer patients have an initial response from their
immune-based therapy, but this response fails with time."
In mice and humans, when the immune system encounters a pathogen or other
foreign invader, it responds by mounting an immune response. Part of this
response involves the recruitment and activation of CD8+ T cells, which are also
called cytotoxic T cells or killer T cells, to help destroy the invader. CD8+ T
cells also play a role in immune responses against tumor cells. Other T cells,
known as CD4+ T regulatory cells, work to suppress CD8+ T cell activity. Immune
suppression by these regulatory T cells helps prevent the body from attacking
its own cells. A high level of CD4+ T regulatory cells is also associated with
poor prognosis of some cancers. Moreover, research in mice has shown that
blocking the immune suppressive activity of these regulatory T cells enhances
the body's immunity against tumors, causing tumor growth to slow and improving
the antitumor immune responses elicited by cancer vaccines.
Recent evidence in mice has suggested that CD8+ T cells can develop suppressive
activities similar to those of CD4+ T regulatory cells. In addition, CD8+
suppressor cells have been found in cancer patients. The presence of these
suppressor cells could explain earlier findings by Hurwitz's team that prostate
tumor-specific CD8+ T cells injected into prostate tumor-bearing mice migrate to
the tumors but then become unresponsive, or tolerized, to the tumor cells. It
remained unclear, however, whether the suppressive CD8+ T cells have suppressor
activity before they reach the tumor or whether they are converted into
suppressor cells by the tumor.
In the new research, Hurwitz's team found that CD8+ T cells acquire immune
suppressive functions after they enter the mouse tumor microenvironment, which
encompasses nearby noncancerous cells and immune cells in addition to tumor
cells. The researchers found that tumor-specific CD8+ T cells isolated from the
tumors were able to suppress the proliferative capacity of nonspecific T cells,
whereas tumor-specific CD8+ T cells isolated from lymph nodes of the mice were
unable to do so.
This anti-proliferative activity appeared to be caused, in part, by substances
secreted by the CD8+ T cells after they had been converted to suppressor cells.
One of these substances, TGF-beta, is a protein that controls cell proliferation
and differentiation and plays a role in cancer and other diseases. TGF-beta is
thought to be involved in the immune-suppressive activity of CD4+ T-regulatory
cells.
Next, the team investigated whether the conversion of tumor-specific CD8+ T
cells to suppressor cells could be prevented. To do this, they administered
tumor-specific CD4+ and CD8+ T cells to prostate tumor-bearing mice. Some CD4+ T
cells act as helper cells and enhance the activity of other immune cells,
including CD8+ T cells. The researchers found that, under these conditions, CD8+
T cells isolated from the prostate tumors no longer suppressed the proliferation
of other T cells. Moreover, these cells produced less TGF-beta than cells that
were not exposed to CD4+ T cells.
The researchers propose that activated CD4+ T cells that enter tumors may
secrete factors that support the CD8+ T cell antitumor functions, or may help
other immune cells located in the tumor block the processes by which CD8+ T
cells acquire their suppressive activity.
Future work by this team will focus on defining the mechanisms by which
tumor-specific CD8+ T cells gain their suppressive functions upon entering the
mouse tumor microenvironment. "It is important to understand how these cells
become suppressive and how they mediate suppression to find approaches to block
these processes," said Hurwitz. "This will enhance our ability to generate more
effective antitumor T cell responses in mice, which then might be translated to
human."
For more information on Dr. Hurwitz's research, please go to
http://ccr.nci.nih.gov/staff/staff.asp'profileid=7740.
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the burden of cancer and improve the lives of cancer patients and their
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------------------------------------ REFERENCE: Shafer-Weaver, KA, Anderson, MJ,
Stagliano K, Malyguine, A, Greenberg, NM, and Hurwitz AA. Cutting Edge:
Tumor-Specific CD8+ T Cells Infiltrating Prostatic Tumors Are Induced to Become
Suppressor Cells. J Immunol. Oct. 15, 2009. Vol. 183, No. 8.
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