Role of alanyl aminopeptidase in growth and function of human T cells (review).

Alanyl aminopeptidase (APN, CD13) is highly expressed in human monocytes, and anti-CD13 monoclonal antibodies are well established routine markers in leukaemia typing. Due to activation or malignant transformation other leukocyte subpopulations including human T cells exhibit significant APN-gene and surface expression. The function of leukocyte APN is poorly understood, especially the knowledge of physiological ligands/substrates of the enzyme is limited. Abnormal expression of APN on malignant lymphocytes, the activation-dependent induction of APN expression in peripheral T cells and the strong anti-proliferative effects of aminopeptidase inhibitors lead to the interesting hypothesis of a linkage of APN expression and/or function to leukocyte growth. In support of this hypothesis we detected mutations in the APN-gene of patients suffering from leukaemia or lymphoma. This review outlines evidence for APN contributing to the regulation and realisation of lymphocyte growth and function by modulating the mRNA expression of IL-2, IL-1 receptor antagonist, and TGF-beta1 and increasing the activity of MAP kinase p42/Erk2.     

The Common vaccine adjuvant aluminum hydroxide up-regulates accessory properties of human monocytes via an interleukin-4-dependent mechanism

Aluminum adjuvants are widely used in human vaccines based on their ability to enhance antibody production. However, the mechanisms underlying these effects remain unknown. In the present study we assessed the direct in vitro effect of aluminum hydroxide on human peripheral blood monocytes, specifically with regard to its impact on the phenotype and functional properties of this cell population. Our results revealed significant changes in the accessory properties of monocytes following short-term exposure of cultured cells to aluminum hydroxide. Thus, flow cytometry analyses showed an increase in the expression of major histocompatibility complex (MHC) class II, CD40, CD54, CD58, CD83, and CD86 molecules on the monocytes. In addition, many cells in the cultures containing aluminum hydroxide acquired typical dendritic morphology. Increased synthesis of interleukin-4 (IL-4) mRNA, but not gamma interferon mRNA, was also noted after exposure to aluminum hydroxide. The increase in cell surface expression of MHC class II did not occur in the presence of neutralizing IL-4 antibody or in cultures of highly purified monocytes or CD4-depleted mononuclear cells. Our findings suggest that aluminum hydroxide directly stimulates monocytes to produce proinflammatory cytokines activating T cells. Activated Th2 cells release IL-4, which in turn can induce an increase in the expression of MHC class II molecules on monocytes. The increase in the expression of antigen-presenting and costimulatory molecules leads to enhanced accessory functions of monocytes. These properties of aluminum hydroxide observed in vitro may explain its potent in vivo adjuvant effect.     

Regulation of the expression of aminopeptidase A,aminopeptidase N/CD13 and dipeptidylpeptidase IV/CD26 in renal carcinoma cells and renal tubular epithelial cells by cytokines and cAMP-increasing mediators

Aminopeptidase (AP) A is a transmembrane type II molecule widely distributed in mammalian tissues. Since APA expression may be absent in renal cell carcinoma (RCC), it is possible that there is an altered regulation or other defect of APA upon malignant transformation of proximal tubular cells. However, investigations into the regulation of APA on tumour cells are rare. We report, for the first time, that both transforming growth factor-beta 1 (TGF-β1) and tumour necrosis factor-alpha (TNF-α) down-regulate APA mRNA as well as protein expression in renal tubular epithelial cells and RCC cells in culture. In addition to this, both cytokines decrease dipeptidylpeptidase (DP) IV/CD26 mRNA, but not APN/CD13 mRNA expression. Otherwise, IL-4 and IL-13 increase CD13 as well as CD26 expression, but do not alter APA expression. Interferon-alpha (IFN-α), IFN-β and IFN-γ increase mRNA expression of all the three membrane ectopeptidases, whereas IL-1, IL-6, IL-7, IL-12 and granulocyte-macrophage colony-stimulating factor (GM-CSF) have been found to be without any significant effect. Treatment of cultured cells with cAMP-increasing agents, such as 8-bromo-cAMP or A23187, results in an increase in APA and DPIV/CD26, but no change in APN/CD13 mRNA expression or even a decrease in it. Furthermore, AP inhibitors can influence APA mRNA expression, since bestatin causes an increase in APA expression in a time- and dose-dependent manner, whereas bestatin does not change CD13 or CD26 expression. No difference could be found with respect to the modulation by different mediators between RCC cells and renal epithelial cells, though permanent tumour cell lines such as Caki-1 and Caki-2 may have lost some of the normally expressed peptidases.     

Apoptotic cells selectively suppress the Th1 cytokine interferon gamma in stimulated human peripheral blood mononuclear cells and shift the Th1/Th2 balance towards Th2

Apoptotic cells are readily recognized and engulfed by phagocytes and usually do not induce inflammation or tissue damage. Furthermore, they can actively suppress a pro-inflammatory response in phagocytes: In the presence of apoptotic cells, activated monocytes/macrophages produce more of the anti-inflammatory and immunoregulatory cytokines IL-10 and TGF-beta, but less of the pro-inflammatory cytokines TNFalpha, IL-1beta and IL-12. This immunoregulatory effect is most likely mediated by several receptors on monocytes/macrophages including the thrombospondin receptor (CD36). In addition to the modulation of cytokine secretion, apoptotic cell material inhibited the expression of MHC class II molecules on the surface of monocytes/macrophages. Decreased MHC II expression appeared to be mediated predominantly by increased IL-10 secretion in a para-/autocrine manner. Here, we show that the functional modulation of antigen-presenting monocytes/macrophages by apoptotic cells also influences T cell activation and function. When human peripheral blood mononuclear cells were stimulated with recall antigens in the presence of apoptotic cells, interferon gamma (IFN gamma) secretion was markedly suppressed, whereas secretion of the Th2 cytokine IL-4 was not significantly altered. Hence, apoptotic cells shift the T cell cytokine secretion pattern towards a Th2-like response. This Th2 shift can largely be prevented by neutralizing IL-10, indicating an important role of this cytokine for modulating T cell cytokine secretion patterns.     

Stimulation of the expression and the enzyme activity of aminopeptidase N/CD13 and dipeptidylpeptidase IV/CD26 on human renal cell carcinoma cells and renal tubular epithelial cells by T cell-derived cytokines, such as IL-4 and IL-13.

Aminopeptidase N (APN) and dipeptidylpeptidase IV (DPIV) are transmembrane type II molecules widely distributed in mammalian tissues. In recent years, the interest in cell surface peptidases has increased considerably because, among other things, several reports indicate roles of ectopeptidases in tumour cell metastasis. Investigations into the regulation of APN and DPIV on tumour cells are rare. We report, for the first time, that IL-4 and IL-13 can up-regulate protein expression as well as enzymatic activity of both the peptidases on renal carcinoma cells and renal tubular epithelial cells in culture. The analysis of mRNA by competitive polymerase chain reaction (PCR) confirmed our results with respect to the APN increase at the level of gene expression. IL-1 beta and tumour necrosis factor-alpha (TNF-alpha) augmented the IL-4-induced effect with respect to APN but not to DPIV. A 5-day incubation with interferon-gamma (IFN-gamma) increased protein expression, especially of APN and, to a lesser extent, also of DPIV, whereas no significant increase in enzymatic activity could be observed. Small concentrations of transforming growth factor-beta 1 (TGF-beta 1) inhibit the expression and enzyme activity of DPIV. IL-6, IL-7, IL-10 and granulocyte-macrophage colony-stimulating factor (GM-CSF) have been found to be without any effect on APN and DPIV. For a prospective therapeutic regimen with T cell-derived cytokines it has to be considered that--besides their effect on tumour cell growth--cytokines might affect surface ectopeptidases involved in tumour cell adhesion processes. The inhibition of APN and DPIV could be a new approach to suppression of cancer spread.     

Differential expression of alternative H2-M isoforms in B cells, dendritic cells and macrophages by proinflammatory cytokines

Major histocompatibility (MHC) class II heterodimers bind peptides generated by degradation of endocytosed antigens and display them on the surface of antigen presenting cells (APCs) for recognition by CD4+ T cells. Efficient loading of MHC class II molecules with peptides is catalyzed by the MHC class II-like molecule H2-M. The coordinate regulation of MHC class II and H2-M expression is a prerequisite for efficient MHC class II/peptide assembly in APCs determining both the generation of the T cell repertoire in the thymus and cellular immune responses in the periphery. Here we show that expression of H2-M and MHC class II genes is coordinately and cell type-specific regulated in splenic B cells, splenic dendritic cells (DCs) and peritoneal macrophages (Mphi) in response to proinflammatory and immunoregulatory cytokines, including GM-CSF, IFN-gamma, TGF-beta2, IL-4, IL-10 and viral IL-10. In addition, ratio-RT-PCR expression analysis of the duplicated H2-Mbeta-chain loci demonstrates for the first time that Mbl and Mb2 genes are differentially expressed in individual APC types. Mb2 is preferentially expressed in IL-4, GM-CSF, IL-10, vIL-10 and IFN-gamma stimulated splenic B cells, whereas splenic DCs express both Mb genes at almost equal levels. In contrast, peritoneal Mphi express predominantly Mb2 but stimulation with IFN-gamma induces a switch towards Mb1 expression. These data suggest a common mechanism that regulates coordinate expression of H2-M and MHC class II genes in professional APCs. Differential expression of Mb1 and Mb2, and by consequence alternative H2-M isoforms (Malphabeta1 or Malphabeta2), may influence the nature of the peptide repertoire presented by different APC types.     

IL-4 decreases the expression of the monocyte differentiation marker CD14, paralleled by an increasing accessory potency.

IL-4 has been found to affect the phenotype and a variety of functions of human monocytes and macrophages and has been discussed as a monocyte activating protein along with other cytokines, such as IL-1 and IL-6. In this study we compared the effects of the cytokines IL-1, IL-6, IL-4, and a combination of IL-1 and IL-6 on the expression of the CD14 antigen, the FcIIIg receptor molecule CD16 and the MHC-class II molecules HLA-DR and HLA-DP. These molecules represent characteristic monocyte surface markers. Furthermore, the CD14 molecule has been described as a surface antigen of high in vivo relevance representing an indirect receptor for LPS. We further analyzed the effect of IL-4 on monocytes and macrophages with respect to their accessory function to initiate T-lymphocyte proliferation. Human peripheral blood monocytes strongly express the antigen CD14 and maintain it as a stable surface molecule during their differentiation to macrophages. Flow cytometry analysis of cultured monocytes demonstrated that cells incubated in the presence of IL-4, but not IL-1 and/or IL-6 revealed a reduced expression of the CD14 antigen in a dose- and time-dependent manner. After 3 days IL-4 treated cells were virtually CD14-negative. At the same time the expression of the CD16 antigen (FcRIIIg) was also strongly reduced, whereas the treatment with IL-4 led to an increased expression of MHC class II antigens such as HLA-DR and HLA-DP. The spontaneous low expression of HLA-DQ antigen on monocytes was not affected by any of the cytokines. Functionally, IL-4 treated CD14-negative monocytes exhibited a more than 2-fold higher activity to stimulate an accessory cell-dependent T cell proliferation. This was found in a mitogenic assay and in MLC when compared to monocytes cultured in the absence of IL-4. These observations provide further evidence that IL-4 is a major modulator of monocyte surface antigen expression. Moreover, IL-4 has an enhancer-effect on monocytes as accessory cells and therefore may be of considerable importance as a regulatory factor during monocyte development to accessory cells. Inasmuch as the CD14 molecule functions as a receptor for LPS-binding protein, our results suggest that IL-4 might also play an important regulatory role in processes initiated by bacterial lipopolysaccharides during inflammation and sepsis.     

Effects of 1 alpha,25-dihydroxyvitamin D3 and cytokines on the expression of MHC antigens, complement receptors and other antigens on human blood monocytes and U937 cells: role in cell differentiation, activation and phagocytosis.

The effect of calcitriol/1 alpha,25-dihydroxyvitamin D3, alone and in combination with cytokines, on the expression of various antigens (Ag) on human peripheral blood monocytes and U937 cells was studied by flow cytometry. Both constitutive and interferon-gamma (IFN-gamma), interleukin-4 (IL-4), IL-6 and tumour necrosis factor-alpha (TNF-alpha)-induced human leucocyte antigen (HLA)-DR, HLA-DP and HLA-DQ Ag expression on monocytes was significantly down-regulated by calcitriol, IL-10 and transforming growth factor-beta (TGF-beta). The effects of calcitriol were concentration dependent and reached maximal inhibitory levels after 3-5 days. Modulation of HLA-DR by calcitriol and IFN-gamma at the protein level correlated with the amount of mRNA specific for the HLA-DR alpha-chain, as judged by Northern blot analysis. The basal as well as IL-4, IL-6, IFN-gamma, TNF-alpha and TGF-beta-driven levels of HLA-ABC Ag were significantly diminished by calcitriol. On U937 cells calcitriol markedly induced CD11a and CD11b expression and weakly up-regulated CD11c whereas on monocytes, constitutive CD11a, CD11b and CD11c expression was significantly down-regulated by calcitriol. The expression of CD14 Ag was strongly induced on U937 cells but only modestly on monocytes. Both the basal level of CD71 and IL-4, IFN-gamma or TNF-alpha-driven expression was diminished on calcitriol-treated U937 cells. In addition, calcitriol suppressed the expression of CD71 Ag on monocytes. The ability of monocytes to phagocytize opsonized Escherichia coli was diminished by calcitriol. Our results demonstrate that calcitriol, alone or in combination with cytokines, modulates expression of MHC, CD11b, CD11c, CD14 and CD71 Ag on both monocytes and U937 cells, and impairs the phagocytic property of monocytes.     

Vitamin D differentially regulates B7.1 and B7.2 expression on human peripheral blood monocytes.

The hormonal active form of vitamin D3, 1,25-dihydroxyvitamin D3 (1, 25(OH)2D3), inhibits (through an unknown mechanism) the ability of monocytes/macrophages to induce T-cell activation. For T cells to be optimally activated, recognition of antigen/major histocompatibility complexes (MHC) by the T-cell receptor (TCR) must be accompanied by a second costimulatory signal. Considerable experimental data now suggest that this costimulatory signal is predominantly generated by B7.1 and/or B7.2 molecules, expressed on antigen-presenting cells (APC), when engaged to their counter-receptor, CD28, present on T cells. To determine whether the inhibitory effect of 1,25(OH)2D3 on monocytes/macrophages might involve modulation of the expression of B7.1 and B7.2 molecules, we analysed (by flow cytometry) the influence of 1,25(OH)2D3 and an analogue, KH 1060, on the expression of these two molecules at the surface of resting human peripheral blood monocytes. In parallel, we tested the effect of these two agents on human monocyte expression of cell-surface markers (CD14 and CD4) and antigen-presenting molecules (MHC class I and MHC class II). Our results showed that both 1,25(OH)2D3 and KH 1060 inhibited the basal expression of B7.2 in a dose- and time-dependent manner, without affecting B7.1. Moreover, these two compounds increased CD14 and reduced MHC class II and CD4 expression. Furthermore, the effect of 1,25(OH)2D3 on B7 molecule expression in combination with lipopolysaccharide (LPS) or cytokines, including interleukin-10 (IL-10), interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha), was studied. The 1,25(OH)2D3-induced B7.2 down-regulation was still detectable when monocytes were activated by IL-10, IFN-gamma and TNF-alpha but not with LPS. Moreover, the induction of B7.1 by TNF-alpha was inhibited by addition of 1, 25(OH)2D3. We conclude that the ability of 1,25(OH)2D3 to decrease B7.2 expression on human monocytes might contribute to its inhibitory effect on APC-dependent T-cell activation and to its immunosuppressive properties observed in autoimmune diseases and organ transplantation.     

Mixed Langerhans cell and interstitial/dermal dendritic cell subsets emanating from monocytes in Th2-mediated inflammatory conditions respond differently to proinflammatory stimuli

The skin harbors two dendritic cell (DC) subsets, Langerhans cells (LC) and interstitial/dermal DC (IDDC), which traffic to lymph nodes after inflammation and ultraviolet stress. To demonstrate that monocytes may act as DC precursors for skin DC in postinflammatory recolonization, we generated LC and IDDC from monocytes by using cytokines related to the T helper cell type 2 environment [granulocyte macrophage-colony stimulating factor/transforming growth factor-beta/interleukin-13/tumor necrosis factor alpha (GM-CSF/TGF-beta/IL-13/TNF-alpha)]. In this study, skin DC [LC as Langerin/CD207(+) cells and IDDC as DC-specific intercellular adhesion molecule-grabbing nonintegrin (SIGN)/CD209(+) cells] displayed desynchronized programs along their differentiation, activation/maturation processes in response to stimuli characteristics of a proinflammatory context. First, we demonstrate that monocytes are able to diverge simultaneously along two distinct pathways toward Langerin(+)-LC-type DC and DC-SIGN(+)-IDDC. Second, as TGF-beta is known to antagonize the TNF-alpha-induced maturation process of DC, we showed that IDDC did not mature and acquired a low CC chemokine receptor 7 (CCR7) receptor expression even when stimulated with prolonged incubation with TNF-alpha. It is striking that the LC subset is able to express a high level of CCR7 expression and the maturation marker DC-lysosome-associated membrane protein (DC-LAMP). Third, mixed LC and IDDC subsets secrete IL-10 and IL-12 when stimulated by CD40 ligand and lipopolysaccharide (LPS) but not after prolonged incubation with TNF-alpha. In contrast, LPS was a better activator of IL-10 secretion than the CD40 ligand for GM-CSF/IL-4-generated DC and for GM-CSF/TGF-beta/IL-13-generated LC and IDDC populations. To summarize, the phenotypic/migratory maturation status of LC may be more easily enhanced by stimuli mimicking a proinflammatory situation, and IDDC are more resistant. Moreover, our culture system provided a means of studying cross-talk between two skin DC outside of their respective skin compartment.     

HIV-1 induces IL-10 production in human monocytes via a CD4-independent pathway

In HIV-infected patients, increased levels of IL-10, mainly produced by virally infected monocytes, were reported to be associated with impaired cell-mediated immune responses. In this study, we investigated how HIV-1 induces IL-10 production in human monocytes. We found that CD14(+) monocytes infected by either HIV-1(213) (X4) or HIV-1(BaL) (R5) produced IL-10, IL-6, tumor necrosis factor-alpha (TNF-alpha), and to a lesser extent, IFN-gamma. However, the capacity of HIV-1 to induce these cytokines was not dependent on virus replication since UV-inactivated HIV-1 induced similar levels of these cytokines. In addition, soluble HIV-1 gp160 could induce CD14(+) monocytes to produce IL-10 but at lower levels. Cross-linking CD4 molecules (XLCD4) with anti-CD4 mAbs and goat anti-mouse IgG (GAM) resulted in high levels of IL-6, TNF-alpha and IFN-gamma but no IL-10 production by CD14(+) monocytes. Interestingly, neither anti-CD4 mAbs nor recombinant soluble CD4 (sCD4) receptor could block IL-10 secretion induced by HIV-1(213), HIV-1(BaL) or HIV-1 gp160 in CD14(+) monocytes, whereas anti-CD4 mAb or sCD4 almost completely blocked the secretion of the other cytokines. Furthermore, HIV-1(213) could induce IL-10 mRNA expression in CD14(+) monocytes while XLCD4 by anti-CD4 mAb and GAM failed to do so. As with IL-10 protein levels, HIV-1(213)-induced IL-10 mRNA expression in CD14(+) monocytes could not be inhibited by anti-CD4 mAb or sCD4. Taken together, HIV-1 binding to CD14(+) monocytes can induce CD4-independent IL-10 production at both mRNA and protein levels. This finding suggests that HIV induces the immunosuppressive IL-10 production in monocytes and is not dependent on CD4 molecules and that interference with HIV entry through CD4 molecules may have no impact on counteracting the effects of IL-10 during HIV infection.     

Regulation of human CD4(+) alphabeta T-cell-receptor-positive (TCR(+)) and gammadelta TCR(+) T-cell responses to Mycobacterium tuberculosis by interleukin-10 and transforming growth factor beta

Mycobacterium tuberculosis is the etiologic agent of human tuberculosis and is estimated to infect one-third of the world's population. Control of M. tuberculosis requires T cells and macrophages. T-cell function is modulated by the cytokine environment, which in mycobacterial infection is a balance of proinflammatory (interleukin-1 [IL-1], IL-6, IL-8, IL-12, and tumor necrosis factor alpha) and inhibitory (IL-10 and transforming growth factor beta [TGF-beta]) cytokines. IL-10 and TGF-beta are produced by M. tuberculosis-infected macrophages. The effect of IL-10 and TGF-beta on M. tuberculosis-reactive human CD4(+) and gammadelta T cells, the two major human T-cell subsets activated by M. tuberculosis, was investigated. Both IL-10 and TGF-beta inhibited proliferation and gamma interferon production by CD4(+) and gammadelta T cells. IL-10 was a more potent inhibitor than TGF-beta for both T-cell subsets. Combinations of IL-10 and TGF-beta did not result in additive or synergistic inhibition. IL-10 inhibited gammadelta and CD4(+) T cells directly and inhibited monocyte antigen-presenting cell (APC) function for CD4(+) T cells and, to a lesser extent, for gammadelta T cells. TGF-beta inhibited both CD4(+) and gammadelta T cells directly and had little effect on APC function for gammadelta and CD4(+) T cells. IL-10 down-regulated major histocompatibility complex (MHC) class I, MHC class II, CD40, B7-1, and B7-2 expression on M. tuberculosis-infected monocytes to a greater extent than TGF-beta. Neither cytokine affected the uptake of M. tuberculosis by monocytes. Thus, IL-10 and TGF-beta both inhibited CD4(+) and gammadelta T cells but differed in the mechanism used to inhibit T-cell responses to M. tuberculosis.     

Cultured human monocytes secrete fibronectin in response to activation by proinflammatory cytokines

We studied the effects of the cytokines IL-1alpha, IL-6, tumour necrosis factor-alpha (TNF-alpha), IL-4, IL-10, IL-13 and transforming growth factor-beta (TGF-beta) on fibronectin (FN) production by cultured-human monocytes. IL-1alpha, IL-6 and TNF-alpha all increased FN production, an indicator of monocyte activation. These cytokines increased FN production in a dose-dependent fashion, with a 4-h treatment being sufficient to measure FN production by radioimmunoassay. Conversely, IL-4, IL-10 and IL-13 strongly inhibited cytokine-induced FN production, while TGF-beta only partially inhibited FN production. The combination of suboptimal doses of cytokines (IL-1alpha + IL-6, IL-1alpha + TNF-alpha, IL-6 + TNF-alpha), which could not singly induce substantial amounts of FN, were able to induce FN production by cultured monocytes. Northern blot analysis with a cDNA specific for FN confirmed the expression of FN mRNA in cultured monocytes stimulated with a single cytokine or a combination of cytokines. Our data demonstrate that monocytes may not always require high concentrations of cytokines for activation in vitro, and that the synergistic or additive action of low levels of cytokines on monocyte activation may be sufficient to promote immune or inflammatory reactions. Our data also suggest that certain T cell cytokines may regulate monocyte activation.     

Diminished responses to IL-13 by human monocytes differentiated in vitro: role of the IL-13Ralpha1 chain and STAT6.

The primary IL-13 receptor complex on human monocytes is believed to be a heterodimer comprised of the IL-4R alpha chain and the IL-2R gamma chain (gamma(c))-like molecule, IL-13R alpha1. mRNA levels for IL-13R alpha1, but not IL-4R alpha, were markedly decreased in in vitro monocyte-derived macrophages (MDMac), and with increasing time of monocytes in culture correlated with the loss of IL-13 regulation of lipopolysaccharide-induced TNF-alpha production. Analysis of cell lines Daudi and THP-1 that differentially express gamma(c) and IL-13R alpha1 showed that IL-13 can activate STAT6 in IL-13R alpha1-positive THP-1 cells but not in gamma(c)-positive, IL-13R alpha1-negative Daudi cells. IL-13 activation of STAT6 was reduced in MDMac which was associated with diminished IL-13-induced expression of CD23 and MHC class II. However, with reduced IL-13R alpha1 expression and low nuclear STAT6 activity, some IL-13-induced responses were unaltered in magnitude in MDMac. In the absence of functional IL-13R alpha1 and gamma(c), IL-13 must signal through an alternative receptor complex on MDMac. Experiments with a blocking antibody to IL-4R alpha showed that this chain remains an essential component of the IL-13 receptor complex on MDMac.     

The Fc receptor, FcRI, and other activation molecules on human mononuclear phagocytes after treatment with interferon-gamma.

Human monocytes and the myeloid cell lines U937 and HL60 have been tested with monoclonal antibodies (MoAbs) reactive with 22 different cell surface molecules before and after treatment with interferon-gamma (IFN-gamma). An increase in the expression of the high affinity Fc receptor, FcRI, and the receptor for interleukin 2, IL-2R, were the most consistent alterations which were observed. In addition, expression of the gp55 molecule recognized by CD14 MoAbs was decreased on monocytes. Of the MHC Class II molecules, there was little expression by the myeloid cell lines and no enhancement after IFN-gamma treatment. In contrast monocytes expressed all three MHC Class II subloci with DR much greater than DQ and DP. However there was much variation in IFN-gamma-mediated increase in expression of the individual subregions. In monocytes, the alteration in expression of FcRI, IL-2R, gp55 and MHC Class II molecules took place in a co-ordinate fashion and reached a plateau only after 48 h. In U937 cells, activation proceeded more rapidly and was at maximum levels between 12-16 h. This increase in FcRI appears to be a hallmark of IFN-gamma activation for mononuclear phagocytes (Mph) as the other alterations are either not found on all types of Mph (gp55, MHC Class II) or are induced by other cytokines on Mph and on other cells (IL-2R, MHC Class II). Conversely, other cytokines do not induce FcRI on Mph. These results also suggest that the cell membrane phenotypic changes induced in Mph by IFN-gamma may not be extensive and that FcRI must play a specific role in the IFN-gamma-activated Mph.     

Apoptotic cells selectively suppress the Th1 cytokine interferon γ in stimulated human peripheral blood mononuclear cells and shift the Th1/Th2 balance towards Th2

Apoptotic cells are readily recognized and engulfed by phagocytes and usually do not induce inflammation or tissue damage. Furthermore, they can actively suppress a pro-inflammatory response in phagocytes: In the presence of apoptotic cells, activated monocytes/macrophages produce more of the anti-inflammatory and immunoregulatory cytokines IL-10 and TGF-β, but less of the pro-inflammatory cytokines TNFα, IL-1β and IL-12. This immunoregulatory effect is most likely mediated by several receptors on monocytes/macrophages including the thrombospondin receptor (CD36). In addition to the modulation of cytokine secretion, apoptotic cell material inhibited the expression of MHC class II molecules on the surface of monocytes/macrophages. Decreased MHC II expression appeared to be mediated predominantly by increased IL-10 secretion in a para-/autocrine manner. Here, we show that the functional modulation of antigen-presenting monocytes/macrophages by apoptotic cells also influences T cell activation and function. When human peripheral blood mononuclear cells were stimulated with recall antigens in the presence of apoptotic cells, interferonγ (IFNγ) secretion was markedly suppressed, whereas secretion of the Th2 cytokine IL-4 was not significantly altered. Hence, apoptotic cells shift the T cell cytokine secretion pattern towards a Th2-like response. This Th2 shift can largely be prevented by neutralizing IL-10, indicating an important role of this cytokine for modulating T cell cytokine secretion patterns.