Multipotent adult germ-line stem cells, like other pluripotent stem cells, can be killed by cytotoxic T lymphocytes despite low expression of major histocompatibility complex class I molecules
- Ralf Dressel†1Email author,
- Kaomei Guan†2,
- Jessica Nolte3,
- Leslie Elsner1,
- Sebastian Monecke1,
- Karim Nayernia3, 4,
- Gerd Hasenfuss2 and
- Wolfgang Engel3
© Dressel et al; licensee BioMed Central Ltd. 2009
Received: 19 August 2009
Accepted: 28 August 2009
Published: 28 August 2009
Multipotent adult germ-line stem cells (maGSCs) represent a new pluripotent cell type that can be derived without genetic manipulation from spermatogonial stem cells (SSCs) present in adult testis. Similarly to induced pluripotent stem cells (iPSCs), they could provide a source of cellular grafts for new transplantation therapies of a broad variety of diseases. To test whether these stem cells can be rejected by the recipients, we have analyzed whether maGSCs and iPSCs can become targets for cytotoxic T lymphocytes (CTL) or whether they are protected, as previously proposed for embryonic stem cells (ESCs).
We have observed that maGSCs can be maintained in prolonged culture with or without leukemia inhibitory factor and/or feeder cells and still retain the capacity to form teratomas in immunodeficient recipients. They were, however, rejected in immunocompetent allogeneic recipients, and the immune response controlled teratoma growth. We analyzed the susceptibility of three maGSC lines to CTL in comparison to ESCs, iPSCs, and F9 teratocarcinoma cells. Major histocompatibility complex (MHC) class I molecules were not detectable by flow cytometry on these stem cell lines, apart from low levels on one maGSC line (maGSC Stra8 SSC5). However, using a quantitative real time PCR analysis H2K and B2m transcripts were detected in all pluripotent stem cell lines. All pluripotent stem cell lines were killed in a peptide-dependent manner by activated CTLs derived from T cell receptor transgenic OT-I mice after pulsing of the targets with the SIINFEKL peptide.
Pluripotent stem cells, including maGSCs, ESCs, and iPSCs can become targets for CTLs, even if the expression level of MHC class I molecules is below the detection limit of flow cytometry. Thus they are not protected against CTL-mediated cytotoxicity. Therefore, pluripotent cells might be rejected after transplantation by this mechanism if specific antigens are presented and if specific activated CTLs are present. Our results show that the adaptive immune system has in principle the capacity to kill pluripotent and teratoma forming stem cells. This finding might help to develop new strategies to increase the safety of future transplantations of in vitro differentiated cells by exploiting a selective immune response against contaminating undifferentiated cells.
This article was reviewed by Bhagirath Singh, Etienne Joly and Lutz Walter.
Pluripotent stem cells could provide the basis for restoring tissue functions by transplantation of cellular grafts in a broad variety of diseases. Embryonic stem cells (ESCs) are the best characterized pluripotent stem cells. They clearly have the capacity to self-renew and to give rise to any cell type of the body . However, the use of human ESCs is restricted due to severe ethical concerns. Therefore, new pluripotent cell types that are derived from adult organisms hold great promises for regenerative medicine. Recently, it has been shown that pluripotent cells can be obtained from spermatogonial stem cells (SSCs) of neonatal  and even adult mice [3–5]. Although SSCs are known to be unipotent  they can undergo reprogramming during in vitro cultivation and give rise to multipotent adult germ-line stem cells (maGSCs) that can differentiate into various cell types in vitro, and form teratomas in vivo [3, 7]. The maGSCs have significant similarities to ESCs [8, 9] and their differentiation capacity into functional cardiomyocytes  and neuronal cells [11, 12] has been shown. Therefore, maGSCs are expected to have a high potential for regenerative medicine . The generation of pluripotent cells from adult human testis has been recently reported [14–16]. Since maGSCs can be obtained without the need of genetic manipulation, they might have advantages even compared to induced pluripotent stem cells (iPSCs) that were obtained by successful reprogramming of mouse and human somatic cells after expression of a limited number of defined transcription factors [17–19].
The possibility to generate autologous pluripotent stem cells from adults could also reduce immunological problems that are expected to occur when allogeneic ESCs are used for regenerative therapies [20, 21]. However, in the case of maGSC-based therapies only male patients could theoretically benefit from fully autologous stem cells. The complexity of maGSC generation and requirements for quality assurance for individual stem cell lines might be a further hurdle for the therapeutic use of individualized maGSCs, as has been discussed in the context of iPSCs [22, 23]. Thus, although maGSCs and iPSC could theoretically allow the generation of autologous pluripotent stem cells, they might also be of use for therapeutic purposes in partially allogeneic settings. However, even when autologous stem cells are utilized for transplantation therapies, cytotoxic T lymphocytes (CTLs) might recognize "oncofetal" antigens  expressed by pluripotent stem cells or differentiation antigens  as has been documented and exploited in the context of tumor immunology. A further concern is that syngeneic or autologous transplantations might be associated with a higher risk of teratoma growth than allogeneic transplantations if grafts are derived from pluripotent cells and are contaminated with undifferentiated cells . Therefore, it seems important to determine whether new pluripotent stem cell types such as maGSCs and iPSCs are protected against CTLs, similarly to what has been reported for ESCs .
In this study, we have analyzed the teratoma growth of maGSCs depending on the conditions used for their culture and in vitro differentiation. Furthermore, the susceptibility of three maGSC lines (maGSC Stra8 SSC5, maGSC 129/Sv, maGSC C57BL) to CTLs has been determined in comparison to corresponding ESC lines (ESC Stra8, ESC 129/Sv R1, ESC C57BL), iPSCs, and F9 teratocarcinoma cells.
The maGSC line Stra8 SSC5 (H2b)  was grown under four different culture conditions that have been previously described in detail . Briefly, under condition I the cells were cultured in dishes, pre-coated with 0.1% gelatine (Sigma-Aldrich, Taufkirchen, Germany) for at least 1 h at 4°C, with basic medium, i.e. Dulbecco's modified Eagle's medium (DMEM) supplemented with 15% fetal calf serum (FCS, selected batches), 2 mM L-glutamine (Invitrogen, Karlsruhe, Germany), 50 μM β-mercaptoethanol (β-ME; Promega, Mannheim, Germany), 1 × nonessential amino acids (NEAA; Invitrogen). For condition II the cells were cultured in gelatine-coated dishes with basic medium containing 103 units/ml leukaemia inhibitory factor (LIF; ESGRO, Millipore, Billerica, MA, USA). Under condition III the cells were cultured on a feeder layer of mitomycin C-inactivated mouse embryonic fibroblasts (MEFs) with basic medium. Cells under condition IV were cultured on a mitomycin C-inactivated MEF feeder layer with basic medium containing 103 units/ml LIF. The cells were used at passages 12 to 18 (condition I), 9 to 20 (condition II), 8 to 22 (condition III), and 10 to 22 (condition IV). The other pluripotent stem cell lines used in this study were cultured under condition IV. These included the lines maGSC 129/Sv (H2b)  (passages 24 to 37), maGSC C57BL (H2b)  (passages 24 to 31), ESC 129/Sv MPI-II (H2b)  (passages 24 to 33), and ESC 129/Sv R1 (H2b)  (passages 19 to 38). The line ESC Stra8 SSC5 (passages 10 to 15) was derived from a Stra8-EGFP/Rosa26 transgenic mouse . A new ESC line (ESC C57BL) (H2b) (passages 9 to 18) was generated as described . The iPSCs (passages 13 to 21) were derived from MEFs (129/Sv × C57BL/6 F1) (H2b) and have been described previously . They were kindly provided by Dr. Rudolf Jaenisch, The Whitehead Institute for Biomedical Research, Cambridge, USA. F9 teratocarcinoma cells (H2b), which originated from an embryonal testicular teratocarcinoma of a 129/Sv mouse were cultured as described . The mouse cell lines YAC-1 (H2a), RMA (H2b), and RMA-S (H2b) were maintained in NaHCO3-buffered DMEM supplemented with 10% FCS (Biochrom, Berlin, Germany), 2 mM L-glutamine, 1 mM sodium pyruvate, 50 μM β-ME, 100 U/ml penicillin, and 100 μg/ml streptomycin.
In vitro differentiation of maGSCs
For differentiation, maGSC Stra8 SSC5 cells cultured under condition III were cultivated as embryoid bodies (EBs) in hanging drops as previously described . Briefly, maGSC Stra8 SSC5 cells were cultured without MEFs for 2 passages before cultivation as EBs. 400 cells in 20 μl of differentiation medium (Iscove's modified Dulbecco's medium supplemented with 20% FCS, 2 mM L-glutamine, 1× NEAA, and 450 μM α-monothioglycerol 3-mercapto-1,2-propandiol (MTG; Sigma-Aldrich, Taufkirchen, Germany)) were placed on the lids of Petri dishes filled with phosphate-buffered saline (PBS) and incubated in hanging drops for 2 days and in bacteriological Petri dishes for another 3 days. At day 5, single EBs were transferred into gelatine (0.1%)-coated 6-cm tissue culture dishes in differentiation medium for 10 further days.
Generation of CTLs and 51Chromium release assays
To obtain peptide-specific CTLs, spleen cells from naive OT-I mice  were stimulated in vitro in the presence of 1 nM SIINFEKL peptide (Ovalbumin 257-264; Bachem Biochemica, Heidelberg, Germany) as previously described . Target cells were labeled by incubating 1 × 106 cells in 200 μl N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered DMEM containing 100 μl FCS and 50 μCi Na251CrO4 (Hartmann Analytic, Braunschweig, Germany) for 1 h at 37°C and washed three times with HEPES-buffered DMEM. Effector cells were added to 5 × 103 51Cr-labeled target cells in triplicate at various ratios in 200 μl HEPES-buffered DMEM/10% FCS per well of round-bottomed microtiter plates. Spontaneous release was determined by incubation of target cells in the absence of CTL. To allow for a peptide-specific killing the respective wells were supplemented with 0.5 μg/ml SIINFEKL. To determine calcium-dependency of killing 2 mM ethyleneglycol-bis(b-aminoethyl ester)-N,N,N',N'-tetraacetic acid (EGTA) and 4 mM MgCl2 were added. The microtiter plates were centrifuged for 5 min at 40 × g, incubated at 37°C for 4 h, and then centrifuged again. Supernatant and sediment were separately taken to determine radioactivity in each well using a Wallac MicroBeta Trilux counter (PerkinElmer Life Sciences, Köln, Germany). Percentage of specific lysis was calculated by subtracting percent spontaneous 51Cr release .
Flow cytometry was performed on a FACScan™ flow cytometer (BD Biosciences, Heidelberg, Germany) using CellQuest™ data acquisition and analysis software. The cell surface expression of major histocompatibility complex (MHC) class I molecules was analyzed using locus and haplotype-specific antibodies (Ab) anti-H2Kb (clone CTKb, mouse IgG2a, phycoerythrin (PE)-conjugated, Caltag Laboratories, Hamburg, Germany), and anti-H2Db (clone CTDb, mouse IgG2a, PE-conjugated, Caltag). The isotype control (mouse IgG2a, PE-conjugated) was purchased from Caltag. To test whether the MHC class I expression can be stabilized at a lower temperature, the stem cells were incubated at 28°C for 24 h before analysis. In order to test for the stabilization of H2Kb cell surface expression by external peptides, the H2Kb-restricted peptide SIINFEKL or a non-binding control peptide (NGLTLKNDFSRLEG) were added for 24 h to cell cultures. The expression analysis of pluripotency markers was done as described previously  using mouse anti-SSEA-1 (clone MC480, Developmental Studies Hybridoma Bank, IA, USA) and mouse anti-Oct4 (clone 9E3.2, Chemicon) mAbs.
Gene expression analysis
Total RNA was extracted from cell lines using the TRIZOL® reagent (Invitrogen, Carlsbad, USA) according to the manufacturer's instructions. The RNA was then treated with RNase free DNase (RQ1, Promega, Madison, USA) for 20 min at 37°C and purified by phenol-chloroform-isoamyl alcohol (25:24:1) extraction and precipitated with 1/10 volume 300 mM sodium acetate (pH 4.8) and 1 volume 2-propanol before washing with 70% ethanol and solving in RNase free water. The quantity of the extracted RNA was determined with a ND-1000 Spectrophotometer (NanoDrop Technologies, Wilmington, USA) and the quality was analyzed using a Bioanalyzer 2100 (Agilent Techonolgies, Santa Clara, CA, USA). For synthesis of cDNA random oligo primers (Promega, Madison, USA) were used. The reverse transcription of RNA was performed for 60 min at 37°C with M-MLV RT polymerase (Promega, Madison, USA) in a total volume of 25 μl. Gene expression levels were analyzed by quantitative real time polymerase chain reaction (qRT-PCR) assays using the following forward and reverse primers generated according to the indicated reference sequences: H2K b (NM_001001892.2, 5'-CCT GGA GTG GAC TTG GTG AC-3' and 5'-GGT GTA GAG GGG TGG ACT GG-3') H2D b (NM_010380.3, 5'-CCC TGT GAG CTT GGG TTC AG-3' and 5'-ACA GGG CAG TGC AGG GAT AG-3'), transporter associated with antigen processing 1 (Tap1) (NM_001161730.1, 5'-CTG CTC TCC CTC TAC CCC TC-3' and 5'-CTG AGT GGA GAG CAA GGA GTC-3'), Tap2 (NM_011530.3, 5'-GCA GAC GAC TTC ATA GGG GA-3' and 5'-GTT GCT TCT GTC CCA CAG C-3'), β2-microglobulin (B2m) (NM_009735.3, 5'-CTC ACA CTG AAT TCA CCC CC-3' and 5'-CAG TAG ACG GTC TTG GGC TC-3'), and Serpinb9 (NM_011452.2, 5'-TGC AGA CAA AAC TTG TGA AGT CCT C-3' and 5'-TGC CTG GAC ACC TCT GCT TC-3') encoding the serine protease inhibitor 6 (SPI-6) protein. The mRNA expression of the housekeeping gene hypoxanthine guanine phosphoribosyl transferase 1 (Hprt1) (NM_013556.2, 5'-GTC CTG TGG CCA TCT GCC TA-3' and 5'-GGG ACG CAG CAA CTG ACA TT-3') was always monitored as internal control. Amplification reactions were carried out in 96-well plates in 25 μl reaction volumes with the Power SYBR® green PCR master mix (Applied Biosystems, Foster City, USA). The PCR reaction plates were preheated for 2 min at 50°C and for 10 min at 95°C followed by 40 cycles of denaturation (15 s at 95°C) and amplification (1 min at 60°C). All reactions were performed in technical triplicates using an ABI 7500 Real Time PCR System. For the data analysis, the ABI 7500 SDS software (Applied Biosystems) was used. The variations in cDNA concentration in different samples were normalized to the housekeeping gene Hprt1. The cycle threshold (ct) values obtained for the genes of interest were corrected by the ct value obtained for Hprt1 in the same sample. The relative level of transcripts was then expressed as Δct value (ct for Hprt1 minus ct for the gene of interest).
Proteins from cultured cells were isolated as described previously . The protein extracts (20 μg per lane) were separated by sodiumdodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and transferred to nitrocellulose (Schleicher und Schüll, Dassel, Germany). The membranes were stained with an anti-PI-9 mAb (clone JM-3544, mouse IgG, MBL, Woburn, MA, USA) that has been described to cross-react with its mouse homologue SPI-6 . An anti-heat shock cognate 70 (HSC70 or HSPA8) mAb (clone 1B5, rat IgG2a, StressGen, Biomol, Hamburg, Germany) was used as loading control. The primary antibodies were diluted 1:2000 in PBS/0.05% Tween 20. Subsequently, blots were incubated with secondary goat anti-mouse IgG (115-005-003, Jackson Laboratories, Dianova, Hamburg, Germany) and goat anti-rat IgG and IgM (112-005-068, Jackson Laboratories) Abs and then with a tertiary peroxidase-conjugated rabbit anti-goat IgG Ab (305-035-045, Jackson Laboratories, Dianova) at dilutions of 1:5000. The substrate reaction was carried out with 0.05% 3,3'-diaminobenzidine/0.0003% H2O2 in PBS/0.05% Tween 20.
All animals were bred in the central animal facility of the Medical Faculty of the University of Göttingen. Severe combined immunodeficient SCID/beige mice (C.B-17/IcrHsd-scid-bg) were kept under pathogen-free conditions. All animal experiments had been approved by the local government. For the analysis of subcutaneous tumor growth undifferentiated maGSC and in vitro differentiated cells, respectively, were suspended in 100 μl PBS (2 × 106 cells/100 μl) and injected into the flank of the animals. Tumor growth was monitored every second day by palpation and size was recorded using linear calipers. The tumor volume was calculated by the formula V = πabc/2, where a, b, c are the orthogonal diameters. Animals were sacrificed at day 100, or before when a tumor volume of 1 cm3 was reached, when a weight loss of more than 10% occurred, or when any behavioral signs of pain or suffering became evident. Autopsies were performed for all animals. Tumor tissue was immediately frozen in liquid nitrogen or placed in phosphate-buffered 4% formalin for 16 h and then embedded in paraffin. Tissue sections (2 μm) were stained with hematoxylin and eosin (HE) for histological examinations.
The data were analyzed using the non-parametric Kruskal Wallis test and the U test for subgroup analysis with the WinSTAT software. A significance level of α = 0.05 was used.
Maintenance of maGSC Stra8 SSC5 cells under different culture conditions
Teratoma formation of maGSC Stra8 SSC5 cells cultured under different culture conditions
Tumor formation of maGSC Stra 8 SSC5 cells in immunodeficient SCID/beige mice
In addition to the undifferentiated cells, in vitro pre-differentiated cells (5 + 10 days of differentiation culture) were injected into 12 SCID/beige mice. In 6 cases an early progressive tumor growth was observed, 2 tumors remained small and stable, and 3 regressed before day 100 (Figure 2, Table 1). Thus, the in vitro pre-differentiation protocol used here is not sufficient to deplete tumorigenic cells from the cultures. This finding is in agreement with the observation that a significant proportion of the in vitro differentiated cells still expressed EGFP, indicating the presence of undifferentiated cells (Figure 1B, C). The histopathological analysis of these tumors showed that the in vitro differentiated cells, similar to undifferentiated maGSCs, formed teratomas (Figure 3D-F). The teratomas contained derivatives of three embryonic germ layers, including epithelium with intestinal differentiation (endoderm, Figure 3D), striated muscle (mesoderm; Figure 3E), smooth muscle, fat, bone and cartilage (mesoderm), and neural tissue (ectoderm, Figure 3F).
The maGSC Stra8 SSC5 cells cultured under condition III (on feeder cells) were also injected subcutaneously into immunocompetent C57BL/6 mice. None of 9 animals receiving 2 × 106 cells developed a teratoma within 100 days. At autopsy no residues of the injected cells were found (data not shown). Thus, maGSCs were readily rejected in immunocompetent allogeneic hosts.
Expression of MHC class I molecules on maGSC Stra8 SSC5 cells cultured under various conditions
MaGSC Stra8 SSC5 cells cultured under different conditions are killed by CTLs whereas in vitro differentiated cells acquire resistance
Next we tested whether the maGSC Stra8 SSC5 cells can be killed by CTLs. We used CTLs from TCR-transgenic OT-I mice as effector cells, which recognize the ovalbumin-derived peptide SIINFEKL in an H2Kb-restricted manner . The target cells, which do not express ovalbumin, were pulsed with the peptide to allow for binding to H2Kb molecules and peptide-dependent killing. RMA cells served as positive control. The results of a representative individual experiment are shown in Figure 4D. RMA cells as well as maGSC Stra8 SSC5 cells were not killed when they were not pulsed with the peptide. They were readily killed in an effector dose-dependent manner when the SIINFEKL peptide was present. In addition, the killing could be blocked by EGTA indicating that the lysis of the targets was mediated by the calcium-dependent granule exocytosis pathway. A summary of the results of 3 experiments with maGSC Stra8 SSC5 target cells cultured under the four conditions and differentiated cells, respectively, is shown in Figure 4E. The lysis of the maGSC Stra8 SSC5 targets did not directly correlate with the H2Kb expression levels since the maGSC Stra8 SSC5 cells cultured under condition III showed the highest expression levels of H2Kb but the lowest killing in comparison to the other three culture conditions. Interestingly, the in vitro differentiated cells (5 + 10 days of differentiation culture) were hardly killed by CTLs.
Further pluripotent cell lines are killed by CTLs despite being negative for MHC class I expression in flow cytometry
In parallel to the killing assays, the MHC class I expression on the target cells was determined by flow cytometry. Interestingly, the maGSC lines maGSC 129/Sv and maGSC C57BL, in contrast to the maGSC Stra8 SSC5 cell line, did not express MHC class I molecules at a level detectable by flow cytometry (Figure 5C, D). They were very similar to the MHC class I negative ESC lines 129/Sv R1 and C57BL (Figure 5C, D). In addition, also the iPSCs and F9 teratocarcinoma cells were negative for MHC class I molecules in flow cytometry (Figure 5C, D). The external peptide pulsing of targets was apparently so efficient that even maGSCs, ESCs, iPSC and F9 cells, on which no H2Kb molecules could be detected by flow cytometry, were readily killed in a peptide-specific manner.
The expression of MHC class I peptides on maGSCs and ESCs cannot be rescued by decreased temperature or external peptides
MaGSCs as well as in vitro differentiated cells do not express the serine protease inhibitor 6 that can protect target cells against CTLs
One advantage of maGSCs over ESCs is that they could theoretically be a source of autologous pluripotent stem cells for individualized medicine. One criterion for assessing the pluripotency of stem cells is the capacity to give rise to teratomas in immunodeficient mice . Therefore, the potential of maGSCs and their differentiation products to form teratomas has to be analyzed very carefully before any therapeutic application can be envisaged. In syngeneic or immunocompromised recipients, the risk of teratoma growth is inherent to transplantation therapies that are based on pluripotent cells. In immunocompetent allogeneic mice, transplanted ESCs do not usually form teratomas , unless very high numbers of pluripotent cells are injected . We have compared the capacity of the maGSC line Stra8 SSC5 to give rise to teratomas depending on the culture conditions used to propagate them, and after differentiation. Cells cultured under four different conditions that varied in the presence of LIF and MEFs as feeder cells did not differ with respect to teratoma growth. This result was consistent with the presence of a similar proportion of undifferentiated cells under the four culture conditions as demonstrated by expression of pluripotency markers SSEA-1 and Oct-3/4, as well as EGFP expression driven by the Stra8 promoter in undifferentiated cells. Thus, the pluripotency of maGSCs was preserved under a variety of culture conditions for over 16 passages including those conditions that do not require feeder cells and might, therefore, have advantages in view of future therapeutic applications. Furthermore, cells differentiated in vitro (5 + 10 days of differentiation culture) without lineage selection still contained about 20% undifferentiated cells and gave rise to teratomas at high frequency in immunodeficient recipients, suggesting that lineage selection is necessary before the differentiated cells can be used for therapeutic application [47, 48].
The risk of teratoma growth after transplantation of pluripotent cells does not only depend on the pluripotency of the injected cells but also on their immunogenicity and their susceptibility to cytotoxic effector mechanisms of the immune system because maGSC Stra8 SSC5 cells did not form teratomas in allogeneic immunocompetent mice. Therefore, we have started to characterize the immunological features of maGSCs and analyzed their susceptibility to killing by CTLs. CTLs recognize peptides presented by MHC class I molecules, which are highly polymorphic in the population. Alloreactive CTLs recognize MHC class I molecules either by direct or indirect allorecognition [20, 21]. The most important immunological barrier for transplantation of pluripotent stem cell-derived allografts is presumably represented by MHC class I molecules [20, 21], but minor histocompatibility antigens have also been shown to contribute to the rejection of ESC-derived grafts by CTL . Interestingly, MHC class I molecules are not detectable by flow cytometry on mouse ESCs [26, 27, 50–54]. This is in contrast to human ESCs [55–57]. However, the expression of MHC class I molecules has been reported to increase during in vitro  or in vivo differentiation of mouse ESCs [54, 58]. MHC class I molecules are inducible by interferon-γ in mouse ESCs [51, 54] or in differentiated cells . Thus, ESCs and their differentiation products might become targets for CTLs after transplantation [49, 53, 54, 59].
Whilst the mouse maGSC line Stra8 SSC5 was found to express comparatively low levels of MHC class I molecules by flow cytometry, the other maGSC lines (maGSC 129/Sv and maGSC C57BL), ESCs, and iPSCs that were used for comparison were negative for these molecules, resembling the phenotype known for mouse ESCs and also F9 teratocarcinoma cells . At the mRNA level H2K and B2m transcripts were detected in all pluripotent stem cell lines, although less abundantly than in RMA cells or MEFs. A low expression of Tap1 and Tap2 mRNA in most pluripotent stem cell lines suggested that a failure of peptide loading could explain the lack of MHC class I molecules at the plasma membrane. However, a decrease of temperature to 28°C or incubation with an H2Kb-restricted peptide did not stabilize the MHC class I molecules on maGSC 129Sv or ECS 129Sv MPI-II cells. A previous report showed a marked increase of expression of H2Dd molecules on mouse embryonic cell lines (including one ESC line) that were transfected with an H2Dd-expression construct and incubated with H2Dd-binding peptides . Thus, the endogenous H2K and B2m transcription in the stem cell lines analyzed here might still be insufficient to produce enough MHC class I molecules that can be detected by flow cytometry after stabilization by external peptides and decreased temperature.
However, all pluripotent cell lines analyzed here were killed by CTLs derived from TCR-transgenic OT-I mice that recognize the peptide SIINFEKL in an H2Kb-restricted manner . This finding is in accordance with studies showing that extremely low numbers of ligands on target cells are sufficient for CTL recognition and killing [62–64]. In addition, it has been reported previously that the MHC class I expression on mouse ESCs is sufficient for recognition by the SIINFEKL-specific T cell hybridoma B3Z [27, 65]. We now show that the MHC class I expression level on ESCs, maGSCs, and iPSCs is functionally relevant since it is also sufficient for peptide-specific CTL killing, although the presence of the peptide did not markedly increase the H2Kb molecules at the cell surface of maGSCs and ESCs. The iPSCs appeared to be less susceptible to CTLs than the ESC and maGSC lines. Whether this is characteristic for iPSCs in general or only for the iPSC line analyzed here has to be determined. Interestingly, in vitro differentiation of maGSC Stra8 SSC5 cells appeared to reduce the sensitivity to CTLs because the in vitro differentiated target cells became much less sensitive to killing compared to their undifferentiated counterparts. The experiments using CTLs from TCR-transgenic mice demonstrated clearly that pluripotent stem cell lines can be killed by this mechanism. However, it has to be further clarified whether the antigen processing machinery of pluripotent stem cells is capable of generating peptide loaded MHC class I molecules for recognition by activated CTLs. Furthermore, it remains to be determined whether alloreactive CTLs that recognize their targets directly can kill the pluripotent stem cells with similar efficacy.
The observation that killing of the maGSCs, ESCs, iPSCs, and F9 cells could be inhibited by EGTA strongly suggests that the granule exocytosis pathway which involves perforin and granzymes [66, 67] was used for killing. Recently, it has been reported that the mouse ESC line CGR8 is resistant to antigen-specific CTLs due to the expression of SPI-6 , which is an endogenous inhibitor of granzyme B . We therefore looked for SPI-6 by western blot in the maGSC and ESC lines but could not detect expression of this protein in any of these cells, even after in vitro differentiation of maGSC Stra8 SSC5 cells. Their relative resistance to CTL cytotoxicity must therefore be caused by another mechanism. Our results are in accordance with recent reports by us  and others  showing that several ESC lines can be readily killed by NK cells. Although the mechanisms of target cell recognitions are different, NK cells and CTLs share the granule exocytosis pathway of target cell killing [66, 67]. Thus, individual ESC lines might differ with respect to SPI-6 expression and resistance to CTL and possibly NK cells.
Several studies have suggested that mouse or human ESCs are immune-privileged. They may suppress immune responses by contact dependent [51, 57] and contact independent mechanisms  or induce apoptosis in T cells . It has also been reported that ESCs are resistant to CTL and NK cell-mediated cytotoxicity [27, 46, 51]. Here we have demonstrated that maGSC, ESC, and iPSC lines can be killed by activated peptide-specific CTLs despite levels of MHC class I expression that were below the detection level of flow cytometry. Therefore, it has to be assumed that these pluripotent cells can become targets for CTLs after transplantation.
MaGSCs represent a new pluripotent cell type derived from adult mouse testis. They retain pluripotency under several cell culture conditions and form teratomas in immunodeficient mice. No expression of MHC class I molecules could be detected by flow cytometry in most maGSC lines and the iPSCs analyzed here, a phenotype shared with ESC lines and F9 teratocarcinoma cells. However, in immunocompetent recipients the immune response could suppress growth of the maGSCs lines as teratomas, and all analyzed pluripotent cells including maGSCs, ESCs, iPSCs, and F9 teratocarcinoma cells could be killed efficiently by activated CTLs. Thus, maGSCs, ESCs, and iPSCs can be recognized as targets for CTLs and they might be rejected by this mechanism after transplantation in immunocompetent recipients. However, the capacity of the immune system to kill pluripotent and teratoma forming stem cells could also be used to develop a new strategy to increase the safety of transplantation of in vitro differentiated cells. Indeed, it might be possible to selectively induce an adaptive immune response in recipients against antigens expressed selectively in pluripotent stem cells in order to kill any teratoma forming cells that could not be depleted before grafting. Further studies are required to validate or disprove this hypothesis.
Reviewers' report 1
Dr. Bhagirath Singh, Department of Microbiology and Immunology, University of Western Ontario, London, Ontario, Canada
In most part this manuscript deals with the immunogenicity of multipotent adult germ-line stem cells (maGSCs) that are derived from spermatogonial stem cells (SSCs) present in adult testis. The authors use these cells as targets for a peptide specific cytotoxic T lymphocytes (CTL) restricted to a major histocompatibility complex (MHC) class I plus OVA (Ovalbumin 257-264) peptide, SIINFEKL.
1) Based upon the data presented the authors conclude that these cells can become targets for CTL even if the expression level of MHC class I molecules is below the detection limit by using flow cytometry. This is not surprising because little as 100 molecules of MHC-peptide complex are sufficient for CTL killing of target cells. Therefore, pluripotent cells might be rejected after transplantation by this mechanism and provides the hope that transplantations of in vitro differentiated cells may be possible and the undifferentiated cells may be killed by their immunogenicity. However, immunologically this approach does not provide any particular benefit as both differentiated cells and undifferentiated cells would be killed in allogeneic conditions. In syngeneic situation it is unclear if there is an advantage to the approach. Authors have to demonstrate this in vivo.
Authors' response: We indeed agree that it is not surprising that pluripotent stem cells can be killed by CTL. However, contrary data have been reported in the literature . Therefore, it was important to demonstrate that pluripotent stem cells are in general not protected against the CTL-mediated cytotoxicity. It remains to be analyzed whether alloreactive CTL that often have lower affinity T cell receptors will similarly kill those targets. Nonetheless, antigens that are specifically expressed in pluripotent teratoma forming stem cells might be relevant targets for CTL and this might be helpful to avoid teratomas after transplantation. However, we completely agree that this is presently a pure speculation that has to be validated or disproved by in vivo experiments in subsequent studies.
2) I agree with the conclusion that suggests that pluripotent cells are not protected against CTL, even if MHC class I molecules are not detectable by flow cytometry. However, authors should explore this further by RT-PCR analysis for the results to be valid. Similarly the SPI-6 data (Fig. 6) should be confirmed by RT-PCR.
3) Finally, it is very important that authors provide a well-balanced analysis to the differences where their results differ from previously published information about the immune-privileged nature of mouse or human ESC as quote in the last paragraph of their Discussion section.
Authors' response: We have altered the discussion in order to clarify these issues.
4) Overall the manuscript is well written and the question posed, methods used are well defined although not new. Additional data is needed to validate the study. The discussion and conclusions need to be refined as outlined above. The title and abstract are well constructed but conclusions need to be modified to identify the shortcomings of the approach and the usefulness of the findings.
Authors' response: We tried also to further clarify the advantages but also the limitations of our present approach in the discussion.
Comment to the revised manuscript
The authors have considerably revised and updated the manuscript based on my comments. Additional experiments have been done and added as new figures. Authors have discussed these results in the text and in my view, the revised manuscript provides a more evidence for the observations and conclusions.
Authors' response: We would like to thank Dr. Singh for his thoughtful review of our manuscript and the helpful suggestions for improvement.
Reviewers' report 2
Dr. Etienne Joly, Equipe de Neuro-Immuno-Génétique Moléculaire, IPBS, UMR CNRS 5089, Toulouse, France
1) Have you considered the possibility that all these pluripotent cells express very low levels of TAP, and have thus very few properly folded MHC molecules on their surface, but probably quite a few empty, peptide-receptive ones. On this subject, I refer you to Bikoff et al.  and to my own paper on neuronal cells .
Authors' response: We have analyzed now the expression of Tap1 and Tap2 at the mRNA level (see the new Figure 6 ) and found rather low levels in most of the pluripotent stem cell lines. H2K and B2m transcripts were more abundant suggesting that indeed a failure of peptide loading could be responsible for the low MHC class I cell surface expression levels. However, culture at 28°C or in the presence of an H2K b -binding peptide did not markedly increase the H2K b cell surface expression (see the new Figure 7 ). Thus, it is unlikely that a low TAP expression alone is the reason for the low MHC class I expression on maGSCs and ESCs.
2) Considering this previous point, and the fact that the effectors in graft rejection would be alloreactive CTL rather than peptide specific ones, I think that you may have found quite a different picture if you had used anti-H2b alloreactive CTLs obtained either after an in vitro mixed lymphocyte reaction, or after priming non-H2b mice with H2b splenocytes. Indeed, whereas OT-I cells have very high affinity TCRs that can kill upon recognition of just a handful of peptide-MHC complexes, the TCRs of alloreactive CTLs would be much more likely to be of low affinity, and would thus not have been triggered by the low levels found at the surface of the various stem cells used in your study.
Authors' response: We agree that using alloreactive CTL might give different results. Therefore, it will be of interest to determine whether the pluripotent cells used in this study can induce an alloreactive immune response and whether they are good targets for alloreactive CTL. However, we think that this question is beyond the scope of the present manuscript. In the present study we wanted to clarify whether the pluripotent cells can in principle be killed by CTL since it has been proposed previously that ESCs are resistant to cytotoxicity mediated by CTL . Therefore, we used a well-controlled and very potent system of peptide-specific CTL derived from T cell receptor-transgenic mice instead of less well-defined alloreactive CTL. Using this system we could clearly demonstrate that the targets were killed and that the killing was peptide-dependent. We clarified the advantages but also the possible limitations of our present experimental approach in the discussion.
3) The last issue is that of the presentation of the FACS data. I find that histograms showing percentages of positive cells or even means of fluorescence are really much less informative than the actual plots. If they all look similar, then you should at least provide one example for each case. And if they are not so similar (for example different shapes), then you should definitely show the plots, and not the percentage of positive cells.
Authors' response: We agree and we have now included representative FACS plots for most of the data in addition to the histograms that summarize several experiments and that are therefore more representative than individual plots.
We would like to thank Dr. Joly for his important and very helpful scientific and editorial suggestions for our manuscript.
Reviewers' report 3
Dr. Lutz Walter, Department of Primate Genetics, German Primate Center - Leibniz Institute for Primate Research, Göttingen, Germany
This manuscript reports on pluripotent stem cells and their susceptibility to killing by CTL. Pluripotent stem cells include ESCs, iPSCs, and maGSCs. The latter type was examined in detail in this paper. MaGSCs are derived from spermatogonial stem cells after reprogramming via in vitro cultivation. These maGSCs harbour enormous potential in regenerative medicine, as they are able to differentiate into a variety of cell types. Additionally, in an autologous transplantation of differentiated maGSCs no immunologic barrier is expected to occur. Yet, a known problem is formation of teratoma after transplantation of pluripotent cells.
The authors compared teratoma growth in immunodeficient SCID/beige mice after in vitro differentiation in various culture conditions. Differences in teratoma formation were not observed. In contrast, immunocompetent mice were able to reject maGSC and did not develop teratoma, suggesting cytotoxic immune cells mediated killing. These cells usually recognise MHC class I molecules. Therefore, the authors tested expression levels of MHC class I on the surface of different maGSC cell lines, which turned out to be either very low or not detectable, similar to what is known for ESCs. Interestingly, all maGSC lines could be killed by CTL, despite their low or undetectable expression level of MHC class I molecules. As the authors used well-defined CTL from TCR-transgenic OT-I mice, they could demonstrate the peptide-dependency of CTL killing and could exclude NK cell-mediated killing. Furthermore, it was found that maGSCs do not express the serine-protease inhibitor 6, which is known to confer protection against the granule exocytosis effector mechanism of cytotoxicity, providing a potential explanation for the observed CTL killing and the lack of CTL inhibition, which can usually be observed for ESCs.
In summary this represents an interesting and carefully conducted study. I have only minor comments.
Methods: The authors should mention the MHC haplotype (H2b) of the studied mice and cell lines.
Results: On page 14 the expression levels of MHC class I molecules were compared between maGSC Str8 SSC5 cells and RMA. It was concluded that the level for maGSC was low. While I have no doubt that this is true, the question arises whether RMA cells are an adequate control. I guess this cell line was used as a control for antibody binding. Would it be possible to compare the level with spermatogonia or spermatogonial cell lines? At the bottom of page 15, last sentence, the authors should add "and absence of NK-mediated killing."
Authors' response: We would like to thank Dr. Walter for his encouraging review of our work. We have added the information on the MHC haplotype of the studied cell lines in the methods section. The RMA cells were indeed used as positive control for MHC class I expression and antibody binding but also for killing by the CTL. Recently, we have analyzed two newly generated cell lines that represent spermatogonial stem cells (SSCs). Similarly to most maGSCs, they did not express H2K or H2D molecules at the cell surface when analyzed by flow cytometry. However, these cell lines are so far not fully characterized. Therefore, we decided not to present these data in this manuscript. The suggested addition on page 15 has been done.
cytotoxic T lymphocyte
enhanced green fluorescent protein
ethyleneglycol-bis(b-aminoethyl ester)-N,N,N',N'-tetraacetic acid
embryonic stem cell
Dulbecco's modified Eagle's medium
fetal calf serum
hematoxylin and eosin
hypoxanthine guanine phosphoribosyl transferase 1
heat shock cognate 70
induced pluripotent stem cell
leukemia inhibitory factor
multipotent adult germ-line stem cell
mouse embryonic fibroblast
mean fluorescence intensity
major histocompatibility complex
nonessential amino acids
quantitative real time polymerase chain reaction
sodiumdodecyl sulphate-polyacrylamide gel electrophoresis
serine protease inhibitor 6
spermatogonial stem cell
stage-specific embryonic antigen 1
transporter associated with antigen processing
T cell receptor
The authors would like to thank Dr. Rudolf Jaenisch, The Whitehead Institute for Biomedical Research, Cambridge, USA, for the iPSCs used in this study. The technical assistance of A. Cierpka, B. Kaltwasser, J. Ulrich, and L. Piontek, is gratefully acknowledged. This work was supported by grants MRTN-CT-2004-512253 (TRANS-NET) from the European Union and 01GN0819 from the German Federal Ministry for Research and Technology (BMBF) to RD, EN 84/22-1 and SSP 1356 from the Deutsche Forschungsgemeinschaft (DFG) to WE, 01GN0601 from BMBF to GH, 01GN0822 from BMBF to KG and GH, and KFO 155 (Clinical Researcher Group 155) from the DFG to KG, WE and GH.
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