Fatty streak formation occurs in human fetal aortas and is greatly enhanced by maternal hypercholesterolemia. Intimal accumulation of low density lipoprotein and its oxidation precede monocyte recruitment into early atherosclerotic lesions

To determine whether oxidized LDL enhances atherogenesis by promoting monocyte recruitment into the vascular intima, we investigated whether LDL accumulation and oxidation precede intimal accumulation of monocytes in human fetal aortas (from spontaneous abortions and premature newborns who died within 12 h; fetal age 6.2+/-1.3 mo). For this purpose, a systematic assessment of fatty streak formation was carried out in fetal aortas from normocholesterolemic mothers (n = 22), hypercholesterolemic mothers (n = 33), and mothers who were hypercholesterolemic only during pregnancy (n = 27). Fetal plasma cholesterol levels showed a strong inverse correlation with fetal age (R = -0.88, P < 0.0001). In fetuses younger than 6 mo, fetal plasma cholesterol levels correlated with maternal ones (R = 0.86, P = 0.001), whereas in older fetuses no such correlation existed. Fetal aortas from hypercholesterolemic mothers and mothers with temporary hypercholesterolemia contained significantly more and larger lesions (758,651+/-87,449 and 451,255+/-37,448 micron2 per section, respectively; mean+/-SD) than aortas from normocholesterolemic mothers (61,862+/-9,555 micron2; P < 0.00005). Serial sections of the arch, thoracic, and abdominal aortas were immunostained for recognized markers of atherosclerosis: macrophages, apo B, and two different oxidation-specific epitopes (malondialdehyde- and 4-hydroxynonenal-lysine). Of the atherogenic sites that showed positive immunostaining for at least one of these markers, 58.6% were established lesions containing both macrophage/foam cells and oxidized LDL (OxLDL). 17.3% of all sites contained only native LDL, and 13.3% contained only OxLDL without monocyte/ macrophages. In contrast, only 4.3% of sites contained isolated monocytes in the absence of native or oxidized LDL. In addition, 6.3% of sites contained LDL and macrophages but few oxidation-specific epitopes. These results demonstrate that LDL oxidation and formation of fatty streaks occurs already during fetal development, and that both phenomena are greatly enhanced by maternal hypercholesterolemia. The fact that in very early lesions LDL and OxLDL are frequently found in the absence of monocyte/macrophages, whereas the opposite is rare, suggests that intimal LDL accumulation and oxidation contributes to monocyte recruitment in vivo.     

Cytokine stimulation of T lymphocytes regulates their capacity to induce monocyte production of tumor necrosis factor-alpha, but not interleukin-10: possible relevance to pathophysiology of rheumatoid arthritis

Previous studies in the laboratory have shown that the pro-inflammatory cytokine tumor necrosis factor (TNF)-alpha plays a pivotal role in the pathogenesis of rheumatoid arthritis (RA). The mechanisms involved in regulating monocyte/macrophage cytokine production are not yet fully understood, but are thought to involve both soluble factors and cell/cell contact with other cell types. We and others have previously demonstrated that T cells activated through the T cell receptor/CD3 complex induce monocyte TNF-alpha production by contact-mediated signals. In this report, we investigated further whether T cells activated by cytokines in the absence of T cell receptor stimulation also regulate monocyte cytokine production. T cells were activated in an antigen-independent manner using the cytokines interleukin (IL)-15 or IL-2 alone, or in combination with IL-6 and TNF-alpha. Subsequently, T cells were fixed and incubated with monocytes. Fixed, cytokine-stimulated T cells induced monocytes to secrete TNF-alpha in a dose-dependent manner, but did not induce secretion of IL-10, a potent endogenous down-regulator of TNF-alpha and other pro-inflammatory cytokines. Stimulation of monocyte TNF-alpha was markedly inhibited when T cells were physically separated from monocytes within the tissue culture well, confirming that T cell contact is necessary. T cell acquisition of monocyte-activating capacity was shown to be dependent on the period of cytokine stimulation, with T cells activated for 8 days more effective than T cells activated for shorter periods. Addition of interferon-gamma or granulocyte/macrophage colony-stimulating factor to the T cell/monocyte cultures enhanced T cell induction of monocyte TNF-alpha by threefold and ninefold, respectively. The results from this model of cognate interaction suggest that cytokine-stimulated T cells, interacting with macrophages in the rheumatoid synovial membrane, may contribute to the continuous excessive production of TNF-alpha observed in the RA joint, and to the imbalance of pro-inflammatory cytokines over anti-inflammatory cytokines.     

Human monocyte ATP-induced IL-1 beta posttranslational processing is a dynamic process dependent on in vitro growth conditions

Despite a large production capacity, freshly isolated lipopolysaccharide (LPS)-activated human monocytes release only a small percentage of their newly synthesized interleukin (IL)-1 beta into the medium. Extracellular ATP, acting via surface P2z-type purino-receptors, increases cytokine posttranslational processing. To explore whether this ATP response was affected by culture conditions, monocytes were maintained for different time periods in the absence and presence of various media components including fetal bovine and human sera and recombinant human cytokines. The ability of monocytes to produce radiolabeled pro-IL-1 beta in response to LPS and to posttranslationally process the procytokine after ATP stimulation was affected both by time in culture and by the presence of specific media components. These observations indicate that ATP's ability to promote human monocyte IL-1 beta posttranslational processing is a dynamic process that is subject to regulation by cytokines and/or growth factors. Changes in monocyte/macrophage ATP responsiveness may provide an important regulatory mechanism for the control of IL-1 biological activity in vivo.     

Alleviation of monocrotaline-induced pulmonary hypertension by antibodies to monocyte chemotactic and activating factor/monocyte chemoattractant protein-1

Administration of monocrotaline (MCT) causes pulmonary vascular lesions consisting of monocyte/macrophage infiltration in the early phase and medial thickening in pulmonary arteries and arterioles associated with pulmonary hypertension (PH) in the later phase. However, the molecular mechanism of monocyte/macrophage infiltration and its roles remain elusive. Herein, we have evaluated the role of a potent monocyte chemotactic and activating chemokine/monocyte chemoattractant protein-1 (MCAF/MCP-1) in MCT-induced PH in rats. A single injection of MCT induced PH at Day 21, as evidenced by increases in the ratio of right ventricular to left ventricular and septum weights (RV/LV+S) and right ventricular systolic pressure (RVSP). A significant increase in macrophage number in lungs started at Day 14, reaching a maximum at Day 21. MCAF/MCP-1 levels in bronchoalveolar lavage fluids were elevated significantly at Day 14 and remained high until Day 28, whereas plasma MCAF/MCP-1 levels increased at Day 7, returning to normal levels by Day 21. Immunoreactive MCAF/MCP-1 proteins were mainly detected in macrophages in alveoli and in perivascular regions of pulmonary arterioles and venules. Intravenous administration of anti-MCAF/MCP-1 antibodies with MCT significantly decreased macrophage infiltration and eventually reduced the increases in RV/LV+S and RVSP, as well as medial thickening of pulmonary arterioles. Thus, MCAF/MCP-1 is essentially involved in MCT-induced PH by recruiting and activating macrophages.     

Monocytes from Wiskott-Aldrich patients display reduced chemotaxis and lack of cell polarization in response to monocyte chemoattractant protein-1 and formyl-methionyl-leucyl-phenylalanine

Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by trombocytopenia, eczema, and progressive decline of the immune function. In addition, lymphocytes and platelets from WAS patients have morphologic abnormalities. Since chemokines may induce morphologic changes and migration of leukocytes, we investigated the monocyte response to chemoattractants in cells from WAS patients with an identified mutation in the WAS protein gene. Here, we report that monocytes derived from four patients with molecularly defined typical WAS have a severely impaired migration in response to FMLP and to the chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha compared with normal donors. Conversely, neither MCP-1 binding to monocytes nor induction of the respiratory burst by MCP-1 and FMLP is significantly different between WAS patients and normal donors. Within a few minutes of stimulation, monocytes respond to chemokines with increased expression of adhesion molecules and with morphologic changes such as cell polarization. Although up-regulation of CD11b/CD18 expression following stimulation with FMLP or MCP-1 is preserved in WAS patients, cell polarization is dramatically decreased. Staining of F-actin by FITC-phalloidin in monocytes stimulated with chemoattractants shows F-actin to have a rounded shape in WAS patients, as opposed to the polymorphic distribution of F-actin in the polarized monocytes from healthy donors. These results suggest that WAS protein is involved in the monocyte response to the chemokines MCP-1 and macrophage inflammatory protein-1alpha.     

Monocyte chemoattractant protein-1 in the intervertebral disc. A histologic experimental model

STUDY DESIGN: Monocyte chemoattractant protein-1 was investigated in an experimental rat model using immunohistochemistry. OBJECTIVE: To ascertain the precise mechanism of macrophage recruitment in the early phase of disc resorption. SUMMARY OF BACKGROUND DATA: In previous studies, many investigators reported that disc herniation was resorbed by monocytic phagocytosis. However, how the recruitment of monocytes was triggered is still unknown. METHODS: The autologous intervertebral discs from tails of Wistar rats were subcutaneously implanted into the abdomen. These discs were obtained on days 2, 3, 7, and 14 after implantation and were used for immunohistochemical study and for quantitative analysis of monocyte chemoattractant protein-1 by sandwich enzyme-linked immunosorbent assay. RESULTS: Monocyte chemoattractant protein-1-positive granulocytes and macrophages were observed surrounding the intervertebral disc, and monocyte chemoattractant protein-1-positive disc chondrocytes were observed in the nucleus pulposus and the inner anulus fibrosus on day 3. By day 7, monocyte chemoattractant protein-1-positive and TRPM-3-positive macrophages appeared in the granulation tissue, and some of these cells invaded the nucleus pulposus and inner anulus fibrosus. The concentration of monocyte chemoattractant protein-1 was highest on day 3. CONCLUSION: Intervertebral disc chondrocytes have chemotactic properties and play an active role in the recruitment of monocytes involved in disc resorption.     

Expression of monocyte chemoattractant protein-1 and other beta-chemokines by resident glia and inflammatory cells in multiple sclerosis lesions

Beta-chemokines induce the directional migration of monocytes and T lymphocytes and are thus associated with chronic inflammation. Using immunocytochemistry and in situ hybridisation (ISH) techniques, we have examined the expression of the beta-chemokines monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, and RANTES (regulated upon activation, normal T cell expressed and secreted) in post-mortem human brain from multiple sclerosis (MS) cases, at different stages of lesion development. In actively demyelinating MS plaques RANTES expression was restricted to the blood vessel endothelium, perivascular cells and surrounding astrocytes, suggesting a role in the recruitment of inflammatory cells from the circulation. MCP-1 was expressed by astrocytes and macrophages within acute MS lesions, but was restricted to reactive astrocytes in the parenchyma surrounding the lesion. MIP-1alpha was expressed by astrocytes and macrophages within the plaque, while MIP-1beta was expressed by macrophages and microglia within the lesion, and by microglia in surrounding white matter. Glial cells may be stimulated to produce chemokines and continue the local inflammatory response by forming chemotactic gradients to attract T cells and mononuclear phagocytes from the circulation and surrounding tissue.     

Unique monocyte subset in patients with AIDS dementia

BACKGROUND: 15-30% of patients infected with HIV will develop a debilitating dementia. Whilst HIV enters the brain soon after infection, presumably within monocyte-derived macrophages, not all patients with HIV become demented. Blood monocytes probably cross the blood-brain barrier and give rise ultimately to parenchyma macrophages. We looked for a specific monocyte subset in AIDS patients with dementia. METHODS: Peripheral blood monocytes from three groups were compared: AIDS patients with (n = 12) and without (n = 11) dementia, and ten HIV seronegative healthy controls. We used flow cytometry to analyse monocytes, and cell lysis and apoptosis assays to examine monocyte effects on human brain cells in vitro. FINDINGS: We found a unique subset of monocytes in patients with AIDS dementia. These monocytes were more dense and granular and expressed CD14/CD16 and CD14/CD69. Means (SD) for CD14/CD16 in HIV-negative controls and in AIDS non-dementia and AIDS dementia patients were 6.5% (4), 16% (13), and 37% (21), respectively (p = 0.008 between the two groups of patients). The corresponding means for CD14/CD69 were 7% (6), 8% (10), and 69% (18) (p < 0.0001). INTERPRETATION: CD69 is a member of the natural-killer-cell gene complex that is expressed after activation. Supernatants from cultures containing these dense cells can trigger apoptosis of human brain cells in vitro. The monocyte subset we found in patients with AIDS dementia might enter the brain and expose neural cells to toxic factors.     

MIP-1alpha as a critical macrophage chemoattractant in murine wound repair

At sites of injury, macrophages secrete growth factors and proteins that promote tissue repair. While this central role of the macrophage has been well studied, the specific stimuli that recruit macrophages into sites of injury are not well understood. This study examines the role of macrophage inflammatory protein 1alpha (MIP-1alpha), a C-C chemokine with monocyte chemoattractant capability, in excisional wound repair. Both MIP-1alpha mRNA and protein were detectable in murine wounds from 12 h through 5 d after injury. MIP-1alpha protein levels peaked 3 d after injury, coinciding with maximum macrophage infiltration. The contribution of MIP-1alpha to monocyte recruitment into wounds was assessed by treating mice with neutralizing anti-MIP-1alpha antiserum before injury. Wounds of mice treated with anti-MIP-1alpha antiserum had significantly fewer macrophages than control (41% decrease, P < 0. 01). This decrease in wound macrophages was paralleled by decreased angiogenic activity and collagen synthesis. When tested in the corneal micropocket assay, wound homogenates from mice treated with anti-MIP-1alpha contained significantly less angiogenic activity than control wound homogenates (27% positive for angiogenic activity versus 91% positive in the control group, P < 0.01). Collagen production was also significantly reduced in the wounds from anti-MIP-1alpha treated animals (29% decrease, P < 0.05). The results demonstrate that MIP-1alpha plays a critical role in macrophage recruitment into wounds, and suggest that appropriate tissue repair is dependent upon this recruitment.     

Gap junctional communication between vascular cells. Induction of connexin43 messenger RNA in macrophage foam cells of atherosclerotic lesions

The structure and function of blood vessels depend on the ability of vascular cells to receive and transduce signals and to communicate with each other. One means by which vascular cells have been shown to communicate is via gap junctions, specifically connexin43. In atherosclerosis, the normal physical patterns of communication are disrupted by the subendothelial infiltration and accumulation of blood monocytes, which in turn can differentiate into resident foam cells. In this paper we report that neither freshly isolated human peripheral blood monocytes nor differentiated monocytes/macrophages exhibit functional gap junctional dye transfer in homo-cellular culture or in co-culture with endothelial cells or smooth muscle cells. By Northern analysis, neither freshly isolated blood monocytes nor pure cultures of differentiated monocyte/macrophages expressed gap junction messenger RNA. However, immunohistochemical staining followed by in situ hybridization on sections of human atherosclerotic carotid arteries revealed strong expression of gap junction connexin43 messenger RNA by macrophage foam cells. These results suggest that tissue-specific conditions present in atherosclerotic arteries induce expression of connexin43 messenger RNA in monocyte/macrophages.     

Sequential dynamics of monocyte chemotactic protein-1 expression in herniated nucleus pulposus resorption

We previously demonstrated that the granulation tissues of herniated nucleus pulposus are composed of a marked infiltration of macrophages that strongly express monocyte chemotactic protein-1. Monocyte chemotactic protein-1 is a chemotactic cytokine that contributes to the activation and recruitment of macrophages. Relatively little is known about its role in the resorption process of herniated nucleus pulposus. To clarify the sequential dynamics of expression of monocyte chemotactic protein-1 in the granulation tissues of herniated nucleus pulposus, we introduced a rat autologous transplantation model of nuclear materials onto its lumbar dura mater and performed immunohistological analysis and competitive polymerase chain reaction assay using the grafted samples. Immunohistological analysis demonstrated that the majority of infiltrating mononuclear cells expressed monocyte chemotactic protein-1. Monocyte chemotactic protein-1 mRNA was expressed in the first 3 weeks after the procedure and was significantly and maximally upregulated at 1 week. To determine whether human recombinant monocyte chemotactic protein-1 facilitates the resorption process of herniated nucleus pulposus, we introduced another model of autologous transplantation, wherein the nuclear materials were grafted to the abdominal subcutaneous tissues and recombinant monocyte chemotactic protein-1 was subsequently applied to these materials. When monocyte chemotactic protein-1 was injected into the murine nucleus pulposus tissues, they reduced in size more rapidly than in the control group. These findings suggest that monocyte chemotactic protein-1 plays an important role in the recruitment of macrophages in the early phase of the resorption process of herniated nucleus pulposus and that its application may physiologically facilitate the resorption process of the nucleus pulposus.     

Effects of monocyte purification and culture on integrin expression

Selective alterations in the surface expression of members of the LeuCAM (leukocyte cell adhesion molecule) family of integrins occur during in vitro culture of human monocytes. Such changes may relate in part to cellular maturation, but also to activation following purification and culture of monocytes. In this paper, we examined the effects of monocyte isolation, adherence during culture and endotoxin exposure on the expression of these molecules and the ligand for LFA-1, ICAM-1 (CD54). Expressions of CD11b, CD18 and CD54, but not CD11a or CD11c, were higher on monocytes freshly isolated by density gradient separation and plastic adherence as compared with cells labelled directly in whole blood. However, the surface expression of the LeuCAMs and CD54 on cultured monocytes was not affected by short-term adherence to plastic for 2 h, as determined by comparisons of their expression on adherence-isolated and elutriated monocytes. In contrast, prolonged adhesion of monocytes for up to 21 days in culture altered expression of CD11a without affecting that of the other LeuCAMs or CD54. Expression of CD11a decreased more rapidly on adherence-maintained cells as compared with suspension-cultured cells. Our results show that cellular manipulations required for in vitro studies of monocyte/macrophages may alter expression of the LeuCAMs.     

Distinct roles of osteopontin in host defense activity and tumor survival during squamous cell carcinoma progression in vivo

Secreted phosphoprotein 1 (spp1), the gene encoding osteopontin (OPN), is expressed in many human carcinomas, although its in vivo functions remain unclear. To delineate the role of OPN during tumor progression, we have subjected OPN null mutant mice to repeated applications of a mutagen/carcinogen to induce cutaneous squamous cell carcinoma. OPN null animals exhibited accelerated tumor growth and progression and had a greater number of metastases per animal compared with wild-type animals. However, metastases in the OPN null animals were significantly smaller than in controls. When injected into nude mice, the growth of OPN null tumor lines and the same lines engineered to reexpress spp1 recapitulated the growth differences observed in the progression study. These differences in tumor growth inversely correlated with the degree of macrophage infiltration. Slower-growing, OPN-producing tumors contained significantly more macrophages, although a higher proportion were mannose receptor positive, a characteristic of differentiated resting macrophages. In vitro, OPN null cell lines displayed decreased survival at clonal density compared with OPN-producing lines, an observation consistent with the smaller metastases of the OPN null mice. Overall, we provide evidence for a model where host-derived OPN acts as a macrophage chemoattractant, whereas tumor-derived OPN is able to inhibit macrophage function and enhances the growth or survival of metastases.     

Mona, a novel hematopoietic-specific adaptor interacting with the macrophage colony-stimulating factor receptor, is implicated in monocyte/macrophage development

The production, survival and function of monocytes and macrophages are regulated by the macrophage colony-stimulating factor (M-CSF or CSF-1) through its tyrosine kinase receptor Fms. Binding of M-CSF results in Fms autophosphorylation on specific tyrosines that act as docking sites for intracellular signaling molecules containing SH2 domains. Using a yeast two-hybrid screen, we cloned a novel adaptor protein which we called 'Mona' for monocytic adaptor. Mona contains one SH2 domain and two SH3 domains related to the Grb2 adaptor. Accordingly, Mona interacts with activated Fms on phosphorylated Tyr697, which is also the Grb2-binding site. Furthermore, Mona contains a unique proline-rich region located between the SH2 domain and the C-terminal SH3 domain, and is apparently devoid of any catalytic domain. Mona expression is restricted to two hematopoietic tissues: the spleen and the peripheral blood mononuclear cells, and is induced rapidly during monocytic differentiation of the myeloid NFS-60 cell line in response to M-CSF. Strikingly, overexpression of Mona in bone marrow cells results in strong reduction of M-CSF-dependent macrophage production in vitro. Taken together, our results suggest an important role for Mona in the regulation of monocyte/macrophage development as controlled by M-CSF.     

Cell surface glycoproteins expressed on activated human T cells induce production of interleukin-1 beta by monocytic cells: a possible role of CD69

In many immunoinflammatory diseases, macrophages, by producing interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha), stimulate protease secretion in fibroblasts, thus contributing to tissue destruction. Monocyte/macrophage activation is prompted by soluble factors released by activated T cells as well as by cell-cell contact. Indeed, previous studies have shown that monocytes exposed to paraformaldehyde (PFA)-fixed, activated T cells produced high amounts of IL-1 beta. In this report, we used the T cell line HUT-78 to further characterize the T cell factor(s) responsible for monocyte activation by cell-cell contact. After subcellular fractionation, most of the activity was found in the cellular membrane fraction of PHA/PMA-stimulated HUT-78 cells, and proved to be due to glycoproteins, following trypsin digestion and tunicamycin treatment. HUT-78 cells acquired the capacity to stimulate monocytic cells after as little as 1h of stimulation. De novo protein synthesis was required for the expression of the IL-1 beta inducing factor, as shown by cycloheximide treatment. When membrane proteins of PHA/PMA-stimulated HUT-78 cells were separated on SDS-polyacrylamide gel, a peak of stimulatory activity was observed at Mr--25-35 x 10(3). By using specific cytokine inhibitors or blocking mAbs, we ascertained that cell-associated cytokines (IL-1, IL-2, IFN gamma and GM-CSF) were not involved in monocyte activation by cell contact. Anti-CD2 and -CD11a (LFA-1) mAbs partially blocked IL-1 beta production by -25% and -35%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of CD11a/CD18, CD11b/CD18, ICAM-1 (CD54) and -2 (CD102) to human monocyte migration through endothelium and connective tissue fibroblast barriers

Recently we reported that monocyte migration through a barrier of human synovial fibroblasts (HSF) is mediated by the CD11/CD18 (beta2) integrins, and the beta1 integrins VLA-4 and VLA-5 on monocytes. Here we investigated in parallel the role of beta2 integrin family members, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) on monocytes, and the immunoglobulin supergene family members, ICAM-1 and ICAM-2 on HSF and on human umbilical vein endothelial cells (HUVEC), in monocyte migration through HSF and HUVEC monolayers. Using function blocking monoclonal antibodies (mAb), when both VLA-4 and VLA-5 on monocytes were blocked, treatment of monocytes with mAb to both LFA-1 and to Mac-1 completely inhibited monocyte migration across HSF barriers, although blocking either of these beta2 integrins alone had no effect on migration, even when VLA-4 and VLA-5 were blocked. This indicates that optimal beta2 integrin-dependent monocyte migration in synovial connective tissue may be mediated by either LFA-1 or Mac-1. Both ICAM-1 and ICAM-2 were constitutively expressed on HSF and on HUVEC, although ICAM-2 was only minimally expressed on HSF. Based on results of mAb blockade, ICAM-1 appeared to be the major ligand for LFA-1-dependent migration through the HSF. In contrast, both ICAM-1 and ICAM-2 mediated LFA-1-dependent monocyte migration through HUVEC. However, neither ICAM-1 nor ICAM-2 was required for Mac-1 -dependent monocyte migration through either cell barrier, indicating that Mac-1 can utilize ligands distinct from ICAM-1 and ICAM-2 on HSF and on HUVEC during monocyte transmigration.     

HIV-1 envelope gp120 inhibits the monocyte response to chemokines through CD4 signal-dependent chemokine receptor down-regulation

Since HIV-1 infection results in severe immunosuppression, and the envelope protein gp120 has been reported to interact with some of the chemokine receptors on human T lymphocytes, we postulated that gp120 may also affect monocyte activation by a variety of chemokines. This study shows that human peripheral blood monocytes when preincubated with gp120 either purified from laboratory-adapted strains or as recombinant proteins exhibited markedly reduced binding, calcium mobilization, and chemotactic response to chemokines. The gp-120-pretreated monocytes also showed a decreased response to FMLP. This broad inhibition of monocyte activation by chemoattractants required interaction of gp120 with CD4, since the effect of gp120 was only observed in CD4+ monocytes and in HEK 293 cells only if cotransfected with both chemokine receptors and an intact CD4, but not a CD4 lacking its cytoplasmic domain. Anti-CD4 mAbs mimicked the effect of gp120, and both anti-CD4 Ab and gp120 caused internalization of CXCR4 in HEK 293 cells provided they also expressed CD4. Staurosporine blocked the inhibitory effect of gp120 on monocytes, suggesting that cellular signaling was required for gp120 to inhibit the response of CD4+ cells to chemoattractants. Our study demonstrates a broad suppressive effect of gp120 on monocyte activation by chemoattractants through the down-regulation of cell surface receptors. Thus, gp120 may be used by HIV-1 to disarm the monocyte response to inflammatory stimulation.