Intratumor Heterogeneity
Cancer exhibits intratumoral heterogeneity, where multiple different cell subpopulations (genotypes, phenotypes) exist within the same tumor, making control difficult with single-target therapies. Cancer cells have reduced levels of MHC class I antigens, which indicate self, so NK cells recognize cancer cells as non-self and attack them. Furthermore, when cells are subjected to stress such as cancer transformation or viral infection, they express NKG2D ligands (NKG2DL) such as MICA, MICB, and ULBP on their cell surface. NK cells reliably recognize and eliminate cells that have both low MHC class I levels and high NKG2DL levels as abnormal cells (targets). However, some cancer cells evade immune responses from NK cells by converting NKG2DL into a soluble form through shedding and releasing it. Currently, methods using 5-FU or sodium valproate (HDAC inhibitory effect) in combination are being considered to activate weakened NKG2DL.

Loss of Tumor Antigens
There are two types of tumor antigens: “tumor-specific antigens (neoantigens)” that exist only on cancer cells, and “tumor-associated antigens” that are also present on normal cells but are overexpressed on cancer cells.
As cancer cells grow, gene mutations are likely to occur, changing tumor antigens. Therefore, T cells (killer T cells and CAR-T cells) cannot recognize or attack these antigens. This is the mechanism by which the effectiveness of CAR-T cell therapy and killer T cell therapy targeting tumor-associated antigens such as CD19, HER2, and Claudin 18.2 is often reduced. On the other hand, neoantigens are abnormal peptides that arise from gene mutations in cancer cells and are not present on normal cells. They are presented on the cell surface via MHC class I molecules. Killer T cells (CD8-positive T cells) recognize these as foreign substances and attack them. As cancer cells grow, they reduce MHC class I molecule expression, preventing neoantigens from being presented on the surface and thus attempting to evade the immune system. However, cells with reduced MHC class I molecule expression are recognized by NK cells and become prey.

Recruitment of Immunosuppressive Cells in the Tumor Microenvironment
Cancer cells do not exist in isolation; they enlist the help of surrounding normal cells to create an “immunosuppressive” environment (tumor microenvironment) that strongly suppresses the activity of immune cells. Cancer cells actively recruit regulatory T cells (Tregs), directly inhibiting the activity of killer T cells or causing them to release suppressive cytokines (TGF-β and IL-10), making it difficult for an immune response to occur against cancer cells. Myeloid suppressor cells (MDSCs), which, like Tregs, possess strong immunosuppressive capabilities, are a population of immature myeloid cells that normally appear temporarily during inflammation, but are recruited from the bone marrow by factors released by cancer cells and proliferate in tumor tissue and lymphoid tissue. MDSCs produce reactive oxygen species (ROS), nitric oxide (NO), etc., paralyzing the proliferation and function of T cells and suppressing the immune response to cancer. Fas ligand expressed on NK cells recognizes the FAS antigen expressed on Tregs and MDSCs, inducing apoptosis and eliminating them. On the other hand, macrophages are cells that normally engulf and eliminate foreign substances, but in a tumor environment, they change into a type called M2 macrophages (tumor-associated macrophages: TAMs) that help cancer cells survive. TAMs not only promote angiogenesis and help supply nutrients to cancer cells, but also release immunosuppressive factors that block attacks by T cells. NK cell exosomes contain miR146a,21,155, which promote the differentiation of M2 macrophages (immunosuppressive type) into M1 macrophages (immunostimulatory type).

Immune Brake of T Cells Caused by PD-L1 Expression
Activated T cells express a receptor called PD-1 on their surface. This is normally a brake switch that prevents damage to normal tissues by excessive immune responses. Cancer cells express large amounts of a molecule called PD-L1, which is a ligand for activating this switch, on their surface. Even when killer T cells recognize cancer cells and approach them, if the PD-L1 on the cancer cell binds to the PD-1 on the T cell, an inhibitory signal is transmitted inside the T cell, causing it to lose its ability to attack. Since NK cells cannot resolve this situation, it is necessary to block the PD-L1 on cancer cells using immune checkpoint inhibitors to release the immune brake.

Chronic Antigen Exposure Leads to T Cell Exhaustion Gene Expression
In cancer tissue, antigens are constantly present, leading to chronic stimulation of T cells. Under these conditions, T cell-derived exhaustion genes (BATF and TOX) are expressed, resulting in a state called “exhaustion.” Exhausted T cells highly express not only PD-1 but also multiple immunosuppressive receptors such as TIM-3 and LAG-3, losing their proliferative capacity, cytokine production capacity, and cytotoxic activity. Exosomes secreted by NK cells contain miR155, which suppresses the expression of exhaustion genes, thus preventing T cell exhaustion and restoring immune function.

Release of Immunosuppressive Cytokines
Cancer cells release cytokines (signaling molecules) that suppress the immune system, either by themselves or by acting on surrounding cells (tumor microenvironment). TGF-β (transforming growth factor beta) is a prime example; it not only directly suppresses the function of killer T cells and NK cells, but also activates fibroblasts to create a physical barrier, preventing immune cells from reaching cancer cells. IL-10 also works to reduce the function of antigen-presenting cells, thereby calming the overall immune response. Furthermore, these cytokines are known to impair dendritic cells, reducing their antigen-presenting ability.