Introduction 

The tumor microenvironment (TME) is a highly complex and dynamic ecosystem comprising malignant cells, stromal cells, extracellular matrix (ECM), and diverse immune-cell populations. Increasing evidence indicates that interactions between these cellular compartments are critical determinants of tumor progression, immune evasion, therapeutic response, and patient outcomes. Among the stromal components, cancer-associated fibroblasts (CAFs) are particularly important in shaping the architecture and functional properties of the TME. Rather than representing a homogeneous population, CAFs comprise multiple phenotypically and functionally distinct subsets with specialized roles in ECM remodeling, tumor growth, angiogenesis, and immune regulation.1,2 

One emerging CAF population of particular interest is the immune-interacting CAF (imCAF) subset. These fibroblasts actively communicate with immune cells through chemokines, cytokines, surface molecules, and ECM-associated mechanisms, thereby influencing immune-cell recruitment, localization, activation, and retention [3]. Although the role of imCAFs has increasingly been recognized in highly desmoplastic tumors such as pancreatic ductal adenocarcinoma (PDAC), hepatocellular carcinoma, and breast cancer, their contribution to immune regulation in lung adenocarcinoma (LUAD) remains comparatively less well characterized.4-7 

CAF heterogeneity and immune regulation 

CAFs constitute a major component of the desmoplastic stroma observed in many solid tumors. Their heterogeneity is reflected by differences in cellular origin, molecular phenotype, spatial localization, and biological function.1,2 Distinct CAF populations can either promote or restrain tumor progression, emphasizing that CAFs should not be considered exclusively tumor-promoting cells. 

In several solid malignancies, immune-interacting CAFs have been associated with profound alterations in the immune composition of the TME. CAF-derived chemokines and cytokines can establish localized gradients that regulate the recruitment and positioning of T cells and other leukocyte populations. CAFs may also influence immune-cell function through antigen presentation, expression of immunoregulatory molecules, and remodeling of the ECM, which can physically restrict immune-cell access to tumor cells.3-7 

Studies in PDAC have identified inflammatory fibroblasts and myofibroblasts as distinct populations with different effects on tumor progression and immune responses.6 Similarly, mesothelial cell-derived antigen-presenting CAFs can promote expansion of regulatory T (Treg) cells, illustrating the capacity of specialized fibroblast populations to actively establish immunosuppressive niches.7 These observations suggest that the functional consequences of CAF activation depend substantially on CAF subtype and tissue context. 

Immune-interacting CAFs in lung adenocarcinoma 

Lung cancer remains a leading cause of cancer-related mortality, with adenocarcinoma representing the most prevalent form of non-small-cell lung cancer (NSCLC).8 Invasive LUAD is frequently associated with a prominent stromal response, providing a favorable environment for the accumulation and functional specialization of CAFs. 

The lung TME is characterized by dynamic changes in immune-cell composition during tumor development. Stage-dependent differences in the distribution of inflammatory and immune cells suggest that tumor-associated stromal niches actively influence antitumor immunity.10 Importantly, fibroblasts within lung tumors express multiple regulatory molecules, cytokines, and chemokines capable of modifying tumor-infiltrating lymphocyte behavior.10-13 

Fibroblast-derived CCL19, for example, has been associated with the organization of local antitumor T-cell responses in lung carcinoma.12 Conversely, specific CAF populations can contribute to T-cell exclusion through ECM remodeling and spatial organization of the tumor stroma.13  Single-cell and spatial analyses have further demonstrated that distinct fibroblast populations are associated with particular molecular and immunological phenotypes of lung cancer and may have independent prognostic significance.1,14,15 

These findings highlight an important concept: the spatial distribution and functional phenotype of CAFs may be as important as their abundance. Consequently, understanding CAF heterogeneity may provide a more accurate framework for predicting tumor behavior and therapeutic response than assessing total fibroblast content alone.1,14,15 

CAF regulation of regulatory T cells 

Among tumor-infiltrating immune populations, Treg cells are particularly important mediators of immune suppression. By suppressing effector T-cell activity and other antitumor immune responses, Tregs can facilitate tumor immune escape and progression.16 Their abundance and spatial localization within tumors have therefore attracted considerable interest as potential determinants of prognosis and therapeutic response. 

Evidence from melanoma, breast, ovarian, and other solid tumors indicates that specific CAF subsets can promote Treg recruitment, retention, and activation.5,17,18 These effects may be mediated through chemokine signaling, direct cell–cell interactions, and specialized receptor–ligand networks. For example, CAF-derived CXCL12 signaling has been implicated in the establishment of immunosuppressive fibroblast niches in ovarian cancer.17 In addition, direct immunological interactions between CAFs and Tregs may contribute to the functional activation of Tregs and promote tumor development.18 

A similar relationship has been observed in LUAD. Tregs preferentially accumulate within the tumor stroma, and their coexistence with CAFs has been associated with poorer clinical outcomes.19 However, the precise CAF populations responsible for Treg recruitment and the molecular mechanisms governing this interaction have remained incompletely understood. 

The CXCL9–CXCR3 axis links imCAFs to immunosuppressive Tregs 

Recent evidence provides important mechanistic insight into the relationship between imCAFs and Tregs in LUAD. A distinct population of immune-interacting CAFs can promote the recruitment of CXCR3-positive Treg cells into the tumor microenvironment. This interaction is mediated, at least in part, through the CXCL9–CXCR3 chemokine axis. 

CXCL9 is a chemokine involved in the recruitment and spatial organization of CXCR3-expressing immune cells. Within the LUAD microenvironment, imCAF-derived CXCL9 can generate a chemotactic signal that facilitates the localization of CXCR3+ Tregs within tumor-associated stromal niches. Once recruited, these Tregs can contribute to local immune suppression through their immunoregulatory functions. 

This mechanism provides a potential explanation for how a stromal cell population can indirectly promote tumor progression by reshaping the immune compartment. Rather than acting solely through ECM deposition or direct effects on malignant cells, imCAFs can function as organizers of immunosuppressive immune niches. 

Experimental observations further indicate that the imCAF–CXCR3+ Treg interaction is associated with increased tumor burden in experimental models. Importantly, comparable associations between these cellular populations and adverse clinical outcomes have been observed in human LUAD. Together, these findings support the potential clinical relevance of the stromal–immune pathway. 

Clinical and therapeutic implications 

The identification of an imCAF–CXCL9–CXCR3+ Treg axis has several potential therapeutic implications. Conventional immunotherapy primarily focuses on activating or reinvigorating antitumor immune cells, particularly through immune-checkpoint blockade. However, the efficacy of such approaches may be limited when immunosuppressive stromal niches prevent effective immune-cell activation or access to tumor cells. 

Targeting CAFs or their interactions with immune cells could therefore represent a complementary therapeutic strategy. Importantly, the objective should not necessarily be the indiscriminate depletion of all CAFs, because CAF populations can have distinct, and sometimes tumor-restraining, functions.1-3 Instead, selective targeting of immunosuppressive CAF subsets or their specific signaling pathways may provide greater therapeutic precision. 

The CXCL9–CXCR3 pathway represents one potential target. Modulation of this axis could theoretically reduce the recruitment or retention of immunosuppressive CXCR3+ Tregs within tumors and thereby enhance local antitumor immunity. However, CXCL9 signaling also participates in the recruitment of effector immune cells, highlighting the complexity of targeting chemokine pathways. Therapeutic strategies will therefore need to account for cellular context, spatial distribution, and the balance between beneficial and detrimental immune responses. 

Integration of single-cell transcriptomics, spatial transcriptomics, multiplex imaging, and functional studies may help identify CAF populations that are most strongly associated with immune suppression and treatment resistance. Such approaches could also facilitate the development of biomarkers capable of identifying patients most likely to benefit from stromal-targeted or combination immunotherapies.1,14,15 

Future perspectives 

Several important questions remain regarding CAF-mediated immune regulation in LUAD. The cellular origins and developmental trajectories of distinct imCAF populations require further investigation. CAFs may arise from multiple precursor populations, and their phenotype may be influenced by tumor-derived signals, inflammatory cytokines, hypoxia, and ECM composition. Defining these trajectories could reveal opportunities to prevent the establishment of immunosuppressive CAF niches at an early stage. 

The spatial organization of imCAFs and Tregs is another important area of investigation. Because chemokine signaling and cellular interactions are strongly influenced by tissue architecture, spatial profiling may provide information that cannot be obtained from bulk or single-cell analyses alone. Furthermore, understanding how imCAF–Treg interactions change following chemotherapy, radiotherapy, targeted therapy, or immune-checkpoint blockade could identify mechanisms of treatment resistance. 

Ultimately, the emerging evidence supports a shift from viewing CAFs as a relatively uniform structural component of tumors toward recognizing them as specialized regulators of tumor immunity. In LUAD, the interaction between imCAFs and CXCR3+ Tregs represents an important example of how stromal heterogeneity can influence immune suppression and disease progression. 

Conclusion 

CAFs are key regulators of the LUAD microenvironment, but their effects are highly dependent on cellular subtype and spatial context. Immune-interacting CAFs appear to have a particularly important role in establishing immunosuppressive niches by regulating T-cell recruitment and function. The CXCL9–CXCR3 axis connecting imCAFs with CXCR3+ Tregs provides a mechanistic link between stromal heterogeneity and impaired antitumor immunity. Further characterization of this pathway may improve prognostic stratification and support the development of CAF-directed or combination immunotherapeutic strategies. A more precise understanding of CAF biology may therefore be essential for overcoming stromal barriers to effective cancer immunotherapy. 

References 

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