Enhancement of recombinant adenovirus vaccine-induced primary but not secondary systemic and mucosal immune responses by all-trans retinoic acid

Vaccination is an important tool for enhancing immune responses against mucosal pathogens. Intramuscularly administered adenovirus (Ad) vectors have been demonstrated to be strong inducers of both systemic and mucosal immune responses. Further enhancement of immune responses following Ad vaccination is highly desirable. All-trans retinoic acid (ATRA), a biologically active vitamin A metabolite, has been explored as an adjuvant for primary immune responses following vaccination. In this study, we investigated the effect of ATRA on a heterologous Ad prime boost regimen. ATRA co-administration during priming increased mucosal and systemic antibody responses as well as mucosal but not systemic CD8(+) T cell responses. However, this effect was no longer apparent after boosting regardless of whether ATRA was administered at the time of priming, at the time of boosting, or at both immunizations. Our findings confirm ATRA as an adjuvant for primary immune responses and suggest that the adjuvant effect does not extend to secondary immune responses.

Retinoic acid-producing, ex-vivo-generated human tolerogenic dendritic cells induce the proliferation of immunosuppressive B lymphocytes

While much is known about tolerogenic dendritic cell effects on forkhead box protein 3 (FoxP3)⁺ regulatory T cells, virtually nothing is known about their effects on another arm of immunoregulation that is mediated by a subpopulation of immunosuppressive B cells. These cells suppress rheumatoid arthritis, lupus and inflammatory bowel disease in mice, and functional defects have been reported in human lupus. We show that co-stimulation-impaired tolerogenic dendritic cells that prevent and reverse type 1 diabetes mellitus induce the proliferation of human immunosuppressive B cells in vitro. We also show that the suppressive properties of these B cells concentrate inside the CD19⁺ CD24⁺ B cell population and more specifically inside the CD19⁺ CD24⁺ CD38⁺ regulatory B cell population. We discovered that B cell conversion into suppressive cells in vitro is partially dependent on dendritic cell production of retinoic acid and also that CD19⁺ CD24⁺ CD38⁺ B regulatory cells express retinoic acid receptors. Taken together, our data suggest a model whereby part of the immunosuppressive properties of human tolerogenic dendritic cells could be mediated by retinoic acid which, in addition to its known role in favouring T cell differentiation to FoxP3⁺ regulatory T cells, acts to convert B cells into immunosuppressive cells.

Retinoic acid expression associates with enhanced IL-22 production by γδ T cells and innate lymphoid cells and attenuation of intestinal inflammation

Retinoic acid (RA), a vitamin A metabolite, modulates mucosal T helper cell responses. Here we examined the role of RA in regulating IL-22 production by γδ T cells and innate lymphoid cells in intestinal inflammation. RA significantly enhanced IL-22 production by γδ T cells stimulated in vitro with IL-1β or IL-18 and IL-23. In vivo RA attenuated colon inflammation induced by dextran sodium sulfate treatment or Citrobacter rodentium infection. This was associated with a significant increase in IL-22 secretion by γδ T cells and innate lymphoid cells. In addition, RA treatment enhanced production of the IL-22-responsive antimicrobial peptides Reg3β and Reg3γ in the colon. The attenuating effects of RA on colitis were reversed by treatment with an anti-IL-22 neutralizing antibody, demonstrating that RA mediates protection by enhancing IL-22 production. To define the molecular events involved, we used chromatin immunoprecipitation assays and found that RA promoted binding of RA receptor to the IL-22 promoter in γδ T cells. Our findings provide novel insights into the molecular events controlling IL-22 transcription and suggest that one key outcome of RA signaling may be to shape early intestinal immune responses by promoting IL-22 synthesis by γδ T cells and innate lymphoid cells.

Retinoid and TGF-β families: crosstalk in development, neoplasia, immunity, and tissue repair

Transforming growth factor-β (TGF-β) isoforms are profibrotic cytokines, par excellence, and have complex multifunctional effects on many systems, depending on the biologic setting. Retinoids are vitamin A derivatives that also have diverse effects in development, physiology, and disease. The interactions between these classes of molecules are, not surprisingly, highly complex and are dependent on the tissue, cellular, and molecular settings.

Regulation of Toll-like receptor 5 gene expression and function on mucosal dendritic cells

Toll-like receptor (TLR) 5 has been shown to maintain intestinal homeostasis and regulate host defense against enterobacterial infection. However, how TLR5 expression is regulated and its function in the intestine have not been fully elucidated. Here we demonstrate that mucosal dendritic cells (DCs), but not splenic DCs, express high levels of TLR5 protein. Alternatively spliced Tlr5 transcripts were identified but it did not explain the selective expression of TLR5 on mucosal DCs. Treatment with various bacterial ligands downregulated BMDC TLR5 expression, while retinoic acid and host stromal cell-derived signals promoted TLR5 expression in a TGF-β-independent mechanism. Signaling through TLR5 restrained regulatory T (Treg) cell generation, and accordingly, TLR5^−/− mice displayed increased frequencies of Foxp3⁺ Treg cells in the intestinal lamina propria. Our data indicate that bacterial and host factors differentially regulate DC TLR5 expression. TLR5 signaling regulates immune responses towards the microbiota via modulation of the Treg/effector T cell balance.

A new look at immune privilege of the eye: dual role for the vision-related molecule retinoic acid

The eye is an immunologically privileged and profoundly immunosuppressive environment. Early studies reported inhibition of T cell proliferation, IFN-γ production, and generation of regulatory T cells (Tregs) by aqueous humor (AH) and identified TGF-β as a critical factor. However, T cell subsets including Foxp3(+) Treg and Th17 were unknown at that time, as was the role of retinoic acid (RA) in Treg induction. Consequently, the effect of the ocular microenvironment on T cell lineage commitment and function, and the role of RA in this process, had not been explored. We now use gene-manipulated mice and highly purified T cell populations to demonstrate that AH suppresses lineage commitment and acquisition of Th1 and Th17 effector function of naive T cells, manifested as reduction of lineage-specific transcription factors and cytokines. Instead, AH promoted its massive conversion to Foxp3(+) Tregs that expressed CD25, GITR, CTLA-4, and CD103 and were functionally suppressive. TGF-β and RA were both needed and synergized for Treg conversion by AH, with TGF-β-enhancing T cell expression of RA receptor α. Newly converted Foxp3(+) Tregs were unstable, but were stabilized upon continued exposure to AH or by the DNA demethylating agent 5-aza-2'-deoxycytidine. In contrast, T cells already committed to effector function were resistant to the suppressive and Treg-inducing effects of AH. We conclude that RA in the eye plays a dual role: in vision and in immune privilege. Nevertheless, primed effector T cells are relatively insensitive to AH, helping to explain their ability to induce uveitis despite an inhibitory ocular microenvironment.

A mathematical model for the reciprocal differentiation of T helper 17 cells and induced regulatory T cells

The reciprocal differentiation of T helper 17 (T_H17) cells and induced regulatory T (iT_reg) cells plays a critical role in both the pathogenesis and resolution of diverse human inflammatory diseases. Although initial studies suggested a stable commitment to either the T_H17 or the iT_reg lineage, recent results reveal remarkable plasticity and heterogeneity, reflected in the capacity of differentiated effectors cells to be reprogrammed among T_H17 and iT_reg lineages and the intriguing phenomenon that a group of naïve precursor CD4⁺ T cells can be programmed into phenotypically diverse populations by the same differentiation signal, transforming growth factor beta. To reconcile these observations, we have built a mathematical model of T_H17/iT_reg differentiation that exhibits four different stable steady states, governed by pitchfork bifurcations with certain degrees of broken symmetry. According to the model, a group of precursor cells with some small cell-to-cell variability can differentiate into phenotypically distinct subsets of cells, which exhibit distinct levels of the master transcription-factor regulators for the two T cell lineages. A dynamical control system with these properties is flexible enough to be steered down alternative pathways by polarizing signals, such as interleukin-6 and retinoic acid and it may be used by the immune system to generate functionally distinct effector cells in desired fractions in response to a range of differentiation signals. Additionally, the model suggests a quantitative explanation for the phenotype with high expression levels of both master regulators. This phenotype corresponds to a re-stabilized co-expressing state, appearing at a late stage of differentiation, rather than a bipotent precursor state observed under some other circumstances. Our simulations reconcile most published experimental observations and predict novel differentiation states as well as transitions among different phenotypes that have not yet been observed experimentally.

In order to perform complex functions upon pathogenic challenges, the immune system needs to efficiently deploy a repertoire of specialized cells by inducing the differentiation of precursor cells into effector cells. In a critical process of the adaptive immune system, one common type of precursor cell can give rise to both T helper 17 cells and regulatory T cells, which have distinct phenotypes and functions. Recent discoveries have revealed a certain heterogeneity in this reciprocal differentiation system. In particular, treating precursor cells with a single differentiation signal can result in a remarkably diverse population. An understanding of such variable responses is limited by a lack of quantitative models. Our mathematical model of this cell differentiation system reveals how the control system generates phenotypic diversity and how its final state can be regulated by various signals. The model suggests a new quantitative explanation for the scenario in which the master regulators of two different T cell lineages can be highly expressed in a single cell. The model provides a new framework for understanding the dynamic properties of this type of regulatory network and the mechanisms that help to maintain a balance of effector cells during the inflammatory response to infection.

All-trans retinoic acid induces TLR-5 expression and cell differentiation and promotes flagellin-mediated cell functions in human THP-1 cells

Toll-like receptor 5 (TLR-5), which is expressed on macrophages and dendritic cells (DCs), is a crucial cell surface molecule that senses microbial-associated molecular patterns and initiates host innate immune responses upon infection with invaders that express flagellin. Little information is known about the induction factors and mechanisms of TLR-5 expression. In this study, we demonstrate that all-trans retinoic acid (ATRA) significantly up-regulated TLR-5 expression in human macrophage THP-1 cells by co-activating NF-κB and the RARα receptor and inducing the differentiation of CD11b(-)CD11c(-) THP-1 cells to CD11b(+)CD11c(low) cells. Furthermore, when stimulated with flagellin, ATRA-induced THP-1 cells expressed multiple cytokines, including TNF-α, IL-1beta, and IL-12p40, and several co-stimulatory molecules, such as CD40, CD80, CD86, and MHC class I and II. We also showed that when ATRA-induced THP-1 cells were stimulated with flagellin, the cells displayed an allostimulatory capacity rather than phagocytic activity. Taken together, our findings suggest that ATRA is a crucial immunostimulatory cofactor that induces the activation of macrophages and their subsequent differentiation into dendritic-like cells.

Retinoic acid: a key player in immunity

For the past 100 years, vitamin A has been implicated as an essential dietary component in host resistance to infectious disease. However, only recently have studies begun to elucidate the cellular and molecular mechanisms of how vitamin A regulates cell-mediated and humoral-mediated immunity. In this review, we present an overview of the recent discoveries of the role that vitamin A and its metabolite, retinoic acid (RA), play in the regulation of immune cells. How RA impacts on leukocyte growth, differentiation, and homing is discussed with special attention to inflammatory responses and solid tumor microenvironment.

Transforming growth factor-beta and all-trans retinoic acid generate ex vivo transgenic regulatory T cells with intestinal homing receptors

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg) mediate immunologic self-tolerance and suppress immune responses. In the gut, a subset of dendritic cells is specialized to induce Treg in a transforming growth factor-beta (TGF-beta)- and retinoic acid (RA)-dependent manner. The aim of this study was to establish if RA synergizing with TGF-beta induced antigen specific CD4(+) CD25(high) Foxp3(+) Treg portraying gut homing receptors. Splenic CD4(+)CD25(-) Foxp3(-) naïve T cells from DO11.10 mice were cocultured with splenic CD11c(+) dendritic cells from Balb/c mice in the presence of TGF-beta, RA, and low levels of an antigenic peptide. After 5 days of culture, cells were analyzed for the expression of Foxp3 and the gut homing receptors CCR9 and alpha4beta7. The number of Foxp3(+) T cells generated with TGF-beta and RA was at least 3 times higher than in the cultures with TGF-beta alone and 15 times higher than in controls without exogenous cytokines. Also, supplementation of the cultures with RA induced the expression of the intestinal homing receptors CCR9 and alpha4beta7. Our results showed that coculture of naïve T cells with antigen-presenting cells in the presence of TGF-beta and RA represents a powerful approach to generate Treg with specific homing receptors.

From the diet to the nucleus: vitamin A and TGF-beta join efforts at the mucosal interface of the intestine

The vitamin A metabolites, including retinoic acid (RA), form ligands for retinoic acid-related nuclear receptors and together they play pleiotropic roles in various biological processes. Recently, we described that RA also functions as a key modulator of transforming growth factor-beta (TGF-beta)-driven immune deviation, capable of suppressing the differentiation of interleukin-17 secreting T helper cells (T(H)17) and conversely promoting the generation of Foxp3(+) T regulatory (Treg) cells. This review will focus on the role of RA in the reciprocal TGF-beta-driven differentiation of T(H)17 and Treg and on the importance of such regulatory mechanism to control a functional immune system, in particular at the mucosal interface of the intestine.

Regulation of CD8(+) T cell functions by RARgamma

Retinoic acid plays a key role in the development and function of the immune system; however, the contribution of each of the three retinoic acid receptors (RARs) to the T cell immune response is not yet well understood. Of these receptors, both RARalpha and RARgamma are expressed in T lymphocytes. While possible functional redundancy thus complicates understanding of the role of each receptor in T cells, emerging data suggest that RARalpha and RARgamma function differently in thymocyte development and that RARgamma is required for both primary and secondary CD8(+) T cell immune responses.

The expression of retinoic acid receptors in thymus of young children and the effect of all-transretinoic acid on the development of T cells in thymus

To explore the effect of retinoic acid on the development of T cells in thymus, we examined the expression and distribution of retinoic acid receptors mRNA in thymus of young children (< or =5 years of age) and investigated the affection of all-transretinoic acid on the differentiation and maturation of the T cells in thymus and cultured in vitro. Twenty thymus samples were collected. By in situ hybridization, we found that four retinoic acid receptor genes expressed in thymus, and the levels of mRNA for retinoic acid receptor genes (detected by reverse transcriptase, fluorescent quantitative PCR), were changed with age. In in vitro culture, all-transretinoic acid promoted the maturation from CD4+CD8+ cells to CD4+ cells but inhibited the differentiation from CD4+CD8+ cells to CD8+ cells. Thus, retinoic acid likely plays important roles in T cell development in thymus and perhaps by affecting the relative expression of retinoic acid receptors.

Effects of inflammatory cytokines IL-1beta, IL-6, and TNFalpha on the intracellular localization of retinoid receptors in Schwann cells

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

Hyporesponsiveness to natural killer T-cell ligand alpha-galactosylceramide in cancer-bearing state mediated by CD11b+ Gr-1+ cells producing nitric oxide

CD1d-restricted natural killer T (NKT) cells are a potential therapeutic target for cancer, for which several clinical trials have already been reported. NKT cells are specifically activated by a synthetic glycolipid, alpha-galactosylceramide (alpha-GalCer). However, it is known that, in human cancer patients, NKT cells express a degree of hyporesponsiveness to alpha-GalCer. In this study, we have examined the mechanism by which hyporesponsiveness to alpha-GalCer can be induced. In cancer-bearing mice, alpha-GalCer-induced NKT cell expansion, cytokine production, cytotoxicity, and antimetastatic effect in vivo were all significantly impaired. In fact, alpha-GalCer could eliminate metastatic disease in naive animals but failed to protect cancer-bearing mice. CD11b(+) Gr-1(+) cells were particularly increased in cancer-bearing mice and were necessary and sufficient for the suppression of the alpha-GalCer response in a nitric oxide-mediated fashion. Administration of a retinoic acid to cancer-bearing mice reduced the population of CD11b(+) Gr-1(+) cells and effectively restored alpha-GalCer-induced protection. These results show a novel feature of NKT cell function in cancer. Furthermore, our data suggest a new strategy to enhance NKT cell-mediated anticancer immune responses by suppressing CD11b(+) Gr-1(+) cell functions.

Vitamin A supplementation and retinoic acid treatment in the regulation of antibody responses in vivo

Vitamin A (VA, retinol) is essential for normal immune system maturation, but the effect of VA(1) on antibody production, the hallmark of successful vaccination, is still not well understood. In countries where VA deficiency is a public health problem, many children worldwide are now receiving VA along with immunizations against poliovirus, measles, diphtheria, pertussis, and tetanus. The primary goal has been to provide enough VA to protect against the development of VA deficiency for a period of 4-6 months. However, it is also possible that VA might promote the vaccine antibody response. Several community studies, generally of small size, have been conducted in children supplemented with VA at the time of immunization, as promoted by the World Health Organization/UNICEF. However, only a few studies have reported differences in antibody titers or seroconversion rates due to VA. However, VA status was not directly assessed, and in some communities children were often breast fed, another strategy for preventing VA deficiency. Some of the vaccines used induced a high rate of seroconversion, even without VA. In children likely to have been VA deficient, oral polio vaccine seroconversion rate was increased by VA. In animal models, where VA status was controlled and VA deficiency confirmed, the antibody response to T-cell-dependent (TD) and polysaccharide antigens was significantly reduced, congruent with other defects in innate and adaptive immunity. Moreover, the active metabolite of VA, retinoic acid (RA) can potentiate antibody production to TD antigens in normal adult and neonatal animals. We speculate that numerous animal studies have correctly identified VA deficiency as a risk factor for low antibody production. A lack of effect of VA in human studies could be due to a low rate of VA deficiency in the populations studied or low sample numbers. The ability to detect differences in antibody response may also depend on the vaccine-adjuvant combination used. Future studies of VA supplementation and immunization should include assessment of VA status and a sufficiently large sample size. It would also be worthwhile to test the effect of neonatal VA supplementation on the response to immunization given after 6 months to 1 year of age, as VA supplementation, by preventing the onset of VA deficiency, may improve the response to immunizations given later on.

Generation of Gut-Homing IgA-Secreting B Cells by Intestinal Dendritic Cells

Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

The anti-tetanus immune response of neonatal mice is augmented by retinoic acid combined with polyriboinosinic:polyribocytidylic acid

Neonates are highly susceptible to infectious diseases and, in general, respond poorly to conventional vaccines due to immaturity of the immune system. In the present study, we hypothesized that the anti-tetanus toxoid (TT) vaccine response of neonatal mice could be enhanced by retinoic acid (RA), a bioactive retinoid, and polyriboinosinic:polyribocytidylic acid (PIC), an inducer of IFN. Early-life treatments with RA and/or PIC were well tolerated and stimulated both primary anti-TT IgG production in infancy and the memory response in adulthood. TT-specific lymphocyte proliferation and type 1/type 2 cytokine production were also significantly augmented. In addition, RA and PIC modulated the maturation and/or differentiation of neonatal B cells, natural killer (NK)/NKT cells, and antigen-presenting cells. Although RA alone increased the neonatal anti-TT antibody response, it selectively increased anti-TT IgG1 and IL-5, resulting in a skewed type 2 response. PIC, a potent adjuvant in adult mice, elevated neonatal anti-TT IgG as well as all IgG isotypes (IgG1, IgG2a, and IgG2b) and induced TT-specific IFN-gamma, an important type 1 cytokine; however, PIC alone failed to benefit the memory response. The combination of RA plus PIC was more potent than either agent alone in elevating primary and secondary anti-TT IgG responses as well as IgG isotypes. Moreover, RA plus PIC increased TT-specific IFN-gamma and IL-5, suggesting the combination effectively promoted both type 1 and type 2 responses in neonatal mice. Thus, RA combined with PIC, a nutritional-immunological intervention, seems promising as an adjuvant for early-life vaccination.

Retinoic acid and polyriboinosinic:polyribocytidylic acid stimulate robust anti-tetanus antibody production while differentially regulating type 1/type 2 cytokines and lymphocyte populations

Retinoic acid (RA), a bioactive retinoid, and polyriboinosinic:polyribocytidylic acid (PIC) are known to promote immunity in vitamin A-deficient animals. In this study, we hypothesized that RA, PIC, and the combination can provide significant immunoadjuvant activity even in the vitamin A-adequate state. Six-week-old C57BL/6 mice were immunized with tetanus toxoid (TT) and treated with RA and/or PIC at priming in three independent studies of short and long duration. RA and PIC differentially regulated both primary and secondary anti-TT IgG isotypes, whereas the combination of RA + PIC stimulated the highest level of anti-TT IgG production and, concomitantly, a ratio of IgG1 to IgG2a similar to that of the control group. The regulation of Ab response was strongly associated with type 1/type 2 cytokine gene expression. Whereas RA reduced type 1 cytokines (IFN-gamma and IL-12), PIC enhanced both type 1 and type 2 cytokines (IL-4 and IL-12) and cytokine-related transcription factors. Despite the presence of PIC, the IL-4:IFN-gamma ratio was significantly elevated by RA. In addition, RA and/or PIC modulated NK/NKT cell populations and the level of expression of the costimulatory molecules CD80/CD86, evident 3 days after priming. Notably, the NKT:NK and CD80:CD86 ratios were correlated with the IL-4:IFN-gamma ratio, indicative of multiple converging modes of regulation. Overall, RA, PIC, and RA + PIC rapidly and differentially shaped the anti-tetanus Ig response. The robust, durable, and proportionate increase in all anti-TT IgG isotypes induced by RA + PIC suggests that this combination is promising as a means to enhance the Ab response to TT and similar vaccines.

Retinoic acid imprints gut-homing specificity on T cells

For a preferential homing of T cells to the gut, expression of the integrin alpha4beta7 and the chemokine receptor CCR9 is essential and is induced by antigenic stimulation with dendritic cells from the gut-associated lymphoid organs. Here, we show that the vitamin A (retinol) metabolite, retinoic acid, enhances the expression of alpha4beta7 and CCR9 on T cells upon activation and imprints them with the gut tropism. Dendritic cells from the gut-associated lymphoid organs produced retinoic acid from retinol. The enhanced alpha4beta7 expression on T cells by antigenic stimulation with these dendritic cells was suppressed by the retinal dehydrogenase inhibitor citral and the retinoic acid receptor antagonist LE135. Accordingly, vitamin A deficiency caused a reduction in alpha4beta7(+) memory/activated T cells in lymphoid organs and a depletion of T cells from the intestinal lamina propria. These findings revealed a novel role for retinoic acid in the imprinting of gut-homing specificity on T cells.

All-trans retinoic acid converts E2F into a transcriptional suppressor and inhibits the growth of normal human bronchial epithelial cells through a retinoic acid receptor- dependent signaling pathway

Retinoids, including retinol and retinoic acid derivatives, maintain the normal growth and differentiation of human bronchial epithelial (HBE) cells and are under investigation as agents for lung cancer prevention. In this study, we examined the biologic effects of retinoids on normal HBE cells and the molecular mechanisms of retinoid actions. At a dose of 10(-6) M, all-trans retinoic acid (t-RA) suppressed the proliferation of normal HBE cells, which accumulated in the G0 phase. No evidence of programmed cell death was observed. The class of retinoid nuclear receptor that mediated the growth arrest was explored. Normal HBE cell growth was suppressed by a retinoid that selectively activates retinoic acid receptors but not by one that activates retinoid X receptors. The E2F transcription factor has demonstrated a role in G0 entry through transcriptional suppression of genes that induce cell cycle progression. To investigate the role of E2F in retinoid signaling, transient transfection assays were performed using reporter plasmids containing E2F-binding sites. Findings from these experiments suggested that t-RA treatment converted E2F into a transcriptional suppressor. Supporting this possibility, t-RA inhibited the expression of the E2F target genes B-myb, cyclin A, and cyclin E. Further, t-RA increased the levels of nuclear E2F-4, p107, and p130 and enhanced the binding of E2F-4 to p107, which have been associated with the conversion of E2F into a transcriptional suppressor in other cells. These findings point to retinoic acid receptor- and E2F-dependent pathways as potential mediators of retinoid-induced growth arrest in normal HBE cells and have implications for the use of retinoids in clinical trials on the prevention of lung cancer.

Long-term in vitro culture and characterisation of avian embryonic stem cells with multiple morphogenetic potentialities

Petitte, J.N., Clarck, M.E., Verrinder Gibbins, A. M. and R. J. Etches (1990; Development 108, 185–189) demonstrated that chicken early blastoderm contains cells able to contribute to both somatic and germinal tissue when injected into a recipient embryo. However, these cells were neither identified nor maintained in vitro. Here, we show that chicken early blastoderm contains cells characterised as putative avian embryonic stem (ES) cells that can be maintained in vitro for long-term culture. These cells exhibit features similar to those of murine ES cells such as typical morphology, strong reactivity toward specific antibodies, cytokine-dependent extended proliferation and high telomerase activity. These cells also present high capacities to differentiate in vitro into various cell types including cells from ectodermic, mesodermic and endodermic lineages. Production of chimeras after injection of the cultivated cells reinforced the view that our culture system maintains in vitro some avian putative ES cells.

HL-60 myeloid leukaemia cells acquire immunostimulatory capability upon treatment with retinoic acid: analysis of the responding population and mechanism of cytotoxic lymphocyte activation

HL-60 myeloid leukaemia cells are ineffective as stimulators of allogeneic lymphocytes in mixed leucocyte culture (MLC). These cells can be induced to differentiate along the monocytic or granulocytic pathways with or without acquisition of major histocompatibility complex (MHC) class II antigen by various agents. Surprisingly, treatment of HL-60 cells with 10 nM all-trans retinoic acid (RA) for 7 days (HL-60-R7) resulted in a marked increase in MLC stimulation although the cells lacked detectable MHC class II antigen expression at the initiation of the MLC. In contrast, treatment with interferon-gamma (IFN-gamma), with or without RA, induced MHC class II antigen expression but failed to enhance MLC stimulation. Lymphocytes responding to HL-60-R7 were predominantly CD8+ and/or CD16+ and displayed enhanced cytolytic capacity for HL-60 and HL-60-R7 cells as well as natural killer (NK)-sensitive K562 cells. Nevertheless, monoclonal antibodies (mAb) to MHC class II antigens substantially inhibited the MLC and some CD4+ lymphocytes in the responding population were required, although this requirement could be replaced by the addition of interleukin-2 (IL-2). HL-60-R7 (and HL-60) cells were shown to acquire detectable MHC class II antigen expression during the first 3 days of the MLC. Thus a low level of activation by MHC class II+ stimulator cells appears to be required for the response. Analysis of the role of cytokines with costimulatory activity for T cells and/or NK cells indicated that tumour necrosis factor-alpha (TNF-alpha) was important in the proliferative response, while interleukins-1, -6 and -12 and stem cell factor did not seem to be involved. Cell interaction molecules lymphocyte function-associated antigen-1 (LFA-1) (CD11a), intracellular adhesion molecule-1 (ICAM-1) (CD54), ICAM-3 (CD50) and B7.2 (CD86) were up-regulated on HL-60-R7. Blocking mAb to LFA-1 and B7.2 potently inhibited the proliferative response indicating a key role for these molecules in the enhanced immunostimulation by HL-60-R7 cells. The results may have implications for the mechanism of the therapeutic effect of RA in acute promyelocytic leukaemia and may also provide valuable information in regard to the immunogenicity of tumour cells in general.

Retinoids as Ig isotype-switch modulators. The role of retinoids in directing isotype switching to IgA and IgG1 (IgE) in association with IL-4 and IL-5

The role of retinoids was analyzed in directing isotype switching to IgA and IgG1 (IgE) by LPS-stimulated murine mu(+)B-cells in the presence of two Th2-type cytokines, IL-4 and IL-5. All trans retinoic acid (RA) enhanced the production of IgA at high concentrations (10-100 nM) in the presence of IL-5. Addition of IL-4 to the system modulated the IgA response in a dose-dependent manner. Namely, IL-4 inhibited the response at concentrations higher than 250 u/ml, but showed slight enhancement at lower concentrations (130 u/ml). IL-4 alone, which is considered to be an IgE isotype-switch inducer, strongly enhanced the IgG1 and IgE responses. Addition of IL-5 to the system showed a synergistic effect which could be attenuated by addition of low concentrations of RA (about 1 nM). Thus, the presence of switch modulators such as IL-4 and IL-5, their concentration ratios, and concentrations of retinoids are crucial factors in initiating and directing isotype switching to IgA and IgG1 (IgE).

Anti-retinoic acid (RA) antibody binding to human premalignant oral lesions, which occurs less frequently than binding to normal tissue, increases after 13-cis-RA treatment in vivo and is related to RA receptor beta expression

Nuclear retinoic acid receptor beta (RAR-beta) expression decreases in human premalignant oral lesions (POLs). RAR-beta suppression could result from a decrease in the cellular level of retinoids because RAR-beta gene transcription is enhanced by retinoids. To explore this hypothesis, we compared the binding of a monoclonal antibody (mAb) against all-transretinoic acid (RA; anti-RA mAbs) to normal oral tissue and POLs. All 7 normal specimens stained positive with the antibody compared to only 20 of 43 POLs; similarly, 7 of 7 normal specimens contained RAR-beta mRNA compared to only 14 of 43 POLs. Twenty-four specimens were available before and after a 3-month treatment with 13-cis-RA in vivo. Anti-RA mAb binding to these specimens increased from 10 of 24 before to 22 of 24 after treatment, and the expression of RAR-beta mRNA increased from 7 of 24 before to 21 of 24 after treatment, respectively. There was a strong agreement between the binding of anti-RA mAbs and the expression of RAR-beta. Thus, we propose that the binding of anti-RA mAbs reflects the level of retinoids in the tissues and that this level is related strongly to RAR-beta expression.

Anti-retinoic acid monoclonal antibody localizes all-trans-retinoic acid in target cells and blocks normal development in early quail embryo

Avian cardiovascular development is vitamin A-dependent, and retinoic acid has been suggested to be active in this important developmental event. We report here that a monoclonal antibody against all-trans-retinoic acid blocks normal embryonic development in the quail causing cardiovascular abnormalities typical of avian vitamin A deficiency. In whole-mount preparations of stage 5 normal quail embryos the fluorescence associated with the antiretinoic acid monoclonal antibody localizes in Hensen's node and in caudal area. In stage 7-8 embryos fluorescence localizes in heart-forming areas as well as in head mesenchyme, in Hensen's node, in nephrotome, and in caudal area. These studies are the first to localize endogenous all-trans-retinoic acid during very early stages of normal avian development. We propose that all-trans-retinoic acid is biosynthesized in its target cells during early avian embryo-genesis and that the availability of this signal molecule is spatiotemporally regulated. We conclude that all-trans-retinoic acid or a closely related metabolite is the physiological form of vitamin A required for normal cardiovascular development and for other very early developmental events in the quail embryo.

Repair of immunoglobulin response in B cell line (JK32.1) originating from immunodeficient patient via implantation of functional plasma membranes

Human-human B cell hybridoma JK32.1, constructed from B lymphocytes of a common variable immunodeficient patient and nonsecreting cell line, retains the defects of B cell immunodeficiency. Efforts to clarify whether the defect is located within the plasma membranes of this cell line were carried out by implanting them with plasma membrane fraction derived from normal functional cells via intact non-infectious Sendai virus. The implanted cells were activated with various mitogens and their Ig responses and isotype switching were examined. Restoration of IgM secretion was achieved in the implanted JK32.1 cells following stimulation with SAC, PWM, or retinoic acid. Augmented IgM response was also obtained in the implanted cells treated with retinoic acid and lipopolysaccharide (LPS) despite their unresponsiveness to LPS alone. No IgG or IgA response could be detected in the implanted JK32.1 cells. These data suggest that this immunodeficient cell line possesses at least two different malfunctions, one located within the plasma membrane moiety of the cells and the other located within the cytoplasmic and/or nucleic components. The plasma membrane moiety defect can be repaired temporarily by delivering proper signals via the implanted plasma membranes. However, this manipulation of the cells could not overcome the intrinsic defect of the cells which blocks isotype switching and secretion of IgG, IgE, and IgA antibodies.

Retinoid repletion of vitamin A-deficient mice restores IgG responses

Vitamin A-deficient (A-) mice produce poor IgG antibody responses due to a helper T cell dysfunction. We performed retinoid repletion studies to determine the minimum dietary retinyl acetate dose and the most active retinoid for supporting immune function. Dietary retinyl acetate repletion at 2 (R2 group) or 4 (R4 group) microgram/g diet restored serum retinol in A- mice to vitamin A-sufficient (A+) control levels within 24 h. However, in R4 mice, liver retinyl palmitate was restored about twofold faster than in R2 mice; liver retinyl palmitate reached A+ control levels by d 30 in R4 mice but not in R2 mice. We challenged the mice with antigen 24 h post repletion; the R4 mice gave an IgG1 response equal to that of A+ controls, but the R2 mice were comparable with the A- controls. We also compared four retinoids for IgG1 response restoration in vitro; 1 nmol/L retinoic acid fully repleted A- cell IgG1 responses and helper T cell frequencies to the unsupplemented A+ control levels. Retinoic acid was at least 10-fold more active than retinyl acetate or retinaldehyde, and 100-fold more active than retinol. Collectively, our results suggest that retinoic acid is probably the physiologically important metabolite for sustaining IgG immune responses in vivo. We discuss the possible relationship between liver retinyl palmitate levels and availability of retinoic acid to support immune function.

Production of a hybridoma cell line secreting retinoic acid-specific monoclonal antibody

A stabilized hybridoma cell line secreting anti-retinoic acid monoclonal antibodies of subclass IgG1 with kappa chains was produced by fusing NS-1 myeloma cells with the spleen cells from BALB/c female mice immunized with all-trans-4-oxoretinoic acid-oxime-chicken IgG conjugate. The antibody titer of mice ascitic fluid ranged from 1/12,800 to 1/25,600, as determined by competitive indirect enzyme-linked immunosorbent assay (ELISA). 50% inhibition dosage of all-trans-retinoic acid at a 1/20,000 dilution of mice ascitic fluid was 6.6 ng/ml, as determined by ELISA. The anti-retinoic acid monoclonal antibody was generated in mice ascitic fluid and purified by protein G affinity chromatography. Cross-reactivity of the monoclonal antibody was determined at 0.1 microgram/ml concentration of retinoids and indicated high specificity to both all-trans-retinoic acid (86% inhibition) and 13-cis-retinoic acid (87% inhibition), and strong cross-reactivity with 4-oxoretinoic acid (77%) and 4-oxoretinoic acid oxime (109%). Specificity was confirmed by the horseradish peroxidase-linked immunostaining method and immunoradioassay. The affinity constant of the monoclonal antibody, K, was determined to be 3.6 X 10(9) l/mol. A calibration curve for retinoic acid using the monoclonal antibody to retinoic acid was developed; the detection limit for all-trans-retinoic acid is 1 ng/ml in the competitive indirect ELISA. The antibody counteracts the effect of retinoic acid on growth inhibition and differentiation in HL-60 cells.

Toxicologic and immunologic evaluations of N-(all-trans-retinoyl)-DL-leucine and N-(all-trans-retinoyl)glycine

Sprague-Dawley rats were dosed by gavage daily for 28 days with 5, 15, or 50 mg/kg of N-(all-trans-retinoyl)-DL-leucine (RL), N-(all-trans-retinoyl)glycine (RG), or all-trans-retinoic acid (RA). On the basis of mortality incidence, fracture incidence, body weight, and histopathologic effects, RG was slightly to moderately less toxic than RA, and RL was significantly less toxic than RA or RG. Doses that had no effect on weight loss and produced no bone fractures were approximately 5 and 15 mg/kg/day for RA administered to males or females, respectively; greater than 15 mg/kg/day for RG administered to males or females; and greater than 50 mg/kg/day for RL administered to males or females. At these doses, RA and RG produced effects, detectable at the microscopic level, of lymphoid hyperplasia and hematopoietic cell proliferation in the spleen, lymphoid hyperplasia in lymph nodes, necrosis of the cortex of the thymus, hypertrophy of the zona fasciculata of the adrenal, a periportal pattern of cytoplasmic vacuolization in hepatocytes, hematopoietic cell proliferation in the liver, epithelial hyperplasia and subacute inflammation in the forestomach, and osteodystrophy. Serological alterations consisted of reduced serum albumin levels and elevated levels of triglycerides and alkaline phosphatase. For RL, similar microscopic effects, dependent on dose level and sex, were observed in spleen, thymus, adrenal, and liver. In vitro, RL was as active as RA in potentiating pokeweed mitogen-induced lymphocyte proliferation; RG was inactive. This study indicates that, relative to RA and RG, RL has less toxicity but similar immunological effects. Since RL and RG expressed little or no binding affinity for cellular RA-binding protein, the immunological effects of these retinoids may be expressed by mechanisms not linked to this protein.

Differential expression of the major histocompatibility antigen complex (MHC) on a series of Burkitt's lymphoma lines

We compared the expressions of class I and class II major histocompatibility antigen complex (MHC) on the surface of Jijoye and P3HR-1 cells of Burkitt's lymphoma sublines. Jijoye cells had a large amount of class I and class II MHC antigens, whereas these antigens were less expressed on P3HR-1 cells. On a subline of P3HR-1 K cells the expression of class I antigen markedly diminished and class II antigen was undetectable. On the other hand, Jijoye, P3HR-1, and P3HR-1 K cell lines were confirmed to be Epstein-Barr virus (EBV) nonproducer, low producer, and high producer, respectively. The chemical activation of EBV genome by treating P3HR-1 cells with 12-O-tetradecanoyl phorbol-13 acetate (TPA) and n-butyrate resulted in inhibition of the expression of class I and II antigens, while the addition of retinoic acid, an inhibitor of virus replication, blocked the decrease in the MHC antigen expression. These findings suggested that there might be an inverse correlation between the virus production and the expression of class I and II MHC antigens.

[Possible role of the thymus in the mechanism of the adjuvant effect of retinoic acid]

Administration of retinoic acid and of its esters into experimental animals stimulated proliferation of thymus lymphocytes, decreased the tetraploid lymphocytes and increased the amount of diploid cells. The thymus mass tended to decrease as a result of contraction in the area of the gland cortical layer. Alterations in blood, which occurred in presence of retinoic acid, may be responsible for the effect observed.

Augmentation of collagen arthritis by synthetic analogues of retinoic acid

We evaluated the effect of administering orally two synthetic analogues of retinoic acid, 13-cis-retinoic acid and all-trans-N-(4-hydroxyphenyl)-retinamide to age-matched female Sprague-Dawley rats immunized with native chick type II collagen in incomplete Freund's adjuvant. Ingestion of a diet containing 13-cis-retinoic acid was associated with a significant increase in the severity of collagen arthritis, but there was no effect on weight gain or hemagglutinating antibody titers and delayed-type hypersensitivity to type II collagen. In two separate trials, ingestion of 4-hydroxyphenyl retinamide also significantly enhanced the severity of arthritis. Monolayer cultures of dissociated synovial cells taken from arthritic rats, but not nonarthritic rats, released prostaglandin E2 (PGE2) and collagenase into the medium. The level of PGE2 production was significantly decreased by in vivo or in vitro exposure to 4-hydroxyphenyl retinamide, whereas the addition of 13-cis-retinoic acid to the cultures had no effect on PGE2 release by the arthritic synovial cells. Five rats fed the 13-cis-retinoic acid-containing diet for 5 mo did not develop clinical or histologic evidence of arthritis. These data demonstrate that both retinoids possess potent enhancing properties for an experimentally inducible autoimmune arthritis, that synovial cells produce PGE2 and collagenase in this model, and that production of PGE2 can be suppressed by 4-hydroxyphenyl retinamide.

Reactivity of vitamin A derivatives and analogues with vitamin A antibodies

Vitamin A antibodies were obtained using retinoic acid conjugated to human serum albumin as an immunogen. The following constraints governed the reactivity of vitamin A analogues with such an anti-serum. The stereochemistry of the side chain is relatively unimportant, and 9- and 13-cis retinal react almost as well as all-trans retinal. The nature of the ring is important; all of the compounds that react readily carry a beta-ionic ring; all of the compounds bearing an aromatic ring react poorly; the two compounds that display intermediate reactivity have non-aromatic 6- and 5-membered rings, respectively.