The extracellular matrix (ECM) acts as a physical scaffold for the microenvironment, facilitates interactions between different cell types and provides signals that elicit a variety of responses, such as cell proliferation, adhesion, migration, apoptosis, and angiogenesis [
1]. Heparan sulfate (HS) is involved in several biological functions due to its interactions with different proteins in the ECM. The glycosaminoglycan chains of heparan sulfate proteoglycan (HSPG) can be degraded by heparanase (HPSE), generating oligosaccharides that intensify the effects of growth factors, cytokines, and angiogenic factors, triggering cell proliferation, cell differentiation, and angiogenesis, which favor tumor development [
2‐
5]. Unlike HPSE, heparanase-2 (HPSE2) has no catalytic activity but binds to heparan sulfate with high affinity and appears to modulate HPSE enzymatic activity [
6]. HPSE stimulates the synthesis and shedding of syndecan-1 proteoglycan (Syn-1) [
7,
8]. Secreted Syn-1 binds to tumor-derived growth factors in the tumor microenvironment and promotes tumor progression [
8]. The combination of HPSE enzymatic activity and Syn-1 shedding provides a powerful tool to better understand tumor development and metastasis [
9,
10]. The tumor metastatic events depend on the cumulative ability of cancer cells to create an appropriate, distinct microenvironment in the primary tumor site and systemic circulation [
11]. The primary tumor microenvironment is composed of different stromal cell types in addition to neoplastic and hematopoietic cells, such as bone marrow-derived cells (BMDCs), including macrophages, mast cells, mesenchymal stem cells (MSCs), myeloid cell-derived suppressor cells (MDSCs) and lymphocytes [
12]. Triple-negative breast cancer represents the worst type of breast cancer and can be classified into four different subtypes: basal-like (BL), mesenchymal (M), luminal androgen receptor (LAR), and immunomodulatory (IM). Some triple-negative breast cancers have high levels of immune cell infiltration, and other triple-negative tumors have low levels of immune cell infiltration. In a recent study, it was demonstrated that among all triple-negative breast cancers, rich lymphocyte infiltration represented a good prognosis that could derive benefit from immune checkpoint inhibitor therapy, enhancing the anticancer activity of the immune system [
13]. Tumor-infiltrating lymphocytes differ in triple-negative breast cancer. Furthermore, the association between the IM subtype and lymphocyte infiltration may influence the response to chemotherapy [
14]. Infiltrating tumor lymphocytes improve immunological activity against tumors and can also influence prognosis. A previous study demonstrated a significant role for CD4
+ T cells as enhancers of lung metastases from breast carcinomas by preventing the reduction or destruction of malignant cells due to the cytotoxic mechanisms present in CD4
+ T cells [
15]. Thus, increasing T cell traffic to the tumor may be a good strategy to enhance responses to immunotherapy in cancer treatment [
16]. The expression of HPSE in the microenvironment can modulate the transmigration and extravasation of hematopoietic stem cells and bone marrow progenitor cells, thus affecting the hematopoietic system [
17]. Moreover, Theodoro and colleagues observed increased levels of HPSE and HPSE2 in peripheral blood mononuclear cells (PBMCs) of breast cancer patients [
18]. It is also conceivable that the exosomes secreted by tumor cells can modulate tumor metastasis and may be involved in microenvironmental signals. During the initial stages, exosomes remain at the primary site of the tumor, and they are secreted into the peripheral blood with tumor progression, subsequently acting on different tissues where tumor metastases develop [
19,
20]. Exosome secretion often increases during tumor cell trafficking, generating a more aggressive phenotype. It has been observed that tumor exosomes can promote a significant acceleration of the metastatic process and increase the recruitment of MDSCs [
21]. Higher exosome levels have been observed in cancer patients’ body fluids compared to those of healthy controls [
22]. The highly malignant nature of tumors and the concentrations of extracellular vesicles in the circulation of patients affected by cancer suggest a prominent role of extracellular vesicles in promoting tumor progression and evading immune surveillance. In recent work, high and low extracellular vesicle concentrations have been shown to have differential effects that dictate the modulatory effects on PBMCs [
23].
Exosomes contain miRNA, mRNA, proteins, and lipids, thereby providing a method for intercellular communication [
24]. Recently, it has been shown that heparan sulfate from Syn-1 is an active modulator of its own shedding by epithelial and tumor cells [
25]. Through interactions with syntenin (syn1-cytoskeleton binding protein), syndecan proteoglycans influence the biogenesis of exosomes [
26]. There is also a direct relationship between increases in the secretion of exosomes and HPSE enzymatic activity. Furthermore, it was demonstrated that the protein composition of exosomes was modified by an increase in heparanase in an aggressive type of tumor. Exosomes secreted by tumor cells, together with high levels of heparanase, not only alter the behavior of tumor cells but also promote alterations to nonneoplastic host cells [
27]. We were interested in investigating the mechanism of interaction between tumor cells and lymphocytes that activates heparanase expression in malignant neoplasms. Belting and colleagues highlighted a role for HSPGs in exosome uptake. Indeed, HSPGs appear to function as internalizing receptors for cancer cell-derived exosomes and to be required for their functional activity, namely, in driving cancer cell migration [
28]. Thompson et al. reported that heparanase stimulated exosome production and affected the composition of exosomes [
27]. Roucourt et al. provided evidence that heparanase activated the syndecan-syntenin-ALIX pathway, which supported the biogenesis of exosomes, affecting specific exosomal cargo [
29,
30]. Since heparanase is known to be involved in tumor progression, several inhibitors of this enzyme have been produced as novel cancer therapeutics. Among the heparanase-inhibiting compounds, we highlight PI88 (a highly sulfonated mannan oligosaccharide), PG545 (a synthetic mixture of oligosaccharides derived from heparin) and SST0001 (a modified heparin saccharide with 100% N-acetylation and 25% glycol split that is also known as roneparstat), which are currently in clinical trials [
31]. An improved understanding of the molecular contexts favoring the action of these agents against cancer would allow the full application of their potential.
The results revealed that heparan sulfate is likely responsible for mediating HPSE and HPSE2 expression in circulating lymphocytes. In addition, exosomes may play a role in heparanase upregulation by carrying secreted heparan sulfate from tumor cells, thus suggesting a mechanism of crosstalk between tumor cells and circulating lymphocytes.