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CUTTING EDGE |
Tularik Inc., Two Corporate Drive, South San Francisco, CA 94080
| Abstract |
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| Introduction |
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and IL-2 and provide protection against
intracellular pathogens. Moreover, Th1 cells mediate delayed-type
hypersensitivity and organ-specific autoimmune diseases
(2). Th2 cells secrete IL-4 as well as IL-5, IL-6, IL-10,
and IL-13, which in turn regulate the production of IgE and IgG1 in B
cells, recruitment of eosinophils, and influence macrophage
function. Therefore, Th2 cells play important roles in allergic and
infectious diseases (1). Th1 and Th2 cells arise from a
common naive CD4+ precursor (Thp). Depending on
the Ag and more importantly the presence of specific exogenous
cytokines, a unique genetic repertoire is activated that leads to the
differentiation of Thp into Th1 or Th2 cells.
IL-12 activates the transcription factor STAT4 and promotes the
differentiation of naive cells into Th1 effector cells. Mice lacking
either the IL-12R
1-chain or STAT4 have impaired Th1 responses
(reviewed in Ref. 3). In contrast, IL-4 activates STAT6,
which is the crucial mediator of Th2 differentiation. Mice that are
deficient in either the IL-4R
-chain or STAT6 fail to develop Th2
cells (reviewed in Ref. 4).
Significant progress has been made in understanding the signals that are required for Th2 differentiation. However, it remains to be seen if all components necessary for the function and development of Th2 cells have been identified. We used the representational difference analysis (RDA)3 method to isolate genes that are selectively expressed in Th2 cells and may have important functions in Th2 cell development. In this report, we describe the identification of a novel gene that we designated FISP (IL-4-induced secreted protein). FISP is a cytokine-like secreted protein and has significant homology to the rat gene mob-5, also named c49a (5, 6), and the human gene melanoma differentiation-associated gene (mda-7) (7). FISP is selectively expressed in Th2 cells but not in Th1 cells, and expression of FISP requires signaling via both the TCR and IL-4R complex.
| Materials and Methods |
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Female BALB/c and AKR/J mice were obtained from Taconic (Germantown, NY). STAT4- and STAT6-deficient mice have been described previously (8, 9). Animals between 4 and 6 wk of age were used for this study. The Th1 clone, AE7, as well as the Th2 cell clone, D10.G4, were kindly provided by Laurie Glimcher (Harward School of Public Health, Boston, MA) and cultured as previously described (10).
Representational difference analysis
In vitro differentiation of total splenocytes or enriched CD4+ cells was performed as previously described (8, 10). Poly(A)+ RNA was prepared from differentiated Th1 and Th2 cells using the Fast Track 2.0 kit (Invitrogen, Carlsbad, CA), and cDNA was synthesized by Superscript Choice System (Life Technologies, Gaithersburg, MD) using a combination of oligo(dT) and 50 ng of random primers. Th2 cDNA was used as tester and Th1 cDNA as driver to perform RDA according to the previously published procedure (11, 12). Briefly, two rounds of hybridizations were conducted using a ratio between tester and driver of 1:100, followed by a ratio of 1:800 for the second round of hybridization. Products were subcloned into pBluescript KS+ (Stratogene, La Jolla, CA) and further analyzed for differential expression.
Construct
Full-length FISP cDNA was isolated from a Th2-specific PCR select subtractive cDNA pool (10) and subcloned into the XbaI site of a pRK5 derived C-terminal flag mammalian expression vector (kindly provided by H. Wesche, Tularik, South San Francisco, CA). The cDNA sequence of full-length FISP was verified by RT-PCR of RNA derived from Th2 and D10.G4 cells.
Northern blot analysis
Total RNA was prepared using TRIzol (Life Technologies) according to the manufacturers instructions. Total RNA (1015 µg) was fractionated on a formaldehyde gel and transferred to nylon membrane. A 302-bp fragment of FISP was obtained by PCR using the primers 6a (5'-GAGTTCCGATTTGGGTCTTGCCAAG) and 6b (5'-GGGCTGTAGTTGTGACATGATGAC) and was used for hybridization. RNA from different tissues was purchased from Ambion (Austin, TX). Hybridization was performed using ExpressHyb (Clontech Laboratories, Palo Alto, CA) according to the manufacturers protocol.
Western blot analysis
Approximately 3 x 107 of D10.G4 cells (maintained for over 3 mo in the presence of 10 ng/ml IL-2) were cultured for 12 h in serum-free RPMI 1640 medium. Conditioned medium was collected and concentrated 60-fold. Twenty microliters of conditioned medium was subjected to SDS-PAGE, and Western blot analysis was performed using a rabbit polyclonal anti-FISP Ab raised against aa 166183 of the protein. Supernatant was also collected from cultured 293 cells, transiently transfected with FISP expression construct. For peptide competition experiments, anti-FISP was incubated with FISP peptide (aa 166183) or control peptide in a 1:10 ratio (w/w) for 2 h at room temperature, before incubation with membrane.
| Results and Discussion |
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To identify genes that are differentially expressed in Th2 cells, we isolated total splenocytes and cultured them under Th1 and Th2 differentiation conditions (8). After 7 days of differentiation, total RNA was extracted and Th1- or Th2-specific cDNA pools were obtained. Th2-specific cDNAs were selected using the RDA method (11). A number of novel cDNAs were identified, and one of them, designated FISP, was further characterized.
The open reading frame of FISP encodes a protein of 220 aa with a
predicted molecular mass of 25 kDa (Fig. 1
). Blast analysis revealed that at DNA
level FISP was 89% identical with rat mob-5 (also designated as c49a)
and 80% identical with human mda-7. At the protein level, FISP shares
93% identity with mob-5/c49a and 69% identity with mda-7. To a lesser
degree, FISP showed homology to IL-10 (33%) and the recently described
IL-20 (13) (30%), suggesting that FISP has a
cytokine-like structure. Analysis with the program SignalP
(14) revealed a potential signal peptide sequence from aa
165.
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Splenocytes are composed of a number of different cell types.
Therefore, we wished to verify differential expression of FISP in
CD4+-enriched Th subsets. When
CD4+ T cells were stimulated under Th2
conditions, FISP expression was detected. Interestingly, T cells
cultured under neutral conditions (anti-CD3 and IL-2) also
expressed FISP mRNA. No transcripts were observed in Th1 cells (Fig. 4
A). These data suggested that
expression of FISP might be inhibited in Th1 cells. Hence, we tested
whether IL-12 or IFN-
can down-regulate FISP expression in
developing Th1 cells. Because STAT4 is a major player in the IL-12
signaling pathway, leading to the differentiation of Th1 cells
(9), we studied FISP expression in STAT4- deficient T
cells cultured under Th1, Th2, and neutral conditions. We also examined
IFN-
-deficient mice for FISP expression under Th1 or Th2 conditions.
Fig. 4
B shows that expression of FISP in STAT4- or in
IFN-
-deficient mice was identical with the expression pattern seen
in wild-type mice. This indicates that FISP was not actively repressed
in Th1 cells by the IL-12-dependent factor STAT4 or by the presence of
IFN-
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Because FISP mRNA was detected under neutral conditions in wild-type
but not in STAT6-deficient T cells, we tested whether IL-4 signaling
occurred at low levels under neutral culture conditions. Enriched
wild-type CD4+ cells were treated for 5 days with
anti-CD3 plus IL-2 in the presence of an anti-IL-4 Ab or a
control Ab. Fig. 4
D shows that the presence of anti-IL-4
completely abolished FISP expression (lane 2),
whereas incubation with the control Ab had no effect (lane
1). These data indicate that FISP is differentially expressed in
Th2 cells and in IL-4-producing T cells developed under neutral
conditions. The latter finding correlates with earlier studies showing
that activated T cells derived from BALB/c mice produce IL-4 under
neutral culture conditions (17). Taken together, the data
indicate that FISP is an IL-4-inducible gene that is
activated by STAT6.
Elevated levels of FISP transcripts are present in activated D10.G4 cells
We further analyzed if FISP was also differentially expressed in
established Th1 and Th2 cell lines. FISP expression was not seen in
untreated D10.G4 cells (Fig. 5
A, lane 1) or AE7
cells (data not shown). However, FISP mRNA was strongly up-regulated
upon activation of D10.G4 cells with plate-bound anti-CD3. Maximal
levels were observed after 3 h of stimulation. Anti-CD3
stimulation of AE7 cells in the presence or absence of IL-4 did not
up-regulate FISP expression (Fig. 5
A, lanes
7 and 8). These data support the notion that FISP
is selectively expressed in Th2 cells. Interestingly, in long-term
cultures of D10.G4 (>3 mo), FISP was constitutively expressed, even in
the absence of anti-CD3 (data not shown), suggesting that FISP
expression may be influenced by the proliferation conditions of D10.G4
cells.
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Protein kinase C (PKC) is one of the downstream signaling
molecules in the TCR activation pathway. Mezerein is a nonphorbol ester
and activates PKC. Human mda-7 was originally found in
melanoma cells treated with IFN-
and mezerein (7). To
examine whether mezerein also induced FISP expression in D10.G4 cells,
we treated cells with anti-CD3, mezerein, or mezerein plus IL-4 for
3 h and performed Northern blot analysis. Mezerein or IL-4
treatment did not induce FISP expression (Fig. 5
, B,
lane 3 and A, lane 2). However,
treatment with mezerein plus IL-4 resulted in elevated FISP expression,
similar to the levels seen with anti-CD3-treated cells (Fig. 5
B). One explanation for this data might be that
anti-CD3 treatment induces IL-4 production in D10.G4 cells, which,
in turn, can positively regulate expression of FISP. In fact,
stimulation of D10.G4 with anti-CD3 but not mezerein induced IL-4
production (Fig. 5
B). Taken together, our data show that
FISP expression requires a combination of at least two different
signaling pathways, TCR engagement and IL-4 stimulation (Fig. 6
).
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Amino acid analysis using the program SignalP (14)
predicted a signal sequence spanning from amino acids 1 to 65 with a
potential cleavage site between aa 65 and 66, which suggested that FISP
could be a secreted protein. Hence, we investigated whether D10.G4
cells secrete FISP. Western blot analysis of conditioned medium of
long-term cultured D10.G4 cells revealed multiple bands, the most
intense one with an approximate molecular mass of 27 kDa. This band was
absent when blocking peptide was included in the Western analysis (Fig. 5
C, lane 2). The specificity of blocking peptide
and FISP Ab was confirmed in transiently transfected 293 cells
(lanes 36). The lower migrating form of
FISP may represent a differentially glycosylated or degraded form of
the protein. As expected, FISP protein was not detected in conditioned
medium from anti-CD3-treated AE7 cells (data not shown).
Recently, it was shown that overexpression of rat mob-5 as well as human mda-7 also leads to the secretion of these proteins into the culture medium (5). The high sequence similarity to mob-5 and mda-7 suggests that FISP might be the mouse homologue of these two genes. Overexpression of mda-7 in breast cancer cells appears to be proapoptotic and is involved in growth arrest (18). In contrast, expression of mob-5 is activated through ras and may play an important role in ras oncogene-mediated neoplasia (5). However, these functional differences suggest that FISP, mda-7, and mob-5 are not homologues but represent closely related members of a gene family with distinct functions. Further studies are necessary to address the function of FISP during Th1/Th2 differentiation.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Chandrasekar Venkataraman, Tularik Inc., Two Corporate Drive, South San Francisco, CA 94080. ![]()
3 Abbreviations used in this paper: RDA, representational difference analysis; FISP, IL-4-induced secreted protein; mda, melanoma differentiation-associated gene; PKC, protein kinase C. ![]()
Received for publication November 28, 2000. Accepted for publication March 22, 2001.
| References |
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