HOW DOES THE IMMUNE SYSTEM
NORMALLY KEEP US HEALTHY?
The “soldiers” of the immune system are white blood
cells, including T and B lymphocytes, which originate
in the bone marrow from hematopoietic stem cells.
Every day the body comes into contact with many
organisms such as bacteria, viruses, and parasites.
Unopposed, these organisms have the potential to
cause serious infections, such as pneumonia or AIDS.
When a healthy individual is infected, the body
responds by activating a variety of immune cells.
Initially, invading bacteria or viruses are engulfed by
an antigen presenting cell (APC), and their component
proteins (antigens) are cut into pieces and displayed on the cell’s surface. Pieces of the foreign
protein (antigen) bind to the major histo compatibility
complex (MHC) proteins, also known as human leukocyte antigen (HLA) molecules, on the surface of the
APCs. This complex, formed by a foreign
protein and an MHC protein, then binds to a T cell
receptor on the surface of another type of immune
cell, the CD4 helper T cell. They are so named
because they “help” immune responses proceed
and have a protein called CD4 on their surface. This
complex enables these T cells to focus the immune
response to a specific invading organism. The antigen- specific CD4 helper T cells divide and multiply
while secreting substances called cytokines, which cause inflammation and help activate other immune cells. The particular
cytokines secreted by the CD4 helper T cells act on cells known as the CD8 “cytotoxic” T cells (because they can
kill the cells that are infected by the invading organism and have the CD8 protein on their surface). The helper T cells can
also activate antigen-specific B cells to produce antibodies, which can neutralize and help eliminate bacteria and viruses
from the body. Some of the antigen-specific T and B cells that are activated to rid the body of infectious organisms become
long-lived “memory” cells. Memory cells have the capacity to act quickly when confronted with the same infectious
organism at later times. It is the memory cells that cause us to become “immune” from later reinfections with
the same organism.
HOW DO THE IMMUNE CELLS OF
THE BODY KNOW WHAT TO ATTACK
AND WHAT NOT TO?
All immune and blood cells develop from multipotent
hematopoietic stem cells that originate in the bone
marrow. Upon their departure from the bone marrow,
immature T cells undergo a final maturation process
in the thymus, a small organ located in the upper
chest, before being dispersed to the body with the
rest of the immune cells (e.g., B cells). Within the
thymus, T cells undergo an important process that
“educates” them to distinguish between self (the
proteins of their own body) and nonself (the invading
organism’s) antigens. Here, the T cells are selected for their ability to bind to the particular
MHC proteins
expressed by the individual. The particular array of
MHCs varies slightly between individuals, and this
variation is the basis of the immune response when a
transplanted organ is rejected. MHCs and other less
easily characterized molecules called minor histocompatibility antigens are genetically determined
and this is the reason why donor organs from relatives of the recipient are preferred over unrelated
donors. In the bone marrow, a highly diverse and random array of T cells is produced. Collectively, these T cells are capable
of recognizing an almost unlimited number of antigens. Because the process of
generating a T cell’s antigen specificity is a random
one, many immature T cells have the potential to
react with the body’s own (self) proteins. To avoid this
potential disaster, the thymus provides an environment where T cells that recognize self-antigens (autoreactive
or self-reactive T cells) are deleted or inactivated in a process called tolerance induction. Tolerance usually ensures that
T cells do not attack the “autoantigens” (self-proteins) of the body. Given the importance of this task, it is
not surprising that there are multiple checkpoints for destroying or inactivating T cells that might react to auto-antigens.
Autoimmune diseases arise when this intricate system for the induction and maintenance of immune tolerance fails. These diseases
result in cell and tissue destruction by antigen-specific CD8 cytotoxic T cells or auto antibodies (antibodies to self-proteins)
and the accompanying inflammatory process. These mechanisms can lead to the destruction of the joints in rheumatoid arthritis,
the destruction of the insulinproducing beta cells of the pancreas in type 1 diabetes, or damage to the kidneys in lupus.
The reasons for the failure to induce or maintain tolerance are enigmatic. However, genetic factors, along with environmental
and hormonal influences and certain infections, may contribute to tolerance and the development of autoimmune disease .
HEMATOPOIETIC STEM CELL
THERAPY FOR AUTOIMMUNE
DISEASES
The current treatments for many autoimmune diseases include the systemic use of anti-inflammatory drugs
and potent immunosuppressive and immunomodulatory agents (i.e., steroids and inhibitor proteins that block the action of inflammatory
cytokines). However, despite their profound effect on immune responses,these therapies are unable to induce clinically significant
remissions in certain patients. In recent years, researchers have contemplated the use of stem cells to treat autoimmune disorders.
Discussed here is
some of the rationale for this approach, with a focus
on experimental stem cell therapies for lupus,
rheumatoid arthritis, and type 1 diabetes.
The immune-mediated injury in autoimmune diseases
can be organ-specific, such as type 1 diabetes
which is the consequence of the destruction of the
pancreatic beta islet cells or multiple sclerosis which
results from the breakdown of the myelin covering of
nerves. These autoimmune diseases are amenable
to treatments involving the repair or replacement of
damaged or destroyed cells or tissue In contrast, non-organ-specific
autoimmune diseases, such as lupus, are characterized by widespread injury due to immune reactions against many different
organs and tissues. One approach is being evaluated in early clinical trials of patients with poorly responsive, life-threatening
lupus. This is a severe disease affecting multiple organs in the body including muscles, skin, joints, and kidneys as well
as the brain and nerves. Over 239,000 Americans, of which more than 90 percent are women, suffer from lupus. In addition,
lupus disproportionately afflicts African-American and
Hispanic women.major obstacle in the treatment of non-organ-specific autoimmune diseases
such as lupus is the lack of a single specific target for
the application of therapy. The objective of hematopoietic stem cell therapy for
lupus is to destroy the mature, long-lived, and auto reactive immune cells and to generate a new,
properly functioning immune system. In most of these
trials, the patient’s own stem cells have been used in
a procedure known as autologous (from “one’s self”)
hematopoietic stem cell transplantation. First, patients
receive injections of a growth factor, which coaxes
large numbers of hematopoietic stem cells to be
released from the bone marrow into the blood stream.
These cells are harvested from the blood, purified
away from mature immune cells, and stored. After
sufficient quantities of these cells are obtained, the
patient undergoes a regimen of cytotoxic (cell-killing)
drug and/or radiation therapy, which eliminates the
mature immune cells. Then, the hematopoietic stem
cells are returned to the patient via a blood transfusion into the circulation where they migrate to
the bone marrow and begin to differentiate to become mature immune cells. The body’s immune system is then restored.
Nonetheless, the recovery phase, until the immune system is reconstituted represents a period of dramatically increased susceptibility
to bacterial, fungal, and viral infection, making this a high-risk therapy. Recent reports suggest that this replacement therapy
may fundamentally alter the patient’s immune system. Richard Burt and his colleagues conducted a long-term follow-up
(one to three years) of seven lupus patients who underwent this procedure and found that they remained free from active lupus
and improved continuously after transplantation, without
the need for immunosuppressive medications. One
of the hallmarks of lupus is that during the natural progression of disease, the normally diverse repertoire
of T cells become limited in the number of different antigens they recognize, suggesting that an increasing proportion of
the patient’s T cells are autoreactive. Burt and colleagues found that following hematopoietic stem cell transplantation,
levels of T cell diversity were restored to those of healthy individuals. This finding provides evidence that stem cell replacement
may be beneficial in reestablishing tolerance in T cells, thereby decreasing the likelihood of disease reoccurrence.
DEVELOPMENT OF HEMATOPOIETIC
STEM CELL LINES FOR
TRANSPLANTATION
The ability to generate and propagate unlimited
numbers of hematopoietic stem cells outside the
body—whether from adult, umbilical cord blood,
fetal, or embryonic sources—would have a major
impact on the safety, cost, and availability of stem
cells for transplantation. The current approach of
isolating hematopoietic stem cells from a patient’s
own peripheral blood places the patient at risk for a
flare-up of their autoimmune disease. This is a potential
consequence of repeated administration of the stem cell growth factors needed to mobilize
hematopoietic stem cells from the bone marrow to
the blood stream in numbers sufficient for transplantation. In addition, contamination of the purified
hematopoietic stem cells with the patient’s mature auto reactive T and B cells could affect the success of the treatment
in some patients. Propagation of pure cell lines in the laboratory would avoid these potential drawbacks and increase the
numbers of stem cells available to each patient, thus shortening the at-risk interval before full immune reconstitution. Whether
embryonic stem cells will provide advantages over stem cells derived from cord blood or adult bone marrow hematopoietic stem
cells remains to be determined. However, hematopoietic stem cells, whether from umbilical cord blood or bone
marrow, have a more limited potential for selfrenewal
than do pluripotent embryonic stem cells.
Although new information will be needed to direct
the differentiation of embryonic stem cells into
hematopoietic stem cells, hematopoietic cells are
present in differentiated cultures from human embryonic stem cells
and from human fetal-derived embryonic germ stem cells .
One potential advantage of using hematopoietic
stem cell lines for transplantation in patients with
auto immune diseases is that these cells could be
generated from unaffected individuals or, as predisposing genetic factors are defined, from embryonic
stem cells lacking these genetic influences. In addition, use of genetically selected or genetically engineered cell types
may further limit the possibility of disease progression or reemergence.
One risk of using nonself hematopoietic stem cells is
of immune rejection of the transplanted cells.
Immune rejection is caused by MHC protein differences between the donor and the patient (recipient).
In this scenario, the transplanted hematopoietic stem cells and their progeny are rejected by the patient’s own T cells,
which are originating from the patient’s surviving bone marrow hematopoietic stem cells. In this regard, embryonic stem
cell-derived hematopoietic stem cells may offer distinct advantages over cord blood and bone marrow hematopoietic stem cell
lines in avoiding rejection of the transplant. Theoretically, banks of embryonic stem cells expressing various combinations
of the three most critical MHC proteins could be generated to allow close matching to the recipient’s MHC composition.
Additionally, there is evidence that embryonic stem cells are considerably more receptive to genetic manipulation than are
hematopoietic stem cells . This characteristic means that embryonic stem cells could be useful in strategies that could prevent
their recognition by the patient’s surviving immune cells. For example, it may be possible to introduce the
recipient’s MHC proteins into embryonic stem cells
through targeted gene transfer. Alternatively, it is
theoretically possible to generate a universal donor
embryonic stem cell line by genetic alteration or
removal of the MHC proteins. Researchers have
accomplished this by genetically altering a mouse
so that it has little or no surface expression of MHC
molecules on any of the cells or tissues. There is no
rejection of pancreatic beta islet cells from these
genetically altered mice when the cells are transplanted into completely MHC-mismatched mice
Additional research will be needed to determine the
feasibility of these alternative strategies for prevention
of graft rejection in humans . Jon Odorico and colleagues have shown that expression
of MHC proteins on mouse embryonic stem cells
and differentiated embryonic stem cell progeny is
either absent or greatly decreased compared with
MHC expression on adult cells. These preliminary
findings raise the intriguing possibility that lines derived from embryonic stem cells may be inherently
less susceptible to rejection by the recipient’s immune
system than lines derived from adult cells. This could
have important implications for the transplantation of
cells other than hematopoietic stem cells.
Another potential advantage of using pure populations
of donor hematopoietic stem cells achieved
through stem cell technologies would be a lower
incidence and severity of graft-versus-host disease, a
potentially fatal complication of bone marrow transplantation. Graft-versus-host disease results from
the immune-mediated injury to recipient tissues that occurs when mature organ-donor T cells remain within the organ at the
time of transplant. Such mature donor alloreactive T cells would be absent from pure populations of multipotent hematopoietic
stem cells, and under ideal conditions of immune tolerance induction in the recipient’s thymus, the donor-derived mature
T cell population would be tolerant to the host.
GENE THERAPY AND STEM CELL
APPROACHES FOR THE TREATMENT
OF AUTOIMMUNE DISEASES
Gene therapy is the genetic modification of cells to
produce a therapeutic effect In most investigational protocols,
DNA containing the therapeutic gene is transferred into cultured cells, and these cells are subsequently administered to the
animal or patient. DNA can also be injected directly, entering cells at the site of the injection or in the circulation. Under
ideal conditions, cells take up the DNA and produce the therapeutic protein encoded by the gene.
Currently, there is an extensive amount of gene
therapy research being conducted in animal models
of autoimmune disease. The goal is to modify the
aberrant, inflammatory immune response that is
characteristic of autoimmune diseases.
Researchers most often use one of two general
strategies to modulate the immune system. The first
strategy is to block the actions of an inflammatory
cytokine (secreted by certain activated immune cells
and inflamed tissues) by transferring a gene into cells
that encodes a “decoy” receptor for that cytokine.
Alternatively, a gene is transferred that encodes an
anti-inflammatory cytokine, redirecting the auto inflammatory immune response to a more “tolerant”
state. In many animal studies, promising results have
been achieved by using these approaches, and the
studies have advanced understanding of the disease
processes and the particular inflammatory cytokines
involved in disease progression.
Serious obstacles to the development of effective
gene therapies for humans remain, however.
Foremost among these are the difficulty of reliably
transferring genetic material into adult and slowly
dividing cells (including hematopoietic stem cells)
and of producing long-lasting expression of the
intended protein at levels that can be tightly controlled
in response to disease activity. Importantly,
embryonic stem cells are substantially more permissive to gene transfer compared with adult cells, &embryonic
cells sustain protein expression during
extensive self-renewal. Whether adult-derived stem
cells, other than hematopoietic stem cells, are
similarly amenable to gene transfer has not yet
been determined. Ultimately, stem cell gene therapy should allow the development of novel methods for
immune modulation in autoimmune diseases. One example is the genetic modification of hematopoietic stem cells or differentiated
tissue cells with a “decoy” receptor for the inflammatory cytokine interferon gamma to treat lupus. For example,
in a lupus mouse model, gene transfer of the decoy receptor, via DNA injection, arrested disease progression. Other investigators
have used a related but distinct approach in a mouse model of type 1 diabetes. Interleukin-12 (IL-12), an inflammatory cytokine,
plays a prominent role in the development of diabetes in these mice. The investigators transferred the gene for a modified
form of IL-12, which blocks the activity of the natural IL-12, into pancreatic beta islet cells (the target of autoimmune
injury in type 1 diabetes). The islet cell gene therapy prevented the onset of diabetes in these mice. Theoretically, embryonic
stem cells
or adult stem cells could be genetically modified
before or during differentiation into pancreatic beta
islet cells to be used for transplantation. The resulting
immune-modulating islet cells might diminish the
occurrence of ongoing autoimmunity, increase the
likelihood of long-term function of the transplanted
cells, and eliminate the need for immunosuppressive
therapy following transplantation. Researchers are exploring similar genetic approachesto prevent progressive
joint destruction and loss of
cartilage and to repair damaged joints in animal
models of rheumatoid arthritis. Rheumatoid arthritis is
a debilitating autoimmune disease characterized by
acute and chronic inflammation, in which the
immune system primarily attacks the joints of the
body. In a recent study, investigators genetically
transferred an anti-inflammatory cytokine, interleukin-4 (IL-4), into a specialized, highly efficient
antigen presenting cell called a dendritic cell, and then
injected these IL-4-secreting cells into mice that can
be induced to develop a form of arthritis similar to
rheumatoid arthritis in humans. These IL-4-secreting
dendritic cells are presumed to act on the CD4
helper T cells to reintroduce tolerance to self-proteins.
Treated mice showed complete suppression of their
disease and, in addition to its immune-modulatory
properties, IL-4 blocked bone resorption (a serious
complication of rheumatoid arthritis), making it a
particularly attractive cytokine for this therapy.
However, one obstacle to this approach is that human
dendritic cells are difficult to isolate in large numbers.
Investigators have also directed the differentiation of
dendritic cells from mouse embryonic stem cells,
indicating that a stem cell-based approach might
work in patients with rheumatoid arthritis . Longerterm follow-up and further characterization will be
needed in animal models before researchers
proceed with the development of such an approach
in humans. In similar studies, using other inhibitors of
inflammatory cytokines such as a decoy receptor for
tumor necrosis factor–_ (a prominent inflammatory
cytokine in inflamed joints), an inhibitor of nuclear
factor–__ (a protein within cells that turns
on the
production of many inflammatory cytokines), and
interleukin-13 (an anti-inflammatory cytokine),
researchers have shown promising results in animal
models of rheumatoid arthritis. Because of the
complexity and redundancy of immune system
signaling networks, it is likely that a multifaceted
approach involving inhibitors of several different
inflammatory cytokines will be successful, whereas
approaches targeting single cytokines might fail or
produce only short-lived responses. In addition, other
cell types may prove to be even better vehicles for
the delivery of gene therapy in this disease.
Chondrocytes, cells that build cartilage in joints, may
provide another avenue for stem cell-based treatment
of rheumatoid arthritis. These cells have been
derived from human bone marrow stromal stem cells
derived from human bone marrow. Little is known
about the intermediate cells that ultimately differentiate into chondrocytes. In addition to adult bone
marrow as a source for stromal stem cells, human embryonic stem cells can differentiate into precursor cells believed to lead
ultimately to the stromal stem cells . However, extensive research is needed to reliably achieve the directed derivation of
the stromal stem cells from embryonic stem cells and, subsequently, the differentiation of chondrocytes from
these stromal stem cells. The ideal cell for optimum cartilage repair may be a more primitive cell than
the chondrocyte, such as the stromal cell, or an intermediate cell in the pathway (e.g., a connective tissue precursor) leading
to the chondrocyte. Stromal stem cells can generate new
chondrocytes and facilitate cartilage repair in a rabbit
model. Such cells may also prove to be ideal
targets for the delivery of immune-modulatory gene
therapy. Like hematopoietic stem cells, stromal stem
cells have been used in animal models for delivery of
gene therapy . For example, a recent study
demonstrated that genetically engineered chondrocytes, expressing a growth factor, can enhance the function
of transplanted chondrocytes. Two obstacles to the use of adult stromal stem cells or chondrocytes are the limited numbers
of these
cells that can be harvested and the difficulties in
propagating them in the laboratory. Embryonic stem
cells, genetically modified and expanded before
directed differentiation to a connective tissue stem
cell, may be an attractive alternative.
Collectively, these results illustrate the tremendous
potential these cells may offer for the treatment of
rheumatoid arthritis and other autoimmune diseases.
