|Mouse Studies Show Tumors Suppress Cells Responsible
for Regulating the Immune System
New research has shown that the function of a type of cell that
helps modulate immune responses is impaired inside tumors in mice.
Researchers also identified several factors that may contribute
to an accumulation of these cells, called T regulatory cells (Tregs),
within and around the tumor, which may be how they respond to their
loss of functionality. The study, by scientists at the National
Cancer Institute (NCI), part of the National Institutes of Health,
appeared online April 18, 2008, in The Journal of Immunology.
"These findings provide insight into the impact of a growing
tumor on the immune system," said Helen Sabzevari, Ph.D.,
of NCI's Center for Cancer Research, an author of the study. "Understanding
the tumor's effects on Tregs and how these cells maintain themselves
inside tumors, and in the environment immediately surrounding tumors,
will be important for designing new immunotherapies."
Tregs are a specialized subset of T cells that help manage the
immune system by suppressing the response of immune cells once
a foreign invader has been defeated. They also prevent autoimmune
diseases by keeping the body from attacking its own cells and tissues
or reacting to its own antigens, called self-antigens. Since tumor-associated
antigens are primarily derived from self-antigens, Treg cells also
play an important role in suppressing immune responses directed
against tumors, yet Tregs are thought to somehow escape the immunosuppressive
effects of the tumor microenvironment.
Previous studies have shown that the suppressive actions of Tregs
require other immune cells to first become activated through the
T cell receptor (TCR), a surface landing site where these cells
recognize and bind to begin an immune response. This triggers the
eventual suppressive activity of Tregs through a step-wise series
of biochemical events called signaling pathways.
In laboratory experiments, Sabzevari's team demonstrated that
Tregs taken from the spleens of mice bearing tumors exhibited a
less suppressive influence on the rate of proliferation of immune
cells than did Tregs from spleens of the same strain of mice without
tumors. In addition, they found that suppression of overall immune
responses decreased about 2.4-fold in tumor-associated Tregs when
compared to normal Tregs in the spleen.
To explore possible mechanisms for a tumor's effects on Treg cell
function, the researchers implanted cancer cells under the skin
of mice. Then they compared gene expression patterns in Tregs collected
from spleen tumors that formed in mice implanted with Treg cells
vs. expression patterns of spleens of implant-free control mice.
Microarray analysis — which allows researchers to examine the
activity of thousands of genes simultaneously — revealed differences
in the gene expression of several types of genes, including those
involved in immune responses, signal transduction, T cell activation,
and the TCR signaling pathway. Comparing individual genes, they
found reduced expression of several molecules that are involved
in TCR signaling in the tumor-associated Tregs when compared to
normal Treg cells.
Further analysis indicated that Treg cells in tumors lose some
of their functionality because they do not become effectively activated. "Our
studies demonstrate that Treg cells from tumors are less capable
of responding to activation through the TCR than are Treg cells
from normal spleens, indicating that the tumor microenvironment
inhibits functionality of Treg cells," said Sabzevari.
In some human cancers, the number of Tregs increases in the peripheral
blood, and these cells accumulate at the site of tumors. Increases
in Treg cell numbers also have been observed in the spleen of animal
Similarly, in this new research, as tumors grew larger in implanted
mice, the number of Treg cells increased in both the spleen and
in the tumors, but, in tumors, the percent of Treg cells actively
copying themselves was 23 to 43 percent of the population of Tregs
compared to 11 to 16 percent in the spleen. Additionally, cell
death in the tumor-associated Tregs was two percent compared to
11 percent for spleen-associated Tregs in the same animals, likely
because of the increased expression of other molecules that interfere
with factors that signal cell death.
Despite their reduced functionality, the accumulation of larger
numbers of Tregs in tumors may still allow the suppression of antitumor
immune responses. Targeting Treg cells may be one way of improving
"Our findings indicate that treatments, such as chemotherapy or
radiation therapy, can directly affect Treg cells," said Sabzevari. "By
decreasing the number of Treg cells at the site of tumors, treatments,
such as immunotherapies, may be more effective."
For more information on Sabzevari's research, please go to http://ccr.cancer.gov/staff/staff.asp?profileid=5674.
For more information about cancer, please visit the NCI website
at http://www.cancer.gov, or call NCI's Cancer Information Service
at 1-800-4-CANCER (1-800-422-6237).
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.
Lutsiak MEC, Tagaya Y, Adams AJ, Schlom J, and Sabzevari H. Tumor-Induced Impairment of TCR Signaling Results in Compromised Functionality of Tumor-Infiltrating Regulatory T Cells. The Journal of Immunology. May 1, 2008.