Defining two subpopulations of marginal zone B cells

Marginal zone (MZ) B cells, which are composed of heterogeneous subpopulations, are participate in the rapid response to antigens. Lee et al. showed that MZ B cells can be divided into two distinct subpopulations based on CD80 expression. These two subpopulations of MZ B cells exhibit differential autoreactivity, radiosensitivity, and functional capacities.

During B-cell development, immature B cells are referred to as transitional B cells (T1 and T2) after they migrate from the bone marrow to the spleen or other secondary lymphoid organs. These immature B cells then grow into mature follicular (FO) B cells if their B cell receptors (BCRs) are strongly activated or into marginal zone (MZ) B cells if their BCRs are weakly activated [1] (Fig. 1). MZ B cells can quickly react to pathogens in the blood and turn into short-lived plasma cells that produce low-affinity antibodies [2]. Autoreactive MZ B cells, which can bind to multiple antigens, have been found to be expanded in patients with several autoimmune diseases [3, 4]. However, it is still debated whether autoreactive MZ B cells exist in healthy individuals or only in those with diseases. In a recent paper published in Cellular & Molecular Immunology, Lee et al. made substantial advances in this area by identifying two different types of MZ B cells that differ in their ability to produce antibodies, radiosensitivity, and autoreactivity [5] (Fig. 1). The authors suggest that the MZ B-cell compartment contains a substantial number of autoreactive B cells with high expression of CD80, which could expand further in autoimmune diseases.

Fig. 1.

Transitional B cells (T1 and T2) can differentiate into follicular B cells (FOs) or marginal zone B cells (MZs), depending on the strength of the activation of their B-cell receptors (BCRs). MZ B cells can be classified into two subpopulations based on CD80 expression. These subpopulations differ in their ability to produce antibodies, radioresistance, and autoreactivity. (This figure was created with BioRender.com.)

B cells are known to upregulate the expression of molecules associated with antigen presentation, such as CD80, CD86 and major histocompatibility complex (MHC) class II, in response to antigenic or innate stimuli [6, 7]. By measuring the expression levels of CD80, CD86, MHC II and other markers on splenic B-cell subpopulations in both C57BL/6 and DBA/1 mice, Lee et al. reported that a proportion of MZ B cells, transitional 2 MZ B-cell precursors (T2-MZPs), and T1 B cells expressed high levels of CD80. The MZ B cells were classified into two subpopulations based on their CD80 expression. These two subpopulations of MZ B cells exhibited distinct gene expression profiles and different BCR-mediated signaling pathways. CD80^high MZ-derived B cells exhibited increased expression of PD-L1, CD5, and CD9, as well as increased basal phosphorylation of the B-cell receptor (BCR) signaling kinases Bruton’s tyrosine kinase (BTK) and AKT. Furthermore, CD80^high and CD80^low MZ B cells also differ in their response to pathogen-associated molecular patterns such as LPS. CD80^high MZ B cells are more autoreactive and have an increased capacity to proliferate and differentiate into plasma cells compared to CD80^low MZ B cells, as evidenced by the greater viability and proportion of plasma cells characterized by CD138⁺ (transmembrane activator and CAML interactor) TACI⁺ cells observed in LPS-stimulated CD80^high MZ B cells.

Lee et al. conducted in vivo and in vitro experiments to investigate the functional disparities between CD80^high and CD80^low MZ B cells. As MZ B cells are known to be radiosensitive [8], they investigated the radiosensitivities of two MZ B-cell subpopulations and found that CD80^high MZ B cells were more radioresistant than CD80^low MZ B cells both in vivo and in vitro. In vivo, the number of CD80^high MZ B cells remained unchanged one week after irradiation, while the number of CD80^low MZ B cells decreased upon irradiation and returned to normal approximately one month later. In vitro, the resolution of phosphorylated histone H2AX (γH2AX), a marker of double-stranded DNA breaks [9], was delayed six hours after 2 Gy irradiation. Furthermore, Lee et al. observed an increase in the severity of collagen-induced arthritis (CIA) and an increase in anti-mouse type II collagen (CII) IgG antibodies in irradiated and bovine CII-immunized mice when CD80^low MZ B cells were selectively depleted by 2 Gy irradiation. These findings indicate that CD80^high MZ B cells promote the progression of CIA and may contribute to the development of radiation-induced autoimmune diseases.

They next conducted a transcriptome analysis and immunoglobulin heavy chain repertoire analysis to compare the differences between CD80^high and CD80^low MZ B cells. The authors found that the gene expression and BCR repertoire differed between the two subpopulations of MZ B cells. The CD80^high MZ B cells were enriched in genes involved in the negative regulation of cell activation, antioxidant activity, the IL-2/STAT5 pathway and complement regulation. The authors analyzed the number of unique clonotypes, lengths of complementary-determining region 3 (CDR3s), frequencies of V segment usage and mutation rates, and Morisita-Horn (MH) similarity indices. MH similarity indices estimate both the number of common clonotypes and the distribution of clone sizes [10]. The results showed that while the frequencies of V segment usage and mutation rates were similar between CD80^high and CD80^low MZ B cells, CD80^high MZ B cells had lower IgH clonal diversity and a greater proportion of clonotypes with shorter CDR3s. The MH indices between B-1a and CD80^high MZ B cells were greater than those between B-1a and CD80^low MZ B or FO B cells. These results suggest that CD80^high MZ B cells have a distinct IgH repertoire from that of CD80^low MZ B cells.

Finally, the authors investigated the developmental pathways of CD80^high and CD80^low MZ B cells by taking advantage of adoptive transfer experiments. The authors showed that adoptively transferred T2-MZPs matured into FOs and mainly into MZ B cells, while T1-MZPs generated small numbers of both CD80^high MZ B cells and CD80^low MZ B cells. Approximately half of the donor CD80^high MZ B cells were CD80^high MZ B cells, while 47% and 26% of the CD80^low MZ B cells were converted into FO B cells and CD80^high MZ B cells, respectively. Thus, the transformation of CD80^low MZ B cells into CD80^high MZ B cells was more significant than was the reverse transformation.

In summary, Lee et al. identified two subpopulations of MZ B cells based on CD80 expression. These subpopulations differ in terms of autoreactivity, radiosensitivity, and functional capacity. CD80^high MZ B cells are more autoreactive and radioresistant than CD80^low MZ B cells. The presence of autoreactive CD80^high MZ B cells suggested that autoreactive MZ B cells already exist in healthy individuals and expand further in autoimmune diseases. Thus, therapies targeting the proliferation or activation of CD80^high MZ B cells may be exploited to treat autoimmune or infectious diseases in the future.

Competing interests

The authors declare no competing interests.