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

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.

Intestinal IgA production and its role in host-microbe interaction

Complex and diverse communities of bacteria establish mutualistic and symbiotic relationships with the gut after birth. The intestinal immune system responds to bacterial colonization by acquiring a state of hypo-responsiveness against commensals and active readiness against pathogens. The resulting homeostatic balance involves a continuous dialog between the microbiota and lymphocytes with the intermediation of epithelial and dendritic cells. This dialog causes massive production of immunoglobulin A (IgA), a non-inflammatory antibody specialized in mucosal protection. Here, we discuss recent advances on the regulation of intestinal IgA responses and their role in host-microbe interaction.

The transforming growth factor beta signaling pathway is critical for the formation of CD4 T follicular helper cells and isotype-switched antibody responses in the lung mucosa

T follicular helper cells (Tfh) are crucial for the initiation and maintenance of germinal center (GC) reactions and high affinity, isotype-switched antibody responses. In this study, we demonstrate that direct TGF-β signaling to CD4 T cells is important for the formation of influenza-specific Tfh cells, GC reactions, and development of isotype-switched, flu-specific antibody responses. Early during infection, TGF-β signaling suppressed the expression of the high affinity IL-2 receptor α chain (CD25) on virus-specific CD4 T cells, which tempered IL-2 signaling and STAT5 and mammalian target of rapamycin (mTOR) activation in Tfh precursor CD4 T cells. Inhibition of mTOR allowed for the differentiation of Tfh cells in the absence of TGF-βR signaling, suggesting that TGF-β insulates Tfh progenitor cells from IL-2-delivered mTOR signals, thereby promoting Tfh differentiation during acute viral infection. These findings identify a new pathway critical for the generation of Tfh cells and humoral responses during respiratory viral infections.

DOI:http://dx.doi.org/10.7554/eLife.04851.001

The influenza virus is thought to cause illness in up to 10% of adults and 30% of children each year worldwide. Most of these cases resolve on their own and don’t require treatment, but three to five million people are hospitalized and up to half a million people die each year.

Unfortunately, the vaccines currently available to protect against influenza only target particular varieties or “strains” of the virus. The strains that circulate vary from year-to-year so it is necessary to develop new influenza vaccines every year. However, it is difficult to correctly predict which strains will circulate, so a more effective solution would be to develop a new vaccine that can help the body defend itself against many, or ideally any influenza strain.

During a viral infection, a type of immune cell in the host can specialize into two different types of cells to help fight the virus: T helper 1 cells and CD4 T follicular helper cells. T helper 1 cells help to kill host cells that have become infected. CD4 T follicular helper cells promote the production of proteins called antibodies, which identify and neutralize the virus.

Here, Marshall et al. studied how T helper 1 cells and CD4 T follicular helper cells form in mice suffering from a lung infection similar to influenza. It was already known that a protein called transforming growth factor beta (TGF-β) helps the immune response to mount an effective defense against an infection without causing too much harm to the host. Marshall et al. show that TGF-β increases the number of CD4 T follicular helper cells in the mice by suppressing the production of another protein—called IL-2—on the surface of CD4 T cells. Treating mice lacking the ability to detect TGF-β with a drug that blocks a protein controlled by IL-2 also allows more CD4 T follicular helper cells to be produced.

Marshall et al.’s findings reveal that TGF-β is involved in controlling the balance of T helper 1 cells and CD4 T follicular helper cells produced during viral infections of the respiratory tract. Since TGF-β also has other roles in immune responses against viruses, it is now an attractive target for the development of a vaccine that may protect us against all strains of the influenza virus.

DOI:http://dx.doi.org/10.7554/eLife.04851.002

Vitamin A supplementation in early life enhances the intestinal immune response of rats with gestational vitamin A deficiency by increasing the number of immune cells

Vitamin A is a critical micronutrient for regulating immunity in many organisms. Our previous study demonstrated that gestational or early-life vitamin A deficiency decreases the number of immune cells in offspring. The present study aims to test whether vitamin A supplementation can restore lymphocyte pools in vitamin A-deficient rats and thereby improve the function of their intestinal mucosa; furthermore, the study aimed to identify the best time frame for vitamin A supplementation. Vitamin A-deficient pregnant rats or their offspring were administered a low-dose of vitamin A daily for 7 days starting on gestational day 14 or postnatal day 1, day 14 or day 28. Serum retinol concentrations increased significantly in all four groups that received vitamin A supplementation, as determined by high-performance liquid chromatography. The intestinal levels of secretory immunoglobulin A and polymeric immunoglobulin receptor increased significantly with lipopolysaccharide challenge in the rats that received vitamin A supplementation starting on postnatal day 1. The rats in this group had higher numbers of CD8⁺ intestinal intraepithelial lymphocytes, CD11_C⁺ dendritic cells in the Peyer's patches and CD4⁺CD25⁺ T cells in the spleen compared with the vitamin A-deficient rats; flow cytometric analysis also demonstrated that vitamin A supplementation decreased the number of B cells in the mesenteric lymph nodes. Additionally, vitamin A supplementation during late gestation increased the numbers of CD8⁺ intestinal intraepithelial lymphocytes and decreased the numbers of B lymphocytes in the mesenteric lymph nodes. However, no significant differences in lymphocyte levels were found between the rats in the other two vitamin A supplement groups and the vitamin A-deficient group. In conclusion, the best recovery of a subset of lymphocytes in the offspring of gestational vitamin A-deficient rats and the greatest improvement in the intestinal mucosal immune response are achieved when vitamin A supplementation occurs during the early postnatal period.

Prenatal vitamin A deficiency impairs adaptive immune responses to pentavalent rotavirus vaccine (RotaTeq®) in a neonatal gnotobiotic pig model

Vitamin A deficiency (VAD) is associated with increased childhood mortality and morbidity in impoverished Asian and African countries, but the impact of VAD on rotavirus (RV) vaccine or infection is poorly understood. We assessed effects of gestational and dietary induced pre- and post-natal VAD and vitamin A supplementation on immune responses to a pentavalent rotavirus vaccine, RotaTeq(®) in a neonatal gnotobiotic pig model. Vaccine efficacy was assessed against virulent G1P[8] human rotavirus (HRV) challenge. VAD and vitamin A sufficient (VAS) piglets were derived from dietary VAD and VAS sows, respectively. VAD piglets had significantly lower levels of hepatic vitamin A compared to that of VAS piglets. RotaTeq(®)-vaccinated VAD piglets had 350-fold higher fecal virus shedding titers compared to vaccinated VAS piglets post-challenge. Only 25% of vaccinated non-vitamin A supplemented VAD piglets were protected against diarrhea compared with 100% protection rate in vaccinated non-supplemented VAS piglets post-challenge. Intestinal HRV specific immune responses were compromised in VAD piglets. Vaccinated VAD piglets had significantly lower ileal HRV IgG antibody secreting cell (ASC) responses (pre-challenge) and duodenal HRV IgA ASC responses (post-challenge) compared to vaccinated VAS piglets. Also, intestinal HRV IgA antibody titers were 11-fold lower in vaccinated VAD compared to vaccinated VAS piglets post-challenge. Persistently elevated levels of IL-8, a pro-inflammatory mediator, and lower IL-10 responses (anti-inflammatory) in vaccinated VAD compared to VAS piglets suggest more severe inflammatory responses in VAD piglets post-challenge. Moreover higher IFN-γ responses pre-challenge were observed in VAD compared to VAS piglets. The impaired vaccine-specific intestinal antibody responses and decreased immunoregulatory cytokine responses coincided with reduced protective efficacy of the RV vaccine against virulent HRV challenge in VAD piglets. In conclusion, VAD impaired antibody responses to RotaTeq(®) and vaccine efficacy. Oral supplementation of 100,000 IU vitamin A concurrent with RV vaccine failed to increase the vaccine efficacy in VAD piglets.

An intrinsic propensity of murine peritoneal B1b cells to switch to IgA in presence of TGF-β and retinoic acid

Aims

In the present study we have investigated the comparative switching propensity of murine peritoneal and splenic B cell subpopulations to IgA in presence of retinoic acid (RA) and TGF-β.

Methods and Results

To study the influence of RA and TGF-β on switching of B cell subpopulations to IgA, peritoneal (B1a, B1b and B2 cells) and splenic (B1a, marginal zone, and B2) B cells from normal BALB/c mice were FACS purified, cultured for 4 days in presence of RA and TGF-β and the number of IgA producing cells was determined by ELISPOT assay or FACS analysis. In presence of TGF-β, peritoneal B1b cells switched to IgA more potently than other peritoneal B cell subpopulations. When TGF-β was combined with retinoic acid (RA), switching to IgA was even more pronounced. Under these conditions, “innate” B cells like peritoneal and splenic B1 cells and MZ B cells produced IgA more readily than B2 cells. Additionally, high frequency of nucleotide exchanges indicating somatic hypermutation in VH regions was observed. Besides IgA induction, RA treatment of sorted PEC and splenic B cells led to expression of gut homing molecules - α₄β₇ and CCR9. Intraperitoneal transfer of RA-treated B1 cells into Rag1^-/- recipients resulted in IgA in serum and gut lavage, most efficiently amongst B1b cell recipients.

Conclusion

Present study demonstrates the differential and synergistic effect of RA and TGF-β on switching of different B cell subpopulations to IgA and establishes the prominence of peritoneal B1b cells in switching to IgA under the influence of these two factors. Our study extends our knowledge about the existing differences among B cell subpopulations with regards to IgA production and indicates towards their differential contribution to gut associated humoral immunity.

The role of transforming growth factor (TGF)-β in modulating the immune response and fibrogenesis in the gut

Transforming growth factor (TGF)-β, a pleiotropic cytokine released by both immune and non-immune cells in the gut, exerts an important tolerogenic action by promoting regulatory T cell differentiation. TGF-β also enhances enterocyte migration and regulates extracellular matrix turnover, thereby playing a crucial role in tissue remodeling in the gut. In this review we describe the mechanisms by which abnormal TGF-β signaling impairs intestinal immune tolerance and tissue repair, thus predisposing to the onset of immune-mediated bowel disorders, such as inflammatory bowel disease and celiac disease. Additionally, we will discuss potential therapeutic strategies aiming at restoring physiologic TGF-β signaling in chronic intestinal diseases.