Larry M. Wahl

Mechanism of suppression of cell-mediated immunity by measles virus

The mechanisms underlying the profound suppression of cell-mediated immunity (CMI) accompanying measles are unclear. Interleukin-12 (IL-12), derived principally from monocytes and macrophages, is critical for the generation of CMI. Measles virus (MV) infection of primary human monocytes specifically down-regulated IL-12 production. Cross-linking of CD46, a complement regulatory protein that is the cellular receptor for MV, with antibody or with the complement activation product C3b similarly inhibited monocyte IL-12 production, providing a plausible mechanism for MV-induced immunosuppression. CD46 provides a regulatory link between the complement system and cellular immune responses.

Identification of complement factor 5 as a susceptibility locus for experimental allergic asthma

The prevalence and severity of allergic asthma continue to rise, lending urgency to the search for environmental triggers and genetic substrates. Using microarray analysis of pulmonary gene expression and single nucleotide polymorphism–based genotyping, combined with quantitative trait locus analysis, we identified the gene encoding complement factor 5 (C5) as a susceptibility locus for allergen-induced airway hyperresponsiveness in a murine model of asthma. A deletion in the coding sequence of C5 leads to C5-deficiency and susceptibility. Interleukin 12 (IL-12) is able to prevent or reverse experimental allergic asthma. Blockade of the C5a receptor rendered human monocytes unable to produce IL-12, mimicking blunted IL-12 production by macrophages from C5-deficient mice and providing a mechanism for the regulation of susceptibility to asthma by C5. The role of complement in modulating susceptibility to asthma highlights the importance of immunoregulatory events at the interface of innate and adaptive immunity in disease pathogenesis.

Genomic-scale measurement of mRNA turnover and the mechanisms of action of the anti-cancer drug flavopiridol

BackgroundFlavopiridol, a flavonoid currently in cancer clinical trials, inhibits cyclin-dependent kinases (CDKs) by competitively blocking their ATP-binding pocket. However, the mechanism of action of flavopiridol as an anti-cancer agent has not been fully elucidated.ResultsUsing DNA microarrays, we found that flavopiridol inhibited gene expression broadly, in contrast to two other CDK inhibitors, roscovitine and 9-nitropaullone. The gene expression profile of flavopiridol closely resembled the profiles of two transcription inhibitors, actinomycin D and 5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole (DRB), suggesting that flavopiridol inhibits transcription globally. We were therefore able to use flavopiridol to measure mRNA turnover rates comprehensively and we found that different functional classes of genes had distinct distributions of mRNA turnover rates. In particular, genes encoding apoptosis regulators frequently had very short half-lives, as did several genes encoding key cell-cycle regulators. Strikingly, genes that were transcriptionally inducible were disproportionately represented in the class of genes with rapid mRNA turnover.ConclusionsThe present genomic-scale measurement of mRNA turnover uncovered a regulatory logic that links gene function with mRNA half-life. The observation that transcriptionally inducible genes often have short mRNA half-lives demonstrates that cells have a coordinated strategy to rapidly modulate the mRNA levels of these genes. In addition, the present results suggest that flavopiridol may be more effective against types of cancer that are highly dependent on genes with unstable mRNAs.

Dysregulation of LPS-Induced Toll-Like Receptor 4-MyD88 Complex Formation and IL-1 Receptor-Associated Kinase 1 Activation in Endotoxin-Tolerant Cells

Prior exposure to LPS induces a transient state of cell refractoriness to subsequent LPS restimulation, known as endotoxin tolerance. Induction of LPS tolerance has been reported to correlate with decreased cell surface expression of the LPS receptor complex, Toll-like receptor 4 (TLR4)/MD-2. However, other results have underscored the existence of mechanisms of LPS tolerance that operate downstream of TLR4/MD-2. In the present study we sought to delineate further the molecular basis of LPS tolerance by examining the TLR4 signaling pathway in endotoxin-tolerant cells. Pretreatment of human monocytes with LPS decreased LPS-mediated NF-κB activation, p38 mitogen-activated protein kinase phosphorylation, and TNF-α gene expression, documenting the induction of endotoxin tolerance. FACS and Western blot analyses of LPS-tolerant monocytes showed increased TLR2 expression, whereas TLR4 expression levels were not affected. Comparable levels of mRNA and protein for myeloid differentiation factor 88 (MyD88), IL-1R-associated kinase 1 (IRAK-1), and TNFR-associated factor-6 were found in normal and LPS-tolerant monocytes, while MD-2 mRNA expression was slightly increased in LPS-tolerant cells. LPS induced the association of MyD88 with TLR4 and increased IRAK-1 activity in medium-pretreated cells. In LPS-tolerant monocytes, however, MyD88 failed to be recruited to TLR4, and IRAK-1 was not activated in response to LPS stimulation. Moreover, endotoxin-tolerant CHO cells that overexpress human TLR4 and MD-2 also showed decreased IRAK-1 kinase activity in response to LPS despite the failure of LPS to inhibit cell surface expression of transfected TLR4 and MD-2 proteins. Thus, decreased TLR4-MyD88 complex formation with subsequent impairment of IRAK-1 activity may underlie the LPS-tolerant phenotype.

Divergent effects of hormone therapy on serum markers of inflammation in postmenopausal women with coronary artery disease on appropriate medical management.

OBJECTIVES The goal of our study was to determine whether hormone therapy alters markers of inflammation in postmenopausal women with chronic stable coronary artery disease (CAD) on appropriate medical management. BACKGROUND Hormone therapy reduces some markers of inflammation associated with atherosclerosis risk (cell adhesion molecules) but increases levels of another marker of inflammation--C-reactive protein-in healthy postmenopausal women. METHODS Ten women (average age 66 years; range 59 to 76 years) with CAD on medical management (including aspirin [9], statin lipid-lowering therapy [7], angiotensin-converting enzyme inhibitors [3]) were randomly assigned to conjugated equine estrogens 0.625 mg (combined with medroxyprogesterone acetate 2.5 mg daily in five women with uterus intact) or placebo(s) daily for one month with crossover to the alternate therapy after one month off of hormone treatment in a double-blind study. At the end of each treatment phase, the following markers of inflammation were measured in serum: interleukin-6, C-reactive protein, E-selectin, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 and matrix metalloproteinase-9. RESULTS Hormone therapy significantly lowered serum levels of cell adhesion molecules E-selectin (46.9+/-18.3 vs. 56.3+/-20.6 ng/mL, p = 0.006), intercellular adhesion molecule-1 (282+/-74 vs. 304+/-78 ng/mL, p = 0.013) and vascular cell adhesion molecule-1 (605+/-218 vs. 657+/-214 ng/mL, p = 0.01) but increased levels of matrix metalloproteinase-9 (648+/-349 vs. 501+/-285 ng/mL, p = 0.02). Interleukin-6 (4.33+/-4.78 vs. 3.04+/-1.47 pg/mL, p = 0.283) and C-reactive protein (0.88+/-1.13 vs. 0.61+/-0.50 mg/dL, p = 0.358) were not significantly elevated on hormone therapy compared with placebo values. CONCLUSIONS Hormone therapy has divergent effects on serum markers of inflammation in women with CAD. Reduction in levels of cell adhesion molecules may reduce attachment of white blood cells to the vessel wall, but increases in matrix metalloproteinase-9 within the vessel wall could digest and weaken fibrous caps of vulnerable plaques, thus provoking thrombosis.

Isolation of human mononuclear cell subsets by counterflow centrifugal elutriation (CCE). I. Characterization of B-lymphocyte-, T-lymphocyte-, and monocyte-enriched fractions by flow cytometric analysis.

Rapid separation of large numbers of human peripheral blood mononuclear cells into fractions enriched for B lymphocytes, T lymphocytes, or monocytes was accomplished by counterflow centrifugal elutriation (CCE). The first fraction contained 98% of the platelets. Ten additional fractions containing subpopulations of mononuclear cells were collected by sequential increases in the flow rate while maintaining a constant centrifuge speed. Analysis of the fractions using monoclonal antibodies revealed that fraction 2, which was free of esterase-positive monocytes, was highly enriched for B cells. T lymphocytes (OKT3+) were the predominant cell type found in fraction 4. No enrichment for T-lymphocyte-helper (OKT4+) or -suppressor (OKT8+) subpopulations was observed in the lymphocyte containing fractions. Three fractions (7-9), highly enriched for esterase-positive cells, were predominantly OKM1+ monocytes with no evidence of selective separation of monocyte subpopulations. Thus, cell fractions enriched for B cells, T cells, and monocytes could be obtained, by utilizing CCE, in large enough quantities to enable analysis of their functional properties. Of particular interest was the ability to separate small, resting B lymphocytes from monocytes.

Cerebrospinal fluid levels of MMP-2, 7, and 9 are elevated in association with human immunodeficiency virus dementia.

Pathological evidence suggests that alterations of the blood–brain barrier (BBB) may occur in association with human immunodeficiency virus (HIV) dementia (HIVD). Increased BBB permeability could contribute to the development of dementia by facilitating the entry of activated and infected monocytes, as well as potentially toxic serum proteins, into the central nervous system. One mechanism by which BBB permeability may be altered is through increased activity of select matrix metalloproteinases (MMPs). In the present study, we examined the possibility that MMPs that target critical BBB proteins, including laminin, entactin, and collagen type IV, are elevated in the cerebrospinal fluid (CSF) of patients with HIVD. We also examined the possibility that such MMPs could be produced by brain‐derived cells, and that MMP production by these cells might be increased by tumor necrosis factor‐α, an inflammatory cytokine that is produced by HIV‐infected monocytes/microglia and is elevated in HIVD. By using western blot and enzyme‐linked immunosorbent assay, we observed that CSF levels of pro–MMP‐2 and pro–MMP‐7 were increased in association with HIVD. In addition, through the use of gelatin substrate zymography, a sensitive functional assay for MMP‐2 and MMP‐9, we observed that MMP‐2 or pro–MMP‐9 activity was more frequently detectable in the CSF of individuals with HIV dementia (9/16) than in the CSF from either nondemented seropositive (2/11) or seronegative (0/11) controls. Although the presence of MMPs in the serum could contribute to elevated levels in the CSF, we also show that brain‐derived cells release MMP‐2, 7, and 9, and that such release is increased after their stimulation with tumor necrosis factor‐α. Together, these results suggest that elevated CSF levels of select MMPs may reflect immune activation within the central nervous system. They also suggest that further studies may be warranted to determine whether these proteins may play a role in the development of symptomatic neurological disease.

TGF-β Regulates Production of Growth Factors and TGF-β by Human Peripheral Blood Monocytes

Transforming growth factor beta 1 (TGF-beta 1) and its closely related homologue, TGF-beta 2, rapidly induce growth factor gene expression by freshly isolated human peripheral blood monocytes. Within 3 h of exposure to TGF-beta, mRNA species specific for interleukin-1 (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha), platelet-derived growth factor (PDGF), and basic fibroblast growth factor (bFGF) were observed. By 14-18 h, cytokine bioactivity and protein were detected in the culture supernatants. Furthermore, not only TGF-beta 1, but also TGF-beta 2 mRNA are expressed constitutively in unstimulated monocytes. However, in response to exogenous TGF-beta (beta 1 or beta 2), only TGF-beta 1 gene expression is upregulated, and the expression of TGF-beta 2 mRNA is unchanged. This selective autoinduction of TGF-beta 1 appears to be controlled at both transcriptional and post-transcriptional levels. These paracrine and autocrine activities of TGF-beta suggest potential mechanisms through which an inflammatory response can be initiated and amplified. In addition, the TGF-beta enhancement of growth factor generation may promote fibrosis and angiogenesis relevant to physiological tissue repair as well as pathological fibrotic sequelae.

Identification of Human Mononuclear Leukocytes Bearing Receptors for Somatostatin and Glucagon

Mononuclear leukocytes (MNL) were isolated from human blood by Ficoll-Hypaque. These cells were further separated into lymphocyte (L) and monocyte (M) enriched fractions. L contained 99% lymphocytes and M contained 74% monocytes, a threefold enrichment over MNL. Specific binding of somatostatin, glucagon, and insulin was measured in the three fractions. Binding of all three hormones in the M fraction was increased by a factor of 3 compared with MNL and was linear with cell number. Binding of glucagon and insulin to the L fraction was very low while, in contrast, somatostatin binding was substantial and linear with lymphocyte number. Autoradiography confirmed the binding of glucagon to monocytes and of somatostatin to both monocytes and lymphocytes. Somatostatin is the first of the peptide hormones shown to bind to both types of circulating mononuclear cells, perhaps complicating quantification of somatostatin binding in disease states in which differential alteration of binding to lymphocytes or monocytes might occur.

Potent suppression of IL-12 production from monocytes and dendritic cells during endotoxin tolerance.

Endotoxin tolerance, the down‐regulation of a subset of endotoxin‐driven responses after an initial exposure to endotoxin, may provide protection from the uncontrolled immunological activation of acute endotoxic shock. Recent data suggest, however, that the inhibition of monocyte/macrophage function associated with endotoxin tolerance can lead to an inability to respond appropriately to secondary infections in survivors of endotoxic shock. IL‐12 production by antigen‐presenting cells is central to the orchestration of both innate and acquired cell‐mediated immune responses to many pathogens. IL‐12 has also been shown to play an important role in pathological responses to endotoxin. We therefore examined the regulation of IL‐12 during endotoxin tolerance. Priming doses of lipopolysaccharide ablate the IL‐12 productive capacity of primary human monocytes. This suppression of IL‐12 production is primarily transcriptional. Unlike the down‐regulation of TNF‐α under such conditions, the mechanism of IL‐12 suppression during endotoxin tolerance is not dependent upon IL‐10 or transforming growth factor‐β, nor is IL‐12 production rescued by IFN‐γ or granulocyte‐macrophage colony‐stimulating factor. Of note, human dendritic cells also undergo endotoxin tolerance, with potent down‐regulation of IL‐12 production. Endotoxin tolerance‐related suppression of IL‐12 production provides a likely mechanism for the anergy seen during the immunological paralysis which follows septic shock.