TLR2 mediates neuroinflammation and neuronal damage

Innate immunity relies on pattern recognition receptors to detect the presence of infectious pathogens. In the case of Gram-positive bacteria, binding of bacterial lipopeptides to TLR2 is currently regarded as an important mechanism. In the present study, we used the synthetic bacterial lipopeptide Pam3CysSK4, a selective TLR2 agonist, to induce meningeal inflammation in rodents. In a 6-h rat model, intrathecal application of Pam3CysSK4 caused influx of leukocytes into the cerebrospinal fluid (CSF) and induced a marked increase of regional cerebral blood flow and intracranial pressure. In wild-type mice, we observed CSF pleocytosis and an increased number of apoptotic neurons in the dentate gyrus 24 h after intrathecal challenge. Inflammation and associated neuronal loss were absent in TLR2 knockout mice. In purified neurons, cytotoxicity of Pam3CysSK4 itself was not observed. Exposure of microglia to Pam3CysSK4 induced neurotoxic properties in the supernatant of wild-type, but not TLR2-deficient microglia. We conclude that TLR2-mediated signaling is sufficient to induce the host-dependent key features of acute bacterial meningitis. Therefore, synthetic lipopeptides are a highly specific tool to study mechanisms of TLR2-driven neurodegeneration in vivo.     

Immune complexes bind to cultured rat glomerular mesangial cells to stimulate superoxide release. Evidence for an Fc receptor

Contractile mesangial cells (MC) possess a number of macrophage-like characteristics, including oxygen radical generation. We suggest that under certain conditions MC may serve as immune effector cells in glomerulonephritis. Immune complex (IC) deposits are a hallmark of glomerulonephritis. Because IC elicit oxygen radicals from other cell types and because oxygen radicals can induce glomerular injury, we measured release of O2- by cultured rat MC in response to IC and, in separate experiments, the binding of IC to MC. Soluble and insoluble IC markedly stimulated dose- and time-dependent, saturable O2- release. Specific antibody (Ab) alone or mixtures of nonimmune Ab and antigen had no significant effect. IC-induced O2- release was not affected by cytochalasin B, an inhibitor of phagocytosis. Binding studies with radioiodinated IC demonstrated specific binding with an affinity of 1.56 X 10(6) M-1 and 1.02 X 10(5) receptors per cell. Both binding and O2- release required the Fc region of Ab. IC formed with F(ab')2 fragments did not bind specifically to or stimulate O2- release by MC. Cultured cells from rats depleted of bone marrow-derived phagocytes by irradiation produced amounts of O2- similar to cells from normal rats. These results provide evidence that IC affect the biology of the contractile glomerular MC in a manner that is dependent on the Fc region of Ab and suggest that MC structure and function may be altered at sites of injury.     

HMGB1 protein promotes glomerular mesangial matrix deposition via TLR2 in lupus nephritis

Lupus nephritis (LN) is the most common complication of systemic lupus erythematosus. Patients with LN mostly die of sclerosing glomerulonephritis and renal failure. The inhibition of glomerular mesangial matrix deposition is an efficient method to restrict the progress of renal injury. By recognizing and binding extracellular and intracellular ligands, Toll-like receptor 2 (TLR2) contributes to the pathogenesis of most immune diseases. However, the relationship between TLR2 and LN is still unknown. Our previous studies confirmed that high-mobility group box 1 (HMGB1), an important ligand of TLR2, promotes the progression of LN by inducing the proliferation of glomerular mesangial cells. However, whether or not HMGB1 participates in the pathogenesis of glomerular mesangial matrix deposition in LN remains unknown. In this study, we observed the upregulated expression of TLR2 in the glomeruli of LN patients and MRL/lpr mice. The inhibition of either TLR2 or HMGB1 inhibited the release of fibronectin and the activation of the MyD88/NF-κB pathway in mesangial cells cultured with LN plasma. In addition, both TLR2- and HMGB1-deficient mice showed reduced 24 hr urine protein levels and improved glomerular histological changes and sclerosis levels. These results indicate that TLR2 regulates glomerular mesangial matrix deposition in LN through the activation of the MyD88/NF-κB pathway by binding to HMGB1.     

β2-Adrenergic agonists bias TLR-2 and NOD2 activated dendritic cells towards inducing an IL-17 immune response

This study tested the hypothesis that activation of β2-adrenoceptors on DCs influences NOD2 signaling along with its cross-talk with Toll-like receptor-2 resulting in altered Th cell priming ability. Th17 cells are a newly discovered lineage of CD4(+) T cells involved in defense against extracellular bacteria and also implicated in autoimmune disorders. Initiation and polarization of the adaptive immune response is controlled by innate immune recognition mediated by DCs. Previous studies demonstrated that adrenergic receptors modulate cytokine production by DCs and affect their Th cell priming ability. We show that the β2-adrenoceptor agonist salbutamol enhanced IL-6 production in murine bone marrow-derived DCs stimulated with the nucleotide-binding oligomerization domain 2 ligand muramyl dipeptide. However, when the Toll-like receptor-2 ligand Pam3CysSK4 was added, salbutamol inhibited IL-12 but did not alter IL-6 and IL-23 expression. Gene expression analysis showed that salbutamol inhibited the p40 subunit as well as IL-12p35, while IL-23p19 and IL-6 were stimulated. Therefore, β2-adrenoceptors modulated cytokine production resulting in a Th17 cell priming cytokine pattern. Indeed, when antigen-pulsed DCs stimulated by muramyl dipeptide or Pam3CysSK4+muramyl dipeptide in the presence of salbutamol were used for in vivo immunization, the resulting Th17/Th1 cell ratio was increased as evaluated by IL-17 and IFN-γ production. In addition, intradermal injection of norepinephrine along with Pam3CysSK4+muramyl dipeptide increased the Th17 response to an immunogenic protein and this effect was reversed by a β2-adrenoceptor antagonist. Thus, β2-adrenoceptors may be involved in the regulation of defense against extracellular bacteria and the pathogenesis of inflammatory diseases.     

Protective roles of mast cells against enterobacterial infection are mediated by Toll-like receptor 4

Toll-like receptors (TLRs) are mammalian homologues of the Drosophila Toll receptors and are thought to have roles in innate recognition of bacteria. We demonstrated that TLR 2, 4, 6, and 8 but not TLR5 were expressed on mouse bone marrow-derived mast cells (BMMCs). Using BMMCs from the genetically TLR4-mutated strain C3H/HeJ, we demonstrated that functional TLR4 was required for a full responsiveness of BMMCs to produce inflammatory cytokines (IL-1beta, TNF-alpha, IL-6, and IL-13) by LPS stimulation. TLR4-mediated stimulation of mast cells by LPS was followed by activation of NF-kappaB but not by stress-activated protein kinase/c-Jun NH2-terminal kinase signaling. In addition, in the cecal ligation and puncture-induced acute septic peritonitis model, we demonstrated that genetically mast cell-deficient W/W(v) mice that were reconstituted with TLR4-mutated BMMCs had significantly higher mortality than W/W(v) mice reconstituted with TLR4-intact BMMCs. Higher mortality of TLR4-mutated BMMC-reconstituted W/W(v) mice was well correlated with defective neutrophil recruitment and production of proinflammatory cytokines in the peritoneal cavity. Taken together, these observations provide definitive evidence that mast cells play important roles in exerting the innate immunity by releasing inflammatory cytokines and recruitment of neutrophils after recognition of enterobacteria through TLR4 on mast cells.     

IFN-gamma production by intrinsic renal cells and bone marrow-derived cells is required for full expression of crescentic glomerulonephritis in mice

The contribution of IFN-gamma from bone marrow (BM) and non-BM-derived cells to glomerular and cutaneous delayed-type hypersensitivity (DTH) was studied in mice. Chimeric IFN-gamma mice (IFN-gamma(+/+) BM chimera), in which IFN-gamma production was restricted to BM-derived cells, were created by transplanting normal C57BL/6 (wild-type (WT)) BM into irradiated IFN-gamma-deficient mice. BM IFN-gamma-deficient chimeric mice (IFN-gamma(-/-) BM chimera) were created by transplanting WT mice with IFN-gamma-deficient BM. WT and sham chimeric mice (WT mice transplanted with WT BM) developed crescentic glomerulonephritis (GN) with features of DTH (including glomerular T cell and macrophage infiltration) in response to an Ag planted in their glomeruli and skin DTH following subdermal Ag challenge. IFN-gamma-deficient mice showed significant protection from crescentic GN and reduced cutaneous DTH. IFN-gamma(+/+) BM chimeric and IFN-gamma(-/-) BM chimeric mice showed similar attenuation of crescentic GN as IFN-gamma-deficient mice, whereas cutaneous DTH was reduced only in IFN-gamma(-/-) BM chimeras. In crescentic GN, IFN-gamma was expressed by tubular cells and occasional glomerular cells and was colocalized with infiltrating CD8(+) T cells, but not with CD4(+) T cells or macrophages. Renal MHC class II expression was reduced in IFN-gamma(+/+) BM chimeric mice and was more severely reduced in IFN-gamma-deficient mice and IFN-gamma(-/-) BM chimeric mice. These studies show that IFN-gamma expression by both BM-derived cells and intrinsic renal cells is required for the development of crescentic GN, but IFN-gamma production by resident cells is not essential for the development of cutaneous DTH.     

FcR-bearing myeloid cells are responsible for triggering murine lupus nephritis

Lupus glomerulonephritis is initiated by deposition of IgG-containing immune complexes in renal glomeruli. FcR engagement by immune complexes (IC) is crucial to disease development as uncoupling this pathway in FcRgamma(-/-) abrogates inflammatory responses in (NZB x NZW)F1 mice. To define the roles of FcR-bearing hemopoietic cells and of kidney resident mesangial cells in pathogenesis, (NZB x NZW)F1 bone marrow chimeras were generated. Nephritis developed in (NZB x NZW)F1 mice expressing activating FcRs in hemopoietic cells. Conversely, recipients of FcRgamma(-/-) bone marrow were protected from disease development despite persistent expression of FcRgamma in mesangial cell populations. Thus, activating FcRs on circulating hemopoietic cells, rather than on mesangial cells, are required for IC-mediated pathogenesis in (NZB x NZW)F1. Transgenic FcRgamma(-/-) mice expressing FcRgamma limited to the CD11b+ monocyte/macrophage compartment developed glomerulonephritis in the anti-glomerular basement disease model, whereas nontransgenic FcRgamma(-/-) mice were completely protected. Thus, direct activation of circulating FcR-bearing myeloid cells, including monocytes/macrophages, by glomerular IC deposits is sufficient to initiate inflammatory responses.     

The long pentraxin PTX3 is synthesized in IgA glomerulonephritis and activates mesangial cells

The long pentraxin PTX3 has been recently involved in amplification of the inflammatory reactions and regulation of innate immunity. In the present study we evaluated the expression and role of PTX3 in glomerular inflammation. PTX3 expression was investigated in the IgA, type I membranoproliferative, and diffuse proliferative lupus glomerulonephritis, which are characterized by inflammatory and proliferative lesions mainly driven by resident mesangial cells, and in the membranous glomerulonephritis and the focal segmental glomerular sclerosis, where signs of glomerular inflammation are usually absent. We found an intense staining for PTX3 in the expanded mesangial areas of renal biopsies obtained from patients with IgA glomerulonephritis. The pattern of staining was on glomerular mesangial and endothelial cells. Scattered PTX3-positive cells were also detected in glomeruli of type I membranoproliferative glomerulonephritis. The concomitant expression of CD14 suggests an inflammatory origin of these cells. Normal renal tissue and biopsies from patients with the other glomerular nephropathies studied were mainly negative for PTX3 expression in glomeruli. However, PTX3-positive cells were detected in the interstitium of nephropathies showing inflammatory interstitial injury. In vitro, cultured human mesangial cells synthesized PTX3 when stimulated with TNF-alpha and IgA and exhibited specific binding for recombinant PTX3. Moreover, stimulation with exogenous PTX3 promoted mesangial cell contraction and synthesis of the proinflammatory lipid mediator platelet-activating factor. In conclusion, we provide the first evidence that mesangial cells may both produce and be a target for PTX3. The detection of this long pentraxin in the renal tissue of patients with glomerulonephritis suggests its potential role in the modulation of glomerular and tubular injury.     

Dendritic cells in glomerulonephritis

Renal biopsies (n = 45) from patients with various forms of glomerulonephritis (GN), comprising mesangial IgA-GN (n = 25), focal glomerular sclerosis (n = 13) and acute GN (n = 7), were examined by double staining immunocytochemistry (APAAP, streptavidinperoxidase) using unconjugated monoclonal antibodies (Ab) against (i) the CD1b antigen expressed on dendritic cells (DCs), (ii) the invariant chain (Ii), and (iii) biotin-conjugated Ab against HLA-DR. In normal control kidneys (n = 7) without interstitial inflammation, CD1b-positive DCs were not detected. Glomerular endothelial cells and a few cells in mesangial areas showed double staining with the Ab against HLA-DR in Ii. In GN without active interstitial inflammation (n = 9), CD1b-positive DCs were not found. In biopsies with interstitial inflammation (n = 36) CDlb-positive DCs were found interspersed among other inflammatory cells. In seven of the biopsies showing IgA-GN DCs were seen in the vicinity of those glomeruli that exhibited either crescents or glomerular sclerosis with splitting of Bowman’s capsule. In proximal tubular epithelial cells de novo expression of HLA-DR/Ii-chain was only seen when DCs were present. We conclude that in different forms of GN: (i) CDlb-positive DCs play an important role in the development of interstitial inflammation, and (ii) their presence may be related to the de novo coexpression of HLA-DR/Ii in tubular epithelial cells, possibly mediated through the production of interferon γ and other cytokines. Supported by the Deutsche Forschungsgemeinschaft (Wa 698/2-1)     

Inhibition of autoimmune diabetes by TLR2 tolerance

We have reported that apoptotic β cells undergoing secondary necrosis, called "late apoptotic (LA) β cells," stimulated APCs and induced diabetogenic T cell priming through TLR2, which might be one of the initial events in autoimmune diabetes. Indeed, diabetogenic T cell priming and the development of autoimmune diabetes were significantly inhibited in TLR2-null NOD mice, suggesting the possibility that TLR2 blockade could be used to inhibit autoimmune diabetes. Because prolonged TLR stimulation can induce TLR tolerance, we investigated whether repeated TLR2 administration affects responses to LA β cells and inhibits autoimmune diabetes in NOD mice by inducing TLR2 tolerance. Treatment of primary peritoneal macrophages with a TLR2 agonist, Pam3CSK(4), suppressed cytokine release in response to LA insulinoma cells or further TLR2 stimulation. The expression of signal transducer IRAK-1 and -4 proteins was decreased by repeated TLR2 stimulation, whereas expression of IRAK-M, an inhibitory signal transducer, was enhanced. Chronic Pam3CSK(4) administration inhibited the development of diabetes in NOD mice. Diabetogenic T cell priming by dendritic cells and upregulation of costimulatory molecules on dendritic cells by in vitro stimulation were attenuated by Pam3CSK(4) administration in vivo. Pam3CSK(4) inhibited diabetes after adoptive transfer of diabetogenic T cells or recurrence of diabetes after islet transplantation by pre-existing sensitized T cells. These results showed that TLR2 tolerance can be achieved by prolonged treatment with TLR2 agonists, which could inhibit priming of naive T cells, as well as the activity of sensitized T cells. TLR2 modulation could be used as a novel therapeutic modality against autoimmune diabetes.     

TLR2 hypersensitivity of astrocytes as functional consequence of previous inflammatory episodes

Precedent inflammatory episodes may drastically modify the function and reactivity of cells. We investigated whether priming of astrocytes by microglia-derived cytokines alters their subsequent reaction to pathogen-associated danger signals not recognized in the quiescent state. Resting primary murine astrocytes expressed little TLR2, and neither the TLR2/6 ligand fibroblast-stimulating lipopeptide-1 (FSL1) nor the TLR1/2 ligand Pam(3)CysSK(4) (P3C) triggered NF-κB translocation or IL-6 release. We made use of single-cell detection of NF-κB translocation as easily detectable and sharply regulated upstream indicator of an inflammatory response or of c-Jun phosphorylation to measure restimulation events in astrocytes under varying conditions. Cells prestimulated with IL-1β, with a TLR3 ligand, with a complete cytokine mix consisting of TNF-α, IL-1β, and IFN-γ, or with media conditioned by activated microglia responded strongly to FSL1 or P3C stimulation, whereas the sensitivity of the NF-κB response to other pattern recognition receptors was unchanged. This sensitization to TLR2 ligands was associated with an initial upregulation of TLR2, displayed a "memory" window of several days, and was largely independent of the length of prestimulation. The altered signaling led to altered function, as FSL1 or P3C triggered the release of IL-6, CCL-20, and CXCL-2 in primed cells, but not in resting astrocytes. These data confirmed the hypothesis that astrocytes exposed to activated microglia assume a different functional phenotype involving longer term TLR2 responsiveness, even after the initial stimulation by inflammatory mediators has ended.     

The synthetic analogue of mycoplasmal lipoprotein FSL-1 induces dendritic cell maturation through Toll-like receptor 2

Granulocyte-macrophage colony-stimulating factor-differentiated bone marrow-derived dendritic cells were stimulated with the synthetic lipopeptide S-(2,3-bispalmitoyloxypropyl)-CGDPKHSPKSF (FSL-1) or the Escherichia coli lipopolysaccharide. FSL-1 induced the production of TNF-alpha and IL-12 by C57BL/6-derived bone marrow-derived dendritic cells but not by bone marrow-derived dendritic cells from Toll-like receptor 2-deficient (TLR2(-/-)) mice. Lipopolysaccharide induced the production of TNF-alpha and IL-12 by bone marrow-derived dendritic cells derived from either type of mice. FSL-1 did not induce production of IL-10 by bone marrow-derived dendritic cells from either type of mice, whereas lipopolysaccharide induced small amounts of IL-10 by bone marrow-derived dendritic cells from both types of mice. The upregulation by FSL-1 of the expression of CD80, CD86 and the MHC class II molecule IA(b) was dose- and time-dependent on the surfaces of C57BL/6-derived bone marrow-derived dendritic cells but not on the surface of TLR2(-/-)-derived bone marrow-derived dendritic cells. Lipopolysaccharide upregulated the expression of these molecules on the surfaces of bone marrow-derived dendritic cells from both types of mice. The expression of CD11c on the surfaces of C57BL/6-derived bone marrow-derived dendritic cells was upregulated by stimulation with both FSL-1 and lipopolysaccharide up to 12 h; thereafter, the expression was downregulated. The results suggest that FSL-1 can accelerate maturation of bone marrow-derived dendritic cells and this FSL-1 activity is mediated by TLR2.     

Dendritic cells in glomerulonephritis.

Renal biopsies (n = 45) from patients with various forms of glomerulonephritis (GN), comprising mesangial IgA-GN (n = 25), focal glomerular sclerosis (n = 13) and acute GN (n = 7), were examined by double staining immunocytochemistry (APAAP, streptavidin-peroxidase) using unconjugated monoclonal antibodies (Ab) against--(i) the CD1b antigen expressed on dendritic cells (DCs), (ii) the invariant chain (Ii), and (iii) biotin-conjugated Ab against HLA-DR. In normal control kidneys (n = 7) without interstitial inflammation, CD1b-positive DCs were not detected. Glomerular endothelial cells and a few cells in mesangial areas showed double staining with the Ab against HLA-DR in Ii. In GN without active interstitial inflammation (n = 9), CD1b-positive DCs were not found. In biopsies with interstitial inflammation (n = 36) CD1b-positive DCs were found interspersed among other inflammatory cells. In seven of the biopsies showing IgA-GN DCs were seen in the vicinity of those glomeruli that exhibited either crescents or glomerular sclerosis with splitting of Bowman's capsule. In proximal tubular epithelial cells de novo expression of HLA-DR/Ii-chain was only seen when DCs were present. We conclude that in different forms of GN: (i) CD1b-positive DCs play an important role in the development of interstitial inflammation, and (ii) their presence may be related to the de novo coexpression of HLA-DR/Ii in tubular epithelial cells, possibly mediated through the production of interferon gamma and other cytokines.     

CD86 (B7-2) can function to drive MHC-restricted antigen-specific CTL responses in vivo

Activation of T cells requires both TCR-specific ligation by direct contact with peptide Ag-MHC complexes and coligation of the B7 family of ligands through CD28/CTLA-4 on the T cell surface. We recently reported that coadministration of CD86 cDNA along with DNA encoding HIV-1 Ags i.m. dramatically increased Ag-specific CTL responses. We investigated whether the bone marrow-derived professional APCs or muscle cells were responsible for the enhancement of CTL responses following CD86 coadministration. Accordingly, we analyzed CTL induction in bone marrow chimeras. These chimeras are capable of generating functional viral-specific CTLs against vaccinia virus and therefore represent a useful model system to study APC/T cell function in vivo. In vaccinated chimeras, we observed that only CD86 + Ag + MHC class I results in 1) detectable CTLs following in vitro restimulation, 2) detectable direct CTLs, 3) enhanced IFN-gamma production in an Ag-specific manner, and 4) dramatic tissue invasion of T cells. These results support that CD86 plays a central role in CTL induction in vivo, enabling non-bone marrow-derived cells to prime CTLs, a property previously associated solely with bone marrow-derived APCs.     

Distinct uptake mechanisms but similar intracellular processing of two different toll-like receptor ligand-peptide conjugates in dendritic cells

Covalent conjugation of Toll-like receptor ligands (TLR-L) to synthetic antigenic peptides strongly improves antigen presentation in vitro and T lymphocyte priming in vivo. These molecularly well defined TLR-L-peptide conjugates, constitute an attractive vaccination modality, sharing the peptide antigen and a defined adjuvant in one single molecule. We have analyzed the intracellular trafficking and processing of two TLR-L conjugates in dendritic cells (DCs). Long synthetic peptides containing an ovalbumin cytotoxic T-cell epitope were chemically conjugated to two different TLR-Ls the TLR2 ligand, Pam(3)CysSK(4) (Pam) or the TLR9 ligand CpG. Rapid and enhanced uptake of both types of TLR-L-conjugated peptide occurred in DCs. Moreover, TLR-L conjugation greatly enhanced antigen presentation, a process that was dependent on endosomal acidification, proteasomal cleavage, and TAP translocation. The uptake of the CpG approximately conjugate was independent of endosomally-expressed TLR9 as reported previously. Unexpectedly, we found that Pam approximately conjugated peptides were likewise internalized independently of the expression of cell surface-expressed TLR2. Further characterization of the uptake mechanisms revealed that TLR2-L employed a different uptake route than TLR9-L. Inhibition of clathrin- or caveolin-dependent endocytosis greatly reduced uptake and antigen presentation of the Pam-conjugate. In contrast, internalization and antigen presentation of CpG approximately conjugates was independent of clathrin-coated pits but partly dependent on caveolae formation. Importantly, in contrast to the TLR-independent uptake of the conjugates, TLR expression and downstream TLR signaling was required for dendritic cell maturation and for priming of na?ve CD8(+) T-cells. Together, our data show that targeting to two distinct TLRs requires distinct uptake mechanism but follows similar trafficking and intracellular processing pathways leading to optimal antigen presentation and T-cell priming.     

Bone marrow-derived cells contribute to contractile dysfunction in endotoxic shock

How infection precipitates depressed contractility is incompletely understood but may involve the immune, nervous, and endocrine systems as well as the heart itself. In this study, we examined the role of Toll-like receptor 4 (TLR4) in LPS-induced myocardial contractile depression. Eighteen hours following endotoxin challenge, we compared contractile responses in hearts from wild-type (WT) and TLR4-deficient mice using modified Langendorff preparations. Unlike hearts from WT mice, TLR4-deficient hearts did not reveal significant contractile dysfunction following LPS administration, as measured by decreased responses in maximal left ventricular pressure, +dP/dtmax, and -dP/dtmax in ex vivo Langendorff preparations. These findings indicate a requirement for TLR4 in LPS-induced contractile depression. To determine the contribution of bone marrow-derived TLR4 function to LPS-induced myocardial dysfunction, we generated TLR4 chimeras using adoptive transfer between histocompatible mouse strains: either TLR4-deficient mice with TLR4+/+ bone marrow-derived cells or TLR4+/+ animals lacking TLR4 in their hematopoietic cells. We then compared the contractile responses of engrafted animals after LPS challenges. Engraftment of TLR4-deficient mice with WT marrow restored sensitivity to the myocardial depressant effects of LPS in TLR4-deficient hearts (P < 0.05). Inactivation of bone marrow-derived TLR4 function, via transplantation of WT mice with TLR4-/- marrow, however, did not protect against the depressant effect of endotoxin. These findings indicate that bone marrow-derived TLR4 activity is sufficient to confer sensitivity to mice lacking TLR4 in all other tissues. However, because inactivation of marrow-derived TLR4 function alone does not protect against endotoxin-triggered contractile dysfunction, TLR4 function in other tissues may also contribute to this response.     

Helper cell factors restore antibody responses of allogeneic bone marrow chimeras: evidence for ineffective cellular interactions

We investigated the nature of deficient antibody responses to SRBC in stable, fully allogeneic bone marrow chimeras. No evidence for a suppressor cell-mediated mechanism was found. Chimera spleens possessed adequate numbers of antigen-reactive B cells to produce a normal antibody response. Using separated chimera cell populations and soluble helper factors, we assessed the functional capabilities of chimera B cells, T cells, and macrophages. Our data suggest that the failure of allogeneic chimeras to produce antibody is not the result of impaired B cell, T cell, or macrophage function, but rather that it is due in ineffective cellular interactions that normally result in the generation of helper factors. In vitro stimulation of chimera macrophages with LPS, and of chimera spleen cells with Con A, resulted in the release of soluble helper factors that were capable of fully restoring chimera B cell responses.