In January, I got to shadow in the hospital for several weeks.
As PhD student, there were many mishaps. Aside from the general awkwardness of not knowing where to stand, I told the wrong patient that they were getting a procedure and I made residents upset by not checking on patients that were assigned to me.
Overall, the experience was very eye opening. I was at a community hospital, meaning that many of the patients came in with typical age related diseases. This is in contrast to a larger hospital system where specialists are required to handle referral cases. Nonetheless, people the in the hospital are very sick. There were many cases of COPD, heart disease, sepsis, etc. When I was in the ICU, I saw several patients pass away from multi-organ failure and become delirious from their hospital stays. But in many other cases, the reason that people are in the hospital sometimes is unrelated to how acute the illness is, but gated by external factors such as availability of nursing home beds. Many patients are there simply because they need to continue to be diuresed.
Throughout the experience, I got to think about all the ways improvements in medical technology or biotechnology could improve patient outcomes. The top three that came up from discussions with classmates were:
However, what ended up being the most important observation from my time on the internal medicine wards was finding that many of the conditions people suffered from were diseases of vasculature. Diabetes, chronic kidney disease, hypertension (general and pulmonary), atherosclerosis, and even sepsis are all disease of vasculature. People die from cardiovascular disease, strokes, peripheral artery disease, all caused by large artery stiffining, microvascular dysfunction, or many other issues stemming from vasculature.
Most patients suffering from these diseases are old. Vascular aging can have several root causes. Elastin is synthesized during development and not replaced throughout aging. Cumulative damage weakens it, and it does not get repaired. Sterile inflammation during aging or endothelium specific autoantibodies can be immune mediated mechanisms. Still, many individuals sustain pristine vascular health throughout old age. For this reason, I was interested in understanding how the vasculature can be targeted therapeutically and categorizing how endothelial cells become dysfunctional during disease.
I think the best way of thinking about dysfunction of a cell type independent of genetic abnormalities is its evolution programmed behavior in healthy individuals. When a cell type becomes ‘dysfunctional’ during disease there is probably a reason for its behavior.
The primary purpose of vasculature is to carry blood and plasma to distribute cells, oxygen, nutrients, and other signaling factors. In order to do this, vasculature first needs to identify areas of poor plasma distribution. The way that it does this is via sensing of oxygen tension and responding to hypoxia. This mechanism is controlled primarily by VEGF signaling where endothelial cells will respond and grow towards tissues that secrete VEGF. These exact same mechanisms become pathogenic during cancer and retinal vascular diseases, where excess angiogenesis drives disease pathology.
Another aspect of proper distribution of oxygen is adequate blood flow. Vasculature constricts and relaxes to delivery the optimal amount of blood to peripheral organs. When the kidney senses that not enough blood is being filtered, vasculature constricts, and when during exercise more blood is needed in the periphery, vasculature relaxes. In diseases where blood pressure is abnormal, endothelial cells are made to constrict or relax suboptimally.
In order to maintain blood flow, there also needs to be smooth laminar flow. This is disrupted during a breakage to the endothelial wall. When this happens, platelets can also encounter the matrix outside of the vessel which provides a surface for initiation of the clot. Disrupted flow increases expression of p-selectin, a platelet adhesion molecule, as well as vWF and other clotting factors.
Finally, vasculature can also function as a sensor of infection. In these circumstances, the natural solution is to recruit cells capable of fixing the plumbing, ie various immune cells. Thus, a major function of vasculature is secretion of immunomodulators like chemokines and cytokines and expression of various adhesion molecules to enable leukocytes to marginate into vessel walls. Conversely, inflammatory cytokines can also cause endothelial activation and expression of adhesion molecules, also enabling leukocyte margination. However, these exact same pathways can be involved when other substances, like cholesterol crystals, mimic what an infection may look like to the immune system.
If we are to think about it as an engineer, endothelial cells have the following inputs, outputs, and sensors:
Sensors:
Inputs:
Outputs:
Using these tools, endothelial cells have the ability to be resilient and recover after injury, as well as defend itself during an infection. Understanding these pathways can also help us understand how to modulate endothelium therapeutically.
A review of indications where vasculature plays a central role will reveal that vasculature is often never the root cause. However, targeting vasculature can still be therapeutically valuable. The following sections provide a brief description of what happens to endothelium during disease and how endothelial cell activities can exacerbate pathology.
Atherosclerosis or “aterial hardening” is the most common age related change that affects virtually everyone and especially those on a Western diet. Cholesterol plaque builds up in vascular wall throughout the body, but clinically this most often manifests as ASCVD or atherosclerotic coronary artery disease, and PAD, peripheral artery disease. Cholesterol deposits not only clog arteries and restrict blood flow, eventually causing ischemic events (ie. heart attacks or leg ischemia) from clot formation, but also cause chronic activation of the NLRP3 inflammasome in phagocytic cells such as macropahges and monocytes that try and clear the plaques. This is one of many sources of ‘sterile inflammation’ that occur through age.
In retinal vascular diseases, like wet AMD, diabetic macular edema (DME), and Retinal vein occlusion (RVO), there is the problem of vascular leakage. There is either too much immature vasculature, or mislocalized neovascularization as seen in wet AMD, chronically damaged vasculature like in DME, or pressure induced peakage in RVO. Leaky vasculature releases tons of protein and cellular debris, making it hard to see through. Treatment for these conditions typically involves stabilization of existing vasculature or inhibiting future vascular growth by neutralizing growth factors like VEGF.
Hereditary angioedema (HAE) is commonly caused by excess bradykinin, a vasodilator that increases vascular permeability and swelling. The most common genetic mechanism is loss of function mutations in C1-INH, which normally inhibits factor XII and plasma kallikrein. Factor XII and kallikrein are proteases that start a proteolytic cycle that leads to production of bradykinin from precursor molecules. Excess swelling causes the characteristic “HAE attacks” that cause intense swelling in HAE patients.
Hereditary Hemorrhagic Telangiectasia (HHT) is a separate condition in which vasculature is malformed. These are caused by mutations to TGFβ pathway proteins including ALK1, ENG, SMAD4, and GDF2, leading to loss of ALK1 signaling and excess Akt signaling. Clustering antibodies that restore ALK1 signaling or Akt inhibitors are in clinical trials and could be curative for these diseases.
Sepsis is a systemic inflammatory disorder characterized by leaky vasculature and massive infiltration of immune cells into tissue, typically caused by an infection. While the infection can be controlled generally by antibiotics, the vascular leak and systemic inflammation can be fatal. Leaky vasculature and a more hypercoagulable state reduce blood flow to peripheral organs, and often the first medication utilized are vasopressors that increase constriction and restore blood pressure. While patients with sepsis often have multiple comorbidities, it is still shocking how high the mortality rate is. Up to 25% of patients hospitalized with sepsis die, most often due to septic shock (from hypotension) and multi-organ failure (often secondary to inadequate blood flow and coagulopathy).
A very simple to understand mechanism of disease is simply vascular injury. This can happen in a variety of different ways ranging from surgery, diabetes, radiation, and high blood pressure. Lymphedema is a common secondary condition caused by injury to lymphatics, the portion of vasculature responsible for carrying lymph (e.g. surgery for cancer where lymph nodes are often dissected out). Diabetes and high blood glucose is damaging to vasculature because high concentrations of glucose lead to excess glycation of nearby proteins and production of reactive oxygen species. Radiation and chemotherapy induced vascular damage often manifests years after cancer therapy. Patients are often children with blood cancers and they have accelerated onset of atherosclerosis and strokes later in life. Speaking of strokes, emboli are the immediate cause but the strongest modifiable risk factor is hypertension. The link between hypertension and clot risk is that high blood pressure can stretch and injury microvasculature, making it more likely for ‘rips’ to form and substrate to be exposed for clots.
In systemic sclerosis, patients have autoantibodies against endothelial cell antigens. These autoantibodies are produced by rogue B cell clones and can be addressed therapeutically by B cell depleting agents. Autoantibodies lead to the characteristic cascade of inflammation (vascular damage -> cytokine release and adhesion molecule expression) and fibrosis (fibroblast activation from TGFβ and hypoxia).
Other causes of vascular inflammation are commonly summarized as ‘vasculidities’. These include ANCA associated vasculitis, immune complex vasculitis, and giant cell arteritis (GCA). ANCA associated vasculitis is caused by neutrophil damage to vasculature. Neutrophils circulate systemically and when patients have ANCAs (antineutrophil cytoplasmic antibodies), neutrophils get activated by ANCAs and release neutrophil extracellular traps, express adhesion molecules to adhere to microvasculature, and release enzymes and reactive oxygen species that damage endothelium. Immune complex vasculitidies have endothelial damage caused by agggregates of immunoglobulins bound to antigen. This typically happens when there is a lot of foreign antigen, antibodies bind to it, and these ‘immune complexes’ randomly deposit throughout the vasculature and their clearance is damaging (complement activation, neutrophil recruitment and damage). Finally, giant cell arteritis is more localized to arteries in the temples (on your head). It is vision threatening and needs to be treated immediately. Giant cell arteritis is a T cell and macrophage mediated disease. There may be viral triggers or there may be age associated vascular antigens that cause T cell targeting of vasculature.
Finally, we have hypertension. This is an age associated condition where blood pressure is simply too high. The two important subsets are pulmonary arterial hypertension (PAH) and primary hypertension, which is systemic. PAH is often caused by dysfunctional TGFβ signaling and mutations in BMPR2, which tilt the balance of signaling towards proliferation in pulmonary vascular cells. The major effective therapy for PAH, sotatercept, binds up activins and other GDFs which drive proliferative signaling, which restores the relative signaling balance.
In primary hypertension, there are often age associated decreases in vascular compliance (how stretchy vessels are), which makes it more difficult for heart beats (and associated transient increases in blood pressure) to be buffered. Sodium intake can also make total blood volume higher. Obesity, heart problems, alcohol, sleep issues, and lack of exercise can all contribute. Therapeutics for primary hypertension (outside of lifestyle intervention) are focused at increasing fluid outflow with diuretics, manipulating the renin angiotensin aldosterone system (RAAS) to excrete more volume, and by directly relaxin endothelial cells with vasodilators.
There are several axes within vasculature that can be harnessed therapeutically:
One fundamental way to intervene is by controlling vascular tone. Vasopressors can increase tone when needed by causing constriction, while nitric oxide or endothelin A antagonists can induce vasodilation. More sophisticated approaches target signaling pathways: PDE5 inhibitors and cGMP modulators (e.g., sildenafil, tadalafil) promote vasodilation in pulmonary arterial hypertension by amplifying NO signaling, while soluble guanylate cyclase stimulators (e.g., riociguat) directly modulate cGMP production. Adrenergic modulation using α1 or β blockers adjusts systemic vascular tone in hypertension through the nervous system.
Tie2 agonists and S1P1 receptor agonists (e.g. fingolimod) stabilize the endothelial barrier. Tie2 agonism has been applied in diseases ranging from wet AMD (e.g., faricimab) to sepsis, reducing pathological leakage and inflammation. S1P1R agonists have ben used in multiple sclerosis.
Endothelial cells exist along a spectrum from quiescent, anti-proliferative states to activated, proliferative phenotypes. Therapies that reprogram endothelial signaling can shift this balance. For example, ALK1-targeted agents (Diagonal Therapeutics) and sotatercept (an activin/GDF ligand trap) modulate TGFβ superfamily pathways to restore quiescence and limit pathological smooth muscle proliferation, particularly in pulmonary hypertension.
Endothelial injury often occurs in the context of inflammation. Cytokine-targeted therapies such as IL-6, IL-1β, and NLRP3 inhibitors can blunt endothelial activation and downstream fibrosis or thrombosis. Broader immunomodulatory approaches, including TNF inhibitors, IL-17/IL-23 blockers, and JAK inhibitors, are used in contexts where vascular inflammation is mediated by systemic or local immune dysregulation, as in vasculitides or autoimmune vascular disease.
Finally, the vascular system can be therapeutically tuned for angiogenesis. VEGF inhibition suppresses abnormal new vessel growth, a strategy used in wet AMD and cancer. Conversely, therapies targeting FLT4/VEGFC can stimulate lymphatic or vascular growth where tissue repair or fluid drainage is desirable, such as in lymphedema or tissue ischemia.