It has long been known that regulatory B cells can downregulate immune responses via the secretion of IL-10 and IL-35. However, the origin and nature of such suppressive B cells were not well understood. Reporting in Immunity, Lino et al. now describe a subset of resident plasma cells that expresses LAG3 and produces IL-10 in response to toll-like receptor (TLR) stimulation.
Previous studies had sought to define regulatory B cells by isolating known B cell subsets and assessing their suppressive capacity. The authors took a different approach. In a model of infection by Salmonella typhimurium, IL-10-producing CD138hi plasmocytes appear rapidly after infection. To characterize these cells, they analysed the transcriptome of IL-10+CD138hi versus IL-10–CD138hi cells on day 1 post infection. A number of genes encoding regulatory transmembrane receptors were found to be overexpressed in the IL-10+ cells, including LAG3, which is also a marker for IL-10 producing T regulatory type 1 cells. LAG3+CD138hi cells were found to produce high levels of IL-10 and had typical plasma cell features, like plasmacytoid morphology, spontaneous antibody secretion and a non-proliferative state.
The rapid emergence of these cells was surprising, given that plasma cell differentiation normally requires proliferation over several days. This led the authors to investigate whether LAG3+CD138hi cells were already present in naive mice — which was indeed the case. Plasma cell differentiation also requires antigen stimulation. Since LAG3+CD138hi cells were also found in mice housed under germ-free or specific pathogen-free conditions, it is likely that they were induced by self-antigen.
Genome-wide methylome analysis, as well as transcriptome and gene-set enrichment analysis of CD138hi cells from naive mice and from mice 1 day post infection, confirmed that LAG3+CD138hi cells are the precursors of IL-10+LAG3+CD138hi cells. It also confirmed that they are in a quiescent state and epigenetically primed for IL-10 expression. Notably, these cells also expressed PD-L1, PD-L2 and CD200, indicating that they can operate multiple immune suppressive mechanisms.
In vivo, LAG3+CD138hi cells were shown to rapidly upregulate IL-10 expression, in a polyclonal fashion, in response to infection. By crossing LAG3–/– mice with Il10eGFP mice and generating mixed bone marrow chimeras, the authors demonstrated that LAG3+CD138hi cells control the expansion of LAG3–CD138hi plasma cells in a LAG3-dependent manner. LAG3+CD138hi cells did not display an amplified response upon re-challenge or undergo isotype switching, indicating that they do not acquire features of memory cells.
Adoptive transfer experiments of different B cell subsets into Rag2–/– mice, as well as cell fate mapping, indicated that LAG3+CD138hi cells can develop from several different B cell subsets and that they have a distinct B cell receptor (BCR) repertoire. Their frequency is strongly reduced in Btk–/– mice, which have defective BCR signalling. In contrast, mice deficient for TLR signalling, CD40 or the αβ T cell receptor, had normal numbers of these cells. This suggests that the development of LAG3+CD138hi cells is BCR-dependent but does not require TLR signalling or T cell help. However, the induction of IL-10 expression was found to be strictly dependent on TLR signalling (except in the bone marrow, where some LAG3+CD138hi cells constitutively expressed IL-10).
Further experiments showed that LAG3+CD138hi cells were also less abundant in mice deficient for CD19, a positive regulator of BCR signalling, and more abundant in mice lacking CD72, which negatively regulates BCR signalling. CD72–/– mice had impaired immunity against salmonella, which could be reversed by treatment with anti-IL-10 or anti-IL-10 receptor.
“LAG3+CD138hi cells … rapidly upregulate IL-10 expression … in response to infection”
Together, these results indicate that LAG3+CD138hi cells are natural regulatory plasma cells that rapidly provide a first layer of B cell-mediated immune regulation in response to TLR signals. The authors observed that frequencies of LAG3+CD138hi cells in mice increase with age, leading them to speculate that they may react against antigens released by damaged cells. This may provide a feedback mechanism that senses the number of damaged cells, downregulating immunity accordingly.
Lino, A. C. et al. LAG-3 inhibitory receptor expression identifies immunosuppressive natural regulatory plasma cells. Immunity 49, 120–133 (2018)
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Flemming, A. Natural regulatory plasma cells. Nat Rev Immunol 18, 540–541 (2018). https://doi.org/10.1038/s41577-018-0057-8