Expression analysis of putative T cell activation inhibitors

Several proteins have been reported to inhibit T cell function. To examine the role of these proteins in PSC-mediated immune inhibition their gene expression in PSCs was analyzed by qPCR.

Except for TGFβ, until now none of these proteins was reported to contribute to PSC-mediated T cell inhibition (see 1.3). Nevertheless, it was analyzed whether these proteins might contributed to T cell inhibition observed in co-culture experiments, since the contribution of previous reported mechanisms such as Arg1, SPI-6, CtsB, IDO and FasL expression could be excluded (see 1.3 and 4.4.1). The transcript amount of RMA cells was chosen as reference, since these cells did not possess an inhibitory effect on T cell activation in co-culture experiments. In addition, spleen and thymus RNA as well as MEF RNA were included as further controls (Figure 23).

Galectin-1 (Gal-1) is as a cytokine-like molecule involved in immune-escape of tumors by regulation of T cell activity. It was shown that Gal-1 expression can induce T cell apoptosis and

RMA

0.24 %

-0.23 %

ESC (MPI-II) ESC (BTL-1)

0.01 %

maGSC (129/Sv)

0.18 %

maGSC (C57Bl/6)

-0.69 %

iPSC (129/Sv)

-0.62 %

iPSC (C57Bl/6)

-0.08 %

69 reduce numbers of infiltrating T cells in tumor tissues (Cooper et al, 2010; Kovacs-Solyom et al, 2010; Le et al, 2005). In both ESC lines high amounts of Gal-1 transcripts were found. The mRNA amounts were comparable to thymus reference RNA that also comprised high amounts of Gal-1 transcripts. In maGSCs as well as iPSCs the expression was even higher (about 160 % to 420 % compared to thymus reference RNA). However, in RMA control cells the expression of Gal-1 mRNA was about 700 % of thymus what suggests that Gal-1 expression was not responsible for the observed T cell inhibition.

Semaphorin-3A (Sema-3A) is a protein with an immunoglobulin-like domain that is expressed in a variety of human tumor cell lines. In mixed lymphocyte culture, recombinant Sema-3A inhibited the activation of cytotoxic activity against K-562 cells. Furthermore, it inhibited T cell proliferation as well as cytokine production of primary human T cells stimulated with anti-CD3 and anti-CD28 (Catalano et al, 2006). Sema-3A transcripts were detectable in various amounts in all analyzed PSCs. Thereby, the mRNA amounts were similar to spleen reference, except for the maGSC line C57Bl/6 in which nearly a 5-fold mRNA amount was detected. However, the Sema-3A transcript level in RMA cells was even 9-fold higher that in spleen reference and consistently higher than in PSCs, suggesting that Sema-3A expression did not confer T cell inhibiting activity to PSCs.

It was reported, that TGFβ expression confers immune suppressive properties to ESCs (see 1.3).

Indeed, moderate levels of TGFβ transcripts were detectable in PSCs. The transcript amount varied slightly between the different PSC lines and origins. However, again the TGFβ transcript amount detected in RMA cells was very high and at least more than 2-fold higher as in PSC-derived RNAs. The high expression of TGFβ in RMA cells, which lacked T cell inhibiting activity, in addition to the finding that the inhibitory activity of PSCs was rather not mediated by a soluble substance (see 4.4), suggests that TGFβ expression did not contributed to PSC-mediated T cell inhibition.

The receptor-binding cancer antigen expressed on Siso cells (RCAS1) induces apoptosis or cell cycle arrest in RCAS1 receptor expressing immune cells. It was demonstrated that soluble RCAS1-induced apoptosis of lymphocytes following co-culture of IL-2 activated peripheral blood lymphocytes with a human oral squamous cell carcinoma cell line (KB cells) (Fukuda et al, 2004).

RCAS1 mRNA amounts were generally low in all analyzed samples, including references. The according Ct values were consistently about 30 and above in PSCs. Nonetheless, RCAS1 transcripts were still detectable but very variable in amount in different PSC lines. In iPSCs and maGSCs derived from C57Bl/6 mice the amounts of RCAS1 mRNA were at least 2-fold higher than in their counterparts derived from 129/Sv mice. However, also in the ESC line MPI-II and the ESC line BTL-1, both derived from 129/Sv mice the RCAS1 gene was rather differently expressed. Whereas the RCAS1 transcript level was comparable to thymus and spleen control in MPI-II cells, the transcript

Results

70 level in BTL-1 cells was only about 40 % compared to spleen control. The highest RCAS1 gene expression was detectable in RMA cells, nearly 4-fold higher than in spleen and thymus control RNA. Therefore, and due to the very low RCAS1 transcript amounts in general, RCAS1 was unlikely responsible for PSC-mediated T cell inhibition.

Figure 23: Expression analysis of genes involved in inhibition of T cell activity

Diagrams represent the fold change in gene expression relative to appropriate reference control RNAs.

Values are efficiency correlated calculated (Pfaffl calculated) as mean of technical triplicates normalized to the housekeeping gene Hprt.

The programmed death ligands (PD-L1 and PD-L2), part of the B7 family, bind to the programmed death receptor (PD-1) expressed by T cells and negatively regulate T cell activation (Dong et al, 1999). It was reported, that up-regulation of PD-L1 leads to increased number of PD-1⁺ CD8⁺ T cells in tumor tissues and that these CD8⁺ T cells are impaired in cytotoxic granule and cytokine

relative geneexpression

71 production (Matsuzaki et al, 2010; Sfanos et al, 2009; Shi et al, 2011; Wu et al, 2009; Zhang et al, 2010). Only very low amounts of PD-L1 mRNA were detectable in spleen and thymus control and in the majority of PSCs nearly no PD-L1 mRNA was detectable. Only in the maGSC line from C57Bl/6 mice, but also in RMA cells trace amounts of PD-L1 transcripts were detectable, albeit only 12 % to 19 % of thymus reference. Due to the relative quantification method and low expression in the reference cells the depiction of true gene expression in the diagram is deceptive. The according Ct values in PSCs were consistently about 30 and above. Therefore, PD-L1 transcripts could be considered to be absent in PSCs, except maGSC (C57Bl/6).

PD-L2 transcripts were generally not detectable in PSCs and in the spleen. Also in thymus reference probes and RMA control only very low amounts of PD-L2 mRNA was detectable. The according Ct values were about 29 in both samples. Therefore, the expression of the PD-L1 and PD-L2 in PSCs is unlikely the reason for the inhibitory activity of PSCs observed in co-culture assays. B7-H3 and B7-H4, further members of the B7 family are also reported to have an inhibitory effect on proliferation, differentiation and cytotoxicity of T cells. Prasad et al. reported that the murine B7-H3 protein inhibited T cell activation and effector cytokine production and the addition of an antagonistic antibody to B7-H3 enhanced T cell proliferation in vitro (Prasad et al, 2004). Furthermore, over-expression of B7 family members is associated with immune modulating properties of a variety of tumors (for review see (Du & Wang, 2011)). However, while most data published so far reported B7-H3-mediated inhibition of T cell function also contradicting data was published. Hashiguchi et al. identified TREML2 as receptor for B7-H3 and reported that this interaction co-stimulates the activation of T cells (Hashiguchi et al, 2008).

Figure 24: Cell surface expression of B7-H3

Representative histograms show the cell surface expression of B7-H3 as detected by flow cytometry. The specific percentages of cells, (B7-H3 positive cells minus cells positive for isotype control) positive for B7-H3 are shown.

macrophages

0.15%

ESC (MPI-II) ESC (BTL-1)

maGSC (C57Bl/6) 35.94 %

OT-I splenocytes

0.88 %

MEFs

0 %

3.43 % 0 %

iPSC (129/Sv) 0.53 %

iPSC (C57Bl/6) 0.17 %

Results negative for B7-H4 expression. Only in maGSC (C57Bl/6) cells minor B7-H4 transcript levels were detectable. In contrast, the amount of detectable B7-H3 mRNA was very high in all PSC lines, about 3-fold up to 8-fold higher compared to J774 positive control cells that also comprised well detectable levels of B7-H3 transcripts. The gene expression in the ESC line MPI-II was even 25-fold higher compared to J774 reference. Interestingly, B7-H3 transcripts were not detectable in RMA cells. The result was confirmed by an independently performed assay, using another set of primers specific for B7-H3. The mean of both assays was calculated and the relative expression is shown in Figure 23. Since B7-H3 transcript levels were undetectable in RMA cells but high in PSCs, B7-H3 was chosen for further investigations.

Figure 25: Proliferation frequencies of CD8⁺ and CD4⁺ T cells after co-culture with PSCs in presence of a B7-H3 blocking antibody

(A) Diagram represents the mean proliferation of CD8⁺ T cells derived from OT-I mice after 4 days of co-culture with PSCs. (B) Diagram shows the mean proliferation of CD4⁺ T cells derived from OT-II mice after 5 days of co-culture with different PSC lines. All cells were supplemented with IL-2 and OVA as antigen. In addition, a B7-H3 blocking antibody in a final concentration of 5 µg/ml was added in order to block B7-H3-mediated interactions.

73 The surface expression of B7-H3 was analyzed by flow cytometry. PSCs as well as MEF control were negative for B7-H3 expression. Only on primary macrophages, which were isolated from femurs of a C57Bl/6 mouse, cells positive for B7-H3 were found. To induce differentiation towards the macrophage lineage, these cells were stimulated with different cytokines gained from the supernatant of L929 cells and 10 ng/ml IFNγ prior to flow cytometric analysis (Figure 24) (Coligan, 2005).

Since the B7-H3 expression on PSCs could have been induced following co-culture with splenocytes, the role of the B7-H3 ligand in modulating T cell activation was further analyzed in co-culture assays with CFSE-stained, antigen-specific T cells. Therefore, a specific antibody against B7-H3 was added (5 µg/ml) in order to block possible interactions between PSCs and T cells.

Co-culture with PSCs again significantly suppressed proliferation of CD8⁺ as well as of CD4⁺ T cells.

In addition co-culture with MEFs also suppressed T cell proliferation albeit less extensively than PSCs. The supplementation with a B7-H3 blocking antibody had no effect on T cell proliferation, neither on CD8⁺ nor on CD4⁺ T cells (Figure 25). In accordance with the finding, that the B7-H3 ligand is absent on the cell surface of PSCs, the co-culture data suggests that B7-H3 did not contribute to PSC-mediated inhibition of T cell proliferation since B7-H3 was not expressed on protein level. Thus, it remains unclear by which mechanism PSCs block T cell proliferation in a contact dependent manner.

4.5 Immunogenicity of OVA-expressing iPSCs in vivo