The TME contains two major classes of immune cells: immuno-suppressive and immuno-stimulating. Their activity is dependent on innate and adaptive immune system responses (10). Several types of immune cells are active in cancer cell escape immune editing, such as tumor infiltrating lymphocytes (TILs). TILs have different functions and transcription factors, and occur as several subtypes, including Th cells (helper cells), CTLs (cytotoxic CD8+ T lymphocytes), and Tregs (regulatory T-cells). Other cells with a role in immune regulation are CD4+ effector T cells, tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), DCs, NKs, and mast cells (11). Several immunosuppressive factors, such as prostaglandin E2 (PGE2), interleukin-10 (IL-10), and transforming growth factor-β (TGF-β), are secreted by these cells to regulate the immunosuppressive network (5).

Studies have shown a high number of TILs in BC, which are composed mainly of T cells of different classes, and fewer B cells (9). The adaptive immune response is supported by CD4+ T cells and CD3+ T cells, as well as CTLs. When APCs (antigen-presenting cells) such as macrophages and DCs detect tumor antigens, CTLs release granzyme and perforin, mediated by interferon-γ (IFN-γ) secretion, to eliminate cancer cells. In addition, IFN-γ and IL-12 signaling activate CD4+ T-cells, which are type 1 helper (Th1) cells, allowing APCs to license the differentiation of CD8 T-cells and clonal expansion. Th1 cell presence is associated with better clinical outcomes in BC patients as these cells activate CD8+ T lymphocytes, triggering their cytotoxic activity by freeing pro-inflammatory cytokines (9). Other T helper cells, Th2 and Th17, play a role in BC progression as well as follicular helper T-cells, which largely regulate the maturation of antigen specific B cells, enhancing local memory and elevating the development of tertiary lymphoid organs, leading to an immune response that targets local tumors (12). Major regulators of immune system homeostasis are Tregs, as their existence in the TME elevates immune system capability to act as an immunosuppressive through direct cell-cell contact suppression and immunosuppressive cytokines (IL-10, TGF-β) (13). Tregs, which express Foxp3, belong to a class of suppressive cells and act to suppress effector T cells, preventing immune-mediated rejection of tumors (14).

NKs are a class of APC which play a major role in immune tolerance in BC. They are cytotoxic innate lymphocytes which act to lysate and eradicate malignant cells, and this eradicating mechanism is independent of major histocompatibility complex I (MHC-I) molecules and antibodies. In order to avoid tumor-invading cytotoxic T lymphocytes detecting the presence of tumor cells, MHC-I expression on the surface of the tumor cell is often inhibited or eliminated. NK cell inhibitory receptors are able to detect this lack of MHC-I, and the immunogenic effect of NK cells is evident in their contribution to regulating the function of multiple immune cells, including DCs, macrophages, and T and B cells (15). Principal components of the TME are DCs, which are thought to be one of the most potent types of APC, and present antigens, including tumor-derived antigens, to T-cells. DCs have two major phenotypes with different surface protein expression, known as myeloid and plasmacytoid populations. DCs become mature and stimulate the immune system by interacting with T cells. Cancer cells, however, have the ability to inhibit the maturation of DCs, which results in tumor-infiltrating DCs having an underdeveloped phenotype. Tumor-derived antigen cross-presentation is therefore inhibited, co-stimulatory molecules show downregulation, so DC antitumor functions can be impaired (9).

Findings show that tumor-infiltrating B cells could have both pro-tumor and anti-tumor effects, which greatly depend on the components of the TME and BC phenotypes. Moreover, tumor-specific antigen recognition, antibody production, and APC functioning have all been shown to affect the anti-tumor properties of B cells (9). On the other hand, B cells have been shown to mediate tumor growth, as regulatory B cells express inhibitory molecules such as programmed cell death-ligand 1 (PD-L1) and FAS ligands, as well as anti-inflammatory mediators such as TGF-β, IL-10, and IL-35, resulting in the suppression of immune responses leading to cancer cell immune escape (16).

One of the main innate immune cells in BC are macrophages, which occur as two polarized phenotypes, the M1 alternative macrophages and the M2-like TAMs (9). M1 macrophages are activated by IFN-γ and tumor necrosis factor-alpha (TNF-α), which are released by Th1 cells, thus causing production of reactive oxygen species and release of pro-inflammatory cytokines (IL-12 and IFN-γ), processes which, in turn, stimulate anti-tumor activity (17). The other activated macrophage phenotype, M2-like TAMs, are switched on by Th2 cells cytokines such as IL-13, IL-10 and IL-4, leading to tumor progression, inducing angiogenesis and metastasis, and inhibiting the anti-tumor response (18). The development of the M2-like TAM macrophage is encouraged by the existence of IL-4 and IL-13 in the TME. Tumor cells can produce macrophage-derived chemokines such as C–C motif chemokine 22, which then bring monocytes into the tumors; if immuno-suppressive conditions prevail, these monocytes will then differentiate into TAMs (5). Immune cells called neutrophils also have the potential to promote or hinder tumor development. As tumor-associated neutrophils (TANs) they function as tumor-infiltrating immune cells. They exhibit different phenotypes (N1 and N2) in which N1 cells have the pro-inflammatory and anti-tumor properties of TANs, triggered by IFN-γ and IFN-β exposure (9). N2 neutrophils are triggered by TGF-β exposure, producing anti-inflammatory and pro-tumor TANs (18).

MDSCs are known as immunosuppressive populations and are characterized into two phenotypes; polymorphonuclear or granulocytic MDSCs and monocytic MDSCs. These populations are precursors of bone marrow and have the ability to suppress the immune response via the repression of CTLs and NKs and the secretion of immunosuppressive cytokines including IL-10 and TGF-β, as well as the induction of expression of PD-L1 (19). Mast cells promote the growth of tumor cells via the secretion of H+, NO, chondroitin sulfate, and oxidized polyamines, leading to the induction of the non-degranulated mode of mast cells, resulting in an immunosuppressive effect. Moreover, the cytokines secreted by mast cells, such as histamine, IL-10, and TGF-β result in the suppression of effector T cells, and the secretion of PGE2 affects the migration and function of DCs, all of which strengthens immunosuppression within tumors (5).

Recent research suggests that certain types of human immune cells exhibit this kind of two-way oppositional mechanism in BC occurrence, that is, alterations in cell composition mean they are able either to encourage tumor growth or suppress it (20). In breast cancer, TILs can affect cancer cells and immune cells in different ways, depending on the stage of the cancer and the specific cell phenotype. This means either a pro- or anti-tumor response, leading either to the promotion of tumor cell proliferation and spread, or its suppression and destruction (21).

Studies in the BC microenvironment show that TILs can affect the response of cancer cells by either promoting tumor generation, or triggering suppression and apoptosis. Both CD4+ and CD8+ T cells can affect the adaptive immune response; however, when activated, CD8+ differentiates into CTLs, while CD4+ cells divide into sub-populations of T helper cells (Th1, Th2, Th17) (20). The CD8+ cell types cause tumor cell destruction and are dominant in the BC microenvironment, whereas CD4+ subpopulations can produce either pro- or anti-tumor activity. For example, Th1 CD4+ cells secrete pro-inflammatory mediators INF-γ and TNF-α, and trigger the anti-tumor activity of NKs, thus activating a powerful anti-cancer immune response (11, 20). In a contradictory fashion, the cell subtype Th2 CD4+ instead promotes tumorgenicity and encourages metastasis by releasing cytokines IL-4, IL-5, and IL-13, but it also releases IL-10, which can influence either the growth or destruction of cancer cells. Another subtype, Th17 CD4+, releases TGF-β, which is known to encourage cancer cell progression (20). Similarly, different phenotypes of TAMs may polarize to either M1-like or M2-like macrophages, which are linked with tumor progression and suppression respectively (22). The M1 phenotype promotes the apoptosis of cancer cells via CTL recruitment and activation of the adaptive immune responses, while M2 attracts Th2 and Treg cells, encouraging cancer cell growth, tissue remodeling and tumor angiogenesis (23). Other clinical research supports findings that M2-like macrophages promote BC cancer cell proliferation and overall negative outcomes (22).

Literature has shown that certain regulatory molecules play a physiological role in the suppression of self-immune responses. When these regulatory molecules, known as immune checkpoints, become dysregulated, they cause evasion of immune-mediated destruction. In pathological conditions such as BC, they include programmed cell death 1 (PD-1), cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and its ligands (PD-L1/2) and T-cell immunoglobulin and mucin domain containing protein-3 (TIM-3) (24). Targeting immune checkpoints with immune checkpoint inhibitors (ICIs) has brought advances in in cancer immunotherapy because ICIs accelerate the anti-cancer immune response to eradicate malignant tumors. ICIs can restore the suppressed immune cells as a recognizers of cancer cells by blocking immune checkpoint interference in the cross-talk between immune cells and cancer cells, thus reactivating natural immune responses in BC patients. Current clinical practice using ICI treatment shows dramatic improvement in survival rates in advanced-stage and metastatic cancers (25).

Curcumin (1) is known as the chief constituent of the culinary spice turmeric. It demonstrates important biological properties, including anti-inflammatory, antimicrobial and anticancer activity. It can regulate immune response in BC as reported by Krishnamurthy et al., where curcumin and mannan, a component of the Aloe vera plant, inhibited the proliferative activity of immune cells, including peripheral blood mononuclear cells (PBMCs) such as lymphocytes, monocytes and macrophages (29). In some conditions the effects of curcumin are limited due to solubility issues, as it has poor solubility in neutral or acidic media and is unstable in alkaline conditions (30). However, delivery of curcumin to the target site in a sustainable and controlled manner can be achieved through nanotechnology, such as curcumin-loaded polymeric nanoparticles (2) and a nano-vaccine containing cytosine-phosphate-guanine and antigenic peptides. Injection of this formula in a BC model triggered immunogenic cell death of cancer cells and activation of DCs. DCs stimulation significantly improved tumor-specific CD8+ T-cell responses resulting in tumor inhibition ( Table 1 ) (31).

Resveratrol (3) belongs to the class of stilbenes, and is recommended for a number of different pathological conditions. Its immunological effect on BC has been tested, revealing that resveratrol inhibits glyco-PD-L1-processing enzymes (α-glucosidase/α-mannosidase) and PD-L1 dimerization and blocks the PD-1/PD-L1 interaction. Thus, it in the process increases cytotoxic T-lymphocyte activity and restores T-cell immune function in tumor tissue (32). HS-1793 (4) is a derivative of resveratrol that has an effect on immune cells through changing lymphocyte proliferation and the Treg cell population in FM3A breast tumor-bearing mice. It was found to promote the activity of concanavalin A-induced lymphocytes in these mice. HS-1793 has also been found to cause changes in the subset of tumor-infiltrating T cells including the CD25+ cells, in a dose-dependent manner (33).

Vanillic acid (5) is an aromatic phenolic compound produced by several different plants, such as vanilla beans. It has been found to be of benefit in different pathological conditions due to its antioxidant and antibiotic effects. This phenolic compound exhibited anti-tumor properties in mouse models with 4 T1 breast tumors, with phagocytosis and apoptosis-induction occurring via the promotion of macrophage polarization to the M1 phenotype through IL-6R/Janus kinase (JAK) signaling (34). XK-81 (6) is a novel bromophenol obtained from Leathesia nana that was found to have an immunotherapeutic effect on BC cell lines. It elevated the number of CD8+ T cells and NKs and modulated the ratio of M1/M2 macrophage in tumor tissues. This was combined with an elevation of immune-related cytokines, including IL-12, TNF-α, and IL-1β in a macrophage cell line (35).

Ursolic acid (7) is triterpenoid compound that exists in different fruit and vegetables and is known for its poor solubility. Research has been conducted to develop liposome-loaded ursolic acid for BC immunotherapy. It has been found to modulate CD25+ forkhead box P3 (Foxp3+) T cells via the inhibition of signal transducers and activators of the transcription (STAT)5 phosphorylation and IL-10 secretion. It also reduced MDSC population and Tregs within tumor tissue, resulting in the correction of immunosuppressive conditions generated by the TME and the inhibition of tumor growth (36). Anemoside A3 (8), from the root of Pulsatilla chinensis, is a triterpenoid glycoside that suppresses progression of TNBC tumors via shifting of the M2-TAM to the M1-TAM phenotype and inhibiting TAM-TNBC crosstalk (37).

Oridonin (9) is a diterpenoid with anti-inflammatory and antitumor activities. It is obtained from the plant Rabdosia rubescens and is used in Chinese herbal medicine. This molecule modulates Treg differentiation both in vitro and in vivo, leading to the attenuation of Treg immunosuppressive ability. This mechanism depends on the reduction of TGF-β receptor expression (38). Crassolide (10) is a natural marine product belonging to the class of cembranoid diterpenes, and is produced by Formosan soft coral, Lobophytum michaelae. Tsai and colleagues investigated its potential immunogenic effects and found that crassolide induced immunogenic cancer cell death. It also reduced the expression of CD24 on the surface of cancer cells and blocked mitogen-activated protein kinase (MAPK) 14 activation and STAT activity (39). Another diterpene, triptolide (11), is derived from the vine Tripterygium wilfordii and is used in traditional Chinese medicine as an immunosuppressant in autoimmune diseases and inflammatory conditions (78). Research has shown the ability of triptolide to act as a controller to promote cancer cell-reactive immune responses via the suppression of interferon-γ-induced PD-L1 surface expression leading to down-regulation of the PD-1/PD-L1 pathway (40).

Naringenin (12) is a flavanone that exists in citrus fruits and has immunomodulatory, antioxidant, antidiabetic, and hypolipidemic properties. Naringenin has been shown to decrease the infiltration of MDSCs and Treg cells in breast cancer cell lines, and to upregulate IL-2 and INF-γ -releasing T cells in spleen and lung tissue, demonstrating its use as an immunomodulatory agent. Qin’s group revealed anticancer activity in tested animal model with an elevation of IL-2 and IFN-γ expressing T cells (41). Other research found that naringenin inhibited the transformation of lymphatic T cells into Tregs, which prevented in vivo pulmonary metastasis triggered by BC through the lowering of TGF-β1 production via the protein kinase C signaling pathway (42). Furthermore, a combination of cryptotanshinone and naringenin (13) caused a switch in immune response towards Th1 cells, thereby enhancing their activities and modulating Treg cell activity through JAK2/STAT 3 pathway in BC (43). Another flavonoid, apigenin (14), which is found in fruit and vegetables such as onions and oranges, has been found to boost immune system functioning. Apigenin has shown inhibitory effects on interferon-γ-induced PD-L1 expression as well as interferon-γ mediated STAT1 activation. One study investigated the immunogenic properties of apigenin and found that it intensified the anti-tumor immune response by increasing the proliferation of T cells (44). Salvigenin (15) is a flavonoid obtained from Salvia miltiorrhiza known to have cytotoxic and immunomodulatory properties. Its activity, in conjunction with the modulation of cytokine production of primed immune cells, was demonstrated by Noori’s group, who found a significant rise in anti-cancer immunity and a reduction of tumor tissues in a BC mouse model. In vivo results showed the reduction of IL-4 and elevation of IFN-c in the models, accompanied by suppression of Foxp3+ Treg cells (45). Another flavonoid, myricetin (16), found in tea and in berry plants, showed significant inhibitory effects on PD-L1 in IFN-γ-treated MDA-MB-231 BC cells (46).

Sativan (17) is an isoflavane produced by Spatholobus suberectus, another plant which is used as a remedy in traditional Chinese medicine. Peng and colleagues revealed that treatment with sativan resulted in the downregulation of the expression of PD-L1 and epithelial-to-mesenchymal transition by up-regulating miR-200c. In addition to the suppression of PD-L1, N-cadherin, snail and vimentin levels decreased, indicating the inhibition of tumor migration and invasion (47). Hesperidin (18) is classified as a flavanone glycoside and has various pharmacological effects on cardiovascular, neurological, and psychiatric conditions as well as on cancer. It is found naturally in citrus fruits. Kongtawelert and colleagues showed the suppressive effects of hesperidin on the mRNA and protein of PD-L1 in the MDA-MB-231 cell line, via inhibition of protein kinase B (Akt) and nuclear factor kappa (NF-κB) signaling (63). Sulaiman et al. investigated the properties of hesperidin via a nanoformulation of hesperidin loaded on gold nanoparticles. The results showed that the nanoformulation stimulated macrophage activity in Ehrlich ascites tumor cell-bearing mice (79).

Berberine (19) is an alkaloid that is naturally present in a variety of plants, including barberry and oregon grape. It has various pharmacological properties, including anti-inflammatory, analgesic, hypolipidemic and antimicrobial activity. Upon exposure of BC 4T1 tumor-bearing mice to berberine, all immune-cell marker levels was significantly reduced except for CD8, and inflammatory markers were down-regulated. Furthermore, the infiltration of NKs was elevated in the treated group, revealing the immunogenic effect of berberine in BC ( Table 1 ) (49). 3,3′-Diindolylmethane (20) is another natural alkaloid that has been investigated for its anti-cancer effects. It is formed during the autolytic breakdown of indole-3-carbinol, a reaction occurring in plants such as cruciferous plants. It possesses anti-tumor properties through its ability to inhibit tumor cell proliferation, suppress metastasis, and induce apoptosis of tumor cells. Moreover, Sun’s group revealed the immunogenic effect of 3,3′-Diindolylmethane as an inhibitor of MDSCs via downregulation of miR-21 levels and subsequent activation of the phosphatase and tensin homolog/PIAS3-STAT3 pathways. In addition, by raising T-cell response, it promoted the production of beneficial PD-1 antibodies and slowed tumor growth, thus indicating its potential for cancer patients undergoing anti-PD-1 treatment (50). Prodigiosin (21) is a microbial alkaloid with a red pigment that is found in the gram-negative bacterium Serratia marcescens. It has anti-microbial and anti-tumor properties, and is able to regulate the TME by controlling immune cells and immune checkpoints. Furthermore, it has been found to reduce the number and differentiation of Tregs, thus preventing immune tolerance and enhancing antitumor functions via inhibition of heat shock protein 90 and survivin, and activation of p53. It also suppresses tumor growth via the inhibition of M2 polarization and induction of TAM infiltration (51).

Polyactin A (22) is an antibiotic belonging to the class of polymannopeptides. It can be isolated after fermentation of the buccal α-hemolytic streptococci strain. This antibiotic affects immune cells via the induction of DCs maturation from PBMCs, and the mature cells in vitro could initiate a potent E75 peptide-specific CD8+ T-cell response. It may have the ability to trigger the E75-specific immunologic response in vivo as well as in vitro, and has been shown to significantly increase positive rates of CD4+ and CD8+ T lymphocytes (52). Enniatin A (23) is a cyclohexadepsipeptide obtained from the Fusarium species of fungi, with the ability to reprogram the TME. It has been shown to trigger immunogenic cell death in TNBC syngeneic mice. Moreover, it can reduce PD-L1 levels and promote CD8+ T cell-dependent antitumor activity by activating the chemokine-related receptor CX3C motif chemokine receptor 1 pathway (53).

Fucoidans (24), are sulfated polysaccharides that can be extracted from brown algae seaweeds such as Cladosiphon okamuranus. They have a range of properties that include antioxidant, immunomodulatory, and anti-cancer activity. In addition to their ability to induce cell cycle arrest and apoptotic death of BC cells, fucoidans have immuno-potentiating effects in immune cells. Moreover, they can modulate the activity of adaptive and innate immune responses via the potentiation of T cells, macrophages, DCs and NKs. A study involving co-culturing co- CD8+ T cells and human breast cancer cells (MCF-7) revealed that CD8+ T cells number and IFN-γ increased more in the fucoidan-treated group. In another study, by Jin et al., similarly immunogenic effects were found to occur through the promotion of both CD4+ and CD8+ T cell responses via the elevation of immunomodulatory mediators of Th1 and CD8+ T cells (IFN-γ and TNF-α). Fucoidans also activate the maturation of DCs, either by raising levels of CD40, CD86, and MHC-I and -II surface molecules or increasing cytokines such as IL-12 and TNF-α (70). Nanoformulations of fucoidan with doxorubicin (25) combine the cytotoxic effects of doxorubicin with the ability of fucoidans to moderate the tumor microenvironment by raising the immune response of Th1 and switching M2 TAM to the M1 TAM phenotype (55). Different formulations composed of oligo-fucoidan and Olaparib (26) were found to synergistically suppress PD-L1, resulting in repression of the oncogenic IL-6/p-epidermal growth factor receptor/PD-L1 pathway. Moreover, it decreased subpopulations of CD44/CD24, suppressed M2 macrophage intrusiveness and repolarized M2 to the M1-like (CD80high and CD86high) phenotypes and induced immunoactivity and antitumoral M1 macrophages (56).

Lentinan (27) is a polysaccharide obtained from the edible mushroom Lentinus edodes, which has antitumor and immuno-stimulating properties. It is able to enhance immune function in the treatment of BC, as demonstrated by Guan’s group. The immunohistochemical findings showed that it reduced the mRNA expression of marker genes related to M2-type macrophage. It also suppressed M2 macrophage polarization induced by IL-4 cytokine. It activated the JAK/STAT signaling pathway, as shown by molecular docking, western blotting and siRNA transfection experiments (57). Polysaccharides from Tetrastigma hemsleyanum (SYQ) (28) have been found to induce the polarization of M2 to anti-tumor M1 phenotypes. This causes promotion of the macrophage polarization leading to the inhibition of BC cell proliferation (58). Polysaccharides (29) from Ganoderma lucidum are known to boost the immune system. Polysaccharide fractions from the plant are reported by Zhao and colleagues to activate macrophages significantly, leading to inhibition of BC cells (80). Moreover, other studies have shown the ability of polysaccharides to promote the function of several immune cells such as APCs and mononuclear phagocytes, as well as increasing humoral and cellular immunity. The latter process includes the production of CTLs and activated macrophages (59).

The steroidal saponin taccaoside A (30) is one of the principal phytochemicals in many herbs used in traditional Chinese medicine, and is known for its anti-cancer activity. It was found to exhibit significant activity against BC cells by increasing granzyme B through improving the signaling of the T lymphocyte mammalian target of rapamycin 1 (mTOR1)-Blimp-1, providing in vivo evidence of anti-tumor efficacy (60). Ginsenosides (31) belong to the class of saponins and are known for neuroprotective and anti-inflammatory properties. The main ginsenosides are obtained from the root of Panax ginseng, one of these being ginsenoside Rg3, extracted from Korean ginseng, which has the ability to induce apoptosis, enhance the activity of NKs and inhibit the NF-κB signaling pathway in BC model (81). Due to its insolubility in water and poor solubility in the intestine, nanoformulations of Rg3 with doxorubicin have been designed, in which Rg3 raises infiltration levels of memory T cells in the tumor microenvironment while the doxorubicin promotes immunogenic cancer cell death (61).

Eribulin mesylate (32) is a natural marine product extracted from the Japanese marine sponge Halichondria okadai which has been approved for use in metastatic cancer. Its immunomodulatory effect was investigated by Goto et al. in conjunction with locally advanced or metastatic breast cancer. Tumor biopsies from patients receiving eribulin treatment were collected and analyzed for the expression of immune markers including PD-L1, PD-L2, CD8 and the Treg marker FOXP3. Results showed a significant reduction of immunosuppressive drivers PD-L1 and FOXP3, leading to reduced immunosuppression in the TME (62). Sesamin (33) is a lignin extracted from the oil of Sesamum indicum, which is recognized for antioxidant and anti-inflammatory activities. Kongtawelert’s group revealed that sesamin downregulated PD-L1 expression in both mRNA and protein in MDA-MB231 breast cancer cells. This was mediated by NF-κB and Akt, and also suppressed extracellular signal-regulated kinase (ERK) and JAK/STAT signaling activity (48).

Artemisinin (34) is sesquiterpene lactone obtained from the plant Artemisia annua that is used as an anti-malarial drug. Cao and colleagues explored its ability to suppress BC growth via its immunomodulatory activity. They found that artemisinin boosted T cell functioning, blocked the immunosuppressive effects of Tregs and MDSCs, and allowed CD4+ IFN-γ+ T cells and cytotoxic T cells to thrive, all of which hindered tumor growth in vivo (64). Oleuropein (35) is a glycosylated seco-iridoid, a type of phenolic bitter compound, extracted from Olea europaea L. It is known for its anti-inflammatory, anti-cancer antioxidant, neuroprotective, and anti-atherogenic properties. Hamed and colleagues revealed that oleuropein controls the miR-194/XIST/PD-L1 loop in TNBC, thus making it a promising nutritional epigenetic agent in cancer immunotherapy (65). Salinomycin (36) is an antibiotic obtained from the bacterial species Streptomyces albus that has been investigated for its anti-tumor activity in BC. It was found to suppress activation of the JAK/STAT pathway by IFN-γ and inhibit IFN-γ-induced activation of the NF-κB pathway by inhibiting IκB degradation and NF-κB phosphorylation. It also inhibited indoleamine 2,3 dioxygenase enzymatic activity, as shown by molecular docking, where salinomycin demonstrated nucleophilic attack in the catalytic domain of indoleamine 2,3 dioxygenase. In tumor tissue, in vivo research found that salinomycin activated cisplatin’s anti-tumor properties and appeared to boost T cell production when co-cultured with BC cells treated with IFN-γ (66).

Metformin (37) is a known hypoglycemic drug that can be extracted from Galega officinalis. Results reported by Cha’s group show that metformin has the potential to lower PD-L1 in breast cancer by activating protein kinases via AMP-activated protein kinase (AMPK). Blocking PD-L1 signaling enhances CTLs activity against tumor cells (67). Alpha-tocopheryl succinate (α-TOS) is one of the forms of vitamin E that is an effective anti-tumor agent. A nanoparticle delivery system was designed for α-TOS (38) which aimed to boost anticancer immunity through the suppression of IFN-γ-induced PD-L1 expression. It also enhanced tumor elimination via the modulation of cytotoxic lymphocyte infiltration into the TME (68). Neo-tanshinlactone (39) is a planar natural molecule having four rings. It is obtained from Salvia miltiorrhiza Bunge and is known as an ICI and as PD-1/PD-L1 interaction inhibitor. Zhang’s group investigated the effect of different analogues of neo-tanshinlactone against TNBC. MZ58 (40) proved to be the best candidate in a subcutaneous transplantation tumor model as it showed less cytotoxicity toward T cells, activated CD8+ T cells, and reduced T cell exhaustion (69).

In some cases, the effect of single compound can be strengthened by combining it with other phytochemicals. This phenomenon has been investigated in studies aimed at remodeling BC cells using the synergistic effects created in combining active chemical constituents ( Table 1 ). For example, a combination of curcumin, resveratrol, and quercetin (RCQ) (41) was designed to manipulate the multi-layered interactions of cells and signaling pathways in a novel approach to phyto-immunotherapy in BC. The RCQ combination has been shown to remodel antitumor immunity in 4T1 breast cancer-bearing mice by shifting the immune balance toward an immune activation state by reversing the superiority of immunosuppressive infiltrating cells in the TME. This is achieved by the inhibition of the development of TILs into immunosuppressive cells, including TAMs cells to M2 TAMs and TANs to the N2 TANs. It also enhanced the T cells accumulation and decreased the recruitment of macrophages and neutrophils in the TME (54).

Ethanolic extracts of Cordyceps militaris (42) have shown immunogenic effects in stimulating the proliferation of tumor-specific T cells without inhibiting DCs functioning and T cell proliferation (82). Yang et al. isolated C. militaris immunoregulatory protein that suppresses the proliferation of 4T1 breast cancer cells. The immunoregulatory protein elevated the mRNA levels of cytokines IL-6, IL-1β and TNF-α in peritoneal macrophages (71). In addition, a traditional Chinese formula (XIAOPI) (43), composed of 10 plants, has shown the ability to reprogram the TME by decreasing the polarization and proliferation of M2-type macrophages. Similarly, other herbal extracts from Cordyceps sinensis, Taraxacum mongolicum, and protein-bound polysaccharides (from the Coriolus versicolor fungus) (44) suppressed progression of TNBC via shifting the M2-TAM to the M1-TAM phenotype, and inhibiting TAM-TNBC-talk (37). Regarding protein-bound polysaccharides from C. versicolor (45), several clinical studies have been conducted showing their significant immunological and oncological activity, with overall improvement of prognosis for BC patients. The mechanism of action involves a Th1 adaptive immune response and modulation of immunosuppressive TME via activation of DCs (72). A polysaccharide fraction obtained from Astragalus membranaceus (46) has been shown to increase immune response. This effect is linked to the inflammatory immune response at the tumor site. Detailed investigation of the mechanism showed reduction of the expression of PD-L1 via the Akt/mTOR/ribosomal protein S6 kinase beta-1 pathway (73).

The fruit extract of Diospyros peregrina (47) has been evaluated for its immunogenic effect on BC models. Findings show that the extract controlled and combatted BC, and suppressed the expression of Foxp3+Treg cells within tumor tissue. This was reflected in immune cell and cytokine activity, as the release of Th2 cytokines was reduced, including IL-10 and IL-4, and the release of Th1 cytokines, including IL-12 and IFN-γ, was elevated. This was accompanied by elevation of the activity of T-box transcription factor TBX21and the suppression of the expression of transcription factor FOXP3 and GATA binding protein 3 (74). Sarcodon imbricatus (48) as an aqueous extract, showed inhibitory effects on the growth, migration, and invasion capacity of BC cells. It decreased the expression of PD-L1 in BC models and elevated IL-2, IL-6, TNF-α, and NKs activity (75). Yang and colleagues prepared an ethyl acetate fraction from a mixture of Hedyotis diffusa and Scutellaria barbata (49) revealing their ability to reduce the expression of PD-L1, β-catenin, and cyclin D1, causing inactivation of MAPK and Akt signaling pathways (76).

Badawy et al. investigated the properties of camel milk (50) and its exosomes nanoparticles. In vitro as well as in vivo tests using oral and local injection, found that camel milk reduced breast tumor progression via several different mechanisms, including apoptosis induction, oxidative stress inhibition, suppression of several types of gene-related inflammation (IL1b, NF-κB), angiogenesis (vascular endothelial growth factor) and metastasis (matrix metalloproteinase-9, intercellular adhesion molecule 1). These anti-tumor effects were accompanied by higher immune response, evidenced by higher numbers of CD+4, CD+8, and NKs (77).