Cardiovascular diseases (CVDs), the leading cause of death worldwide, claims ~18 million lives each year and has high morbidity rates (1-4). Atherosclerosis is the main pathologic mechanism of most CVDs (5). These chronic plaques in vessel, composed mainly of cholesterol, fat, calcium and blood cells, gradually harden over time, leading to arterial stenosis and thereby restricting the flow of oxygen-rich blood to various parts of the body (6). Atherosclerosis has been found to be the central mechanism of this pathological process and the leading cause of CVDs worldwide (7).

In recent years, atherosclerosis has been recognized as a chronic inflammatory disease and leads to arterial stenosis and plaque formation due to the accumulation of lipids and inflammatory cells within the arterial wall (8-11). The role of adaptive immunity in atherosclerosis has increasingly attracted attention (12). The adaptive immune system participates in the development of atherosclerosis through various cellular and molecular mechanisms, including the interactions of T cells (13), B cells (14), antigen-presenting cells (15) and cytokines (16). Inflammation plays a key role in the progression of atherosclerosis, and adaptive immune responses influence plaque formation and stability by modulating the activity of inflammatory cells and the secretion of cytokines. These studies further confirm the key role of inflammation and immunity in atherosclerosis and provide a theoretical basis for immune-modulating therapies.

Epigenetics focuses on heritable changes in gene expression that are not derived from alterations in the nucleotide sequence, such as DNA or RNA methylation modifications, as well as post-translational modifications of proteins (17). Lactylation, an emerging post-translational modification, has garnered widespread attention in biological and medical research (18-20). Lactylation plays a crucial role in various biological processes, including the regulation of macrophage function, modulation of glycolytic activity and the promotion of tumorigenesis (21). As a key regulatory factor in numerous biological processes and disease mechanisms, lactylation modification has become a hot topic in molecular biology studies (22,23).

Although the potential role of lactylation modification in CVD has been demonstrated (24), its role in regulating immunity in the context of atherosclerosis remains largely elusive. During the pathological process of atherosclerosis, the accumulation of lactate in local tissues obviously impacts this process. Therefore, vast research efforts have explored the interaction between lactylation modifications and adaptive immunity within the atherosclerotic microenvironment. This review aims to clarify the role of lactylation in immunity in the context of atherosclerosis and summarize the driving mechanisms. This article not only explores the roles of lactate and lactylation in atherosclerosis but also systematically analyzes their impacts on various components of the immune system, such as macrophages, T cells and B cells. To the best of our knowledge, this multidimensional perspective is relatively rare in previous reviews (12,25-27), as most studies have focused solely on a single cell type or mechanism. By closely integrating the metabolic characteristics of lactate with the regulation of immune responses, the article highlights the crucial role of metabolic reprogramming in atherosclerosis. This integrative approach provides a more comprehensive framework for understanding the pathogenesis of atherosclerosis. These innovative aspects not only enrich our understanding of the pathogenic mechanisms of atherosclerosis but also offer new directions for future therapeutic strategies.

Lactate exists in two isomeric forms: D-lactate and L-lactate, with L-lactate being the most common form in living organisms (28). Lactate transport mainly relies on monocarboxylate transporters (MCTs) (29,30). In organisms, glucose is metabolized to lactate via the glycolytic pathway under a short supply of oxygen. Although only 2 ATP molecules are generated per glucose molecule during this process, the energy output is relatively low and the rapid production of lactate quickly provides energy under hypoxic conditions to maintain normal cellular functions (31). As an acidic substance, lactate plays an important role in the acid-base balance of the body. Furthermore, tumor cells often exhibit aerobic glycolysis, producing large amounts of lactate even in the presence of oxygen (32,33). This metabolic phenomenon is known as the 'Warburg effect' and the accumulation of lactate facilitates the growth and invasion of tumor cells (34,35).

Under normal physiological conditions, the production and clearance of lactate are in a dynamic equilibrium. It has been found that lactate is a metabolic byproduct of glycolysis and an important signaling molecule (36-39). The metabolic pathways of lactate vary with the cell type and environment. Lactate can be produced through anaerobic glycolysis, leading to its accumulation, or it can be oxidized to acetyl-coenzyme A (CoA) and enter the tricarboxylic acid cycle (TCA) cycle during aerobic respiration (40). Additionally, lactate can be generated from pyruvate via the Warburg effect (41). These distinct metabolic fates highlight the versatile roles of lactate in different physiological contexts. For instance, in the immune system, lactate modulates the activity of immune cells, inhibiting the overactivation of certain immune cells while promoting the proliferation and differentiation of others (42-44). In addition, lactate is involved in various physiological and pathological processes as an energy source and a signaling molecule and participate in epigenetic modifications (e.g., lactylation) (19).

In the heart, lactate is a byproduct of glycolysis and a key energy source for cardiomyocytes. Under normal physiological conditions, lactate oxidation provides >50% of the energy demands of cardiomyocytes. Even at rest, the proportion of energy supplied by lactate oxidation can reach 10%, and during exercise, this proportion can exceed 50% (45-47). Lactate enters the mitochondria through the lactate oxidation complex, is converted into pyruvate, and then enters the TCA cycle to ultimately generate ATP. Furthermore, lactate activates a series of transcriptional networks, affecting the expression and subcellular localization of metabolism-related proteins in cardiomyocytes, such as hexokinase-2, pyruvate kinase M2 (PKM2) and lactate dehydrogenase A/B, thereby regulating metabolic homeostasis in cardiomyocytes (48-52). Lactate and pyruvate can also scavenge free radicals, protecting cardiomyocytes from oxidative stress damage (53-55). A cross-sectional study involving 1,496 participants reported that blood lactate levels were independently associated with carotid atherosclerosis, suggesting that lactate is involved in atherosclerosis development (56). During the progression of atherosclerosis, lactic acid accumulates in the microenvironment of atherosclerotic plaques as an end product of glycolysis and leads to local acidification. This acidic microenvironment facilitates the binding of oxidized low-density lipoprotein (oxLDL) to aortic proteoglycans, triggers local immune responses and recruits immune cells, thereby exacerbating inflammatory reactions (57). Furthermore, lactic acid stabilizes hypoxia-inducible factor 1α (HIF-1α) and N-myc downstream regulated gene 3. This further promotes the expression of pro-angiogenic factors such as vascular endothelial growth factor and forms a positive feedback loop that promotes atherosclerosis development (58) (Fig. 1).

Lactate participates in cellular metabolism to provide energy, serves as a donor for lactylation modification and contribute to the regulation of the local tissue microenvironment.

Atherosclerosis is a chronic inflammatory disease triggered by lipid metabolism disorders and characterized by the formation of atherosclerotic plaques beneath the intima of large and medium-sized arteries (5,11,12). During the progression of atherosclerosis, the glycolysis levels of cells within the plaque significantly increase, leading to excessive production and release of lactic acid, which in turn acidifies the extracellular environment (59-61). This metabolic change further influences the development of atherosclerosis through multiple mechanisms.

There is a close and complex interrelationship between atherosclerosis and the immune system (25,62,63). As a chronic inflammatory disease, the inflammatory process in atherosclerosis is typically initiated by the deposition of oxLDL and damage to endothelial cells. After endothelial injury, cytokines and chemokines are released as inflammatory mediators and then attract monocytes and other inflammatory cells into the vascular wall, thereby initiating the inflammatory response (Fig. 2) (64). The immune system plays a critical role in atherosclerosis through the interplay of innate and adaptive immune responses, immune cell metabolism and cytokine networks, driving inflammation and plaque progression.

The involvement of various immune cells plays a crucial role in atherosclerosis. Macrophages engulf oxLDL to form foam cells that accumulate within the vascular wall and serve as the foundation for plaque formation (65-67). Lipidomics and transcriptomics analyses have shown that foam cells themselves do not possess pro-inflammatory properties. However, the increased oxidative stress in the arterial wall and the accumulation of modified lipoproteins reprogram the metabolic processes of macrophages, thereby triggering inflammatory responses that promote atherosclerosis (68,69). After cell death, lipid-overloaded macrophages are usually phagocytized by other macrophages (70). However, when phagocytic cells become overwhelmed, they release pro-inflammatory components. The retention of macrophages in the arterial wall is promoted by adhesion molecules (e.g., vascular cell adhesion molecule 1) and neuroguidance factors (such as netrin 1), while high-density lipoprotein facilitates the egress of macrophages from plaques through a C-C motif chemokine receptor (CCR)7-mediated mechanism (71-74). Macrophages in atherosclerotic plaques exhibit pro-inflammatory or anti-inflammatory characteristics but do not follow the classical M1/M2 classification (25,75). Single-cell studies have identified at least five distinct macrophage subpopulations in the mouse aorta, including inflammatory macrophages, type I interferon-inducible cells, foam cells with high expression of triggering receptor expressed on myeloid cells 2, aortic intima-resident macrophages and embryonic-derived resident macrophages (68,69,76,77). These subpopulations play diverse roles in atherosclerosis. These cells exert both pro-inflammatory and anti-inflammatory effects during disease progression and their functions are regulated by multiple factors, such as cholesterol metabolism, oxidative stress and cell-to-cell interactions. Efforts should be invested to further characterize the functions of these macrophage subpopulations and explore their specific mechanisms of action in atherosclerosis.

Adaptive immunity is a key modulator of atherosclerosis. T cells play a complex role in atherosclerosis and different subsets influence disease progression through numerous mechanisms. Type 1 T-helper (Th1) cells are the most prominent CD4⁺T-cell subset in atherosclerotic plaques, promoting lesion development and plaque instability by secreting IFNγ and expressing T-bet (78,79). The mechanisms include inducing the uptake of oxLDL, foam cell formation, polarization of macrophages to a pro-inflammatory phenotype and proliferation of vascular smooth muscle cells (13,80,81).

Lactylation is an epigenetic modification that involves post-translational modifications of both histone and non-histone proteins and plays a key role in the regulation of gene transcription and protein function (114). There are three isomers generated in this process: Lysine L-lactylation (Kl-la), N-ε-(carboxyethyl) lysine and D-lactyl lysine. It has been shown that Kl-la is the primary form of lactylation within cells and significantly contributes to glycolysis and the Warburg effect (115). Lactylation tends to occur at nucleophilic sites, such as amino groups (-NH₂), interacting with corresponding functional groups, and this high level of selectivity allows lactylation to precisely target specific proteins, causing significant changes in their properties and functions (114) (Fig. 3).

Induced by the accumulation of lactate, lactylation modifies proteins and alters the spatial conformation of histones, thereby affecting gene transcription and the regulation of gene expression. Similarly, it affects the function of non-histone proteins by altering their spatial conformation. Lactylation involves a series of enzyme-catalyzed, reversible chemical reactions, rather than spontaneous chemical activity (116). The 'writers' of lactylation refer to enzymes or proteins capable of catalyzing lactylation reactions, transferring lactoyl groups to target proteins after lactate is converted into lactoyl-CoA. The known lactylation 'writers' proteins mainly include histone acetyltransferases Sirtuin (SIRT)1 (117), P300 (117), lysine acetyltransferase 2A (KAT2A) (118), Tat-interactive protein 60 kDa (119,120) and KAT8 (121,122). 'Erasers' include enzymes or proteins that remove or erase lactoyl groups through hydrolysis, restoring the target molecule to its original state, with histone deacetylases (123) and SIRTs (124,125) being known 'erasers' that remove lactylation modifications. In biology, 'readers' are defined as proteins or domains capable of recognizing and interacting with lactoyl groups. Although no specific lactylation readers have been discovered, this may be related to the recent discovery of lactylation and an incomplete understanding of its underlying molecular mechanisms. Lactylation, a post-translational modification first reported in 2019 by Professor Yingming Zhao's group at the University of Chicago, has a shorter research history compared to classical modifications like acetylation and phosphorylation (19). Although the identification of specific 'reader' proteins for lactylation is still in progress, their functional roles have been confirmed through various studies. For instance, histone H3K18 lactylation influences macrophage polarization and tumor microenvironment regulation (126), lactylation of cyclic GMP-AMP synthase inhibits its immune activity, and lactylation in NK cells causes mitochondrial dysfunction (127). The lack of identified 'readers' does not diminish lactylation's biological significance but rather highlights the field's rapid development. Current evidence underscores its crucial roles in gene regulation, immune responses and disease. Future technological advancements and in-depth research are expected to uncover lactylation-specific 'readers', further clarifying its molecular mechanisms. As research progresses, more information about lactylation readers will be revealed in the future, leading to a more comprehensive understanding of the molecular mechanisms of lactylation.

Lactylation is an important epigenetic modification mechanism that plays a key role in various physiological and pathological processes by affecting protein stability, enzyme activity, protein-protein interactions, protein distribution, structure and function (128). Specifically, lactylation involves a series of complex physiological activities, including somatic cell reprogramming (129,130), embryonic development (131-133), neural excitation (134), osteoblast differentiation (135,136), pyroptosis (137,138), macroautophagy (139), decidualization (140), homologous recombination (119,141), cuproptosis (142), myogenesis (143) and oxidative phosphorylation (52,144). In addition, lactylation is closely related to the occurrence and development of various diseases, including but not limited to malignant tumors (22,119), inflammation-related diseases (145-147), fibrosis-related diseases (148,149), Alzheimer's disease (150), heart disease (60,117,151), insulin resistance (152), nonalcoholic fatty liver disease (153), cerebral infarction (154,155), viral infections (156,157) and endometriosis (158).

In summary, this review comprehensively analyzes the multifaceted roles of lactate and lactylation in shaping the immune system within the context of atherosclerosis. As a chronic inflammatory disease, atherosclerosis is characterized by the complex interplay between lipid metabolism, immune cell infiltration and the local microenvironment. The results highlight the critical roles of lactate and lactylation in modulating immune cell functions, metabolic reprogramming and epigenetic regulation, thereby influencing the progression and stability of atherosclerotic plaques.

The involvement of lactate and lactylation in the regulation of macrophage polarization, T cell differentiation and B cell metabolism underscores their potential as therapeutic targets. Specifically, lactate-induced metabolic reprogramming and lactylation modifications have been shown to significantly impact the phenotypes and functions of immune cells and contribute to pro-inflammatory and anti-inflammatory responses. These insights provide a deeper understanding of the immune metabolic mechanisms underlying atherosclerosis and pave the way for novel therapeutic strategies targeting lactate metabolism and lactylation pathways.

However, despite the progress made in elucidating the roles of lactate and lactylation in atherosclerosis, several gaps remain in our knowledge. The complex microenvironment of atherosclerotic plaques, characterized by hypoxia, nutrient deprivation and acidosis, is likely to influence the behavior of immune cells and the efficacy of potential treatments. Yet, the precise mechanisms through which these microenvironmental factors interact with lactate and lactylation to modulate immune responses are still not fully understood. Additionally, the diverse metabolic profiles and functional states of immune cells within plaques suggest that a more detailed characterization of immune cell subsets and their specific roles is needed.

Furthermore, the dynamic changes in the atherosclerotic microenvironment over time, from plaque initiation to rupture, add another layer of complexity to the study of immune regulation. The impact of these temporal changes on lactate production, lactylation modifications and immune cell function remains elusive. Furthermore, the potential compensatory mechanisms and feedback loops involving other metabolic pathways and epigenetic modifications in response to lactate accumulation and lactylation need further investigation.

In conclusion, although significant advancements have been made in understanding the roles of lactate and lactylation in atherosclerosis, the intricate interplay between the microenvironment, metabolic changes and immune regulation remains an area of considerable research need. Future studies should focus on unraveling the detailed mechanisms through which the atherosclerotic microenvironment influences lactate metabolism and lactylation, as well as their downstream effects on immune cell function. These will enhance our understanding of the pathophysiology of atherosclerosis and provide new avenues for developing targeted therapies aimed at modulating immune responses and improving clinical outcomes.