between T Cells and Dendritic Cells Protein in Antigen-Driven ...

The Journal of Immunology

Functional Implication of Cellular Prion Protein in Antigen-Driven Interactions between T Cells and Dendritic Cells1 Clara Ballerini,2,3* Pauline Gourdain,2* Ve´ronique Bachy,* Nicolas Blanchard,4† Etienne Levavasseur,* Sylvie Gre´goire,5* Pascaline Fontes,6* Pierre Aucouturier,* Claire Hivroz,† and Claude Carnaud7* The cellular prion protein (PrPC) is a host-encoded, GPI-anchored cell surface protein, expressed on a wide range of tissues including neuronal and lymphoreticular cells. PrPC may undergo posttranslational conversion, giving rise to scrapie PrP, the pathogenic conformer considered as responsible for prion diseases. Despite intensive studies, the normal function of PrPC is still enigmatic. Starting from microscope observations showing an accumulation of PrPC at the sites of contact between T cells and Ag-loaded dendritic cells (DC), we have studied the contribution of PrPC in alloantigen and peptide-MHC-driven T/DC interactions. Whereas the absence of PrPC on the DC results in a reduced allogeneic T cell response, its absence on the T cell partner has no apparent effect upon this response. Therefore, PrPC seems to fulfill different functions on the two cell partners forming the synapse. In contrast, PrPC mobilization by Ab reduces the stimulatory properties of DC and the proliferative potential of responding T cells. The contrasted consequences, regarding T cell function, between PrPC deletion and PrPC coating by Abs, suggests that the prion protein acts as a signaling molecule on T cells. Furthermore, our results show that the absence of PrPC has consequences in vivo also, upon the ability of APCs to stimulate proliferative T cell responses. Thus, independent of neurological considerations, some of the evolutionary constraints that may have contributed to the conservation of the Prnp gene in mammalians, could be of immunological origin. The Journal of Immunology, 2006, 176: 7254 –7262.

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ransmissible spongiform encephalopathies (TSE)8 are fatal neurodegenerative conditions including scrapie and bovine spongiform encephalopathy in animal species and Creutzfeldt-Jakob disease in humans (1). TSE are characterized by the presence in the brain and the lymphoid tissues of a misfolded protein termed scrapie prion protein (PrPSc), which is viewed as

*Universite´ Pierre et Marie Curie-Paris6 and Unite´ Mixte de Recherche (UMR) Institut National de la Sante´ et de la Recherche Me´dicale (INSERM) Unite´ (U)-712, Paris, France; and †INSERM U-653, Institut Curie, Paris, France Received for publication August 8, 2005. Accepted for publication March 28, 2006. 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 by INSERM and Universite´ Pierre et Marie Curie-Paris6, and by specific grants from Groupement d’Inte´ret Scientifique-Maladies a` Prions and European Union Grant no. QLKS-CT-2002-01044. C.B. was the recipient of a poste vert INSERM and a fellowship from Universite´ Pierre et Marie Curie-Paris6; P.G. is the recipient of a thesis fellowship from the French Ministry of Research and Technology; N.B. was supported by a fellowship from Fondation pour la Recherche Me´dicale (FRM); S.G. was supported by a fellowship from FRM. 2

C.B. and P.G. contributed equally to this work.

3

Current address: Laboratory of Neuroimmunology, Department of Neurological Sciences, University of Firenze, 50134 Florence, Italy. 4 Current address: Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720. 5 Current address: Centre National de la Recherche Scientifique UMR 7087, Hoˆpital Pitie´-Salpeˆtrie`re, 75005 Paris, France. 6

INSERM U-431, Universite´ Montpellier 2, 34095 Montpellier, France.

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Address correspondence and reprint requests to Dr. Claude Carnaud, INSERM U-712, Hoˆpital Saint-Antoine, 184 rue du Faubourg Saint-Antoine, 75571 Paris Cedex 12, France. E-mail address: [email protected] 8 Abbreviations used in this paper: TSE, transmissible spongiform encephalopathy; PrPSc, prion protein scrapie; PrPC, cellular prion protein; LAT, linker for activation of T cell; DC, dendritic cell; BM, bone marrow; SAF, scrapie-associated fibril.

Copyright © 2006 by The American Association of Immunologists, Inc.

the major (if not the unique) pathogenic element responsible for the neurodegenerative process and disease transmissibility (2). PrPSc proceeds from the posttranslational conversion of a highly conserved, host-encoded glycoprotein, termed cellular prion protein (PrPC) (3), constitutively expressed on many tissues, including the CNS and cell subsets of hemopoietic origin. PrPC is essentially present at the cell surface, concentrated in sphingolipid and cholesterol-enriched microdomains, and linked to the plasma membrane by a GPI moiety (4). The normal biological function of PrPC is still enigmatic (5, 6). Besides a complete resistance to TSE infectious propagation (7), mice lacking PrPC (PrP⫺) display only minor phenotypic anomalies (8, 9). Yet, the remarkable conservation of Prnp, the PrPCencoding gene (⬎85% homology between mouse and human sequences) and its universal expression in vertebrate species (10, 11), suggests that the gene product fulfills either directly or indirectly, some vital function(s). Deciphering the biological role of PrPC is therefore a major challenge for an evolutionary interpretation of the Prnp gene conservation and for a better understanding of TSE pathogenesis. The GPI insertion of PrPC suggests at least three putative functions: capture of an exogenous ligand, adhesion to cells or to extracellular matrices, and signaling. All three possibilities have been abundantly documented (12). Several groups have reported that PrPC binds and internalizes copper ions that in turn enhance the activity of superoxide dismutase enzymes, resulting in better resistance against oxidative stress (13–15). Other groups have shown that PrPC might exert neuroprotection through alternative pathways including the binding to laminin or to the precursor of the laminin-specific receptor (16, 17), to chaperones and stress proteins (18 –20), or to members of the antiapoptotic Bcl-2 family (21). Signaling has been demonstrated using anti-PrP Ab as a substitute of a presumptive natural ligand. The 0022-1767/06/$02.00

The Journal of Immunology oligomerization of PrPC on neuronal cell lines results in a succession of events including phosphorylation of protein kinases, production of reactive oxygen species, mobilization of protein kinase C, and, ultimately, activation of MAPKs ERK1/2 (22). These cascades are generally considered as pathways leading to neuronal differentiation or survival, but some authors have also suggested a possible delivery of apoptotic messages, when PrPC-mediated signaling exceeds a certain threshold (23, 24). Lymphoid tissues represent the second compartment, next after the brain, where PrPC is most abundantly expressed. Although no obvious immunological defect has been reported in PrP⫺ mice (25), there is a good indication that PrPC might contribute to the development and normal functioning of the immune system. The protein appears to be tightly regulated on certain lymphoreticular subsets such as T cell, monocytes, and medullary precursors (26 – 28), and anti-PrPC Abs cause partial inhibition of mitogen-driven T cell proliferation (29 –32). More recent data based on confocal imaging and immunoprecipitation have documented, shortly after T cell polyclonal activation, a shift of GPI-anchored PrPC within lipidic rafts, in physical association with a cohort of molecules with signaling functions such as Src, fyn, lck, Zap70, linker for activation of T cells (LAT), NADPH, and MAPKs (33–38). Dendritic cells (DC), which are the natural partner of T cells in initiating primary responses, are also good candidates for being the support of PrPC functions. In addition to their implication in the replication and propagation of PrPSc in the transmitted forms of TSE (39 – 41), mature DC express significant amounts of PrPC along with class II and costimulatory molecules (28, 42, 43). Yet, as in the case of T cells, the precise role of the prion protein on DC remains unclear. Because so little is known about the role of PrPC on DC, and because most data on T cells have been generated in polyclonal systems of activation, we thought it was important to re-evaluate the contribution of PrPC in more physiological conditions, imply-

FIGURE 1. Cell surface PrPC is up-regulated on activated T cells and maturing DC. Con A-activated spleen T cells and BM-derived DC from either B6 or BALB/c mice were generated as described in Materials and Methods. CD4⫹ and CD8⫹ T cell subsets were examined at days 0, 1, and 2 of culture (A and B for B6 and BALB/c, respectively). BM-derived DC collected at days 4, 6, and 8 were stained and analyzed by flow cytometry after gating on CD11c-positive cells (C and D for B6 and BALB/c, respectively).

7255 ing conventional alloantigen or MHC-peptide-driven interactions between T cells and DC. We have examined the impact of PrPC genetic knockout or that of Ab-mediated coating, on either partner of the immunological synapse, using as readout the proliferation of the stimulated T cells. We have also investigated the impact of PrPC upon in vivo responses. Our results show that PrPC has a definite effect on both sides of the synapse, but that this effect might be of a different nature depending on whether it is expressed on DC or T cell membrane.

Materials and Methods Mice PrP⫺ mice were from the original Zu¨rich strain (25) (with permission from C. Weissmann, Institute of Neurology, Medical Research Council Prion Unit, London, U.K.) and have been iteratively backcrossed in our facility to the C57BL/6 (B6) background. The mice designated as PrP⫺ in this study were homozygous offsprings derived from backcross 10. The wildtype mice used as controls came from the same B6 breeding stock and were fully histocompatible with the knockout mice. Mutants and controls were raised and maintained under strictly identical conditions. In some transfer experiments, Ly5.1 mice (either PrP⫺ or wild-type) were used as recipients. These mice were generated by appropriate backcrossings with B6 Ly5.1 breeders. TCR transgenic Marilyn B6 female mice with a RAG2 null mutation (44) were obtained at the Centre de De´veloppement des Techniques Avance´es pour l’Expe´rimentation Animale (CDTA)-Centre National de la Recherche Scientifique (CNRS) (Orle´ans la Source, France). BALB/c mice were purchased from a commercial supplier (Janvier). All of the animals were housed in individual ventilated cages, in compliance with European recommendations, and were maintained under strict specific pathogen-free conditions. The sanitary status was regularly monitored at the CDTACNRS and the Virology Reference Center of Nimegue (Netherlands).

Cell separations T cells were enriched from spleen and pooled lymph nodes by negative magnetic cell sorting. Mechanically dispersed suspensions were freed from red cells by hemolysis in ammonium chloride buffer (ACK), and then incubated with a mixture of anti-CD11b (Mac1) and anti-CD19 hybridoma

7256 supernatants, followed by an incubation with magnetic particles coupled to goat anti-rat Ig Ab (Ademtech). Washed suspensions were submitted to a magnetic field, and the nonretained cells, containing ⬎85% T cells, were carefully decanted. Spleen DC were purified by positive magnetic cell sorting. Spleens were perfused with 3 ml of collagenase D (Roche) at 1 mg/ml in PBS, cut into small fragments, and incubated for 45 min at 37°C. Cells were dispersed on cell strainers (BD Biosciences), hemolysed in ACK, washed, and incubated for 12 min at 4°C with magnetic particles coupled to anti-CD11c Ab (20 ␮l for 1 ⫻ 108 cells) (Miltenyi Biotec). Cell suspensions were then deposited on a magnetic column, washed, and the CD11c⫹-retained cells were flushed out. Passage through columns was repeated twice for a better purity. The percentage of CD11c⫹ cells at the end of the procedure was verified by flow cytometry and was ⬃90%.

In vitro differentiation of DC from bone marrow (BM) precursors BM-derived DC were generated from primary cultures of femoral marrow from 8- to 10-wk-old female wild-type and PrP⫺ mice. Cells were cultivated in RPMI 1640 supplemented with 10% FCS and GM-CSF at 200 U/ml (PeproTech), added at days 0 and 3. At day 6, cells were collected and maturated for 48 h in fresh GM-CSF containing medium plus TNF-␣ at 500 U/ml (PeproTech). Maturation was monitored by FACS analysis of CD86 expression on electronically gated CD11c⫹ cells.

In vitro stimulation of spleen DC Spleen DC isolated as described above were plated in 96-well plates (BD Falcon) at a concentration of 1 ⫻ 106/ml, in a total volume of 200 ␮l. Cells were incubated for 24 h in GM-CSF containing medium supplemented with either 1 ␮g/ml LPS (Sigma-Aldrich), 2 ␮g/ml bacterial CpG motifs (Sigma-Aldrich), 15 ␮g/ml poly(I:C) (Amersham Biosciences), or nothing. After 24 h, DC were collected, washed, and resuspended in FACS buffer. DC were then immunostained with anti-CD11c-FITC, anti-IAb-PE, and either anti-CD40-biotin, anti-CD86-biotin, or anti-CD80-FITC. Biotinylated Abs were revealed with streptavidin-APC (BD Pharmingen, BD Biosciences).

In vitro T cell activation and proliferation assays B6 or BALB/c T cells suspended at 1 ⫻ 106/ml in DMEM, supplemented with 10% SVF, 1% 1 nM sodium pyruvate, 1% 2 mM L-glutamin, 1% penicillin (100 U/ml), 1% streptomycin (100 ␮g/ml) (all reagents were obtained from Invitrogen Life Technologies), and 0.05 mM 2-ME (SigmaAldrich) were polyclonally activated with 2 ␮g/ml Con A. For MLR, responder T lymphocytes from either BALB/c or B6 origin, enriched as above described, were suspended at 2 ⫻ 106/ml in supplemented DMEM. Purified stimulating DC from B6 wild-type or PrP⫺ donors or from BALB/c mice were suspended in DMEM at various concen-

FIGURE 2. Visualizing PrPC at the zones of T/DC contacts. TNF-␣-matured BM-derived DC loaded with the H-Y peptide were incubated together with Marilyn TCR transgenic T cells. PrPC is stained in red. A–C, Represent attempts at colocalizing the prion protein with CD3, LFA-1, and CD43, respectively (in green). ⴱ, Mark the Marilyn T lymphocytes associated to an Ag-loaded DC, seen in phase contrast microscopy. White arrows point at the sites of T/DC contacts, where an enrichment in PrPC is seen. Blue arrows underline the accumulation or the exclusion of T cell markers associated with the supramolecular complex.

ROLE OF PrPC AT THE IMMUNOLOGICAL SYNAPSE trations (from 3 ⫻ 105 down to 3 ⫻ 103 cells/ml). Equal volumes of 100 ␮l/well of responding and stimulating cells were distributed in flat-bottom, 96-microtiter plates (BD Falcon), which were incubated at 37°C in humidified 5% CO2 air for 5 days. The absence of proliferation of purified DC populations alone, or with syngeneic T cells, made irradiation unnecessary. Cultures were pulsed with 1 ␮Ci [3H]thymidine per well for the last 18 h of culture (Amersham Biosciences). Incorporated thymidine was collected on cellulose filters with an automated harvester (Tomtech MacIII; PerkinElmer) and was measured by scintillation (MicroBeta 1450 Trimux; Wallac). Marilyn T cell proliferation in response to male Ag was assayed under similar conditions. T cells from transgenic female donors were collected from pooled lymph nodes and spleens and enriched by elimination of CD11b⫹ (Mac1 positive) cells. They were suspended in supplemented DMEM and distributed in flat-bottom microplates together with various concentrations of spleen DC from female B6 mice and the H-Y peptide (NAGFNSNRANSSRSS) (a gift from Dr. O. Lantz, Institut Curie, Paris, France), in a total volume of 200 ␮l/well. Plates were incubated for 4 days, pulsed with [3H]thymidine for the last 18 h, and processed as described above.

In vitro blocking experiments with mAb Anti-PrP mAb including clone scrapie-associated fibril (SAF)83, SAF61 (45), and Fab of SAF61 (a gift from J. Grassi, Commissariat a` l’Energie Atomique, Saclay, France) were assayed in parallel with their respective IgG1 and IgG2a isotype controls (BD Pharmingen). The Abs were added at the onset of the cultures, under a fixed volume of 20 ␮l/well, and left for the whole duration of the experiment.

In vivo assay for Marilyn T cell proliferation Enriched T lymphocytes from RAG2⫺/⫺ transgenic Marilyn mice were labeled with CFSE at 4 ␮M (Sigma-Aldrich) for 5 min in PBS. The reaction was stopped by addition of chilled FCS. After 2 consecutive washes in PBS, 3 ⫻ 106-labeled cells were injected i.v. into wild-type or PrP⫺ Ly5.1 B6 female recipients challenged within the next 2 h with H-Y peptide (50 ␮g/mouse) in IFA injected at the base of the tail. Mice were sacrificed 3 days later. Regional (inguinal plus lumboaortic) and mesenteric nodes were collected, homogenized separately, labeled with anti-Ly5.2 and anti-CD4 Ab, and analyzed by flow cytometry. Statistical analysis was made between pairs of PrP⫺ and wild-type mice assayed under the same circumstances, using the nonparametric Wilcoxon paired test.

Flow cytometry Fluorescence analyses were performed on a two-laser FACSCalibur (BD Biosciences). Cell samples, usually 1 ⫻ 106, were washed in FACS buffer (PBS 1⫻, 0.5% BSA, 0,1% azide). Fc receptor blocking was achieved in a saturating solution of 2.4G2 anti-CD16/CD32 Ab. Staining Ab directly coupled to fluorochromes were added at pretitrated dilutions, for 20 min at


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FIGURE 3. Whether PrPC is absent on DC or on T cells has a different impact upon allogeneic MLR. Enriched T cells were stimulated in a fully allogeneic MLR model. PrPC was missing on stimulating DC (A and B) or on responding T cells (C and D). A, A typical experiment comparing PrPC-positive DC (u) vs PrP-deprived DC (䡺) (mean cpm of triplicate wells ⫾ SD). B, The compilation of five independent experiments showing comparative responses induced by PrP-positive vs PrP-negative allogeneic DC at a concentration of 5 ⫻ 104 cells/well. C, A typical experiment comparing PrPC-positive (u) vs PrPC-deprived (䡺) responding T cells. D, A compilation of four identical experiments.

⫹4°C. SAF61 and SAF83 anti-PrP Ab (45) were biotinylated according to routine procedures (EZ link Sulfo-NHS-LC-biotin; Pierce) and revealed with streptavidin PE or allophycocyanin. Cell fluorescence was acquired and analyzed using CellQuest software (BD Biosciences). CFSE fluorescence acquisitions were treated in addition with FlowJo software (Tree Star).

Confocal analysis of T/DC conjugates Coverslips were covered with 1 ⫻ 105 BM-derived or spleen DC loaded with 10 nM of the H-Y peptide. After 30 min at 37°C, Marilyn T cells were added at a 1:1 ratio. After 45 min of incubation at 37°C, the coverslips were washed with PBS, fixed 10 min with 4% paraformaldehyde, and permeabilized with a PBS solution containing 0.05% saponin (ICN Biomedicals) and 2% BSA (Sigma-Aldrich). Primary and secondary Abs were diluted in PBS, 2% BSA, and 0.05% saponin, and incubated for 2 h and 1 h, respectively. Abs used were as follows: biotin-conjugated hamster anti-mouse CD3␧ (clone 145-2C11; BD Pharmingen) followed by Alexa 488-conjugated streptavidin (Molecular Probes); anti-LAT (rabbit polyclonal IgG; Upstate Biotechnology) followed by Alexa 488-conjugated goat anti-rabbit IgG (Molecular Probes); monoclonal rat anti-mouse LFA1 (American Type Culture Collection; TIB-127) followed by Alexa 488-conjugated goat anti-rat IgG (Molecular Probes); anti-PrP SAF83 followed by Cy3-conjugated donkey anti-mouse (Fab(⬘)2; Jackson ImmunoResearch Laboratories). Images of conjugates were acquired on a Leica TCS SP2 confocal scanning microscope (Leica), equipped with a 100⫻ 1.4 aperture HCX PL APO oil immersion objective. “En face” view of the T-DC contact zone (xz) was reconstructed from series of xy sections spaced by 0.3 ␮m (Metamorph software; Universal Imaging).

Results Up-regulation of cell surface PrPC on activating T cells and on maturing DC The rapid up-regulation of cell surface PrPC, following T cell activation, has been reported in several studies (29 –32). We confirmed that PrPC was increased on both CD4⫹ and CD8⫹ T cell subsets from B6 or BALB/c mice using Con A as a polyclonal T cell activator (Fig. 1, A and B). Although PrPC up-regulation on differentiating DC has been less well studied, there is indication that PrPC is tightly regulated in this lineage too (42). To provide further evidence, we followed prion protein expression in cultures

of maturing BM-derived DC of B6 and BALB/c origin. As shown in Fig. 1, C and D, cell surface PrPC increased steadily together with CD86 costimulatory molecules, between day 4 and day 8. A similar steady increase of the costimulation molecule CD80 and of MHC class II, was observed between day 4 and day 8 (data not shown).

PrPC migrates at the sites of contact between T cells and DC To substantiate the idea that PrPC is involved in Ag-driven interactions, we looked at the distribution of the protein in T/DC synapses. We took advantage of the Marilyn model, where the recognition by the transgenic Marilyn TCR of DC loaded with male H-Y peptide in the I-Ab context can be readily visualized by the formation of conjugates (44). Complexes formed between Agloaded, fully matured BM-derived DC, and Marilyn T cells were subsequently labeled with anti-PrP Abs and examined by confocal microscopy. More than 70% of such complexes showed an accumulation of PrPC fluorescence at the sites of contact between T cells and DC (Fig. 2). Marilyn T cells alone were stained as controls. They presented a diffuse and even distribution of PrPC on their surface (data not shown). To define more precisely where PrPC migrates within the supramolecular complex that structures the immunological synapse, we costained the conjugates for PrPC (in red) and for molecules such as CD3, LFA-1, or CD43 (in green), which accumulate at the center, the periphery, or are excluded from the supramolecular complex, respectively (46). As can be seen in Fig. 2, no clear colocalization was evidenced between PrPC and those markers, although PrPC was enriched in the zones of contact. No colocalization was observed either with LAT, another central marker of the complex, or with CD90, a GPI-anchored T cell marker associated with activation (data not shown). Thus, PrPC appears to be mobilized at the immunological synapse, at a still unidentified location.

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ROLE OF PrPC AT THE IMMUNOLOGICAL SYNAPSE

Different impact of PrPC absence on the two partners of the MLR As a first attempt to evaluate the contribution of PrPC in Ag-driven T/DC interactions, we examined the consequences of prion protein invalidation on allogeneic MLR. Having verified that B6 and BALB/c strains behaved similarly in terms of PrPC expression (see Fig. 1), we first compared the stimulating potential of wildtype vs PrPC-null DC of B6 origin cultured with responding BALB/c T lymphocytes. As shown in Fig. 3A, T cell stimulation induced by DC deprived of PrPC was less vigorous than that caused by wild-type DC. This was true at all tested concentrations of stimulating cells, ruling out a marginal effect due to suboptimal conditions of stimulation. Fig. 3B shows the results of five independent experiments, each time confirming the lower stimulating efficiency (from 30 to 55% decrease) of PrPC-deprived allogeneic DC. Interestingly, the release of IL-2 by the same responding T cells was not affected, suggesting that the reduced proliferation was not a direct consequence of a lack of growth factor (data not shown). In reciprocal experiments, we compared PrPC-deficient vs wildtype B6 T cells stimulated by allogeneic BALB/c DC. Here, at variance with what had been previously observed, the absence of prion protein on the T cell partner had no impact on the proliferative response, neither in the experiment shown in Fig 3C, nor in four similar assays compiled in Fig. 3D. These experiments therefore revealed a difference in the functional status of dendritic vs lymphocytic PrPC. The absence of PrPC does not affect DC maturation A trivial explanation for the lower efficiency of PrPC-deprived DC could have been that the gene invalidation indirectly affected maturation, reducing the expression of both MHC class II or costimulatory molecules. To rule out this possibility, we compared the phenotypes of spleen DC isolated from PrP⫺ or wild-type mice and matured in vitro for 24 h with LPS. Starting from comparable populations of positively selected CD11c⫹ DC, we found that the absence of PrPC had no detectable influence upon the expression of MHC class II, CD80 and CD40 costimulation molecules (Fig. 4). Other agents of DC maturation such as TNF-␣, oligo-CpG, or poly(I:C) led to similar conclusions (data not shown), suggesting that, irrespective of the TLR pathway being used, the absence of PrPC does not interfere with DC maturation. IL-12p70 production by LPS or CpG-activated spleen DC was not altered either by the absence of PrPC (data not shown). PrPC coating by Ab partially blocks MLR To gain further insight into the respective roles of PrPC on both partners of allogeneic MLR, we looked at the effects of SAF83, an IgG1 mAb that binds to cell surface PrPC. An isotype control was used in parallel to rule out a possible implication of Fc receptors expressed on DC and activated T cells. As can be seen in Fig. 5, Ab inhibited in a dose-dependent manner, alloantigen-driven T cell proliferation. It did so in MLR, where PrPC was expressed on both cell partners (Fig. 5A) or on the stimulating DC only (Fig. 5B). But rather unexpectedly, anti-PrP Ab were also effective under conditions where PrPC was expressed on T cells only, thus revealing an implication of the prion protein on both sides of the synapse and notably on T cells where the mere absence of PrPC had shown no effect (Fig. 5C). Finally, to rule out a destabilizing effect of the Abs on the immunological synapse, we tested in parallel Fab and total Ig of SAF61, an IgG2a mAb with similar specificity as SAF83 for mouse PrPC. The results of such an experiment, shown in Fig. 5D, indicate clearly that Fab are as effective as total Ig in inhibiting

FIGURE 4. Absence of PrPC on spleen DC does not affect their maturation. CD11c⫹ DC were isolated from spleens of wild-type or PrP⫺ mice and cultured overnight with GM-CSF plus LPS (bold histogram) or GMCSF alone (light histogram) as a control. Cells were phenotyped on the following day for class II, CD80, and CD40 markers.

allogeneic MLR. Therefore, it seems that anti-PrP Abs do not mediate their effect by steric hindrance, and that PrPC does not necessarily need to be cross-linked to modify the proliferative T cell response. Absence of PrPC on DC or its mobilization by Ab affects the in vitro response of Marilyn T cells To extend the conclusions from allogeneic to peptide-MHC driven T/DC interactions, we came back to the Marilyn model where PrPC accumulation at the sites of conjugation had been initially observed. Naive transgenic T cells were cocultured for 4 days with PrP⫺ or wild-type female DC loaded with H-Y peptide. Experiments shown in Fig. 5, A and B, replicated the results of allogeneic MLR, in that DC devoid of PrPC were systematically less efficient in stimulating T cells than wild-type DC. This was true at all the experimental conditions tested, whether the doses of peptide (Fig. 6A) or the numbers of loaded DC were varied (Fig. 6B). The reciprocal experiment was unfortunately not feasible, because the PrP null mutation could not be passed onto Marilyn RAG2⫺/⫺ mice due to the close vicinity of RAG and Prnp loci on chromosome 2. Still, it was possible to study the effects of Ab coating in situations where PrPC was present either on both partners or on T cells only. As shown in Fig. 6, C and D, SAF83 caused definite inhibition of T cell proliferation, irrespective of whether PrPC was present on both partners (Fig. 6C) or on the T cells only (Fig. 6D). Here again, the isotype control did not cause significant inhibition, thus ruling out an effect of anti-PrP Ab due to Fc receptor binding. Absence of PrPC on APCs affects Ag-driven T cell proliferation in vivo Having found that PrPC-deprived DC stimulated T cells less efficiently in allogenic and peptide-MHC-driven in vitro interactions, we sought to extend this result in vivo, by comparing the efficiency of the APCs in PrP⫺ vs wild-type mice. Recipients of both types were first transferred i.v. with purified CFSE-labeled Marilyn T cells, and subsequently challenged with H-Y peptide in IFA. Control mice received emulsified PBS instead. Ag-driven T cell proliferation was evaluated 3 days later by measuring the decrease in


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FIGURE 5. Anti-PrP Ab reveal a functional implication of PrPC on both stimulating DC and responder T cells. T cells were stimulated as in Fig. 3. A, The inhibitory effect of anti-PrP mAb SAF83 in a MLR where PrPC is expressed on both partners. In B, PrPC is present on DC only and in C on T cells only. F, Represent values of thymidine uptake in cultures with SAF83; E, represent values in control cultures with the IgG1 isotype control (mean cpm of triplicates ⫾ SD). D, Fab and total Ig of SAF61 have been tested in parallel with an IgG2a isotype control. F, SAF61; E, isotype control; f, Fab of SAF61.

CFSE fluorescence of the transferred T cells, collected in the draining lumboaortic and inguinal lymph nodes or in the mesenteric chain. The percentages of retrieved T cells were similar in wildtype and PrP⫺ recipients. As expected, the T cells in control mice that had not received male Ag, manifested maximal fluorescence intensity, whereas in mice challenged with the H-Y peptide, they displayed several peaks of decreasing fluorescence corresponding to successive waves of cell division (Fig. 7A). A close comparison of the patterns seen in a PrP⫺ vs a wild-type recipient mouse (Fig. 7B) showed, however, a delayed proliferation of the T cells implanted in the PrP-deficient host. There are, for instance, four times more cells (32 vs 9%) in peak 1 of the PrPC-deprived mouse than in the wild-type control, whereas the reverse is seen (5 vs 21%) in peak 4 corresponding to T cells that have undergone more divisions. This experiment was repeated four times, with a total of six PrP⫺ and five wild-type mice. In five of the six PrPC-deficient recipients, Marilyn T cells proliferated less promptly than in the wild-type controls. The difference between the two groups was statistically significant (Fig. 7C). Thus, the absence of PrPC on APCs has a definite impact upon in vivo Ag-driven proliferation of responding T cells.

Discussion The objective of this study was to assess the functional implication of PrPC in T/DC interactions, by examining the behavior of cell partners on which the prion protein was either missing or was coated by mAb. Several teams are currently contributing to the elucidation of PrPC function(s) in the immune system. The novelty of our approach resides in the following points. First, we have considered simultaneously the two cell types of the immunological synapse, T cells and DC, whereas most studies published so far have focused on the T cell partner only (29 –37). Second, at variance with other studies examining polyclonal T cell activation by mitogenic lectins (29, 30), Ab cross-linking (35–37), or hypothermic shock (38), we have looked at more physiological conditions. Even if allogeneic stimulation or MHC-peptide activation of TCR transgenic T cells mimic only the normal development of an immune response, the two models imply the formation of T/DC synapses, specific recognition of antigenic patterns, and physiological

signal transduction pathways. Last, we demonstrate that the lack of PrPC has in vivo repercussions, which could provide an explanation for the selective advantage of the Prnp gene. An interesting result from the present study is that the absence of PrPC does not have the same consequences on T cells and DC. Lack of PrPC on T lymphocytes has no visible influence on their capacity to proliferate in response to allogeneic APCs, whereas the lack of PrPC on DC results in a significant reduction of proliferation by the responding T cells. This difference may account for some of the discrepancies noted in the literature regarding the consequences of Prnp gene knockout on polyclonal T cell responses (30 –31). It probably reflects differences in function and in signaling properties of dendritic vs lymphocytic PrPC. Regarding the DC side, we have ruled out an effect of PrPC absence on the expression of MHC and CD80/CD86 or CD40 costimulation molecules. The production of IL-12p70 by DC is not modified either by the absence of PrPC. A more likely eventuality, comforted by the observation that the prion protein is mobilized at the supramolecular complex, could be that PrPC stabilizes the synapse, affecting in turn the duration and the efficiency of T/DC interactions. In a recent study using the Marilyn transgenic model (44), it was shown that the dynamics of conjugation, which differs between immature and full-fledged DC, had an impact on T lymphocyte activation. One of our future objectives will be to document, through imaging experiments, the possibility that the absence of PrPC on DC affects the quality of T/DC conjugates. The GPI anchoring, which confers flexibility and mobility to the prion protein, would certainly be compatible with a role of PrPC in the physical shaping of the synapse. Furthermore, it will be important to find out whether PrPC acts exclusively as an element of physical cohesion between T cells and DC or also as a signaling molecule transducing messages inside the DC. Preliminary data regarding synapses formed between Marilyn T cells and DC from knockout mice suggest that lymphocytic PrPC migrates more readily when PrPC is also present on the DC partner (data not shown). The discrepancy between the lack of functional effect of PrPC invalidation on T lymphocytes and the inhibition of their proliferation after Ab-mediated PrPC recruitment is a strong indication

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FIGURE 6. Implication of PrPC in peptideMHC-driven T/DC interactions. A and B, The in vitro proliferation, expressed as stimulation index, of Marilyn T cells stimulated with Ag-loaded female spleen DC expressing either PrPC (F) or PrPC⫺ (E). A, T cells were stimulated with a constant number of spleen DC pulsed with decreasing concentrations of H-Y peptide. B, The amount of peptide was maintained constant at 10 nM, while the number of DC was decreased (mean stimulation indexes of triplicates ⫾ SD). C and D compare T cell proliferation at two concentrations of H-Y peptide, in the presence of SAF83 (䡺), isotype control (u), or no Ig (f). C, PrPC is present on both partners, whereas in D PrPC is present on T cells only (mean stimulation indexes of triplicates ⫾ SD).

that lymphocytic PrPC exerts signaling functions. Abs do not simply mask or strip off PrPC on T cells, like genetic invalidation. By mobilizing PrPC, they probably induce a cascade of biochemical events resulting in partial inhibition of T cell proliferation. Results already exist, both in neuron and in lymphocyte cell lines, suggesting that the mobilization of PrPC leads to signaling pathways (22, 37, 47). Still, the physiological consequences of PrPC engagement, whether it results into differentiation, expansion, acquisition or inhibition of functions, or to apoptosis, remain to be properly evaluated. The latter possibility is of particular interest in view of the fact that both pro- and antiapoptotic effects have been attributed to PrPC in neuronal cells (23, 24). Thus, it is possible that similar pathways are at work in T lymphocytes, depending upon

the intensity, duration, and timing of PrPC signaling, together with an eventual synergy with TCR/CD3 signaling. Another line of thoughts is provided by studies dealing with the Ab-mediated recruitment of GPI-anchored proteins on T cells. Such studies have revealed profound similarities between all these molecules, and notably their capacity, following Ab-mediated mobilization, to inhibit clonal T cell expansion through the IL-2R pathway, while preserving the functions of the lymphocytes (48, 49). An important issue will be to find out whether PrPC follows the signaling pathway common to most GPI-anchored proteins on T cells, a pathway that results in clonal size control, while leaving intact effector functions such as cytotoxicity or lymphokine production, or whether PrPC initiates its own specific signaling pathway.

FIGURE 7. Marilyn T cells proliferate less readily upon antigenic challenge in a PrP⫺ host. CFSE-labeled Marilyn T cells were injected into PrP⫺ or wild-type female mice and subsequently stimulated with the H-Y peptide in IFA. Control mice received emulsified PBS. A, CFSE fluorescence profiles of transferred, but not stimulated, Marilyn T cells vs Ag-stimulated T cells retrieved 3 days later from the mesenteric nodes of recipients. B, A quantitative cycle analysis of the Marilyn T cells retrieved from Ag-challenged PrP⫺ vs wildtype mice. C, The compilation of four independent experiments comparing pairs of PrP⫺ vs wild-type female mice assayed in parallel. Statistical difference between the two groups was assessed by Wilcoxon paired test.

The Journal of Immunology The last part of this study was aimed at evaluating the effects of Prnp gene invalidation in vivo. Although the present experimental setting gives only a partial view on this issue, by focusing exclusively on the Ag-presenting side of immune responses, it shows nevertheless that the absence of PrPC on APCs has a definite impact on Ag-driven T cell proliferation. Whether this is sufficient to qualify PrP⫺ mice as immunocompromised is still too preliminary. More focused experiments will have to be performed to find out whether, at variance with what had been initially observed (25), the knockout of the Prnp gene represents a true selective disadvantage, especially when PrPC-deprived mice are, for instance, confronted by harmful pathogens. A better understanding of PrPC implication in vivo might provide a clue regarding the evolutionary conservation of a gene whose only known function so far is its contribution to a fatal neurodegenerative condition. The fact that the same molecule might be involved in immunological and neurological synapses is not unprecedented. MHC class I molecules and agrin, a glycoprotein present at synaptic and neuromuscular junctions as well as in the sphingolipid microdomains of lymphoid cells, are obvious examples of molecules with functional properties in the two systems (50, 51). Semaphorins and their receptors, which were originally identified as molecules involved in axonal guidance during CNS development, also seem to modulate T/DC interactions (52, 53). For instance, expression of plexin-A1, a receptor of semaphorins present at the surface of mature DC, is tightly regulated with transcription factor CIITA, which controls MHC class II expression and optimizes T lymphocyte activation. Abs against neuropilin-1, another member of the semaphorin family present at the T and DC surfaces, inhibit T cell proliferation in a way reminiscent of our present results (54). The implication of PrPC in two independent physiological systems does not necessarily mean that the protein fulfills equivalent functions, but it provides the ground for future investigations aimed at a better understanding of the physiological and eventually the pathogenic role of the prion protein.

Acknowledgments We thank I. Renault for the mouse breeding and the management of the animal facility; Dr. C. Weissmann for the PrP⫺ breeders; Dr. O. Lantz for advice, discussion, and gift of H-Y peptide; Dr. J. Grassi for the gift of anti-PrP mAbs and Fab; and Dr. M. Rosset-Bruley for critical reading of the manuscript.

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Disclosures The authors have no financial conflict of interest.

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