# Radiotherapy and Immunotherapy--A Future Partnership towards a New Standard
## 2 Tumor Immune Phenotype--A Classification with Therapeutic Implications  
The presence and distribution of immune cells in the tumor microenvironment (TME) can be a factor that classifies a tumor into three basic immune phenotypes: **desert**, **excluded**, and **inflamed**. Three characteristics make a tumor \"hot\" from an immunological point of view: a large amount of infiltrated T lymphocytes, interferon-\(\gamma\) signaling, a tumor mutation burden (TMB), and expressions of Programmed death-ligand 1 (PD-L1). From this last class of tumors are selected the most responsive to ICI treatment. The other two immune phenotypes place the tumors in the \"cold\" category, these cases being much less or nonresponsive to immunotherapy. Both of these categories show a reduced mutational load and a lower expression of PL-L1. The differences between the \"immune-excluded\" and \"desert\" subtypes are generated by the presence of T lymphocytes. In the first case, CD8+ T are located at the periphery of the tumor, but do not infiltrate it, and in the case of tumors with a \"desert\" phenotype, T lymphocytes are absent or present in reduced number. Both \"cold\" immune phenotypes are also characterized by the presence of some categories of cells with immunosuppressive potential. Among these, the most important are tumor-associated macrophages (TAM), derived from myeloid suppressor cells (MDSC). The presence of a larger number of cells with immunosuppressive potential counterbalances the effect of CD8+ and makes tumors belonging to these phenotypes inert to the action of CTLA-4 and PD-1/PD-L1 inhibitors [1; 6]. Tumor cells express PD-L1 (also called B7-H1 or CD274) and the action of inhibiting immunity is produced by the interaction of PD-1/PD-L1 and the inhibition of T-lymphocyte action. The blockage of T-lymphocyte action and the release of cytotoxic mediators limiting the killing of cancer cells are mechanisms that normally tilt the balance in favor of immunosuppressive action, promoting tumor progression. For this reason, blocking this PD-1/PD-L1 interaction was conceived as a strategy to inhibit immunosuppressive mechanisms [7; 8; 9]. T cell-inflamed TME shows increased activity of type 1 interferon and chemokines, the end result being the recruitment of activated CD8+ effector T cells. Endogenous T cell priming against tumor antigens is strongly correlated with the tumor response to ICI, and type 1 interferon is a bridge to adaptive immunity. It should also be mentioned the involvement of the STING pathway in the innate immune sensing of cancer. Modification of TME metabolism by some enzymes including indoleamine-2,3-dioxygenase, but also the recruitment of FOXP3+ Treg can still have a negative effect on the response to immunotherapy [10; 11]. CTLA-4 is a cell surface molecule expressed especially in CD8+ and CD4+ T lymphocytes, having an essential role in the homeostasis of the T lymphocyte and in the immune response mediated by the T lymphocyte, being essential both in the early and late stages of its activation [12]. In the case of resting naive T cells, CTLA-4 is located in the intracellular compartment. Stimulatory signals can lead to exocytosis and expression of CTLA-4 on the cell surface. Due to the competitive competition for these ligands (CD80 and CD86) on antigen-presenting cells APCs, CD28, and CTLA-4 activate feedbacks of T lymphocyte activation and inhibition, respectively [13; 14]. Tregs have the role of suppressing abnormal/excessive immune response and in the case of tumors, it has the role of stimulating progression by inhibiting the anti-tumor immune response. There is a relationship between Tregs and CTLA-4, CTLA-4 deficiency on the surface of Tregs able to affect their suppression capacity [14; 15; 16]. PD-1 binding to its ligand PD-L1 has an inhibitory effect similar to CTLA-4, being associated with the reduction of the production of IL-2, interferon-\(\gamma\), tumor necrosis factor-\(\alpha\), and with the inhibition and limitation of T lymphocyte survival.