The constitutive levels PD-L1 and RelB are correlated with the aggressiveness of PCa. RelB has been shown that RelB associates with PCa and breast cancer progression [26, 27]. In consideration of the putative role of RelB in the regulation of cancer immune escape, we examined the Oncomine™ and TCGA datasets for the enrichment of the potential relationship between PD-L1 and RelB signatures in cancer progression. PD-L1 uniquely expresses at high levels in multiple types of cancer tissues compared to their corresponding normal tissues (Fig. S1a). In addition, the correlation of PD-L1 with RelA and RelB in PCa was assessed. The results indicated that PD-L1 is highly associated with RelB compared to RelA in PCa tumor tissues, but no clear correlation was found in peritumor tissues (Fig. S1b). Interestingly, similar results were observed in breast cancer, indicating that the noncanonical NF-κB pathway is critical for the development of sex hormone relative cancers (Fig. S1c). In contrast, the correlation of PD-L1 with RelA is higher than RelB in lung cancer. There is no apparent relationship of PD-L1 with RelA or RelB in liver cancer (Fig. S1d-e).
To verify if the correlation of PD-L1 and RelB is also associated with the aggressiveness of PCa, tumor and normal prostate tissues were examined by IHC with specific antibodies against RelB and PD-L1. As expected, PD-L1 and RelB were consistently elevated, corresponding to increased patients’ Gleason scores (Fig. 1a-d). The correlation between PD-L1 and RelB was associated with the pathological grads of tumor tissue samples (Fig. 1e). The results are consistent with the aspects from the Oncomine™ database, suggesting that RelB may participate in the regulation of the CD274 gene expression during PCa progression.
RelB upregulates PD-L1 expression in PCa cells.
Although the activation of RelA-based canonical NF-κB pathway is involved in chemo- or radio-therapy mediated PD-L1 induction in cancer cells [29, 30], there is no evidence showing that the RelB-based noncanonical pathway contributes to PD-L1 expression in cancers. To elucidate the effect of RelB on PCa immune evasion, RelB was silenced in two aggressive AR-negative PCa cell lines (PC-3 and DU-145) using a lentiviral shRelB expressional construct. The reduction of NF-κB binding activity in RelB-silenced cells was confirmed. However, since other NF-κB members can also bind to the probe with the consensus NF-κB sequence, the effect of RelB silence on total NF-κB binding activity was diluted, especially in DU-145 cells (Fig. 2a). RNA-Seq was applied to analyze mRNA expression profiles in the RelB-silenced PC-3 cells. The expression profiles of transcripts relevant to cytokines/chemokines production, inflammatory signaling pathway, immune response were selected as illustrated in Fig. 2b. Compared to the scramble control cells, most mRNA expression levels were reduced in response to the silence of RelB. KEGG-enriched signaling pathway analysis showcased that the silence of RelB led to reductions in advanced PCa-associated cytokines/chemokines (Fig. S2a). Inflammation recognition-associated TLRs, NLRs and LTM were also declined in the RelB-silenced cells (Fig. S2b). Additionally, the silence of RelB in PCa cells led to inhibition of receptors associated with immune response (Fig. S2c). Furthermore, the mRNA expression profiles related to the immune response in the RelB-silenced cells were listed in Table S2. Remarkably, the mRNA level of the CD274 gene was declined in the RelB-silenced PC-3 cells.
To verify that RelB is able to regulate PD-L1 in PCa cells, the mRNA and protein levels of PD-L1 in the RelB-silenced PC-3 and DU-145 cells were quantified. RelB was efficiently knocked down in the two cell lines, but no change in RelA expression was observed. Consistent with the decline of RelB, the levels of PD-L1 mRNA and protein were also reduced (Fig. 2c-d). In addition, IFN-γ remarkably induced PD-L1 expression in PC-3 cells, but the induction was alleviated when RelB was silenced (Fig. 2e). Consistent with the decreased level of nuclear RelB, the induction of PD-L1 protein was impeded (Fig. 2f). The Correlation of nuclear RelB and cytosolic PD-L1 was further confirmed by confocal microscope (Fig. 2g). These results suggested that IFN-γ induces PD-L1 expression partially via RelB-mediated transcriptional activation.
A proximal NF-κB element was identified to be responsive to RelB-mediated the CD274 gene transcriptional regulation.
Although several lines of studies demonstrated that the NF-κB signaling regulates PD-L1 expression, there is a lack of experimental evidence that NF-κB directly regulates PD-L1 expression via the cis/trans transcriptional regulatory manner. To elucidate how RelB regulates the CD274 gene expression, a 2000-bp 5’-flanking fragment region containing a core promoter was cloned to drive the luciferase reporter gene expression. The luciferase activity was reduced in RelB-silenced PC-3 cells (Fig. 3a). Additionally, the IFN-γ-induced reporter response was declined when RelB was silenced (Fig. 3b). Three putative NF-κB binding sites (E1, E2, E3) located in the 5’-finking region were identified by analyzing the Jaspar transcription factor database (http://jaspar.genereg.net). Accordingly, a ChIP assay was performed to verify each binding site using a RelB antibody. As shown in Fig. 3c, the proximal site (E3) appeared to be more susceptible to ChIP than E2 and distant E1 sites. Consistently, the amount of pull-down E3 element was reduced when chromatin extracted from RelB-silenced cells was used (Fig. 3d). Moreover, RelB binding to the E3 site was validated by EMSA. Nuclear extract was capable of shifting the probe containing the E3 site but not the mutant E3 site. An unlabeled probe was able to compete with the E3 binding, but an unlabeled mutant probe was found no such competition. In addition, the RelB antibody was able to reduce the E3 binding activity (Fig. 3e). Furthermore, Cell transfection with the mutated E3 site resulted in a reduction in the RelB-activated reporter response (Fig. S3a and Fig. 3f). Altogether, these results suggest that RelB transcriptionally regulates the human CD274 gene expression via a proximal NF-κB element located in the core promoter region, which is conserved in human, mouse and rabbit (Fig. S4a-b).
Tumor-derived RelB contributes to inactivation of CD4 + and CD8+ T cells.
Cancer immunotherapy aims to promote the cytotoxic effect of T lymphocytes within the tumor microenvironment. The signaling process is mainly relayed from CD4+ and CD8+ T cells by specific dendritic cells to optimize the immune response of T lymphocytes [31, 32]. Thus, it is important to define the immune responsiveness of T cells to RelB-depleted PC-3 cells. To this end, we collected blood samples and isolated the primary T cells from healthy donors who participated in this study. The T cells were activated by pretreating with anti-CD3, anti-CD28 and IL-2, and then co-cultured with RelB-silenced PC-3 cells. The percentages of CD4+ and CD8+ T cells and their proliferation were quantified by flow cytometry with relative specific antibodies and CFSE dye. The results showed that the stimulation by CD3 and CD28 efficiently increased the numbers of T cells. Interestingly, co-culture with PC-3 cells significantly reduced the activated CD4+ and CD8+ T cells, but its effect was alleviated when RelB was depleted in PC-3 cells (Fig. 4a-b). Consistently, the proliferation of CD4+ and CD8+ T cells was highly increased by the stimulation but further precluded after co-culturing with PC-3 cells. Nevertheless, the inhibitory effect of the PC-3 cells on the T-cell activation was favorably diminished by abrogating RelB (Fig. 4c-d).
Moreover, to testify whether tumorous RelB promotes T-cell immune compromise by increasing PD-L1, after co-culture, the survival of PC-3 cells was quantified by clonogenic assay. The activated T cells efficiently eliminated the PC-3 cell colony number compared to no co-culture control. Intriguingly, the T-cell immune response was further enhanced when RelB was silenced in PC-3 cells, suggesting that the high level of RelB in PC-3 cells contributes toward immune evasion (Fig. 4e).
RelB deprivation enhances the immune checkpoint blockade by anti-PD-L1 inhibitor.
Meanwhile, a CRISPR/Cas9 gene edition system was applied to knockout RelB in murine PCa RM-1 cells to verify that RelB contributes to immune evasion in vivo. Additionally, mouse PD-L1 was ectopically expressed in the RelB-KO cells to restore immune suppression (Fig. S3b and Fig. 5a). The NF-κB binding activity was measured in the gene manipulated RM-1 cells (Fig. 5b). The T cells were prepared from mouse spleens and then activated by CD3 and CD28 stimulation before co-cultured with RM-1 cells (Fig. 5c). Consistent with the results from PC-3 cells above, the RelB-KO cells were more susceptible to T cells than control cells, but the T-cell activation was further eliminated by enforced expression of PD-L1 in RelB-KO cells (Fig. S5a). Similarly, co-culture with RelB-KO cells led to recovered CD4+ and CD8+ T cells compared to co-culture with RM-1 cells. In turn, the RelB-KO effect was further attenuated by expressing PD-L1 (Fig. S5b-c). In addition, the proliferation of CD4+ and CD8+ T cells further confirmed that the T-cell growth was virtually regulated by administrating RelB and PD-L1 in RM-1 cells (Fig. S5d-e).
Moreover, after co-culture with the activated T cells, RM-1 cells were treated with an anti-PD-L1 mAb. The results showed that the mAb strikingly enhanced the immune response of the T cells. Notably, RelB deprivation enhanced mAb therapeutic efficiency. Nevertheless, the enforced expression of PD-L1 in the RelB-KO cells led to partially rescues the cells (Fig. 5d). Correspondingly, the apoptotic cell rate was increased in RelB-KO cells treated with T-cell plus PD-L1 inhibitory mAb, but the effect was further alleviated as the PD-L1 level was elevated (Fig. 5e). These results suggest that the immune checkpoint blockade of PD-1/PD-L1 can be modulated by manipulating RelB.
RelB contributes to PCa tumor immune evasion in mice.
A mouse tumor xenograft model was applied to define the role of RelB in the immune checkpoint of PD-1/PD-L1. RM-1, RM-1:RelB-KO and RM-1:RelB-KO/PD-L1 cell lines were used for tumor formation by subcutaneously injecting into mice. In the control group, 3–5 days after injection, tumors were formed and then rapidly grew to reach the maximal tumor volume (3000 mm3) within three weeks. In contrast, the tumor formation in the RelB-KO group was delayed and the tumor growth was also slow. Nevertheless, the tumor growth in the RelB-KO/PD-L1 group was restored due to increased PD-L1 (Fig. 6a-b). All mice were executed when the average tumor volume in the control group reached the maximum and tumor tissues were excised. Regardless of the endogenous mouse PD-L1, the levels of PD-L1 in RelB-KO tumors were strikingly reduced, but the levels were raised in RelB-KO/PD-L1 tumors (Fig. 6c-d). According to the manipulated PD-L1, the levels of CD4 and CD8 proteins increased via knock out of RelB, but the levels further decreased as PD-L1 was expressed (Fig. 6e). In addition, the mice serum samples were collected for T-cell activation (Fig. S6a). Compared to the control group, the numbers of T cells were increased in the RelB-KO group (Fig. S6b). After tumor formation, the amounts of CD4+ and CD8+ T cells slightly increased, but in turn, they were rapidly deceased as tumors consistently growing. Although CD4+ and CD8+ T cells were high in the RelB-KO group, the cell numbers were declined as the increase of PD-L1 in tumor cells (Fig. 6f).
Moreover, to examine whether RelB deprivation also inhibits PCa metastasis by down-regulation of PD-L1, the three tumor cell lines were further injected into mice through tail veins. Three weeks after the injection, metastatic lung tumors were detected in the control group, but not in the groups injected with RelB-KO tumor cells irrespective of PD-L1 expressed in the cells, indicating that RelB is critical for PCa metastasis (Fig. 7a). Consistent with the reduction of RelB, the PD-L1 amounts also decreased in the RelB-KO group, but increased in the PD-L1 expressed group (Fig. 7b-c). Accordingly, CD4+ and CD8+ T-cell numbers in serums increased in mice injected with RelB-KO RM-1 cells, but the T-cell number dereased in injected mice by restoring PD-L1 in RM-1 cells (Fig. 7d). Taken together, the results from the present study delineated that the high levels of RelB in advanced PCa cells promote immune evasion by transcriptional upregulation of PD-L1, as illustrated in Fig. 7e.