The Emerging Role of B Cells in the Pathogenesis of NAFLD

Abstract

NAFLD is one of the leading causes of abnormal liver function worldwide. NAFLD refers to a group of liver conditions ranging from nonalcoholic fatty liver to NASH, which involves inflammation, hepatocellular damage, and fibrosis. Triggering of inflammation in NASH is a key event in the progression of the disease, and identifying the factors that initiate or dysregulate this process is needed to develop strategies for its prevention or treatment. B cells have been implicated in several autoimmune and inflammatory diseases. However, their role in the pathogenesis of NAFLD and NASH is less clear. This review discusses the emerging evidence implicating intrahepatic B cells in the progression of NAFLD. We highlight the potential mechanisms of B‐cell activation during NAFLD, such as increased hepatic expression of B‐cell–activating factor, augmented oxidative stress, and translocation of gut‐derived microbial products. We discuss the possible effector functions by which B cells promote NAFLD, including the production of proinflammatory cytokines and regulation of intrahepatic T cells and macrophages. Finally, we highlight the role of regulatory and IgA⁺ B cells in the pathogenesis of NASH‐associated HCC. In this review, we make the case that future research is needed to investigate the potential of B‐cell–targeting strategies for the treatment of NAFLD.

Abbreviations

BAFF

B‐cell–activating factor

BCR

B‐cell receptor

Bregs

regulatory B cells

cDCs

classical dendritic cells

DCs

dendritic cells

HFD

high‐fat diet

IFN

interferon

IR

insulin resistance

KCs

Kupffer cells

LPS

lipopolysaccharide

MOMFs

monocyte‐derived macrophages

MYD88

myeloid differentiation primary response 88

NAFL

nonalcoholic fatty liver

OSEs

oxidative‐stress–derived epitopes

sc‐RNAseq

single‐cell RNA sequencing

Th

T helper

TLRs

Toll‐like receptors

NAFLD is estimated to affect 30% of the population and is one of the leading causes of abnormal liver function.^{( 1 )} NAFLD covers a wide spectrum of liver pathology ranging from simple lipid accumulation to NASH, characterized by steatosis, inflammation, hepatocellular injury, and fibrosis.^{( 2 )} Hepatic inflammation, driven by liver immune cells, is a critical component in the initiation and progression of NASH.^{( 3 )} Notably, the liver is a unique immunological site where immune cells encounter antigen‐rich blood from the gastrointestinal tract.^{( 4 )} NASH is characterized by a robust recruitment of immune cells into the liver where they become activated and release molecules that cause inflammation.^{( 5 )} Innate immune mechanisms are a major contributing factor to the inflammatory process in NASH. However, recent evidence indicates that adaptive immunity plays an essential role in the progression of NASH, fibrosis, and HCC.^{( 6 )} B lymphocytes are emerging as central mediators of NAFLD because of their ability to secrete cytokines and antibodies and promote inflammation. In this review, we discuss the evidence for a role of B cells in the progression of NAFLD, including the mechanisms of their activation, their contribution to NASH‐associated HCC, and the potential for B‐cell–specific therapeutic strategies.

The NAFLD Inflammatory Environment at a Glance

Role of Innate immune cells

The liver receives a constant influx of gut‐derived products and acts as a firewall for the antigen‐rich mesenteric circulation when the mucosal–blood barrier is compromised.^{( 7 )} NAFLD is associated with increased intestinal permeability that facilitates the translocation of bacterial products and intact bacterium into the liver that triggers inflammation in the liver.^{( 8 )} Innate immune responses, particularly those initiated by macrophages, are a major contributing factor to disease progression. Hepatic macrophages are a heterogeneous population of cells that can be categorized into tissue‐resident Kupffer cells (KCs) and monocyte‐derived macrophages (MoMFs). KCs originate primarily from the yolk sac and are maintained in the liver through self‐renewal.^{( 9 )} During NASH, activation of KCs by lipopolysaccharide (LPS) and fatty acids promotes hepatic inflammation and fibrosis through the production of TNFα^{( 10 )} and IL‐1β.^{( 11 )} The prevailing dogma is that KC‐derived factors promote the infiltration and expansion of MoMFs, which further aggravate the disease through their inflammatory functions.^{( 12 )} Recent single‐cell RNA sequencing (sc‐RNAseq) of intrahepatic immune cells shows that NASH is characterized by a loss of embryonically derived KCs and increased infiltration of MoMFs.^{( 13 )} In addition, a remarkable subset of macrophages expressing the lipid receptor, triggering receptor expressed on myeloid cells 2, that expands in the NASH liver to coordinate hepatocyte energy supply and mitochondrial function, has been identified.^{( 14 )} The liver contains a reservoir of classical (cDCs) and plasmacytoid (pDCs) dendritic cells (DCs) strategically located in the subcapsular space, between hepatocytes, and in the vasculature.^{( 15 )} In human NASH, the ratio of cDC subsets shifts toward increased type 2 cDCs that could activate pathogenic T cells.^{( 16 )} However, the function of DCs in NASH is less clear given that animal studies suggest that cDCs, particularly classical type 1 DCs, prevent fibrosis and inflammation.^{( 17} , ^{18 )} Furthermore, interferon (IFN)α‐producing pDCs expand in the livers of high‐fat diet (HFD)‐fed mice and contribute to hepatic insulin resistance (IR).^{( 19 )} Neutrophils promote NASH through the secretion of elastase^{( 20 )} and myeloperoxidase.^{( 21 )} Accordingly, antibody‐mediated depletion of neutrophils in mice ameliorated liver steatosis and inflammation in mice fed an HFD.^{( 22 )} Given that neutrophils remove dead vasculature and create new channels for vascular regrowth in sterile liver injury,^{( 23 )} a better understanding of their reparative function in NASH is needed.

Role of Adaptive Immune Cells

Although the innate immune response in NASH has been the focus of research, growing evidence indicates that T and B cells play essential roles.^{( 6 )} Upon activation by type 1 IFN signals and intestinal antigens, CD8⁺ T cells promote nonalcoholic fatty liver (NAFL)^{( 24 )} and NASH^{( 25 )} through the secretion of IFNγ and TNFα.^{( 24 )} Indeed, depletion of CD8⁺ T cells can restore hepatic insulin sensitivity,^{( 24 )} decrease liver damage,^{( 25 )} and reduce fibrosis.^{( 26 )} During NASH, elevated levels of hepatic acetate and extracellular ATP stimulate resident CD8⁺ T cells that show an exhausted phenotype and enhanced hepatocyte killing activity.^{( 27 )} NASH promotes increased intrahepatic CD4⁺ T‐cell polarization toward IFNγ secreting T helper (Th)1 and IL‐17 secreting Th17 subsets.^{( 28 )} Elevated levels of intrahepatic IL‐17 accelerate the pathogenesis of NASH and HCC.^{( 29 )} As a result, an increased peripheral and intrahepatic Th17/regulatory T cell ratio has been proposed as a biomarker of progression from NAFL to NASH.^{( 28 )} Furthermore, obesity‐associated endotoxemia can also result in the expansion of IL‐17‐secreting γδ T cells in the liver, sustained by microbial antigens that enter through the portal vein.^{( 30 )} Natural killer T cells, a subclass of T cells that express an invariant T‐cell receptor, infiltrate the NASH liver and secrete proinflammatory cytokines, such as LT‐related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator on T cells^{( 24 )} and IFNγ,^{( 31 )} and contribute to hepatic fibrosis through the activation of HSCs.^{( 32 )} Despite the substantial evidence showing the diverse functions of adaptive immune cells, such as T cells in NAFLD, the role of B cells in the pathogenesis of the disease is less understood.

B Cells and Their Subsets

B cells can be categorized primarily into B1 and B2 subsets according to the expression of cell‐surface markers (Table 1). B2 cells arise from hematopoietic progenitors in the bone marrow.^{( 33 )} The development of B2 cells is antigen independent and involves rearrangements of the V‐D‐J regions of the heavy‐ and light‐chain Ig gene to form an immature B cell.^{( 34 )} Immature B cells expressing IgM exit the bone marrow to migrate into the spleen where they mature into the marginal zone and follicular B cells in a process that requires the survival factor B‐cell–activating factor (BAFF).^{( 33 )} Splenic marginal zone B cells are involved in thymus‐dependent and ‐independent responses to blood‐borne pathogens and lipid antigens.^{( 35 )} Mature follicular B cells make up most of the B‐cell populations in the periphery and secondary lymphoid organs where they survey for antigens.^{( 35 )} B1 cells arise from the fetal liver and primarily reside in peritoneal and pleural cavities.^{( 36 )} B1 cells are classified into B1a cells that produce natural antibodies against self‐antigens and B1b cells, which can class switch to IgG3 or IgA after antigen binding.^{( 37 )} A functionally distinct subpopulation of B cells, known as regulatory B cells (Bregs), are strong producers of IL‐10 and IL‐35 that suppress inflammation.^{( 38 )} Following antigen recognition, B cells undergo thymus‐dependent or ‐independent activation, affinity maturation, and class switching.^{( 39 )} Activated B cells can subsequently differentiate into antibody‐secreting plasma cells and memory B cells.^{( 40 )} During thymus‐dependent activation, the antigen is bound by the B‐cell receptor (BCR), internalized, and presented to follicular Th cells, which become activated and provide signals to the B cell through CD40/CD40 ligand interactions and cytokines.^{( 41 )} In contrast, thymus‐independent B‐cell activation does not require a second signal from T cells, but instead involves signaling through Toll‐like receptors (TLRs) or cross‐linking of the BCR.^{( 42 )} Although their precise function at steady state is less clear, most B‐cell subsets, such as mature, immature, B2, B1, and plasma cells, have been detected in the human liver.^{( 43} , ^{44 )}

TABLE 1.

Murine B‐Cell Subsets and Their Markers

B‐Cell Subset	Markers	Refs.
B1 B cells
B1a	CD19+ CD5+ CD11b+/− B220lo IgMhi IgDlo CD23− CD43+	( 35 )
B1b	CD19+ CD5− B220lo IgMhi IgDlo CD23− CD43+	( 35 )
B2 B cells
Follicular	CD19+ B220+ IgMlow IgDhi CD21med CD1dlow CD23+	( 35 )
Marginal zone	CD19+ B220+ IgMhi IgDlow CD1dhi CD21hi CD23‐	( 35 )
Memory	CD19+ B220low/+ CD21+ CD95+ CD40+ CD273+ CD73+ CD80+	( 83 )
Plasma cells	CD138+ CD93+ CXCR4+ CD44+ VLA‐4+ BCMA+ IL6‐R+	( 84 )
Regulatory	CD19+ PD‐L1+/− Tim‐1+/− FasL+/− LAG‐3+/−	( 38 )

Abbreviations: BCMA, B‐cell maturation antigen; FasL, Fas ligand; LAG‐3, lymphocyte activation gene‐3; Tim‐1, T‐cell immunoglobulin and mucin domain 1; VLA‐4, very late antigen‐4.

Adipose Tissue B Cells in Metabolic Disease

B2 cells accumulate in the obese adipose tissue where they contribute to local and systemic IR through pathogenic IgG antibodies^{( 45 )} and activation of T cells and macrophages.^{( 46 )} In humans, adipocyte‐specific IgG against antigens released by dying cells are found locally and in the plasma, suggesting that obesity promotes B‐cell responses against “self.”^{( 47 )} During obesity, IgG undergoes hyposialylation and can promote IR through activation of its receptor, functionally impaired Fcγ receptor IIB.^{( 48 )} Although the mechanisms underlying the expansion or infiltration of B cells into obese adipose tissue are poorly defined, signaling through the chemokine, leukotriene B4, and its receptor is required.^{( 46 )} B cells are among the first immune populations to infiltrate the mesenteric adipose tissue of mice fed an HFD from where they migrate to the liver and promote hepatic inflammation.^{( 49 )} In contrast to B2 cells, Bregs maintain adipose tissue homeostasis through IL‐10, and their loss during obesity leads to adipose tissue inflammation and IR.^{( 50 )} In mice fed an HFD, adipose tissue B1 cells ameliorate disease attributable to their production of IL‐10 and polyclonal IgM^{( 51 )}; however, a role for B1 cells in adipose tissue is yet to be confirmed in humans. Furthermore, whether B cells travel between adipose tissue and the liver as a strategy to coordinate whole‐body metabolism during obesity or NAFLD is still being uncovered.

Intrahepatic B Cells in the Progression of NAFLD

Although limited, increasing evidence suggests that intrahepatic B cells participate in the progression of NAFLD (Table 2; Fig. 1). Patients with NAFL or NASH show increased intrahepatic B cells in association with higher levels of circulatory IgG against oxidative‐stress–derived epitopes.^{( 52 )} In patients with NASH, intrahepatic B cells are more abundant in the portal tract than in the lobules, suggesting that they participate in the portal inflammatory process.^{( 53 )} Studies using rodent models suggest a pathogenic function for these intrahepatic B cells in the pathogenesis of NAFLD. Mice fed an HFD that develop liver steatosis show an increased accumulation of B cells that express increased amounts of proinflammatory cytokines and have a higher capacity to activate T cells.^{( 54 )} Whether this accumulation and activation of B cells are causal or consequential to the triggering of NASH, or are merely bystander effects of the inflammatory environment, has been less clear. Our laboratory recently showed that intrahepatic B cells, primarily B2 cells, accumulate in the liver of NASH mice where they express a proinflammatory gene expression profile and secrete increased amounts of IL‐6 and TNFα, thereby promoting inflammation and fibrogenesis.^{( 55 )} Although we show that B‐cell–deficient mice have improved glucose tolerance during NASH, whether intrahepatic B cells can directly promote local IR, similar to their adipose tissue counterparts, remains to be determined. Recent transcriptome and surface proteome assessments of immune cells in mice fed a NASH‐inducing Western diet showed a distinct cluster of CD38⁺ B cells whose proportions were altered with NASH progression.^{( 56 )} Using sc‐RNAseq with superloading and multiplexing, we were able to identify three distinct clusters of B cells, including a predominant mature B‐cell population as well as two less‐abundant subsets of immature and metabolically active cells enriched with mitochondrial genes.^{( 55 )} Supporting a fibrogenic function of B cells, sc‐RNAseq and ligand‐receptor network analysis of NASH livers shows that activated HSCs express increased C‐X‐C motif chemokine ligand 12 with C‐X‐C motif chemokine receptor 4 (Cxcr4) in B cells as the potential target.^{( 57 )} Mature and plasma B‐cell clusters have been also detected by sc‐RNAseq within areas of scarring in fibrotic livers of patients with cirrhosis.^{( 58 )} Consistently, B cells have been shown to directly contribute to the evolution of inflammation and fibrosis in mouse models of NASH and hepatotoxicity. In mice fed a methionine‐choline–deficient diet, depletion of B2 cells reduces Th1 responses and partially prevents NASH‐related inflammation.^{( 52 )} B cells also seem to promote hepatic fibrosis in the CCl₄‐induced mouse model of hepatotoxicity.^{( 59} , ^{60 )} B‐cell–deficient mice treated with CCl₄ have substantially attenuated liver fibrosis with reduced collagen deposition, infiltration of immune cells, and HSC activation, suggesting that B cells are required for liver fibrogenesis.^{( 60 )} In turn, HSC‐derived retinoid acid induces B‐cell survival and innate‐like proinflammatory function in a feed‐forward mechanism that drives liver fibrosis.^{( 60 )} The profibrotic role of B cells during CCl₄‐induced liver injury depends on the adaptor protein, myeloid differentiation primary response 88 (MYD88), which is indispensable for innate immunity and involved in the retinoid acid signaling pathway.^{( 60 )}

TABLE 2.

Direct Evidence of B‐Cell Involvement in the Pathogenesis of NAFLD

	Key Finding	Refs.
Human studies
NAFL‐NASH patients	B cells found in T‐cell–rich aggregates in the liver of NAFLD patients. B‐cell infiltration was associated with elevated levels of IFNγ, IgG against OSE antigens, and lobular inflammation and fibrosis.	( 52 )
NAFL‐NASH patients	Elevated serum BAFF levels correlated with hepatocyte ballooning and fibrosis.	( 61 )
NAFL‐NASH patients	Elevated serum IgA was associated with and predictive of liver fibrosis.	( 67 )
Mouse models
High‐fat, high‐carbohydrate diet‐induced NASH, muMT (B‐cell–deficient) mice, anti‐CD20 B‐cell depletion, B‐cell–specific MYD88‐deficient mice	Intrahepatic B cells drive NASH progression through activation by microbial triggers in a MYD88‐dependent mechanism and through modulation of T‐cell–mediated inflammation.	( 55 )
HFD‐induced NAFL, muMT (B‐cell–deficient mice)	Mesenteric adipose tissue (MAT) B cells promote MAT inflammation by stimulating proinflammatory macrophages. MAT B cells also migrate into the liver where they induce decreased IL‐10 and increased TNFα and MCP‐1 expression in hepatocytes.	( 49 )
HFD‐induced NAFL	Intrahepatic B cells promote inflammation by producing TNFα, IL‐6, and IgG2a and stimulating Th1 responses.	( 54 )
Methionine‐choline–deficient diet‐induced NASH, TACI‐Ig (B2‐cell deficient), BAFF neutralizing monoclonal antibody	B2 cells mature into plasma cells. Increased circulating anti‐OSE IgG. B2‐cell deficiency ameliorates NASH progression and reduces IFN‐γ+ CD4 T cells in the liver.	( 52 )

Abbreviations: MCP‐1, monocyte chemoattractant protein‐1; TACI, transmembrane activator and CAML interactor.

FIG. 1.

Role of B cells in the progression of NAFLD. Intrahepatic B cells promote disease through cytokine and antibody production. BAFF and OSEs are involved in B‐cell activation. Intestinal‐derived microbial factors resulting from dysbiosis activate intrahepatic B cells in a Myd88‐dependent manner. B‐cell activation may also occur in an antigen‐specific manner through BCR signaling. Activated B cells promote NASH progression through modulation of CD4 and CD8 T‐cell activation and IFNγ responses. The mechanisms of B‐cell–mediated activation of HSCs leading to fibrosis in NASH are unknown.

Potential Mechanisms for B‐Cell Activation

A central objective in the field is the identification of the factors that trigger inflammation in the transition from NAFLD to NASH.^{( 3 )} In this regard, limited research has been conducted to determine the intrinsic and extrinsic mechanisms of B‐cell activation. In a mouse model of NASH without obesity, B‐cell activation preceded T‐cell responses and was correlated with increased hepatic expression of BAFF, suggesting that this cytokine is involved in B‐cell survival and maturation.^{( 52 )} Compared with subjects with simple steatosis, NASH patients have higher BAFF serum levels, which are positively correlated with hepatocyte ballooning and liver fibrosis.^{( 61 )} Despite increased weight gain and adiposity, BAFF‐deficient mice fed an HFD have improved insulin sensitivity, decreased adipose tissue inflammation, and reduced hepatic steatosis in an adipokine‐like role.^{( 62 )} In mice fed a methionine‐choline–deficient diet to induce NASH, antibody‐mediated BAFF neutralization prevented liver plasma maturation and ameliorated parenchymal damage and lobular inflammation.^{( 52 )} These findings suggest either direct effects of BAFF on hepatic and adipose tissue metabolism or indirect effects mediated by B‐cell survival and maturation.

Oxidative stress may also trigger B‐cell immune responses as shown in a cohort of NAFLD patients with increased levels of IgG antibodies against oxidatively damaged molecules known as oxidative‐stress–derived epitopes (OSEs).^{( 63 )} In mice, inducing NASH with a methionine‐deficient diet leads to the expansion of liver plasma cells and increased circulatory anti‐OSE IgG, whereas B2 cell deficiency blunts this response.^{( 52 )} Another possibility is that gut‐derived antigens are involved in the activation and subsequent proinflammatory function of B cells. Indeed, fecal microbiota transfer from NAFLD patients into lean mice results in increased B‐cell accumulation in the liver accompanied by increased liver steatosis and fibrosis.^{( 55 )} LPS is the main bacterial product implicated in NAFLD pathogenesis upon translocation across the intestinal epithelium in a TLR4‐dependent mechanism.^{( 64 )} Although TLR4 activation in NAFLD has been mainly attributed to KCs,^{( 65 )} TLR4 signaling can also activate B cells through the MYD88 pathway.^{( 55 )} In addition, we have shown that NASH promotes a strong BCR activation, which suggests a direct antigen stimulation of the BCR independent from inflammatory signals.^{( 55 )} Whether B‐cell activation in the liver during NAFLD is antigen dependent, the identity of the antigenic stimuli, and the potential T‐dependent or ‐independent mechanisms involved in B‐cell responses are unknown. Alternatively, changes in the microbiota can influence B cells through mechanisms that are independent of antigen translocation. For instance, gut‐microbiota–derived metabolites, including tryptophan, short‐chain fatty acids, and bile acids, are potent immunomodulatory molecules that can affect host metabolism.^{( 66 )}

Effector Functions of Intrahepatic B Cells

B cells are important effector cells in immune‐mediated inflammatory diseases because of their ability to differentiate into antibody‐producing cells and modulate immune responses through antigen presentation and secretion of cytokines. Although the cellular sources and mechanisms leading to their production are unclear, some patients with NAFL and NASH show increased serum IgA, IgM, and IgG.^{( 67 )} Indeed, patients with NASH have elevated anti‐OSE IgG antibodies, which are positively correlated with hepatic inflammation, suggesting that antibody‐mediated responses against oxidative stress are involved in disease progression.^{( 63} , ^{68 )} In mice fed a methionine‐choline–deficient diet to induce experimental NASH, the production of anti‐OSE IgG correlates with the severity of liver injury and hepatic inflammation.^{( 69 )} Furthermore, inducing an immune response against OSEs worsens hepatic inflammation and parenchymal injury and activates Th1 cells and macrophages, suggesting a causative role for anti‐OSE IgGs in NASH progression.^{( 69 )} In addition to anti‐OSE IgGs, patients with NAFLD and NASH have elevated IgA, which is an independent predictor of advanced fibrosis.^{( 67 )} In mice and humans with NASH, liver‐resident IgA⁺ cells suppress the protective role of CD8⁺ cytotoxic T cells leading to HCC.^{( 70 )}

Beyond their traditional role as antibody‐producing cells, intrahepatic B cells use cytokine‐mediated responses to promote inflammation and regulate the function of neighboring T cells. In NASH patients, liver biopsies show evidence of B‐cell aggregation within T‐cell–rich regions, suggesting crosstalk between these cell types.^{( 52 )} Intrahepatic B cells have been shown to secrete greater amounts of IL‐6 and TNFα and promote the activation of CD4 Th cells and their differentiation into Th1 cells in steatotic^{( 54 )} and NASH^{( 55 )} livers. These data suggest a proinflammatory, innate‐like function of intrahepatic B cells in the progression of NAFLD. Whether this secretion of proinflammatory cytokines by B cells promotes the transition from NAFLD to NASH, is the consequence of earlier inflammatory cascades, or simply a bystander response are unknown.

A key event in liver fibrogenesis is the activation of quiescent HSCs, which differentiate into myofibroblasts and secrete large amounts of collagen and profibrosis factors.^{( 71 )} Multiple mechanisms have been implicated in HSC profibrotic functions, but cytokine‐mediated activation is indispensable.^{( 72 )} Intrahepatic B cells from CCl₄‐treated fibrotic mice show increased activation and production of proinflammatory cytokines whereas their genetic ablation results in suppressed fibrosis.^{( 59} , ^{60 )} Mice with a normal number of B cells, but impaired Ig production, are not protected from CCl₄‐induced liver fibrosis, suggesting an antibody‐independent mechanism.^{( 59 )} Importantly, TNF signaling has a prominent role in promoting fibrosis, given that TNFα and its receptor 1 are key regulators of HSC activation.^{( 73 )} Our findings show that B cells are strong producers of TNFα and that their activation promotes fibrosis in NASH, independent from lipid accumulation.^{( 55 )} Together, this evidence suggests that B cells may be strong inducers of HSC activation through the release of proinflammatory cytokines. However, the precise mechanisms through which B cells contribute to NASH‐related fibrosis have not been investigated.

Role of B Cells in NASH‐Associated HCC

B cells, particularly IgA⁺ B cells, have been shown to play a critical role in the progression of NASH‐driven HCC given that the number of tumor‐infiltrating B cells correlate with tumor progression in patients with HCC.^{( 74 )} In humans, sc‐RNAseq shows that B cells, predominantly plasma cells, are enriched in HCC‐infiltrating immune cells (24.26%) compared to the cirrhotic (5.41%) and healthy liver (5.82%).^{( 75 )} In agreement, patients with HCC who have a lower proportion of tumor‐infiltrating plasma cells present a better prognosis.^{( 75 )} Inflammation‐induced IgA⁺ B cells seem to disrupt the antitumor immune response during NASH‐induced HCC, as shown in MUP‐μPA mice fed an HFD.^{( 70 )} Genetic ablation of B cells has been shown to strongly reduce tumor formation in a different mouse model where the animals fail to secrete phospholipid and develop portal inflammation and HCC.^{( 74 )} Mechanistically, IgA⁺ B cells inhibit the activation and cytotoxic activity of CD8⁺ T cells through expression of programmed death‐ligand 1 (PD‐L1) and production of the immunosuppressive cytokine, IL‐10.^{( 70 )} Bregs can also inhibit antitumor immunity through IL‐10 production and promote HCC growth through direct interactions with tumor cells.^{( 76 )} However, liver B cells might play multifaceted roles given that several studies show that patients with HCC with increased infiltration of total B cells have a favorable prognostic outcome.^{( 77} , ^{78 )} In addition, in mice transplanted with murine hepatomas or diethylnitrosamine‐induced cancer, tumor growth was enhanced when all B cells were depleted.^{( 78 )} These contradictory results might be attributed to the stages of human HCC studied, different mouse models, specificity for B‐cell subsets, and composition of microbiotas. Future studies are needed to clarify the precise role of liver B cells in NASH‐related HCC.

Concluding Remarks and Future Directions

NAFLD is a heterogeneous liver disease with a spectrum ranging from simple lipid accumulation to inflammation, liver injury, fibrosis, and increased risk of HCC. Activation of the immune system during NASH and the recruitment of proinflammatory cells into the liver is a key event in the pathogenesis of the disease. Although B lymphocytes are central mediators of autoimmune and inflammatory disease because of their ability to secrete cytokines and antibodies, their role in NAFL and NASH is poorly understood. Future research is needed to identify additional antigen‐dependent and ‐independent mechanisms by which intrahepatic B cells become activated and promote NASH, particularly as activators of HSCs. Advances in single‐cell omics are likely to help elucidate the functional relevance of B1 subsets in disease progression as well as the crosstalk between B cells and other intrahepatic immune and nonimmune cells. Given the role of B cells in the progression of NAFLD, it is conceivable that B‐cell–directed therapies can modulate disease development. Such strategies may include B‐cell depletion, impairment of B‐cell survival and proliferation, modulation of BCR signaling, immunization, and approaches targeting B‐cell inflammatory mediators such as TNFα. B‐cell–directed therapies, such as rituximab (anti‐CD20) and belimumab (BAFF signaling blocker), have been used for years and are effective for the treatment of several autoimmune diseases.^{( 79} , ^{80 )} Although B‐cell depletion therapies might ameliorate NASH progression, unwanted side effects may involve immunosuppression and toxicity. Therefore, more selective strategies that target the costimulatory pathways, CD80/CD86, and inhibit the interaction between T cells and B cells could be viable.^{( 81 )} Another possibility is to modulate BCR signaling using a monoclonal antibody that binds CD22, currently in trials for the treatment of systemic lupus erythematosus.^{( 82 )} However, more preclinical evidence is needed before considering the BCR signaling pathway as a therapeutic target in NAFLD.

Author Contributions

F.B., S.K., H.W., and X.S.R. contributed to the writing of the manuscript and the generation of figures. X.S.R. conceptualized the topic, acquired funding for, and supervised the writing of the article.