in vivo

Laboratory for
Lymphocyte Development
Thymic microenvironments critically support
the development of T lymphocytes.
The stromal cell types composing these
microenvironments are epithelial in origin, and
form as non-polarized cells a three-dimensional
(3-D) organized network (Fig.1).
This type of organization is unique for
epithelial cells in the thymus, because in other
organs epithelial cells are polarized and placed
on a basal lamina forming sheets of cells. The
3-D organization of thymic epithelial cells (TECs)
forms the basis of microenvironments, allowing
both for migration of thymocytes through
the epithelial network, as well as for lymphostromal
interaction. In this way, TECs control
various steps in T cell development, like clonal
expansion, and also positive and negative
selection. Reciprocally, the integrity of thymic
microenvironments depends on the physical
presence of thymocytes within the epithelial
network. We have previously shown that removal
of thymocytes from the thymic environment,
either by genetic manipulation or by biochemical
methods leads to a dramatic shift in the
phenotype and organization of TECs, inducing
the formation of epithelial cell types which are
normally found in the gastro-intestinal tract and
the respiratory tract (Fig.2). These experiments
have uncovered a mutual interdependency
between TECs and thymocytes, a phenomenon
designated as “thymic crosstalk” (Fig.3). Thus,
thymic crosstalk regulates the integrity and
maintenance of the thymic stroma, and in this
respect, thymocytes continuously promote their
own development (see references 1-4).
Unit leader
Willem van Ewijk
Technical Staff
Student trainees
: Eric Vroegindewey
: Stijn Crobach
Janneke Rood
Nienke Grotenhuis
Cellular and molecular analysis of thymic
crosstalk.
The actual mechanisms of thymic crosstalk
have remained obscure for many years. Using a
“gain of function” approach we have started to
analyze the role of the Notch signaling pathway
in TEC development. Using fetal thymic organ
culture (FTOC), we first showed that thymic
crosstalk is a T cell specific phenomenon.
Although progenitor B-lymphocytes may reside
and develop in between TEC’s, they are only
marginally able to influence the organization of
thymic microenvironments. We then showed
that B cells infected with a DLL-1 expressing
retrovirus induce proper development of the
thymic epithelial reticulum. These experiments
particularly reveal that Notch activation by the
DLL expressing B lymphocytes induces the 3-D
non polarized phenotype in TECs (5).
Our FTOC experiments have also shown that
complete loss of crosstalk leads to a dramatic
shift in the development of TECs. Within a period
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