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The Journal of Immunology, 2006, 176: 5240-5246.
Copyright © 2006 by The American Association of Immunologists

CTLA-4 Engagement and Regulatory CD4+CD25+ T Cells Independently Control CD8+-Mediated Responses under Costimulation Blockade1

Jeroen J. A. Coenen*, Hans J. P. M. Koenen*, Esther van Rijssen*, Louis Boon{ddagger}, Irma Joosten* and Luuk B. Hilbrands2,{dagger}

* Department of Bloodtransfusion and Transplantation Immunology and {dagger} Department of Nephrology, University Medical Center Nijmegen, Nijmegen, The Netherlands; and {ddagger} Bioceros, Utrecht, The Netherlands


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Blockade of costimulatory signals is a promising therapeutic target to prevent allograft rejection. In this study, we sought to characterize to what extent CTLA-4 engagement contributes to the development of transplantation tolerance under the cover of CD40/CD40L and CD28/CD86 blockade. In vitro, we found that inhibition of the primary alloresponse and induction of alloantigen hyporesponsiveness by costimulation blockade was abrogated by anti-CTLA-4 mAb. In addition, regulatory CD4+CD25+ T cells (TREG) were confirmed to play a critical role in the induction of hyporesponsiveness by anti-CD40L and anti-CD86 mAb. Our data indicated that CTLA-4 engagement is not required for activation or suppressor function of TREG. Instead, in the absence of either CTLA-4 signaling or TREG, CD8+ T cell division was enhanced, whereas the inhibition of CD4+ T cell division by costimulation blockade remained largely unaffected. In vivo, the administration of additional anti-CTLA-4 mAb abrogated anti-CD40L- and anti-CD86 mAb-induced cardiac allograft survival. Correspondingly, rejection was accompanied by enhanced allograft infiltration of CD8+ cells. We conclude that CTLA-4 signaling and TREG independently cooperate in the inhibition of CD8+ T cell expansion under costimulation blockade.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Elucidating the mechanisms underlying the regulation of T cell responses is crucial for the improvement of tolerogenic strategies in clinical transplantation. Blockade of costimulatory signals is a promising therapeutic target, for blockade of the CD28/B7 and CD40/CD40L pathways results in profound T cell hyporesponsiveness and application of costimulation blockade in vivo results in long-term allograft survival ( 1, 2, 3).

An important regulatory mechanism in T cell responses is the engagement of the CD28 homolog CTLA-4 ( 4, 5). CTLA-4 is up-regulated upon T cell activation, and subsequent binding of this molecule to B7 molecules CD80 or CD86 results in down-regulation of the T cell response. CTLA-4 signaling has been implicated in the acquirement of transplantation tolerance, but exactly how CTLA-4 exerts its effect in this process remains a matter of debate ( 6, 7). It is of note that CTLA-4 is constitutively expressed on regulatory CD4+CD25+ T cells (TREG),3 which are by now firmly established as critical mediators of peripheral allograft tolerance ( 8). Various studies suggest an association between the engagement of CTLA-4 on these cells and their regulatory function ( 9, 10, 11, 12). However, there are also studies that support the contrary ( 13, 14, 15).

Recently, we demonstrated that combined blockade of the CD40L and the CD86 pathways by anti-CD40L and anti-CD86 mAb resulted in a reduction of the alloreactive clone size, allowing immunoregulatory mechanisms mediated by TREG to emerge ( 16). Here, we sought to characterize to what extent CTLA-4 engagement may contribute to the development of transplantation tolerance under the cover of CD40/CD40L and CD28/86 blockade, either by activation of TREG, by containment of the alloreactive effector pool size, or a combination of both.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Disclosures
 References
 
Cell purification

BALB/c responder cells were purified from pooled peripheral lymph node and spleen cells. Cells were depleted of erythrocytes by osmotic shock in lysis buffer containing 0.15 µM NH4Cl (Merck), 10.0 mM KHCO3 (Merck), and 0.1 mM Na2EDTA (Sigma-Aldrich). T cells were enriched by depletion of MHC class II-positive cells by coating with anti MHC class II magnetic microbeads (Miltenyi Biotec), followed by passage through a LD microbead column (Miltenyi Biotec). Purification of CD4+CD25+ T cells was conducted by magnetic cell separation. To this end, the T cell-enriched population was inoculated with saturating amounts of biotin-labeled anti-CD25 Ab (7D4; BD Biosciences). Cells were subsequently incubated with antibiotin microbeads and separated using MS columns according to the manufacturer’s instructions, yielding >90% CD25+ positivity of the CD4 T cell fraction. CD25+ T cell-depleted populations typically contained <1% CD4+CD25+ T cells. C57BL/6 or BALB/c stimulator cells were enriched by culturing splenocytes in 9-cm petri dishes (Allegiance) for 2 h and further overnight culture of the adherent cell fraction. After 18–24 h, the nonadherent cells were obtained by harvesting the supernatant and gently washing the petri dish.

Proliferation assay

For primary MLR, responder cells were cultured with gamma-irradiated (27.5 Gray) stimulator cells at a ratio of 1 x 105 responders to 2.5 x 104 stimulator cells in flat-bottom 96-well plates (Costar) containing RPMI 1640 (Invitrogen Life Technologies) with 10% FCS (Invitrogen Life Technologies), 50 µM 2-ME (Bio-Rad), 1 mM sodium pyruvate, glutamax, and antibiotics (100 U/ml penicillin; 100 mg/ml streptomycin; all from Invitrogen Life Technologies). In specified cases, 1 µg/ml anti-CD40L (American Type Culture Collection (ATCC) no. MR1; CRL-2580),1 µg/ml anti-CD80 (ATCC no. HB301), and 1 µg/ml anti-CD86 (ATCC no. HB253) were added. Blocking anti-CTLA-4 Abs (ATCC no. 4F10; UC10–4F10-11) were added in various concentrations as described. All Abs were from Bioceros. Isotype-matched control Abs rat IgG2a and hamster IgG (R35-95 and A19-3; both from BD Biosciences) were used in equal concentrations. F(ab')2 of the anti-CTLA-4 Ab were generated with the Immunopure F(ab')2 Preparation kit (Pierce) and the Spectra/Por Float-A-Lyzer using the instructions of the manufacturer. To study antigenic rechallenge, cells were harvested at day 7 of culture and washed and subsequently rested in 5% FCS culture medium (see above). At day 10, the cells were added to fresh stimulator cells at the indicated ratios. For plate-immobilized anti-CD3 stimulation, anti-CD3{epsilon} mAb (145-2C11; hamster IgG; BD Pharmingen) 2.5 µg/ml was precoated on flat-bottom 96-well plates by overnight incubation at 4°C. Before these anti-CD3-coated plates were used, they were washed with sterile PBS. CD25+ T cell-depleted T cells were plated in anti-CD3 mAb-coated plates at 5 x 104 cells/well. Cell proliferation was monitored by tritiated thymidine incorporation. To this end, the cells were pulsed with tritiated thymidine (0.5 µCi) during the last 16–18 h of the culture. Tritiated thymidine incorporation was analyzed by a gas scintillation counter and is expressed as mean count per 5 min and SD of at least triplicate measurements.

Flow cytometry

Cells were phenotypically analyzed by three-color fluorescence. Cells were washed twice with PBS supplemented with 0.2% BSA (Sigma-Aldrich). The following Abs were used: PE-conjugated anti-CD4 (L3T4); anti-CD8 (53-6.7) both from BD Biosciences. All incubations were conducted for 20 min at room temperature and thereafter the cells were washed twice. The samples were run on a Coulter Epics (Beckman Coulter) and at least 100,000–150,000 live gate events were collected based on live lymphocyte gating, as indicated by 5 µg/ml propidium iodide staining. Isotype-matched Abs were used to define marker settings. Data were analyzed using Coulter Epics Expo 32.

Immunofluorescence

Two-micrometer sections were cut from cardiac allograft tissue samples embedded in Tissue-Tek OCT (Sakura Finetek) and mounted on glass microscope slides. Sections were fixed for 10 min with 90% acetone, rinsed in PBS, and incubated with: isotype control FITC-conjugated IgG2a, {kappa} (R35-95); isotype control PE-conjugated IgG2b, {kappa} (A95-1); FITC-conjugated anti-CD3 (17A2); FITC conjugated Ly-6G+ (RB6-8C5); or PE-conjugated anti-CD8 (53-6.7) (all from BD Biosciences) in 2% BSA-PBS for 1 h. Finally, the sections were rinsed in PBS, postfixed with 1% paraformaldehyde-PBS, rinsed in PBS, and embedded in Vecta-Shield mounting medium H-1000 (Vector Laboratories). The slides were investigated by fluorescence microscopy (Zeiss Axioscop microscope equipped with an epi-illuminator) and photographed with a digital camera (Nikon Coolpix DXM 1200). Intragraft infiltration was examined by calculating the average number of infiltrating cells of 5–10 section areas. To asses CD4+ T cell infiltration, section areas were coscored for the presence of CD3+CD8– cells. To asses neutrophil and CD8+ T cell infiltration, Ly-6G+ and CD8+ cells were scored.

Discrimination of subpopulations through CFSE dilution

The cell division rate of alloreactive T cells was studied by labeling responder T cells with carboxyfluorescein diacetate succinimidyl ester (CFDA-SE, Molecular Probes). A total of 1 x 107 T cells was labeled with 0.25–0.5 µM CFSE for 10 min. Subsequently, an equal volume of FCS was added, and the cells were washed with medium. Simultaneous labeling of T cells by fluorochrome-conjugated mAb enables the study of division kinetics of different T cell subsets by flow cytometry. Data, preferentially 100,000–150,000 live gate events, were analyzed using Coulter Epics Expo 32.

Murine cardiac allograft transplantation

Experiments were performed with completely MHC-mismatched combinations of C3H recipients (H-2k) and BALB/c (H-2d) donors. Mice were purchased from Charles River Laboratories. Mice were used at 8–12 wk of age and housed in a specified pathogen-free facility. Cardiac allografts were placed in an intra-abdominal location using the technique described by Corry et al. ( 17). Graft function was assessed every 2 days by palpation. In at least five mice per group, animals received three doses of MR1 (ATCC no. CRL-2580) at days 0, 2, and 4 (0.25 mg each) by i.p. injection and two doses of HB253 (ATCC no. HB253) at day 0 (0.2 mg) and day 1 (0.1 mg) by i.v. injection. In specified cases, animals received three doses of additional 4F10 (ATCC no. UC10-4F10-11) at days 0, 2, and 4 (0.1 mg each) by i.p. injection. The day of rejection was defined as the day of cessation of palpable heartbeat. All mAb were obtained from Bioceros. Experiments performed were approved in accordance with the institutional animal care and utilization committee approved criteria and protocols.

Statistical analysis

Statistical analysis was performed using the Statistical Product and Services Solutions (SPSS) package version 10.0 and Graphpad Prism 3.0. For comparisons between groups, we used the Student t test, assuming unequal variances because of small sample size. Graft survival curves were analyzed by a log-rank test. A p value of <0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Disclosures
 References
 
CTLA-4 signaling is required for inhibition of the allogeneic response by anti-CD40L and anti-CD86 mAb

Typically, the addition of anti-CD40L and anti-CD86 mAb to a primary MLR leads to a substantial inhibition of the T cell response. Because this approach leaves the CTLA4-CD80-signaling pathway intact, we set out to assess whether in this system CTLA-4 engagement is involved in the restraint of the alloreactive T cell response. Also, we addressed the question of whether blockade of CD80 would result in similar effects. To this end, we first examined the effect of CTLA-4 blockade by anti-CTLA-4 mAb in costimulation-blocked primary MLR cultures. Fig. 1A shows that, whereas addition of anti-CD40L and anti-CD86 mAb to a primary MLR resulted in 65% inhibition of the alloresponse at the proliferative peak of culture, additional anti-CTLA-4 mAb abrogated this inhibition. Blockade of CD80 in addition to anti-CD40L and anti-CD86 mAb resulted in enhanced inhibition by CD40L and CD86 costimulation blockade (Fig. 1A). Equivalent quantities of isotype control Abs did not modify the proliferation of the cultures (Fig. 1A). Thus, blockade of CTLA-4 signaling but not CD80 abrogates the inhibition of the alloresponse by CD40L and CD86 costimulation blockade. This effect was found to be dose dependent (Fig. 1B.). The use of F(ab')2 of the anti-CTLA-4 mAb yielded similar results as using the whole Ab (data not shown).


Figure 1
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  		FIGURE 1. In vitro efficacy of anti-CD40L and anti-CD86 costimulation blockade rests on CTLA-4 engagement. A, Primary MLR was set up with 1 x 105 responder BALB/c whole T cells and 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes. Cells were cultured with anti-CD40L and anti-CD86 mAb (1 µg/ml each), with anti-CD40L, anti-CD86 mAb (1 µg/ml each), and anti-CTLA-4 mAb (5 µg/ml) or with anti-CD40L + anti-CD86 mAb + anti-CD80 mAb (1 µg/ml each). Matched isotype control Abs were tested in equivalent quantities. Polyclonal hamster IgG was the control Ab for anti-CD40L, anti-CD80, and anti-CTLA-4, and rat IgG2a was the control Ab for anti-CD86. The maximum amount of proliferation as measured by [3H]thymidine incorporation is shown on the x-axis. *, p < 0.05 for difference between groups. B, A total of 1 x 105 responder BALB/c T cells was cultured with 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes. Cells were cultured in the presence of anti-CD40L + anti-CD86 mAb with increasing amounts of anti-CTLA-4 mAb. The amount of proliferation as measured by [3H]thymidine incorporation is shown on the y-axis. *, p < 0.05 for difference between groups.

 
CTLA-4 signaling and TREG function are both essential for the induction of alloantigen hyporesponsiveness by anti-CD40L and anti-CD86

In addition to inhibiting a primary MLR, costimulation blockade-based strategies are known to induce secondary hyporesponsiveness to alloantigen ( 1, 18). We analyzed the importance of CTLA-4 signaling and TREG function in the induction of alloantigen hyporesponsiveness by our costimulation blockade protocol. We first showed that whereas the addition of anti-CD40L and anti-CD86 to a primary MLR resulted in the induction of hyporesponsiveness to antigenic rechallenge, the coadministration of a dose of 5 µg/ml anti-CTLA-4 mAb to the primary MLR was already sufficient to compromise the development of this hyporesponsive state (Fig. 2A). The addition of isotype control Abs during the primary culture did not alter the secondary response (data not shown). Second, induction of hyporesponsiveness to antigenic rechallenge by anti-CD40L and anti-CD86 mAb was dependent on TREG ( 18). Whereas whole T cell populations treated with anti-CD40L and anti-CD86 mAb displayed hyporesponsiveness to allogeneic restimulation, T cells that were depleted for CD25+ cells at the start of primary culture (in the presence of anti-CD40L and anti-CD86 mAb) responded vigorously to allogeneic restimulation (Fig. 2B). Taken together, these data indicate that active signaling by CTLA-4 and TREG function are both required to establish hyporesponsiveness to alloantigens by anti-CD40L and anti-CD86 mAb.


Figure 2
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  		FIGURE 2. Efficacy of costimulation blockade depends on CD4+CD25+ T cells. A, Secondary responses were examined by antigenic restimulation of 5 x 104 responder BALB/c whole T cells by 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes. Cells were obtained from primary MLR cultured with anti-CD40L + anti-CD86 mAb (1 µg/ml each), with anti-CD40L + anti-CD86 (1 µg/ml each) + anti-CTLA-4 mAb (5 µg/ml), or with no mAb. Proliferation as measured by [3H]thymidine incorporation is shown on the y-axis. A typical example of three independent experiments is shown. *, p < 0.01 for difference between groups a and b. #, p < 0.01 for difference between groups b and c. B, Secondary responses were examined by antigenic restimulation of 5 x 104 responder BALB/c T cells by 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes. Responder cells were obtained from primary MLR in which the responder fraction consisted of either 1 x 105 BALB/c responder whole T cells or CD4+CD25+ T cell-depleted T cells. In both cases, primary MLR was conducted under the cover of anti-CD40L + anti-CD86 mAb. Cells obtained from a regular primary MLR with whole T cells and without mAb were used as control. Proliferation in time (x-axis) is shown on the y-axis. A typical example of three independent experiments is shown. *, p < 0.05 for difference between groups a and b. #, p < 0.01 for difference between groups b and c.

 
CTLA-4 engagement is not required for the activation or the suppressor-effector function of CD4+CD25+ T cells

The observation that both CD4+CD25+ TREG and CTLA-4 signaling are essential for the induction of hyporesponsiveness to alloantigen was suggestive of a role for CTLA-4 signaling in TREG. TREG constitutively express CTLA-4, which we confirmed by showing that TREG express CTLA-4 with increasing expression upon allogeneic stimulation (data not shown). Thus, we investigated whether the stimulatory effect of the anti-CTLA-4 Ab was due to an impaired TREG function. TREG require activation via their TCR to become suppressive, but once activated, their suppressor effector function is completely nonspecific ( 19). Therefore, suppression can be viewed as a two-step phenomenon. We wished to distinguish whether CTLA-4 engagement might be implicated in either the stage of induction of suppression or the suppressor-effector stage of TREG. Purified CD4+CD25+ T cells were preactivated with allogeneic splenocytes for 5 days in the presence or absence of anti-CTLA-4 mAb. Cells were harvested, washed, and subsequently added to a MLR consisting of CD25– responder T cells and fresh splenocytes of the original stimulator type. The presence of anti-CTLA-4 mAb during the preculture did not compromise the suppression of CD25– responder T cell proliferation by the CD4+CD25+ T cells, ruling out an active role for CTLA-4 in the activation of CD4+CD25+ T cells (Fig. 3A).


Figure 3
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  		FIGURE 3. CTLA-4 signaling is not required for activation and suppressor-effector function of TREG. A, BALB/c CD4+CD25+ T cells were preactivated with 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes in the absence or presence of 5 µg/ml anti-CTLA-4 mAb. Cells were harvested at day 5 of MLR, extensively washed, and cocultured in increasing numbers with CD25– T cells and 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes to asses their regulatory capacity. Proliferation as measured by [3H]thymidine incorporation on day 6 is shown on the y-axis and expressed as mean cpm ± SEM. B, A total of 1 x 105 BALB/c CD25– T cells was cocultured with increasing numbers of freshly isolated CD4+CD25+ T cells and stimulated with 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes in the presence of the indicated amounts of anti-CTLA-4 mAb. Proliferation as measured by [3H]thymidine incorporation on day 6 is shown on the x-axis. A typical example of three independent experiments is shown.

 
Subsequently, we analyzed a role for CTLA-4 in the suppressor-effector function of CD4+CD25+ T cells. Freshly purified TREG were added to a coculture of purified CD25– T cells and allogeneic splenocytes in the absence or presence of anti-CTLA-4 mAb. The addition of CTLA-4, even at very high dose, to the coculture did not disrupt the dose-dependent suppression displayed by the TREG (Fig. 3B). We excluded the possibility that the observed decrease in proliferation was due to crowding, by showing that the addition of equal amounts of CD25– effector T cells did not result in inhibition.

We conclude that CTLA-4 engagement does not significantly contribute to the activation nor the suppressor-effector function of TREG.

Blockade of CTLA-4 signaling enhances CD25– effector proliferation in the presence or absence of APC

Because CTLA-4 engagement did not appear to be directly involved in the suppressor function of TREG, the question remained of whether anti-CTLA-4 mAb could interfere in the tolerance inducing effect of costimulation blockade by acting on effector T cells. We observed that especially the addition of high doses of anti-CTLA-4 mAb augmented the proliferative response of purified CD25– cells to allogeneic splenocytes (Fig. 4A). Similar results were obtained when purified CD25– T cells were stimulated with immobilized anti-CD3 mAb, a strong TCR signal that led to the expression of CD80 and CD86 on the activated T cells (data not shown). Also in this case, CTLA-4 blockade increased the proliferative response of CD25– T cells (Fig. 4B). In the absence of CD3 stimulation, the anti-CTLA-4 mAb had no effect (data not shown), indicating that T cell activation is a prerequisite for the anti-CTLA-4 mAb to be effective. Thus, it appeared that anti-CTLA-4 mAb directly acts on activated T cells.


Figure 4
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  		FIGURE 4. Blockade of CTLA-4 signaling enhances CD25– effector proliferation in the presence or absence of APC. A, A total of 1 x 105 BALB/c CD25– T cells was stimulated with 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes in the presence of the indicated amounts of anti-CTLA-4 mAb. Proliferation as measured by [3H]thymidine incorporation is shown on the y-axis. B, A total of 5 x 104 BALB/c CD25– T cells was stimulated with immobilized anti-CD3 mAb in the absence or presence of anti-CTLA-4 mAb. Proliferation (y-axis) is shown at day 4 of culture. A typical example of three independent experiments is shown. *, p < 0.05 for difference between groups.

 
TREG and CTLA-4 signaling cooperate in controlling CD8+ T cell expansion during costimulation blockade

Summarizing the findings above, we have demonstrated a restraining effect of CTLA-4 engagement on the proliferation of CD25– T cells, while TREG function was independent of CTLA-4 signaling. The counteracting effects of anti-CTLA-4 mAb on costimulation blockade, as shown in Fig. 1, might thus well be explained by a lack of control of CD25– effector T cell division, rather than a disturbed TREG function. Nevertheless, the question remained whether CTLA-4 signaling and TREG can have additive effects during costimulation blockade. Therefore, we studied the separate as well as combined effects of CTLA-4 and TREG on CD4+ and CD8+ responder T cell division under the cover of anti-CD40L and anti-CD86 mAb. The responder T cell populations that were analyzed either contained the natural proportion of TREG (the whole T cell population), or were depleted for this population. Anti-CTLA-4 mAb was added to the appropriate cultures. CFSE analysis revealed that the degree of inhibition of CD4+ T cell proliferation already obtained by costimulation blockade as such remained largely unaffected by the additional presence of CTLA-4 signaling or TREG, for neither CTLA-4 blockade nor TREG depletion substantially increased CD4+ T cell division (Fig. 5). In contrast, inhibition of CD8+ T cell division by costimulation blockade was markedly compromised when either CTLA-4 engagement was blocked or TREG were depleted. The CD8+ T cell proliferation was even further enhanced when blockade of CTLA-4 was combined with depletion of CD4+CD25+ T cells. These data demonstrate that CTLA-4 engagement and TREG have cooperative effects in facilitating the anti-CD40L and anti-CD86 mAb-mediated inhibition of CD8+ T cell expansion.


Figure 5
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  		FIGURE 5. During costimulation blockade CTLA-4 signaling and TREG function have complementary effects on CD8+ effector expansion. A total of 1 x 105 responder BALB/c T cells of either whole T cells (including TREG; indicated as "CD4+CD25+ Present") or CD25+-depleted T cells (indicated as "CD4+CD25+ Depleted") was CFSE-labeled and cultured with 2.5 x 104 C57BL/6 gamma-irradiated (27.5 Gy) splenocytes with anti-CD86 + anti-CD40L mAb (1 µg/ml each). Either condition of whole T cells (including TREG) or CD25+-depleted T cells was examined in the presence of anti-CTLA-mAb (5 µg/ml; indicated as "CTLA-4 engagement No"), or absence of anti-CTLA-4 mAb (indicated as "CTLA-4 engagement Yes"). Histograms are shown of the dividing T cells within the live gate in the CD4+ and CD8+ T cell populations analyzed by flow cytometry at day 7 of culture. A typical example of three independent experiments is shown.

 
Also in vivo, CTLA-4 signaling contributes to CD8+ T cell containment during costimulation blockade

Previously, we established that the CD40L and CD86 costimulation blockade protocol facilitates TREG function in vivo ( 16). Here, we found that in vitro CTLA-4 signaling serves to further constrain CD8+ T cell expansion in the presence of TREG during CD40L and CD86 costimulation blockade, and appears not directly involved in the activation of TREG. We questioned whether the same holds true in an in vivo model. Therefore, BALB/c cardiac allografts were transplanted into C3H recipients under the cover of anti-CD40L and anti-CD86 mAb, typically resulting in a mean survival of >90 days. In correspondence to our in vitro observations, we demonstrated that blockade of CTLA-4 signaling abrogated the ability of anti-CD86 and anti-CD40L mAb to prevent allograft rejection. Coadministration of anti-CTLA-4 mAb resulted in the rejection of 80% of the cardiac allografts within 90 days after transplantation (Fig. 6A).


Figure 6
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  		FIGURE 6. In vivo anti-CTLA-4 mAb abrogates allograft acceptance by anti-CD40L and anti-CD86 mAb. A, Graft survival of BALB/c cardiac allografts in C3H recipients. Recipients received either anti-CD40L + anti-CD86 mAb, or anti-CD40L + anti-CD86 mAb with additional anti-CTLA-4 mAb, or no mAb (control); p < 0.01 for differences between groups a and b, b and c, and a and c. B, Immunofluorescence staining for CD8+ cell infiltration in rejected allografts of control recipient mice and anti-CD40L mAb, anti-CD86 mAb and anti-CTLA-4 mAb-treated mice (magnification x250). Grafts were harvested within 1 day after complete cessation of palpable heartbeat. A typical example of four independent experiments is shown. C, The average number of infiltrating Ly-6G+, CD4+ and CD8+ cells per section area was determined for rejected allografts of control recipient mice and anti-CD40L mAb, anti-CD86 mAb, and anti-CTLA-4 mAb-treated mice. A typical example of two independent experiments is shown. *, p < 0.01 for difference between groups.

 
Correspondingly, rejected allografts of recipient mice treated with anti-CD40L mAb, anti-CD86 mAb, and anti-CTLA-4 mAb displayed enhanced CD8+ cell infiltration as compared with rejected allografts of control recipient mice (Fig. 6B). Cell counting indicated that CD8+ cell infiltration was significantly higher in recipient mice treated with anti-CD40L mAb, anti-CD86 mAb, and anti-CTLA-4 mAb compared with untreated recipients, whereas Ly-6 G+ (neutrophil marker) and CD4+ cell infiltration were similar under both conditions (Fig. 6C). Thus, also in vivo, CTLA-4 signaling serves to constrain CD8+ T cell expansion in the presence of CD40L and CD86 costimulation blockade.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Disclosures
 References
 
In the present study, we demonstrate that the efficacy of costimulation blockade in the prevention of allograft rejection relies on mechanisms that are integral in maintaining normal immune homeostasis ( 20). The effects of costimulation blockade were markedly impaired if CTLA-4 engagement was abrogated or if TREG were depleted. These regulatory mechanisms were found to act independently. CTLA-4 engagement was not required for TREG action, while in the absence of TREG, blockade of CTLA-4 signaling resulted in enhanced effector cell proliferation, predominantly by compromised inhibition of CD8+ T cell expansion. Moreover, these pathways acted in concert because intact CTLA-4 engagement in the presence of TREG resulted in the highest inhibition of CD8+ T cell proliferation by costimulation blockade.

In our model, anti-CTLA-4 mAb compromised the inhibition of the in vitro alloresponse by anti-CD40L and anti-CD86 mAb. Moreover, an intact CTLA-4 pathway in the primary MLR was required to induce a hyporesponsive phenotype. Notably, in vivo, the prolongation of allograft survival by anti-CD40L and anti-CD86 mAb was dependent on the integrity of the CTLA-4 pathway, in line with the importance of CTLA-4 signaling in attenuation of T cell responses ( 21, 22). Initially, we had three reasons to assume that these observations should be explained by interference of the anti-CTLA-4 mAb with the activation or suppressive effect of TREG. First, like others, we had observed that the in vitro effects of costimulation blockade were dependent on the presence of CD4+CD25+ T cells ( 18). Second, CD4+CD25+ T cells were shown to play an important role in experimental models of peripheral allograft tolerance ( 12, 23, 24). Finally, CTLA-4 is constitutively expressed on TREG, and several studies have postulated that engagement of CTLA-4 on CD4+CD25+ T cells indeed is essential for their regulatory function ( 9, 11). However, in cocultures with alloreactive CD25– cells, we were unable to reverse the suppressive effect of purified CD4+CD25+ T cells by CTLA-4 blockade. Because it has been suggested that lower concentrations of anti-CTLA-4 mAb may facilitate cross-linking of CTLA-4 instead of blocking engagement of CTLA-4 with its ligands ( 9), we also tested a high concentration of anti-CTLA-4 mAb (100 µg/ml). Even at this high dosage, anti-CTLA-4 mAb did not abrogate the suppression of CD4+CD25+ T cells. These data indicated that CD4+CD25+ T cell function is not dependent on CTLA-4 engagement on these cells.

Interestingly, high doses of anti-CTLA-4-mAb increased the proliferative response of the CD25– effector cell population in both the presence and absence of APC, indicating that the inhibition of T cell proliferation by CTLA-4 signaling was CD25– T cell specific. Previous in vivo studies reporting a functional role for CTLA-4 on TREG may have been confounded by this effect of CTLA-4 blockade on the CD25– T cells ( 11, 12, 25). Also in cocultures of CD25+ and CD25– cells, the effects of anti-CTLA-4 are difficult to interpret because the Ab may theoretically affect the TREG, the effector T cells, or both populations. This problem was circumvented in studies that used cocultures of CTLA-4-deficient effector T cells and TREG from wild-type mice ( 9, 26). In these studies, addition of anti-CTLA-4 mAb abolished the suppressive effect of TREG, which strongly indicates a functional role for CTLA-4 on these cells. As an alternative approach to uncouple the effects of anti-CTLA-4 on TREG from those on effector T cells, we incubated TREG with anti-CTLA-4 in a preactivation culture, before adding them to the coculture with CD25– cells in the absence of CTLA-4. In agreement with findings of Thornton et al. ( 13), we did not observe an effect of anti-CTLA-4 in this situation. Several other recent studies support our conclusion that the main effect of anti-CTLA-4 is on the level of effector T cells rather than TREG ( 6, 14, 21). In addition, CTLA-4-deficient CD4+CD25+ cells retain their suppressor function in vitro, which indicates that CTLA-4 is not indispensable ( 9, 15, 26). We cannot readily explain the diverging findings obtained in the studies with CTLA-4 deficient effector cells. However, it is known that these cells show strong signs of activation ( 5), which may be accompanied by up-regulation of B7 molecules ( 27). One can imagine that the interaction with CTLA-4-bearing TREG and the effects of anti-CTLA-4 are different in this situation.

The stimulating effect of anti-CTLA-4 mAb on the proliferation of CD25– T cells was especially observed during blockade of CD40L and CD86. In this situation, addition of anti-CTLA-4 mAb resulted in a marked increase in proliferation, predominantly of CD8+ T cells. In vivo, this was reflected by rejection that was mediated by an enhanced infiltration of CD8+ cells in cardiac allografts of recipients treated with anti-CD40L, anti-CD86, and anti-CTLA-4 mAb as compared with untreated recipients. Current dogma holds that CD40-CD40L interactions are required for the CD4+ Th cell-dependent stimulation of naive CD8+ T cells ( 28). Apparently, in the allogeneic response CTLA-4 engagement is still essential to inhibit the proliferation of CD8+ T cells despite the absence of CD40L and CD86 signaling during costimulation blockade. However, it is of note that a CD4 help-independent pathway of alloreactive CD8+ T cell activation has been demonstrated ( 29). This may allow CD8+ T cell activation in the absence of CD4+ T cell help in the condition of anti-CD40L and anti-CD86 costimulation blockade. Moreover, in contrast to CD4+ T cells, CD8+ T cells were shown to be less dependent on CD28/CD40L interactions with respect to cell cycle entry and initial expansion ( 30). Because CTLA-4 is up-regulated upon sufficient T cell activation ( 20), it appears that under the cover of costimulation blockade with anti-CD40L and anti-CD86 mAb, activated CD8+ T cells are more responsive to CTLA-4 blockade than the inhibited CD4+ T cells.

The most obvious ligand for CTLA-4 during CD40L and CD86 blockade would be CD80. Indeed, biophysical studies indicate that CD80 is predominantly inhibitory by preferential engagement of CTLA-4. Conversely, CD86 was found to be more costimulatory due to more effective interactions with CD28 ( 31). However, CD80 does not exclusively interact with CTLA-4, it can also bind to CD28. Because the peripheral homeostasis of TREG is controlled by CD28 engagement ( 32), the interaction between C80 and CD28 may be essential for the survival and self-renewal homeostasis of TREG during treatment with anti-CD86. An interaction between CD80 and CD28 on effector T cells is supported by our finding that addition of anti-CD80 to the combination of anti-CD40L and anti-CD86 during a primary MLR further decreased the proliferative response. Of note, it was recently demonstrated that in particular in the presence of CD28 signaling the CTLA-4 pathway provides a negative signal to alloreactive CD8+ cells ( 33). Possibly, CTLA-4 signaling can serve as a negative regulator in our protocol by virtue of the intact CD80:CD28 signaling. Yet, our data and results from other studies indicate that costimulation blockade protocols that allow the negatively signaling CD80-CTLA-4 interaction are preferable for tolerance induction ( 34, 35).

Previously, we established that costimulation blockade contributes to allograft tolerance by inducing functional dominance of TREG ( 16). In line with others, we here underscore the role of CTLA-4 signaling on effector cells during costimulation blockade and not on TREG ( 13, 14, 15). We extend these findings by the observation that the counteracting effect of CTLA-4 blockade appears specifically due to an increase in CD8+ T cell proliferation, despite the presence of costimulation blockade by anti-CD40L and anti-CD86 mAb. When CTLA-4 engagement is blocked in this protocol, CD8+ T cell expansion overcomes CD4+CD25+ T cell-mediated suppression. We propose that CTLA-4 signaling and TREG function constitute fully complementary immunoregulatory mechanisms and have additive effects in the containment of the alloreactive effector pool. Correspondingly, in a tumor vaccination model maximal tumor eradication by CD8+ effector T cells was achieved by combining CTLA-4 blockade and CD4+CD25+ T cell depletion ( 36). Also, TREG and CTLA-4 signaling were found to act cooperatively to maintain tolerance to self-Ag in a transgenic mouse model of diabetes ( 37).

Thus, CTLA-4 engagement limits the expansion of deleterious alloreactive T cells, which facilitates the ability of CD4+CD25+ T cells to maintain the balance between regulatory vs effector cells. We thus propose that strategies targeting costimulatory molecules without interfering in CTLA-4 signaling might prove of added value in the induction of transplantation tolerance.


   Acknowledgments
 
We thank the animal facility laboratory for the mice cardiac allograft transplantation and Marinka Bakker-van Bebber for technical support in the immunofluorescence experiments.


   Disclosures
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
References
 
The authors have no financial conflict of interest.


   Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 This work was supported in part by a grant from the Roche Organ Transplantation Research Foundation (ROTRF 903539494) and the Dutch Kidney Foundation. Back

2 Address correspondence and reprint requests to Dr. Luuk B. Hilbrands, Department of Nephrology (464), Radboud University Medical Center Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail address: l.hilbrands{at}nier.umcn.nl Back

3 Abbreviation used in this paper: TREG, regulatory CD4+CD25+ T cell. Back

Received for publication June 15, 2005. Accepted for publication February 15, 2006.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 

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