The differentiation process of CD4⁺ T cells is governed by multiple factors, including antigen-specific stimulation, T cell receptors(TCR), cytokines, and transcription factors (13). Initially, upon detection of “non-self” or foreign substances by the immune system, antigens are presented to CD4⁺ T cells via the TCR, initiating the differentiation process (14). The type of antigen presenting cell(APC) determines the antigen type, while its affinity and quantity influence the nature and strength of TCR signaling, collectively regulating the activation and differentiation of CD4⁺ T cells in conjunction with co-stimulatory molecules (13, 14).

Subsequently, cytokines secreted by antigen-presenting cells and differentiated CD4⁺ T cells play crucial roles in differentiation. For instance, interleukin-12(IL-12) and interferon-gamma(IFN-γ) promote type 1helper T(Th1) cell differentiation (15, 16), IL-4 induces Th2 cell differentiation (17), IL-4 and transforming growth factor beta(TGF-β) enhance Th9 cell differentiation (18, 19), and IL-6 and TGF-β drive Th17 cell differentiation (20). These cytokines activate distinct signaling pathways, guiding the formation of specific T cell subgroups (13).

Finally, under the regulation of specific cytokine signals and cellular environments, master transcription factors contribute to shaping and maintaining the balance and diversity of the immune system by activating specific gene expression patterns (16). Each T cell subset is governed by lineage-specific master transcription factors, such as T-bet, GATA binding protein 3(GATA3), interferon regulatory factor 4(IRF4), and retinoid-related orphan receptor gamma t(RORγt), which control the expression of subset-specific genes, thereby determining the direction of cell differentiation (18, 21–24). Therefore, the variegated landscape of the tumor microenvironment impels T cells toward distinct subtypes, underscoring the critical importance of the types and functional states of T cell subtypes in shaping the immune response to tumors (13).

Under antigen stimulation, naïve CD8⁺ T cells generate effector and memory T cells, with the effector CD8⁺ T cells referred to as CD8⁺ CTLs (13). CTLs assume a pivotal role in the vigilant immune surveillance against neoplastic entities, recognizing cell surface antigens on tumor cells through the discerning receptors of the TCR (44, 45). The primary mechanisms through which CTLs coordinate their anti-tumor capabilities involve the granzyme/perforin pathway and cytotoxicity mediated by Fas receptors (46). The granzyme/perforin cascade involves the liberation of granules containing granzymes and perforin, thereby directly instigating apoptotic cascades within targeted cellular domains (47–49). Usually, perforin serves as the conduit for ingressing granzymes into tumor cells, thereby facilitating the demise of targeted cells (50). Therefore, the absence or impairment of perforin may diminish the tumor-suppressive efficacy of CTLs (47). In the latest literature, it has been discovered that endosomal sorting complexes required for transport can repair the plasma membrane pores caused by perforin. This rehabilitative action restores membrane integrity, effectively preventing the invasion of granzymes (51) ( Figure 1 ).

In instances where the integrity of the granzyme/perforin pathway is compromised, there emerges a heightened prominence of Fas-mediated processes (46) ( Figure 1 ). FasL triggers apoptosis through the intricate activation of caspases (52). Nonetheless, noteworthy observations posit that FasL expressed by exosomes might exert divergent effects, potentially fostering tumor invasion instead of inducing apoptotic signals (53).

Furthermore, CTLs also possess the capability to modulate the metabolic dynamics of neoplastic cells through the secretion of cytokines ( Figure 1 ). Notably, factors such as IFN-γ, originating from CTLs, downregulate certain components of the glutamate-cystine antiporter system, subsequently influencing lipid metabolism within the tumor cell milieu and promoting tumor cells’ apoptosis (54, 55). Another secreted product, TNF-α, despite its potential derivation from various immune cells, undeniably plays a role in inducing the rupture of tumor blood vessels, promoting cell infiltration, and maintaining an ischemic state in tumors (56) ( Figure 1 ). However, under typical circumstances, the contact of individual CTLs with tumor cells fails to eliminate the tumor cells effectively. And it is only through sequential interactions with multiple CTLs that elimination occurs (57).

In addition, tumor cells heavily impact the function of CTLs ( Figure 1 ). Tumor-derived lactic acid efficaciously reduces the activity of monocarboxylate transporter -1, which weakens cellular metabolism and diminishes the cytotoxicity of IFN-γ, granzymes, and perforin in CTLs (58). Moreover, the secretion of TGF-β by tumor cells directly impedes the immune activity of CTLs by inducing the upregulation of miR-23a and simultaneous downregulation of B-lymphocyte-induced maturation protein 1(Blimp-1) (59). Notably, Blimp-1, as a pivotal transcriptional repressor, plays a fundamental role in the differentiation and memory response of effector CD8⁺ T cells (60). Consequently, this mechanism undermines the immune function mediated by CTLs. Tumor cells also induce the expression of FasL in endothelial cells via vascular endothelial growth factor-A (VEGF-A), IL-10, and prostaglandin E2 (PGE2), thereby eliciting specific cytotoxic effects in effector T cells (61).

γδT cells and αβT cells are the two main types of T cells within the adaptive immune system. γδT cells have T-cell receptors composed of γ and δ chains and recognize a broader range of antigens, while αβT cells bear T-cell receptors made of α and β chains and primarily respond to peptide antigens presented by major histocompatibility complex (MHC) molecules (62).

Despite being a minority among peripheral blood cells, γδT cells assume a pivotal role in the detection and eradication of tumor cells (63). In a previous study focused on squamous cell carcinoma of the head and neck (SCCHN), it was observed CD56⁺ γδT cells, isolated from peripheral blood mononuclear cells (PBMCs) expanded under the stimulation of isopentenyl pyrophosphate (IPP) and IL-2, could effectively destroy SCCHN cell lines in a dose-dependent manner, in contrast to CD56^- γδT cells (64). What’s more, the cytotoxicity of γδT cells underwent a notable suppression following treatment with concanamycin A (CMA), an inhibitor of the granzyme/perforin pathway (64), which concurrently functions as a downregulator (65). Additionally, γδT cells also exhibit a lytic effect on MCF-7 breast tumor cells. Subsequent research revealed that MCF-7 tumor cells were surrounded by a substantial number of γδT cells, forming a tight conjugate, and were subsequently eliminated within a span of ten seconds. Furthermore, γδT cells were empirically demonstrated to possess the capability to lyse autologous primary tumor kidney cells, a phenomenon alleviated upon the application of CMA (66). From the above, it can be deduced that the perforin/granzyme pathway occupies an irreplaceable position in the cytotoxic activity of γδT cells (67) ( Figure 1 ).

Antibody-dependent cell-mediated cytotoxicity (ADCC) constitutes another crucial mechanism (68) ( Figure 1 ). Classified by their maturation levels, γδT cells categorize into four functionally distinct subpopulations: naïve γδT cells, central memory γδT cells, effector memory γδT cells, and terminally differentiated effector memory γδT cells (69). The latter two subpopulations express CD16, a surface receptor that efficiently facilitates tumor cell killing, even in the absence of antibody engagement (68, 70). Vγ9Vδ2 T cells, isolated from PBMCs of healthy donors, undergo activation, leading to the expression of CD16, a phenomenon not observed in their unstimulated counterparts (71). Furthermore, when TCR-activated Vγ9Vδ2 T cells are cross-linked to plastic wells with anti-CD16 monoclonal antibodies, substantial TNF-α production occurs, a response mitigated by the addition of soluble anti-CD16 monoclonal antibodies (71).

The cytotoxic activity of γδT cells is also ascribed to the expression of TRAIL and FasL, which bind to corresponding receptors on tumor cells (72, 73) ( Figure 1 ). TRAIL’s interaction with different receptors produces varied outcomes: knockdown of TRAIL-Receptor 4(TRAIL-R4) in Colo357 and MDA-MB-231 cells significantly reduces sensitivity to γδT cell-induced cytotoxicity, whereas TRAIL-R4 knock-in HeLa cells show reinforced cytotoxicity (74). Furthermore, serum TRAIL levels hold clinical significance, as evidenced in a study involving eighteen patients with refractory prostate cancer, where higher serum TRAIL levels at nine months correlated with improved clinical outcomes (75). Additionally, the upregulation of Fas on the surface of osteosarcoma cells effectively increases the cytotoxicity of γδT cells (76).

Finally, γδT cells serves as potent producers of IFN-γ and TNF-α, exerting anti-tumor effects through various mechanisms, including the inhibition of tumor vascular growth (77) ( Figure 1 ). Blocking TNF-α or its receptor significantly diminishes cytotoxicity, while knocking down miR-125b-5p could increase the secretion of IFN-γ and TNF-α, thereby enhancing anti-tumor effects (78, 79). Studies focusing on solid tumors, particularly breast cancer, nasopharyngeal carcinoma, and melanoma, have demonstrated a positive correlation between the production of TNF-α by peripheral γδT cells and their contribution to tumor defense (77).

NK cells have consistently been acknowledged as effector cells proficient in lysing tumor cells or viruses, albeit with a non-specific targeting of cells. Upon recognizing target cells, NK cells exhibit directed movement of their abundant granules toward the binding site of target cells with the assistance of dynein motors (80, 81). The aggregation of these granules enhances efficiency in secretion while reducing the killing of surrounding cells. However, the cytotoxic impact of granules is contingent upon the presence of perforin. Mouse experiments have demonstrated that defective perforin leads to diminished cellular cytotoxicity, expedited tumor growth, and heightened metastasis, underscoring the crucial role of perforin in this process (82–84) ( Figure 1 ). Currently, it is possible to induce the expression granzymes and perforin genes to augment the cytotoxic effects of NK cells.

Termed as “serial killers”, NK cells frequently shift towards cell destruction contingent upon FasL and TRAIL once their reservoirs of granzymes and perforin are depleted (85, 86) ( Figure 1 ). Subsequent investigations have revealed NK cells deficient in perforin, previously considered lacking cytotoxicity, effectively eliminate MHC class I-deficient tumor cells due to the upregulation of FasL (87). FasL, in turn, interacts with the CD95 receptor on target cells, thereby initiating the apoptotic signaling cascade intrinsic to target cells (88). Intriguingly, the cleaved soluble form of FasL proves to be devoid of cytotoxic efficacy. Furthermore, NK cells harvested from the murine hepatic milieu distinctly express TRAIL, with their cytotoxicity potential markedly attenuated upon the introduction of anti-TRAIL monoclonal antibodies (85, 89).

Besides, IFN-γ secreted by NK cells has been demonstrated independently to exert anti-tumor functions, irrespective of perforin ( Figure 1 ). Its collective influence plays a crucial role in governing the initiation, proliferation, and metastasis of tumors (90). Furthermore, while the specific anti-tumor mechanism of IFN-γ in particular tumors remains incompletely understood, its capabilities to inhibit tumor angiogenesis and modulate the sensitivity of tumor cells have long been reported (89).

In contrast, neoplastic cells may indeed serve as a crucial force driving the anti-tumor effects innitiated by NK cells ( Figure 1 ). In melanoma, lactate derived from tumor cells significantly reduces the quantity and activation of NK cells. This is accomplished by suppressing the upregulation of the nuclear factor of activated T cells (NFAT) with NK cells, leading to a noticeable reduction in IFN-γ production and a simultaneous alleviation of the cytotoxic impact on tumors (91). In another study, it was revealed that lactate derived from tumor cells also directly diminishes the expression of perforin and granzyme, thereby impeding their cellular lytic functionality (92).

ILCs earn their name due to their absence of adaptive antigen receptors. In addition to NK cells, other subsets include ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer cells (LTi) (93). They predominantly inhabit tissues and maintain close associations with the extracellular matrix (93). Typically, within the tumor microenvironment, ILC1s release significant levels of IFN-γ. This cytokine acts on tumor cells, inducing the upregulation of MHC-I and MHC-II, thereby directly stimulating tumor cell apoptosis and pyroptosis (93). Moreover, both ILC1s and ILC3s possess the ability to directly eliminate tumor cells by expressing TRAIL, thereby imbuing these cells with the potential of anti-tumor effector cells (94) ( Figure 1 ).

While traditionally recognized for their potent anti-tumor effects, γδT lymphocytes also possess the potential to accelerate the progression and invasive tendencies of solid tumors (140). Nonetheless, compelling evidence suggests that γδT cells may expedite the development and invasion of solid tumors (141). At the core of their tumorigenic impact is the pivotal mediator IL-17, a molecule that not only drives neoplastic cell proliferation through intricate IL-6/STAT3 and NF-κB signaling cascades but also triggers metastasis by inducing the secretion of VEGF and MMP (142). Furthermore, under specific circumstances, epithelial Vδ1 T cells have been observed to secrete notable quantities of TGF-β, initiating the transformation of epithelial cells into mesenchymal cells and thereby amplifying the invasiveness of malignancies (143) ( Figure 2 ).