Despite CD8+ T cell responses against tumor cells are well-understood, information about the role of CD4+ T cell immunity in cancer is limited. different factors dynamically regulate different steps of the cancer immune process thereby negatively interfering with the T cell Cmediated anti-tumoral immune responses. Therefore, this review will summarize the current knowledge of the different players involved in inhibiting tumor immunogenicity and mounting resistance to checkpoint inhibitors with focus on the role of tumor T cell interaction. A better insight INH1 of this process might lead to the development of strategies to revert these inhibitory processes and represent the rational for the design of novel immunotherapies and combinations in order to improve their efficacy. strong class=”kwd-title” Keywords: T cells, tumor growth, tumor microenvironment, microbiome, inflammation, checkpoint inhibitors Introduction It has been generally accepted that the development and progression of tumors is a result of an altered crosstalk between the tumor and the host immune system (1C3). The immune system not only suppresses tumor growth by destroying tumor cells or inhibiting their outgrowth, but also promotes tumor progression by either selecting for tumor escape variants or by establishing conditions within the tumor microenvironment (TME) and periphery that facilitate tumor outgrowth, which has been classified as a hallmark of cancer (4). These include an increased frequency of immune suppressive cells, metabolites, cytokines and soluble factors, hypoxia and acidic pH (5, 6). Further changes of the TME during neoplastic transformation are a selective ablation of immune effector cells and deletion or neutralization of cytokines, like interferon (IFN)- (7). Despite interferon (IFN)- exert pro-tumorigenic effects under certain circumstances dependent on the cellular and molecular context (8, 9), it represents a key mediator of immunosurveillance produced by natural killer (NK) cells and T cells known to promote cytotoxic activity of INH1 macrophages and enhance the expression of immune modulatory molecules on tumor cells (7). This results in the release of tumor associated antigens (TAA) for cross presentation by dendritic cells (DCs), which uptake and process these antigens into peptides then presented via the major histocompatibility complex (MHC) class I and class II molecules to CD8+ and CD4+ T cells, respectively. However, elimination of transformed cells can be incomplete due to a decreased tumor immunogenicity (10). This results first in an equilibrium state characterized by a balance between proliferation and killing of tumor cells by CD8+ T cells thereby maintaining the tumor at a subclinical INH1 stage, followed by the generation of tumor cells, which are resistant to immune rejection due to constant selective pressure of the immune system (2). These immune escape mechanisms are associated with the loss or downregulation of TAA and/or HLA class I surface molecules or aberrantly expression of the non-classical HLA-G and HLA-E antigens as well as co-inhibitory molecules (Table 1). This might be at least partially mediated by the induction of oncogenic pathways (11, 12) and changes in the tumor cell metabolism (13, 14). Table 1 Immune escape mechanisms. thead th valign=”top” align=”left” rowspan=”1″ colspan=”1″ Tumor /th th valign=”top” align=”left” rowspan=”1″ colspan=”1″ Microenvironment /th th valign=”top” align=”left” rowspan=”1″ colspan=”1″ Periphery /th /thead HLA I Suppressive cytokines CTL frequency and function HLA-G/-E Suppressive metabolites NK cells, frequency, and function IFN signaling pH DCs, frequency and function Oncogenic signaling Hypoxia TAN, TAM iCP MicrobiomeTreg, frequency Metabolism MDSC, frequency Open in a separate window em upregulation /em , em downregulation /em . However, interventions, such as chemotherapy, radiotherapy (RT), physico-chemical, and thermal ablation can promote the release of TAA RICTOR and might overcome the dominant immune suppressive pathways leading to an increased immunogenicity (15C18). Therefore, the combination of immunotherapies with other strategies offers novel opportunities to recover immune activity and increase INH1 their efficacy, which result in a better patients’ outcome. Indeed, this approach is currently INH1 investigated in a number of experimental models and clinical trials (19). Players involved in mounting anti-tumor immune responses include in particular cells of the adaptive immune system, which protect and/or control tumor outgrowth and the interaction of the host against viral/pathogen infections and neoplastic transformation. The therapeutic.