Young-Saeng Jang

Retinoic acid, acting as a highly specific IgA isotype switch factor, cooperates with TGF‐β1 to enhance the overall IgA response

The present study demonstrates that RA has activity of an IgA switch factor and is more specific than TGF‐β1. RA independently caused only IgA switching, whereas TGF‐β1 caused IgA and IgG2b switching. We found that RA increased IgA production and that this was a result of its ability to increase the frequency of IgA‐secreting B cell clones. Increased IgA production was accompanied by an increase of GLTα. RA activity was abrogated by an antagonist of the RAR. Additionally, RA affected intestinal IgA production in mice. Surprisingly, RA, in combination with TGF‐β1, notably enhanced not only IgA production and GLTα expression but also CCR9 and α4β7 expression on B cells. These results suggest that RA selectively induces IgA isotype switching through RAR and that RA and TGF‐β have important effects on the overall gut IgA antibody response.

Retinoic acid acts as a selective human IgA switch factor

Retinoic acid (RA) is known to have several functions that lead to a potent mucosal IgA response. Nevertheless, its exact role in human IgA synthesis has yet to be elucidated. Thus, we investigated the role of RA in promoting IgA isotype switching in human B cells. We found that RA increased IgA production and the expression of germ-line IgA1 and IgA2 transcripts (GLTα1 and GLTα2). This induction occurred alongside an increase in the frequency of IgA1-secreting B cell clones, as assessed by limiting dilution analysis. Under the same conditions, RA did not increase IgM and IgG production. Am80, an agonist of RA receptor α (RARα), increased IgA production. In addition, RA activity was abrogated by LE540, an antagonist of RAR, suggesting that the RAR pathway is involved in RA-induced IgA production. Taken together, these results indicate that RA induces IgA isotype switching mainly through RARα in human B cells.

Alum Directly Modulates Murine B Lymphocytes to Produce IgG1 Isotype

Aluminum hydroxide (alum) is the most widely used adjuvant in human vaccines. Nevertheless, it is virtually unknown whether alum acts on B cells. In the present study, we explored the direct effect of alum on Ig expression by murine B cells in vitro. LPS-activated mouse spleen B cells were cultured with alum, and the level of isotype-specific Ig secretion, IgG1 secreting cell numbers, and Ig germ-line transcripts (GLT) were measured using ELISA, ELISPOT, and RT-PCR, respectively. Alum consistently enhanced total IgG1 production, numbers of IgG1 secreting cells, and GLTγ1 expression. These results demonstrate that alum can directly cause IgG1 isotype switching leading to IgG1 production.

IL-4 stimulates mouse macrophages to express APRIL through p38MAPK and two different downstream molecules, CREB and Stat6.

APRIL (a proliferation-inducing ligand) is primarily expressed by macrophages and dendritic cells, and has profound effects on B cell physiology. In this study, we investigated the role of IL-4 in APRIL expression by mouse macrophages and the signaling mechanism involved. IL-4 markedly enhanced APRIL expression in mouse macrophages at the transcriptional and protein level. The p38MAPK inhibitor SB203580 completely abolished the IL-4 effect, whereas overexpression of CREB with IL-4 augmented APRIL expression. This increase was abolished by SB203580 treatment, indicating that p38MAPK may activate CREB. Overexpression of Stat6 also augmented IL-4-induced APRIL expression; this effect was partially abolished by SB203580 but not by the Jak inhibitor AG490, indicating that Stat6 mediates IL-4-induced APRIL expression in a Jak-independent manner and that p38MAPK acts as the intermediate. Our results demonstrate that IL-4 up-regulates APRIL expression through two divergent pathways in mouse macrophages, p38MAPK-CREB and p38MAPK-Stat6.

Lactoferrin Combined with Retinoic Acid Stimulates B1 Cells to Express IgA Isotype and Gut-homing Molecules.

It is well established that TGF-β1 and retinoic acid (RA) cause IgA isotype switching in mice. We recently found that lactoferrin (LF) also has an activity of IgA isotype switching in spleen B cells. The present study explored the effect of LF on the Ig production by mouse peritoneal B cells. LF, like TGF-β1, substantially increased IgA production in peritoneal B1 cells but little in peritoneal B2 cells. In contrast, LF increased IgG2b production in peritoneal B2 cells much more strongly than in peritoneal B1 cells. LF in combination with RA further enhanced the IgA production and, interestingly, this enhancement was restricted to IgA isotype and B1 cells. Similarly, the combination of the two molecules also led to expression of gut homing molecules α4β7 and CCR9 on peritoneal B1 cells, but not on peritoneal B2 cells. Thus, these results indicate that LF and RA can contribute to gut IgA response through stimulating IgA isotype switching and expression of gut-homing molecules in peritoneal B1 cells.

TGF-β1 stimulates mouse macrophages to express APRIL through Smad and p38MAPK/CREB pathways

A proliferation-inducing ligand (APRIL), a new TNF family member, supports B-cell survival and tumor cell proliferation. APRIL is secreted as a soluble protein by macrophages, dendritic cells and activated T cells. However, factors involved in regulation of APRIL expression are as yet unknown. In this study, we investigated the effect of TGF-β1 on APRIL expression in P388D1, a mouse macrophage cell line. TGF-β1 induced APRIL mRNA expression in a time- and dose-dependent manner. One nanogram per milliliter of TGF-β1 was optimal and APRIL transcripts appeared as early as 3 h after stimulation. Based on our studies, which included overexpression of Smad3, DN-Smad3, and sh-Smad3, we found that Smad3 mediates APRIL transcription at least partially. Further, experiments using inhibitors revealed that p38MAPK and CREB are also involved in TGF-β1-induced APRIL expression. These results suggest that TGF-β1, through Smad3 and p38MAPK/CREB signaling pathways, stimulates APRIL expression in macrophages.

Mechanism underlying the suppressor activity of retinoic acid on IL4-induced IgE synthesis and its physiological implication

The present study extends an earlier report that retinoic acid (RA) down-regulates IgE Ab synthesis in vitro. Here, we show the suppressive activity of RA on IgE production in vivo and its underlying mechanisms. We found that RA down-regulated IgE class switching recombination (CSR) mainly through RA receptor α (RARα). Additionally, RA inhibited histone acetylation of germ-line ε (GL ε) promoter, leading to suppression of IgE CSR. Consistently, serum IgE levels were substantially elevated in vitamin A-deficient (VAD) mice and this was more dramatic in VAD-lecithin:retinol acyltransferase deficient (LRAT-/-) mice. Further, serum mouse mast cell protease-1 (mMCP-1) level was elevated while frequency of intestinal regulatory T cells (Tregs) were diminished in VAD LRAT-/- mice, reflecting that deprivation of RA leads to allergic immune response. Taken together, our results reveal that RA has an IgE-repressive activity in vivo, which may ameliorate IgE-mediated allergic disease.

TGF-β and BAFF derived from CD4+CD25+Foxp3+ T cells mediate mouse IgA isotype switching

TGF-β1 is generally accepted as the physiological IgA isotype switch factor. Nevertheless, it is unclear as to which cells in mucus-associated lymphoid tissue provide this cytokine to B cells. Regulatory T cells (Tregs) play immune-suppressive roles by secreting inhibitory cytokines such as TGF-β and IL-10. Thus, it is plausible that Tregs are involved in IgA class switch recombination (CSR) in MALT. We explored, in the present study, the possibility that CD4+CD25+ T cells facilitate IgA CSR in murine B cells. In cocultures, CD4+CD25+Foxp3+ T cells stimulated IgA production by splenic B cells to a greater extent than did CD4+CD25−Foxp3− T cells. This effect was markedly abrogated by the addition of anti-TGF-β1 Ab. Additionally, IgA production was paralleled by an increase in germ line transcript α (GLTα), an indicator of IgA CSR. In contrast, CD4+CD25−Foxp3− T cells were more potent at inducing GLTγ1 and GLTɛ production by cocultured splenic B cells than were CD4+CD25+Foxp3+ T cells. Consistent with these results, phenotypic analyses revealed that TGF-β1 and IL-4 were predominantly expressed by CD4+CD25+Foxp3+ T cells and CD4+CD25−Foxp3− T cells, respectively. Furthermore, CD4+CD25+ T cells strongly expressed BAFF, which led to activation-induced deaminase (AID) expression in B cells. Taken together, our results suggest that CD4+CD25+ Tregs have an important effect on IgA isotype commitment by expressing TGF-β1 and BAFF in MALT.

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

Lactoferrin (LF) and retinoic acid (RA) are enriched in colostrum, milk, and mucosal tissues. We recently showed that LF-induced IgA class switching through binding to betaglycan (transforming growth factor-beta receptor III, TβRIII) and activation of canonical TGF-β signaling. We investigated the combined effect of LF and RA on the overall IgA response. An increase in IgA production by LF was further augmented by RA. This combination effect was also evident in Ig germ-line α (GLα) transcription and GLα promoter activity, indicating that LF in cooperation with RA increased IgA isotype switching. We subsequently found that RA enhanced TβRIII expression and that this increase contributed to LF-stimulated IgA production. In addition to the IgA response, LF and RA in combination also enhanced the expression of the gut-homing molecules C-C chemokine receptor 9 (CCR9) and α4β7 on B cells. Finally, peroral administration of LF and RA enhanced the frequency of CCR9+IgA+ plasma cells in the lamina propria. Taken together, these results suggest that LF in cooperation with RA can contribute to the establishment of gut IgA responses.

Murine γδ T Cells Render B Cells Refractory to Commitment of IgA Isotype Switching

γδ T cells are abundant in the gut mucosa and play an important role in adaptive immunity as well as innate immunity. Although γδ T cells are supposed to be associated with the enhancement of Ab production, the status of γδ T cells, particularly in the synthesis of IgA isotype, remains unclear. We compared Ig expression in T cell receptor delta chain deficient (TCRδ−/−) mice with wild-type mice. The amount of IgA in fecal pellets was substantially elevated in TCRδ−/− mice. This was paralleled by an increase in surface IgA expression and total IgA production by Peyers patches (PPs) and mesenteric lymph node (MLN) cells. Likewise, the TCRδ−/− mice produced much higher levels of serum IgA isotype. Here, surface IgA expression and number of IgA secreting cells were also elevated in the culture of spleen and bone marrow (BM) B cells. Germ-line α transcript, an indicator of IgA class switch recombination, higher in PP and MLN B cells from TCRδ−/− mice, while it was not seen in inactivated B cells. Nevertheless, the frequency of IgA+ B cells was much higher in the spleen from TCRδ−/− mice. These results suggest that γδ T cells control the early phase of B cells, in order to prevent unnecessary IgA isotype switching. Furthermore, this regulatory role of γδ T cells had lasting effects on the long-lived IgA-producing plasma cells in the BM.