PD-L1 (CD274) contributes to functional exhaustion of T cells and limits immune responses in patients with cancer. In this study, we report the identification of an human leukocyte antigen (HLA)-A2-restricted epitope from PD-L1, and we describe natural, cytolytic T-cell reactivity against PD-L1 in the peripheral blood of patients with cancer and healthy individuals. Notably, PD-L1-specific T cells were able not only to recognize and kill tumor cells but also PD-L1-expressing dendritic cells in a PD-L1-dependent manner, insofar as PD-L1 ablation rescued dendritic cells from killing. Furthermore, by incubating nonprofessional antigen-presenting cells with long peptides from PD-L1, we found that PD-L1 was rapidly internalized, processed, and cross-presented by HLA-A2 on the cell surface. Apparently, this cross-presentation was TAP-independent, as it was conducted not only by B cells but in addition by TAP-deficient T2-cells. This is intriguing, as soluble PD-L1 has been detected in the sera from patients with cancer. PD-L1-specific CTL may boost immunity by the killing of immunosuppressive tumor cells as well as regulatory cells. However, PD-L1-specific CTLs may as well suppress immunity by the elimination of normal immune cells especially PD-L1 expressing mature dendritic cells. Cancer Res; 73(6);

Purpose: To investigate targeting of indoleamine 2,3 dioxygenase (IDO) enzyme using a synthetic peptide vaccine administered to patients with metastatic non-small cell lung cancer (NSCLC).Experimental Design: In a clinical phase I study, we treated 15 HLA-A2-positive patients with stage III-IV NSCLC in disease stabilization after standard chemotherapy. Patients were treated with imiquimod ointment and subcutaneous vaccinations (100 mg IDO5 peptide, sequence ALLEIASCL, formulated in 900 mL Montanide). Primary endpoint was toxicity. Clinical benefit and immunity were assessed as secondary endpoints.Results: No severe toxicity was observed. One patient developed a partial response (PR) after one year of vaccine treatment, whereas long-lasting stable disease (SD) ! 8.5 months was demonstrated in another six patients. The median overall survival (OS) was 25.9 months. Patients demonstrated significant improved OS (P ¼ 0.03) when compared with the group of patients excluded because of HLA-A2 negativity. IDOspecific CD8 þ T-cell immunity was demonstrated by IFN-g Elispot and Tetramer staining. Fluorescenceactivated cell sorting analyses demonstrated a significant reduction of the Treg population (P ¼ 0.03) after the sixth vaccine (2.5 months) compared with pretreatment levels. Furthermore, expression of IDO was detected in nine of ten tumor biopsies by immunohistochemistry. High-performance liquid chromatography analyses of kynurenine/tryptophan (Kyn/Trp) ratio in sera were performed. In long-term analyses of two clinical responding patients, the ratio of Kyn/Trp remained stable. Conclusions:The vaccine was well tolerated with no severe toxicity occurring. A median OS of 25.9 months was demonstrated and long-lasting PRþSD was seen in 47% of the patients.

The presence of natural killer (NK) cells in the tumor microenvironment correlates with outcome in a variety of cancers. However, the role of intratumoral NK cells is unclear. Preclinical studies have shown that, while NK cells efficiently kill circulating tumor cells of almost any origin, they seem to have very little effect against the same type of tumor cells when these have extravasated. The ability to kill extravasated tumor cells is, however, is dependent of the level of activation of the NK cells, as more recent published and unpublished studies, discussed below, have demonstrated that interleukin-2–activated NK cells are able to attack well-established solid tumors.