Visualizing T cell activation around the blood-brain barrier Dissertation

T cells recognizing myelin auto-antigens penetrate into the CNS to
induce inflammatory autoimmune disease following complex sequential
interactions with individual components of the vascular blood-brain
barrier (BBB), particularly endothelial cells, and perivascular
phagocytes. To determine the functional consequences of these
processes, two-photon intravital imaging was performed to compare
the behavior of three myelin-specific GFP-expressing T cell lines
with different potentials for transferring Experimental Autoimmune
Encephalomyelitis. Imaging documented that, irrespective of their
pathogenic potential, all T cell lines reached the CNS and
interacted with vascular endothelial cells indistinguishably,
crawling on the luminal surface, preferably against blood flow,
before crossing the vessel wall. In striking contrast, after
extravasation the T cell motility and their interactions with
perivascular antigen presenting cells (APCs) varied dramatically.
While highly encephalitogenic T cells showed a low motility, made
stable contacts with local APCs and became activated, the
corresponding contacts of weakly encephalitogenic T cells remained
short, their motility high and their activation marginal. Supplying
auto-antigen, via either local injection or by transfer of
antigen-pulsed meningeal APCs, lowered their motility and prolonged
the contact duration of weakly encephalitogenic T cells to values
characteristic for highly pathogenic ones. Only after exogenous
antigen supply, the weakly encephalitogenic T cells became
activated, infiltrated the CNS parenchyma, and triggered clinical
EAE, suggesting that the strength of the antigen-dependent signals
received by immigrating effector T cells from leptomeningeal APCs
is crucial for their pathogenic effect within the target tissue. To
directly correlate the activation of encephalitogenic T cells with
their dynamic behavior in the CNS, a truncated fluorescent
derivative of nuclear factor of activated T cells (NFAT) was
introduced as a real-time activation indicator. Two-photon imaging
documented the activation of the auto-reactive T cells extravasated
into the perivascular space, but not within the vascular lumen.
Activation correlated with reduced T cell motility, and it was
related to contacts with the local APCs. However, it did not
necessarily lead to a long-lasting arrest, as individual, activated
T cells SUMMARY 2 were able to sequentially contact other APCs. A
spontaneous cytosol-nuclear translocation of the marker was noted
only in T cells with a high pathogenic potential. The translocation
implied the presentation of an auto-antigen, as the weakly
pathogenic T cells, which remained silent in the untreated hosts,
were activated upon the instillation of exogenous auto-antigen. It
is proposed here that the presentation of local auto-antigen by
BBB-associated APCs provides stimuli that guide autoimmune T cells
to the CNS destination and enable them to attack the target tissue.
In addition, a theoretical, physicist approach was used for
modeling T cell activation in the leptomeningeal space. Assuming
that T cells have evolved to gain their activation signal in a way
that is energetically optimal for them, two possible scenarios for
T cell activation were compared. The first one assumes that, after
finding an APC presenting the epitope of interest, the T cell will
stop and interact with the APC until it becomes fully activated.
The second model considers the possibility that a T cell can
accumulate activation signals from different APCs while scanning
them without stopping, until a certain threshold is exceeded and
the T cell becomes activated. Using this approach, it is proposed
that the T cells in EAE are more likely to become activated
following the first scenario. However, in a more natural
environment such as a lymph node, the second scenario could give
them some advantages