Pleural effusion, characterized by the accumulation of fluid in the pleural space, can be categorized as either benign or malignant. Benign pleural effusion commonly occurs due to congestive heart failure, infection and connective tissue diseases. Malignant pleural effusion (MPE) is frequently associated with advanced stages of lung cancer [1]. MPE is characterized by the presence of malignant cells and a significant number of lymphocyte cells, including αβ T cells (CD4+ and CD8+ T cells) [2], [3], [4], [5], [6], NK cells [7] and γδ T cells [8] within the pleural space. γδ T cells have showing promising antitumor activity and potent cytotoxicity in preclinical models of various solid cancers and hematological malignancies [9], [10], [11]. Unlike αβ T cells, γδ T cells can recognize antigens independently of major histocompatibility complex (MHC) peptide presentation [12]. Moreover, γδ T cells exhibit characteristics of both innate and adaptive immunity, enabling rapid activation, cytotoxicity production, migration to peripheral tissues, and interaction with other immune cells [12].

Human γδ T cells can be classified into Vδ1 and Vδ2 subsets based on the type of Vδ chain [13]. Vδ2 T cells, primarily found in peripheral blood, exert potent antitumor activity [14]. In contrast, Vδ1 cells are mainly present in tissues, including solid tumor infiltrates [15]. Both subsets possess cytotoxic effector capacities, leading to the secretion of IFN-γ, perforin, or granzymes [16]. Regulatory receptors on γδ T cells, such as PD-1 and TIM-3, play crucial roles in regulating inflammatory responses by inhibiting cytotoxic effector [17]. Additionally, the expression of CD56 serves as an activation marker in lymphocytes [18], [19] and CD56+γδ T cells have demonstrated efficacy in combating human squamous cell carcinoma and producing IFN-γ [20]. Wei. et al observed IL-17A-producing γδ T cells inhibit MPE formation in mouse models with MPE [8]. However, the phenotypic characterization of γδ T cells from MPE patients remains unexplored.

The migration of γδ T cells to target organs is facilitated by soluble chemokines and their corresponding receptors. In rheumatoid arthritis synovium, γδ T cells are recruited to the joint cavity by CCR5-CCL3/CCL4 and CXCR3-CXCL9/CXCL10 [21]. In pancreatic ductal adenocarcinoma, γδ T cell recruitment depends on elevated levels of CCR2, CCR5, and CCR6 [22]. In MPE, the concentrations of CCL2 and CCL20, the main ligands of CCR2 and CCR6, were found to be significantly higher compared to transudative pleural effusion (PE), tuberculous pleural effusion (TPE), and corresponding blood [23], [24]. These chemokines are associated with pleural mesothelial cells (PMCs) permeability [25], [26], [27] and MPE formation [28]. However, the recruitment of γδ T cells are to the pleural space in MPE is still unknown.

Complement C5a, known for its dual effects on tumors, can influence tumor cell proliferation, migration, and invasion by inhibiting apoptosis [29], while also enhancing the immune response through the attraction and activation of immune cells such as natural killer cells and macrophages [30]. Furthermore, C5a can stimulate the release of inflammatory cytokines and chemokines, further enhancing the anti-tumor immune response [30]. Our previous research has shown significantly elevated levels of C5a in MPE compared to PE and induced PMCs and monocytes to secrete cytokines and chemokines such as CCL2 and CCL7 in MPE [26], [28]. However, the role of C5a in regulating the cytotoxicity and chemokine production of γδ T cells in MPE remains unclear.

In view of the above considerations, the major focus of this study is to elucidate the phenotypic characterization of γδ T cells in MPE, investigate the recruitment of circulating γδ T cells to the pleural cavity, and explore the involvement of C5a in regulating these processes.