cancers Review Natural Killer Cells and Dendritic Cells: Expanding Clinical Relevance in the Non-Small Cell Lung Cancer (NSCLC) Tumor Microenvironment Pankaj Ahluwalia 1, Meenakshi Ahluwalia 2, Ashis K. Mondal 1 , Nikhil S. Sahajpal 1, Vamsi Kota 3 , Mumtaz V. Rojiani 4 and Ravindra Kolhe 1,* 1 Department of Pathology, Medical College of Georgia, Augusta University, Augusta, GA 30912, USA; [email protected] (P.A.); [email protected] (A.K.M.); [email protected] (N.S.S.) 2 Department of Neurosurgery, Medical College of Georgia, Augusta University, Augusta, GA 30912, USA; [email protected] 3 Department of Medicine, Medical College of Georgia, Augusta University, Augusta, GA 30912, USA; [email protected] 4 Department of Pharmacology, Penn State University College of Medicine, Hershey, PA 17033, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-706-721-2771; Fax: +1-706-434-6053 Simple Summary: Cancer is one of the leading causes of mortality around the globe. In the past decades, there has been rapid progress in the development of tools to detect, screen, and treat several cancers. For its benefit to reach a wider patient population, significant challenges such as tumor Citation: Ahluwalia, P.; Ahluwalia, heterogeneity, resistance to therapies, and lack of biomarkers should be addressed. The immune M.; Mondal, A.K.; Sahajpal, N.S.; system holds the key to a greater understanding of these complex barriers. Natural Killer cells are Kota, V.; Rojiani, M.V.; Kolhe, R. cytotoxic cells of innate immunity that can kill multiple tumorigenic cells. Dendritic cells link innate Natural Killer Cells and Dendritic and adaptive immunity by processing and presenting tumor-derived antigens to initiate anti-tumor Cells: Expanding Clinical Relevance T cell response. These immune cells and associated gene signatures have emerged as potential in the Non-Small Cell Lung Cancer (NSCLC) Tumor Microenvironment. biomarkers with prognostic and predictive potential in several cancers. In this review article, we have Cancers 2021, 13, 4037. https:// discussed the biological roles of NK cells and DCs along with their translational relevance in NSCLC. doi.org/10.3390/cancers13164037 Abstract: Non-small cell lung cancer (NSCLC) is a major subtype of lung cancer that accounts for Academic Editors: David Entenberg, almost 85% of lung cancer cases worldwide. Although recent advances in chemotherapy, radiother- Panagiota Filippou and apy, and immunotherapy have helped in the clinical management of these patients, the survival rate George Karagiannis in advanced stages remains dismal. Furthermore, there is a critical lack of accurate prognostic and stratification markers for emerging immunotherapies. To harness immune response modalities for Received: 14 July 2021 therapeutic benefits, a detailed understanding of the immune cells in the complex tumor microenvi- Accepted: 5 August 2021 ronment (TME) is required. Among the diverse immune cells, natural killer (NK cells) and dendritic Published: 11 August 2021 cells (DCs) have generated tremendous interest in the scientific community. NK cells play a critical role in tumor immunosurveillance by directly killing malignant cells. DCs link innate and adaptive Publisher’s Note: MDPI stays neutral immune systems by cross-presenting the antigens to T cells. The presence of an immunosuppressive with regard to jurisdictional claims in milieu in tumors can lead to inactivation and poor functioning of NK cells and DCs, which results published maps and institutional affil- iations. in an adverse outcome for many cancer patients, including those with NSCLC. Recently, clinical intervention using modified NK cells and DCs have shown encouraging response in advanced NSCLC patients. Herein, we will discuss prognostic and predictive aspects of NK cells and DC cells with an emphasis on NSCLC. Additionally, the discussion will extend to potential strategies that seek to enhance the anti-tumor functionality of NK cells and DCs. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article Keywords: NK cells; natural killer cells; DCs; dendritic cells; immunotherapy; prognostic; predictive; distributed under the terms and cancer; gene expression; lung cancer; therapeutics; NSCLC conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Cancers 2021, 13, 4037. https://doi.org/10.3390/cancers13164037 https://www.mdpi.com/journal/cancers Cancers 2021, 13, 4037 2 of 18 1. Introduction Lung carcinoma is one of the deadliest cancers globally, with almost 1.8 million deaths annually [1]. Lung cancer consists of heterogenous subtypes that can be differentiated at epidemiological, histological, and molecular levels. Small-cell lung cancer (SCLC) accounts for 13%, while non-small cell lung cancer (NSCLC) causes 76% of lung cancer cases [2]. Tobacco smoking has a well-documented association, with a high prevalence of lung carcinoma. Furthermore, air pollution (household and outdoor) is the second most leading risk factor of lung cancer [3]. According to one estimate, more than 91% of the global population lives under WHO air quality guideline levels [4]. Air pollution consists of fine particulate matter (PM10 and PM2.5), gases (oxides of sulfur and carbon), and metals (lead, nickel, and vanadium). According to one meta-analysis, an increase in every 10 µg/m3 of PM10 levels leads to a 22% higher risk of lung cancer [5]. The incidence and mortality of lung cancer are falling in the United States due to strengthened tobacco control and public education, but are rising in developing countries [6]. In the United States, from 2008 to 2016, NSCLC related incidence decreased at an annual rate of 3.1%. The specific survival in lung cancer patients improved from 26% in 2001 to 35% in 2014 [2]. The advances in treatment regimens including the incorporation of targeted therapies and immunotherapies are major factors in this improvement. Surgical resection of the tumor is most effective for stage I, stage II, and some stage III NSCLC cases [7]. Unfortunately, most of the patients have tumor recurrence with a 5-year survival rate that falls from 83% in stage IA to 36% for stage IIIA patients [8]. Platinum compounds have been used against cancer since the 1978 approval by the FDA [9]. Platinum drugs cause damage to tumor cells by cross-linking DNA. In healthy cells, efficient DNA damage response can repair the damage, but due to changes accumulated through the process of cancer evolution, most of the cancer cells lack DNA repair pathways and are subsequently killed [10]. Platinum-based doublet therapies (cisplatin combined with docetaxel or other cytotoxic compounds) have been standard of care for advanced NSCLC patients with a good performance status [11]. Besides these, platinum triplets (addition of a third component—bevacizumab, an anti-VEGF moAb) are recommended for advanced metastatic NSCLC [12,13]. For NSCLC patients with activating EGFR mutations (30–50% of Asians and 15% Caucasians), tyrosine kinase inhibitors (TKIs) comprise the first-line therapeutic strategy [14]. Encouragingly, recent breakthroughs in immunotherapies have provided a new direction to the management of lung cancer patients. Currently, patients with higher expression of PD-L1 are administered pembrolizumab (anti-PD-1) as a first- and second-line therapy in patients with advanced NSCLC [15]. Atezolizumab (anti-PD-L1) and Nivolumab (anti-PD-1) are approved as a second-line therapy, while Durvalumab (anti-PD-L1) is used as a maintenance therapy in unresectable stage III NSCLC patients [15–17]. The tumor microenvironment is composed of malignant cells along with fibroblasts, extracellular matrix (ECM), endothelial cells, adipocytes, and immune cells [18] (Figure1). Tumors can be divided into two major immunological subtypes: hot (high T cell activity) and cold (lack of T cell priming/activation) based on inflammatory cytokine profile [19]. In the cancer progression cycle, excessive secretion of PD-L1 in the tumor and enhanced PD-1 signaling inactivates T cells leading to tumor growth and metastasis [20]. The use of novel immunotherapeutic agents, such as nivolumab (anti-PD-1), pembrolizumab (anti- PD-1), and atezolizumab (anti PD-L1), has led to the improved overall survival of NSCLC patients compared to treatment with docetaxel [16,21,22]. Further, the combination of cyto- toxic therapy and immunotherapy has shown synergistic improvements in the anti-tumor response. The combination of pemetrexed (folate antimetabolites), carboplatin (platinum- based antineoplastic drug), and pembrolizumab (anti-PD-1) led to the improvement of the objective response rate and progression-free survival [23,24]. This striking difference was exhibited in the benefits of immunotherapy, as hot tumors show a higher response rate to immunotherapy. The immune checkpoint blockade (ICB) is effective in patients with expression of PD-L1 or high TMB (tumor mutation burden), but there are different approaches required to target cold tumors. TMB is the number of nonsynonymous coding Cancers 2021, 13, x 3 of 18 Cancers 2021, 13, 4037 3 of 18 of nonsynonymous coding mutations per million bases (Mb) and is a predictive biomarker mutationsfor ICB therapy per million efficacy. bases It is (Mb)associated and is with a predictive increased biomarker secretion of for neoantigens ICB therapy and efficacy. cyto- It istoxic associated T cell activity with increased in several secretion cancers