RESEARCH HIGHLIGHTSIN BRIEF

T- C E L L D E V E LO P M E N T

Understanding why T-cellreceptors remain single

CHEMOKINESThe role of CCL21 in recruitment of T-precursors tofetal thymi.
Liu, C. et al. Blood 105, 31–39 (2005).

To ensure that T cells express a single
T-cell receptor β-chain (TCR-β) —that is, allelic exclusion — TCR-β
gene rearrangement is tightly regulated. Downregulation of Vβ genesegment accessibility was thought tobe crucial to the process of preventing secondary rearrangement after afunctional TCR-β has been generated. However, data published in
Nature Immunology indicate that
additional mechanisms regulate thisprocess.
TCR-β allelic exclusion is regulated at two stages of thymocyte development: at the double negative (DN)
stage, when the TCR-β alleles must
rearrange asynchronously; and at thedouble positive (DP) stage — which isreached only after a maturation signal
from the pre-TCR indicates successful
TCR-β gene rearrangement — whenfurther TCR-β gene rearrangement
must be prevented. In this study,Jackson et al. set out to investigate theproposed role of Vβ gene-segment
accessibility in the inhibition of Tcr-β
gene rearrangement after transition
to the DP stage. They generated
knock-in mice expressing a modified
Tcr-β allele in which the Tcr-αenhancer (Eα) was positioned downstream of the Vβ12 gene segment.
Consistent with their hypothesis thatthe introduced Eα (Eαi) would maintain the Vβ chromatin in an active
configuration in DP thymocytes, DP
thymocytes from knock-in mice
showed increased Vβ germline transcription, histone modification andrestriction-endonuclease accessibility compared with DP thymocytes
from wild-type animals.

Vβ-to-DJβ rearrangement isdownregulated in DP thymocytes
from wild-type mice compared withrearrangement in DN thymocytes
from the same animals. Surprisingly,
despite the presence of Eαi increasing
Vβ chromatin activation (and therefore accessibility) in DP thymocytes,
Vβ-to-DJβ rearrangement in thesecells was also downregulated compared with rearrangement in DN
thymocytes.
Although inhibition of Tcr-β generearrangement after transition to theDP stage was not affected by the presence of Eαi, Vβ12 rearrangement wasincreased in DN thymocytes fromEαi-knock-in mice. This correlatedwith an increased proportion ofperipheral T cells expressing botha TCR-β containing Vβ12 and a
TCR-β containing a distinct Vβ. Bothβ-chains were products of chromosomal rearrangement, indicating that thepresence of Eαi disturbed allelic exclusion in DN thymocytes. This defect inallelic exclusion occurred despite thefact that the presence of Eαi did notalter Vβ gene-segment accessibility in
DN thymocytes.
This study shows that factors
other than chromatin accessibility
regulate allelic exclusion in both theDN and the DP thymocyte compartments. Future studies will focus onidentifying these additional controls
at the molecular level.
Karen Honey
References and links
ORIGINAL RESEARCH PAPER Jackson, A.,

Kondilis, H. D., Khor, B., Sleckman, B. P. &Krangel, M. S. Regulation of T cell receptor β
allelic exclusion at a level beyond accessibility.
Nature Immunol. 6, 189–197 (2005).

In this study, Liu et al. developed a new in vitro time-lapse
visualization technique to investigate the mechanisms that
regulate the recruitment of T-cell precursors to the fetal thymus.
Initial analysis showed that a proportion of fetal liver cells, bloodcells or DN1 thymocytes were actively attracted towards analymphoid fetal thymic lobe and that these cells were T-cellprecursors. Recruitment of the T-cell precursors was mediatedby MHC-class-II-expressing thymic epithelial cells and wasabrogated in the presence of an inhibitor of G-protein-coupledreceptors. Subsequently, the chemokines CCL21 and CCL25 wereshown, using several assays, to contribute significantly to theactive recruitment of T-cell precursors to the fetal thymus.
Further studies are required to determine the molecularmechanism by which these chemokines mediate this effect.
I N N AT E I M M U N I T Y

Drosophila peptidoglycan recognition protein LC(PGRP-LC) acts as a signal-transducing innate
immune receptor.
Choe, K.-M. et al. Proc. Natl Acad. Sci. USA 102, 1122–1126 (2005).

Drosophila melanogaster peptidoglycan-recognition protein LC(PGRP-LC) is a transmembrane protein that is required for
antibacterial responses mediated by a signalling pathway thatinvolves the death-domain protein IMD and the nuclear factor-κBfamily member Relish. Because the cytoplasmic domain ofPGRP-LC has no sequence homology to characterized proteins,
it is not known how it activates intracellular signalling. Using
truncated PGRP-LC mutants, the authors show that its
cytoplasmic domain binds IMD and is crucial for the inductionof downstream signalling. The cytoplasmic domain also promotes
dimerization of PGRP-LC, which might be required for its
activation and might increase the diversity of ligand recognition
through heterodimerization with other PGRP-LC isoforms.
DENDRITIC CELLS

Selective Rac1 inhibition in dendritic cells diminishes
apoptotic cell uptake and cross-presentation in vivo.
Kerksiek, K. M. et al. Blood 105, 742–749 (2005).

Previous in vitro studies have indicated that the RHO guanosine
triphosphatase RAC1 is important for endocytosis and crosspresentation by dendritic cells (DCs). In this paper, the authors
confirm these results in vivo. Because knocking out RAC1
function in mice is embryonic lethal, they generated transgenic
mice that express a DC-specific dominant-negative formof RAC1. Transgenic mice showed defective CD8+ DC
development and function, and although DC migration wasunaffected, apoptotic-cell uptake and cross-presentation byCD8+ DCs was severely impaired. This reduced their ability toinduce bacteria-specific T-cell responses, indicating animportant role for RAC1 in CD8+ DC function in vivo.

NATURE REVIEWS | IMMUNOLOGYVOLUME 5 | FEBRUARY 2005 | 9 5© 2005 Nature Publishing Group