Lactoferrin causes IgA and IgG2b isotype switching through betaglycan binding and activation of canonical TGF-β signaling

Abstract

Lactoferrin (LF), a pleiotropic iron-binding glycoprotein, is known to modulate the humoral immune response. However, its exact role in Ig synthesis has yet to be elucidated. In this study, we investigated the effect of LF on Ig production by mouse B cells and its underlying mechanisms. LF, like transforming growth factor (TGF)-β1, stimulated B cells to produce IgA and IgG2b, while downregulating other isotypes. Using limiting dilution analysis, LF was shown to increase the frequency of IgA-secreting B-cell clones. This was paralleled by an increase in Ig germ-line α (GLα) transcripts, indicating that LF plays a role as an IgA switch factor. Interestingly, LF directly interacted with betaglycan (TGF-β receptor III, TβRIII) and in turn induced phosphorylation of TβRI and Smad3 through formation of the TβRIII/TβRII/TβRI complex, leading to IgA isotype switching. Peroral administration of LF increased intestinal/serum IgA production as well as number of IgA plasma cells in lamina propria. Finally, we found that LF has an adjuvant activity when nontoxigenic Salmonella typhimurium was inoculated perorally, conferring protection against intragastrical infection of toxigenic S. typhimurium. These results suggest that LF has an important effect on the mucosal/systemic IgA response and can contribute to protection against intestinal pathogens.

Retinoic acid enhances lactoferrin-induced IgA responses by increasing betaglycan expression

“…13 Additionally, we recently reported that LF enhanced IgA CSR by binding to TbRIII (betaglycan) and activating canonical TGF-b signaling. 19 The present study extends these findings. Thus, similar to our findings with TGF-b1 plus RA, LF and RA synergized to increase IgA CSR.…”

“…This LF/RA combination effect was expected to some extent because LF functions through canonical TGF-b signaling as noted above. 19 Nevertheless, the mechanisms underlying IgA CSR caused by the combination of TGF-b1/RA and LF/RA will not be entirely identical because the role played by RA is believed to differ from the TGF-b1/RA and LF/RA combinations. Previously, we observed that RA could directly increase IgA CSR on its own and that the RA signal transduced through RAR.…”

“…13 Additionally, LF induces IgA and IgG2b CSR through TGF-b signaling. 19 Therefore, we assessed the effect of LF and RA on Ig expression. Similar to TGF-b1, LF increased IgA and IgG2b production by mouse splenic B cells.…”

“…28 Because LF induces IgA production by binding to TbRIII, 19 it was important to investigate whether RA increased TbRIII expression in mouse B cells and whether this increase affected LF-dependent IgA production. RA markedly increased the expression of TbRIII on the surface of mouse splenic B cells (Figure 3a) in a dose-dependent manner (Supplementary Figure 1).…”

“…17,18 We recently showed that LF caused IgA and IgG2b isotype switching by binding to TbRIII and activating canonical TGF-b signaling. 19 Although LF and RA are abundant inmucosal secretions and are known to cause IgA isotype switching, their combined effect on the overall gut IgA response has not been characterized. In this study, we found that LF and RA synergized to enhance IgA isotype switching and the expression of gut-homing molecules on B cells.…”