Biomedical Engineering Reference
In-Depth Information
detecting immune responses in preclinical animal models
and clinical trial patients. Because of the diverse nature of
therapeutic proteins, testing strategies should be consid-
ered on an individual basis. An aspect of risk-based
analysis concerns the design of the molecule, encompass-
ing in silico identification of any immunological “hot
spots”andtheirreengineeringsoastoreduceoreliminate
these hot spots. Performing binding, in vivo, and cell-based
assays to understand the functional role of drug attributes
in a physiological context can be an additional component
of the risk assessment. Clinical evaluation to confirm
acceptable immunogenicity is always required because
the in silico and in vitro methods cannot account for all
sources of variability. The types of tools for risk assess-
ment and mitigation described herein can increase the
stringency of selecting which therapeutics to move forward
into clinical trials, making the process safer and more
efficient.
autologous proteins. The link between the HLA-restricted
T-cell immune response and the development of auto-
antibodies is still being defined, but early evidence points
to the reduction of T reg immune responses and to the
induction of T eff responses as significant contributors in
the context of unwanted immunogenicity [16].
During the last several decades, a variety of approaches
have been developed for the induction of tolerance in
experimental animals. Some of these approaches have pro-
gressed to clinical trials. Our goal herein is not to cover this
area of research, which has been extensively reviewed
elsewhere [15,60]. Rather, the fundamental principles favor-
ing tolerance are outlined, following which we focus on
approaches developed in our
lab and those of our
collaborators.
As noted in the introductory section, the route of immu-
nization with a protein significantly influences its immuno-
genicity and tolerogenicity. An intravenous injection favors
tolerance, whereas a subcutaneous or intramuscular injec-
tion leads to direct uptake by dendritic cells. These cells
travel to the lymphoid follicles, where antigens are proc-
essed, and immune responses follow. In contrast, intra-
venous antigen is taken up by APCs in the spleen,
primarily B cells. Oral administration of antigens can
lead to unresponsiveness due to processing of antigens in
the gut immune system. This route may also deviate the
response toward mucosal immune responses similar to IgA
formation [61].
Alternatively, if co-stimulation is blocked (e.g. by CTLA-
4-Ig or antibodies to CD80/CD86), anergy may ensue. The
strategy is to provide “signal 1” (T-cell epitopes bound
to MHC) to the T cell in the absence of “signal 2”
(co-stimulation). This approach has been widely employed
in transplant models but has not been successful for other
applications. Additional methods are based on treatments to
block or subvert T-cell signaling using either antibodies to
the CD3 co-receptor or drugs such as rapamycin that inhibit
downstream signaling pathways. Clinical trials of drugs
employing these approaches also are in progress [62,63].
Recently, the use of so-called tolerogenic APCs, which
are tolerogenic immature dendritic cells (iDC) pulsed with
target antigens, has gained popularity. Such iDC express low
amounts of CD80/CD86 and thus provide little of the co-
stimulation necessary to generate a robust adaptive response
[64]. However, maintaining this immature phenotype in vivo
can be challenging. In a related approach, B cells as tol-
erogenic APC in combination with gene therapy are being
explored. Similar to iDC, na ıve B cells are low in co-
stimulatory molecules [65]. Even mature or activated B
cells can still be tolerogenic under certain circumstances.
This approach has been successful in a variety of mouse and
rat models for autoimmune diseases and hemophilia; it is
now moving forward to proof of concept in nonhuman
primates.
5.4.2 De-immunization by Epitope Modification
Prediction of immunogenic epitopes has led to the strategy
of de-immunization by epitope modification to disrupt HLA
binding, an underlying requirement for T-cell stimulation.
Indeed, rational epitope modification is based on the natural
process that occurs when tumor cells and pathogens escape
immune pressure by accumulating mutations to reduce
binding of their constituent epitopes to host HLA, rendering
the host cell unable to alert T cells to the presence of the
tumor or pathogen [58,59]. As de-immunized protein ther-
apeutics make their way into the clinic, the initial
results appear to support this approach to reducing immu-
nogenicity risk.
5.4.3 Tolerance Induction
Clusters of epitopes with MHC binding potential can stim-
ulate an immune response through T eff cell induction, but the
ability of a peptide to bind to MHC does not necessarily
imply an effector immune response. Alternatively, a peptide
with MHC binding potential may interact with regulatory T
cells to modulate an immune response. For example, poten-
tial epitopes in autologous proteins may trigger T cells that
are absent from the peripheral circulation. While the major-
ity of autoreactive T cells are thought to be deleted during
thymic development, it is now well accepted that some T
cells specific for autologous proteins escape thymic deletion
to become natural regulatory T cells (nT reg ). This subset of T
cells serves as a suppressor of autoimmune and auto-reactive
immune responses [18]. Just as the inadvertent addition of
stimulatory T-effector epitopes to proteins may lead to
increased immunogenicity, removal or alteration of regula-
tory T-cell epitopes in the drug development process may
alter the natural T reg immune response to recombinant
Search WWH ::




Custom Search