Research Report 2000 - MDC

Cellular Immunology
Kirsten Falk
Olaf Rötzschke
The newly established group will start
operation in spring 2000. One of the
major areas of interest is the
investigation of mechanisms relevant
for the induction or suppression of
immune tolerance.
The thymal selection of T cells, in
concert with peripheral tolerance
mechanisms, generates and maintains
a T cell repertoire, which responds to
foreign antigens but not to peptides
derived from self-proteins. Under
certain circumstances, however, these
tolerance mechanisms can fail.
Autoreactive T cells escape the
selection-process and cause the
induction of chronic autoimmune
diseases, such as multiple sclerosis,
diabetes mellitus or rheumatoid
arthritis. In most of these autoimmune
diseases the typical tissue-specific
damage is caused by the activity of
autoreactive CD4 + T cells (and not by
CD8 + T cells). In the case of
experimental autoimmune
encephalomyelitis (EAE) the CD4 + T
cells are responsible for the removal
of the isolating myelin sheath from
the axons of the CNS. They inflict the
damage either by a direct attack of the
myelin sheath or indirectly by
activating B cells, which produce
myelin-specific antibodies. While in
these chronic autoimmune diseases
the effect of autoreactive CD4 + T cells
is extremely harmful, it could be very
beneficial if the response could be
directed against self-proteins
expressed in transformed tissue.
In order to break tolerance it is
necessary to increase the sensitivity of
the T cells to the autoantigen. One
way is to manipulate co-stimulatory
signals. For instance, the blockade of
CTLA-4, an attenuator of the T cell
activation cascade, can significantly
lower the threshold for the T cell
activation. However, a general
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disadvantage of this approach is the
complete lack of selectivity. Our
efforts, therefore, concentrated on
ways to address autoreactive T cells in
an antigen–specific manner. In
previous studies we showed that
multimers containing repeats of a
peptide antigen derived from the
influenza hemagglutinin protein can
trigger an antigen-specific T cell
response at almost 1000 fold lower
concentrations than the peptide. The
enhancement was strictly antigenspecific
and appeared to result from
the cross-linking of MHC/peptide/TCR
complexes. To adapt this approach to
a true autoimmune model system
multimerized forms of
encephalitogenic peptides derived
from the myelin basic protein (MBP)
or proteolipid protein (PLP) were
generated and tested in the EAE
system. The trials revealed that the
multimerization increased the in vivo
potency of these epitopes to such an
extent that EAE was induced even in
strains of mice normally not effected
by the monomeric peptides.
Furthermore utilizing immunization
protocols, which aim at a tolerization
rather than the activation of the T
cells, relatively small amounts of
these multimers were found to be
sufficient to suppress the disease (the
monomeric peptides did not show any
suppressive effect). In vitro
experiments in the hemagglutinin
system indicated that this suppression
results from the apoptotic elimination
of overstimulated CD4 + T cells (‘highzone
tolerance’), one of the main
mechanisms of peripheral tolerance.
Another tool to enhance the sensitivity
of CD4 + T cells was found during
biochemical studies of two
conformational variants of MHC class
II/ligand complexes. Binding
experiments revealed that the on-rate
for the formation of the peptide/MHC
complex was significantly increased if
certain small molecular compounds
were present during the binding
reaction. Subsequent studies revealed
that these compounds catalyze the
ligand exchange in mechanism similar
to HLA-DM molecules. In contrast to
HLA-DM, however, these compounds
facilitated the peptide exchange
directly on the surface of antigen
presenting cell, increasing the
sensitivity of an antigen-specific T
cell response by almost 2 logs. This
project is still at an early stage and
will be continued utilizing
combinatorial chemistry. The control
of autoimmune reactions is crucial for
the treatment of autoimmune diseases
as well as for the development of
tumor immunotherapies. To achieve
this goal the group will continue to
investigate the underlying
mechanisms in vitro, in vivo as well
as on the molecular level.
Selected Publications
Rötzschke, O., Falk, K., and
Strominger, J. L. (1997) Superactivation
of an immune response
triggered by oligomerized T cell
epitopes. Proc. Natl. Acad. Sci. U S A.
94, 14642-14647.
Rötzschke, O., Falk, K., Mack, J.,
Lau, J. M., Jung, G., and Strominger,
J. L. (1999) Conformational variants
of class II MHC/peptide complexes
induced by N- and C-terminal
extensions of minimal peptide
epitopes. Proc. Natl. Acad. Sci. U S A.
96, 7445-7450.
Falk, K., Rötzschke, O.,
Santambrogio, L., Dorf, M. E.,
Brosnan, C., and Strominger, J. L.
(2000) Induction and suppression of
an autoimmune disease: Enhanced in
vivo potency of oligomerized T cell
epitopes. J. Exp. Med. 191, 1-14.
Structure of the Group
Group leaders
Dr. Kirsten Falk
Dr. Olaf Rötzschke