Pleural innate response activator B cells protect against pneumonia via a GM-CSF-IgM axis.
Bottom Line: We show that in response to lung infection, B1a B cells migrate from the pleural space to the lung parenchyma to secrete polyreactive emergency immunoglobulin M (IgM).The strategic location of these cells, coupled with the capacity to produce GM-CSF-dependent IgM, ensures effective early frontline defense against bacteria invading the lungs.The study describes a previously unrecognized GM-CSF-IgM axis and positions IRA B cells as orchestrators of protective IgM immunity.
Affiliation: Center for Systems Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 Department of Visceral, Thoracic and Vascular Surgery, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, 01307 Dresden, Germany email@example.com firstname.lastname@example.org.Show MeSH
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Mentions: IgM production is a defining feature of innate-like B cells (Ehrenstein and Notley, 2010; Baumgarth, 2011; Cerutti et al., 2013). We have previously shown that IRA B cells are B1a-derived GM-CSF and IgM-producing cells (Rauch et al., 2012), whereas others have documented that GM-CSF can induce immunoglobulin secretion (Snapper et al., 1995). IgM and GM-CSF co-expression by the same cell prompted us to test for a direct link between the antibody and the growth factor. We sorted B1a B cells from serosal cavities (peritoneal and pleural), locations known to contain B1a B cells. After in vitro LPS stimulation, B1a B cells gave rise to GM-CSF–producing IRA B cells, defined as CD19+ IgMhigh CD43+ CD5+ CD138+ CD93+ MHCII+ (Fig. 1 A). B1a B cells also expressed the common β chain high-affinity receptor for GM-CSF (Csf2rb, also known as CD131) at high levels (Fig. 1 B), which corresponded with transcriptional profiling data obtained by the Immunological Genome Project (ImmGen) and suggested that B cell–derived GM-CSF might be acting in an autocrine manner to produce IgM. To test this, we placed sorted B1a B cells from WT, Csf2−/− (i.e., GM-CSF–deficient), and Csf2rb−/− mice into culture and used flow cytometry to detect intracellular IgM reservoirs (Fig. 2, A and B). In response to LPS, WT but neither Csf2−/− nor Csf2rb−/− B1a B cells gave rise to a large population of IgM-producing cells (Fig. 1, C and D). GM-CSF was necessary but not sufficient to elicit robust IgM production because adding GM-CSF to WT cells in medium had no effect. Adding recombinant (r) GM-CSF to Csf2−/− B1a cell cultures partially restored, whereas adding rGM-CSF to WT B1a cells augmented IgM production (Fig. 1 D). Although the rescue effect was partial in absolute values, the ∼10-fold increase of IgM by Csf2−/− cells after rGM-CSF was similar to that observed in WT cells. These data suggest that despite GM-CSF’s absence during B1 cell development in Csf2−/− mice, which might affect the cells’ ability to respond to LPS, a relatively robust response nevertheless occurs, providing evidence that GM-CSF stimulates IgM production. To illustrate the link between GM-CSF and IgM in WT cells, we focused on Stat5, which is crucial in the GM-CSF signaling pathway (Mui et al., 1995). Adding a Stat5 inhibitor to B1a cell cultures abrogated IgM production. B2 cells cultured with LPS likewise produced IgM, albeit at much lower levels. The intracellular IgM reservoirs correlated with secreted IgM, as measured by ELISA (Fig. 1 E) and ELISPOT (Fig. 1 F). RT-PCR confirmed that WT and Csf2rb−/− but not Csf2−/− cells produced GM-CSF in response to LPS (Fig. 1 G). When compared with Csf2−/− and Csf2rb−/− B1a B cells, stimulated WT B1a B cells expressed moderately higher levels of Prdm1 (Fig. 1 H), the gene which codes for Blimp-1, which is an essential transcription factor in plasma cell generation (Shapiro-Shelef et al., 2003; Savitsky and Calame, 2006; Fairfax et al., 2007; Nutt et al., 2007). In sum, the data show that B1a-derived IRA B cell secretion of GM-CSF promotes IgM production via CD131. This is a previously unrecognized GM-CSF-IgM axis that may be central to the early response to bacterial infection.
Affiliation: Center for Systems Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 Department of Visceral, Thoracic and Vascular Surgery, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, 01307 Dresden, Germany email@example.com firstname.lastname@example.org.