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Contents
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CD4-CD8 Differentiation in the Thymus: The cKrox of the Matter
To examine cKrox function during T-cell development, we generated transgenic mice in which this protein is expressed in all developing and mature T cells. Remarkably, these mice had CD4 but not CD8 T cells. This raised the possibility that cKrox might impose CD4 choice to MHC-I–signaled precursors that are normally CD8-bound and, thus, might be one of the long sought-after effectors of CD4-CD8 differentiation. Although this was an appealing perspective, it was also possible that the cKrox transgene simply prevented the differentiation of CD8 T cells without affecting their lineage direction. To distinguish between these possibilities, we generated cKrox transgenic mice whose T cells all carry the same TCR specificity for a defined MHC-I–peptide complex. Normally, T-cell precursors in such mice fail to express cKrox and develop into CD8 cells. In the presence of the cKrox transgene, however, these precursors were redirected into CD4 cells, indicating that cKrox promoted CD4 choice at the expense of CD8 choice. Importantly, cKrox also imposed the functional helper differentiation characteristics of CD4 cells: Whereas MHC-I–specific CD8 T cells normally are cytotoxic, the MHC-I–specific CD4 T cells that developed in cKrox transgenic mice lacked cytotoxic properties (such as expression of the enzyme perforin) and had gained attributes of helper function. These findings indicate that cKrox is a master developmental regulator that imposes CD4 differentiation to developing thymocytes. In parallel to this work, the laboratory of Dietmar Kappes, PhD, Fox Chase Cancer Center, independently showed that a spontaneous point mutation in the gene encoding cKrox (which these authors called Thpok and that is now officially referred to as Zbtb7b) resulted in a phenotype mirroring the one observed in the cKrox transgene: Mice carrying this mutation lacked CD4 T cells and had MHC-II–specific cytotoxic CD8 T cells. The identification of cKrox as a master switch of CD4-CD8 lineage differentiation raises many questions. One key issue will be to investigate how cKrox is upregulated during the development of T cells recognizing MHC-II–peptide but not MHC-I–peptide complexes. The search for cKrox target genes should provide insight into the mechanism of lineage differentiation in the thymus. It is interesting that some cKrox homologs repress gene expression by recruiting enzymes (histone deacetylases) that promote the closure of chromatin to transcription. The possibility that cKrox affects lineage differentiation by altering chromatin is intriguing. Indeed, whereas many differentiation processes mediated by changes in chromatin organization are intimately associated with cell division (during which chromatin reorganization occurs), this is not the case with CD4-CD8 lineage differentiation.
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D4
and CD8 T cells are essential components of the adaptive immune system and are
critical for defense against infection. CD4 T cells recognize peptides of extracellular
origin in complex with class II major histocompatibility complex (MHC-II) molecules,
and generally provide help to other immune-competent cells. In contrast, CD8
T cells recognize peptides of intracellular origin in complex with MHC-I molecules
and differentiate into cytotoxic effectors. Besides their role in infection
defense, CD8 T cells have attracted the attention of tumor immunologists, who
seek to harness their killing power to eliminate cancer cells. CD4- and CD8-lineage
T cells arise in the thymus from nonfunctional precursors upon interaction with
MHC-II or MHC-I peptide complexes expressed on the thymic stroma, respectively.
How this choice of lineage occurs has puzzled immunologists and others for years
and is the main focus of our laboratory. Although signals initiated by the binding
of T-cell receptors (TCR) to intrathymic MHC molecules are now largely accepted
as critical for lineage choice, the intra-cellular effectors of this process
have remained mysterious. To gain insight into this question, we hypothesized
that genes encoding such effectors would be upregulated during lineage choice,
and we conducted a microarray screen to compare gene expression in T-cell precursors
during and immediately before lineage differentiation. One top candidate that
emerged from this search encodes a zinc finger DNA binding protein known as
cKrox, a member of a large family of transcription factors involved in many
differentiation processes. We found that cKrox was specifically upregulated
during the differentiation of CD4 but not of CD8 T cells in the thymus, and
that its expression remained CD4 specific in mature T cells.