Blockade of CCL2 expression overcomes intrinsic PD-1/PD-L1 inhibitor-resistance in transglutaminase 2-induced PD-L1 positive triple negative breast cancer

Anti-PD-1/PD-L1 immunotherapy, as a treatment for many tumors, has shown good efficacy. However, responses to immunotherapy did not always occur or last long., i.e. primary or acquired resistance, even tumors were PD-L1 positive. Several oncogenic pathways, including PI3K/AKT activation by PTEN loss and NF-κB activation, induce PD-L1 expression and PD-L1 inhibitor-resistance. They also induce expression of CCL2, an inhibitory chemokine that blocks T cell tracking into the tumor by binding to CCR2 on the T cell surface. In this study, we showed that transglutaminase 2 (TG2), a post-translational modification enzyme, induced ubiquitin-proteasome dependent degradation of tumor suppressors including PTEN and IκBα by peptide cross-linking, inducing CCL2 as well as PD-L1 expression via PI3K/AKT and NF-κB activation. It also induced PD-L1 inhibitor-resistance because CCL2 was expressed despite increased PD-L1, which was blocked by PD-L1 inhibitor. We also revealed that inhibition of TG2, instead of PD-L1, restored T cell-dependent killing effect by blocking expression of both PD-L1 and CCL2 in PD-L1(+) triple negative breast cancer (TNBC) cells. In addition, the TG2-expressing TNBC patient group showed higher PD-L1 expression incidence than did the TG2-negative TNBC patient group. In conclusion, TG2 induces primary PD-1/PD-L1 inhibitor-resistance by inducing CCL2 expression. TG2 blockade can be utilized as an excellent therapeutic strategy to overcome PD-L1 inhibitor-resistance in PD-L1(+) TNBC patients. Our study suggested that PD-L1 expression alone might not always be a predictive biomarker for PD-L1(+) TNBC, but TG2 could be a useful predictive marker to select PD-L1 inhibitor-resistant TNBC patients.