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CUTTING EDGE |

*
Department of Immunology, University of Washington, Seattle, WA 98195; and
Bristol-Myers Squibb, Princeton, NJ 06540
| Abstract |
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1.8-fold increase in CD40L mRNA abundance. In response
to trinitrophenol (TNP)-keyhole limpet hemocyanin and
tetanus/diphtheria vaccine, CD40Ltg+ mice developed higher
titers of high-affinity IgG and IgG1 Ab than wild-type mice. In
contrast, the Ab response of CD40Ltg+ and control mice was
similar in response to the T-independent Ag TNP-Ficoll. These results
suggest that a modest increment in expression of CD40L accelerates the
development of T-dependent responses, and that CD40L plays a limiting
role in the induction of high-affinity Ab and Ab-class
switching. | Introduction |
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These results indicate that CD40L plays an important role in B cell responses to T cell-dependent Ags but it is not known whether the abundance of CD40L expressed on activated T cells is in excess or is a critical factor limiting T cell-dependent B cell responses. To evaluate this question, transgenic mice were engineered with the murine CD40L gene under the control of the human IL-2 promoter.
| Materials and Methods |
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C57BL/6 and BDF1 mice were purchased from The Jackson Laboratory (Bar Harbor, ME) and maintained at the University of Washington (Seattle, WA) Specific Pathogen-Free animal facility. Mice lacking the CD40L gene were kindly provided by R. Flavell (Yale University, New Haven, CT).
IL-2 promoter-CD40L-HGx transgenic construct
The cDNA for the murine CD40L gene (nucleotides 615 to 1435) was cloned downstream of the human IL-2 promoter/enhancer region from -568 to +47 relative to the transcriptional start site (7, 8) and upstream from a mutated version of the human growth factor (HGx) gene in the pBluescript plasmid (Stratagene, La Jolla, CA). HGx have been shown to increase transgene expression, providing a source of introns and poly(A) signal, but the mutation prevents expression of functional human growth hormone (9).
Generation of transgenic mice
Transgenic mice were generated by microinjection of the construct described above into BDF2 single cell embryos, using previously published protocols (10). Founders were identified by PCR and Southern blotting and crossed sequentially to C57BL/6 mice. Of the 11 founder lines generated, 1 expressing line was selected for further analysis.
Preparation of Ags and immunization of mice
Trinitrophenol-keyhole limpet hemocyanin (TNP14-KLH) and TNP-Ficoll were kindly provided by R. J. Noelle (Dartmouth Medical School, Lebanon, NH). TNP4-BSA and TNP16-BSA were conjugated in our laboratory (11). Mice were immunized by i.p. injection at day 1 with 10% alum in normal saline (NS), TNP14-KLH (50 µg/mouse) in 10% alum in NS, or TNP-Ficoll (50 µg/mouse) in PBS. Sera were collected from the mice before and 7 and 21 days after immunization. Mice were immunized by i.p. injection at day 1 and day 42 with 1 Lf tetanus and 1.3 Lf diphtheria toxoids adsorbed to alum (Connaught Laboratories, Swiftwater, PA). Sera were collected from mice before and at days 7, 21, 47, and 52 after immunization.
Ab measurement
TNP4-BSA and TNP16-BSA (10 µg/m; were added to the wells (50 µl each) of polystyrene microtiter trays (MaxiSorp, Nalge Nunc International, Naperville, IL) and incubated at 4°C overnight. Wells were then blocked by incubation for 2 h with 3% BSA in PBS-0.5% Tween-20. After washing the plates six times with PBS-0.5% Tween-20, diluted samples were added to the plates and incubated for 2 h at room temperature (RT) (sera were initially diluted 1:100 and then diluted 2-fold serially). After washing, a 1:2000 dilution of HRP-conjugated anti-mouse IgM (Biosource International, Camarillo, CA), anti-IgG, anti-IgG1, or anti-IgG2a (Southern Biotechnology Associates, Birmingham, AL) was added for 1 h at RT. After washing, reactions were developed with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) peroxidase substrate: H2O2 (Kirkegaard & Perry Laboratories, Gaithersburg, MD) for 1015 min. The A405 was read in a Beckman Coulter (Fullerton, CA) Biomek spectrophotometer. Ab titers were determined by endpoint titrations after subtraction of the values for the preimmune samples. The last dilutions at which the net OD405 exceeded 0.2 for IgM and 0.1 for total IgG and IgG1 were taken as the titer. Total Ab titers were determined in plates coated with TNP16-BSA, and high-affinity Ab titers were determined on plates coated with TNP4-BSA.
Cell preparations
Cell suspensions were prepared from various mouse tissues by gently teasing the cells from the organs and macerating the tissue through a wire-mesh screen. Freshly isolated thymocytes, splenocytes and lymph node cells were resuspended in RPMI 1640 (Life Technologies, Grand Island, NY) supplemented with 2 mM L-glutamine, 100 U/ml of penicillin, and 100 µg/ml of streptomycin and 10% FCS (RPMI complete). The RPMI complete medium was supplemented when indicated with 10 µg/ml of PMA and 0.5 µM ionomycin and/or biotinylated MR-1 mAb, specific for murine CD40L, was added (12). The cultures were maintained at 37°C in a humidified atmosphere of 5% CO2 for 3, 5, 8, or 24 h.
Flow cytometry
All Abs were titrated and used at saturating concentrations for flow cytometric studies. Single cell suspensions were prepared from thymus, spleen, or lymph nodes. RBC were removed from spleen cell preparations by NH4Cl lysis. Cells were washed and then stained for 20 min with various combinations of the following mAbs specific to murine Ags: FITC anti-B220, PE-conjugated anti-CD3, PE-conjugated anti-CD4, FITC-anti-CD8, biotin (BIO)-conjugated anti-CD80, and BIO-conjugated anti-CD86. MR-1 was detected with strepavidin-PE or strepavidin-FITC. Cells were fixed in PBS containing 2% parafomaldehyde. A minimum of 10,000 events per sample was analyzed using a FACScan flow cytometer (Becton Dickinson, San Jose, CA). Analysis of the data was performed using CellQuest software (Becton Dickinson).
Northern blot analysis
Total cellular RNA was isolated from resting and activated
splenocytes using Trisol reagent (Biotecx Laboratories, Houston, TX),
and 10 µg of total RNA per sample were electrophoreted and
transferred as described (13). The 311-bp probe containing
part of the third and fourth exons of the CD40L was used to identify
CD40L mRNA, and a 1.5-kb probe for elongation factor-1
was used as a
control. RNA abundance was quantitated by PhosphoImager analysis after
correction for background.
| Results |
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The human IL-2 promoter fragment used to generate the
CD40Ltg+ mice has been shown previously to drive
expression of the murine IL-10 gene (14) and
LacZ transgenes in activated T cells (7).
Consistent with these reports, the fraction of cells in the lymph nodes
that expressed CD40L in response to stimulation with PMA/ionomycin was
modestly but consistently greater (1.1- to 2-fold, mean 29% positive
cells) in four matched pairs of CD40Ltg+ mice
than in their littermate controls (mean 18% positive cells) (Fig. 1
A). Two-color analysis showed
that nearly all of the cells that expressed CD40L were
CD4+ T cells (Fig. 1
B). In contrast,
nearly all PMA/ionomycin-stimulated cells from lymph nodes of
CD40Ltg+ and control mice expressed CD69 (94 and
90%, respectively). Few cells expressed CD40L in the absence of
stimulation, but this number was also greater in
CD40Ltg+ than in control mice (mean 2.8 and
1.5%, respectively). The mean proportion of cells in the lymph nodes
of control and CD40Ltg+ mice, respectively, which
were CD4+ (40%, 46%),
CD8+ (20%, 23%), and
B220+ (32%, 31%), was similar as was the
proportion and numbers of these cell populations in the spleens.
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To determine whether the modest increase in CD40L expression in
the tg+ mice affected Ab production, mice were
immunized with the T cell-dependent Ag, TNP-KLH. The
CD40Ltg+ mice had significantly higher titers of
high-affinity anti-TNP IgM Ab at day 7. The
CD40Ltg+ mice also had significantly higher
titers for total and high-affinity IgG and IgG1 anti-TNP at day 7
(4- and 5- fold more, respectively) and day 21 (3.6- and 1.6- fold
more, respectively) than littermate controls (Fig. 3
). These results suggested that
CD40Ltg+ mice developed a more robust overall
response, which reflected in part a more rapid affinity maturation and
isotype switch. To determine whether this was true for another T
cell-dependent Ag, the response to tetanus toxoid was evaluated. The
results paralleled those observed with TNP-KLH: the
CD40Ltg+ mice produced more IgG1 and similar
amounts of IgM Ab than controls (Fig. 4
).
In contrast to these T cell-dependent responses, there was no
difference in the production of Ab in response to the T
cell-independent Ag TNP-Ficoll (Fig. 5
).
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| Discussion |
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Our results, in which the abundance but not the pattern of CD40L expression was altered in CD40Ltg+ mice, contrast with those obtained in mice in which this protein was expressed constitutively in T cells (22, 23). Constitutive expression of a CD40L transgene in developing T cells under the proximal lck promoter disrupted thymic architecture and perturbed T cell development (22). Transduction of T cell progenitors from CD40L knockout mice with a retrovirus caused low-level constitutive expression of CD40L and was followed by the development of T lymphoblastic lymphomas in >50% of the mice by 69 mo of age. In contrast, the thymus and peripheral T cell compartments were normal in mice expressing the CD40L transgene under the IL-2 promoter. The CD40Ltg+ mice have remained healthy and appear normal at necropsy at >9 mo of age (our unpublished observations). This suggests that disruption of the normal pattern of transient, activation-induced expression accounted for the aberrant development and malignant transformation observed in the other studies.
The finding that CD40L expression by activated T cells is limiting has implications for situations in which the abundance of CD40L may be reduced below levels normally expressed by activated T cells. T cells from murine and human neonates (24, 25, 26), and to some extent naïve T cells from mature mice (27), express lower amounts of CD40L than do T cells from adults in general, and memory/effector T cells in particular (27). Lower level of expression of CD40L by neonatal T cells may underlie, at least in part, their somewhat slower development of T cell-dependent Ab responses and greater bias toward the production of Th2 cytokine-dependent isotypes (28, 29). Decreased expression of CD40L may also be a factor in the greater sensitivity of neonatal and naïve T cells to induction of tolerance (27, 30). Consistent with this, administration of anti-CD40 Abs as a surrogate for CD40L expression by activated T cells abrogates neonatal tolerance induction (24). Similarly, presentation of Ags by mature dendritic cells (31), or the use of adjuvants that may in part bypass the need for CD40L to prime APC (26), may be useful strategies to enhance T cell-dependent responses in cases where the initial expression of CD40L may be limiting.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Christopher B. Wilson, Department of Immunology, Box 357650, 1959 NE Pacific Street, University of Washington, Seattle, WA 98195. E-mail address: ![]()
3 Abbreviations used in this paper: CD40L, CD40 ligand; TNP, trinitrophenol; KLH, keyhole limpet hemocyanin; WT, wildtype. ![]()
Received for publication April 16, 1999. Accepted for publication June 1, 1999.
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