3.1 Origin of ABCs

In the lymphoid lineage, mature B cells are divided into different developmental subgroups by applying phenotypic definitions (Figure 2). By using the same method, mature B cells can be further divided into the B lymphoid FO subgroup, which is characterized by B220⁺CD19⁺CD11b^-CD21^intCD1d^int; the MZ B-cell subgroup is defined as B220⁺CD19⁺CD11b^-CD21^highCD11d^high; and the B1 B-cell subgroup, which is defined as CD5⁺B220^lowCD19⁺CD11b^low. ABCs are a newly formed B-cell pool that is distinct from FO B cells, MZ B cells, and B1 B-cell subsets in terms of phenotypic criteria, function, and other aspects (Table 1). First, they have specialized anatomical sites. FO B cells are predominantly present in the lymphoid follicles of the lymph nodes, while the marginal region of the spleen is the main habitat of MZ B cells. B1 B cells, as one of the peripheral B-cell subsets, are confined mainly to the abdominal cavity and mucosal tissues. In contrast to FO B cells, ABCs are not present in most lymph nodes or lymphatic vessels, which allows them to be effectively distinguished. Second, ABCs respond to TLR9 and TLR7 ligands but not to BCR or CD40, which are important factors for stimulating FO and MZ cell proliferation and survival. Third, the survival rates of FO B cells and MZ cells are affected by B lymphocyte stimulators (Blys), whereas those of ABCs are not (Hao et al. 2011). Fourth, the secretion of antichromatin IgG is exclusive to ABCs, whereas other B-cell populations predominantly secrete IgM (Rubtsov et al. 2011). Fifth, ABCs express histocompatibility complex-II (MHC-II) and some costimulatory molecules on their surfaces and may act as antigen-presenting cells (APCs) for T-cell differentiation (Hao et al. 2011; Rubtsov et al. 2015).

ABCs constitute a previously uncharacterized new cell subpopulation that is somewhat similar to exhausted B cells but differs from FO B cells, MZ B cells, and naïve B cells in that the number of cells increases with age (Moir et al. 2008). Hao et al. (2011) studied peripheral B-cell subsets after autoreconstruction in aging mice irradiated at sublethal doses. The results showed that the aging B-cell progenitor population did not skew toward ABC generation in terms of internal and microenvironment characteristics, but the FO B-cell or MZ B-cell subset was restored. Whereas ABCs are similar to FO B cells, they used flow cytometric analyses of cell-surface Ags, concluding that they express similar levels of IgM and CD40. Additionally, they employed various in vivo and in vitro techniques, including adoptive B-cell transfer and sublethal irradiation, to confirm that under the synergistic stimulation of TLRs and BCR, FO B cells can acquire the ABC phenotype. ABCs may constitute a subpopulation that has undergone FO B-cell stimulation and differentiation.

Furthermore, as the proportion of ABCs increases, the number of FO-like B cells decreases in aged mice. However, ABCs induced the apoptosis of pre-B cells through the direct action of TNF-α (Frasca et al. 2012; Ratliff et al. 2013). FO-like B cells can “block” the effects of TNF-α on B-cell precursors by secreting IL-10²⁸ (Figure 2). ABCs in aged mice suppress the growth of B-cell precursors through multiple mechanisms. Therefore, the pool of naïve B cells remains unchanged, whereas ABCs gradually accumulate and replace FO B cells in their niche.

3.2 Differentiation Signals of ABCs

3.2.1 Participation via TLR7 and TLR9 Signals

Research findings of Hao et al (Hao et al. 2011). suggest that ABCs are insensitive to stimulation related to BCR and CD40 costimulation in adaptive immunity. Additionally, qPCR analysis revealed that ABCs expressed TLR7 and were enriched in the expression of TLR9. Furthermore, in in vitro experiments, ABCs exhibited a special response to TLR7/9 ligands. Interestingly, they showed the maximum degree of cell division in response to combined BCR and TLR stimulation; this may be because some studies have shown that mild self-reactivity of BCR plays a certain role in promoting B-cell development (Zikherman, Parameswaran, and Weiss 2012). Rubtsov demonstrated for the first time that the TLR7 signaling pathway directly regulates ABCs to produce anti-chromatin IgG antibodies in a mouse model of autoimmune disease (Rubtsov et al. 2013). TLR9 is a receptor that senses endogenous DNA and is associated with autoimmune diseases. Previous studies have shown that when BCRs bind to TLR9 agonists, they trigger primary proliferation of B cells (Leadbetter et al. 2002), followed by cell apoptosis. The activation of TLR9 leads to the activation of p38 MAPK, which subsequently causes cell cycle arrest and mitochondrial apoptosis, such as when DNA-bound antigens bind to BCRs to trigger B-cell proliferation and cell cycle arrest. However, B cells can be saved during this process; how they are saved determines their subsequent fate of being split in half. Researchers have concluded through in vitro experiments that if BLyS is added to save B cells, they will differentiate into cells that secrete antibodies. However, if CD40 costimulation and IFN-γ or IL-21 are used to save B cells, they will express the T-bet⁺ phenotype (Sindhava et al. 2017). Leibler et al. subsequently reported through studies on lupus-prone mice with TLR9 point mutations that TLR9 can negatively regulate the pro-inflammatory signal transduction of TLR7 by controlling the endoplasmic reticulum localization of TLR7. Additionally, TLR9 has anti-inflammatory effects that are dependent on ligands that are independent of MyD88 signaling (Leibler et al. 2022) (Figure 3).

In addition to the classical regulation of ABCs by TLR7/9, recent studies have shown that in the cells of lupus patients, the mammalian target of rapamycin complex 1 (mTORC1) pathway is significantly activated (Wu et al. 2019). By blocking the effect of rapamycin on mTORC1 signaling, the differentiation of T-bet⁺ B cells can be inhibited (Figure 3).

3.2.3 Multiple Cytokines Promote the Differentiation of ABCs

Naïve B cells must receive direct stimulation from TLR7/9 to differentiate into ABCs (Figures 3, 4). However, only these innate signals are insufficient; adaptive signals, such as IL-21 and IFN-γ, are also required (Wang et al. 2018; Naradikian et al. 2016).

Naradikian et al. (2016) discovered through an H. polygyrus infection model and a Helicobacter pylori infection model that type 1 helper T cells (Th1 cells) secrete IL-21 and that IFN-γ can directly promote the expression of T-bet in B cells, whereas IL-4 has an inhibitory effect on the induction of T-bet. In other words, IL-4 blocked the upregulation of T-bet driven by IL-21 but enhanced the upregulation of T-bet mediated by IFN-γ. Furthermore, while IL-21 can induce the expression of CD11c in B cells, IFN-γ does not perform this function. The interactions between these cytokines and their regulation of the process of T-bet and CD11c⁺ cell formation in B cells stimulated by TLRs were demonstrated in a previous study (Naradikian et al. 2016). In 2017, Michela Manni et al. used gene knockout technology to produce mice lacking the SWEF protein (a member of the Rho GTPase regulating protein family, including Def6 and Swap-70) and reported that the SWEF protein controls the proliferation of ABCs by regulating the IL-21‒IL-21R signaling pathway (Manni, Ricker, and Pernis 2017). Levack et al. subsequently used the E. muris mouse model to demonstrate that early postinfection, the cytokines IL-21 and IFN-γ produced by follicular helper T cells (Tfh cells) drive the generation of CD11c⁺T-bet⁺ B cells. In contrast, in the later stages of infection, this process is mainly dependent on the Tfh costimulatory signal CD40:CD40L (Levack et al. 2020). Studies have shown that Th1 cells stimulate B cells to promote the generation of CXCL9/10-producing T-bet⁺ effector B cells (Nakayama et al. 2021).

4.1 Autoantibody Production

Analysis of the transcripts of ABCs revealed that they highly expressed the plasma cell markers CD80 and CD138 and highly expressed CXCR6 and CXCR3, which are involved in lymphocyte homing to inflammatory sites (Rakhmanov et al. 2009; Tipton et al. 2015). They also secrete cellular factors that promote plasma cell differentiation, such as Prdm1, Irf4, and Xbp1 (Rubtsov et al. 2011). These data suggest that ABCs may be precursors of plasma cells.

Researchers have demonstrated through in vitro culture and preparation of B-cell hybridoma experiments that ABCs are under differentiated plasma cells (Figure 5). Only when stimulated (e.g., by a TLR7 agonist) can they secrete anti-chromatin IgG autoantibodies (Rubtsov et al. 2011; Jenks et al. 2018). Through ELISPOT experiments and autoantibody microarray tests, Song et al. reported that T-bet⁺CD11c⁺ B cells can secrete virus-specific antibodies and autoantibodies after LCMV infection (Song et al. 2022). In female DKO mice with an autoimmune propensity, the absence of B-cell T-bet significantly reduces the production of anti-dsDNA IgG2c but does not correspondingly increase the production of anti-dsDNA IgG1. Therefore, B-cell T-bet is not essential for ABC generation or differentiation but has a specific requirement for the production of IgG2c autoantibodies (Ricker et al. 2021; Rubtsova et al. 2013; Stone et al. 2019; Peng, Szabo, and Glimcher 2002).

4.2 Antigen Presentation

The anatomical location of ABCs is located at the boundary of the spleen, thus providing conditions for their interaction with Tfh cells (Figure 5). Research has shown that ABCs can act as antigen-presenting cells, helping activate naïve T cells to favor Th17 differentiation (Hao et al. 2011; Rubtsov et al. 2015).

By sequencing the VH and Vk genes of unimmunized 22-month-old C57BL/6 mice, it was found that ABCs expressed diverse recombination sequences in their VH and Vκ genes, suggesting that their high mutation rate may enable them to recognize and respond to a variety of different antigens (Russell Knode et al. 2017). T-bet⁺CD11c⁺ B cells can undergo Ig somatic hypermutation and class switch recombination, both of which occur outside the germinal centers (Song et al. 2022; Karnell et al. 2017; Knox, Myles, and Cancro 2019). Ramirez De Oleo et al. (2023) have shown that ABCs express a variety of receptors related to phagocytosis, such as CD36 and AXL. These receptors help ABCs effectively phagocytose antigens. Additionally, ABCs express high levels of the antigen-presenting molecule MHC II, which can present the ingested antigens to T cells. By using metformin (a drug that can block the process of B-cell terminal differentiation into plasma cells) to reduce antigen uptake and thus inhibit the presentation process in ABCs, Tfh differentiation can be reduced (Lee et al. 2017). These findings demonstrate that ABCs can act as antigen-presenting cells (APCs) to present ingested antigens to T cells and initiate an immune response. Similarly, the proliferation of Tfh cells requires CD11c⁺T-bet⁺ ABCs (Zhang et al. 2019). The experimental data revealed that when cocultured with OVA protein, both ABCs and FO B cells could induce T-cell proliferation and activation (Ramirez De Oleo et al. 2023). In the present study, ABCs together with chicken egg albumin (OVA) were cultured with naïve CD4⁺ T cells, resulting in stronger T-cell receptor (TCR) signal induction. ABCs promote the differentiation of Tfh cells by inducing the production of IL-21 and the expression of Tfh cell markers such as CXCR5, PD-1, and Bcl-6 (Ramirez De Oleo et al. 2023).

4.3 Cytokine/Chemokine Production

ABCs are activated B cells that have been shown in ex vivo experiments to preferentially secrete IL-4, IL-10, and TNF-α after TLR7 or TLR9 agonist stimulation (Hao et al. 2011; Ratliff et al. 2013) (Figure 5). Furthermore, ABCs produce more IL-1β and IL-6 than FO B cells (Ramirez De Oleo et al. 2023). Additionally, Th1 cells stimulate the BCR/CD40 and IFN-γ signaling pathways of B cells, thereby promoting the secretion of CXCL9-11 by T-bet effector B cells (Nakayama et al. 2021; Long, Kleiner, and Looney 2023). When CXCL9-11 binds to CXCR3 on the surface of Th1 cells, it can attract them to inflamed tissues and further induce an increase in the release of IFN-γ, thus further intensifying the inflammatory response (Long, Kleiner, and Looney 2023). Additionally, a study revealed that the proliferation of Th1 cells is associated with ABCs (Unger et al. 2018).

4.4 Migration to End Organs and Associated Tissue Damage

ABCs can be detected in both viral infection models and autoimmune disease models at corresponding validation sites. This is because ABCs express CD11c (an integrin involved in cell adhesion and migration) on their surface and highly express CCR7 (Rubtsov et al. 2015) (which regulates the migration and localization of lymphocytes), CXCR6 and CXCR3 (Rakhmanov et al. 2009; Tipton et al. 2015) (which participate in homing to inflammatory sites); thus, after being guided by inflammation signals, ABCs can specifically migrate to the corresponding inflammatory sites and accumulate.

4.5 Inhibiting the Survival of B-Cell Precursors

During coculture of ABCs and FO B cells, ABCs did not inhibit the growth of other B cells (Hao et al. 2011). Frasca et al. discovered through experiments such as an IL-7 colony formation assay, that ABCs induce various biological mediators and cytokines, such as PGE2 and IL-1, in macrophages and adipocytes by secreting TNF-α, thereby altering the bone marrow microenvironment and inhibiting the survival of B-cell precursors (Berg and Scherer 2005). In 2015, researchers reported that these cells did not alter the level of pro-B cells in the bone marrow when ABCs or FO B cells were transplanted from aged mice to young mice. Instead, there is an interaction between age-related B cells in aged mice and alternative light chains λ5 (SLCs), which may lead to the apoptosis of pre-B cells in the bone marrow and the generation of autoimmune B cells (Ratliff et al. 2015).

5.1 ABCs in Cell Aging

ABCs can promote inflammatory responses. B cells play an important role in producing antibodies in humoral immunity. With age, the differentiation and function of B cells may change, leading to a decline in humoral immunity. In addition, aging can lead to the redistribution of B-cell subsets, increasing the frequency and number of inflammatory B cells. Moreover, aging can reduce the expression of immunoglobulin class switching and recombination involved in high-affinity antibody production and the expression of highly mutated molecules in somatic cells, as well as reduce germinal center formation and the diversity of immune pools (Frasca et al. 2020).

Studies have shown that the number and proportion of ABCs increase in older individuals and that they also exhibit an inflammatory response. ABCs secrete TNF-α, which can promote an inflammatory microenvironment in the bone marrow and inhibit the growth or survival of pre-B cells and common lymphoid progenitor cells (Ratliff et al. 2013; Riley, Khomtchouk, and Blomberg 2017). In this study, CFSE-labeled OTII T cells were cultured under different polarization conditions in the presence of aged FO or ABCs. The results showed that ABCs were able to effectively present antigens to T cells and promote T-cell proliferation under all polarization conditions (Hao et al. 2011). Since ABCs express high levels of MHC II and costimulatory molecules, they may promote the differentiation of Th17 cells by presenting self-antigens to T cells (Rubtsov et al. 2011). Th17 cells mainly produce inflammatory cytokines such as IL-17 and IL-22, thereby promoting inflammatory responses (Miossec and Kolls 2012). Older people have a weaker antibody response to the flu vaccine. The diminished antibody response of elderly individuals to the influenza vaccine is closely related to the age-related expansion of B-cell populations and decreased levels of PAX5 (Nipper et al. 2018). The enhancement of B-cell function in elderly individuals may therefore be a necessary strategy to optimize the efficacy of vaccines that rely on humoral immune protection.

5.2 ABCs in Autoimmune Disease

B cells play important roles in autoimmune diseases, including producing autoantibodies, presenting antigens, providing costimulatory signals, and producing cytokines. Some autoimmune diseases are closely related to B cells. Rubtsov was the first to link ABCs with autoimmune diseases (Rubtsov et al. 2011). The ABC subgroup was initially identified in a mouse model of autoimmune disease. In clinical studies, researchers have reported that ABCs tend to accumulate in various autoimmune diseases, such as systemic lupus erythematosus (SLE), primary Sjögren's syndrome (pSS), systemic sclerosis, and rheumatoid arthritis (RA) (Figure 6).

5.3 ABCs in Viral Infection

Some studies have demonstrated that ABCs are found in the marginal zone of Ehrlichia muris infection models (Trivedi et al. 2019). In a viral infection model, ABCs proliferate in the spleen and the infected site, and the spleen becomes the main storage depot for ABCs (Mouat and Horwitz 2022; Johnson et al. 2020; Mouat, Shanina, and Horwitz 2022b). Following the clearance of viral infection in mice, the quantity and proportion of ABCs in their spleens and infection sites (such as the lungs and mediastinal lymph nodes) decline, and the majority of ABCs merely reside in the spleen (Johnson et al. 2020).

During acute infection, ABCs produce antibodies specific to the virus (Song et al. 2022). Patients with common variable immunodeficiency (CVID) are often susceptible to recurrent bacterial infections, especially respiratory infections (Quinti et al. 2007). A study by Mirzokhid Rakhmanov et al. indicated that in CVID patients, B cells with low expression of CD21 show signs of activation and proliferation and have limited calcium signaling and proliferation ability in response to B-cell receptor stimulation (Rakhmanov et al. 2009). Recent studies have shown that in patients with CVID-ILD (CVID-related interstitial lung disease), Th1-CD21^low B cells interact with their bronchoalveolar lavage fluid (BALF), with the number of ABCs (CD21^low B cells) markedly increasing in the BALF (Friedmann et al. 2021). In other pulmonary diseases, such as sarcoidosis (SarcP), similar findings have been reported in patient populations (Phalke et al. 2020). Moreover, the frequency of T-bet⁺ B cells increased in chronic HCV-infected patients.

Kugler-Umana et al. (2022) discovered that immunizing elderly mice with an inactivated vaccine, known as iABCs, could elicit an infection-induced antibody-mediated immune response, which closely resembled the immune response triggered by live influenza virus infection. Nucleic acid-based vaccines for COVID-19, such as mRNA vaccines, can stimulate innate immunity by activating TLR7/8 or TLR9, thereby inducing ABC generation and triggering autoimmune responses (Mahroum and Shoenfeld 2022). Furthermore, ABCs express high levels of inhibitory FcγRIIB receptors, and the Fc region of antigen‒antibody complexes can bind to FcγRIIB receptors to form immune complexes, thereby reducing vaccine responses (Yam-Puc et al. 2023). Steuten et al. reported that after secondary immunization, ABCs rapidly proliferate, but their numbers gradually decrease over time (Steuten et al. 2023). Researchers have analyzed the regulation of endogenous retroviruses (ERVs) by T-bet⁺ B cells through the control of TLR signaling-dependent spontaneous germinal centers and IFNγ-dependent regulation by infecting Moloney-Murine leukemia virus-marked GFP (ERV-GFP) in mouse GC-derived cell lines (Rauch et al. 2024). ABCs begin to accumulate antiviral humoral responses during the peak of the antiviral response (Rubtsova et al. 2013). These cells are maintained during infection and can significantly reduce their frequency after antiviral treatment (Chang, Li, and Kaplan 2017). In addition, there are other infectious diseases, such as LCMV (Rubtsova et al. 2013) and HIV infection (Knox et al. 2017), which are most closely related to ABCs.

5.4 ABCs in Organ Transplant

Lei Wang and others reported that the number and percentage of ABCs significantly increased in patients with end-stage renal disease before and after kidney transplantation, especially in patients who experienced viral or bacterial infections after the transplant operation (Wang et al. 2021). The immune system of elderly individuals is compromised, namely, immunosenescence, which leads to impairment of the development and antibody-producing capacity of B cells (Franceschi et al. 2000). In elderly patients undergoing kidney transplantation, the B-cell population in their body transforms more mature memory cells, especially with an increase in ABCs, which might influence the immune response of the patients (Wang et al. 2021). Consequently, an increase in the proportion of ABCs in transplant patients might be related to the adaptability of the immune response and the response to infections, but the specific benefits and mechanisms still await further research.