Mononuclear phagocyte activation: activation-associated antigens

Mononuclear phagocyte activation is characterized by alterations in cellular metabolism and plasma membrane composition. In rodent and human systems, antibodies (conventional heteroantibodies or monoclonal reagents) that identify plasma membrane antigens selectively expressed by activated macrophages and monocytes have been generated. Among these activation-associated determinants is Mo3e (p50,80), a protease-sensitive antigen that is expressed by human monocytes activated in culture by exposure to bacterial lipopolysaccharide, muramyl dipeptide, or phorbol myristate acetate (PMA) (as well as other biologically active phorbol compounds). Mo3e is also expressed by the monoblastic cell line U-937 after culture in medium containing PMA and other pharmacological activators of protein kinase C (4 beta-phorbol-12,13-dibutyrate, 4 beta-phorbol-12,13-didecanoate, mezerein, and cell-permeable 1,2-diacylglycerol). The human promyelocytic cell line HL-60 becomes Mo3e positive after exposure in vitro to certain inducers of monocytic differentiation (PMA, dibutyryl cyclic AMP, and cholera toxin plus 3-isobutyl-1-methylxanthine). The surface expression of Mo3e is blocked by inhibitors of protein synthesis, N-linked glycosylation, and protein kinase activation, as well as by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and calcium antagonists. These data suggest the involvement of glycoprotein synthesis, protein kinase activation, and calcium ions in the stimulated expression of Mo3e by activated human mononuclear phagocytes. Anti-Mo3e antibody blocks the human monocyte response to migration inhibitory factor (MIF), which indicates an association between the expression of Mo3e antigen and responsiveness to MIF.     

Modulation of the expression of chondroitin sulfate proteoglycan in stimulated human monocytes

Proteoglycan biosynthesis was studied in human monocytes and monocyte-derived macrophages (MDM) after exposure to typical activators of the monocyte/macrophage system: interferon-gamma (IFN-gamma), lipopolysaccharide (LPS), and phorbol 12-myristate 13-acetate (PMA). By morphological examination, both monocytes and MDM were stimulated by these activators. Treatment with IFN-gamma resulted in a slight decrease in the expression of [35S]chondroitin sulfate proteoglycan (CSPG) in both monocytes and MDM, whereas LPS treatment increased the [35S]CSPG expression 1.8 and 2.2 times, respectively. PMA, in contrast, decreased the CSPG expression 0.4 times in monocytes, whereas MDM were stimulated to increase the biosynthesis 1.9 times. An increase in the sulfate density of the chondroitin sulfate chains was evident following differentiation of monocytes into MDM due to the expression of disulfated disaccharide units of the chondroitin sulfate E type (CS-E). However, monocytes exposed to PMA did also express disaccharides of the chondroitin sulfate E type. Furthermore, the expression of CS-E in MDM was increased 2 times following PMA treatment. An inactive phorbol ester, phorbol 12,13-diacetate, did not affect the expression of CS-E in either monocytes or MDM when compared with control cultures, suggesting that protein kinase C-dependent signal pathways may be involved in the regulation of sulfation of CSPG. Exposure to LPS or IFN-gamma did not lead to any changes in the sulfation of the chondroitin sulfate chains.     

LPS-stimulated release of prostaglandin E and thromboxane B2 from the U937 cell line

Human monocytes are known to metabolize arachidonic acid (AA) and to release prostaglandins upon stimulation. Previous data indicate that in vitro maturation and differentiation of monocytes result in alteration of this property with greatly diminished response to stimulators of release of prostaglandin E (PGE) and thromboxane B2 (TxB2) occurring after cells have been cultured. To further study the effects of differentiation on human monocyte AA metabolism, a model system was established based upon the human histiocytic cell line U937. Among tested stimulants, which included opsonized zymosan, complement fragment C3b, phorbol myristate acetate (PMA), calcium ionophore A23187, and concanavalin A, it was found that Escherichia coli lipopolysaccharide (LPS) was unique in that it stimulated increased release of TxB2 from U937 cells. The effect of the phorbol ester PMA, a compound commonly used to induce differentiation of U937, on the ability of U937 to respond to LPS was examined. Following 48 hr of treatment with PMA, U937 became capable of releasing both PGE and TxB2 in response to small doses of LPS. As previously observed for human monocytes, the release of PGE was delayed for several hours following stimulation and failed to reach maximal cumulative levels in culture until 24-48 hr following stimulation. In contrast to human monocytes, PMA-induced U937 were capable of maintaining their responsiveness to LPS for several days. Thus, the U937 cell line provides a useful model for study of the effects of differentiation of human mononuclear phagocytes on their ability to metabolize AA, and for the effects of LPS on histiocytic tumor cell prostaglandin release.     

Cultured human monocytes require exposure to bacterial products to maintain an optimal oxygen radical response

Freshly isolated human blood monocytes displayed a vigorous oxygen radical response, measured as release of superoxide anion (O2-), after stimulation with phorbol myristate acetate (PMA) or opsonized zymosan. High O2- release was observed with cells isolated by using a variety of procedures. Monocytes cultured in endotoxin-free medium M199 with or without 5% heat-inactivated autologous serum gradually lost this ability to produce O2- in response to PMA over the course of 4 days. The decreased responsiveness to PMA was accompanied by decreased adherence and viability. The loss of function, adherence, and viability was prevented by supplementing the culture medium with either bacterial lipopolysaccharide (LPS) or muramyl dipeptide (MDP). The O2- response of monocytes cultured for several days without bacterial products could be partially restored by the addition of LPS on day 2 or 3 of culture. Partial restoration could be detected in monocytes after only 1 hr of exposure to LPS, although a maximal response required a 2-day exposure. The minimum effective concentration of MDP was 1 ng/ml; stereoisomers of MDP, which are inactive as adjuvants, had no effect at 1 micrograms/ml. The minimum effective concentration of LPS was 1 pg/ml, corresponding to fewer than 10 molecules of LPS per monocyte. These results suggest that exposure to LPS or other bacterial products, represented here by MDP, may be required to preserve the microbicidal potential of human monocyte-macrophages in vivo.     

Lipopolysaccharide exposure during purification of human monocytes by adherence increases their recovery and cytolytic activity

Exposure of monocytes to lipopolysaccharide (LPS) during, but not after, adherence purification increased their cytolytic activity in short-term 51Cr-release assays against K562 target cells. In the absence of LPS only a minority of monocytes could be recovered by adherence. With 1 ng/ml to 10 micrograms/ml LPS present during the 1-hr adherence procedure, however, monocytes spread more extensively on serum-coated plastic and glass surfaces and virtually all of the monocytes in a mononuclear leukocyte preparation were recovered in the adherent fraction. While increasing the recovery of monocytes threefold, LPS exposure during adherence also increased monocyte purity as assessed by peroxidase staining, morphology, and indirect immunofluorescence with monoclonal Mo2. The proportion of Leu-11-positive NK cells in the adherent fraction did not change. Depletion of NK cells by treatment with anti-Leu-11b and complement eliminated cytolytic activity from the nonadherent, but not from the adherent, fraction isolated with LPS. Thus, addition of LPS during adherence produced a monocyte preparation with enhanced cytolytic activity not attributable to NK contaminants. To test whether LPS caused production of lymphokines that activate monocytes, we tested supernatants of unseparated mononuclear leukocytes for the capacity to stimulate purified monocytes for cytolysis. Such supernatants stimulated monocytes more effectively than LPS alone. We conclude that LPS stimulates monocytes for cytolysis most effectively during adherence purification because LPS allows the recovery of weakly adherent monocytes with high cytolytic capacity; also, LPS may stimulate production of lymphokines that further augment monocyte cytolytic activity.     

A monoclonal antibody specific for a monocyte-macrophage membrane component blocks the human monocyte response to migration inhibitory factor

In our study of the human monocyte response to migration inhibitory factor (MIF), we screened murine anti-monocyte monoclonal antibodies (MAb) for their ability to affect the MIF response. When blood monocytes were preincubated for 30 min at 37 degrees C with MAb of various cell specificities, it was found that Mo3e-MAb could inhibit the monocyte response to MIF. This effect was observed at an antibody concentration of 0.37 microgram/10(6) monocytes. This blocking activity appeared to be specific for Mo3e-MAb, since MAb specific for Mo1, Mo2, Mo4, alveolar macrophage (PAM-1), and platelet (Plt-1) antigens were unable to block the MIF response. Specificity was also shown by experiments performed with MAb similar to Mo3e-MAb in their immunofluorescence staining patterns. Two MAb of this type were not able to block the MIF response. All of these MAb are of the IgM isotype, and therefore it is unlikely that the blocking activity of Mo3e-MAb is mediated by the Fc receptor. These studies suggest that Mo3e-MAb may represent an anti-MIF receptor antibody. Mo3e-MAb recognizes a protease-sensitive antigen of 80 kD and 50 kD that is weakly expressed on fresh blood monocytes but is up-modulated by stimulation of monocytes with bacterial lipopolysaccharide, muramyl dipeptide, or phorbol compounds. Mo3e is also expressed by human myeloid cell lines, U-937 and HL-60, that have been stimulated in culture by biologically active phorbol diester compounds.     

Regulation of the synthesis of the third component of complement and factor B in cord blood monocytes by lipopolysaccharide

We compared the regulation of C3 and factor B synthesis in cord blood and adult monocytes by using techniques for identification and quantification of newly synthesized proteins, lipopolysaccharide (LPS) from several Gram-negative organisms, and precursors of LPS. Synthesis of C3 and factor B in cord blood monocytes was unaffected by lipid A (the active moiety of LPS extracted by the Westphal procedure). In contrast, adult monocytes increased C3 synthesis by 11.5-fold and factor B synthesis by 3.1-fold in response to LPS. This difference in cord blood monocyte response to LPS was specific in that other LPS-induced monocyte functions (superoxide production and phagocytosis) were stimulated comparably in both cord blood and adult monocytes by LPS. To characterize further this regulatory difference, the roles of LPS precursors, arachidonic acid metabolites, and of factor(s) released by adult monocytes were examined. Precursors of the lipid portion of LPS (lipid X and lipid Y), LPS isolated by trichloroacetic acid extraction, and endotoxin-associated protein (EAP) increased C3 and factor B synthesis in cord blood monocytes. Inhibitors of the lipoxygenase pathway (dexamethasone, ETYA) but not of the cyclooxygenase pathway (indomethacin) abrogated the response of adult monocytes to lipid A and EAP and of cord blood monocytes to EAP. Finally, co-incubation of adult monocytes and cord blood monocytes in LPS-containing medium resulted in enhancement of C3 and factor B synthesis in cord blood monocytes. These data suggest that the difference in LPS response between cord blood and adult monocytes may result from differences in lipid processing or protein recognition of LPS, differences in the production of lipoxygenase pathway products, and/or one or more regulatory factors. The availability of human mononuclear phagocytes which exhibit distinct differences in biosynthetic responsiveness to LPS should permit investigation of the molecular mechanism(s) by which LPS affects C3 and factor B gene expression.     

Inhibition of macrophage priming by sulfatide from Mycobacterium tuberculosis

Sulfatide from the outer surface of Mycobacterium tuberculosis blocked priming in cultured human monocytes. Monocytes were primed in vitro with either lipopolysaccharide (LPS) or interferon-gamma. Primed monocytes released increased amounts of superoxide anion (O2-) when stimulated with formyl-methionyl-leucyl-phenylalanine or with phorbol myristate acetate. Primed monocytes also showed increased phagocytosis of sheep erythrocytes and increased release of interleukin 1. When primed monocytes were treated with 10 micrograms/ml of sulfatide, these enhanced functions, characteristic of primed monocytes, returned to levels found in unprimed monocytes. (With respect to these functions and others, monocytes or macrophages primed in vitro by exposure to LPS or interferon-gamma resemble macrophages activated in vivo by infection. In vivo, activated macrophages provide non-specific resistance to infection). Inhibition of priming by sulfatide could be detected within 10 min, but maximum effect of sulfatide required 3 to 5 hr. Sulfatide had no effect on O2- release, if it was added after the cells had been stimulated by PMA, suggesting that sulfatide did not inhibit enzymes involved in formation of O2-, but rather that sulfatide inhibited priming. Increasing the amounts of LPS or interferon-gamma did not counteract the effects of sulfatide. Sulfatide did cause monocytes to release some prostaglandin E2 (less than 1 nM), but the amount was not sufficient to inhibit monocyte functions. The effect of sulfatide was not blocked by indomethacin. Other sulfated compounds and other products of mycobacteria did not produce the sulfatide effect. We conclude that M. tuberculosis has on its outer surface a chemical that directly interferes with monocyte priming. In vivo, M. tuberculosis might use sulfatide to block macrophage activation and thereby resist being killed by macrophages.     

Monoclonal antibody-defined surface markers of effector cells involved in human monocyte cytotoxicity

Human adherent peripheral blood mononuclear cells were cytotoxic in vitro against the murine TU5 line in a 48-hr [3H]thymidine-release assay. Monocyte-enriched adherent cell preparations contain a small and variable (usually less than 5%) contamination with large granular lymphocytes as assessed by morphology and staining with monoclonal antibody markers B73.1 and HNK1. To assess whether killing was in fact mediated by monocytes, mononuclear cells or monocyte-enriched preparations were separated using monoclonal antibodies directed against mononuclear phagocytes (Mo2, UCHM1, B44.1) or natural killer (NK) cells (B73.1 and HNK1), and a fluorescence-activated cell sorter. Cells positive for monocyte markers were highly cytotoxic against TU5, whereas negative cells were not. B73.1+ or HNK1+ cells had little or no activity. Cytotoxicity of cells positive for monocyte markers (Mo2, UCHM1, B44.1) was augmented by in vitro exposure to lymphokines or less frequently to interferon (IFN). However, cells negative for these monocytes markers were also stimulated to kill TU5 by lymphokine or IFN to an extent similar or greater than that of positive ones. IFN or lymphokines induced killing of TU5 by monocyte-depleted, B73.1-positive, lymphoid cells. These observations demonstrate that human monocytes do kill tumor cells, either in the absence of deliberate stimulation or after exposure to agents such as lymphokines. However, the possible contribution to "monocyte" cytotoxicity of minor NK cell contaminants must be taken into account particularly when agents such as IFN and lymphokines are applied, even when a relatively NK-cell-resistant target such as TU5 is used.     

Uptake of lactoferrin by mononuclear phagocytes inhibits their ability to form hydroxyl radical and protects them from membrane autoperoxidation.

Human mononuclear phagocytes do not contain the iron-binding protein lactoferrin that we have previously demonstrated inhibits the potential for human neutrophils to generate hydroxyl radical in the presence of an exogenous iron catalyst of the Haber-Weiss reaction. Previous work by other investigators has suggested that mononuclear phagocytes (monocytes and monocyte-derived macrophages (MDM] have the capacity to bind exogenous lactoferrin via lactoferrin-specific membrane surface receptors. Accordingly, we examined the possibility that uptake of iron-free (apo) lactoferrin by human mononuclear phagocytes could play a role in limiting the potential for generation of hydroxyl radical during the monocyte/MDM respiratory burst. When monocytes or MDM were incubated in the presence of apo-lactoferrin, cell-associated lactoferrin increased in proportion to the concentration of lactoferrin provided. Similar results were obtained with iron-loaded (diferric) milk lactoferrin. Consistent with the in vivo importance of these findings, we found that lactoferrin was intimately associated with human alveolar macrophages obtained by bronchoalveolar lavage. The fucose polymer fucoidan inhibited lactoferrin uptake whereas exogenous transferrin or MDM exposure to IFN-gamma was without effect. Scatchard binding analysis confirmed the presence of a lactoferrin-specific receptor with a calculated kDa of 3.56 x 10(-6) M and 3.4 x 10(7) binding sites per cell. Subcellular fractionation studies indicated that twofold more of the lactoferrin which became cell-associated over the 1-h incubation time could be found in the cytoplasmic fraction compared to the plasma membrane-containing fraction, consistent with previous evidence by others for internalization of lactoferrin by mononuclear phagocytes. When lactoferrin-loaded monocytes/MDM were incubated in lactoferrin-free media, evidence for release of lactoferrin was obtained by SDS-PAGE and immunoblot analysis, suggesting the presence of a recyclable pool of cell-associated lactoferrin. To assess the impact of lactoferrin loading on monocyte/MDM hydroxyl radical formation, lactoferrin-loaded phagocytes were stimulated with PMA in the presence of catalytic iron. Hydroxyl radical generation by lactoferrin-loaded cells was decreased to about 50% of control cells. Similarly, monocytes that had been lactoferrin-loaded demonstrated a 28% decrease in autooxidation of their membrane when stimulated in the presence of catalytic iron. These data suggest that lactoferrin binding may play an important role in maintaining optimal mononuclear phagocyte function and protecting adjacent tissue from untoward phagocyte-associated hydroxyl radical generation.     

Human monocyte adherence to cultured vascular endothelium: monoclonal antibody-defined mechanisms

We have evaluated the binding of human peripheral blood monocytes to cultured vascular endothelium as an in vitro model of monocyte interaction with the vessel wall. Monocytes were purified (91% +/- 4 SE esterase positive) by elutriation to avoid contact with surfaces before assay. Adherence of 51Cr-labeled monocytes after 45 min (36% +/- 11 SE) was significantly higher than that observed with autologous radiolabeled neutrophils (9% +/- 5 SE) and was greater on monolayers of human umbilical vein endothelium than on bovine aortic endothelium. Peripheral blood mononuclear cells treated with monoclonal antibody (MoAb) 60.3, a reagent that binds leukocyte membrane complex CDw18, implicated in multiple adherence-dependent functions, failed to adhere and flatten on artificial surfaces. Mononuclear cells treated with MoAb 60.3 simulated cells from a patient with recurrent infections whose phagocytes failed to react with MoAb 60.3 and failed to emigrate to extravascular sites in vivo. Incubation of monocytes with MoAb 60.3 inhibited (by 32 to 61%) monocyte adherence to endothelium in a dose-dependent manner for periods up to 24 hr, but had negligible effects on basal (unstimulated) neutrophil adherence. Basal monocyte adherence in the presence of MoAb 60.3 remained significantly greater than basal neutrophil adherence. Augmentation of phagocyte adherence to endothelial monolayers by autologous plasma or phorbol ester (PMA) was abrogated by incubation with MoAb 60.3. Studies with immunofluorescence flow cytometry indicated that PMA stimulation of monocytes resulted in a specific 40% increase in monocyte surface expression of the epitope recognized by MoAb 60.3. These in vitro findings, in conjunction with observations from two patients, support the hypothesis that monocyte adherence to endothelium and emigration to tissues is mediated by mechanisms both dependent upon and independent of the CDw18 complex and the epitope recognized by MoAb 60.3.     

Neutral proteinases of human mononuclear phagocytes. Cellular differentiation markedly alters cell phenotype for serine proteinases, metalloproteinases, and tissue inhibitor of metalloproteinases

Mononuclear phagocytes have the capacity to directly participate in extracellular matrix turnover via secretion of neutral proteinases. We have studied the effects of in vivo and in vitro differentiation upon cellular content or secretion of a spectrum of neutral proteinases, along with a counter-regulatory metalloproteinase inhibitor (TIMP). We found 1) matrix-degradative serine proteinases (leukocyte elastase and cathepsin G) were lost during cellular maturation and/or differentiation; 2) the 92-kDa type IV/type V collagenase and TIMP were secreted earliest in mononuclear phagocyte differentiation, whereas stromelysin secretion was observed only by LPS-stimulated alveolar macrophages; 3) exposure of alveolar macrophages, but not monocytes, to phorbol esters and LPS resulted in markedly augmented secretion of all studied metalloproteinases and TIMP; 4) monocyte-derived macrophages partially (but not completely) mimicked the metalloproteinase secretory phenotype of alveolar macrophages; and 5) the secretory phenotype of alveolar macrophages for interstitial collagenase (but not TIMP) was largely lost during in vitro culture. These results underscore the complexity of the process of differentiation in human mononuclear phagocytes, and provide insights into the variable capacity of mononuclear phagocytes to degrade extracellular matrix components. Moreover, we anticipate that human mononuclear phagocytes at various stages of differentiation will provide a useful model system for study of the variable regulation of secretion of human matrix-degrading metalloproteinases.     

Identification and characterization of a hamster monoclonal antibody anti-2.28 directed against a 70-kilodalton activation antigen on human monocytes

Hamster mAb against activated human monocytes were examined for their reactivities against monocyte activation Ag. One mAb, anti-2.28, stained only monocytes activated with LPS plus IFN-gamma, but not unactivated peripheral blood monocytes, polymorphonuclear leukocytes, lymphocytes, RBC, and platelets. However, it stained peripheral blood T cells activated with PMA plus anti-CD3 and peripheral blood and tonsillar B cells activated with PMA plus anti-mu. Of the 35 cell lines of diverse origin examined for immunofluorescence staining by anti-2.28, only EBV-transformed cell lines showed strong staining by this mAb. One pre-B cell line, Nalm-12, could be induced by PMA to exhibit intermediate staining. Immunoprecipitation studies identified the 2.28 Ag as a 70- to 85-kDa monomer. Immunofluorescence staining, immunoprecipitation, and peptide mapping studies indicated that 2.28 was different from a number of monocyte and lymphocyte surface Ag including Mo3e, B-4 (CD19), B-5, CD39, and the G28-8 Ag Bgp 95. These studies suggest that 2.28 may be a novel hemopoietic non-lineage-specific activation Ag.     

Muramyl dipeptide enhances osteoclast formation induced by lipopolysaccharide, IL-1 alpha, and TNF-alpha through nucleotide-binding oligomerization domain 2-mediated signaling in osteoblasts

Muramyl dipeptide (MDP) is the minimal essential structural unit responsible for the immunoadjuvant activity of peptidoglycan. As well as bone-resorbing factors such as 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) and PGE2, LPS and IL-1alpha stimulate osteoclast formation in mouse cocultures of primary osteoblasts and hemopoietic cells. MDP alone could not induce osteoclast formation in the coculture, but enhanced osteoclast formation induced by LPS, IL-1alpha, or TNF-alpha but not 1alpha,25(OH)2D3 or PGE2. MDP failed to enhance osteoclast formation from osteoclast progenitors induced by receptor activator of NF-kappaB ligand (RANKL) or TNF-alpha. MDP up-regulated RANKL expression in osteoblasts treated with LPS or TNF-alpha but not 1alpha,25(OH)2D3. Osteoblasts expressed mRNA of nucleotide-binding oligomerization domain 2 (Nod2), an intracellular sensor of MDP, in response to LPS, IL-1alpha, or TNF-alpha but not 1alpha,25(OH)2D3. Induction of Nod2 mRNA expression by LPS but not by TNF-alpha in osteoblasts was dependent on TLR4 and MyD88. MDP also enhanced TNF-alpha-induced osteoclast formation in cocultures prepared from Toll/IL-1R domain-containing adapter protein (TIRAP)-deficient mice through the up-regulation of RANKL mRNA expression in osteoblasts, suggesting that TLR2 is not involved in the MDP-induced osteoclast formation. The depletion of intracellular Nod2 by small interfering RNA blocked MDP-induced up-regulation of RANKL mRNA in osteoblasts. LPS and RANKL stimulated the survival of osteoclasts, and this effect was not enhanced by MDP. These results suggest that MDP synergistically enhances osteoclast formation induced by LPS, IL-1alpha, and TNF-alpha through RANKL expression in osteoblasts, and that Nod2-mediated signals are involved in the MDP-induced RANKL expression in osteoblasts.     

Effects of disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) on interleukin 1 production by macrophages

The effects of disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) on the in vitro functions of guinea pig macrophages were studied. A high dose (1 mg/ml) of EHDP inhibited interleukin 1 (IL 1) production by oil-induced peritoneal macrophages stimulated with muramyl dipeptide (MDP), lipopolysaccharide (LPS), phorbol myristic acetate (PMA), heat-aggregated IgG2 or calcium ionophore A23187. On the other hand, low doses (less than 0.125 mg/ml) of EHDP augmented the MDP induced IL 1 production by macrophages. This biphasic effect was also observed when macrophages were exposed to EHDP at 37 C for 24 hr and then stimulated with IL 1 inducers. Superoxide anion generation induced by formyl peptide or PMA was not affected by preincubation of the macrophages with doses of EHDP up to 1 mg/ml. Adherence and spreading of macrophages was inhibited by EHDP in a dose dependent manner without affecting cell viability. These results demonstrated that EHDP acted on macrophages directly and modulated IL 1 production in vitro.     

Functional Characteristics of Enhanced Fc Receptor Expression of β2 Integrin-Deficient Bovine Mononuclear Phagocytes

Fc receptor expression, cytoplasmic Ca²⁺ signaling, chemiluminescent (CL) response, and electron spin resonance (ESR) combined with spin trapping of blood mononuclear phagocytes from control heifers and a heifer with leukocyte adhesion deficiency (LAD) were evaluated to elucidate the relationships between complement receptor type 3 (CR3) and Fc receptor expression and their functional responses. The mean fluorescence intensity of fluorescein isothiocyanate (FITC)-conjugated anti-bovine IgG bound to mononuclear phagocytes from the heifer with LAD was 1.8-fold higher than that of control heifers. The mean increments of cytoplasmic Ca²⁺ concentrations of mononuclear phagocytes from the heifer with LAD stimulated with OPZ, Agg-IgG, and PMA were 39.4 (P<0.05), 118, and 71.6% compared with those of control heifers. A 1.27-fold increase in the CL response relative to control heifers was detected when mononuclear phagocytes from the heifer with LAD were stimulated with Agg-IgG. The OPZ-induced CL response of mononuclear phagocytes from the heifer with LAD was significantly (P<0.05) decreased, whereas the PMA-induced CL response was similar to that of control heifers. The ESR spectrum of mononuclear phagocytes from the heifer with LAD was increased when stimulated with Agg-IgG, and was impaired when stimulated by OPZ compared with that of control heifers. The ESR spectrum of mononuclear phagocytes stimulated with PMA was similar in control heifers and the heifer with LAD. Fc receptors on mononuclear phagocytes from the heifer with LAD were enhanced, and their cytoplasmic Ca²⁺ signaling, CL response, and ESR-spin trapping when stimulated with Agg-IgG and OPZ appeared to be associated with enhanced Fc receptors.     

Disparities in TLR5 expression and responsiveness to flagellin in equine neutrophils and mononuclear phagocytes

As sentinel cells of the innate immune system, neutrophils and mononuclear phagocytes use specific TLRs to recognize the conserved molecular patterns that characterize microbes. This study was performed to compare the responses of equine neutrophils and mononuclear phagocytes to LPS and flagellin, components of bacteria that are recognized by TLR4 and TLR5, respectively. Neutrophils and mononuclear phagocytes isolated from healthy horses were incubated in vitro with LPS, flagellin, or pronase-inactivated flagellin in the presence or absence of polymyxin B. Production of reactive oxygen species and expression of mRNA for proinflammatory cytokines were used as readouts for activation of neutrophils; production of TNF-α was used for the mononuclear cells. Western blot analysis and flow cytometry were used to detect TLR5 protein in both cell types. Although the neutrophils responded to both LPS and flagellin by producing reactive oxygen species and expressing mRNA for proinflammatory cytokines, flagellin had no stimulatory effect on monocytes or macrophages. Although both neutrophils and monocytes expressed mRNA for TLR5, it appeared to be translated into protein only by the neutrophils. Incubation with neither LPS nor IFN-γ altered TLR5 expression by the monocytes. These findings indicate that flagellin has disparate effects on neutrophils and mononuclear phagocytes isolated from horses, a species that is exquisitely sensitive to the TLR4 ligand, LPS, and that equine mononuclear phagocytes, unlike corresponding cells of other mammalian species, lack surface expression of TLR5 and do not respond to flagellin.