At
the most basic level, the immune system must distinguish self from
non-self, that is, it must discriminate between the molecular signatures
of invading pathogens (non-self antigens) and cellular constituents
that usually pose no risk to health (self-antigens).
The
system is far from foolproof. Cancer cells can undergo unchecked
proliferation, producing self-antigens that are tolerated by the immune
system, rather than being targeted for destruction. At the opposite
extreme, a range of so-called autoimmune disorders can result when
healthy cells in the body are misidentified as hazards. The immune
system has developed a further line of protection against such
autoimmune responses in order to limit the pathology that can result.
Essentially, the immune system is programmed to ‘turn itself off’ after
prolonged recognition of an antigen.
In a new study appearing in the current issue of the journal Science,
Dr. Joseph Blattman, a researcher at Arizona State University’s
Biodesign Institute examines how CD8 T cells—critical weapons in the
body’s defensive arsenal—are regulated when they transition from this
tolerant state to an activated state and back. “We have previously shown
that prolonged stimulation of T cells in disseminated cancers or
chronic viral infections results in a tolerant state to the tumor or
pathogen. It was never clear if this ‘immune exhaustion’ was a
reversible fate or if ‘resting’ the T cells by removing them from the
cancer or infection environment could restore their function” said Dr.
Blattman. “These results show that even if you can temporarily rescue a
tolerant T cell, it is hard-wired to become tolerant again.”
Lymphocytes
or white blood cells are central players in the immune systems of all
vertebrates, and come in various types. Large granular lymphocytes
include natural killer cells (NK cells), while small lymphocytes consist
of T cells and B cells. Cytotoxic T cells (also called CD8 T cells)
take their name from their place of maturation in the thymus gland and
the CD8 glycoprotein adorning their surfaces. These cells help the
immune system identify infected or malignant cells and are the main
cells responsible for eliminating them.
In
the thymus, T cells undergo both positive and negative selection. In
this process, T cells that bind too weakly or too strongly to
self-antigens are weeded out, undergoing cell death. The first group
would result in a deficient immune response to foreign invasion while
the latter would tend to overreact to self proteins, leading to
autoimmunity. Only about 2% of these developing T cells or thymocytes
will survive. This dual process of selection generally produces cells
capable of recognizing foreign threats while maintaining a tolerance for
self-antigens.
However,
some self-reactive CD8 T cells do make it out of the thymus and are
exposed to self-antigen. In order to avoid causing autoimmune disease,
the stimulation by self cells results in T cell tolerance. This could be
for a number of reasons including lack of costimulation, the presence
of regulatory T cells that inhibit CD8 T cell responses, or continuous
stimulation by self antigens. This essential safeguard however can
become an Achilles’ heel, causing unresponsiveness in CD8 T cells to
certain cancer antigens, many of which are self-antigens. One of the
central challenges in tumor immunology is to somehow short-circuit T
cell tolerance to tumor/self-antigens, without provoking autoimmunity.
Dr.
Blattman and his group sought to illuminate the underlying molecular
mechanisms of self-tolerance and the regulatory programs that maintain
or break it. Contrary to prevailing theory, the group demonstrated in a
mouse model that T cells return to the tolerant state even in the
absence of self-antigen. Further, such cells could be induced to
proliferate and become functional if the lymphocyte cell numbers fell to
appropriately low levels—a condition known as lymphopenia—and that this
effect is observed even when self-antigen is present.
Because
T cells are known to proliferate in lymphopenic environments, such as
after chemotherapy and/or irradiation in cancer patients, the
researchers used this to ‘trick’ T cells into proliferating in order to
reset their function. This strategy did restore their function
temporarily, but within a month afterwards, the T cells once again
became tolerant even if they did not continue to encounter the tumor
antigen.
The
current research overturns a central paradigm regarding T-cell
tolerance to self-antigens and may provoke a fundamental rethinking of
the underlying mechanisms that govern the immune response. The results
will help identify the molecular events that lead to T cell tolerance to
tumor antigens, which should aid in development of strategies to
permanently restore the function of T cells. This, in turn, should
suggest new approaches for the treatment of cancer and chronic viral
infections that employ adoptive transfer of modified cancer-specific T
cells that make these cells resistant to becoming tolerant.
“Adoptive
immunotherapy with T cells is an exciting strategy for combating cancer
because the transferred T cells don’t kill all dividing cells, but
instead only target the cells expressing the cancer antigen. The problem
has been that the transferred T cells usually become tolerant to the
cancer” said Dr. Blattman. “By knowing the rules governing T cell
tolerance, we will be able to identify what regulates this process and
design ways of overcoming it in order to provide more effective cancer
therapies.”
The
rescue of CT8 T cell functionality was indeed transient, in the mouse
studies undertaken. When lymphocyte numbers in mice rebounded (having
been reduced through irradiation), CD8 T cell tolerance snapped back
into place, and the genetic master plan for these cells was
reestablished. This fact implies that while a genetic blueprint oversees
T cell tolerance, this characteristic is not entirely fixed, but may be
subject to epigenetic regulation, that is, non-genetically encoded
regulation that is transferred to each dividing cell.
Using
techniques of genome-wide mRNA and microRNA profiling, Dr. Blattman and
his colleagues uncovered a tolerance-specific gene profile for CD8 T
cells, further demonstrating that this gene-based regulatory system
could be overridden under lymphopenic conditions.
The
rescue of CD8 T cells through this method has been dubbed
homeostasis-driven proliferation. The mechanism operates even in the
absence of a cognate antigen, but apparently shuts down once T cell
homeostasis has been reestablished. Rescued T cells showed a reduction
or down-regulation of tolerance-specific genes as well as an
up-regulation of some 475 “rescue-associated” genes.
Evidentially,
T cells are able to recall the tolerance program initially established
after their first encounter with self-antigen, returning to it as a
default, following repletion of lymphocyte numbers. While the precise
mechanisms that account for this tolerant memory remain unclear, various
forms of epigenetic gene regulation, not reliant on DNA sequence, are
implied.
Future
research will attempt to identify the signaling pathways associated
with the interruption and reacquisition of T cell tolerance, which
appears to operate independently of surface T cell receptors. Further,
use of lymphopenia-mediated rescue of CD8 T cells for cancer therapy
will require that tolerance-specific epigenetic memory somehow be
erased.
Finally,
lymphopenia-based T cell proliferation and activation also provides a
model to describe heightened autoimmunity following organ
transplantation, particularly cases of graft-host rejection. Such
autoimmunity is often of a transient nature, again suggesting that T
cell tolerance is reset once lymphocyte populations rebound following
surgery.
“These
results clearly suggest that epigenetic mechanisms are in place to
maintain tolerance in T cells specific for self antigens. Uncovering
precisely which key molecules and genes are important in this process
should help us to improve T cell based approaches to the treatment of
cancer, as well as to induce tolerance in T cells causing autoimmunity,”
said Dr. Blattman.
