ITGB6 as an inhibitor of T-cell killing via modulation of paracrine TGFβ signaling in pancreatic ductal adenocarcinoma.

Abstract

e16334 Background: Immunotherapy has experienced limited success in PDAC, with αPD-1 therapy being approved only in a subset that makes up < 2% of all PDAC cancers. TGFβ is a contributor to T-cell suppression in the PDAC tumor microenvironment. However, the success of blocking TGFβ in preclinical models has failed to translate into the clinic due to the off-target effects of systemic TGFβ blockade. Integrin ITGB6 is an activator of the inert latent TGFβ into the active, immunosuppressive form. Consequently, inhibition of ITGB6 is a potent and targeted therapeutic approach for disrupting TGFβ to overcome T-cell suppression in PDAC. Methods: Analysis of TCGA molecular and survival data was performed to determine the specificity and prognostic value of ITGB6 in PDAC. Subsequently, candidate cell lines were screened for ITGB6 expression using flow cytometry to facilitate an in vitro model. Using a lentiviral system, PDAC cells were transduced with a doxycycline-inducible short hairpin RNA (shRNA) knockdown (KD) of ITGB6. Both KD and control shRNA cell lines were induced with doxycycline and subsequently co-cultured with TALL-104 T-cells. Furthermore, conditioned media was analyzed for secreted active TGFβ levels using a luciferase-based TGFβ reporter cell line. Finally, a clonogenic assay was performed to determine the autocrine effects of TGFβ activation in PDAC. Results: ITGB6 is highly specific to PDAC and also shows upregulation in comparison to normal pancreatic tissue (FC 3.38, p < 0.0001). Capan-2 cells were chosen as a model for high integrin PDAC (Capan-2 99.73% cells ITGB6+, negative control: HCT116WT 0.96% cells ITGB6+). Fluorescent microscopy of the co-culture revealed that the KD of ITGB6 increased T-cell killing over 24 hours (Capan-2 CTRL shRNA 7.71%±3.63 dead vs. Capan-2 ITGB6 shRNA 21.68%±4.57 dead, p < 0.05). Media from ITGB6 KD cells contained significantly decreased TGFβ (+TGFβ: 673000 RLU vs. CTRL: 175500 RLU, p = 0.015). A clonogenic assay of Capan-2 cells treated with active-TGFβ demonstrates that PDAC cells resist the autocrine growth inhibitory effects of TGFβ through loss of SMAD4. TCGA survival data revealed ITGB6 as prognosis marker in PDAC (high ITGB6 median survival 16.79 mos, vs. low ITGB6 21.88 mos, HR = 2.730, p < 0.004). Conclusions: Survival data that clearly demonstrates the poor prognosis of ITGB6 expression, as well as the presently proposed mechanism of immunosuppression, provide ample justification for deploying anti-ITGB6 combination therapies to in vivo studies that facilitate translation of our findings into therapeutic clinical trials.