Despite the rapid development of delivery platforms and theranostic tools, malignancies are still one of the leading causes of human death globally. Currently, most clinically targeted-therapies mainly target cancer cells, while ignoring their surrounding substances in microenvironments. The microenvironments mainly contain stromal cells (e.g., cancer-associated fibroblasts (CAFs), immune cells, pericytes and endothelial cells), cancer cells, cytokines, chemokines, extracellular matrix (ECM) and vasculature [1]. In malignant tumors, the microenvironments of desmoplastic tumors (e.g., cholangiocarcinoma, pancreatic ductal adenocarcinoma, ovarian cancer, breast cancer, colorectal cancer, prostate cancers and lung cancer) are especially complex composed of abundant stromal cells and deposited ECM, which help establish pathological barrier to hinder the effective transport of therapeutic agents to the cancer site [2,3,4,5,6,7,8,9,10,11]. Meanwhile, the barrier also can compress blood vessels to reduce blood perfusion, further decreasing the delivery and extravasation of immune cells and therapeutic agents. Thus, aiming at the treatment of such tumors, targeting the complex tumors desmoplastic microenvironments are promising therapeutic avenues.

Notably, CAFs are the most abundant of all stromal cells in desmoplastic microenvironments, which contribute to 60–90% ECM proteins and remodel ECM through exerting a physical force on cancer cells as well as depositing, crosslinking and degrading the ECM proteins [1, 12, 13]. The ECM after CAFs remodeling is rigid, which not only serves as pathological barrier to hinder the effective delivery of drug and immune cells infiltration, but also offers scaffold for the migration and invasion of tumor cells [5, 14]. Additionally, CAFs can secret abundant cytokines and ECM proteins that activate associated signaling pathways to talk with immune cells and cancer cells, promoting immune evasion of tumor cells and cancer stem cells (CSCs)-mediated resistance of therapeutic agents [1]. Therefore, CAFs play significant role in mediating the formation of complex tumor microenvironments.

Currently, the therapies of targeting CAFs mediated desmoplastic microenvironments in most of tumors are mainly performed by synthesized nanoparticles (e.g., liposomes, micelles and polymer-based synthetic nanoparticles) that can improve the solubility, efficiency, blood circulation time of chemotherapeutic agents [2]. However, desmoplastic environment presents unique and complex pathological abnormality that limit drug delivery even in nanoscale, so there is a need to a tailored targeted-delivery system for desmoplastic tumors that can penetrate pathological barrier and may be less likely to trigger an immune response. Compared with synthesized nanoparticles, exosomes originate from biological systems can inherit abundant information (e.g., nucleic acids, lipids and proteins) from the patient cells and own good biocompatibility, biodegradability, intercellular communication and low immunogenicity, which endow them with unprecedented potential as carriers for drug delivery [15,16,17]. Additionally, exosomes also exhibit excellent deep penetration due to their specific phospholipid bilayer structure and naturally small size, the mediation of transcytosis, and the carriers with ECM remodeling components, which are suitable for the treatment of desmoplastic tumors [18,19,20]. To gain multiple functional properties, engineered modifications can be flexibly exerted on outer membrane or inner cargoes of exosomes, further improving their specific targeting and accumulation of at desired sites [15, 21, 22]. Meanwhile, with the rapid development of engineered exosomes as communicator or messenger in surrounding cells and substances have endowed them with powerful potential in the theranostic of desmoplastic tumors.

In this review, we illustrate how CAFs can be significant role in tumors desmoplastic microenvironments and their detailed mechanism, while also elaborate that engineered exosomes could target CAFs mediated desmoplastic microenvironment for the effectively delivery and theranostic of such complex tumor in the future.

The significance of CAFs in tumors desmoplastic microenvironmentsHow CAFs constitute the complexity of desmoplastic microenvironment

Desmoplastic tumors with high-grade malignancy are characterized by fibrotic stroma and accompanied by abundant stromal cells and ECM deposition [2,3,4], including cholangiocarcinoma, pancreatic ductal adenocarcinoma, ovarian cancer, breast cancer, colorectal cancer, prostate cancers and lung cancer [5,6,7,8,9,10,11]. The dense stroma can reduce blood perfusion by compressing blood vessels, further decreasing the effective delivery and extravasation of therapeutic agents. Additionally, the surrounding substances of desmoplastic tumors establish a pathological barrier that hinders the effective transport of therapeutic agents to the cancer site, leading to the poor distribution and penetration in desmoplastic tumors [2, 3]. Thus, chemotherapy and nanomedicine therapy failed more than once to treat desmoplastic tumors even though they are sufficient to eliminate tumor cells in vitro experiments.

The complex desmoplastic microenvironments are composed of abundant stromal cells (e.g., CAFs, infiltrating immune cells, pericytes and endothelial cells), cancer cells, cytokines, chemokines, ECM and vasculature [2, 3]. Notably, CAFs, the most abundant of all stromal cells merged in tumor tissues, are activated fibroblasts embedded within ECM during cancer development, which exhibit enhanced proliferative, migratory and secretory properties compared with quiescent fibroblasts and normal activated fibroblasts [1, 12, 23]. The most prominent feature of CAFs is that they can secrete a great deal of ECM proteins and remodel ECM via exerting a physical force as well as depositing, crosslinking and degrading ECM proteins during desmoplastic tumors progression, ultimately leading to the stiffness of ECM and tumors tissue [1, 13]. Subsequently, the CAFs constructed rigid ECM not only serves as pathological barrier to hinder drugs delivery and immune cells infiltration, but also provides a scaffold for the invasion and migration of tumor cells (Fig. 1) [5, 14]. Additionally, CAFs also secrete larger numbers of cytokines and chemokines to instigate immune cells into their friends and establish an immunosuppressive microenvironment [1]. As cancer cells’ henchmen, it’s not surprising that CAFs can talk directly with cancer cells by secreting cytokines and ECM proteins. It is reported that CAFs not only can promote the dedifferentiation of cancer cells toward CSCs, but also participate in the maintenance of CSCs self-renew, further promoting resistance of therapeutic agents [24, 25]. Notably, cancer cells affected by CAFs can secrete cytokines, which further triggering the recruitment and activation of fibroblasts, leading to the accumulation of high activity of CAFs and forming a positive feedback loop [1, 14, 26]. Meanwhile, the accumulation of abundant CAFs also can compress blood vessels and form pathological barriers to impede the effective drugs delivery to the inner of desmoplastic tumors [2]. In view of mentioned above, CAFs can mediate the formation of tumors desmoplastic microenvironments. Therefore, aiming at the treatment of such tumors, targeting CAFs mediated microenvironments are promising therapeutic avenues.

Fig. 1

The significance of CAFs in desmoplastic tumors. CAFs secrete abundant ECM proteins and exert physical force to remodel ECM and establish pathological barrier that hinders drugs delivery and immune cells infiltration and promotes cancer cells invasion. CAFs also secret abundant cytokines and ECM proteins to induce immunosuppress and CSCs. Cancer cells affected by CAFs can secrete abundant cytokines to promote CAFs accumulation that form barrier and compress blood vessels. CSF1, colony-stimulating factor 1; FGF5, fibroblast growth factor 5; EMT, epithelial mesenchymal transition

Heterogeneity of CAFs

CAFs are heterogeneous cell populations with diverse subpopulations and functions in many desmoplastic tumor types, which might be ascribed to their various cellular origins, mainly including tissue-resident fibroblasts, mesenchymal stem cells, quiescent stellate cells, endothelial cells, epithelial cells and adipocytes (Fig. 1) [1, 27,28,29]. These precursor cells are activated, recruited, or trans-differentiated into CAFs under the stimuli of multiple factors in tumors desmoplastic microenvironments, including oxidative stress, local hypoxia, physical changes in the ECM, exosomes and DNA damage during radiation therapy, cancer-derived cytokines such as transforming growth factor-β (TGF-β), interleukin-6 (IL-6), platelet-derived growth factor (PDGF), stromal derived factor 1 (SDF1) and hepatocyte growth factor (HGF) [1, 26, 27, 30]. Activated CAFs can be identified by markers, including but not limited to fibroblast activation protein (FAP), PDGF receptors (PDGFRs) and alpha smooth muscle actin (αSMA) [1, 12, 30]. FAP is a serine protease with overexpression on CAFs in more than 90% of human cancer, which not only takes part in ECM remodeling, but also induces the immunosuppressive microenvironment [31,32,33]. PDGFRs, tyrosine kinase receptors, are mainly classified as two types-PDGFRα and PDGFRβ [34]. PDGF can recruit and activate fibroblasts into a CAF-like state to promote desmoplastic tumor growth and stimulate angiogenesis via interacting with PDGFRs [8, 34]. α-SMA, a cytoskeletal protein, is related to the secretion of TGF-β and the regulation of myofibroblast contractility, which has been served as a most frequent marker for the identification of CAFs and the prognosis factor of desmoplastic tumors [5, 35, 36]. Unfortunately, none of these markers are specifically expressed by CAFs because they are shared with at least one cell subpopulation, which also emphasizes CAFs heterogeneity that mainly exhibited by the existence of various CAF subpopulations in different desmoplastic tumor and accompanied by different markers [28].

In recent years, abundant CAF subpopulations have been identified using single-cell RNA sequencing in desmoplastic tumors and some of them exhibit different markers and biological functions (Table 1). In pancreatic ductal adenocarcinoma, researchers first identified two CAF subpopulations derived from pancreatic stellate cells, myofibroblast CAF and inflammatory CAF, which can dynamically reverse between them in vitro [37]. Among them, myofibroblast CAF expresses high level of αSMA and associated with an ECM-producing, conversely, inflammatory CAF expresses low level of αSMA, which increasingly secretes inflammatory factors (such as IL-6 IL-8, IL-11 and LIF) and characterizes by an inflammatory phenotype. Subsequently, these two CAF subpopulations also were found in breast cancer and cholangiocarcinoma patient samples [38, 39]. Four CAFs subpopulations (CAF-A to CAF-D) were identified in human pancreatic ductal adenocarcinoma with different biomarkers, in which periostin was a biomarker for subtype A and associated with tumor invasion and shorter survival, myosin-11 was biomarker for subtype B and related to lymph-node metastasis, podoplanin was biomarker for subtype C and associated with immune promotion and good prognosis, and no marker was selected in subtype D [40]. In breast cancer and ovarian cancer, four CAF subpopulations, CAF-S1 to CAF-S4, were identified with many markers, such as PDGFRβ, FAP, CD29, αSMA and S100A4 [35, 41, 42]. CAF-S1 and CAF-S4 subpopulations exhibited pro-invasive characteristics and accompanied with high expression of αSMA. Among them, CAF-S1 promotes cancer cell invasion mainly through secreting TGF-β and C-X-C chemokine ligan (CXCL) 12, while CAF-S4 through Notch pathway. Another difference is that in terms of immunosuppression, CAF-S1 can enhance the function of CD4+T and CD25+T lymphocytes and increase the capacity of Treg Cell, while CAF-S4 does not have these characteristics. In breast cancer, Brechbuhl et al. reported two CAF subtypes, CD146posCAFs and CD146negCAFs. Among them, CD146posCAFs promote oestrogen-dependent proliferation and tamoxifen sensitivity of cancer cells, whereas CD146negCAFs suppress oestrogen receptor expression and enhance tamoxifen resistance [43]. These examples, and many more (Table 1), are leading us to reveal the significant relationship between CAF subpopulations and desmoplastic tumors progression.

Table 1 CAF subpopulations and markers were identified in different desmoplastic tumors

The above studies showed that in addition to the majority of CAF subpopulations exert tumor-promoting functions, there are also a few tumor-repressing CAF subpopulations in certain cancers. Under the circumstances, nonselective targeting of CAF subpopulations may be off-target or even adverse clinical outcomes in cancer treatment. Such as, the depletion of αSMA+ myofibroblasts in pancreatic ductal adenocarcinoma can suppress immune surveillance with increased CD4+Foxp3+ Tregs, further reducing survival of patients [44]. Consequently, it is critical to identify more specific markers to distinguish tumor-promoting and tumor-repressing CAF subpopulations and design specific targeted-exosomes to treat desmoplastic tumors in the future. But here, the functions of tumor-promoting CAFs in tumors desmoplastic microenvironment were emphasized.

Briefly, CAFs paly significant role in mediating the formation of tumors desmoplastic microenvironment, resulting in an abundance of dense and fibrous tissue that acts as pathological barrier, which limit the efficacy of drugs by blocking the deep delivery to tumor sites. Clinically, desmoplastic tumors with high mortality present a great challenge for recurrence and metastasis. Thus, there is a need for a biocompatible targeted drug delivery system that can penetrate pathological barrier, further reducing mortality of desmoplastic tumors.

myCAFs myofibroblast CAFs, iCAFs inflammatory CAFs, Lox lysyl oxidase, PDPN podoplanin, TPM tropomyosins 1, POSTN periostin, MMP matrix metallopeptidase, HAS hyaluronan synthases, MHC II major histocompatibility complex class II

Functions of CAFs in desmoplastic tumors progression

In desmoplastic tumor progression, CAFs secrete abundant ECM proteins and exert physical force to remodel ECM and establish pathological barrier that hinders drugs delivery and immune cells infiltration and promotes cancer cells invasion. Furthermore, CAFs also secret abundant cytokines and ECM proteins to talk with immune cells and cancer cells, inducing immunosuppression and promoting CSCs-mediated resistance for therapeutic agents (Fig. 1).

CAFs remodel ECM

ECM, a non-cellular component, consists of macromolecules including collagen, elastin, fibrin and proteoglycan, which is a significant supporter of the tumor tissues and stromal cells in desmoplastic tumor. During desmoplasia, CAFs can secrete abundant ECM proteins and remodel ECM via exerting a physical force on cancer cells as well as continuously depositing, crosslinking and degrading the ECM proteins, resulting in the stiffness of desmoplastic tumor tissue and matricellular fibrosis that obstruct drugs delivery and immune cells infiltration and provide scaffold for the invasion and migration of tumor cells [1].

ECM remodeling also can be achieved by CAFs-mediated physical force, further promoting the invasion of tumor cells. Study reported that CAFs not only produce rich fibronectin, but also modulate fibronectin matrix by increasing contractility and traction forces that are exerted by PDGFRα and nonmuscle myosin II [50]. Then the contractile and tractive forces are transmitted to fibronectin via α5β1 integrin. Finally, CAFs organize fibronectin as parallel fibers, further promoting directional migration of prostate tumor cells. Additionally, by cell–cell contact, CAFs also exert physical forces to promote the joint invasion of CAFs and cancer cells, further remodeling ECM. The published study showed that force transmission was exerted through heterophilic adhesion between N-cadherin on CAFs and E-cadherin on tumor cells [51].

CAFs can deposit ECM proteins, such as I, III, IV and V types of collagens, fibronectins, hyaluronan, laminins, glycoproteins and proteoglycans, which contribute to the stiffness of ECM and go on as pathological barrier that hinders drugs delivery and the infiltration of immune cells [1, 52]. Pietilä et al. demonstrated that increased ECM stiffness can protect ovarian cancer cells from cisplatin mediated apoptosis via focal adhesion kinases and yes-associated protein signaling pathway [7]. The process of deposition often matches with matrix crosslinking enzymes. For instance, lysyl oxidase-like 2 is responsible for crosslinking collagen and elastin leading to the stiffness of ECM, which can be used as a biomarker of poor prognosis in cholangiocarcinoma and pancreatic ductal adenocarcinoma [5, 53]. Intriguingly, instead of isolating cancer cells, the barrier for ECM stiffening facilitates their invasion and spread [7, 54]. The overexpression of FAP in pancreatic cancer CAF can remodel ECM through modulating fibronectin levels and increasing the organization of collagen fiber, further forming a hardened and parallel fiber that increases the directionality of tumor cell invasion [31]. Simultaneously, in the process of desmoplasia, CAFs produce ECM-degrading proteases, such as MMPs, which can remodel ECM and promote the metastasis and invasion of tumor cells. The overexpressed MMPs (such as MMP-2, MMP7, MMP-9, MMP-11 and MMP-14) are frequently induced by TGF-α/β, NF-κB and WNT, which are related to poor prognosis for cholangiocarcinoma and breast cancer [5, 55, 56]. Therefore, desmoplasia is a dynamic stromal alteration process via CAFs-mediated local remodeling of ECM.

CAFs induce immunosuppression

Except for the pathological barrier caused by CAFs and ECM to hinder immune cells infiltration, CAFs can secrete larger numbers of cytokines and chemokines to recruit immunosuppressive cells or reduce the activities of immune effector cells, promoting the immune escape of cancer cells [1, 27, 57].

In desmoplastic tumors progression, the most common immunosuppressive cells, such as tumor associated macrophages (TAM) 2 and myeloid-derived suppressor cells (MDSCs), are related to poor clinical prognosis of patients [58,