Immunological aspects of retinoids in humans. II. Retinoic acid enhances induction of hemolytic plaque-forming cells

Immunomodulator polyinosinic-polycytidylic acid enhances the inhibitory effect of 13-cis-retinoic acid on neuroblastoma through a TLR3-related immunogenic-apoptotic response

High-risk neuroblastoma is associated with low long-term survival rates due to recurrence or metastasis. Retinoids, including 13-cis-retinoic acid (13cRA), are commonly used for the treatment of high-risk neuroblastoma after myeloablative therapy; however, there are significant side effects and resistance rates. In this study, we demonstrated that 13cRA has a better antiproliferative effect in MYCN-amplified neuroblastoma cells than in MYCN-nonamplified neuroblastoma cells. In MYCN-amplified SK-N-DZ cells, 13cRA induced significant upregulation of toll-like receptor 3 (TLR3) and mitochondrial antiviral-signaling protein (MAVS) expression in a time-dependent manner. Furthermore, poly (I:C), a synthetic agonist of TLR3, effectively synergized with 13cRA to enhance antiproliferative effects through upregulation of the innate immune signaling and the mitochondrial stress response, leading to augmentation of the apoptotic response in 13cRA-responsive cancer cells. In addition, the 13cRA/poly (I:C) combination induced neural differentiation through activation of retinoic acid receptors beta (RAR-β), restoring expression of α-thalassemia/mental retardation syndrome X-linked (ATRX) protein, and inhibiting vessel formation, leading to retarded tumor growth in a mouse xenograft model. These results suggest that the combination of poly (I:C) and RA may provide synergistic therapeutic benefits for treatment of patients with high-risk neuroblastoma.

Vitamin A potentiates CpG-mediated memory B-cell proliferation and differentiation: involvement of early activation of p38MAPK

Foreign CpG-DNA from viruses and bacteria can activate memory B cells through binding to toll-like receptor 9, and this pathway has been hypothesized to be involved in the continuous activation of memory B cells ensuring life-long humoral immunity. In this study, we demonstrate that retinoic acid (RA) is a potent coactivator of this pathway in human B cells. RA enhanced the CpG-mediated proliferation of CD27(+) memory B cells, and the proliferative response was accompanied by increased immunoglobulin (Ig) secretion indicative of plasma-cell formation. The RA-induced proliferation was preceded by enhanced expression of cyclin D3, and both the expression of cyclin D3 and the induced Ig secretion were found to be dependent on IL-10. Of importance, RA increased the CpG-induced phosphorylation of ERK1/2, p38MAPK, and IkappaB as early as 30 minutes after stimulation. By using specific inhibitors, all the RA-mediated events, including proliferation, cyclin D3 expression, IL-10 secretion, and Ig secretion, were shown to be dependent on p38MAPK. Hence, we propose that RA can strengthen humoral immunity by promoting CpG-mediated stimulation of CD27(+) B cells via activation of p38MAPK resulting in increased proliferation and differentiation to Ig-secreting plasma cells.

Therapeutic strategies in common variable immunodeficiency

The treatment of common variable immunodeficiency (CVID) is currently based on the early recognition of the condition and replacement immunoglobulin combined with prompt treatment of infections and complications. The route of administration, dose and frequency of administration of immunoglobulin still vary between centres and countries. Other interventions aimed at overcoming the immunological defects in CVID such as interleukin-2 therapy are being studied but there is as yet insufficient evidence to support their routine use. The treatment of complications such as suppurative lung disease uses principles broadly similar to those used for cystic fibrosis, whereas the granulomatous complications involving the lungs and other organ systems are in need of much more research to define optimum therapies.

The effects of retinoic acid on immunoglobulin synthesis: role of interleukin 6

Retinoic acid (RA) and its parent compound, retinol (ROH, vitamin A), have been recognized as important immunopotentiating agents. Previous studies from our laboratory have demonstrated that RA can augment formalin-treated Staphylococcus aureus (SAC)-stimulated immunoglobulin (Ig) synthesis of cord blood mononuclear cells (CBMC). To determine the mechanism(s) by which RA modulates Ig synthesis, we studied the effects of RA on B cells and cytokine production. The addition of RA (10(-5) to 10(-10) M) to Epstein-Barr virus (EBV)-transformed B-cell clones derived from either adult or cord blood B cells augmented Ig secretion twofold. In contrast, cell proliferation was inhibited as measured by 3H-thymidine incorporation. We evaluated two cytokines known to be constitutively produced by EBV cell lines, IL-1 and IL-6. While RA had no effect on IL-1 production, IL-6 synthesis was greatly enhanced (20- to 45-fold), which was also reflected by an increase in steady-state mRNA levels for IL-6 but not TNF-alpha or TGF-beta on Northern blot analysis. Polyclonal rabbit anti-IL-6 antibodies were used to block the augmenting effects of RA on Ig synthesis of adenoidal B cells. RA-induced augmentation in IgG and IgA synthesis was blocked 58 and 29%, respectively, by anti-IL-6 antibodies. These studies suggest that the enhancing effects of RA on Ig synthesis are mediated, at least in part, by the autocrine or paracrine effects of IL-6 on B-cell differentiation.

In vivo retinoic acid modulates expression of the class II major histocompatibility complex and function of antigen-presenting macrophages and keratinocytes in ultraviolet-exposed human skin

Because retinoic acid (RA) can alter photoaging of the skin and repeated ultraviolet (UV)-induced immunologic injury may play a role in chronic photoaging, we asked whether RA alters the acute photoimmunologic effects of UV radiation. Two sites from each volunteer were treated with 0.1% RA or vehicle continuously for 24 h before and 24 h after a 4-minimal erythema dose UVB exposure. RA did not function as a sunscreen, as determined by quantitating the increase in redness after 1 minimal erythema dose to vehicle- and RA-pretreated sites (n = 12). By flow cytometric analysis of epidermal cell suspensions harvested 3 d after the UV-EC, RA treatment did not protect CD1+ Langerhans cells from being depleted by UV light and did not modify the number of UV-induced infiltrating CD36+CD11b+CD1-DR+ macrophages. RA treatment did, however, result in a 40% downregulation of human leukocyte antigen (HLA)-DR expression on these infiltrating macrophages (p = 0.016) (n = 11), in conjunction with a decrease in alloantigen-presenting cell activity of RA-treated UV-EC as measured by T-cell proliferations. RA also induced a 72% inhibition of the autologous T suppressor-inducer cell proliferation induced by UV-EC (vehicle: 21,813 +/- 7,302 cpm; RA; 5,299 +/-635 cpm) (n = 3). The downregulation could be due to RA-modulated keratinocytes; RA-treated UV-EC keratinocytes depleted of CD1a+ and DR+ antigen-presenting cells displayed a greater ability, relative to similarly treated vehicle-EC keratinocytes, to inhibit alloantigen presentation.

IN CONCLUSION

(i) in vivo RA treatment did not protect human Langerhans cells from being depleted by UV and did not block infiltration of macrophages into sunburned skin; and (ii) RA did decrease autologous and allogeneic T-cell reactivity induced by macrophage antigen-presenting cells in UV-exposed epidermis, at least in part by downregulating their HLA-DR expression and by upregulating inhibitory signals from UV-irradiated keratinocytes.

Retinoic acid induces translocation of protein kinase C (PKC) and activation of nuclear PKC (nPKC) in rat splenocytes

Retinoic acid (RA), a vitamin A metabolite, has marked effects on growth of normal and malignant cells; however, the exact mechanism of action remains unclear. The effect of two RA analogs, 13-cis-RA and all-trans-RA, on transmembrane signalling processes was investigated in rat splenocytes. Treatment of rat splenic cells with these retinoic acid analogs resulted in translocation of protein kinase C (PKC) from the cytosol to the membrane. Previous studies have described nuclear RA receptors (RARs and RXRs) for several species and the biologic activity of RA has been shown to be mediated by specific interaction with these nuclear receptors. Thus, activation of nuclear pool(s) of protein kinase C (nPKC) by RA analogs was also studied. Rat splenocyte nuclei pure by enzymatic and electron microscope criteria demonstrated a biphasic pattern of bell-shaped curves for both cis- and trans-RA with maximum statistically significant peak of phosphate incorporation into endogenous substrates at 10(-16) M cis-RA and 10(-16)-10(-17) M trans-RA. A monoclonal antibody to PKC and the PKC inhibitors, H-7, sphingosine, and staurosporine, blocked the RA-stimulated nuclear phosphorylation. The ability of RA to activate cell membrane PKC resulting in an increase in particulate PKC activity correlates well with the activation of nPKC since the particulate fraction would include nuclear enzyme systems. This ability of RA to activate nPKC and possibly affect the growth status of a cell may provide a missing link to our understanding of the cellular sites of action for this vitamin.

Administration of 13-cis-retinoic acid to dairy cattle. 1. Plasma retinoid concentrations and lymphocyte blastogenesis

Effects of administration of 13-cis-retinoic acid on concentrations in plasma of retinoic acid isomers, retinol, and beta-carotene and on in vitro mitogen-stimulated lymphocyte blastogenesis of peripheral blood mononuclear leukocytes were evaluated in Holstein cattle. Treatments consisted of i.m. injections of 0 (vehicle only), 100, 200, or 400 mg of 13-cis-retinoic acid in dimethylsulfoxide for 7 d. Concentrations of 13-cis- and all-trans-retinoic acids in plasma and in mononuclear leukocytes were elevated in a dose-dependent manner by administration of 13-cis-retinoic acid. Treatment with 13-cis-retinoic acid, however, had no effect on concentrations of retinol and beta-carotene in plasma. In vitro DNA synthesis in unstimulated and mitogen-stimulated mononuclear leukocyte cultures was unaffected by in vivo administration of 13-cis-retinoic acid. In contrast, in vitro supplementation of unstimulated and mitogen-stimulated mononuclear leukocyte cultures with 13-cis-retinoic acid inhibited DNA synthesis relative to unsupplemented cultures. In conclusion, elevated plasma and intracellular concentrations of 13-cis- and all-trans-retinoic acids produced by repeated i.m. administrations of 13-cis-retinoic acid did not affect peripheral blood mononuclear leukocyte function, as measured by the in vitro blastogenic response of peripheral blood lymphocytes.

Retinoid-induced modulation of immunoglobulin M secretion by bovine mononuclear leukocytes in vitro

Effects of trans and cis isomers of retinol and retinoic acid on IgM secretion by bovine peripheral blood mononuclear leukocytes were evaluated in vitro. Mononuclear leukocyte cultures that were unstimulated or stimulated by pokeweed mitogen were supplemented with isomers of retinol and retinoic acid at 10(-10) to 10(-6) M. Concentrations of polyclonal IgM in supernatants from 14-d cultures were measured by an ELISA. Cultures stimulated by pokeweed mitogen consistently secreted more IgM than parallel, unstimulated cultures. Retinoid supplementation did not affect basal IgM secretion by unstimulated cultures. However, each retinoid affected IgM secretion by cultures stimulated by mitogen. The nature of the effect was dependent on the concentration of the specific retinoid. All-trans-retinoic acid enhanced secretion at 10(-10) M and inhibited secretion at 10(-6) M. The other retinoids, however, did not inhibit IgM secretion at any concentration. Each retinoid enhanced IgM secretion at one or more concentrations, although enhancement was produced by much lower concentrations of retinoic acid isomers than retinol isomers. These results indicate that retinol and retinoic acid modulate polyclonal IgM secretion by cultures of bovine mononuclear leukocytes stimulated by mitogen. Future research will determine which subsets of the mononuclear leukocyte population are affected and whether trans-retinoic acid is the metabolite that produces these effects.

The effects of retinol on in vitro immunoglobulin synthesis by cord blood and adult peripheral blood mononuclear cells

In this study we examined the effects of retinol (ROH), a metabolic precursor of retinoic acid (RA), on Staphylococcus aureus Cowan I (SAC)-induced immunoglobulin synthesis of cord blood mononuclear cells (CBMC) and adult peripheral blood mononuclear cells (PBMC). ROH augmented SAC-induced IgM synthesis of CBMC by 5.9 +/- 1.5-fold (n = 7, mean +/- s.d.), and IgG synthesis of adult PBMC by 16.3 +/- 5.1-fold (n = 3) at optimal concentrations of 10(-6) M and 10(-11) M, respectively. No augmenting effects could be demonstrated for the other immunoglobulin isotypes. Time-course studies showed that the synthesis of IgM by CBMC was accelerated with detectable immunoglobulin in supernatant fluids starting on day 3. ROH augmented immunoglobulin synthesis of CBMC stimulated by Epstein-Barr virus (EBV), a T cell-independent polyclonal activator, and of EBV-transformed B cell clones (2.5 +/- 0.2 and 4.1 +/- 1.5-fold increase, respectively), which suggests that ROH can act directly on B cells to enhance immunoglobulin synthesis. In contrast, when ROH was preincubated with cord blood T cells, washed and added to the B cell-enriched fraction with SAC, no increase (0.9-1.8-fold) in IgM synthesis was obtained. Thus, the principal mechanism(s) by which ROH augments immunoglobulin synthesis is by acting on B cells. This is in contrast to the immunoglobulin-enhancing effects of RA which is mediated by T cells, or T cell products, e.g. cytokine. Our studies suggest that RA and ROH may have different pathways of immunoglobulin-enhancing effects, perhaps mediated by different retinoid binding proteins resulting in gene activation and immunoglobulin synthesis.

Retinoid augmentation of bioactive interleukin-1 production by murine keratinocytes

The effect of retinoids on the production of interleukin-1 (IL-1) by murine epidermal keratinocytes was investigated. Freshly isolated keratinocytes were cultured in the presence of etretinate, acitretin, all-trans retinoic acid or 13-cis retinoic acid at concentrations of 8 x 10(-9)-8 x 10(-6) mol/l. Exposure of keratinocytes to retinoids increased IL-1 bioactivity in culture supernatants and cell extracts at concentrations as low as 8 x 10(-9) mol/l, as assessed by T-cell proliferation. Prolongation of the culture period enhanced the augmentative effect of retinoids. All-trans retinoic acid and 13-cis retinoic acid had a greater ability to induce IL-1 production than the two aromatic retinoids, etretinate and acitretin. Treatment with 8-methoxypsoralen plus ultraviolet A radiation and treatment with triamcinolone acetonide both reduced the effect of retinoids on the production of bioactive IL-1.

13-cis retinoic acid enhances in vivo B-lymphocyte differentiation in patients with common variable immunodeficiency

Retinoic acid (RA) has been demonstrated to drive both phenotypic and functional in vitro differentiation of B cell hybridomas from patients with common variable immunodeficiency (CVI) who manifest an "intrinsic" defect in terminal B cell differentiation (J Exp Med 1988;168: 55-71). Therefore, we conducted an open trial to determine the effects of oral 13-cis RA (0.5 mg/kg/day; 12 weeks receiving and 12 weeks without drug) on in vivo B cell differentiation in subjects with CVI. At various times before, during, and after drug administration, patients' B cells were tested for changes in cell-surface phenotype and in vitro immunoglobulin production in response to recombinant cytokines. Before 13-cis RA, all patients had decreased Leu-8 coexpression on CD20+ cells. Seven of eight subjects demonstrated "normalization" of this phenotype after 8 to 16 weeks of 13-cis RA administration. Patients whose B cells demonstrated more than normal CD20 display also had a fall toward normal in this parameter. These effects persisted for 6 to 12 weeks after drug was stopped. It appears that 13-cis RA drives B cells of patients with CVI to express a more differentiated cell-surface phenotype and may promote functional differentiation in some patients.

The effects of retinoic acid on immunoglobulin synthesis by human cord blood mononuclear cells

Derivatives of vitamin A have attracted considerable attention as agents which have immune potentiating properties and possibly tumor-suppressive effects. Recent investigations have shown that retinoic acid (RA) can augment immunoglobulin production of B-cell hybridomas from patients with immune deficiency. In this study we examined the ability of RA to modify the mitogen-induced polyclonal immunoglobulin synthesis of cord blood mononuclear cells (CBMC). RA in concentrations ranging from 10(-5) to 10(-7) M augmented IgM synthesis of CBMC in response to formalinized Cowans I strain Staphylococcus aureus (SAC) up to 45.6-fold which was greater at suboptimal responses to SAC. There were no changes in IgG or IgA synthesis and minimal effects on SAC-induced proliferative responses. RA did not produce similar changes in IgM synthesis of SAC-stimulated adult peripheral blood mononuclear cells (PBMC), and RA had no effect on the immunoglobulin synthesis of Epstein-Barr virus (EBV)-stimulated CBMC or adult PBMC. Time course studies showed that peak enhancement occurred when RA was added between 4 and 24 hr after culture initiation and required prior activation by SAC for augmentation of IgM synthesis. Cell separation experiments showed that prior incubation (18 hr) of an enriched T-cell fraction with RA enhanced the IgM synthesis of a T-cell-depleted B-cell fraction. These experiments and the findings that RA-induced augmentation of IgM production in response to SAC, but not to EBV suggest that the immunoregulatory effects of RA may be mediated by either T cells or T-cell products. Further studies will be necessary to understand the mechanism by which RA augments IgM synthesis of CBMC.

Enhancing effects of retinoic acid on monoclonal antibody production of human-human hybridomas

The effects of retinoids on the production of monoclonal antibody of human-human hybridomas were examined. IgG antibody secretion of a hybridoma CLNH11 was enhanced up to about two- to fourfold by retinoic acid (RA) at concentrations ranging from 10(-9) to 10(-5) M, where RA had little effect on the growth rate and saturation density of the cell. Among other retinoids, retinol magnified the antibody production as well as RA. Retinal and retinyl acetate had weak effects. Retinyl palmitate showed no effect. RA also enhanced the production of monoclonal antibodies from other human-human hybridomas: SLNF10, IgG-producing; CoLNE10, IgA-producing; TOS/H8, IgM-producing. RA and human hybridomas provide a defined system to study the effects of retinoids on immune responses at a molecular level.

Differential effects of retinoids on pokeweed mitogen induced B-cell proliferation vs immunoglobulin synthesis

The effects of three retinoids, all-trans-retinoic acid (RA), 13-cis-retinoic acid (cRA), and N-(4-hydroxyphenyl) retinamide (4-HPR) on mouse splenocyte responses to pokeweed mitogen (PWM) and Escherichia coli lipopolysaccharide (LPS) in vitro were evaluated. All three retinoids caused a dose dependent increase in the proliferative response to PWM. The retinoids hierarchy of efficacy based on potentiation of PWM-induced splenocyte proliferation was RA greater than cRA greater than 4-HPR. 13-cis-retinoic acid and 4-HPR also resulted in significant increases in Ig secretion in response to PWM. However, RA did not produce a significant increase in secretion compared with cells treated with PWM alone. The efficacy hierarchy of retinoids ability to potentiate Ig secretion was 4-HPR greater than cRA greater than RA. All three compounds did not affect Ig secretion from LPS-stimulated splenocytes and produced dose dependent decreases in proliferation. Both inhibition of LPS-induced proliferation and potentiation of PWM-induced proliferation were maximal when the retinoids were added during the first hour of culture. These results indicate that retinoids have a differential effect on Ig secretion and B-cell proliferation based on structural differences of the retinoids. Potentiation of proliferation and Ig secretion are both T-cell dependent and could be a result of increased lymphokine synthesis by the T-cells or increased responsiveness to the effects of the T-cell produced lymphokines.

Effects of 13-cis retinoic acid therapy on human antibody responses to defined protein antigens

We have evaluated the in vivo effects of 13-cis retinoic acid (13-cis RA) on human antibody responses to immunization with tetanus toxoid (TT) and keyhole limpet hemocyanin (KLH). Subjects with severe cystic acne were immunized with suboptimal doses (10 micrograms) of KLH 7 d and 3 months after starting retinoid therapy (13-cis RA, 1 mg/kg/day for 4 mo). A standard booster immunization with TT was given along with the initial KLH sensitization. A control group of acne patients received identical immunization regimens, but no 13-cis RA. Plasma retinoid levels were evaluated by reverse-phase HPLC and confirmed that blood-level concentrations of 13-cis RA and metabolites in these acne patients reached values previously demonstrated to be immunomodulatory in vitro. The retinoid had no effect on responses to TT as reflected by the characteristics of increased anti-TT IgG levels or the isotype distribution of the antibody. In contrast, the anti-KLH response was significantly enhanced in the 13-cis-RA-treated group. Whereas anti-KLH antibody was detected in only 4 of 13 control subjects after the secondary immunization, 10 of 13 retinoid-treated subjects had measurable levels of anti-KLH IgG (p less than 0.05). Among the responders, no differences were noted in the isotype distribution of anti-KLH antibody. These results showing enhanced anti-KLH responses induced by 13-cis RA therapy represent the first demonstration in humans that in vivo administration of a retinoid can modulate antigen-specific immune responses.

Retinoic acid upregulates interleukin-2 receptors on activated human thymocytes

It has previously been demonstrated that retinoic acid (RA) enhances the blastogenic responses of human thymocytes. We have now delineated the cellular mechanism of this activity. When RA was added to resting thymocyte cultures in the presence of recombinant interleukin-2 (rIL-2), blastogenesis was increased two- to fourfold. By assessing the proportion of cells that became Tac-positive and showed DNA synthesis early in the activation process, we determined that the augmentation by RA was not caused by an increased recruitment of resting cells that are activated to undergo blast transformation. Instead, RA markedly potentiated the growth rate of long-term rIL-2-dependent thymocyte blasts and, correspondingly, increased the Tac expression on these proliferating cells. Thus, RA enhancement of thymocyte responses appears to be mediated by an increase in IL-2-receptor expression on thymocyte blasts, resulting in augmented IL-2-dependent growth. This effect is independent of the original activating stimulus since enhancement of thymocyte responses to phytohemagglutinin (PHA) was also shown to be caused solely by increased proliferation of IL-2-dependent blast growth. In contrast to these effects on thymocytes, peripheral blood lymphocyte (PBL) proliferative responses were unaffected by RA treatment and, correspondingly, RA affected neither IL-2 receptor expression on PBL blasts nor the growth of these cells. Taken together, the results of this study suggest that RA can modulate IL-2-dependent immune responses, in part, by upregulating the expression of IL-2 receptors on proliferating T lymphoblasts generated from cells at restricted stages of development.

Retinoic acid induces the differentiation of B cell hybridomas from patients with common variable immunodeficiency

Human-human B cell hybridomas constructed from B lymphocytes of common variable immunodeficiency (CVI) patients and the nonsecreting cell line WIL2/729 HF consistently secrete low levels of Ig and appear to retain a defect characteristic of the CVI patient's B cells. We assessed the differentiative capacity of retinoic acid (RA) on these hybridomas, as well as on hybridomas constructed from normal B cells and from patients with selective IgA deficiency. RA at concentrations varying between 10(-5) and 10(-9) M augmented IgM secretion 4-20-fold from four of four CVI hybridomas tested, but did not affect Ig secretion from normal or IgA-deficiency hybridomas. In support of this elevated Ig secretion, RA enhanced the de novo synthesis of biosynthetically labeled light (kappa) and heavy (mu) Ig (up to 4- and 15-fold, respectively) in the CVI hybridoma line JK32.1. The increase in IgM synthesis/secretion could not be accounted for by RA-induced alteration in the cell cycle. In inducing this increase in IgM production, RA was found to affect two aspects of Ig gene expression: (a) the steady-state levels of heavy and light chain mRNAs were enhanced, and (b) the processing of mu heavy chain transcripts to the secreted mRNA form became favored over the membrane mRNA form. We also show that expression of Leu-17 (CD38), a surface marker that is re-expressed in the late pre-plasma stage of B cell development, was increased by RA from less than 20% to greater than 90% of the total cell population, with a concomitant 4-10-fold augmentation in the mean fluorescence intensity. Changes in both Leu-17 expression and de novo Ig synthesis were prominent by 24 h, but could be observed as early as 8 h after induction. Taken together, our study demonstrates that RA affects a marked alteration in the differentiated state of the CVI hybridoma clones. This finding suggests that retinoids can enhance the functional capabilities of B cells with defects in maturation and support further studies to evaluate their clinical potential in CVI.

In vitro effects of retinoids on murine thymus-dependent and thymus-independent mitogenesis

The effects of three retinoids: all-trans-retinoic acid (RA), 13-cis-retinoic acid (13-cis-RA), and N-(4-hydroxyphenyl)retinamide (4-HPR) on murine splenic lymphocyte proliferative responses to mitogens were evaluated. The responses to T-cell mitogens, PHA and Con A, and a T-cell-dependent B-cell mitogen, PWM were significantly potentiated by these retinoids. However proliferative responses to a B-cell mitogen, Escherichia coli LPS were unaffected or inhibited. All three retinoids at concentrations ranging from 10(-6) to 10(-15) M significantly potentiated Con A-induced proliferative responses. In response to PWM, 10(-13) M RA, 10(-12) M 13-cis RA, and 10(-11) M 4-HPR were the lowest concentrations producing significant potentiation. Endpoint concentrations of retinoids significantly potentiating responses to PHA were; 10(-9) M RA, 10(-8) M 13-cis RA, and 10(-6) M 4-HPR. These responses were independent of retinol contained in fetal calf serum supplemented medium since responses were reproduced in serum-free medium devoid of retinol. Optimal potentiation by retinoids of responses to these T-cell-dependent mitogens were found at superoptimal concentrations of mitogen suggesting a selective inhibition of T-suppressor cells. Thus, potentiation of T-cell-dependent mitogen responses provides the most sensitive biological assay yet described for detection of retinoid activity and is a reproducible system to explore the cellular and molecular mechanisms of retinoid-mediated immunopotentiation.