Blood vessel endothelial cells (ECs) have long been known to modulate inflammation by regulating immune cell trafficking, activation status and function. However, whether the heterogeneous EC populations in various tissues and organs differ in their immunomodulatory capacity has received insufficient attention, certainly with regard to considering them for alternative immunotherapy. Recent single-cell studies have identified specific EC subtypes that express gene signatures indicative of phagocytosis or scavenging, antigen presentation and immune cell recruitment. Here we discuss emerging evidence suggesting a tissue-specific and vessel type-specific immunomodulatory role for distinct subtypes of ECs, here collectively referred to as ‘immunomodulatory ECs’ (IMECs). We propose that IMECs have more important functions in immunity than previously recognized, and suggest that these might be considered as targets for new immunotherapeutic approaches.
Blood vessels had long been viewed as passive bystander conduits, with their sole function being the supply of blood to and the drainage of blood from organs. Whereas lymph vessels are known to regulate various aspects of immunity1,2, a potentially similar role for blood vessels has not received sufficient attention to date. Interestingly, endothelial cells (ECs), the cells that line blood vessels, share a common ancestor with immune cells (Box 1), intuitively supporting a role for ECs in immune responses.
Research from more than 100 years ago showed that ECs from the sinusoids of the liver, spleen and other organs can act as scavenger ECs, complementing the activity of macrophages in eliminating circulating waste macromolecules3,4. Indeed, scavenger ECs were proposed 10 years ago to be “an integral component of the innate immune system”3, and like immune cells, liver sinusoidal ECs (LSECs) in rats can arise from bone marrow precursors in response to liver injury and during liver regeneration3. In addition, a combined single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing study identified an “immune cell-like EC (EndICLT)” subpopulation among mouse aortic ECs, which is induced by disturbed blood flow. Induction of EndICLT marker genes was confirmed in vitro in human aortic ECs under disturbed flow-mimicking conditions5. In addition, it was found that during mouse embryonic development, aortic ECs can bud off from the ventral aorta and transition into haematopoietic cells; this was in part dependent on the transcription factor RUNX1 (ref.6). Moreover, adult mouse ECs can be reprogrammed in vivo into haematopoietic stem cell-like cells through transient expression of the transcription factors FOSB, GFIL, RUNX1 and SPI1, and vascular-niche derived angiocrine factors7.
Emerging evidence indicates that subsets of ECs in different tissues and organs exert immunomodulatory activities beyond their well-known role in alloimmunity, immune cell recruitment, immune tolerance and vascular inflammation8,9,10. Furthermore, several subtypes of ECs have been shown to display features that are typical of immune cells. These include the expression of co-stimulatory and co-inhibitory receptors11, the capacity to induce apoptosis in other cells (for example, they have been shown to kill ovarian tumour-homing cytotoxic T cells via FAS ligand (FASL) in human co-cultures and mice12), secretion of cytokines and their acting as (semi-professional) antigen-presenting cells (APCs). They can also act as phagocytes and scavengers of circulating waste macromolecules and participate in efferocytosis4,11,12,13,14. Notably, immunomodulation by ECs can be influenced by cytokines, such as interleukin-35 (IL-35)15 and IL-17A16. Given that ECs are among the first cells to come into contact with circulating pathogens and are the first cells that immune cells interact with when invading tissue parenchyma, they are strategically ideally positioned as a first-line defence system to participate in immune responses.
In this Perspective, we first provide an overview of some of the well-known ‘traditional’ immunomodulatory functions of ECs, such as immune cell recruitment and semi-professional antigen presentation. We then examine recent advances in our understanding of the context-dependent role of ECs in immunomodulation in different organs, which are based mainly on scRNA-seq analyses. These studies indicate that immunomodulation by specific subsets of ECs, which we collectively refer to as ‘immunomodulatory ECs’ (IMECs), can have a prominent role in tissue-specific immunity, as well as in cancer, neurodegeneration and infectious diseases such as COVID-19. Some of these IMECs may have constitutive immunomodulatory activities (such as LSECs), while other IMECs may refer to (transitory) plastic phenotypes, induced by particular contextual conditions (such as EndICLTs).
Immune cell recruitment by ECs
In the late 1990s and early 2000s, ECs were discovered to function as local gatekeepers of immunity8. By interacting with circulating innate and adaptive immune cells and controlling their extravasation from the circulation into the tissue parenchyma, ECs can indeed control tissue and lymph node inflammation11,17. This process involves the differential expression of adhesion molecules (such as vascular cell adhesion molecule 1 (VCAM1)), selectins (such as E-selectin and P-selectin), addressins (such as peripheral node addressins, mostly in mucosal and lymphoid tissue) and chemokines (such as CCL2 and CXCL10) by ECs. During immune homeostasis, they allow patrolling immune cells to extravasate into tissue, and during inflammation, ECs can become activated and capable of actively recruiting effector immune cells11,18. EC activation can be induced by cytokines such as IL-6, IL-1β and tumour necrosis factor (TNF), but also by pathogen-associated molecular patterns, such as lipopolysaccharide19,20. The surface repertoire of adhesion molecules, selectins and addressins on ECs as well as their repertoire of secreted chemokines, in combination with the differential expression of cognate integrins, selectin ligands and chemokine receptors by immune cells, determines which circulating immune cells invade which tissue21. Some aspects of immune cell recruitment by ECs might differ between species (as is also the case for antigen presentation (see the next section and Box 2)).
Antigen presentation by ECs
Some EC subtypes are considered semi-professional APCs as they express genes involved in antigen capture, processing and presentation. For example, human renal vascular ECs express the major histocompatibility complex class II (MHC-II) surface molecule HLA-DR, which allows them to present antigens to CD4+ T cells22,23,24, and in vitro experiments showed that human umbilical vein ECs can activate allogenic T cells22,23,24,25. However, unlike professional APCs (such as dendritic cells), ECs generally do not express the surface receptors CD80 and CD86 (ref.26), which bind to CD28 on naive T cells and are required for their activation. ECs therefore primarily activate antigen-experienced T cells, although experiments in mice have shown that naive T cells can also be activated by ECs in the context of alloimmunity27,28. Importantly, not all molecules/processes related to APC function in ECs are conserved between species29 (Box 2). Interferon-γ (IFNγ) and TNF induce immunomodulatory processes in human and mouse ECs in vitro, including antigen uptake, processing and presentation9,10. Antigen presentation and immune cell recruitment by ECs contribute to alloimmunity and kidney/heart transplantation failure, for example through CD8+ T cell-induced lysis of ECs in the donor tissue8,30,31,32,33. Moreover, antigen presentation by human ECs has been implicated in autoimmune diseases such as rheumatoid arthritis34.
There are estimated to be more than 1013 ECs in the human body35; thus, even if only a fraction of ECs acts as semi-professional APCs, they form a large reservoir of potential APCs. ECs contextually present intracellular and extracellular antigens depending on the EC subtype and activation status9,36. For the presentation of intracellular antigens by ECs, nitric oxide37 and IFNγ can induce a modified proteasome38,39, called the ‘immunoproteasome’, which facilitates antigen degradation and antigen loading39. ECs share many features with professional APCs, but differ from them in other aspects (Table 1). For instance, ECs are exposed to shear stress40, which has been found to increase intercellular adhesion molecule 1 (ICAM1) expression41,42,43. ICAM1 binds to T cell integrins, which are capable of increasing T cell receptor signalling44. Moreover, shear stress increases the binding of selectins45,46,47, upregulates E-selectin expression in response to IL-1β48 and inhibits E-selectin expression in response to TNF42. Through binding to P-selectin glycoprotein ligand 1 on T cells, E-selectin can increase T cell receptor signalling, co-inhibitory molecule expression and T cell proliferation in the context of antigen presentation by ECs49. The roles of non-conventional MHC molecules such as MR1 (activating mucosal-associated invariant T cells50) and BTN3A1 (presenting phosphoantigens to Vγ9Vδ2+ T cells51) in antigen presentation in ECs have yet to be determined.
Tissue-specific immunomodulation by ECs
Studies from the past two decades examined possible roles of ECs in immunomodulation at the bulk population level11,18,35,52,53,54. A recent transcriptomic and epigenomic study on bulk mouse ECs reported tissue-specific patterns of gene transcription, with notable differences in expression patterns of co-stimulatory molecules as well as chemokines and cytokines, suggesting tissue-specific immunomodulation by ECs55. Single-cell studies have now allowed deeper insights to be obtained into the role of EC immunomodulation in (1) the recruitment and homing of immune cells to lymph nodes, (2) the modulation of immunity in response to external challenges in the liver and lung, (3) the detection and clearance of immune complexes in the liver and kidney and (4) the shielding of the brain tissue parenchyma from immune cell invasion in healthy conditions.
Secondary lymphoid organs, such as lymph nodes and Peyer patches, and tertiary lymphoid organs that arise in response to chronic inflammation are of particular interest in the context of immunomodulation by ECs, as these form ‘hubs’ in the lymphatic system where cells of the innate and adaptive immune systems interact56. Lymph nodes contain a vascular bed with a heterogeneous composition of ECs that line arterioles, capillaries and venules. Notably, lymph nodes also contain high endothelial venules (HEVs); these are a subtype of postcapillary venules (PCVs) that are lined by high (tall and plump) ECs that are specialized in recruiting immune cells such as monocytes, plasmacytoid dendritic cell precursors, neutrophils, B cells and T cells17,57,58,59. Naive T cells in the circulation home to lymph nodes, a process that, under non-inflamed conditions, is mediated by the adhesion molecule L-selectin, which binds to addressins on HEVs. These include adhesion molecules such as CD34, podocalyxin, GLYCAM1 or MADCAM1 containing the 6-sulfo sialyl Lewis X glycan modification. These modified adhesion molecules can be detected by antibodies binding peripheral node addressins, such as MECA-79 (refs60,61,62). A combination of addressins and chemokines such as CCL21 facilitates the capture and tethering of naive T cells on HEVs and promotes their extravasation17 (Fig. 1a). HEVs are extensively remodelled upon infection and the subsequent expansion of draining lymph nodes17, but their phenotypic plasticity is only beginning to be explored.
An outstanding question is whether the interaction between HEVs and immune cells is sufficiently long to allow immunomodulation by the ECs. For T cells, which can reside in lymph node ‘pockets’ close to HEVs17, the interactions may be long enough to allow HEVs to modulate T cell activity and differentiation through the expression of co-inhibitory or co-stimulatory receptors and the secretion of cytokines. However, this might be a T cell/HEV-specific phenomenon, given that transendothelial migration of immune cells across conventional PCVs, which are the primary site of immune cell recruitment in many organs, is rapid63,64,65 (for example, 6 min for mouse neutrophils in vivo66), which limits sustained interactions with ECs. In the liver, lungs and kidneys, however, immune cell recruitment occurs primarily in capillaries, which are often only a few micrometres in diameter63,67. This causes immune cells to crawl, slows down extravasation and prolongs interactions with ECs, potentially allowing immunomodulation by ECs.
The characterization of HEVs at single-cell resolution under inflammatory conditions has strengthened the concept that HEVs can modulate immune cells (Fig. 1d). Indeed, scRNA-seq analysis of enriched mouse MECA-79+ HEVs from lymph nodes, isolated after oxazolone-induced inflammation (which promotes HEV activation68), revealed an upregulation of EC activation markers and the co-stimulatory molecule CD137, which can suppress the activation of immune cells that express CD137L such as dendritic cells69. Activated HEVs from oxalozone-exposed mice also express higher levels of macrophage migration inhibitory factor (MIF), which regulates context-dependent M1/M2 macrophage polarization70,71, and thrombospondin 1 (TSP1), which can impair T cell activation72. Together, these findings suggest that HEVs have immunomodulatory functions beyond immune cell recruitment73. Another scRNA-seq study of mouse lymph nodes implied that non-HEV ECs can recruit myeloid cells to lymph nodes during inflammation in a MECA-79-independent, but P-selectin and E-selectin-dependent manner74, implying that not only HEVs are important for (myeloid) immune cell recruitment during inflammation (Fig. 1b). Single-cell studies in mouse and human tumours further revealed that there is no clear phenotypic separation between HEVs and (postcapillary) venous ECs in tumours, which express a selected set of canonical and non-canonical HEV markers75,76,77.
Interestingly, a combination therapy consisting of anti-VEGF therapy (which facilitates vessel normalization) and anti-PDL1 immunotherapy promotes HEV formation and T cell recruitment, and improves antitumour immunity in preclinical tumour models78. Similarly, the treatment of mice with anti-PD1 in combination with delivery of vascular-targeted LIGHT proteins that induce non-canonical NF-κB signalling, which is required for differentiation of ECs into the HEV phenotype, induces HEV biogenesis and improves tumour immunity and immunotherapy in preclinical tumour models79,80 (Fig. 1c). Thus, in addition to the established function of HEVs in immune cell trafficking to lymph nodes during infections, HEVs may also have direct immunomodulatory effects. Further insight into this additional immunomodulatory potential and the extralymphatic biogenesis of HEVs during (chronic) inflammation, cancer and other diseases may offer new immunotherapeutic opportunities for these conditions.
Organs controlling immunity versus tolerance to external danger
Several organs, such as the liver, intestines, lung and skin, are exposed to airborne or nutrient-derived antigens, pathogens and toxins and to their microbiota, as well as microbiota-derived antigens (Fig. 1e). These organs must both protect the organism against harmful attacks by raising an adequate immune response and, at the same time, prevent uncontrolled or excessive immune attacks against harmless agents by inducing tolerance — a delicate balance that requires fine-tuned immunoregulation.
The liver is exposed to microbial and dietary antigens from the gut via the portal vein. Specialized EC subpopulations in the liver contribute to immune tolerance, most notably LSECs. LSECs are equipped with a repertoire of molecules for the detection and uptake of extracellular antigens (microbial products and viruses), including Toll-like receptor 1 (TLR1), TLR2, TLR3, TLR4, TLR6, TLR8, TLR9 (refs81,82) and scavenger receptors such as the C-type lectin receptor mannose receptor83,84. In mice, LSECs take up and cross-present extracellular antigens on MHC-I molecules to CD8+ T cells, but have a tolerogenic function because they express high levels of co-inhibitory molecules such as PDL1 and do not express (or express at only low levels) the co-stimulatory receptors CD80 and CD86, which are necessary for the activation of naive T cells85,86,87. Similarly, exogenous antigens, acquired through mannose receptor-mediated endocytosis and presented on MHC-II molecules to naive CD4+ T cells, induce tolerance by promoting differentiation of regulatory T cells (Treg cells)88,89 (Fig. 1e). Additionally, LSECs are also involved in Fc receptor-mediated phagocytosis and degradation of (primarily large) antibody–antigen immune complexes from the circulation3,90 (Fig. 1f).
LSECs recruit different immune cells via different molecular mechanisms. For example, Treg cells migrate through the liver sinusoidal endothelium primarily by interacting with the scavenger receptor stabilin 1 and the adhesion molecules ICAM1 and VAP1, whereas CD8+ T cell extravasation into the liver is mediated primarily by ICAM1 (refs91,92,93). As LSECs exhibit zone-dependent heterogeneity in liver lobules94,95, these findings raise the question of whether LSEC heterogeneity might contribute to zone-specific recruitment of Treg cells and accompanying immunosuppression in the liver. A recent study showed that resident myeloid and lymphoid cells cluster around periportal hepatic zones96 owing to MYD88-dependent signalling in LSECs. This is induced by gut commensal bacteria and changes the composition of the LSEC glycocalyx layer and hence the gradients of chemokines (such as CXCL9) binding to components of the glycocalyx (such as glycosaminoglycans) (Fig. 1g). The resulting periportal concentration of immune cells was more efficient than a uniform distribution of immune cells in protecting against systemic bacterial dissemination. This demonstrates that LSECs actively orchestrate the localization of immune cells, which optimizes host defence.
However, single-cell studies revealed confounding results. Indeed, the transcriptome of periportal LSECs differs from that of central vein LSECs in the human liver. Central vein LSECs upregulate the expression of CD32B (also known as FCGR2B; encoding an inhibitory receptor) and STAB1 (encoding stabilin 1) and of genes involved in innate immunity, phagocytosis and leukocyte activation, whereas periportal ECs exhibit a TNF activation signature and express other immunomodulatory genes95. However, a paired-cell RNA-seq study of livers from healthy mice, in which mRNA from pairs of ECs attached to hepatocytes was sequenced and gene expression from one cell type was used to infer the tissue coordinates of the cell pair, reported opposite findings, indicating low levels of STAB1 transcription in central vein LSECs94. Moreover, this report identified close interactions between LSECs and Kupffer cells (liver-resident macrophages) through colony-stimulating factor 1 (CSF1)–CSF1 receptor and CD93–C1qa signalling94 (Fig. 1g). Overall, although all these studies documented regional LSEC heterogeneity and interactions between LSECs and immune cells, further protein-level validation is needed to confirm their relevance.
LSECs also affect disease outcome. For example, LSECs present cancer cell-derived apoptotic bodies to naive CD8+ T cells. However, as LSECs act as semi-professional APCs, they impair the differentiation of naive CD8+ T cells into cytotoxic effector T cells, which are capable of killing cancer cells, thereby hampering tumour immunity1. It was shown that breaking LSEC-induced immune tolerance (using nanoparticles to deliver melittin, a host defence peptide with immunomodulatory activity) leads to LSEC activation and a changed hepatic chemokine and cytokine milieu, which inhibits metastasis in melanoma, breast cancer and colon cancer models97. In mouse models of hepatocellular carcinoma, malignant hepatocyte-derived VEGF induces the expression of the EC-specific transmembrane protein PLVAP in LSECs, which promotes the recruitment of FOLR2+ immunosuppressive tumour-associated macrophages and the creation of an immunosuppressive niche by interacting with Treg cells98. This suggests that LSECs form a communication hub in the liver tumour microenvironment that promotes immunosuppression and thereby facilitates tumour growth (Fig. 1h).
LSECs can also promote excessive inflammation in mice and humans and contribute to organ damage in conditions such as autoimmune hepatitis99,100 and fibrosis101, suggesting that immunomodulation by LSECs is critical for maintaining an immunological balance and tissue homeostasis in the liver. Furthermore, an scRNA-seq study of healthy and cirrhotic human livers showed that the latter contained a disease-specific EC population in the fibrotic niche101, which was enriched in ACKR1 transcripts101 (Fig. 1i), encoding the atypical chemokine receptor 1 (ACKR1). This chemokine receptor is primarily expressed by PCV ECs (and small venule ECs102), and transports basal chemokines for presentation at the luminal surface of ECs and in paracellular junctions, where it regulates different stages of immune cell diapedesis103 and recruitment104. Moreover, in silico analyses predicted that ACKR1+ ECs interact with disease-specific macrophages via multiple chemokines (such as CXCL12 and CCL2) and the macrophage differentiation factors GAS6 and PROS1 (ref.101). This suggests that ACKR1+ ECs might recruit disease-specific immune cells, and raises the question of whether liver ECs might be therapeutic targets to treat cirrhosis. In mice with experimentally induced portal hypertension, LSECs express lower levels of MHC-I and MHC-II molecules105, suggesting that immune responses in the liver may be altered in this disease. Finally, an scRNA-seq study in aged mice revealed decreased expression of Mrc1 (encoding the C-type lectin receptor CD206) in LSECs, which might contribute to their decrease in endocytic capacity with age106. However, in situ RNA staining for Mrc1 and the classical LSEC marker Pecam1 (encoding CD31) in the same study showed that the number of Mrc1-expressing LSECs actually increases with age in mice, raising the question of whether LSECs in aged individuals have a reduced or a similar immunomodulatory potential. Overall, LSECs differ from ECs in other tissues by their constant exposure to dietary and pathogen-derived antigens, exert a predominantly tolerogenic APC function and show zonal heterogeneity.
The lung is highly vascularized with a specialized composition of ECs, consisting largely of microvascular ECs that facilitate gas exchange between the circulation on the apical side and the air in alveoli on the basal side. Inhalation of large volumes of air exposes the lung to pathogens and pollutants, to which appropriate immune responses are required that do not put the vital gas exchange apparatus at risk. The lung has elaborate mechanisms to ensure homeostasis and dampen immune activation following lung damage107. Immunomodulation by ECs might play a more important role in the lung than originally anticipated.
Indeed, compared with mouse ECs from the heart or brain, the gene expression signature as detected by bulk RNA-seq of lung ECs showed a marked upregulation of transcripts involved in immune regulation108. Moreover, subsets of lung ECs express MHC-II, and in humans this feature appears to be restricted to capillary ECs75,109. A recent scRNA-seq study revealed that human bronchial ECs form a transcriptomically distinct population from alveolar ECs, although the genes involved in immunomodulation do not appear to be their most distinguishing feature110. Another single-cell study suggested that human alveolar capillary ECs can be divided in two populations on the basis of their transcriptome and location, where ECs termed ‘aerocytes’ (which are located close to alveolar type 1 epithelial cells) are specialized in gas exchange and immune cell recruitment, whereas general capillary ECs can activate CD4+ T cells through MHC-II (ref.111), suggesting that these alveolar ECs might facilitate an adequate immune response against harmful antigens.
Though yet to be confirmed, VEGF may contribute to preventing uncontrolled, detrimental immune responses to the (commensal) microbiota (Fig. 2c). Indeed, a single-cell analysis of alveolar cell populations (conserved in humans, mice, rats and pigs) predicted capillary ECs to be the cell type most responsive to VEGF (released primarily by alveolar type 1 cells and secretory epithelial cells112). Given the immunosuppressive effects of VEGF113, the aforementioned finding raises the question of whether VEGF signalling in the alveolar microenvironment might contribute to EC-mediated tolerance to airborne pathogens and toxins in the lung. Whether additional molecular mechanisms contribute to the tolerogenic nature of lung ECs with immunomodulatory features requires further study.
Emerging evidence also indicates that immunomodulation by pulmonary ECs may co-determine disease severity and progression in lung cancer. Tumour ECs (TECs) from individuals with untreated, non-metastatic non-small-cell lung cancer of the squamous cell or adenocarcinoma subtype exhibit decreased expression of genes encoding ICAM1, the chemokines CCL2 and CCL18, the cytokine IL-6 and HLA-I/HLA-II (ref.114), suggesting an immunosuppressive environment115. Additionally, TECs of human and mouse lungs show elevated expression of genes encoding FASL, a cell death regulator capable of inducing cell death in cytotoxic T cells12, and of co-inhibitory molecules such as PDL1, further indicating an immunosuppressive role116 (Fig. 2a). Another single-cell study, of human and mouse lung tumours, illustrated a complex immunomodulatory gene signature75. In line with earlier studies, lung capillary TECs expressed lower levels of immunomodulatory genes (involved in antigen presentation and processing) than peritumoural capillary ECs, suggesting that certain TEC subpopulations might become more tolerogenic75. However, tumours had fewer capillaries, which suggests that further research is required to investigate the exact immunomodulatory role of lung capillary TECs75. Furthermore, mice with a deficiency of MHC-II in non-haematopoietic cells had fewer Treg cells in the lung and a lower pulmonary metastasis burden in lung tumour models109, which may suggest that antigen presentation by pulmonary ECs contributes to immune tolerance in lung cancer, although EC-selective knockout approaches are required to confirm this. However, another population of activated PCV lung ECs that was enriched in human non-small-cell lung cancer and mouse lung tumours was shown to upregulate a HEV-like gene signature and ACKR1 expression, suggesting that there may be different populations of TECs that either promote or suppress tumour immunity75. Notably, mass cytometry revealed high surface expression of HLA-DRA on healthy capillary lung ECs, which was comparable to that on immune cells in general. This finding requires further functional validation, but highlights the immunomodulatory potential of these ECs as non-professional APCs75.
The role of lung ECs has also been investigated in various infection models. For example, in a mouse model of Plasmodium berghei-induced malaria, lung ECs were shown to cross-present malaria parasite antigens to CD8+ T cells (this was also shown in vitro) in response to stimulation by IFNγ, which is presumably secreted by CD8+ T cells (and possibly CD4+ T cells and natural killer cells). This process is associated with vascular leakage and lung damage117 (Fig. 2b), indicating that antigen presentation by lung ECs can have detrimental effects. Vascularized lung-on-chip models allow investigation of the role of lung ECs in infections such as COVID-19. These showed that lung ECs underlying epithelial cells can be directly infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and contained viral RNA (however, without signs of active viral replication), and infected ECs exhibited a decreased barrier integrity118. In aged mice, pulmonary capillary ECs have been shown to upregulate various cytokine transcripts (such as Il1b, Tnf and Tgfb1)119, which suggests that capillaries might contribute to lung diseases that are more prevalent in individuals >65 years of age, such as chronic obstructive pulmonary disease and lung cancer120, and possibly might contribute to the severity of COVID-19 (ref.121). Given that aged individuals are more prone to severe COVID-19, it is possible that SARS-CoV-2 infection of ECs in aged individuals might lead to a more pronounced loss of barrier function and increased hyperinflammation in the lung121. On the other hand, SARS-CoV-2 infection of ECs in a human lung-on-a-chip model has also been shown to decrease CD31 expression and thus impair immune cell recruitment to the lung118.
Similarly, in influenza virus infection, ECs may contribute to the cytokine storm that characterizes severe infection122. Viral replication in mouse ECs has been shown for specific influenza virus strains123, and this might impair the barrier function of the lung epithelium. Hence, viral replication in specific subtypes of ECs, such as capillary ECs, might induce viral antigen presentation and contribute to a rapid recall response of intravascular or perivascular memory T cells124. Together, emerging evidence indicates that pulmonary ECs are involved in immune responses, but whether they promote immunity (and potential tissue pathology in infections) or tolerance appears to be contextual and requires further study.
Kidney ECs represent a particularly heterogeneous population, where cortical, glomerular and medullary ECs exert distinct functions in the renal vascular bed and are exposed to different microenvironments depending on where they are located alongside the nephron125,126. Glomerular and peritubular ECs have fenestrations and are exposed to different concentrations of uraemic toxins, which are filtered from blood, and different osmolalities, which may affect their phenotype and their responses to vasoregulation by the renin–angiotensin–aldosterone system125. Indeed, in vitro, elevated sodium chloride concentrations increase the expression of VCAM1 and E-selectin in human ECs and promote the transmigration of mononuclear immune cells and monocytes, and in vivo, higher salt concentrations enhance myeloid cell binding to ECs127,128. In agreement with these observations, newly identified subpopulations of cortical and medullary capillary ECs in healthy kidneys of mice express an interferon-regulated gene expression signature, including an upregulation of MHC-II, the functional consequences of which need to be validated129 (Fig. 2g). Interestingly, medullary capillary ECs from dehydrated mice, which are exposed to non-physiologically high osmolalities, lower their transcriptional response to IFNβ129, indicating that different osmolalities may influence inflammatory responses via their effects on kidney ECs.
To date, studies of the immunomodulatory potential of ECs in the kidney have focused mainly on glomerular ECs. Glomeruli are the blood-filtering hubs of the nephron and contain fenestrations, which allows them to be selectively permeable to water, salts and specific macromolecules. Compared with other ECs, glomerular ECs have a particularly thick filamentous glycocalyx that contributes to the regulation of fluid balance, but also prevents interactions with immune cells. Upon activation of glomerular ECs in response to infection or as a consequence of disease, such as lupus nephritis, shedding of the glycocalyx exposes surface molecules on ECs that facilitate the extravasation of immune cells into the glomeruli125,130,131 (Fig. 2d). This can contribute to immune cell-mediated damage of glomeruli when immune cells such as neutrophils infiltrate the glomeruli and release their granules125. Glomerular ECs also participate in immune responses by filtering circulating immune complexes from the blood into the glomeruli via transcellular transport, where these are removed by glomerular macrophages, which can also initiate an inflammatory response if appropriately stimulated4 (Fig. 2e).
Immunomodulation by renal ECs is of particular interest in the context of organ transplantation. Renal microvascular ECs are frequently targets of donor-specific antibodies that bind to HLA molecules expressed by the transplanted kidney, and ECs contribute to alloimmunity by upregulating HLA-II genes after transplantation132,133 (Fig. 2f). A recent study of transplanted human kidneys documented a not further specified subpopulation of donor ECs in the transplanted kidney that showed signs of activation134 (suggesting that it is a target of donor-specific antibody-mediated rejection) and an upregulation of genes involved in phagocytosis134, which may indicate antibody uptake. Also, under stress conditions, renal ECs (subtype to be specified) produce transforming growth factor-β (TGFβ)135 and can secrete large amounts of IL-6 (ref.136). These cytokines can promote the differentiation of naive CD4+ T cells into either immunosuppressive Treg cells (when only TGFβ is present) or pro-inflammatory T helper 17 (TH17) cells (when TGFβ and IL-6 are present)137. As antigens presented by MHC-II molecules on renal ECs can skew CD4+ T cell differentiation towards either Treg cells or TH17 cells138,139,140, the inflammatory context that renal ECs are exposed to might have an impact on kidney transplantation success.
Thus, different renal EC populations appear to exert distinct immunomodulatory functions during homeostasis and inflammation and require further study. Therapeutic strategies targeted at ECs in donor kidneys before transplantation may allow the tweaking of EC-mediated immunomodulation in such a way that alloimmunity is decreased and transplantation success increased. Finally, in Wilms tumours, a cancer affecting the kidneys, renal TECs upregulate ACKR1 transcription141. Whether the potential for immune cell recruitment by ACKR1+ TECs can be exploited by tuning additional TEC populations to acquire ACKR1 expression to stimulate tumoricidal immune cell infiltration might be of interest as anticancer therapy, given the generally immunosuppressive features of TECs.
In healthy conditions, the brain is poorly infiltrated by immune cells owing to the low expression of adhesion molecules by the specialized capillary and PCV ECs of the blood–brain barrier (BBB)142 and the abundance of tight junctions between these ECs. Brain ECs thus exhibit a larger level of immune anergy and contribute to the maintenance of the immune privileged state of the brain54. Unlike liver and renal ECs, BBB ECs lack fenestrations and form continuous intercellular junctional complexes, limiting paracellular leakage of molecules from the circulation into the brain. Further, BBB ECs not only express low levels of adhesion molecules (such as ICAM1) but also express lower levels of cytokines and chemokines (such as IL-8 and CCL2), regulated in part by astrocyte-derived sonic hedgehog, which, via hedgehog receptors, induces immune quiescence in ECs, impairing immune cell migration143.
However, in models of infection or inflammatory disease, BBB ECs upregulate adhesion molecules (such as E-selectin and P-selectin) and chemokines (such as CXCL1), thereby promoting immune cell infiltration and inflammation in the brain53,144,145 (Fig. 2h). For example, after transmigration, extravasated monocytes differentiate into TH17-polarizing dendritic cells in response to brain EC-derived granulocyte–macrophage colony-stimulating factor (GM-CSF) and TGFβ146, suggesting a tight regulation of immune cells that interact with brain ECs in mouse models. Intriguingly, depression due to chronic stress alters BBB integrity in animal models, allowing the passage of monocytes and IL-6 from the circulation, and raising the question of whether compromised BBB integrity and depression may indeed be linked147. Interestingly, brain ECs have phagocytotic capacity148, and microvascular ECs of the spinal cord can phagocytose myelin debris and recruit macrophages in vivo14, raising the question of whether specialized brain ECs may process antigens and promote brain inflammation in neurological diseases with an inflammatory component. Indeed, even though BBB ECs have low rates of pinocytosis (suggesting that this is not the main route for extracellular antigens to be acquired), they can present antigens on MHC-I and express MHC-II under inflammatory conditions149,150, which may facilitate adaptive immune responses in the brain by promoting T cell activation and potentially allowing antigen-specific T cells to enter the brain.
scRNA-seq analyses of mouse and human brains provided further insights into the regional heterogeneity of ECs in the brain, in particular in the context of ageing and age-related neurodegenerative disease (Fig. 2i). For example, brain ECs from hippocampi of aged mice upregulate the expression of VCAM1 in a vascular bed-specific pattern151. Indeed, venous and arterial VCAM1+ ECs expressed Tnfrsf1a, Il1r1, Il6ra and Il6st (generally considered to be pro-inflammatory), whereas venous VCAM1+ ECs additionally upregulated genes involved in immune cell infiltration, differentiation and antigen presentation (including Tspo, Lrg1 and B2m) and in pathways involved in TNF and NF-κB signalling151. This suggests that venous brain ECs are the most activated, and thus likely the immune cell-recruiting EC population in aged brains.
Another scRNA-seq study reported VCAM1 expression in a mixed mouse EC population (exhibiting arterial and venous features) but found that it was unaltered in brain ECs from aged brains compared with young brains152. However, aged capillary ECs had increased expression of genes involved in VCAM1-mediated immune cell migration152. Moreover, IFNγ response genes were downregulated in aged arterial and venous ECs compared with young controls, TLR-signalling was upregulated in aged arterial and venous–capillary ECs, and interleukin signalling was predominantly upregulated in aged capillary, venous and capillary–venous ECs152, suggesting a large heterogeneity in inflammatory signalling in ECs from different parts of the aged brain vasculature.
Other scRNA-seq studies document that ageing affects immunomodulation by capillary ECs by upregulating pathways involved in immune cell recruitment to the BBB, but also in innate immunity, TGFβ signalling and antigen processing144, or that ECs from aged mouse brains upregulate the expression of Cxcl12 (ref.153) (encoding a chemotactic ligand for CXCR4-expressing cells154) and Cd9 (ref.153) (encoding a surface protein that promotes the adhesion of immune cells to VCAM1 and ICAM1 (ref.155)). In the entorhinal cortex of patients with Alzheimer disease, ECs upregulated genes involved in the regulation of cytokine secretion and inflammation, including HLA-E (encoding a known natural killer cell modulator), MEF2C and NFKBIA156, indicating that ECs from brain regions affected by Alzheimer disease have a stronger inflammatory signature than brain ECs from age-matched healthy controls. These conflicting reports suggest that ECs from aged brains generally display immunological features that are atypical for ECs from non-aged brains, with the activation of specific subpopulations of brain ECs that are likely to promote the recruitment and functional modulation of immune cells. However, it is unclear which subtypes of brain ECs are most affected by ageing.
We have described the immunomodulatory functions of many different subsets of ECs, which we propose to collectively refer to as ‘IMECs’. The findings discussed herein suggest that (1) IMECs in tissues that are infiltrated by immune cells have specific immune cell-recruiting properties, a feature that can be induced by chronic inflammatory stimuli in non-lymphoid tissues; (2) IMECs in the lung and liver not only promote immune homeostasis but also mediate a careful balance between tolerance and inflammation (their role in immunomodulation may be partially determined by their anatomical location); (3) IMECs in the kidney and liver closely interact with resident immune cells, which may allow swift responses to circulating immune complexes; and (4) IMECs of immune privileged tissues such as the healthy brain form a tight and low immunomodulatory barrier to minimize infiltration of the tissue parenchyma. The capacity of IMECs to facilitate immune homeostasis might be more diverse than realized to date, and appears to depend on the specific subpopulation of ECs in a given tissue and their location in the vascular bed, and may change with age and in response to infection and disease.
However, there are a number of important outstanding questions. For example, it remains to be determined whether IMECs in tumours are tolerogenic or immunostimulatory, and whether they can be rendered more immunostimulatory by promoting their antigen-presenting function. If so, how could this be achieved? Does antigen presentation by IMECs in specific (which?) contexts, organs or conditions promote inflammation or tolerance? And when is antigen specificity a prerequisite for efficient immune cell migration157,158,159? Is the repertoire of antigens (presented by semi-professional antigen-presenting ECs) unique or generic compared with that of professional APCs? How important are IMECs as semi-professional APCs, considering their abundance compared with professional APCs? What is the main mechanism of antigen uptake for the different subtypes of IMECs? Does the apical–basolateral polarity of ECs affect antigen uptake from the circulation or tissue parenchyma? A related question is whether apically expressed MHC and adhesion molecules, which are the first molecules to which recruited T cells bind11, facilitate a sufficiently long interaction between the T cell and the IMEC to allow immunomodulation. Another question is whether some of these molecules are redistributed basolaterally and thereby prolong the duration of IMEC–T cell interaction. What is the contribution of IMECs interacting with perivascular immune cells to tissue immune homeostasis? And adding another layer of complexity, what is the relevance of bone marrow-derived endothelial progenitor cells, which might be recruited to replace injured IMECs3,160, and do these acquire tissue-specific immunomodulatory features similar to those of pre-existing IMECs? Do IMECs develop a form of trained immunity, as observed in in vitro experiments with human aortic ECs161,162,163? EC metabolism affects interferon-stimulated gene expression in ECs via effects on gene methylation, raising the question of how EC metabolism regulates IMEC function across tissues164. Are IMECs polarized towards a pro-inflammatory or an anti-inflammatory phenotype in a tissue-specific manner upon priming by specific pathogen-associated molecular patterns? What are the mechanisms of HEV biogenesis in non-lymphoid tissues? And how do HEVs regulate immunity beyond immune cell recruitment?
The observation that subsets of ECs are involved in immune cell recruitment and vascular inflammation is not novel, but the concept that specific subpopulations of ECs are non-haematopoietic partners in an active immune response is an emerging concept, raising the translationally important question of whether the immunomodulatory capacity of IMECs can be targeted for immunotherapeutic purposes.
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The authors thank L. de Rooij for assistance in searching for literature on single-cell RNA sequencing analyses of endothelial cells, and M. Dewerchin for critically reviewing the manuscript. J.A. is supported by a Marie Skłodowska-Curie individual fellowship and the Fonds Wetenschappelijk Onderzoek (FWO). The work of P.C. is supported by long-term structural Methusalem funding by the Flemish Government, FWO, the Foundation Against Cancer (2016-078), a European Research Council (ERC) Advanced Research Grant (EU-ERC743074) and an ERC Proof of Concept grant (ERC-713758), VIB TechWatch and a Novo Nordisk Foundation Laureate Research Grant (NNF19OC0055802).
P.C. declares associations with Montis Biosciences, Leuven, Belgium, of which he was a scientific co-founder. The remaining authors declare no competing interests.
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Amersfoort, J., Eelen, G. & Carmeliet, P. Immunomodulation by endothelial cells — partnering up with the immune system?. Nat Rev Immunol 22, 576–588 (2022). https://doi.org/10.1038/s41577-022-00694-4
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