The initial step in immunosurveillance is the recognition of tumor associated antigens in the TME. Dendritic cells (DCs) play a key role in this process. After engulfing TAAs, DCs process and present these peptides on their surface MHC molecules before migrating to draining lymph nodes to function as antigen-presenting cells (APCs) (44). The migratory process is initiated by chemotaxis and is guided by the CCR7 receptor on DCs, which binds to CCL21—a ligand constitutively expressed by lymphatic endothelial cells. Ongoing inflammation in the TME releases TNF-α, which further elevates CCL21 levels. This increase in CCL21 in turn, promotes LFA-1 activation in DCs, mediating their transendothelial migration into the afferent lymphatic vessels, ultimately guiding them to the draining lymph nodes (41).

Once in the lymph nodes, DCs orchestrate an effective antitumor response by priming and activating naïve tumor-specific CD4+ and CD8+ T cells. These naïve T cells rely on LFA-1 to migrate and enter the lymph nodes through HEVs. Chemokines CCL19 and CCL21 from the lymph nodes as well as within HEVs, facilitate T cell migration by binding to CCR7 on T cells, triggering inside-out signaling that activates LFA-1. Once activated, LFA-1 on T cells binds to ICAM-1 on HEVs enabling their transendothelial entry into the lymph nodes (19). Within the lymph nodes, LFA-1 further aids effective antigen scanning by T cells, helping them locate their cognate antigen presented by APCs (48). Here, the adhesive state of LFA-1 in T cells is regulated by bidirectional signaling—inside-out signaling adjusts its affinity, and outside-in signaling provides feedback from LFA-1 engagement (49, 50). This dual signaling mechanism modulates T cell adhesion, ensuring that T cells migrate effectively within lymphoid tissue, optimizing their surveillance and interactions with tumor antigens. Notably, unlike chemokine-induced inside-out signaling, which alone can activate LFA-1 to a high-affinity state in T cells, full LFA-1 activation through TCR signaling requires LFA-1 to bind to ICAM-1 on APCs (51). In DCs, however, regulation of LFA-1’s activity plays a key role in DC/T cell conjugate formation, with mature DCs expressing elevated levels of cytohesin-1–interacting protein (CYTIP) to maintain LFA-1 in an inactive state (48). This inactivity is essential for the release of T cells and their subsequent priming. Additionally, the lower lateral mobility of ICAM-1 on mature DCs compared to immature DCs increases the likelihood of binding between LFA-1 on naïve T cells and ICAM-1 on mature DCs (52). The strength of LFA-1 binding to ICAM-1 on DCs, combined with CD2/CD58 interactions, improves T cell antigen discrimination (53). This process likely ensures that T cells bind specifically to non-self-antigens, including tumor antigens, through their TCR. Furthermore, LFA-1 binding to ICAM-1 within the lymphoid parenchyma increases T cell retention in the lymph nodes, preventing their immediate exit and allowing more time for T cells to encounter their cognate antigen (54). While our review paper focuses on the role of integrins, it is important to note that interstitial migration of leukocytes within lymph nodes may occur independently of integrins, as has been extensively demonstrated in other studies (55–58).

In vitro studies indicate that LFA-1/ICAM-1 interactions may promote CD4+ T cell differentiation towards the Th1 phenotype (59). Th1 cells are crucial for tumor elimination as they produce TNF-α and IFN-γ, which stimulate the recruitment and activation of cytotoxic CD8+ T cells (CTLs), NK cells, and monocytes to the tumor microenvironment, while also negatively regulating tumor-promoting Th17 cells (60). CD4+ T cells further support CD8+ T cell expansion by forming LFA-1-dependent transient homotypic synapses with adjacent activating CD8+ T cells (61, 62). Following differentiation, tumor-specific effector T cells exit the lymph nodes and migrate to peripheral tissues to perform their immune functions.

Unlike T cells though, NK cells can directly identify tumor cells without prior priming by APCs. NK cells become “educated,” meaning they gain the ability to recognize tumor cells during their development. Mature NK cells are characterized by high LFA-1 expression, as well as the expression of receptors such as NKp46, NKG2D, CD16, and elevated levels of perforin and granzyme B (63, 64). LFA-1, along with other cell-surface adhesion receptors including Mac-1, regulate the priming of NK cells and facilitates their homing and migration to both lymphoid and non-lymphoid organs (65, 66).

T cells exit the lymph nodes via the efferent lymphatic vessels and drain into the bloodstream through the thoracic duct (67). Their exit from the bloodstream into the TME is dependent on the interaction between LFA-1 and ICAM-1 on endothelial cells. A recent study in mice models highlighted the intriguing influence of the circadian clock on the transmigration of CTLs into tumors, with increased ICAM-1 expression on endothelial cells being observed in the morning, thereby enhancing T cell infiltration during this time (68). Similarly, NK cells are recruited to the tumor site through inflammation-induced signaling in response to interleukins (IL-12, IL-15, and IL-18) from target cells, with LFA-1 playing a crucial role in their migration (69, 70).

In addition, tumor-associated HEVs exhibit elevated ICAM-1 expression (71, 72), which may further facilitate lymphocyte infiltration into tumors and support the formation of tertiary lymphoid structures. Notably, a study of resected primary human breast carcinomas revealed that the density of ICAM-1-expressing HEVs predicted both the extent of T and B cell infiltration and clinical outcomes in breast cancer (72).

Within the TME, migrated CTLs recruit LFA-1 to engage with ICAM-1 on tumor cells, forming a cytolytic synapse ultrastructure that facilitates their effector functions. This includes the directed delivery of cytotoxic granules, which contain granzymes and perforin, through secretory clefts of the mature lytic synapse. The efficient delivery of these cytolytic granules depends on the arrangement of the microtubule-organizing center (MTOC), which is also influenced by Mg²⁺ and Ca²⁺ (73–75). Importantly, the conformational activation of LFA-1 is essential for licensing the exocytosis process, which enhances the local concentration of cytotoxic molecules to optimize tumor cell recognition and potentiate the antitumor immune response (74). What is more, earlier studies have underscored the importance of functional CTLs in antitumor responses. In particular, a study involving LFA-1-deficient mice demonstrated that these mice were unable to clear tumors injected into their footpads, which was attributed to a defective CTL response against the tumor cells (76). LFA-1 is also crucial for the mechanosensitive recognition of tumor cells by NK cells and facilitates the formation of lytic synapses between them (65). Upon successful synapse formation, NK cells release lytic granules, including perforin and granzymes (77, 78). Perforin creates pores in the target cell membrane, allowing granzymes to enter and trigger apoptosis, effectively killing tumor cells. The efficiency of NK cell degranulation and cytotoxicity is influenced by LFA-1 and a high Ca²⁺ influx (79). Moreover, activation with IL-2 further enhances NK cells’ broad antitumor cytotoxicity. NK cells can also destroy tumors through Antibody-Dependent Cellular Cytotoxicity (ADCC), where Fc receptor-bearing NK cells recognize and kill antibody-coated tumor cells. However, in the absence of LFA-1, NK cells are unable to effectively destroy their targets. A study of four patients with LFA-1 deficiency supports this, showing that their NK cells exhibited impaired cytotoxicity in cellular assays, despite retaining functional Fc receptors (80).

From a functional perspective, the role of LFA-1 in mediating immune surveillance is heavily dependent on its proper activation within T cells. Several proteins and factors regulate LFA-1 activation, conformational changes, and functions. For instance, layilin, a membrane glycoprotein expressed by certain tumor-specific CTLs, enhances LFA-1 activation by stabilizing its interaction with ICAM-1. Together with talin, layilin supports LFA-1-mediated adhesiveness and the cytotoxic activity of CTLs, playing a critical role in promoting immune synapse formation and tumor cell killing, as demonstrated in melanoma (81). However, further research is needed to fully elucidate the layilin-mediated pathways affecting LFA-1 expression and activation before these findings can be applied to therapeutic interventions. Also, certain co-receptors, such as CD226 (DNAM-1), regulate LFA-1 function by enhancing its adhesive properties through the regulation of its affinity conformation. Evidence shows that the loss of CD226 impairs LFA-1’s high-affinity conformation in CTLs, a change associated with malignancy, including multiple myeloma and lung adenocarcinoma (82). In CLL, T cells exhibit significantly reduced LFA-1 expression or impaired LFA-1 function, leading to their failure to adhere to and transmigrate across VCAM-1, ICAM-1, and CXCL12-expressing endothelium, which results in diminished trafficking to lymph nodes (83, 84). Taken together, it is apparent that proper functioning of LFA-1 is crucial for optimal antitumor CTL responses during immunosurveillance.

The role of LFA-1 in mediating immune surveillance against cancer by innate immune cells beyond NK cells remains underexplored. Nevertheless, some studies suggest that certain tumors are targeted by specific subsets of monocytes and neutrophils, despite differences in LFA-1 regulation and activation between these cell types (85). For example, through intravital microscopy studies in mice, non-classical monocytes have been shown to patrol the vasculature and maintain endothelial homeostasis by scavenging microparticles (42). This patrolling behaviour is facilitated by the expression of LFA-1 on these cells, enabling interactions with ICAM-1-rich endothelial cells (42). In lung cancer mouse models, these “intravascular housekeepers” have been observed removing metastatic tumor cells from vascularized tissues (86). Upon detecting primary tumor invasion, non-classical monocytes decrease their patrolling activity, migrate toward tumor particles, and engulf them (86). Similarly, tumor-entrained neutrophils (TENs) can suppress metastatic seeding in the premetastatic lung in mice underscoring their roles in limiting tumor spread (87). In addition, ICAM-1 expression on breast cancer cells in mice facilitates interactions with neutrophils that suppress lung metastasis, independent of cytotoxic T cell activity (88).

Leukocyte trafficking into the TME relies heavily on cell movement toward chemokine gradients and the use of integrins like LFA-1 for adhesion to vasculature and subsequent extravasation into the tumor (43, 97). However, there is evidence that tumors can disrupt these processes. For instance, non-inflammatory tumors with inactivated LFA-1 exhibit impaired T cell priming and reduced recruitment into the TME (98, 99). Although the exact mechanisms of LFA-1 inactivation are not fully understood, these tumors are known to release chemokines such as vascular endothelial growth factor (VEGF), which suppress the expression of endothelial ICAM-1, a crucial ligand for LFA-1 (100–102). This suppression hinders LFA-1-mediated leukocyte recruitment into the TME. Moreover, reduced secretion of chemokines like CXCL12 further impairs T cell infiltration by decreasing the inside-out signaling required for effective LFA-1 activation (98). Normally, these chemokines promote LFA-1 expression on the cell surface and enhance the ability of T cells to infiltrate the TME (98). A recent study on human melanoma patients revealed that tumors secrete the cytokine growth differentiation factor 15 (GDF-15), which hinders LFA-1-mediated adhesion of T cells to activated endothelial cells—a crucial step for T cell extravasation (103). GDF-15 does this by curtailing the interaction of LFA-1 with the actin cytoskeleton via the protein talin. The study also emphasized the significance of LFA-1 in the success of immunotherapy, showing that elevated serum GDF-15 levels in cancer patients were linked to poor outcomes following PD-1 therapy, and that inhibiting GDF-15 improved T cell recruitment and enhanced therapeutic efficacy (103).

The complement receptor C3aR has also been shown to interfere with LFA-1 activation in T cells and other leukocytes, disrupting their trafficking to tissues (104). In line with this, research by Nandagopal et al. revealed that high levels of the inflammatory factor C3a, released within the TME, bind to C3aR, causing the accumulation of high-affinity LFA-1 at the uropod (tail region) of NK cells (105). This suboptimally increases NK cell adhesion to the endothelium, thereby preventing effective infiltration into the TME (105). Also, impaired LFA-1 function in non-classical monocytes results in inefficient killing of metastatic tumor cells during vasculature patrolling (86). These monocytes fail to express Kindlin-3, a key intermediary molecule required for LFA-1 activation, leading to dysfunctional LFA-1 signaling. Consequently, the weakened LFA-1:ICAM-1 interactions cause the monocytes to ‘slip’ rather than firmly adhere to the endothelium, reducing their effectiveness in infiltrating and killing metastatic cells (86).

Regulating cell adhesion and motility is a complex process that involves the intracellular trafficking of integrins to and from adhesion sites, particularly in fast-moving cells like leukocytes (84, 106). In CLL, T cell movement is disrupted due to impaired LFA-1 resulting from continuous direct contact with the tumor cells. Although the precise receptors on T cells and ligands on CLL tumors involved in transmitting these immunosuppressive signals are not fully understood, this contact alters Rho GTPase signaling in T cells by downregulating RhoA and Rac1 and upregulating Cdc42, leading to defective T cell adhesion and motility (84). Further research is needed to fully understand how these impaired T cells migrate into secondary solid tumor niches and whether this process is equally impaired in CLL patients.

After infiltrating the tumor microenvironment, leukocytes are met with a barrage of dysregulating factors from the tumor that impair their ability to effectively use LFA-1 for immunological synapse formation and cytotoxic functions. These disruptions occur through multiple mechanisms, including altered LFA-1 activation and clustering, impaired shifting of LFA-1 to its high affinity conformation, and changes in the mechanical properties of the tumor cell surface (46, 47, 107, 108). Collectively, these factors significantly hinder the leukocyte’s capacity to engage and destroy tumor cells via LFA-1 within the TME.

In solid tumors, metastatic extravasation involves increased invasiveness through adhesion and transendothelial migration of disseminated cancer cells (126). Tumor-associated leukocytes, such as pro-tumorigenic macrophages, can exacerbate this process. For example, in nasopharyngeal carcinoma, tumor-associated macrophages (TAMs) interact with interferon-stimulated gene 15 (ISG15) via LFA-1 (127). This interaction triggers SRC family kinase (SFK) signaling in the TAMs, leading to the secretion of CCL18 into the TME, which plays a crucial role in promoting tumor cell dissemination (127). Equally, tumor-associated neutrophils (TANs) are known to promote tumor growth and angiogenesis in breast cancer. A study using murine models showed that estradiol increased pro-tumorigenic neutrophil recruitment to the tumor site, in a process facilitated by the overexpression of TGFβ1 and LFA-1 (128).

Microglia, the resident immune cells in the brain that express LFA-1, also play a crucial role in the TME, contributing to tumorigenesis and neoplastic cell growth in low-grade gliomas (LGGs) (129). Although the exact mechanism by which LFA-1 interaction mediates metastasis in LGGs is unclear, genetic ablation of CD11a (a subunit of LFA-1) has been shown to reduce microglia infiltration, proliferation, and CCL5 secretion, thereby decreasing the likelihood of LGG progression (129).

In CLL, leukemic B cells exhibit enhanced adhesion due to LFA-1 activation, which is triggered by CXCL12, leading to the dissemination of CLL cells to secondary lymphoid organs (130, 131). During the pathogenesis of CLL, CXCL12 activates protein tyrosine kinase (PTK), Bruton’s tyrosine kinase (BTK), and Janus Kinase (JAK), which in turn activate the small GTPase RhoA (132). This signaling cascade shifts LFA-1 from an inactive to a high-affinity conformation, thereby promoting adhesion to ICAM-1 and spreading of CLL B cells into secondary niches (131). Elsewhere, human peripheral blood mononuclear cells (PBMCs) from healthy donors or CLL patients were injected into non-irradiated mice, and three hours later, cells were collected from lymph nodes, bone marrow, and spleen. Using flow cytometry, the study found that CLL B cells had reduced LFA-1 expression, impairing homing to lymph nodes but allowing re-entry into the bone marrow via VLA-4 (83). Both normal and CLL B cells could home to the spleen independently of these integrins, suggesting that integrin targeting may block CLL cells from survival niches in lymph nodes and the bone marrow (83).

T regulatory cells (Tregs) serve as an obstacle to effective antitumor immunity (133), and there is evidence suggesting that LFA-1 may influence tumorigenesis through them. A study using LFA-1 knockout mice demonstrated that the absence of LFA-1 significantly inhibited tumor growth in both subcutaneous melanoma and intestinal adenocarcinoma models (134). This tumor suppression was linked to a substantial decrease in Treg cell numbers in the spleen, blood, and mesenteric lymph nodes. Similarly, treatment of tumor-bearing wild-type mice with the LFA-1 inhibitor BIRT377 reduced both tumor growth and Treg cell accumulation. Moreover, analysis of tumor databases revealed a positive correlation between LFA-1 expression, Treg cell infiltration, and tumor progression. These findings suggest that LFA-1 contributes to a pro-tumorigenic microenvironment by promoting Treg cell-mediated immune suppression, drawing attention to its potential as a therapeutic target for cancer immunotherapy.

Interestingly, certain tumors progress and metastasize by leveraging LFA-1’s interaction with its other ligands, particularly ICAM-3 (135, 136). Shen et al. demonstrated that in human lung carcinoma and breast adenocarcinoma, signaling through ICAM-3 promoted cancer cell stemness (135). Their investigation revealed that ICAM-3 upregulation in these tumors correlated with higher tumor grades and played a critical role in driving metastasis via LFA-1 (136). They further showed that ICAM-3 binds LFA-1 through its extracellular domain and associates with the structural protein ERM, while its intracellular domain links to lamellipodia. This interaction generates mechanical tension, facilitating cell separation and metastasis (136).

ICAM-2 has also been implicated in angiogenesis, as shown in a study using ICAM-2–deficient mice and endothelial cells. The absence of ICAM-2 impaired angiogenesis both in vitro and in vivo by disrupting homophilic interactions that likely contributed to endothelial tube formation (137). ICAM-2–deficient cells exhibited defective migration and increased apoptosis in response to stressors such as serum deprivation, anti-Fas antibody, or staurosporine, underlining its critical role in supporting endothelial cell function during angiogenesis (137). In contrast to its role in promoting angiogenesis, ICAM-2 expression in neuroblastoma has been linked to a reduced metastatic potential. A study using neuroblastoma cells from mouse models demonstrated that ICAM-2 interacted with α-actinin, which strengthened the membrane-cytoskeleton link (138). This interaction reduced cellular invasiveness and motility, thereby supporting a more favorable clinical outcome, suggesting that ICAM-2 might act as a tumor suppressor in neuroblastoma by limiting metastasis. While both studies show ICAM-2’s involvement in regulation of cellular adhesion and actin dynamics, they highlight its context-dependent functions—supporting tumor suppression in neuroblastoma while aiding endothelial migration and survival in angiogenesis.

Altogether, these studies underscore the dual role of LFA-1 in immunosurveillance. They reveal the intricate dependence of leukocytes on this integrin, while also highlighting how tumors and pro-tumorigenic leukocytes, including Tregs, can exploit LFA-1 and its ligands to drive tumorigenesis and metastasis. This duality poses several challenges in understanding LFA-1 dynamics in tumors and leukocytes and complicates the development of LFA-1-targeting therapeutics in oncology.

Increased leukocyte infiltration into tumors has long been linked to improved tumor clearance and better cancer prognosis, a correlation that holds true even following immunotherapy (97, 99, 141, 142). Proof-of-concept studies using mouse models have demonstrated that LFA-1-activating interventions can convert a T cell–exclusionary tumor microenvironment into one enriched with T cells (98). Hickman et al. showed that in melanoma, treatment with 7HP349, a small molecule that activates LFA-1, significantly increased the infiltration of tumor-specific CD8+ T cells into tumors by promoting chemoattraction via CXCL12. Furthermore, when 7HP349 was combined with anti–CTLA-4 therapy, infiltration of CD8+ effector T cells in anti–programmed death 1–resistant (anti–PD-1)-resistant tumors was enhanced, and facilitated tumor regression (98). By activating LFA-1, 7HP349 enhances T cell adhesion to ICAM-1, leading to improved cell spreading and migration from the vasculature into the tumor microenvironment.

In contrast, the drug BIRT377, which inhibits the high-affinity conformation of LFA-1 by blocking domain I in the β2 subunit, enhances NK cell infiltration into colorectal carcinoma and drives tumor regression in mice models (105). This mechanism is particularly important because, as discussed in the previous chapter, Nandagopal et al. demonstrated that C3a/C3aR signaling induces high-affinity LFA-1 in the uropods of NK cells, leading to reduced tumor infiltration, likely due to NK cells becoming trapped on the endothelium (105). By preventing this high-affinity LFA-1 state, BIRT377 effectively reduces excessive adhesion of NK cells to the endothelium and supports them to migrate into the tumor microenvironment.

The immunomodulatory drug lenalidomide has been shown to restore impaired immune synapse formation between T cells and CLL B cells, as demonstrated in in vitro studies using human samples (46). These defective synapses are primarily due to disruptions in the localization of signaling molecules, including the clustering of LFA-1 at the synapse, which impairs F-actin remodeling—a critical process for the formation of mature synapses (46). Lenalidomide normalizes Rho GTPase signaling, thereby restoring proper downstream signaling of LFA-1, which facilitates effective actin dynamics and the formation of functional immunological synapses (46, 84).

Elsewhere, treatment with the monoclonal antibody elotuzumab, which targets signaling lymphocytic activation molecule family member 7 (SLAMF7) on tumor cells, was shown to enhance conjugation between NK cells and ovarian adenocarcinoma target cells in cell culture (139). This was correlated with upregulated expression of active LFA-1 and ICAM-1 on NK cells, potentially accounting for the increased conjugation. Moreover, there was a notable elevation in the levels of the activating receptor NKG2D on NK cells. These changes collectively facilitated improved NK cell adhesion and cytotoxicity against tumor targets (139). On top of that, combining elotuzumab with lenalidomide significantly increased ICAM-1 expression in both NK cells and multiple myeloma cells, surpassing the effects of either treatment alone (140). This elevated ICAM-1 expression was associated with the formation of stable contacts with tumor cells, leading to enhanced NK cell-mediated cytotoxicity. Overall, these reports emphasize the fundamental role of ICAM-1/LFA-1 interactions in leukocyte-driven tumoricidal processes and demonstrate how LFA-1 functions can be enhanced through combinatorial therapy.

As discussed in the previous chapter, LFA-1 can adversely influence tumor progression through its interaction with interferon-stimulated gene 15 (ISG15), which activates tumor-associated macrophages (TAMs) (127). These macrophages subsequently release CCL18, a promiscuous chemokine known to drive tumor progression and metastasis (127). In this regard, Chen et al. demonstrated using in vitro assays and a murine model that inhibiting LFA-1 with the small allosteric inhibitor A286982 effectively blocked CCL18 secretion from macrophages. Additionally, the increased migration of nasopharyngeal carcinoma cells, induced by CCL18 from the supernatants of ISG15-treated macrophages, was significantly reduced following treatment with this LFA-1 inhibitor (127).

In human lung carcinoma and breast adenocarcinoma, tumor progression and metastasis have been linked to the interaction between ICAM-3 and LFA-1 (135, 136). Lifitegrast, a drug that disrupts the binding between LFA-1 and ICAM-3, was shown through in vitro assays and mouse models to inhibit cell migration in both tumor cell lines (136). Furthermore, combining Lifitegrast with an LFA-1 antibody resulted in a significant reduction in tumor cell migration (136).

Taken together, these studies, primarily based on in vitro experiments and in vivo work in murine models, highlight the critical role of targeting ICAM-1/LFA-1 interactions between leukocytes and tumors for therapeutic purposes. This approach can either enhance leukocyte-mediated tumoricidal activity or, conversely, inhibit LFA-1-driven tumorigenesis and metastasis. Moreover, the findings suggest that in certain cancers, LFA-1 functionality can be optimized through combinatorial therapies, offering a more effective treatment strategy. Nonetheless, to fully realize the clinical potential of these approaches, further studies are essential to validate their utility in clinical settings.