The heterogeneity of Breg subsets leads to the assumption that there are either various distinct Breg lineages or that IL-10 can be induced by external stimuli at different stages of B cell development [
9]. The precise mechanisms and required signals for Breg differentiation still need to be elucidated. However, several murine and human studies revealed that B cells of different maturational and differentiation stages are able to differentiate into Bregs upon antigen recognition and/or several kinds of stimuli [
69‐
71]. CpG stimulation results in the generation of human CD19
+CD27
intCD38
+ plasmablasts. Additionally treating B cells with IL-2, IL-6, and especially with the type I interferon IFN-α results in the generation of IL-10 secreting human CD19
+CD27
intCD38
+ plasmablasts [
16]. IFN-α was described to induce CD38
+ expression on human naïve B cells [
72] and promote differentiation into plasma cells [
73]. In this regard, it is likely that IFN receptor signaling is required for IL-10-producing human plasmablasts [
16]. Indeed, treatment with IFN-β, which is another type I IFN, increases IL-10 expression of human B cells after BCR and CD40 ligation [
74]. Another study in mice revealed that stimulation with IL-1β, IL-6, and anti-CD40 in combination promotes the differentiation into IL-10-producing Bregs. Interestingly, B cell stimulation with TNF-α or IL-17 with or without anti-CD40 has no effect on IL-10 production [
70]. It was also demonstrated that the TLR-MyD88-STAT3 pathway not only leads to antibody production of human B cells, but also regulates IL-10 production of human B cells by TLR7/8, which is enhanced by IFN-α [
75]. The BATF/IRF-4/IRF-8-axis was also shown to play a role in IL-10 and IL-35 expression of murine regulatory B cells [
76]. Matsumoto et al. also showed that IRF4 is required for the differentiation into IL-10-expressing murine plasmablasts, both in vitro and in vivo [
16]. Moreover, IL-21 was described to induce IL-10 expression of murine B10 cells [
71] and human GrB
+ Bregs [
65]. Also, in vitro stimulation of murine B cells with other molecules such as LPS, phorbol myristate acetate (PMA), and ionomycin generates IL-10-expressing B cells [
77‐
79]. Menon et al. revealed that also plasmacytoid dendritic cells (pDCs) can drive the differentiation of human CD19
+CD24
hiCD38
hi Bregs and plasmablasts that express IL-10, IL-6, and TNF-α [
69].
As another mechanism for Breg induction, overexpression of IL-10 in primary human B cells was shown to upregulate suppressor of cytokine signaling 3 (SOCS3), glycoprotein A repetitions predominant (GARP), IL-2 receptor α chain (CD25), and programmed cell death ligand 1 (PD-L1). Moreover, IL-10 overexpressing human B cells secrete less pro-inflammatory cytokines such as TNF-α or IL-8, whereas production of anti-inflammatory IL-1 receptor antagonists (IL-1RN) and vascular endothelial growth factor (VEGF) is increased [
80]. In addition, the aryl hydrocarbon receptor (AhR) was shown to regulate the differentiation and function of IL-10-producing murine Bregs [
81] and A proliferation-inducing ligand (APRIL) drives the differentiation of naïve human B cells to IL-10-producing Bregs [
82]. Human type 3 innate lymphoid cells (ILC3s) were also shown to play a role in the induction of human IL-10-producing immature transitional Bregs [
83], whereas IL-35 was described to induce IL-35-producing human Bregs (IL-35
+ Bregs) through activation of STAT1/STAT3 [
84].