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Immunology
Regulators of T-Cell Development
and Lymphocyte Function
J. Kaye, P. Aliahmad, O. Goularte,
P. Han, J.-M. Yang
Precursor
cells in the thymus undergo a complex developmental program before seeding peripheral lymphoid
organs as mature T lymphocytes. T-cell development is an inefficient process; the thymus generates
a vast excess of cells that do not complete maturation. This inefficiency is the result of developmental
checkpoints termed β-selection,
positive selection, and negative selection that combined select for a mature T-cell population
that is not overtly self-reactive but can still function in recognition of pathogen-derived antigens.
We are interested in the mechanisms that determine the fate of developing T cells and the control
of gene expression during these developmental processes. Our identification of a cell-surface
protein that is upregulated on developing thymocytes also led to studies on regulation of the immune
response and the potential of this protein as a novel therapeutic target.
A Nuclear Protein Involved in Regulation of Thymocyte Selection
We identified thymocyte selection–associated
high mobility group (HMG) box protein (TOX) several years ago. Members of the HMG box protein superfamily
share one or more copies of a sequence-related and structurally related DNA-binding domain. These
proteins can modify chromatin by bending and unwinding DNA. In general, HMG box proteins function
as architectural factors that regulate gene expression by promoting formation of transcriptional
complexes. We showed that TOX is indeed a DNA-binding factor, and more detailed analysis of its
binding specificity is under way.
TOX is a member of a small subfamily of proteins
in rodents and humans that share almost identical HMG box sequences. In addition, we identified
genes in the pufferfish and malarial mosquito genomes that encode proteins with a TOX-type HMG
box. Most likely the TOX subtype of HMG box domain first appeared in invertebrates, was duplicated
in early vertebrates, and took on new functions in mammalian species, including in the immune system.
To our knowledge, our studies on TOX were the first on any member of this highly conserved protein
family.
Expression of TOX in the thymus is tightly
regulated. The protein is expressed in early thymocyte progenitor cells and then transiently
upregulated during β-selection
and positive selection. Using transgenic technology, we showed that TOX is sufficient to initiate
the differentiation of immature thymocytes to the CD8 T-cell lineage, even in the absence of signals
mediated by T-cell antigen receptors. Other data suggest that TOX plays a role in the regulation
of CD4 gene silencing in the thymus. In contrast, TOX is not expressed during B-cell development.
Signaling through the serine/threonine
phosphatase calcineurin is required for positive selection of thymocytes. We found that the gene
for TOX is a downstream target for this signaling pathway. On the basis of these findings, we propose
that upregulation of TOX is a critical component of cell-lineage decisions during positive selection.
Experiments are under way to further test this idea.
A Cell-Surface Lymphocyte and Antigen-p\Presenting Cell Protein with Regulatory Function
We isolated a previously unidentified
gene that encodes a cell-surface protein of the immunoglobulin superfamily that is upregulated
during positive selection. On the basis of analysis of differential gene expression in mature
T cells, other researchers also identified this gene, which is now designated Btla (B-
and T-lymphocyte attenuator). Our studies indicated a wider expression pattern for BTLA, the
protein encoded by Btla, than was originally reported. We found that BTLA is expressed
on all lymphocytes, macrophages, and mature dendritic cells. Both murine and human BTLA have a
cytoplasmic domain containing tyrosine-based signaling motifs that are involved in negative
signaling. Using gene knockout and other technology, we are exploring the role of BTLA in the immune
system and are testing the potential of antibodies to BTLA as inhibitors of transplant rejection.
Publications
Aliahmad, P., O’Flaherty,
E., Han, P., Goularte, O.D., Wilkinson, B., Satake, M., Molkentin, J.D., Kaye, J. TOX
provides a link between calcineurin activation and CD8 lineage commitment. J. Exp. Med. 199:1089,
2004.
Han, P., Goularte, O.D., Rufner,
K., Wilkinson, B., Kaye, J. An inhibitory Ig superfamily
protein expressed by lymphocytes and APCs is also an early marker of thymocyte positive selection.
J. Immunol. 172: 5931, 2004.
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