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of June 17, 2017.
Differential Roles of IL-2−Inducible T Cell
Kinase-Mediated TCR Signals in
Tissue-Specific Localization and Maintenance
of Skin Intraepithelial T Cells
Mingcan Xia, Qian Qi, Yan Jin, David L. Wiest, Avery
August and Na Xiong
J Immunol published online 17 May 2010
http://www.jimmunol.org/content/early/2010/05/17/jimmun
ol.1000453
http://www.jimmunol.org/content/suppl/2010/05/17/jimmunol.100045
3.DC1
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Supplementary
Material
Published May 17, 2010, doi:10.4049/jimmunol.1000453
The Journal of Immunology
Differential Roles of IL-2–Inducible T Cell Kinase-Mediated
TCR Signals in Tissue-Specific Localization and Maintenance
of Skin Intraepithelial T Cells
Mingcan Xia,* Qian Qi,*,1 Yan Jin,*,1 David L. Wiest,† Avery August,* and
Na Xiong*
U
nlike conventional ab T cells that primarily reside in
secondary lymphoid organs for adaptive immune
responses, various subsets of gd T cells preferentially
reside in epithelial tissues, such as the skin, reproductive tract, respiratory tracts, and intestines, where they function as the first line of
defense (1). The different tissue-specific gd T cells preferentially
use different subsets of TCRs. In mice, skin-specific intraepithelial
gdT lymphocytes (sIELs, also called dendritic epidermal T cells),
a prototype of the tissue-specific T cells, almost exclusively express
canonical gd TCRs composed of Vg3–Jg1Cg1 and Vd1–Dd2–
Jd2Cd chains, whereas vaginal epithelial gd T cells express Vg4/
Vd1+ TCRs. By comparison, gd T cells in secondary lymphoid
organs express more diverse TCRs, predominantly of Vg2 and
Vg1.1 associated with several Vd chains. The preferential usage
of specific TCRs by the different tissue-specific gd T cells is suggested to be important for their tissue-specific functions. sIEL-
*Department of Veterinary and Biomedical Sciences, Center for Molecular Immunology and Infectious Disease, Pennsylvania State University, University Park, PA
16802; and †Blood Cell Development and Cancer Program, Fox Chase Cancer Center, Philadelphia, PA 19111
1
Q.Q. and Y.J. contributed equally to this work.
Received for publication February 9, 2010. Accepted for publication April 8, 2010.
This work was supported by grants from the National Institutes of Health (to N.X)
and in part by a grant from the Pennsylvania Department of Health using Tobacco
Settlement Funds (to N.X.).
The Department of Veterinary and Biomedical Sciences specifically disclaims responsibility for any analyses, interpretations, or conclusions.
Address correspondence and reprint requests to Dr. Na Xiong, Department of Veterinary and Biomedical Sciences, Pennsylvania State University, 115 Henning Building, University Park, PA 16802. E-mail address: [email protected]
The online version of this article contains supplemental material.
Abbreviations used in this paper: ITK, IL-2–inducible T-cell kinase; N.D., no difference;
PECy5, R-phycoerythrin-Cy5; S1P, sphingosine-1-phosphate; S1PR1, sphingosine-1phosphate receptor 1; sIEL, skin-specific intraepithelial gdT lymphocyte; Tg, transgenic;
WT, wild-type.
Copyright Ó 2010 by The American Association of Immunologists, Inc. 0022-1767/10/$16.00
www.jimmunol.org/cgi/doi/10.4049/jimmunol.1000453
specific Vg3+ gd TCRs react with Ags upregulated on diseased skin
cells and play an important role in tumor surveillance and wound
healing among others to maintain the integrity of the skin.
Precursors for the different tissue-specific gd T cells are generated
in the thymus at different stages of ontogeny. The Vg3+ sIEL precursors are generated exclusively in the early fetal thymus, where
they are the first T cell population to arise during ontogeny (around
day 15 of embryonic gestation, E15). Once out of the thymus, they
take residence in the skin epithelium, where they expand and sustain
locally for the life span of mice (2–4). In contrast, fetal thymic Vg4+
gd T cells localize to peripheral destinations, such as the reproductive tract. In adults, the generation of Vg3+ and Vg4+ gd T cells is
completely suppressed, whereas Vg2+ and Vg1.1 gd+ T cells are
predominately generated and preferentially emigrate to secondary
lymphoid organs, among other tissues. However, mechanisms regulating tissue-specific development of the various gd T cell subsets
are not well understood.
It has become clear recently that selection is involved in the
development of tissue-specific gd T cells, at least in the case of
sIELs (5–7). We reported that fetal thymic gd T cell populations
that display activated or memory phenotypes correlated with their
development into sIELs (7). Compared to other gd T cells, fetal
thymic Vg3+ gd T cells express a unique set of chemokine and
cytokine receptors, including high levels of sphingosine-1phosphate (S1P) receptor 1 (S1PR1) and CCR10 (7), which are
potentially important for their thymic egress and skin localization
(8–11), and the cytokine receptor CD122 (IL-15Rb), which is
critical for their survival/expansion in the skin (12, 13). In
absence of positive selection, as observed in a substrain of FVB
mice (FVB/Taconic) that express mutated Skint1, a selecting
molecule for the Vg3+ sIEL precursors, these cells could not
develop into sIELs (14, 15). However, if transgenic (Tg) fetal
thymic gd T cells are positively selected to express the proper
chemokine and cytokine receptors, then they could develop into
sIELs (7, 16). These findings suggest that the TCR-dependent positive selection of fetal thymic gd T cells is critical for their
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Tissue-specific innate-like gd T cells are important components of the immune system critical for the first line of defense, but
mechanisms underlying their tissue-specific development are poorly understood. Our study with prototypical skin-specific intraepithelial gdT lymphocytes (sIELs) found that among different thymic gd T cell subsets fetal thymic precursors of sIELs specifically
acquire a unique skin-homing property after positive selection, suggesting an important role of the TCR selection signaling in
“programming” them for tissue-specific development. In this study, we identified IL-2–inducible T cell kinase (ITK) as a critical
signal molecule regulating the acquirement of the skin-homing property by the fetal thymic sIEL precursors. In ITK knockout mice,
the sIEL precursors could not undergo positive selection-associated upregulation of thymus-exiting and skin-homing molecules
sphingosine-1-phosphate receptor 1 and CCR10 and accumulated in the thymus. However, the survival and expansion of sIELs in
the skin did not require ITK-transduced TCR signaling, whereas its persistent activation impaired sIEL development by inducing
apoptosis. These findings provide insights into molecular mechanisms underlying differential requirements of TCR signaling in
peripheral localization and maintenance of the tissue-specific T cells. The Journal of Immunology, 2010, 184: 000–000.
2
Materials and Methods
Mice
ITK2/2, TCRd2/2, Vav12/2, and b2M2/2 and KN6 Tg mice were previously described (32, 42, 43). CCR10 knockout/EGFP knockin
(CCR10EGFP/EGFP) mice were generated in the Xiong laboratory at Pennsylvania State University. ITK2/2KN6 and ITK2/2CCR10+/EGPP mice were
generated by crossing ITK2/2 mice with KN6 and CCR10EGFP/EGFP mice,
respectively. All of the mice were kept in specific pathogen-free conditions
and used for experiments at ages of 6–8 wk unless indicated otherwise in the
text or figure legends. Experiments were approved by the Institutional Animal Care and Use Committee at Pennsylvania State University.
Cell preparations
Epidermal cells were prepared as previously described (44). Briefly, hair
was removed from the skin with Nair (Church & Dwight, Princeton, NJ).
The treated skin was excised and trimmed of s.c. fat. Skin strips were
digested with 0.3% trypsin/glucose/NaCl/KCl solution for 45 min at 37˚C.
Epidermal layers of skin strips were gently removed and incubated with
the 0.3% trypsin/glucose/NaCl/KCl solution containing 0.0001% DNase
for 10 min at 37˚C. The epidermal cells were washed with medium and
purified with Ficoll (GE Healthcare, Piscataway, NJ). Cells were cultured
overnight in media containing IL-2 (20 U/ml) and used for analyses.
Abs and reagents
FITC-conjugated anti-Vg2 (or Vg3), R-phycoerythrin-Cy5 (PECy5)conjugated anti-CD3, and PE-conjugated anti-CD24 Abs were purchased
from BD Biosciences (San Jose, CA). FITC-conjugated anti-CD122 and
biotin-conjugated anti-BrdU, anti-CD122, and anti-gdTCR Abs were
purchased from eBioscience (San Diego, CA). PE- or FITC-conjugated
streptavidin was purchased from Invitrogen. S1P was purchased from
VWR (San Diego, CA), and CCL27 was purchased from PeproTech (Rocky
Hill, NJ).
Flow cytometry
Cells were incubated with fluorescent Abs for 30 min at 4˚C. For biotinlabeled Abs, streptavidin–PE was added in the second step and incubated
for 20 min at 4˚C. All of the samples were analyzed using the flow cytometer FC500 (Beckman Coulter, Miami, FL).
Immunofluorescent microscopy of ear epidermal sheets to detect
sIELs
The epidermal sheets were prepared as described (45). Briefly, 6-wk-old
mice were sacrificed; the ears were cut off, mechanically split into dorsal
and ventral sides, and then placed in EDTA solution. After incubation,
the epidermis was peeled off as a single sheet and stained with FITCconjugated anti-Vg2 or Vg3 TCR Abs and analyzed on a fluorescence
microscope (BX61, Olympus, Center Valley, PA, or Eclipse TE300,
Nikon, Melville, NY).
Semiquantitative and real-time RT-PCR
Total RNAwas extracted from sorted cells or skin tissue with TRIzol reagent
(Invitrogen) according to the manufacturer’s instructions. The first-strand
cDNA was synthesized from the RNA using SuperScript III Reverse Transcriptase (Invitrogen). For semiquantitative PCR, serial 5-fold dilutions
of cDNA were subjected to PCR with primer sets for rearranged Vg3,
CCL27, or b-actin. Quantitative real-time PCR was performed using
SYBR Green Master Mix (Invitrogen). Primer sets for individual genes
are as follows: CCR6F, 59-AGAACTCCAAGAGGCACAGAGCAA-39;
CCR6R, 59-TGTTGTGAGGGATCTGACAAGCCA-39; CCR10F, 59-TTCCTAGCCTGTATCAGCG; CCR10R, 59-TAGAGCCAGAAACAGCGAC-39; S1PR1F, 59-GTGTAGACCCAGAGTCCTGCG-39; S1PR1R, 59AGCTTTTCCTTGGCTGGAGAG-39; KLF2F, 59-TGTGAGAAATGCCTTTGAGTTTACTG-39; KLF2R, 59-CCCTTATAGAAATACAATCGGTCATAGTC-39; b-actinF, 59-CCCATCTACGAGGGCTAT-39; b-actinR,
59-TGTCACGCACGATTTCC-39; L3 and J1 primers were previously
described (46).
CFSE cell proliferation assay
Cells were loaded with CFSE by incubation at a concentration of 1 3 107
cells per milliliter in PBS for 10 min with 2.5 mM CFSE (Molecular
Probes, Eugene, OR) at 37˚C, then washed with cold complete medium
twice, and resuspended in culture medium. The labeled cells (5 3 106 cells
per well, 2 ml) were cultured for 3 d in 12-well tissue culture plates
(Becton Dickinson Labware, Franklin Lakes, NJ) in the presence of
IL-2 (10 U/ml) only, IL-15 (50 ng/ml) and IL-2 (10 U/ml), or anti-gd
TCR Ab (GL4, 1 mg/ml) and IL-2 (10 U/ml). After the culture, the cells
were stained with PE-conjugated anti-gd TCR (GL3) and PECy5conjugated anti-CD3 Abs and analyzed by flow cytometry.
BrdU incorporation assay
Mice were injected i.p. with 100 ml BrdU dissolved in PBS (10 mg/ml) at
the onset of experiments. At the same time, mice were fed water that
contained 0.8 mg/ml BrdU and 5% glucose. For the incorporation of BrdU
into 2-wk-old mice, the mice were injected i.p. with 50 ml of 10
mg/ml BrdU every other day. Nine days later, sIELs were isolated and
assessed for BrdU incorporation by staining with biotin-conjugated antiBrdU Ab/PE–streptavidin and FITC-conjugated anti-Vg3 and PECy5conjugated anti-CD3 Abs and flow cytometric analysis.
Downloaded from http://www.jimmunol.org/ by guest on June 17, 2017
development into sIELs by promoting the expression of proper
homing and cytokine receptors for epidermal localization and
expansion.
Previous studies using various knockout mice found that multiple TCR signaling molecules, including Lck, Syk, and ZAP-70,
are important for sIEL development (17–20). Although these molecules are involved in TCR signaling in general, they may differentially affect the development of sIELs and other T cell
populations. For example, mice deficient in Syk, a kinase downstream of the TCR signaling, have normal development of conventional ab T cells and splenic gd T cells but impaired sIEL
development, suggesting that there is a unique molecular signaling
requirement for sIEL development (17, 21). However, mechanisms by which TCR signaling molecules affect tissue-specific
sIEL development are poorly understood.
IL-2–inducible T-cell kinase (ITK) is a Tec family nonreceptor
tyrosine kinase that plays multiple roles downstream of TCR signaling. During TCR signaling, ITK forms a complex with the
adaptor molecule Slp-76 and is involved in the phosphorylation
of phospholipase Cg and intracellular Ca2+ mobilization. The
ITK-regulated signal is also involved in the activation of the
ERK/MAPK pathway and the activation of transcription factors
AP-1 and NFAT. In addition, ITK-mediated signals modulate the
TCR-induced reorganization of the actin cytoskeleton by interfacing with the guanine nucleotide exchange factor Vav1, another
important TCR signaling molecule (22–24). Besides its role in
the TCR signaling, ITK could transduce integrin and chemokine
receptor-initiated signaling (22, 25–29).
ITK deficiency affects the development of various T cell populations differentially (30–34). Although the absence of ITK impairs
the development of conventional CD4+ and CD8+ T cells, the development of nonconventional or “innate memory phenotype” CD4+ and
CD8+ T cells remains intact. The nonconventional CD8+ T cells in
ITK-deficient mice exhibit activated/memory phenotypes, including
the expression of memory markers CD44, CD122, and NK1.1, rapid
production of cytokines, and dependency on IL-15, features shared by
innate lymphocytes (35–37). ITK is also required for the development
and function of invariant NKT cells (38, 39). More recently, we and
others have reported that ITK deficiency increased the generation of
an IL-4–producing Vg1.1+ T cell population (40, 41). These studies
suggest that ITK is a key signal component that differentially regulates
the development of various T cell populations.
In this study, we identified ITK as a critical signaling molecule
specifically involved in the positive selection-associated acquirement of the unique skin-homing property by the fetal thymic sIEL
precursors for their specific peripheral location, but dispensable for
their maintenance, which provides molecular insights into differential requirements for TCR signaling in peripheral localization
and survival/expansion of the tissue-specific gdT cells.
ITK IN SKIN-RESIDENT T CELL DEVELOPMENT
The Journal of Immunology
3
Chemotaxis assay
The assay was performed using 24-well chemotaxis chambers (Corning Costar, Lowell, MA). E16 fetal thymocytes (2 3 105 cells per well, 100 ml)
were added to the upper chamber, and 100 nM S1P, 100 nM CCL27, or
fetal skin culture medium was placed in the bottom chamber. Cells were
then incubated for 4 h at 37˚C and 5% CO2, and the cells in the upper and
lower chambers were collected and analyzed. The percentage of migration
was determined from the original cell input.
In situ TUNEL staining of ear epidermal sheets to detect
apoptotic sIELs
These experiments were performed using the TMR red In Situ Cell Death
Detection Kit (Roche Applied Science, Indianapolis, IN). Briefly, freshly
isolated ear epidermal sheets were fixed with 4% paraformaldehyde for
20 min and permeablized with 0.1% Triton X-100 for 2 min on ice, followed by culture with the TUNEL reaction mixture for 30 min and FITCconjugated anti-Vg2 Ab overnight. The stained epidermal sheets were
analyzed by Olympus FluoView TM FV300 laser scanning microscope.
Adoptive transfers
Statistical analysis
All of the data are expressed as mean 6 SD. Statistical significance was
determined by two-tailed Student t tests. p , 0.05 is considered significant.
Results
Defective development of epidermal gd T cells in ITK knockout
mice
To evaluate the role of ITK-mediated signaling in sIEL development, we first assessed the sIEL populations in ITK2/2 and
wild-type (WT) mice by flow cytometry (Fig. 1A). Compared to
the WT controls of the same ages, 6- to 8-wk-old ITK2/2 mice
had reduced percentages of Vg3+ sIELs (14.73 6 0.41% versus
3.56 6 0.32%, p , 0.0001). In situ examination of sIELs on skin
epidermal sheets by immunofluorescent microscopy confirmed the
impaired development of sIELs in ITK2/2 mice (181.8 6 19.5
[WT] versus 66.3 6 4.3 [ITK2/2] cells per field, p , 0.05),
demonstrating that ITK-mediated signaling is important in sIEL
development (Fig. 1B, 1C). In addition, although the remaining
sIELs in ITK2/2 mice displayed the normal dendritic morphology
(Fig. 1B), they produced less IFN-g when stimulated in vitro with
anti-gd TCR Ab (Supplemental Fig. 1), suggesting that the ITKtransduced TCR signaling is also important for the function of
sIELs.
Vav1 lies downstream of ITK and has been suggested to act
directly with ITK to transduce TCR-activated signals (24). In
addition, Vav1 has been found previously to be critical for
gdTCR-mediated T cell proliferation (47), raising the possibility
that it is also involved in sIEL development. To test this, we
analyzed the development of sIELs in Vav1 knockout mice. Surprisingly, unlike ITK2/2 mice, 6- to 8-wk-old Vav12/2 mice had
similar numbers of sIELs in the skin as WT controls (60.5 6 6.3
[WT] versus 54 6 7.1 [Vav12/2] cells per field) (Fig. 1D–F),
suggesting that Vav1-regulated signals are not involved in the
ITK-mediated sIEL development.
Signals regulated by ITK could be potentially involved in
multiple stages of sIEL development, from the TCR-mediated positive selection of the fetal thymic sIEL precursors to their peripheral
expansion in the skin. We therefore determined whether the seeding
of fetal skin by fetal thymic Vg3+ sIEL precursors was impaired in
ITK2/2 mice. Compared to the WT controls, transcripts of rearranged Vg3 TCR in the fetal skin were dramatically decreased in
FIGURE 1. Impaired development of sIELs in ITK2/2 but not Vav12/2
mice. A, Skin cell preparations from 6- to 8-wk-old ITK2/2 and WT mice
were stained with anti-CD3 and Vg3 Abs and analyzed for percentages of
the CD3+Vg3+ population by flow cytometry. One representative of three
independent experiments is shown. B and C, Ear epidermal sheets from
WT and ITK2/2 mice were stained with fluorescent anti-Vg3 Ab and
observed under a fluorescent microscope (Olympus BX61) for the Vg3+
sIELs (B), average numbers of which per field at 3200 magnification were
plotted (C). Data were obtained from three independent experiments. pp ,
0.05. D–F, Flow cytometric and immunofluorescent microscopic analysis
of skin Vg3+ gd T cells from Vav12/2 mice as performed in the A–C,
except that the Vg3+ sIELs on the epidermal sheets were visualized under
a different fluorescent microscope (Nikon Eclipse TE 300) that has
a smaller field at the same 3200 magnification (E). Experiments were
repeated twice for both flow cytometric and immunofluorescent analyses.
Average numbers of sIELs per field obtained in E were plotted in F. G,
Total RNA from fetal and adult mouse skin was reverse-transcribed to
cDNA. Serially 5-fold–diluted cDNA was subjected to semiquantitative
PCR to determine expression levels of the rearranged TCRg3 gene.
b-Actin was used as a control. Data shown were obtained from three
independent experiments. N.D., no difference.
ITK knockout mice (.10-fold reduction) (Fig. 1G), suggesting
that the defective sIEL development originates at the fetal stage,
likely due to impaired generation, selection, or both of the fetal
thymic sIEL precursors. As a control, the reduction of transcripts
of rearranged Vg3 TCR in the skin of 6- to 8-wk-old ITK2/2 mice
was ∼3-fold and correlated with the reduction of the Vg3+ sIEL
numbers (Fig. 1A–C, 1G).
sIEL precursors undergo a seemingly normal positive selection
process but accumulate in the fetal thymus of ITK2/2 mice
To address how ITK deficiency affects the development of sIEL
precursors, we characterized the fetal thymic Vg3+ gd T cells of
ITK2/2 mice. First, we determined whether ITK deficiency affects
their positive selection and maturation. As previously reported, the
positive selection and maturation of the fetal thymic Vg3+ cells
are associated with upregulation of CD122 and downregulation of
CD24 in WT mice (7, 48). Surprisingly, this was also the case in
fetal thymic Vg3+ cells of ITK2/2 mice (Fig. 2A), indicating that
ITK deficiency does not affect their general selection and maturation processes.
Generation of the fetal thymic Vg3+ sIEL precursors was not
impaired significantly in ITK2/2 mice either. Although there was
a slight delay in the appearance of Vg3+ gd T cells in E15 fetal
thymi of ITK2/2 mice, this was no longer the case by E16 (Fig.
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KN6 Vg2+ Tg gd T cells were purified from E16 fetal thymus of ITKsufficient or knockout KN6 mice by a cell sorter and injected i.p. into 1wk-old b2M2/2TCRd2/2 recipients (5 3 105 cells per mouse). Eight
weeks after the transfer, ear epidermal sheets of the recipients were analyzed for donor-derived sIELs by in situ immunofluorescent staining.
4
ITK IN SKIN-RESIDENT T CELL DEVELOPMENT
Vg3+ cells, the number of Vg3+ cells continued to increase in the
ITK2/2 fetal thymi, resulting in significantly more of these cells
in ITK2/2 mice than in WT mice (Fig. 2B). These findings raise
a possibility that the ITK deficiency might affect the proper migration of the sIEL precursors, which would correlate with their
impaired seeding in the fetal skin (Fig. 1G).
ITK-deficient fetal thymic sIEL precursors cannot undergo a
proper switch in the expression of thymus-exiting and
skin-homing molecules
2B). In fact, as the fetuses aged, there was gradual accumulation of
Vg3+ cells in the ITK2/2 fetal thymi. By E18 when the number
of Vg3+ cells was decreased in WT fetal thymi due to the egress
of mature Vg3+ gd T cells and the reduced generation of new
FIGURE 3. ITK2/2 fetal thymic sIEL precursors exhibit altered migration molecule expression and defective migration capability. A, Real-time RT-PCR
analysis of the expression of indicated molecules in purified CD122+ and CD1222CD3+Vg3+ cells of E16 WT and ITK2/2 fetal thymi. Data shown were
obtained from three independent experiments. B, E14 or E16 fetal thymocytes of ITK2/2CCR10+/EGFP and ITK+/2CCR10+/EGFP mice were analyzed for
CCR10(EGFP) expression on Vg3+ cells. Percentages and numbers of CCR10(EGFP)+Vg3+ cells are shown. Data presented are one representative from at
least six mice of each genotype. C, In vitro migration of WT and ITK2/2 E16 fetal thymic Vg3+ gd T cells to S1P, CCL27, and conditioned medium of fetal
skin cultures. The migration index was calculated as a ratio of numbers of Vg3+ cells migrating into the bottom chamber in presence of attractants versus
medium only. Data shown were obtained from two independent experiments. pppp , 0.001.
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FIGURE 2. Vg3+ sIEL precursors undergo a normal maturation process
but accumulate in the fetal thymus of ITK2/2 mice. A, Flow cytometric
analysis of CD122 and CD24 expression on gated E16–E17 fetal thymic
Vg3+ gd cells. One representative of three independent experiments is
shown. B, Numbers of Vg3+ gd T cells in WT and ITK2/2 fetal thymi of
different gestation ages. The numbers were calculated based on total
numbers of thymocytes and percentages of Vg3+ cells per thymus. Data
presented were means and SDs from three to five experiments. pp , 0.05;
ppp , 0.01; pppp , 0.001.
TCR-mediated positive selection of the fetal thymic Vg3+ sIEL
precursors promotes a coordinated switch in expression of multiple “migration” molecules, including the upregulation of S1PR1
and CCR10 and downregulation of CCR6, which are important for
their thymic egress and skin-homing (7). To determine whether
ITK-mediated signals regulate their expression, we sorted
CD1222 and CD122+ Vg3+ gd T cells from ITK2/2 and WT fetal
thymocytes and analyzed the expression of these migration molecules by real-time RT-PCR. Compared to the WT controls, the
upregulation of CCR10 and S1PR1 expression in the ITK2/2 fetal
thymic CD122+ Vg3+ gd T cells was significantly impaired (Fig.
3A). Therefore, although its deficiency did not affect the maturation of fetal thymic sIEL precursors, ITK is required for promoting the proper expression of the migration molecules in the mature
fetal thymic sIEL precursors. The mature fetal thymic CD122+
Vg3+ T cells of ITK2/2 mice also expressed a lower level of
KLF2 (Fig. 3A), a transcription factor critical in the regulation
of chemokine receptor expression in positively selected ab
T cells (49, 50), suggesting that KLF2 might be involved in the
ITK-mediated chemokine receptor expression. In contrast to the
defect in upregulation of the migration molecules in the mature
The Journal of Immunology
as severe (Fig. 1A–C, 1G). This suggests that there might be
a homeostatic compensation by which the few ITK-deficient sIEL
precursors that make it to the skin are capable of extensive expansion, which would result in the reduced difference in numbers
of sIELs between ITK2/2 and WT mice when they grow older.
Consistent with this, compared with WT controls of same ages, 3to 4-wk-old ITK2/2 mice had on average 6-fold reduction in the
number of sIELs (8.5 6 1.5% [WT] versus 1.5 6 0.7% [ITK2/2],
p , 0.01, n = 5 each), whereas the reduction was 3- to 4-fold in
the 6- to 8-wk-old adults (Fig. 1A–C).
To directly assess whether the ITK deficiency affected the
proliferation of sIELs, we performed in vivo BrdU labeling
experiments of WT and ITK2/2 mice and found that the ITKdeficient sIELs incorporated BrdU at a level similar as, if not
higher than, WT sIELs (Fig. 4A). Therefore, although ITK is
critical for the regulated expression of multiple migratory molecules on fetal thymic sIEL precursors for their epidermal localization, it is not required for their peripheral proliferation.
Correlating with their normal in vivo proliferation, there was no
significant difference in the in vitro proliferation of the ITK2/2 and
WT fetal thymic sIEL precursors in response to the anti-gd TCR
Ab stimulation (Fig. 4B), which was in contrast with the
requirement of Vav1 in the TCR signaling-mediated proliferation
of gd T cells (Fig. 4C) (47). Together with the fact that the Vav1
knockouts did not have any defects in sIEL development (Fig.
1D–F), these results support a notion that TCR-mediated signaling
is not required for the peripheral expansion and maintenance of
sIELs. Likely, the expansion of sIELs in the skin is driven by IL15/receptor signaling (12, 13). Consistent with this, both ITK2/2
and Vav12/2 fetal thymic gd T cells proliferated normally in response to IL-15 (Fig. 4D).
ITK-regulated TCR signals are not required for the expansion
of sIELs in the skin
Continuous ITK-transduced, TCR/ligand-initiated signals in sIELs
impair their maintenance in the skin due to activation-induced
apoptosis
We noted that even though the ITK2/2 fetal thymic sIEL precursors exhibited severe defects in skin-seeding at the fetal stage,
the reduction in the number of sIELs in adult ITK2/2 mice is not
To directly characterize the role of ITK-regulated TCR signaling in
the development of sIELs, we crossed ITK2/2 mice with KN6
gdTCR Tg mice (42). The Vg2+ KN6 gdTCR recognizes ligands
FIGURE 4. ITK2/2 sIELs and
their fetal thymic precursors have
normal proliferation capacities. A,
Similar in vivo proliferation rates of
WT and ITK2/2 sIELs. The 2-wk- or
1-mo-old mice were treated with
BrdU for 9 d, and then sIELs were
isolated and analyzed for BrdU incorporation by flow cytometry. Data presented were means and SDs from
three to five experiments. B–D,
CFSE-labeled E16 fetal thymocytes
from ITK2/2, Vav12/2, or WT mice
were stimulated with anti-gd TCR
Ab (1 mg/ml, GL4) or IL-15 (50 ng/
ml) for 3 d and analyzed by flow
cytometry for the proliferation of
CD3+ gd T cells. One representative
of three independent experiments was
shown.
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ITK2/2 CD122+ Vg3+ T cells, there was little difference in their
expression on the immature CD1222 Vg3+ T cells of the ITK2/2
and WT mice (Fig. 3A), suggesting that the ITK-mediated signaling is mainly involved in regulation of the positive selectionassociated acquisition of the skin-homing property.
To further confirm that the ITK deficiency impaired the CCR10
upregulation in the positively selected sIEL precursors, we crossed
ITK2/2 mice with CCR10 knockout/EGFP knockin mice that use
EGFP as a reporter for CCR10 expression. The resultant ITK2/2
CCR10+/EGFP mice had a significantly reduced percentage of
CCR10(EGFP)+ Vg3+ gd T cells in the early fetal thymus (Fig. 3B,
top panels). However, as the fetus aged, this reduction disappeared
(Fig. 3B, bottom panels). Considering that the ITK2/2 mature Vg3+
gd T cells are defective in the upregulation of S1PR1, one plausible
explanation for this is that although the ITK2/2 Vg3+ T cells have
impaired upregulation of CCR10 after positive selection, the smaller
numbers of CCR10+ Vg3+ cells are unable to emigrate and accumulate in the thymus. By contrast, WT mature CCR10+ Vg3+ sIEL
precursors constantly migrate out of the thymus.
To further dissect this, we assessed the in vitro migration
capabilities of ITK2/2 fetal thymic Vg3+ gd T cells toward S1P,
a ligand for S1PR1 involved in mature thymic T cell emigration
(9, 50–52). As shown in Fig. 3C, ITK2/2 fetal thymic Vg3+ gd
T cells migrated much less efficiently than those in WT controls
toward the S1P attraction. The ITK2/2 Vg3+ gd T cells also had
defects in migration toward CCL27 and culture media of the fetal
skin (Fig. 3C), suggesting that they have impaired ability in
CCL27-mediated skin homing. Together, these results demonstrate that ITK-mediated signaling is important for positive
selection-associated acquisition of the unique homing property
in the fetal thymic Vg3+ sIEL precursors for their egress from
the thymus and localization in the skin.
5
6
analysis of epidermal sheets found significantly lower percentages
of apoptotic Tg sIELs in ITK2/2 KN6 than those in ITK-sufficient
KN6 mice (Fig. 5D, Supplemental Fig. 3).
These data suggest the increased number of Tg sIELs in adult
ITK2/2 KN6 mice to be a result of improved peripheral maintenance, which overcomes the reduced initial skin-seeding by the
ITK2/2 fetal thymic Tg gd T cells. To further test this, we eliminated
the effect of peripheral TCR/ligand interaction-induced signaling
by transferring ITK2/2 or ITK+/+ fetal thymic KN6 Tg gd T cells
into ligand-negative b2M2/2TCRd2/2 recipients, which lack endogenous gd T cells. Eight weeks after the transfer, the recipients
were analyzed for donor-derived sIELs. As shown in Fig. 5E, the
adoptively transferred ITK2/2 fetal thymic KN6 Tg gd T cells
gave rise to fewer sIELs in the ligand-negative recipients than the
ITK+/+ donor cells, a difference that is a reversal from that seen in
ITK2/2 KN6 mice. In addition, the sIELs that developed in the
recipients also displayed normal dendritic morphology (Fig. 5E).
Together, these results demonstrate that the continuous peripheral
TCR/ligand interaction, signaling through ITK, impairs the maintenance of Tg sIELs by promoting their apoptosis, which could be
corrected by removing ITK-mediated signals.
Discussion
Although it is increasingly realized that the various subsets of
tissue-specific gd T cells are important components of the immune
FIGURE 5. ITK-mediated TCR/ligand-induced signaling in the skin impairs the development of sIELs by promoting their apoptosis. A, Skin cell
preparations of KN6 Tg mice of WT and ITK2/2 backgrounds were analyzed for Tg Vg2+ sIELs by flow cytometry. Percentages of the Tg sIELs are
indicated. One representative of three independent experiments is shown. B and C, Ear epidermal sheets of ITK-sufficient and knockout KN6 mice were
stained and observed under a fluorescent microscope (Olympus BX61) for Tg Vg2+ sIELs (B), average numbers of which per field at the 3200
magnification were plotted (C). Data were obtained from three experiments. ppp , 0.01. D, Lower percentages of apoptotic KN6 Tg sIELs on ITK2/2
than WT background. The percentages of apoptotic sIELs were calculated based on ratios of numbers of apoptotic versus total sIELs from in situ TUNEL
analyses of ear epidermal sheets, as shown in Supplemental Fig. 3. Data shown were obtained from at least four mice of each genotype in two independent
experiments. pp , 0.05. E, The development of KN6 Tg sIELs in b2M2/2TCRd2/2 recipients from adoptively transferred fetal thymic ITK-sufficient or
knockout fetal thymic KN6 Tg gd T cells. Ear epidermal sheets of the recipients were analyzed for donor-derived sIELs by in situ immunofluorescent
staining (Olympus BX61). Data presented is one representative from three mice of each genotype.
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T10/T22, two nonclassical MHC class I molecules whose expression is high in C57BL/6 (B6), low in BALB/c, but absent in
b2M2/2 mice (53, 54).
As observed in ITK2/2 Vg3+ sIEL precursors, the fetal thymic
Tg Vg2+ T cells of WT and ITK2/2 KN6 mice on the high-ligand
B6 background could be positively selected to undergo the Vg3like maturation process, but the ITK2/2 Tg Vg2+ cells had defects
in seeding the fetal skin (Supplemental Fig. 2). Surprisingly, the
absence of ITK had opposite effects on the development of the Tg
Vg2+ and natural Vg3+ sIELs in adult mice. Compared to the
ITK-sufficient KN6 mice, ITK2/2 KN6 mice had significantly
increased numbers of Tg Vg2+ sIELs (Fig. 5A–C), whereas ITK
deficiency impaired Vg3+ sIEL development (Fig. 1A–C).
Closer microscopic examination found a morphological difference in the Tg sIELs of ITK2/2 and WT background. Although the
Tg sIELs in ITK2/2 KN6 mice displayed the normal dendritic
morphology, those of ITK-sufficient KN6 mice looked more
rounded (Fig. 5C, 3400 magnification), resembling activated
sIELs (19). Because ligands for the KN6 gdTCR, unlike those
for the natural sIEL-specific Vg3+ gdTCR, are highly expressed
in the skin of B6 mice (53), this suggests that the continuous TCR/
ligand-mediated signaling in Tg sIELs, transduced via ITK, may
lead to their reduction, likely due to persistent activation-induced
apoptosis, whereas a reduction of such signaling in the absence of
ITK reverses the effect. Supporting this idea, the in situ TUNEL
ITK IN SKIN-RESIDENT T CELL DEVELOPMENT
The Journal of Immunology
Not only is the sIEL development independent of the peripheral
TCR signaling, but also the continuous stimulation of TCRs in the
sIELs impairs their development by promoting apoptosis. Such
enhanced cell death was reduced by ITK deficiency, suggesting the
involvement of ITK in the TCR-induced activation of sIELs that
would result in the apoptosis if persisting. Consistent with this,
ITK2/2 sIELs are defective in producing IFN-g in response to
TCR stimulation. Therefore, although the ITK-transduced TCR
signaling in the peripheral sIELs is not required for their normal
maintenance, it is important for their activation, suggesting that
the ITK-transduced thymic and peripheral TCR signals are differentially involved in the development and function of sIELs. However, to fully understand these, how the ITK signaling is involved
in the in vivo functions of sIELs needs to be addressed.
There are increasing types of tissue-specific lymphocytes that
function in various roles to provide the first lines of defense (1,
58–60). In humans, the preferential distribution of specific T cell
subsets in the skin was also reported (61–63). Although the human
and murine skin T cells use different TCR compositions, they seem
to perform the similar functions. It was reported that like the murine
sIELs, the human skin gd T cells could lyse skin tumor cells and
produce similar cytokines in response to stimulation in vitro (61). In
addition, human epidermal gd T cells, as well as epidermal ab
T cells, were shown to contribute to wound healing (63), suggesting
that even though they have different TCR usages, the human and
murine epidermal T cells share similar functional properties and
might develop similarly. Our findings with the murine skinspecific sIELs would aid in understanding how ITK and other
TCR-associated signaling is involved in the development of the
human skin T cells as well as other different tissue-specific lymphocytes. In addition, in light of the role of ITK in regulating CCR10
expression in the sIEL precursors, whether ITK is involved in its
expression in other cells of the skin, such as melanocytes and melanoma tumor cells, under physiological and pathological conditions
would also be interesting questions.
Acknowledgments
We thank Joonsoo Kang for critical comments and Christina Saylor for technical support. The experiments with Vav12/2 mice were initiated in the laboratory of David Raulet (University of California at Berkeley, Berkeley, CA).
Disclosures
The authors have no financial conflicts of interest.
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system critical for the first line of defense, mechanisms regulating
their development are poorly understood. Our recent studies found
that thymic “educational” processes of different gd T cell subsets
promote their acquirement of unique homing properties (7, 48),
suggesting a critical role of the TCR-mediated selection signaling
in programming thymic gd T cells for their tissue-specific development. In this study, we investigated molecular mechanisms underlying the involvement of TCR signaling in tissue-specific
development of the skin-specific gd sIELs and identified ITK as
a critical signaling molecule that specifically controls the skinhoming property of fetal thymic sIEL precursors and their seeding
into the skin. The fetal thymic sIEL precursors from ITK2/2 mice
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migration into the skin (9, 50–52). In addition, considering previous studies that ITK is also involved in chemokine receptormediated signaling (22, 26–29), ITK deficiency might potentially
impair the migration of the sIEL precursors by directly affecting
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TCR-mediated selection signaling in the fetal thymic sIEL precursors not only promotes their acquirement of the unique skinhoming property but also endows them with capacities to survive
and expand in the skin, such as upregulation of CD122 (IL15Rb)
that is critical for the survival/expansion of sIELs (12, 13). Interestingly, the absence of ITK does not affect the CD122 upregulation and normal maturation of the fetal thymic sIEL precursors. In addition, although there were fewer sIELs in the ITK
knockout mice, their in vivo proliferation rates were the same
as, if not higher than, those of WT mice, indicating that the
ITK-mediating signaling is not involved in controlling the survival/proliferation capacity of the fetal thymic sIELs in the skin.
These suggest that different TCR signaling molecules are responsible for promoting the skin-homing and survival/proliferation
properties of the selected fetal thymic sIEL precursors. In this
regard, it is likely that although other TCR downstream signaling
molecules, such as Lck, Syk, and Zap-70, are all involved in the
sIEL development (17–20), they could affect different aspects of
the development. Consistent with this, sIELs in ZAP-702/2 mice
displayed significant morphological changes, whereas sIELs in
ITK2/2 mice maintain the normal dendritic shape (17–19). This
difference might reflect the fact that ZAP-70, which is located at
the upstream of the ITK signal pathway, may regulate a larger
subset of TCR signals than ITK, with correspondingly greater
effect (55).
ITK2/2 fetal thymic sIELs proliferate normally in response to
TCR stimulation in vitro, consistent with the normal peripheral
survival/expansion of sIELs in vivo. In addition, even in Vav12/2
mice whose Vg3+ fetal thymic sIELs are defective in TCR-mediated
proliferation, they still had normal sIEL development, suggesting
that TCR signaling-mediated proliferation is not required for the
maintenance of sIELs in the skin. This agrees with the notion that
TCR-specific ligand(s) of sIELs are not expressed in normal skin but
are upregulated on “stressed” or “diseased” keratinocytes that
would activate the sIELs for proper functions (56, 57). Therefore,
there is a close interplay between the establishment of sIELs and
their subsequent function. Considering that positively selected fetal
thymic sIEL precursors display an activated “memory”-like phenotype and are independent of TCR signaling for survival and proliferation in the periphery, this suggests that their development is
intrathymically programmed through TCR signaling moleculemediated selection for their specific function.
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ITK IN SKIN-RESIDENT T CELL DEVELOPMENT