Effect of retinoic acid on the proliferation and phagocytic capability of murine macrophage-like cell lines

Photothermal enhancement of chemotherapy mediated by gold-silica nanoshell-loaded macrophages: in vitro squamous cell carcinoma study

Moderate hyperthermia (MHT) has been shown to enhance the effects of chemotherapeutic agents in a wide variety of cancers. The purpose of this study was to investigate the combined effects of commonly used chemotherapeutic agents with MHT induced by near-infrared (NIR) activation of gold nanoshell (AuNS)-loaded macrophages (Ma). AuNS-loaded murine Ma combined with human FaDu squamous cells, in hybrid monolayers, were subjected to three cytotoxic drugs (doxorubicin, bleomycin, cisplatin) with or without NIR laser irradiation. For all three drugs, efficacy was increased by NIR activation of AuNS-loaded Ma. The results of this in vitro study provide proof-of-concept for the use of AuNS-loaded Ma for photothermal enhancement of the effects of chemotherapy on squamous cell carcinoma.

Vitamin A deficient mice exhibit increased viral antigens and enhanced cytokine/chemokine production in nasal tissues following respiratory virus infection despite the presence of FoxP3+ T cells

The World Health Organization (WHO) estimates that 250 million children under the age of five suffer from vitamin A deficiencies (VAD). Individuals with VAD experience higher rates of mortality and increased morbidity during enteric and respiratory infections compared with those who are vitamin A sufficient. Previously, our laboratory has demonstrated that VAD mice have significantly impaired virus-specific IgA and CD8(+) T-cell responses in the airways. Here, we demonstrate that VAD mice experience enhanced cytokine/chemokine gene expression and release in the respiratory tract 10 days following virus infection compared with control vitamin A sufficient animals. Cytokines/chemokines that are reproducibly up-regulated at the gene expression and protein levels include IFNγ and IL-6. Despite previous indications that cytokine dysregulation in VAD animals might reflect low forkhead box P3 (FoxP3)-positive regulatory T-cell frequencies, we found no reduction in FoxP3(+) T cells in VAD respiratory tissues. As an alternative explanation for the high cytokine levels, we found that the extent of virus infection and the persistence of viral antigens were increased on day 10 post-infection in VAD animals compared with controls, and consequently that respiratory tract tissues had an increased potential to activate virus-specific T cells. Results encourage cautious management of viral infections in patients with VAD, as efforts to enhance FoxP3(+) T cell frequencies and quell immune effectors could potentially exacerbate disease if the virus has not been cleared.

Retinoic acid, immunity, and inflammation

Vitamin A (also called retinol), absorbed in the intestine and stored mainly in the liver and fat, is normally maintained at significant concentrations in the human blood plasma. Vitamin A is constitutively metabolized at high levels in certain tissues such as the small intestine and eyes. Retinoic acid (RA) produced at high levels in the intestine plays important roles in mucosal immunity and immune tolerance. RA at basal levels is required for immune cell survival and activation. During immune responses, enzymes metabolizing vitamin A are induced in certain types of immune cells such as dendritic cells (DCs) and tissue cells for induced production of RA. As a result, induced gradients of RA are formed during immune responses in the body. RA regulates gene expression, differentiation, and function of diverse immune cells. The cells under the influence of RA in terms of differentiation include myeloid cells such as neutrophils, macrophages, and DCs. Also included are lymphoid cells such as effector T cells, regulatory T cells, and B cells. Our current understanding of the function of RA in regulation of these immune cells is reviewed in this chapter.

Vitamin A and HIV infection: disease progression, mortality, and transmission

Among HIV-infected individuals, many nutritional factors that influence disease progress, mortality, and transmission are not well understood. Of particular interest is the role of vitamin A. The benefits of vitamin A have been recognized since ancient times by Egyptian physicians who successfully treated night blindness with vitamin A. Contemporary scientists have since recognized the importance of vitamin A and have provided evidence that it may help in repairing damaged mucosal surfaces; what remains unclear, however, is its role during HIV infection. In this review, we examine the evidence provided in both observational studies and randomized controlled trials that assessed the effect of vitamin A during HIV infection.

Effect of all-trans-retinoic acid on cytokine production in a murine macrophage cell line

All-trans-retinoic acid (RA) is a cancer chemopreventive agent and a pluripotent morphogen. It belongs to the class of retinoids that, besides being inducers of differentiation and growth-inhibitos, exert immunomodulatory and anti-inflammatory functions by mechanisms that are not clearly understood. Macrophages play different roles in diverse physiological processes, including ones in orchestrating immune and inflammatory responses. Products of activated macrophages such as interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNFalpha), interleukin-6, interleukin-8, and nitric oxide (NO) are important regulators of inflammatory reactions. In this study J774A. 1 cells, a murine macrophage cell line, was used to study the effects of RA on the production of NO, TNFalpha and IL-1beta. Cells were stimulated with lipopolysaccharide (LPS) with or without RA. RA depressed the levels of NO in a dose-dependent manner. NO production and subsequent nitrite accumulation in the media peaked at 24 h, plateaued at 48 h, and remained at the same level through 72 h. The presence of RA decreased TNFalpha levels, measured by both bioassay and enzyme-linked immunosorbent assay (ELISA), but these did not correlate with increased mRNA expression measured by reverse-transcriptase polymerase chain reaction at 6 h after LPS stimulation. IL-1beta protein production measured by both ELISA and bioassay decreased with RA treatment. IL-1beta mRNA expression was not affected by RA except at low doses. This study indicated that RA modulates cytokine production in J774A.1 macrophage cells. Inhibition of inflammatory cytokine production may play a role in the anti-inflammatory activity of RA. The results suggested that effects of RA are complex and are time and concentration dependent.

Dietary vitamin A modulates the concentrations of RRR-alpha-tocopherol in plasma lipoproteins from calves fed milk replacer

The practice of supplementing milk replacers fed to neonatal calves with high concentrations of vitamin A has raised concerns regarding the effect of excess vitamin A on the bioavailability of vitamin E. A 4 x 2 factorial experiment evaluated the effects of four dietary amounts of vitamin A [0, 1.78 [National Research Council (NRC)(6) requirement, control], 35.6 and 71.2 micromol daily as retinyl acetate] and two forms of vitamin E (RRR-alpha-tocopherol and RRR-alpha-tocopheryl acetate, 155 micromol daily) on plasma RRR-alpha-tocopherol and RRR-gamma-tocopherol and RRR-alpha-tocopherol associated with plasma lipoproteins (Lp) from milk replacer-fed Holstein calves from birth to 28 d of age. The VLDL, LDL, HDL and very high-density lipoprotein (VHDL) fractions were separated by ultracentrifugal flotation, and the amount of vitamin E associated with each fraction was determined by normal-phase HPLC. The amount and distribution of RRR-alpha-tocopherol in Lp fractions were unaffected by the form of dietary vitamin E. Plasma and Lp RRR-alpha-tocopherol concentrations increased with age (P < 0.0001) and were maximal at 28 d of age. Concentrations of RRR-alpha-tocopherol associated with Lp were 25% (P < 0.01) to 39% (P < 0.0001) lower in calves fed 35.6 and 71.2 micromol of vitamin A daily than in control calves at 28 d of age. The RRR-gamma-tocopherol concentrations were unaffected by dietary vitamin A (P >/= 0.05). In conclusion, dietary vitamin A modulated the amount and distribution of RRR-alpha-tocopherol in the circulation of milk replacer-fed neonatal calves. Because of the essential antioxidant role of vitamin E, the health-related consequences associated with the depression of the LP RRR-alpha-tocopherol concentrations in calves fed vitamin A at 35.6 and 71.2 micromol need to be investigated.

Retinoic acid stimulates plasminogen activator inhibitor 2 production by blood mononuclear cells and inhibits urokinase-induced extracellular proteolysis

Retinoids have been shown to modulate several functions of mononuclear phagocytes. We investigated the in vitro effect of all-trans-retinoic acid (ATRA) on the production of two major fibrinolytic components, urokinase-type plasminogen activator (u-PA) and PA inhibitor 2 (PAI-2), by human blood mononuclear cells (MNC). ATRA caused a dose-dependent (range 0.01-10 microM) accumulation of PAI-2 antigen and activity into the cell culture medium, with a maximal increase (about 5-fold over control) at a concentration of 1-10 microM. Similarly, a dose-dependent increase in PAI-2 antigen was observed in cell extracts upon ATRA stimulation. Northern blot analysis showed a parallel increase in the amount of PAI-2 mRNA in ATRA-treated cells. Time-course experiments with 1 microM ATRA showed enhanced PAI-2 mRNA expression as early as 2 h, reaching a maximum at 4-6 h and then declining at 18-24 h, and a time-dependent increase in PAI-2 antigen in the cell culture medium. At variance with PAI-2, u-PA was not influenced by the drug. To establish whether ATRA-induced changes influenced the fibrinolytic process, we evaluated the effect of MNC stimulated with ATRA on u-PA-induced degradation of diluted plasma clots. ATRA-treated cells markedly inhibited clot lysis induced by low concentrations of u-PA. The effect was due to enhanced extracellular PAI-2 accumulation since it was observed with conditioned medium from ATRA-treated cells; it was abolished by the addition of neutralizing anti-PAI-2 antibodies and was negligible when single-chain t-PA was used instead of u-PA. Since monocyte/macrophage-mediated, plasminogen-dependent extracellular proteolysis has been proposed as an important mechanism of tissue damage in several inflammatory states, our findings might contribute to better explain the anti-inflammatory properties of retinoids.

Strain differences in natural killer cell-mediated immunity among mice: a possible mechanism for the low natural killer cell activity of A/J mice

Natural killer (NK) cells are well established as fundamental elements in the early eradication of aberrant cells potentially leading to neoplasia. Moreover, it has also long been known that inbred strains of laboratory mice, as well as human individuals, demonstrate a wide range of NK cell-mediated immune response even to the same tumor. In the present study, various parameters which could lead ultimately to high, or low, NK cell-mediated functional activity have been assessed. Mice of the A/J strain demonstrate very low NK cell tumor-lytic activity and correspondingly high incidence of lymphoma. By contrast, C57B1/6 mice demonstrate relatively high NK cell activity and virtually never develop lymphomas. The results of this study have revealed that the absolute numbers of splenic NK cells were significantly lower in A/J vs C57B16/mice. Furthermore, the blood of A/J mice contained significantly fewer (30%) NK cells than did that of C57B1/6 mice. However, no significant difference between the 2 strains was found in the numbers of lymphocytic cells from NK cell-enriched fractions from the spleens, which possessed either the homing receptor MEL-14, or the integrin Mac-I, both essential surface molecules for transendothelial migration of lymphocytic cells from the circulation into organ parenchyma. Moreover, NK cells from both strains responded similarly to the NK cell stimulants, ATRA, indomethacin and interleukin-2. Finally, there was no significant difference between the 2 strains, in the numbers of lymphocytic cells in the bone marrow (including NK cells), which were radiolabelled with the DNA synthetic precursor, 3H-thymidine, indicative, thus, of equivalent levels of lymphocyte production by the bone marrow in the 2 strains. The observations collectively suggest that the low peripheral (spleen, blood) levels of NK cell-mediated functional activity found in the A/J strain of mouse at least, reflects either post-production, large-scale NK cell abortion/death, or a bone marrow-based microenvironmental deficiency which inhibits NK cells' exit from the bone marrow birth site.

Retinoids upregulate phagocytosis by human dermal microvascular endothelial cells

Animals fed a diet deficient in vitamin A show severe physiological changes that often result in death. At the cellular level, retinoids have been shown to induce differentiation of cells derived from a wide spectrum of tissues, including the vasculature. To understand further the mechanisms for these events, we studied the effects of 13-cis-retinoic acid, all-trans-retinoic acid, all-trans-retinol, and all-trans-retinol acetate on human dermal microvascular endothelial cells (HDMEC). Concentrations of retinoids in the physiological range from 0 to 1 microM were used in our experiments. These concentrations were nontoxic to HDMEC. Here we report that in addition to the known effect of retinoids on keratinocytes and sebacytes, retinoids induced morphological and functional changes in HDMEC that gave these cells macrophage like characteristics. 13-Cis-retinoic acid and all-trans-RA induced HDMEC to phagocytize and to increase the production of hydrogen peroxide and superoxide anion. These two retinoids also changed the morphology of endothelial cells from typical small compact cuboidal epithelioid cells to cells with larger cytoplasm and indistinct cell membranes. The retinoid-stimulated HDMEC deposited increased amounts of extracellular matrix. All-trans-retinol and all-trans-retinol acetate did not significantly affect HDMEC in all parameters tested. The induction of these properties provides a new model with which to study how retinoids regulate gene expression using a normal, nontransformed cell line.

A mechanism of retinoid potentiation of murine T-cell responses: early upregulation of interleukin-2 receptors

The capacity of retinoids to amplify the proliferative response of BALB/c lymphocytes to concanavalin A (Con A)2 in the presence of exogenous interleukin-2 (IL-2) and the induction of IL-2 receptors (IL-2R) on L3T4+ and Lyt-2+ T-cells was evaluated. Preincubation with Con A for 8 h in the presence of retinoids resulted in a greater than two-fold increase in spleen cell proliferative response to Con A plus rIL-2 over the following 72 h relative to the response of cells preincubated with Con A alone. Peak potentiation of IL-2 responses occurred over a pharmacologic range of retinoic acid (RA) concentration (10(-10)-10(-8) M) in the presence of 20 U/ml rIL-2. This potentiation of the response to IL-2 was likewise observed after 8 h prestimulation with Con A with splenic T-cells enriched by passage over nylon wool. Preincubation of the spleen cells with Con A plus RA without the subsequent addition of IL-2 resulted in a proliferative response that was potentiated nearly to the level of the response produced by subsequent addition of IL-2 to Con A-activated cells. Preincubation of the cells with Con A in the presence of RA produced a true synergy with IL-2; the resulting increase in response was greater than the sum of the increases produced by RA or IL-2 alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of IL-2-induced t-cell proliferation by retinoids

We evaluated the capacity of retinoids to potentiate proliferative responses of murine T-cells to recombinant human interleukin 2 (rIL-2). Concanavalin A (Con A) prestimulated spleen cells responded in a dose-dependent manner to added rIL-2. All-trans-retinoic acid (RA) at 10(-8) M potentiated the proliferative response by fivefold at saturating levels of IL-2. In similar experiments, two closely related retinamides, all-trans-(phenyl)retinamide (PR) and N-(4-hydroxyphenyl)retinamide (4-HPR), also potentiated murine splenocyte rIL-2 responses. Potentiation of IL-2-induced proliferation was dose-responsive to the concentration of added retinoid with peak potentiation occurring at 10(-10) - 10(-8) M in the presence of 10 U/ml rIL-2. Significant potentiation was observed at retinoid concentrations as low as 10(-14) M. Fluorescence flow cytometry of the responding cells revealed that among L3T4+, Lyt-2+ or total T-cells, at 72 h following Con A stimulation, essentially all of the cells expressed IL-2 receptors (IL-2R). This apparently represents near maximum IL-2R expression and treatment of the cells with retinoids did not increase IL-2R expression at that time point. The potentiation of IL-2 responses by retinoids was also observed with IL-2-dependent HT-2 cells, 98% of which were IL-2R positive. HT-2 proliferative responses to rIL-2 were potentiated as much as fourfold by 10(-10) M RA. HT-2 proliferative responses to rIL-2 were potentiated by all three retinoids dose dependently. Significant potentiation was observed with as little as 10(-14) M retinoid. Retinoids in the absence of IL-2 induced no proliferative responses. These data suggest that retinoids can augment the capacity of IL-2 to induce T-cell proliferation using Con A-activated murine splenic T-cell blasts and a long-term-cultured T-cell line.

Impaired T lymphocyte immune response in vitamin A depleted rats and chicks

Vitamin A deficiency results in decreased immune responses; the objective of the present study was to investigate the involvement of T lymphocytes in the depression of immune responses resulting from vitamin A depletion. This objective was achieved by evaluating antigen-specific T lymphocyte proliferative responses in vitro as vitamin A depletion developed. The evaluation was performed in both rat and chick to examine the generality of immune effects due to vitamin A depletion. Our findings show that vitamin A depletion led to severe impairment of T lymphocyte activity in both animal models, and that this was directly related to the vitamin A status in both species. Immune response impairment was found to precede other manifestations of vitamin A deficiency, and was rapidly corrected by feeding retinyl acetate boluses. This implied a possible regulatory, rather than constitutive, role of vitamin A in immune responsiveness.

Effects of retinoids on human thymus-dependent and thymus-independent mitogenesis

The effects of all-trans-retinoic acid (RA), 13-cis-retinoic acid, and N-(4-hydroxyphenyl)retinamide on the mitogenic responses of various populations of human lymphocytes have been evaluated. Superoptimal concentrations of mitogens allowed the greatest RA-induced potentiation of lymphocyte proliferation. All three retinoids at concentrations as low as 5 x 10(-14)M significantly potentiated the proliferation of adenoidal and tonsillar lymphocytes stimulated by pokeweed mitogen (PWM). However, the responses of adenoidal and tonsillar lymphocytes to Staphylococcus aureus Cowan strain A were not potentiated by retinoids. Retinoids also caused significant potentiation of proliferation of PWM-stimulated peripheral blood lymphocytes (PBL). However, endpoint concentrations of retinoids required to significantly potentiate PBL proliferative responses to PWM were much higher than required for potentiation of adenoidal or tonsillar lymphocytes. PBL responses to concanavalin A (Con A) were significantly potentiated by retinoid concentrations as low as 10(-8) to 10(-10) M. Retinoid-potentiated responses were also observed wi Con A-stimulated thymocytes, but the endpoint concentrations required for significant potentiation were 10-fold higher than required to potentiate PBL responses to Con A. These data indicate that the sensitivity of lymphocytes to the retinoid-mediated potentiation of mitogenesis depends on the lymphoid compartment from which the cells are obtained. Tonsillar and adenoidal lymphocytes were the most responsive of the lymphocytes tested to the retinoid-induced potentiation of PWM responses. In addition, retinoids appear to selectively potentiate T cell-dependent proliferative activity.

Characteristics of the beta-glucan receptor of murine macrophages

Phagocytosis of heat-killed yeast (HK-yeast), zymosan, and glucan particles by thioglycollate-elicited mouse peritoneal macrophages (Tg-macrophages) was inhibited by soluble glucan polymers/oligomers. The inhibitory capacity of soluble glucans decreased steeply with the decrease in the degree of polymerization (DPn); i.e., the concentration at which 50% inhibition of phagocytosis was attained was 0.23 microgram/ml for glucan 1 (DPn 24.8), 0.8 microgram/ml for glucan 2 (DPn 21.9), and greater than 40 micrograms/ml for glucan 3 (DPn 13.8). The glucan polymers were obtained by partial hydrolysis of glucan particles with formic acid (90%, 95 degrees C, 20 min) and fractionation according to solubility in ethanol water mixtures. A short preincubation (5 min, 4 or 37 degrees C) of Tg-macrophages with glucan 1 led to a subsequent inhibition of HK-yeast phagocytosis. Recovery of the phagocytic function was slow (27% in 3 h; 68% in 5 h) and required protein synthesis. beta-Glucan receptor expression was also suppressed by dexamethasone treatment. Mannan exerted at high concentrations (5 mg/ml) a partial inhibitory activity which was totally abrogated by beta-glucanase treatment. Treatment of macrophages with glucan together with mannan did not enhance the inhibitory capacity of glucan beyond the component abrogated by enzyme treatment. Contribution of local opsonization of HK-yeast to the phagocytic response (involvement of complement receptors) was indirectly negated; (a) glucan 1 which inhibits HK-yeast phagocytosis by up to 95% is not an activator of complement and therefore could not compete for the opsonizing proteins; (b) cycloheximide treatment in itself inhibited only partially HK-yeast phagocytosis whereas it inhibited the reexpression of the glucan receptors; (c) glucan 1 did not affect the phagocytosis of serum opsonized HK-yeast. Thus under the experimental conditions described, phagocytosis of HK-yeast by murine macrophages is mediated by and large by the beta-glucan receptors, while the mannose receptors and complement receptors do not contribute to the process.

The effect of 13-cis retinoic acid on hematopoiesis in human long-term bone marrow culture

The modulatory effect of 13-cis retinoic acid (RA) on the growth, differentiation and function of hematopoietic cells in human long-term cultures was studied. RA (5 X 10(-8) M) induced enhancement of myeloid progenitor cell growth in the non-adherent layer throughout 6 weeks of incubation while it did not affect the number of myeloid progenitors in the adherent layer. The vitamin did not alter the differentiation pattern of colony forming unit-culture (CFU-C). The addition of RA to cultures for 5 weeks did not alter the cellular composition of the adherent layer. Prolonged exposure of hematopoietic cells to RA did not affect the functional activity of neutrophils and macrophages, i.e. the cells were active in phagocytosing Candida albicans (CA).

Modulation of transglutaminase activity in mononuclear phagocytes and macrophage-like tumor cell lines by differentiation agents

The effect of glucocorticosteroids, retinoids, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and the tumor promoter phorbol myristate acetate (TPA) on the expression of transglutaminase activity in vitro differentiating bone marrow-derived mouse and rat mononuclear phagocytes (BMDMP) and mouse and human myeloid leukemia cell lines was assessed. Dexamethasone was found to induce an increase of about 100% in transglutaminase activity in mouse and rat BMDMP. The effect was time- and dose-dependent, and specific for steroids with glucocorticoid activity. Retinoic acid (RA) suppressed transglutaminase activity in mouse BMDMP (approximately 50%) and enhanced it in rat BMDMP (100-200%). Other retinoids were less effective. 1,25(OH)2D3 had little effect on transglutaminase expression in mouse BMDMP and suppressed it in rat BMDMP (approximately 60%). TPA exerted a suppressive effect (approximately 50%) on transglutaminase activity of both rat and mouse BMDMP. In murine (P388D1 and J774.2) and human (ML3, HL-60, KG-1, HEL, U937) myeloid leukemia cell lines, dexamethasone enhanced transglutaminase activity to a varying degree (100-1,000%), RA suppressed it in P388D1 cells (approximately 70%) and enhanced it in the other cell lines (100-1,500%), 1,25(OH)2D3 induced a rather small augmentation of enzyme expression, whereas TPA suppressed enzyme expression (70-100%). The species-specific differences previously observed by us for the effect of RA, dexamethasone and 1,25(OH)2D3 on the formation of BMDMP from mouse and rat bone marrow progenitor cells are now shown to extend also to effects on expression of transglutaminase activity. From a mechanistic point of view it is of interest that dexamethasone uniformly enhanced transglutaminase activity, whereas TPA suppressed it. RA and 1,25(OH)2D3 induced either suppression or enhancement in the various cell types, with no correlation between the direction of the effect of the two agents. The data suggest that modulation of transglutaminase activity by the four agents occurs via disparate mechanisms.

Enhancement of colony-stimulating-factor--dependent clonal growth of murine macrophage progenitors and their phagocytic activity by retinoic acid

The effect of retinoic acid (RA) on the colony-stimulating-factor-dependent clonal growth of myeloid progenitors was assessed in semisolid agar cultures of mouse bone marrow cells using L-cell-conditioned medium that gave rise to macrophage colonies, granulocyte colonies, and mixed macrophage-granulocyte colonies and clusters. RA was found to enhance the overall formation of myeloid colonies (about 50%) and clusters in 7-day cultures. The increase was due to an enhanced formation of macrophage colonies (70-250%) and clusters which reached a maximal value at about 3 microM RA. In 4-day cultures, the effect of RA on macrophage colony formation was biphasic with a maximal enhancement at 10 nM. RA suppressed granulocyte-colony formation in 4-day cultures. RA increased the phagocytic activity of bone-marrow-derived macrophages at all stages of differentiation and/or maturation in culture. The Fc-receptor-mediated erythrophagocytosis as well as the phagocytosis of heat-killed yeast cells (HK-yeast) and starch particles increased by RA treatment in a dose-dependent manner, reaching an increase of 100-200% of the activity expressed in the absence of RA. Peritoneal exudate macrophages likewise exhibited an increased phagocytic response to a variety of particles, at both physiological and pharmacological concentrations of RA. Expression of an RA-mediated increase in phagocytic activity required a prolonged incubation with RA (greater than 19 hr). The data suggest that RA may be of physiological relevance in the regulation of proliferation and function of hemopoietic cells. Therapeutic doses of RA may potentiate macrophage proliferation and function, elements that are crucial at all phases of the various defense mechanisms that the organism possesses.

1,25-Dihydroxyvitamin D3 and the regulation of the differentiation and function of macrophages and granulocytes

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) exerts a differential inhibitory effect on the formation of granulocyte, granulocyte/macrophage, and macrophage colonies grown from mouse bone marrow precursor cells; 50% inhibition was attained at 1.1, 2.3, and 23 nM 1,25(OH)2D3, respectively. The inhibition of colony formation, as well as phagocyte proliferation in liquid cultures, requires the presence of 1,25(OH)2D3 in the early stages of culture (up to 72 h after culture initiation). 1,25(OH)2D3 induces a dose- and time-dependent augmentation of the phagocytic capability of mononuclear phagocytes (up to 100%) towards both heat-killed yeast cells and IgG-coated sheep red blood cells. The augmentation of the phagocytic capability of the mononuclear phagocytes depends critically on when 1,25(OH)2D3 is added. It is effective when added up to 72 h after culture initiation, while at later stages (greater than or equal to 96 h) the cells are no longer induced to express enhanced phagocytic capability. We suggest that these phenomena may be relevant to hemopoietic processes.