Targeting coagulation to unlock antitumor immunity?

Besides a number of physical consequences (reduced blood supply, stabilization of circulating tumor microemboli, shielding from the attack of immune cells), the coagulation cascade may specifically regulate antitumor immunity. We recently applied systems biology and genomics to explore the regulation of the tumor immune microenvironment by coagulation.

Coagulation is a vital biochemical cascade that is almost systematically activated in human tumors.1 As a consequence of this hypercoagulant state, solid tumors often induce systemic complications, such as venous thromboembolism, that account for significant mortality and morbidity (especially in glioblastoma, lung carcinoma or pancreatic adenocarcinoma).[1] The role of the coagulation cascade as a source of hemostatic complications has motivated the development of pharmacological anticoagulant agents, such as low-molecular weight heparins or direct oral anticoagulants (DOAC). Despite an attractive preclinical rationale, preventing thomboembolic accidents in cancer patients has proven difficult and current anticoagulation protocols have failed to broadly extend the survival of ambulatory cancer patients.[1] Lack of personalization and the intrinsic narrow therapeutic window of the available treatments (that target the common effectors of coagulation, thus inducing hemorrhage and bleeding), could explain this.[1]

Acute activation of coagulation contributes to the installation of an inflammatory response, with the recruitment and activation of myeloid cells (polymorphonuclear cells and macrophages).[2] Coagulation also contributes to chronic inflammation, as for example shown in the central nervous system at the neurovascular interface.[3] In cancer, coagulation likely contributes to the establishment of an inflammatory tumor microenvironment (TME), a hallmark of solid tumors that has been popularized by the concept of the “wound that does not heal”.[4] The contribution of acute coagulation to the recruitment of myeloid cells was shown experimentally by coinjecting cancer cells with blood clots of variable composition.[5] Unfortunately, analyzing the role of the coagulation cascade in the clinical setting has turned out to be difficult for multiple reasons, the first being the difficult access to tumor material in primary tumors characterized by strong procoagulation, especially in the context of pharmacological anticoagulation (Figure 1).

Figure 1.

The consequences of the activation of coagulation on the tumor immune microenvironment.

Experimental studies showed a possible direct regulation of the monocyte/macrophage lineage by coagulation and fibrinolysis. Kubala et al. reported that the Plasminogen Activator Inhibitor-1 (PAI-1) modulates the functional polarization of Tumor-Associated Macrophages (TAM) toward the pro-tumoral M2 phenotype.[6] Monocytic cells were also shown to produce activated coagulation factor X (FXa) able to interact with specific receptors on their surface in a cell-autonomous manner.[7] Following these studies, the role of coagulation was recently addressed using systems biology approaches to explore the human tumor ecosystems. Pan-cancer studies using The Cancer Genome Atlas (TCGA) showed that tumors present extremely variable coagulomes, i.e. profiles of coagulation and fibrinolysis gene expression, between tumor types and individual tumors.[8] Surprisingly, the most “pro-coagulant” tumors are not necessarily the ones with the highest risk of venous thromboembolism. Of all primary tumors, Oral Squamous Cell Carcinoma (OSCC), the most frequent tumors of the oral cavity, have the highest expression of F3, the gene encoding Tissue Factor (TF), a key initiator of the coagulation cascade.[8,9] The role of coagulation should therefore be reconsidered for each tumor, outside of the pre-conceived ideas generated by the study of systemic vascular complications of cancer.[9] Myeloid cells that infiltrate the tumors could be both a source and a target of coagulation factors. Indeed, monocytic cells contribute greatly to the expression of the fibrinolysis genes, since these cells express high mRNA levels of PLAU (encoding the urokinase type-plasminogen activator, uPA) and SERPINE1 (encoding PAI-1).[8,9] Importantly, the coagulation cascade does not simply correlate with the tumor recruitment of immune cells.[8,9] In OSCC for example, there is no clear link between tumor infiltration by the major families of immune cells and the expression of F3.[9] Single-cell analyses however point to the existence of an activated TAM phenotype, characterized by high expression of the cytokine CXCL2 (C-X-C Motif Chemokine Ligand 2) in tumors that express high levels of F3. In this subset of tumors, dendritic cells, key players in antitumor adaptive immunity, also express high levels of the immune checkpoint molecule CD274/PD-L1.[9] The significance of these modifications remains unclear and the findings are correlative at this stage. Nevertheless, they are suggestive of a possible defective immune stimulation in this context. Whether the corresponding observations are related to hypoxia, a specific cytokine context or the result of direct activation of Protease-Activated Receptors (PAR) on the surface of immune cells remains to be determined (Figure 1).

The recent success of Immune Checkpoint Blockers (ICB) provides a strong motivation to identify the mechanisms that limit adaptive tumor immunity. We argue that re-exploring coagulation in this framework is important, given the existence of an actionable interplay between the coagulome and the TME.[1] Importantly, potent anticoagulants are readily available. While their ability to extend the survival of cancer patients as single agents is questionable, they might be useful as an adjunct to ICB to boost anti-tumor immunity. Interestingly, some of the most pro-coagulant tumor types, such as glioblastoma or pancreatic adenocarcinoma, are resistant to ICB. We propose that a focus of future studies should be to identify the tumors and the situations (for example the perioperative period) in which the coagulome might constitute a bottleneck to anti-tumor immunity. Instead of globally considering all anticoagulants, precise molecular mechanisms might need to be envisioned. Indeed, specific targeting of coagulation factors, such as FXa, might be required to achieve superior antitumor immune stimulation, as suggested by recent preliminary clinical studies suggesting that the DOAC rivaroxaban boosts the effects of ICB in malignant melanomas.[10] Finally, the coagulation cascade is a source of clinical biomarkers, many of which are broadly-available in clinics to monitor hemostasis. The existence of a rich, active interplay between coagulation and the TME, as discussed elsewhere,[1] opens the enticing possibility that coagulation/fibrinolysis biomarkers might be revisited to non-invasively explore the TME and the immune characteristics of human tumors. It is time to reconsider coagulomics in the quest for precision immunology in cancer, instead of considering it merely as a source of hemostatic complications in cancer patients.