Maximo Agustin Schiavone: HDL Beyond Cholesterol – Linking Lipids and Immunity

Maximo Agustin Schiavone, Head of Clinical Research – Bariatric and Metabolic Surgery Department at Hospital Universitario Austral, shared a post on LinkedIn:

“High-density lipoprotein (HDL) is traditionally known for its role in cholesterol transport and cardiovascular protection. However, emerging evidence suggests that HDL also plays a crucial role in the immune system.

This article aims to explore the multifaceted functions of HDL in innate and adaptive immunity, the behavior of HDL in various inflammatory states, the impact of immunosuppressants on HDL levels, and its association with all-cause mortality positioning HDL as a potential marker of immune activity.

The Role of HDL in the Immune System

Integration in Innate Immunity

HDL participates actively in innate immunity. It binds and neutralizes lipopolysaccharides (LPS) and lipoteichoic acid (LTA), which are components of the cell walls of Gram-negative and Gram-positive bacteria, respectively.

By neutralizing these endotoxins, HDL prevents excessive inflammatory responses that can lead to sepsis.

HDL also enhances the clearance of apoptotic cells by macrophages, a process known as efferocytosis, which is crucial for resolving inflammation.

Integration in Adaptive Immunity

HDL’s role in adaptive immunity is increasingly critical for maintaining immune balance.

Adaptive immunity involves a more specific and long-lasting response to pathogens, primarily mediated by lymphocytes, including T-cells and B-cells.

• Modulation of Dendritic Cells (DCs): HDL influences the function of dendritic cells (DCs), pivotal for antigen presentation and the initiation of adaptive immune responses. HDL-associated apolipoproteins, such as ApoA-I, can modulate DC activity by inhibiting the expression of co-stimulatory molecules. This inhibition reduces the DCs’ ability to activate T-cells, promoting a more tolerogenic environment. This mechanism is essential in preventing overactive immune responses and autoimmune diseases.
• Impact on T-Cells: T-cells are crucial for adaptive immunity, and HDL affects their function and activation. HDL can alter the lipid raft composition of T-cell membranes, which is essential for T-cell receptor (TCR) signaling. By modulating these lipid rafts, HDL can influence T-cell activation and differentiation. This regulation is particularly important in controlling excessive immune responses and maintaining immune tolerance.
• Influence on B-Cells: HDL also affects B-cells responsible for antibody production. HDL particles can bind to B-cell receptors and modulate their activity. This interaction can impact the production of antibodies and the overall humoral immune response.HDL’s role in B-cell function underscores its importance in adaptive immunity, particularly in the context of vaccination and immune memory.
• Regulation of Cytokine Production: HDL can influence the production of cytokines, which are signaling molecules critical for immune responses. By modulating cytokine production, HDL helps balance pro-inflammatory and anti-inflammatory signals, ensuring a controlled immune response. This regulation is vital in preventing chronic inflammation and autoimmune conditions.
• Impact on Monocytes/Macrophages: HDL plays a significant role in modulating the activity of monocytes and macrophages, which are key players in innate and adaptive immunity. HDL promotes cholesterol efflux from these cells, reducing lipid accumulation and preventing foam cell formation, a critical step in atherosclerosis development. Additionally, HDL can inhibit the expression of adhesion molecules and the release of pro-inflammatory cytokines by macrophages, thereby reducing inflammation. HDL also enhances the anti-inflammatory properties of macrophages, promoting the M2 macrophage phenotype, which is associated with tissue repair and resolution of inflammation.

HDL Behavior in Different Inflammatory States

Sepsis

During sepsis, HDL levels decrease significantly, and the composition of HDL particles is altered. Acute inflammation leads to the remodeling of HDL, resulting in the loss of anti-inflammatory and antioxidative properties.

Low HDL levels during sepsis are associated with poor outcomes, including higher mortality rates. Therefore, HDL could serve as a valuable marker for the severity of sepsis and the effectiveness of therapeutic interventions.

Chronic Systemic Inflammation

Chronic systemic inflammation, as seen in metabolic syndrome and cardiovascular diseases, affects HDL levels and function. Patients with metabolic syndrome often exhibit low HDL levels and dysfunctional HDL particles.

These dysfunctional particles have a reduced capacity for cholesterol efflux and antioxidant activity, contributing to the persistence of inflammation and atherosclerosis.

Impact on Cardio-Renal-Metabolic Diseases:

Chronic systemic inflammation is pivotal in disease progression and complications in cardio-renal-metabolic diseases.

HDL is critically involved in modulating this inflammatory milieu. In patients with cardiovascular diseases, such as coronary artery disease and heart failure, low HDL levels and dysfunctional HDL particles are common findings.

These HDL particles often exhibit reduced antioxidant and anti-inflammatory capacities, leading to enhanced oxidative stress and endothelial dysfunction, key drivers of atherosclerosis and cardiac events.

In renal diseases, such as chronic kidney disease (CKD), HDL functionality is significantly impaired. CKD is associated with a pro-inflammatory state that alters HDL composition and reduces its protective properties.

Uremic toxins, commonly elevated in CKD, can modify HDL particles, reducing their ability to promote cholesterol efflux from macrophages and impairing their anti-inflammatory functions.

This dysfunctional HDL contributes to the accelerated atherosclerosis observed in CKD patients, increasing their risk of cardiovascular events.

Metabolic diseases, including type 2 diabetes and obesity, also impact HDL levels and functionality. Chronic low-grade inflammation is prevalent in these conditions, and HDL particles often become oxidized and glycated. These modifications reduce HDL’s effectiveness in cholesterol transport and its anti-inflammatory properties.

Furthermore, insulin resistance, a hallmark of metabolic syndrome, is associated with reduced HDL synthesis and increased HDL catabolism, further exacerbating the dyslipidemia and inflammatory state.

Monitoring HDL levels and functionality in patients with cardio-renal-metabolic diseases can provide valuable insights into disease activity and progression.

Therapeutic strategies aimed at improving HDL function and raising HDL levels may significantly reduce inflammation and improve cardiovascular and renal outcomes in these patients.

Rheumatic Diseases

In rheumatic diseases, such as rheumatoid arthritis and systemic lupus erythematosus (SLE), HDL levels are often reduced.

The chronic inflammation associated with these diseases leads to the oxidation and modification of HDL, impairing its protective functions.

Monitoring HDL levels and functionality in rheumatic diseases could provide insights into disease activity and the effectiveness of anti-inflammatory treatments.

Cancer

Cancer and its associated treatments can profoundly impact lipid metabolism. Patients with cancer often experience dyslipidemia, including alterations in HDL levels and functionality.

Chemotherapy and radiation therapy can exacerbate these changes, further complicating the management of inflammation and cardiovascular risk in cancer patients.

Evaluating HDL levels and functionality in cancer patients could help assess the overall health status and guide supportive care strategies.

The Impact of Immunosuppressants on HDL Levels

Immunosuppressants Targeting T-cells

• Immunosuppressants that target T-cells, such as calcineurin inhibitors (e.g., cyclosporine and tacrolimus), can significantly affect HDL levels. These drugs often lead to dyslipidemia, characterized by elevated low-density lipoprotein (LDL) and reduced HDL levels. The reduction in HDL can impair the body’s ability to modulate inflammation and may increase susceptibility to infections and cardiovascular diseases in patients undergoing immunosuppressive therapy.

Immunosuppressants Targeting B-cells

• Rituximab, a monoclonal antibody targeting CD20 on B-cells, also affects lipid metabolism. Studies have shown that rituximab treatment can lead to transient increases in HDL levels.The mechanism behind this increase is not fully understood, but it may be related to reduced systemic inflammation and altered cytokine profiles following B-cell depletion.

Broad-Spectrum Immunosuppressants

• Broad-spectrum immunosuppressants, such as corticosteroids, have complex effects on lipid metabolism. While corticosteroids can increase HDL levels, they also raise triglycerides and LDL levels, which can offset the beneficial effects of HDL. The overall impact of corticosteroids on cardiovascular risk and immune function depends on the balance of these changes in lipid profiles.

HDL and All-Cause Mortality

• Extensive research has examined the association between HDL levels and all-cause mortality. Studies have consistently shown that low HDL levels are linked to higher all-cause mortality, independent of other cardiovascular risk factors. This relationship underscores the protective role of HDL not only in cardiovascular health but also in overall mortality risk.
• Cardiovascular Mortality: Low HDL levels are strongly associated with increased cardiovascular mortality. HDL’s role in reverse cholesterol transport, antioxidative activities, and anti-inflammatory properties are critical in preventing atherosclerosis and subsequent cardiovascular events. Dysfunctional HDL, which lacks these protective properties, contributes to the progression of cardiovascular diseases and higher mortality rates.
• Non-Cardiovascular Mortality: Interestingly, low HDL levels are also linked to non-cardiovascular mortality, including deaths due to infections and cancer. HDL’s role in the immune system, particularly its ability to neutralize pathogens and modulate immune responses, is likely a key factor in this association. Inflammatory states and immune dysfunction, common in chronic diseases and cancer, can lead to reduced HDL levels and impaired functionality, increasing susceptibility to infections and mortality.

Last thoughts

HDL is more than just a cholesterol transporter; it is a dynamic player in the immune system. Its roles in neutralizing endotoxins, modulating immune cell functions, and resolving inflammation underscore its importance in maintaining immune homeostasis.

The impact of immunosuppressants, the behavior of HDL in various inflammatory states, and its association with all-cause mortality highlight its potential as a marker of immune activity. Future research should focus on elucidating the mechanisms behind HDL’s immunomodulatory functions and exploring its use as a therapeutic target and biomarker in immune-related diseases.”

Stay updated with Hemostasis Today.