**3. Tumor-induced immunosuppression and immunoresistance**

Although immunoediting may eliminate tumor cells with alterations in their antigenic epitope profile, many immunoresistant variants escape from the immune system of the host by the following immunosuppressive molecular and cellular mechanisms [15]. Not the immunoef‐ fectors but only the immunosuppressive regulators are supported by the heterogeneous tumor microenvironment, which contains tumor cells, extracellular matrix (ECM) cells, local bone marrow-derived stromal progenitor cells, pericytes, endothelial cells, proteins, matrix degrad‐ ing enzymes, chemokines, cellular factors, immune cells, tumor-associated fibroblasts, and angiogenic cells, which may cause desmoplasia after stromal cell infiltration and ECM deposition [16–18].

Tumors escape eradication from the immune system by mechanisms of the first category, which consists of the development of tumor immunoresistance, including the promotion of oncogenicity of tumor stem cells that causes resistance to conventional anticancer treatments and immune responses of the host due to tumor dormancy that cause tumor relapse by selfrenewal, continuous ability of proliferation, incomplete differentiation, and production of immunosuppressive factors causing immunoresistance due to inhibition of apoptosis or type I PCD [19].

Another mechanism of tumor immunoresistance is the loss of abnormal surface antigens on the tumoral plasma membrane due to mutations and immunoescape of epitope loss tumor variants, which occurs due to the genetic instability of tumors, leading to continuous altera‐ tions of their surface molecules, hiding their antigenic profile by losing their epitope, especially after they sense the presence of cytotoxic T lymphocytes (CTL) in the tumor microenvironment. Thus, the immune system eradicates only the tumor cells that express the specific epitope, circumventing invisible epitope-negative tumor cells that become extremely resistant to CTL elimination [20].

One more immunoresistant mechanism exerted by the tumor cells is the lack of susceptibility to immune effector cells, such as natural killer cells (NK), cytotoxic T lymphocytes (CTL), macrophages, and dendritic cells (DC), which promote antibody-induced cytotoxicity, phagocytosis, or vaccine effects in cancer immunotherapy [21].

The second category of tumor immunoescape mechanisms consists of the interference with the antitumor-induced immune responses, such as reduced expression of costimulatory molecules on tumor cells or antigen presenting cells (APCs). This downregulation of costimu‐ latory molecules on tumor cells or professional APC may inactivate or eliminate TAA-specific CTLs, put in an immature state the dendritic cells conditioned by the tumor cells, and inactivate T cells leading to tumor tolerance by circumventing productive immune responses against the tumor cells [22]. Also, the tumor for escaping the immune system of the host alters the T-cell receptor (TCR) on the tumor infiltrating lymphocytes (TIL), especially in cases with advanced cancer, leading to reduced mediation of tumor cytotoxicity and decreased production of Th1 type cytokines [23–25].

Circumventing immune destruction is one of the hallmarks of cancer pathogenesis in addition to evading growth suppressors, deregulating cellular energetics, enabling replicative immor‐ tality, inducing angiogenesis, activating invasion and metastasis, sustaining proliferative signaling, and resisting cell death, which may lead to the uncontrollable promotion of tumor

Although immunoediting may eliminate tumor cells with alterations in their antigenic epitope profile, many immunoresistant variants escape from the immune system of the host by the following immunosuppressive molecular and cellular mechanisms [15]. Not the immunoef‐ fectors but only the immunosuppressive regulators are supported by the heterogeneous tumor microenvironment, which contains tumor cells, extracellular matrix (ECM) cells, local bone marrow-derived stromal progenitor cells, pericytes, endothelial cells, proteins, matrix degrad‐ ing enzymes, chemokines, cellular factors, immune cells, tumor-associated fibroblasts, and angiogenic cells, which may cause desmoplasia after stromal cell infiltration and ECM

Tumors escape eradication from the immune system by mechanisms of the first category, which consists of the development of tumor immunoresistance, including the promotion of oncogenicity of tumor stem cells that causes resistance to conventional anticancer treatments and immune responses of the host due to tumor dormancy that cause tumor relapse by selfrenewal, continuous ability of proliferation, incomplete differentiation, and production of immunosuppressive factors causing immunoresistance due to inhibition of apoptosis or type

Another mechanism of tumor immunoresistance is the loss of abnormal surface antigens on the tumoral plasma membrane due to mutations and immunoescape of epitope loss tumor variants, which occurs due to the genetic instability of tumors, leading to continuous altera‐ tions of their surface molecules, hiding their antigenic profile by losing their epitope, especially after they sense the presence of cytotoxic T lymphocytes (CTL) in the tumor microenvironment. Thus, the immune system eradicates only the tumor cells that express the specific epitope, circumventing invisible epitope-negative tumor cells that become extremely resistant to CTL

One more immunoresistant mechanism exerted by the tumor cells is the lack of susceptibility to immune effector cells, such as natural killer cells (NK), cytotoxic T lymphocytes (CTL), macrophages, and dendritic cells (DC), which promote antibody-induced cytotoxicity,

The second category of tumor immunoescape mechanisms consists of the interference with the antitumor-induced immune responses, such as reduced expression of costimulatory molecules on tumor cells or antigen presenting cells (APCs). This downregulation of costimu‐ latory molecules on tumor cells or professional APC may inactivate or eliminate TAA-specific

phagocytosis, or vaccine effects in cancer immunotherapy [21].

**3. Tumor-induced immunosuppression and immunoresistance**

burden at the expense of the immune system [14].

20 Immunopathology and Immunomodulation

deposition [16–18].

I PCD [19].

elimination [20].

The next tumor immunoescape mechanism of this category consists of death receptor/ ligand signaling and tumor-induced counterattack on immune cells that induces apopto‐ sis or type I PCD, in the majority of circulating CD8+ effector T cells in cancer patients, due to the overexpression of Fas (CD95) receptor on the plasma membrane of activated T cells cross-linked by FasL, which is overexpressed on tumor cells [26]. The tumor cells release immunosuppressive factors, such as PGE2, which downregulates Jak3, blocking the IL-2R downstream signaling pathway that downregulates the prosurvival members of the oncogenic bcl-2 family, leading to a defective signaling which inactivates T cells with subsequent circumvention of tumor cells [27].

Another immunosuppressive mechanism of this category consists of dendritic cell (DC) dysfunction in tumor-associated antigen (TAA) cross presentation to T cells, which leads to a deficient immune response against tumor cells, which may deplete dendritic cells (DCs) by inhibiting the induction of TAA-specific immunity that consists of cytokines and chemokines, such as interleukins (IL-1, IL-12, IL-15, IL-18, and IL-23), interferons, and costimulatory molecules, which are required as growth factors, and signals for T-cell proliferation, differen‐ tiation, and memory development [28–30]. The tumor cells may inhibit the maturation of dendritic cells (DCs) by utilizing VEGF and block their differentiation with exosomes. Also, tumor-associated gangliosides (TAG) may downregulate proteasomal constituents of antigen processing machinery (APM) of dendritic cells (DCs) [31–34]. Furthermore, there is another tumor-induced immunosuppressive mechanism consisting of induction of apoptosis or type I PCD of dendritic cells (DCs) in the tumor microenvironment (TME), leading to their elimi‐ nation by the downregulation of antiapoptotic oncogene bcl-2, the production of nitric-oxide (NO), which downregulates cellular inhibitors of apoptotic proteins (cIAPs or cFLIP), the release of ceramide, which blocks PI3K-mediated survival signals, and alterations in intrinsic apoptotic pathways [35].

One more tumor-induced immunosuppressive category consists of insufficient function of effector cells in the tumor microenvironment (TEM). Its first mechanism consists of suppres‐ sion of T-cell immune responses by regulatory T cells (Treg), such as CD4+CD25 highFOXP3+, which accumulate in tumors, and in the peripheral circulation of cancer patients [36]. They downregulate the immune response of the effector T cells by releasing TGF-b1 and IL-10 and involve the Fas/FasL and pathways linked to granzyme/perforin, and enzymatic ATP degra‐ dation to adenosine exerting immunosuppressive effects, which create tumor resistance [37,38]. The second mechanism of this immunosuppressive category consists of suppression of immune cells by bone marrow myeloid-derived immature suppressor cells (MDSC), such as CD13+, CD33+, and CD34+, which are located in the peripheral circulation of cancer patients, and they are recruited to the tumors after they release soluble immunosuppressive factors, such as PGE2, IL-6, GM-CSF, IL-10, VEGF, and TGF-b1, which produce the arginase-1 enzyme that metabolizes L-arginine, activate iNOS, and control the tumor release of indoleamine-2,2 dioxygenase (IDO), which catabolizes the essential for the differentiation of T-cell amino acid tryptophan, leading to the immunosuppression of T-cell responses that promotes the survival of tumor cells [39–41]. The third immunosuppressive mechanism of this third category consists of tumor-derived microvesicles (MV) or exosomes, which express TAA, HLA class I molecules, and death ligands, which exert their immunosuppressive action by the induction of apoptosis or type I PCD in activated CD8+ effector T cells, eradicating their antitumor action. Also, these tumor-derived exosomes exert an additional immunosuppressive action by blocking the differentiation of monocytes to dendritic cells. Subsequently, the monocytes are transformed by the tumor-induced exosomes (MV) into CD14-negative HLA-DR low TGF-b+ myeloid suppressor cells (MSC), blocking the differentiation of immune cells, which inactivates their antitumor properties by releasing TGF-b, downregulating HLA class II molecules, and inhibiting the proliferation of lymphocytes [42].

The fourth mechanism of this immunosuppressive mechanism consists of induction of apoptosis or type I PCD in effector T cells in the tumor and its periphery. Tumor cells may cause apoptotic DNA fragmentation in a proportion of activated CD8+ T lymphocytes and their effector subpopulations, such as CD8+CD28– and CD8+CD45RO+CD27–, in the tumor site, and the peripheral circulation of cancer patients may lead to tumor progression due to apoptotic death of effector T-cell functions, which compromises significantly the antitumor immune responses [43–46].

The last tumor-induced immunosuppressive category consists of insufficiency in tumor recognition signals consisting of four mechanisms. The first one consists of the downregulation of expression of HLA molecules on the surface of tumor cells. As the tumor progresses, it downregulates all HLA class I allospecificities, HLA-A, HLA-B, and HLA-C loci [47,48]. The tumor cells may cause alterations in the expression of the APM components and defects in the b2-microglobulin, and HLA class I heavy chain synthesis due to the deregulation of mecha‐ nisms involving the expression of HLA class I antigen and epigenetic alterations in the HLA class I heavy chain loci, creating resistance to adoptive T-cell-based immunotherapy due to defects into HLA class I which circumvents immune recognition, leading to tumor progression that reduces significantly survival rates of cancer patients [49]. The second mechanism consists of the downregulation of antigen processing machinery (APM) components in tumor cells or antigen presenting cells (APCs) that affect all the peptides, which are presented by HLA class I molecules to T cells enhancing tumor resistance to CTL lysis. The downregulation of total loss of expression of the HLA class I/peptide complexes circumvents the recognition and subsequent destruction of tumor cells by CTL, significantly reducing the disease-free interval and survival rate of cancer patients. The third mechanism consists of the suppression of natural killer cells (NK) in the tumor microenvironment (TME). The downregulation of the cytolytic activity against tumor cells is mediated by the action of inhibitory receptors, such as ILT2/LIRI, CD94/NKG2A, and KIR, which blocks lysis of cells expressing normal HLA class I [50]. The NK cells respond spontaneously to cytokines by expressing IL2Rβγ, such as IFN-a, IFN-γ, IL-2, and IL-15. Upon activation, NK cells release TNF-a and IFN-γ for eradicating tumor cells. They also interact with dendritic cells (DCs) for exerting synergistic apoptotic cell death in tumor cells [51]. However, tumor cells release TGF-b1, which downregulates the expression of NKG2D on NK cells impairing their antitumor activity, especially in advanced stages [52]. Thus, tumors may escape the cytolytic activity of NK cells by the inhibition of interactions between receptors and ligands, the downregulation of tumoral ligands MICA or MICB, the eradication of activated NK cells mediated by overexpression of tumoral death-ligands, and the suppression of interactions between NKs and DCs in the tumor microenvironment (TME) promoting tumor growth and subsequent metastasis, which may kill the cancer patient [53]. The final mechanism of the last immunosuppressive category consists of loss or downregula‐ tion of surface antigens TAA by tumor cells, which evade the host's immune system by circumventing the cytolytic action of effector T cells (CTLs) due to genetic or epigenetic alterations, which may alter the tumoral protein expression, misleading recognition by the immune system, which promotes uncontrollable tumor growth. Thus, the loss or downregu‐ lation of epitopes, such as TAA, and differentiation antigens, such as TRP-1, tyrosinase, MART-1, gp100, and MUC-1, may promote tumoral growth due to escape from the host immune system [54,55]. Furthermore, mutations caused by the tumor in the TAA may circumvent the generation of epitopes, which are recognized immunogenically by cognate CTL regardless of the expression of TAA. These genetic alterations of tumor cells at the coding RNA level may affect posttranslational mechanisms at the protein level, including glycosylation, ubiquitination, and proteolytic enzymes, such as endopeptidases and metaloproteinases (MMPs), which degrade extracellular-matrix (ECM), leading to the downregulation or even total loss of TAA, which mediates tumor escape from the immune system of the host promoting tumor growth.

such as PGE2, IL-6, GM-CSF, IL-10, VEGF, and TGF-b1, which produce the arginase-1 enzyme that metabolizes L-arginine, activate iNOS, and control the tumor release of indoleamine-2,2 dioxygenase (IDO), which catabolizes the essential for the differentiation of T-cell amino acid tryptophan, leading to the immunosuppression of T-cell responses that promotes the survival of tumor cells [39–41]. The third immunosuppressive mechanism of this third category consists of tumor-derived microvesicles (MV) or exosomes, which express TAA, HLA class I molecules, and death ligands, which exert their immunosuppressive action by the induction of apoptosis or type I PCD in activated CD8+ effector T cells, eradicating their antitumor action. Also, these tumor-derived exosomes exert an additional immunosuppressive action by blocking the differentiation of monocytes to dendritic cells. Subsequently, the monocytes are transformed by the tumor-induced exosomes (MV) into CD14-negative HLA-DR low TGF-b+ myeloid suppressor cells (MSC), blocking the differentiation of immune cells, which inactivates their antitumor properties by releasing TGF-b, downregulating HLA class II molecules, and

The fourth mechanism of this immunosuppressive mechanism consists of induction of apoptosis or type I PCD in effector T cells in the tumor and its periphery. Tumor cells may cause apoptotic DNA fragmentation in a proportion of activated CD8+ T lymphocytes and their effector subpopulations, such as CD8+CD28– and CD8+CD45RO+CD27–, in the tumor site, and the peripheral circulation of cancer patients may lead to tumor progression due to apoptotic death of effector T-cell functions, which compromises significantly the antitumor

The last tumor-induced immunosuppressive category consists of insufficiency in tumor recognition signals consisting of four mechanisms. The first one consists of the downregulation of expression of HLA molecules on the surface of tumor cells. As the tumor progresses, it downregulates all HLA class I allospecificities, HLA-A, HLA-B, and HLA-C loci [47,48]. The tumor cells may cause alterations in the expression of the APM components and defects in the b2-microglobulin, and HLA class I heavy chain synthesis due to the deregulation of mecha‐ nisms involving the expression of HLA class I antigen and epigenetic alterations in the HLA class I heavy chain loci, creating resistance to adoptive T-cell-based immunotherapy due to defects into HLA class I which circumvents immune recognition, leading to tumor progression that reduces significantly survival rates of cancer patients [49]. The second mechanism consists of the downregulation of antigen processing machinery (APM) components in tumor cells or antigen presenting cells (APCs) that affect all the peptides, which are presented by HLA class I molecules to T cells enhancing tumor resistance to CTL lysis. The downregulation of total loss of expression of the HLA class I/peptide complexes circumvents the recognition and subsequent destruction of tumor cells by CTL, significantly reducing the disease-free interval and survival rate of cancer patients. The third mechanism consists of the suppression of natural killer cells (NK) in the tumor microenvironment (TME). The downregulation of the cytolytic activity against tumor cells is mediated by the action of inhibitory receptors, such as ILT2/LIRI, CD94/NKG2A, and KIR, which blocks lysis of cells expressing normal HLA class I [50]. The NK cells respond spontaneously to cytokines by expressing IL2Rβγ, such as IFN-a, IFN-γ, IL-2, and IL-15. Upon activation, NK cells release TNF-a and IFN-γ for eradicating tumor cells. They

inhibiting the proliferation of lymphocytes [42].

immune responses [43–46].

22 Immunopathology and Immunomodulation

Thus, there is a continuous struggle between the tumor promoting and the antitumor immune components of the cancer patient where immune promoters of tumor growth and survival include Th17 cell, Cd4+Foxp3+ Treg cells, MDSC, TAM, and their associated chemokines/ cytokines, such as TGF-b, IL-23, IL-1b, TNF, and IL-6, while inhibitors of tumor development and growth consists mainly of CD8+ T, Th1, and CD4+ [56]. The inhibitory signaling pathways to the immune system must be suppressed by cancer immunotherapy [57]. Furthermore, the complexity of cancer involves a crosstalk between tumor microenvironment that interferes with the anticancer activities of the immune system, which in part is caused by the deregulation of the epigenetic machinery that involves methylation-mediated silencing, chromatin remod‐ eling, and microRNA regulons, which may affect immune invasion, tumor–stromal interac‐ tions, and tumor angiogenesis [58]. Epigenetic silencing of coding RNA genes, such as retinoblastoma (Rb) gene mediated by histone deacetylase-2(HDAC-2), may regulate immune responses in cancer, which are facilitated by myeloid cells, such as myeloid-derived suppressor cells (MDSCs), polymorphonuclear MDSCs (PMN-MDSCs), and monocytic MDSCs (M-MDSCs), which are the normal counterparts of inflammatory monocytes that differentiate into macrophages, and dendritic cells whose dysfunction in cancer is a severe mechanism of immunosuppression [59,60]. Furthermore, tumor microenvironment (TME) may convert plasmacytoid dendritic cells by complex molecular pathways into tolerogenic immunosup‐ pressive cells [61].

Other tumor microenvironment (TME)-induced immunosuppressive factors, which we must target with cancer immunotherapy not only in solid tumors but also in hematologic malig‐ nancies, include tumor intrinsic immunosuppressing ectoenzyme CD37, which is a disulfidelinked homodimer that regulates negatively the proinflammatory effects of extracellular ATP; activates P2X7R, which is a coactivator of the NLRP3 inflammasome-releasing proinflamma‐ tory cytokines such as IL-18 and IL-1b; and blocks antitumor T-cell immunity via upregulation of the adenosine receptor (AR) signaling, promoting tumor angiogenesis, growth, and metastasis [62–66].
