Magnitude of response of histocompatibility-restricted T-cell clones is a function of the product of the concentrations of antigen and Ia molecules

Assessing antigen specific HLA-DR+ antibody secreting cell (DR+ASC) responses in whole blood in enteric infections using an ELISPOT technique

Antibody secreting cells (ASCs) generate antibodies in an antigen-specific manner as part of the adaptive immune response to infections, and these cells increase their surface expression of HLA-DR. We have studied this parameter (HLA-DR+ ASC) in patients with recent diarrheal infection using immuno-magnetic cell sorting and an enzyme linked immunospot (ELISPOT) technique that requires only one milliliter of blood. We validated this approach in adult patients with cholera (n = 15) or ETEC diarrhea (n = 30) on days 2, 7 and 30 after showing clinical symptom at the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) hospital in Dhaka, and we compared responses to age-matched healthy controls (n = 7). We found that HLA-DR+ ASC (DR+ASC) responses specific both for T cell-dependent (cholera toxin B subunit), and T cell-independent (lipopolysaccharide) antigens were elevated at day 7 after showing clinical cholera symptom. Similarly, DR+ASCs were elevated against both heat-labile toxin and colonization factors following ETEC infection. We observed significant correlations between antigen-specific DR+ASC responses and antigen-specific, gut homing ASC and plasma antibody responses. This study demonstrates that a simple ELISPOT procedure allows determination of antigen-specific ASC responses using a small volume of whole blood following diarrhea. This technique may be particularly useful in studying DR+ASC responses in young children and infants, either following infection or vaccination.

TCR/pMHC Interaction: Phenotypic Model for an Unsolved Enigma

TCR-pMHC interaction is the keystone of the adaptive immune response. This process exhibits an impressive capacity of speed, sensitivity, and discrimination that allows detecting foreign pMHCs at very low concentration among much more abundant self-pMHC ligands. However, and despite over three decades of intensive research, the mechanisms by which this remarkable discrimination and sensitivity is attained remain controversial. In kinetic proofreading mechanisms (KPR), an increase of specificity occurs by reducing the sensitivity. To overcome this difficulty, more elaborate models including feedback processes or induced rebinding have been incorporated into the KPR scheme. Here a new approach based on the assumption that the proofreading chain behaves differently for foreign- and self-pMHC complexes has been integrated into a phenotypic model in which the complexes responsible for T cell activation stabilize (for foreign peptides) or weaken (for foreign peptides), resulting in a dramatic increase in sensitivity and specificity. Stabilization and destabilization of complexes may be caused by conformational changes, rebinding, or any other process leading to variations in the dissociation rate constants of the complexes transmitting the activation. The numerical solution and the analytical expression for the steady-state response as a function of k_off(i) (i = 0, 1, …, N, where C₀, C₁, …, C_N are the complexes in the proofreading chain) are provided. The activation chain speeds up, and larger increases in sensitivity and discrimination are obtained if the rate of activation along the proofreading chain increases for foreign pMHCs and decreases for self-ligands. Experimental implications and comparison with current models are discussed.

Initial viral load determines the magnitude of the human CD8 T cell response to yellow fever vaccination

CD8 T cells are a potent tool for eliminating intracellular pathogens and tumor cells. Thus, eliciting robust CD8 T-cell immunity is the basis for many vaccines under development. However, the relationship between antigen load and the magnitude of the CD8 T-cell response is not well-described in a human immune response. Here we address this issue by quantifying viral load and the CD8 T-cell response in a cohort of 80 individuals immunized with the live attenuated yellow fever vaccine (YFV-17D) by sampling peripheral blood at days 0, 1, 2, 3, 5, 7, 9, 11, 14, 30, and 90. When the virus load was below a threshold (peak virus load < 225 genomes per mL, or integrated virus load < 400 genome days per mL), the magnitude of the CD8 T-cell response correlated strongly with the virus load (R(2) ∼ 0.63). As the virus load increased above this threshold, the magnitude of the CD8 T-cell responses saturated. Recent advances in CD8 T-cell-based vaccines have focused on replication-incompetent or single-cycle vectors. However, these approaches deliver relatively limited amounts of antigen after immunization. Our results highlight the requirement that T-cell-based vaccines should deliver sufficient antigen during the initial period of the immune response to elicit a large number of CD8 T cells that may be needed for protection.

Receptor Pre-Clustering and T cell Responses: Insights into Molecular Mechanisms

T cell activation, initiated by T cell receptor (TCR) mediated recognition of pathogen-derived peptides presented by major histocompatibility complex class I or II molecules (pMHC), shows exquisite specificity and sensitivity, even though the TCR-pMHC binding interaction is of low affinity. Recent experimental work suggests that TCR pre-clustering may be a mechanism via which T cells can achieve such high sensitivity. The unresolved stoichiometry of the TCR makes TCR-pMHC binding and TCR triggering, an open question. We formulate a mathematical model to characterize the pre-clustering of T cell receptors (TCRs) on the surface of T cells, motivated by the experimentally observed distribution of TCR clusters on the surface of naive and memory T cells. We extend a recently introduced stochastic criterion to compute the timescales of T cell responses, assuming that ligand-induced cross-linked TCR is the minimum signaling unit. We derive an approximate formula for the mean time to signal initiation. Our results show that pre-clustering reduces the mean activation time. However, additional mechanisms favoring the existence of clusters are required to explain the difference between naive and memory T cell responses. We discuss the biological implications of our results, and both the compatibility and complementarity of our approach with other existing mathematical models.

Secretory IgA: Designed for Anti-Microbial Defense

Prevention of infections by vaccination remains a compelling goal to improve public health. Mucosal vaccines would make immunization procedures easier, be better suited for mass administration, and most efficiently induce immune exclusion - a term coined for non-inflammatory antibody shielding of internal body surfaces, mediated principally by secretory immunoglobulin A (SIgA). The exported antibodies are polymeric, mainly IgA dimers (pIgA), produced by local plasma cells (PCs) stimulated by antigens that target the mucose. SIgA was early shown to be complexed with an epithelial glycoprotein - the secretory component (SC). A common SC-dependent transport mechanism for pIgA and pentameric IgM was then proposed, implying that membrane SC acts as a receptor, now usually called the polymeric Ig receptor (pIgR). From the basolateral surface, pIg-pIgR complexes are taken up by endocytosis and then extruded into the lumen after apical cleavage of the receptor - bound SC having stabilizing and innate functions in the secretory antibodies. Mice deficient for pIgR show that this is the only receptor responsible for epithelial export of IgA and IgM. These knockout mice show a variety of defects in their mucosal defense and changes in their intestinal microbiota. In the gut, induction of B-cells occurs in gut-associated lymphoid tissue, particularly the Peyer's patches and isolated lymphoid follicles, but also in mesenteric lymph nodes. PC differentiation is accomplished in the lamina propria to which the activated memory/effector B-cells home. The airways also receive such cells from nasopharynx-associated lymphoid tissue but by different homing receptors. This compartmentalization is a challenge for mucosal vaccination, as are the mechanisms used by the mucosal immune system to discriminate between commensal symbionts (mutualism), pathobionts, and overt pathogens (elimination).

T cell vaccinology: exploring the known unknowns

The objective of modern vaccine development is the safe generation of protective long-term immune memory, both prophylactic and therapeutic. Live attenuated vaccines generate potent cellular and humoral immunity [1-3], but numerous problems exist with these vaccines, ranging from production and storage issues to adverse reactions and reversion to virulence. Subunit vaccines are safer, more stable, and more amenable to mass production. However the protection they produce is frequently inferior to live attenuated vaccines and is typically confined to humoral, and not cellular immunity. Unfortunately, there are presently no subunit vaccines available clinically that are effective at eliciting cellular responses let alone cellular memory [4]. This article will provide and overview of areas of investigation that we see as important for the development of vaccines with the capacity to induce robust and enduring cellular immune responses.

T cell triggering: insights from 2D kinetics analysis of molecular interactions

Interaction of the T cell receptor (TCR) with pathogen-derived peptide presented by the major histocompatibility complex (pMHC) molecule is central to adaptive immunity as it initiates intracellular signaling to trigger T cell response to infection. Kinetic parameters of this interaction have been under intensive investigation for more than two decades using soluble pMHCs and/or TCRs with at least one of them in the solution (three-dimensional (3D) methods). Recently, several techniques have been developed to enable kinetic analysis on live T cells with pMHCs presented by surrogate antigen presenting cells (APCs) or supported planar lipid bilayers (two-dimensional (2D) methods). Comparison of 2D versus 3D parameters reveals drastic differences with broader ranges of 2D affinities and on-rates and orders of magnitude faster 2D off-rates for functionally distinct pMHCs. Here we review new 2D data and discuss how it may impact previously developed models of T cell discrimination between pMHCs of different potencies.

Antigen-induced B cell apoptosis is independent of complement C4

Deficiencies in early complement components are associated with the development of systemic lupus erythematosus (SLE) and therefore early complement components have been proposed to influence B lymphocyte activation and tolerance induction. A defect in apoptosis is a potential mechanism for breaking of peripheral B cell tolerance, and we hypothesized that the lack of the early complement component C4 could initiate autoimmunity through a defect in peripheral B lymphocyte apoptosis. Previous studies have shown that injection of a high dose of soluble antigen, during an established primary immune response, induces massive apoptotic death in germinal centre B cells. Here, we tested if the antigen-induced apoptosis within germinal centres is influenced by early complement components by comparing complement C4-deficient mice with C57BL/6 wild-type mice. We demonstrate that after the application of a high dose of soluble antigen in wild-type mice, antibody levels declined temporarily but were restored almost completely after a week. However, after antigen-induced apoptosis, B cell memory was severely limited. Interestingly, no difference was observed between wild-type and complement C4-deficient animals in the number of apoptotic cells, restoration of antibody levels and memory response.

Modulation of Antigenic Phenotype in Cultured Human Osteoblast-like Cells by FGFb, TGFβ1, PDGF-BB, IL-2, IL-1β, LPS and IFNγ

Background/Aims

Recent reports demonstrated that osteoblast-like cells can also exert activities directly associated with the immune system (cytokine synthesis, antigen presentation, phagocytosis and stimulation of T lymphocytes). The present study aimed to analyze the effect of Transforming growth factorβ1 (TGFβ1), Fibroblast growth factor basic (FGFb), Platelet-derived growth factor-BB (PDGF-BB), Interleukin-1β (IL-1β), Interleukin-2 (IL-2), Lipopolysaccharide (LPS) and Interferon-γ (IFNγ) on the expression on osteoblast-like cells of antigens involved in antigen presentation.

Methods

Flow cytometry was used to investigate whether the growth factors FGFb, TGFβ1, PDGF-BB, IL-2, IL-1β, LPS and IFNγ modulate the expression on cultured human osteoblast-like cells of different antigens involved in antigen-presentation and T cell activation.

Results

TGFβ1 treatment significantly reduced the expression of CD54 and CD86. IL-1β treatment significantly enhanced the expression of CD54, CD86 and HLA-DR. LPS and IFNγ treatments produced a major increase in CD54, CD80, CD86 and HLA-DR expression. Expression of these antigen-presenting molecules was not significantly modified by FGFb, PDGF-BB or IL-2 treatment.

Intratumor CD4 T-Cell Accumulation Requires Stronger Priming than for Expansion and Lymphokine Secretion

T cells need to migrate to and accumulate inside tumors before mediating rejection of the tumor. The number of specific T cells inside tumors may depend on the efficiency of priming in the draining lymph node (DLN), intratumor deletion, suppressive phenomena, or both. We used monoclonal anti-male antigen CD4 (Marilyn) T cells and tumor cell lines expressing or not the corresponding antigen (Dby) to analyze CD4 T-cell accumulation in tumors. Priming by MHC II(+) or MHC II(-) male splenocytes or Dby(+) tumor cells induced similar Marilyn T-cell expansion in the DLN and recirculation in other lymph nodes and capacity to produce IFN-gamma. However, intratumor accumulation was different for each priming condition. In mice with Dby(-) tumors, MHC II(+) male splenocyte priming induced greater, although not statistically significant, Marilyn T-cell accumulation in the tumors than MHC II(-) male splenocyte priming. In mice with Dby(+) tumors, priming in the tumor DLN induced less Marilyn T-cell intratumor accumulation than priming by MHC II(+) male splenocytes. We saw comparable differences for Marilyn T-cell accumulation in gut lamina propria, suggesting that priming affects effector T-cell accumulation in inflamed tissues. Mature dendritic cells were loaded with graded doses of Dby peptide to control for antigen-presenting cell characteristics during priming. We observed similar proliferation, with higher concentrations inducing higher intratumor accumulation. Thus, intratumor accumulation requires stronger stimulation than for proliferation or the capacity to secrete lymphokines. In this system, priming intensity alone can explain the number of intratumor T cells without having to call for intratumor deletion or suppression phenomena.

Mucosal B cells: phenotypic characteristics, transcriptional regulation, and homing properties

Mucosal antibody defense depends on a complex cooperation between local B cells and secretory epithelia. Mucosa-associated lymphoid tissue gives rise to B cells with striking J-chain expression that are seeded to secretory effector sites. Such preferential homing constitutes the biological basis for local production of polymeric immunoglobulin A (pIgA) and pentameric IgM with high affinity to the epithelial pIg receptor that readily can export these antibodies to the mucosal surface. This ultimate functional goal of mucosal B-cell differentiation appears to explain why the J chain is also expressed by IgG- and IgD-producing plasma cells (PCs) occurring at secretory tissue sites; these immunocytes may be considered as 'spin-offs' from early effector clones that through class switch are on their way to pIgA production. Abundant evidence supports the notion that intestinal PCs are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists concerning the relative importance of M cells, major histocompatibility complex class II-expressing epithelial cells, and professional antigen-presenting cells for the uptake, processing, and presentation of luminal antigens in GALT to accomplish the extensive and sustained priming and expansion of mucosal B cells. Likewise, it is unclear how the germinal center reaction in GALT so strikingly can promote class switch to IgA and expression of J chain. Although B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, the cues directing preferential homing to different segments of the gut require better definition. This is even more so for the molecules involved in homing of mucosal B cells to secretory effector sites beyond the gut, and in this respect, the role of Waldever's ring (including the palatine tonsils and adenoids) as a regional inductive tissue needs further characterization. Data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, eyes, oral cavity, small and large intestines, and urogenital tract.

HLA-DRB5*0101 and -DRB1*1501 expression in the multiple sclerosis-associated HLA-DR15 haplotype

The HLA region, and particularly the DR15 haplotype (containing the two DRB* genes DRB1*1501 and DRB5*0101 and the tightly linked DQ alleles DQA*0102 and DQB1*0602, which together form the DQw6 molecule) in Caucasians, shows the strongest genetic association with multiple sclerosis (MS). In the DR15 haplotype, two beta-chains HLA-DRB1*1501 and -DRB5*0101 are co-expressed resulting in two different surface HLA-DR alphabeta heterodimers, DR2b and DR2a. Most previous studies focused on DRB1*1501, however, both DR2a and DR2b may contribute to MS pathogenesis via antigen presentation to myelin-specific T lymphocytes. We therefore analyzed the expression of the two DR15 genes in various antigen presenting cells (APCs), central nervous system and thymic tissues. Transcript levels were higher for DRB5*0101 in all cell types and tissues. Both HLA-DR heterodimers were expressed at significant levels on the cell surface, where they showed a differential expression pattern in different APCs. They were similarly regulated after stimulation with interferon-gamma and interleukin-4. Finally, immunohistochemistry experiments indicated that both molecules were expressed in thymic tissue. Our results encourage future research to investigate the potential functional relevance of both genes for the pathogenesis of MS.

Molecular characterization of major histocompatibility complex class II alleles in wild tiger salamanders (Ambystoma tigrinum)

Major histocompatibility complex (MHC) class II genes are usually among the most polymorphic in vertebrate genomes because of their critical role (antigen presentation) in immune response. Prior to this study, the MHC was poorly characterized in tiger salamanders (Ambystoma tigrinum), but the congeneric axolotl (Ambystoma mexicanum) is thought to have an unusual MHC. Most notably, axolotl class II genes lack allelic variation and possess a splice variant without a full peptide binding region (PBR). The axolotl is considered immunodeficient, but it is unclear how or to what extent MHC genetics and immunodeficiency are interrelated. To study the evolution of MHC genes in urodele amphibians, we describe for the first time an expressed polymorphic class II gene in wild tiger salamanders. We sequenced the PBR of a class II gene from wild A. tigrinum (n=33) and identified nine distinct alleles. Observed heterozygosity was 73%, and there were a total of 46 polymorphic sites, most of which correspond to amino acid positions that bind peptides. Patterns of nucleotide substitutions exhibit the signature of diversifying selection, but no recombination was detected. Not surprisingly, trans-species evolution of tiger salamander and axolotl class II alleles was apparent. We have no direct data on the immunodeficiency of tiger salamanders, but the levels of polymorphism in our study population should suffice to bind a variety of foreign peptides (unlike axolotls). Our tiger salamander data suggest that the monomorphism and immunodeficiencies associated with axolotl class II genes is a relic of their unique historical demography, not their phylogenetic legacy.

Increase of antigen-presenting cells in the gastric mucosa of Helicobacter pylori-infected children

BACKGROUND

Infection with Helicobacter pylori leads to an increase of T cells in the gastric mucosa of children. In contrast to peripheral blood, where monocytes are the most abundant antigen-presenting cells, CD14+ macrophages are very rare in infected gastric mucosa. We postulated that other types of antigen-presenting cells must be present in infected gastric mucosa.

MATERIAL AND METHODS

Antral biopsies were obtained from 56 children. The cellular expression of major histocompatibility complex (MHC) class II molecules, CD1a/b, and CD23, which are involved in antigen presentation were analyzed by immunohistochemistry. In addition, T cells (CD4, CD8, CD25, and gamma/delta-TCR), B cells (anti-IgM), macrophages (CD14) and granulocytes (CD15) were quantified.

RESULTS

Twenty-eight children were H. pylori-infected. Thirteen children were healthy, 15 had other gastric pathologies. T cells (p<.0001), B cells (p<.0001), CD23+ (p<.0001), and CD1a/b+ (p<.005) cells were significantly increased in the lamina propria of H. pylori-infected children, whereas macrophages were rare without significant differences among the groups. Within the epithelium, CD8+ T lymphocytes predominated clearly over CD4+ cells. H. pylori-negative children had only few MHC class II-positive cells within the gastric epithelium, whereas MHC class II antigens were strongly expressed on epithelial cells (p<.0001) of all H. pylori-infected children.

CONCLUSION

Helicobacter pylori infection leads to an enhanced expression of antigen-presenting molecules together with a parallel rise of T cells in the lamina propria. This may represent an effort of the immune system to optimize local immune responses against H. pylori. We speculate that the epithelium participates in the initiation of a local immune response against H. pylori.

Systemic immunomodulation of autoimmune disease using MHC-derived recombinant TCR ligands

Human autoimmune disease involves local activation of antigen-specific CD4(+) T cells that produce inflammatory Th1 cytokines leading to the further recruitment and activation of lymphocytes and monocytes, resulting ultimately in the destruction of target tissue. Antigen presenting cells (APCs) initiate activation of CD4(+) T cells in a multistep process that minimally involves co-ligation of the TCR and CD4 by the MHC class II/peptide complex and costimulation through additional T cell surface molecules such as CD28. Disruption of this highly orchestrated series of events can result in the direct modulation of CD4(+) T cell behavior. The interaction between MHC and TCR holds unique promise as a focal point for therapeutic intervention in the pathology of CD4(+) T cell-mediated diseases, and MHC class II-derived Recombinant TCR Ligands ("RTLs") have emerged as a new class of therapeutics with potent clinical efficacy in a diverse set of animal models for multiple sclerosis. Here I review the systemic effect that RTL therapy has on the intact immune system and present an overview of a molecular mechanism by which RTL therapy could induce these systemic changes.

Mechanism of modulation of T cell responses by N-palmitoylated peptides

Small structural changes in the antigenic peptides recognized by TCR can alter the biological properties of those peptides and convert them into weak agonists, partial agonists, or antagonists of these receptors. These altered peptide ligands (APL) are usually generated by conservative amino acid substitutions at TCR contact residues. Here, we show that APL with therapeutic properties can also be generated by attachment of palmitic acid at the N terminus of the peptide without the need to modify the peptide's primary sequence. Using N-palmitoylated pigeon cytochrome-c peptide 81-104 (PALPCC(81-104)), we were able to induce T cell hyporesponsiveness to the wild-type peptide in vitro. More importantly, administration of the PALPCC(81-104 )to mice reduced the responsiveness to the native peptide when tested ex vivo. Biochemical and functional experiments indicated that the action of N-palmitoylated peptides was due to the conversion of the native peptide into a weak agonist that could then induce T cell anergy. Our results demonstrate that N-palmitoylation of antigenic peptides is a feasible strategy to generate APL, as it avoids the need to screen multiple amino acid variants of each specific antigen to identify those with therapeutic properties.

The [173–196] fragment of ovalbumin suppresses ovalbumin-specific rat IgE responses

Peptides and protein hydrolysates are attractive tools for the induction of tolerance or regulation of targeted B and/or T cell responses. In vivo, peptides are mainly produced by the action of digestive enzymes or following the processing of exogenous antigens by antigen-presenting cells (APCs). In vitro, these molecules are generally produced by enzymatic digestion and chemical hydrolysis of proteins. We investigated the T and B cell determinants of the major food allergen ovalbumin (nOVA) in rat by analyzing (1) the stimulatory effect of nOVA peptides generated by cyanogen bromide (CNBr) cleavage on nOVA-specific T cells, and (2) the potential of CNBr-derived OVA fractions to induce oral tolerance to nOVA. Peptide fractions of the CNBr-hydrolysated OVA were isolated by high-pressure liquid chromatography and tested for their ability to stimulate nOVA-specific T cells isolated from rats parenterally immunized with nOVA. The nOVA fractions containing the stimulatory determinants were then intragastrically administered to rat to test their potential to induce oral tolerance. The hole CNBr hydolysate stimulated proliferation of nOVA-specific T cells. Three out of the five HPLC-purified peptidic fractions were also able to stimulate proliferation and cytokine production by nOVA-specific T cells. A peptide fraction exhibiting a single peak by HPLC contained the 173-196 nOVA segment and stimulated nOVA-specific T cells. This segment also promoted oral tolerance to nOVA and reduced IgE responses. CNBr hydrolysis releases several peptides with stimulatory effect on nOVA-specific T cells including a new nOVA [173-196] T cell determinant which induces oral tolerance to nOVA.

Up-regulation by feline interleukin-4 and down-regulation by feline interferon-gamma of major histocompatibility complex class II on cat B-lymphocytes

Interleukin-4 (IL-4) exhibits numerous biological and immunoregulatory functions on B- and T-lymphocytes, monocytes, and dendritic cells in both mice and humans. In the present study, we show that IL-4 also has a regulatory function in the cat species. Cells transfected with IL-4 DNA expressed a biologically active protein as demonstrated by the up-regulation of MHC class II molecules on B-lymphocytes (CD21(+)) in a flow cytometric assay. Increased levels of MHC class II expression on CD21(+) cells were seen in 11 out of 12 cats (p<0.05). In addition, 12 out of 12 cats showed up-regulation of MHC class II on CD21(-) cells, mainly consisting of T-lymphocytes (p<0.05). In contrast, concanavalin A (ConA)-induced culture supernatant from peripheral blood mononuclear cells (PBMCs) containing high levels of interferon-gamma (IFN-gamma) transcripts induced down-regulation of MHC class II molecules on CD21(+) cells of all samples (p<0.05). Variable results were observed for CD21(-) cells incubated with ConA-conditioned medium (p=0.71). The nature of the cytokine(s) responsible for these effects remains to be determined. However, the fact that down-regulation of MHC class II molecules on B cells occurred in all cats tested suggests that IFN-gamma may be involved. These data provide further insight into the mechanism by which MHC class II expression is regulated in feline lymphocytes, and suggest that the Th1/Th2 paradigm is also present in the cat.

The proliferative capacity of individual naive CD4(+) T cells is amplified by prolonged T cell antigen receptor triggering

Strong antigenic encounter by T cells rapidly induces immunological synapse formation and surface T cell receptor (TCR) downregulation. Although surface TCR expression can remain low for several days, T cells can still sustain antigenic signaling. It has been unclear whether prolonged antigenic signaling occurs in the absence of surface TCR replenishment, being maintained by a few "nondownregulatable" surface TCRs that might reside in a synaptosomal structure. Alternatively, the low surface TCR level induced by antigen might represent a dynamic state of expression involving continual surface TCR replenishment, reengagement by antigen, and ongoing downregulation. To resolve this issue, we studied in vivo-generated, dual-specificity primary naive CD4(+) T cells. On these cells, antigenic stimulus exclusively downregulated antigen-specific, but not antigen-nonspecific, TCRs. In addition to providing a means to track TCR engagement, this also allowed us to use the antigen nonspecific TCR to track TCR expression in isolation from TCR engagement by antigen. Surface TCR replenishment began within the first day of stimulation, and occurred synchronously with continuous antigen-specific TCR engagement and downregulation. Furthermore, by enhancing CD25 expression, extended signaling through surface-replenishing TCRs significantly amplified the number of daughter cells generated by naive CD4(+) T cells that had already committed to proliferate. This effect required TCR engagement and could not be substituted for by interleukin 2. These data demonstrate that TCR triggering and consumption can occur over an extended period of time, with a significant impact on the effector responses evoked from naive CD4(+) T cells.

Activation of monocyte cyclooxygenase-2 gene expression by human herpesvirus 6. Role for cyclic AMP-responsive element-binding protein and activator protein-1

Prostaglandin E(2) (PGE(2)) is an arachidonic acid metabolite mainly produced by activated monocytes/macrophages (Mo/Mphi) that display broad immunomodulatory activities. Several viruses capable of infecting Mo/Mphi modulate PGE(2) synthesis in a way that favors the infection processes and the spread of virions. In the present work, we studied the effect of human herpesvirus 6 (HHV-6) infection of Mo/Mphi on PGE(2) synthesis. Our results indicate that HHV-6 induces COX-2 gene expression and PGE(2) synthesis within a few hours of infection. We mapped the different promoter elements associated with COX-2 gene activation by HHV-6 to two cis-acting elements: a cyclic AMP-responsive element and an activator protein-1 element. HHV-6 immediate-early protein 2 was identified as a modulator of COX-2 gene expression in Mo/Mphi. Finally, addition of PGE(2) to HHV-6-infected peripheral blood mononuclear cells cultures was found to increase significantly viral replication. Overall, these results further contribute to the immunomodulatory properties of HHV-6 and highlight a potential role for eicosanoids in the replication process of this virus.

Strength of prior stimuli determines the magnitude of secondary responsiveness in CD8+ T cells

CD8+T cells can become anergic following activation, though the cellular mechanism, as compared to CD4+ T cells, remains poorly understood. Here, we examined the effects of different antigen-dose, peptide ligands, and engagement of costimulatory molecules on the induction of CD8+ T cell anergy. We observed that increasing strengths of signals delivered to CD8+ T cells by varying the antigen-dose and the nature of peptide ligands induced increasing degrees of non-responsiveness to secondary stimulation. Furthermore, higher levels of LFA-3 engagement of CD2 rendered CD8+ T cells unresponsive to secondary antigenic re-challenge. This pattern of secondary responsiveness lasted up to 2 weeks following primary stimulation and was not correlated with prior cell division history. These results indicate that the strength of prior stimuli, which is determined by the sum of signals from both TCR and costimulatory molecules, determines the activation threshold and magnitude of CD8+ T cell responses.

Reduced alloreactive T-cell activation after alcohol intake is due to impaired monocyte accessory cell function and correlates with elevated IL-10, IL-13, and decreased IFNgamma levels

BACKGROUND

Immunosuppression associated with chronic alcohol use is characterized by reduced antigen-specific T-cell response and impaired delayed type hypersensitivity. Increasing evidence suggests in chronic alcohol consumption models that reduced antigen-specific T-cell proliferation is due to insufficient accessory cell function. Accessory cell function, a critical step in recognition of viral antigens, is reduced in chronic hepatitis C. The severity of hepatitis C is increased by alcohol consumption. Thus, we investigated the effects of alcohol consumption on accessory cell activity of monocytes in supporting alloreactive T-cell proliferation.

METHODS

Alloreactive T-cell proliferation was evaluated in a one-way mixed lymphocyte reaction (MLR). Mononuclear cells were isolated by Ficoll density gradient and monocytes by adherence. Alcohol (0.8 g/kg body weight, an equivalent of approximately three drinks) was given to nonalcohol-consuming individuals and blood samples were collected before, 4 hr, or 18 hr after alcohol consumption. Alcohol in vitro was administered at concentrations of 25-100 mM.

RESULTS

T-cell proliferation in MLR was significantly reduced in the presence of physiologically relevant concentrations of alcohol in vitro (25-100 mM ethanol) (p < 0.05). In vivo alcohol consumption also depressed proliferation in the MLR when stimulator cells were obtained 4 hr after alcohol consumption. MLR was not decreased, however, in the presence of alcohol-exposed responder cells and normal stimulator cells, suggesting that the accessory cell population and not T cells are affected by alcohol. Decreased accessory cell function was further evidenced by reduced superantigen-induced (SEB) but not mitogen-induced (PHA) T-cell proliferation in samples obtained 18 hr after alcohol intake (35% reduction). Reduced accessory cell function was not due to changes in surface expression of monocyte costimulatory molecules (HLA class I, HLA-DR, CD80, CD86, CD40). We found reduced IFNgamma, elevated IL-10, and unchanged IL-4 levels during T-cell proliferation in samples obtained 18 hr after alcohol consumption. Acute alcohol treatment resulted in increased IL-13 in the MLR.

CONCLUSION

These data suggest that even on one occasion moderate alcohol intake can reduce allostimulatory T-cell activation via decreasing accessory cell function. Increased IL-10 and IL-13 plus the reduced IFNgamma production after acute alcohol use are likely to contribute to both the reduced T-cell proliferation and monocyte accessory cell function. These accessory cell mediated defects in T-cell activation may result in impaired antiviral and antitumor immunity after moderate acute alcohol use.

The expression and significance of intracellular T helper cytokines in systemic lupus erythematosus

To analyze the Th1 and Th2 paradigm of peripheral T helper cells in patients with systemic lupus erythematosus (SLE). The intracellular Th1 and Th2 cytokines were analyzed in fresh blood T cells from 20 SLE patients who had not yet received any treatment. Th1 and Th2 cells were quantitated based on their intracellular cytokine content as assessed by flow cytometry. Cytokine expressions were correlated with clinical features, laboratory findings, and disease activities. There was no difference in the expression of intracellular IFN-y, or IL-4 between SLE patients and healthy controls. However, the IL-2 and IL-10 levels were significantly higher and lower respectively in the lupus patients than in the control group. In addition, patients with arthritis had higher IFN-gamma expression than patients without arthritis. Moreover, patients with serositis or CNS involvement had higher IL-4 expression than in patients without these manifestations. There was no correlation between the SLEDAI scores and the cytokine expression levels. However, patients with serum anti-ds DNA antibodies had higher IL-10 levels than in those without these antibodies. The present study demonstrates that a Th1 pattern of intracellular cytokines predominates in patients with SLE prior to treatment. The pattern of particular intracellular T cell cytokines may suggest specific clinical manifestations and disease progression of SLE.

Increased binding of IFN regulating factor 1 mediates the synergistic induction of CIITA by IFN-gamma and tumor necrosis factor-alpha in human thyroid carcinoma cells

Expression of MHC class II molecules is restricted to professional antigen-presenting immune cells, but it can be induced by IFN-gamma in other cell types. Thyroid cells have been shown to induce class II expression (mainly HLA-DR) following stimulation with IFN-gamma and addition of tumor necrosis factor (TNF)-alpha synergistically enhanced this expression. Class II transactivator (CIITA) has been implicated as the master regulator of MHC class II molecules and its transcription has been shown to be regulated from four different promoters, one of which is responsible for its induction by IFN-gamma. The aim of this study was to find whether CIITA is synergistically induced by IFN-gamma and TNF-alpha in the human thyroid MRO-87-1 cell line, and to investigate the molecular mechanisms responsible for this synergism. We have demonstrated that IFN-gamma and TNF-alpha synergistically induce HLA-DRalpha and CIITA mRNAs, but prolonged incubation resulted in the inhibition of CIITA mRNA accumulation. Several potential mechanisms that could explain the synergistic effect were explored. NF-kappaB did not bind the CIITA inducible promoter and addition of SN50, which inhibits NF-kappaB translocation to the nucleus, did not change the synergistic effect. Furthermore, IFN-gamma did not induce IkappaBalpha degradation. Synergistic activation of signal transducer and activator of transcription (STAT)-1 or IFN regulating factor (IRF)-1 was not observed, and STAT-1 did not bind the CIITA inducible promoter. IRF-1, although not synergistically induced or activated, bound synergistically to its specific cis element on the CIITA type IV promoter. Thus we propose that IRF-1 binding mediates the synergistic induction of HLA-DRalpha and CIITA in thyroid cells.

Slow proliferation as a survival advantage: an attempt to resolve a paradox

A model is proposed for immunological high zone- and self-tolerance that is based on the concept of overstimulation of replication: It is demonstrated that if proliferation is hindered by delayed response to nutritional deficiency, then under competitive conditions slower proliferation can become a survival advantage. This offers a resolution of the paradox that self-antigens promote both positive and negative selection in the development of lymphocytes. It also suggests that maintaining nutrition and caloric intake at a minimal level can prolong the survival of cancer patients if the cancer cells replicate more rapidly than healthy ones.

Expression of HLA-DR, CAM and co-stimulatory molecules on cord blood monocytes

Umbilical cord blood (CB) transplantations are associated with a lower risk of severe graft-versus-host disease (GVHD) compared to BMT. GVHD is an immune reaction that involves interaction between cell surface molecules resulting in cell activation and release of many cytokines. Monocytes are known to be an important source of cell adhesion (CAM) and co-stimulatory molecules which play a crucial role in the efficient activation of T and B cells. We analyzed the phenotype of CB monocytes in the presence or absence of an inflammatory signal (rIFN-gamma) and compared them to adult blood (AB); the expression of HLA-DR and 17 different markers (CD11a, CD11b, CD11c, CD18, CD29, CD40, CD44, CD49a, CD49d, CD49e, CD49f, CD54, CD58, CD62L, CD80, CD86 and CD102) was measured by flow cytometry. Statistical analysis showed that, compared to AB, CB monocytes did not express CD11b, CD11c, CD49d and after stimulation with rIFNgamma, they lost the expression of CD58 and CD102, whereas CD80 and CD86 expression was induced. The analysis of fluorescence intensity (MFI) revealed that CB monocytes expressed some CAM (CD29, CD54, CD102) with a lower intensity than AB monocytes except CD44. In conclusion, absence and reduced expression of some markers argue for a different phenotypic profile of CB monocytes compared to AB monocytes, which might partly contribute to their impaired immune response and to the low incidence of GVHD observed after CB transplantations. However, CB monocytes expressed CD80 and CD86 co-stimulatory molecules, but this expression did not prove a normal co-stimulatory function.

Review: the role of omega 3 fatty acids in intestinal inflammation

The role of polyunsaturated fatty acids (PUFAs) in inflammatory lesions of the intestines is the subject of increasing research. This review begins with a background discussion of the source, elongation, and desaturation of PUFAs, as well as the role they have played in the human diet through evolution. The available data and hypotheses as to how manipulation of PUFAs might effect the various components of the immune system are then provided. Possible mechanisms by which PUFAs result in immunomodulation include alterations in eicosanoid synthesis, membrane fluidity, signal transduction, intraluminal bacteria, and gene expression. Attention is then turned to the known effects that these polyunsaturated fatty acids have on the various individual components of the immune system including lymphocytes, neutrophils, and antigen presenting cells, as well as the immunoregulatory process of apoptosis. Finally, laboratory data on the role of PUFAs in necrotizing enterocolitis, and to a greater extent inflammatory bowel disease, first as demonstrated in animal models of the disease, and second in human studies are then summarized.

Designing lymphocyte functional structure for optimal signal detection: voilà, T cells

One basic task of immune systems is to detect signals from unknown "intruders" amidst a noisy background of harmless signals. To clarify the functional importance of many observed lymphocyte properties, I ask: What properties would a cell have if one designed it according to the theory of optimal detection, with minimal regard for biological constraints? Sparse and reasonable assumptions about the statistics of available signals prove sufficient for deriving many features of the optimal functional structure, in an incremental and modular design. The use of one common formalism guarantees that all parts of the design collaborate to solve the detection task. Detection performance is computed at several stages of the design. Comparison between design variants reveals e.g. the importance of controlling the signal integration time. This predicts that an appropriate control mechanism should exist. Comparing the design to reality, I find a striking similarity with many features of T cells. For example, the formalism dictates clonal specificity, serial receptor triggering, (grades of) anergy, negative and positive selection, co-stimulation, high-zone tolerance, and clonal production of cytokines. Serious mismatches should be found if T cells were hindered by mechanistic constraints or vestiges of their (co-)evolutionary history, but I have not found clear examples. By contrast, fundamental mismatches abound when comparing the design to immune systems of e.g. invertebrates. The wide-ranging differences seem to hinge on the (in)ability to generate a large diversity of receptors.

Prevention of experimental autoimmune encephalomyelitis by encephalitogenic epitope sequence simplified derivatives

The encephalitogenic epitope P81-100 from mouse myelin basic protein was used to generate two simplified derivatives with glycine substitutions in alternating positions which were tested for their biological activity in a murine model of multiple sclerosis, experimental autoimmune encephalomyelitis. While both derivatives were unable to induce in mice the disease at the same parent peptide P81-100 dosage, T cell proliferation assays demonstrated their ability to compete with the parental peptide in a dose related manner. Experiments of cell surface binding and T cell tolerance revealed a different behavior of the two derivatives, suggesting different roles in the MHC blockade or T cell tolerance. On induction of encephalomyelitis in animals by P81-100 treatment, one variant proved in vivo to be very effective in protecting from the disease.

n-3 polyunsaturated fatty acids inhibit the antigen-presenting function of human monocytes

Diets rich in n-3 polyunsaturated fatty acids (PUFAs) are associated with suppression of cell-mediated immune responses, but the mechanisms are unclear. We hypothesized that n-3 PUFAs can inhibit the function of human antigen-presenting cells. A prerequisite for this role of blood monocytes is the cell surface expression of major histocompatibility complex (MHC) class II molecules [human leukocyte antigen (HLA)-DR, -DP, and -DQ], aided by the presence of intercellular adhesion molecule-1 (ICAM-1) and leukocyte function associated antigens 1 and 3. We showed previously that the n-3 PUFA eicosapentaenoic acid (EPA) inhibits the expression of HLA-DR on unstimulated human monocytes in vitro, but that docosahexaenoic acid (DHA) enhances its expression. However, both n-3 PUFAs suppress the expression of HLA-DR, HLA-DP, and ICAM-1 on interferon-gamma-activated monocytes. We also established that dietary fish-oil supplementation can inhibit the expression of these surface molecules on circulating human monocytes. We subsequently showed that when EPA and DHA were combined in the same ratio as is commonly found in fish-oil-supplement capsules (3:2), there was no significant effect in vitro on the expression of HLA-DR on unstimulated monocytes, but the expression on activated monocytes remained significantly inhibited. In the same in vitro system, the ability of activated monocytes to present antigen to autologous lymphocytes was significantly reduced after culture with the combined n-3 PUFAs. These findings provide one potential mechanism for the beneficial effect of fish oil in the treatment of rheumatoid arthritis, a disorder associated with elevated expression of MHC class II and adhesion molecules on monocytes present within affected joints.

Beta-cell activity and destruction in type 1 diabetes

Type 1 diabetes is the result of a chronic inflammatory process that causes elimination of insulin-producing beta-cells, resulting in insulin deficiency and hyperglycemia. The destruction is thought to be mediated by an autoimmune process involving cytotoxic T cells recognizing beta-cell autoantigens in the context of MHC class I-peptide complexes. Autoantibodies against insulin, glutamic acid decarboxylase (GAD) and and ICA 512 protein tyrosine phosphatase are frequently found. At the clinical onset of diabetes, some beta-cells remain and after initiation of insulin treatment, most patients enter a period of remission, a phenomenon that may reflect diminished autoimmune activity in the islets. There is evidence to suggest that a further loss of beta-cells can be curtailed, and that patients, who maintain endogenous insulin production, have better glycemic control and less risk of complications. This is the basis for our current research. We are characterizing the remission phenomenon in epidemiological studies in order to identify determinants of beta-cell survival. In randomized, prospective multicenter trials, we are evaluating the benefit of beta-cell secretory rest for rescue of insulin production in patients at onset of clinical disease. In experimental studies, we are investigating expression and regulation of the key molecules of an autoimmune process in the islets. Further, selective beta-cell damage is induced in rat islets and measures to enhance beta-cell resistance and repair are being examined. We have recently identified a remarkable, beta-cell protective effect of K(ATP)-channel opening.

Inhibition of interleukin 2 signaling and signal transducer and activator of transcription (STAT)5 activation during T cell receptor-mediated feedback inhibition of T cell expansion

Limitation of clonal expansion of activated T cells is necessary for immune homeostasis, and is achieved by growth arrest and apoptosis. Growth arrest and apoptosis can occur passively secondary to cytokine withdrawal, or can be actively induced by religation of the T cell receptor (TCR) in previously activated proliferating T cells. TCR-induced apoptosis appears to require prior growth arrest, and is mediated by death receptors such as Fas. We tested whether TCR religation affects T cell responses to interleukin (IL)-2, a major T cell growth and survival factor. TCR ligation in activated primary human T cells blocked IL-2 induction of signal transducer and activator of transcription (STAT)5 DNA binding, phosphorylation of STAT5, Janus kinase (Jak)1, Jak3, and Akt, and kinase activity of Jak1 and Jak3. Inhibition was mediated by the mitogen-activated protein kinase kinase (MEK)-extracellular stimulus-regulated kinase (ERK) signaling pathway, similar to the mechanism of inhibition of IL-6 signaling we have described previously. TCR ligation blocked IL-2 activation of genes and cell cycle regulatory proteins, and suppressed cell proliferation and expansion. These results identify TCR-induced inhibition of IL-2 signaling as a novel mechanism that underlies antigen-mediated feedback limitation of T cell expansion, and suggest that modulation of cytokine activity by antigen receptor signals plays an important role in the regulation of lymphocyte function.

Competition for antigenic sites during T cell proliferation: a mathematical interpretation of in vitro data

By fitting different mathematical T cell proliferation functions to in vitro T cell proliferation data, we studied T cell competition for stimulatory signals. In our lymphocyte proliferation assays both the antigen (Ag) availability and the concentration of T cells were varied. We show that proliferation functions involving T cell competition describe the data significantly better than classical proliferation functions without competition, thus providing direct evidence for T cell competition in vitro. Our mathematical approach allowed us to study the nature of T cell competition by comparing different proliferation functions involving (i) direct inhibitory T-T interactions, (ii) Ag-specific resource competition, or (iii) resource competition for nonspecific factors such as growth factors, and access to the surface of Ag-presenting cells (APCs). We show that resource competition is an essential ingredient of T cell proliferation. To discriminate between Ag-specific and nonspecific resource competition, the Ag availability was varied in two manners. In a first approach we varied the concentration of APCs, displaying equal ligand densities; in a second approach we varied the Ag density on the surface of the APCs, while keeping the APC concentration constant. We found that both resource competition functions described the data equally well when the Ag availability was increased by adding APCs. When the APC concentration was kept constant, the nonspecific resource competition function yielded the best description of the data. Our interpretation is that T cells were competing for "antigenic sites" on the APCs.

Ethanol impairs major histocompatibility complex (MHC) class II molecule-mediated but not MHC class I molecule-mediated T cell response in alcohol-consuming mice

The goal of this study was to determine whether alcohol affects alloantigen-induced proliferative and cytolytic activity of T cells in mice, and whether the altered immune response was in part due to a defect of IL-2 activity. The ability of spleen cells from individual alcohol-consuming C57BL/6 mice to generate allo-specific mixed lymphocyte response (MLR) and cytotoxic T lymphocyte (CTL) was compared to that of mice fed on an isocaloric maltose diet and regular diet. Allospecific MLR and CTL were generated by sensitizing spleen cells of C57BL/6 mice against spleen cells from BALB/c mice, and the allo-specific CTL activity was determined by the ability of the CTL to kill 51Cr-labeled P815 mastocytoma target cells. Our results showed that the allo-specific MLR of the responder cells from alcohol-consuming mice was significantly reduced (40% reduction, p<0.0 1), and the addition of exogenous interleukin 2 (IL-2) could not reverse the suppression of MLR induced by ethanol. However, our results clearly showed that ethanol has little suppressive effect on allo-reactive CTL of alcohol-consuming mice as compared to the alloreactivity of the control mice (P>0.05). Finally, we also demonstrated that ethanol did not impair the alloantigen-induced IL-2 production in the mixed lymphocyte cultures (P>0.1).

Different binding of NF-Y transcriptional factor to DQA1 promoter variants

Polymorphism in the HLA-DQA1 promoter (QAP) sequences could influence the gene expression through a differential binding of transcriptional factors. Considering the main role played by the Y-box in the transcription, we focused on the QAP4 variants differing for a G vs A transition from the QAP Y-box consensus sequence. Electrophoretic Mobility Shift Assay using the two Y-box sequences was performed to determine whether this mutation could be reflected in an allele-specific binding of transcriptional factors. Indeed, the NF-Y specific band, recognised by supershift experiments, was clearly observed using the Y-box consensus probe but it was barely detectable with the QAP4 one. On the contrary, two other complexes were found to more strongly interact with QAP4 Y-box in comparison to the consensus sequence. The analysis of a selected panel of HLA homozygous lymphoblastoid cell lines by competitive RT-PCR and by Northern blotting revealed that the DQA1 *0401, *0501,*0601 alleles regulated by the QAP4 promoters were less expressed at the mRNA level than the DQA1* 0201 allele regulated by the QAP2.1 variant. In conclusion, these results show an evident reduction of NF-Y binding to the mutated QAP4 Y-box and a decreased mRNA accumulation of the DQA1 alleles regulated by these variants.

In vitro induction of naive cytotoxic T lymphocytes with complexes of peptide and recombinant MHC class I molecules coated onto beads: role of TCR/ligand density

We previously reported that complexes of peptide with soluble single-chain recombinant MHC (SC-MHC) class I molecules are able to induce cytotoxic T lymphocytes (CTL) in vitro in a murine system with an efficiency comparable to that observed with peptide-pulsed dendritic cells as antigen-presenting cells. In this report, we have assessed the capacity of preformed peptide/SC-Kd complexes in monomeric or dimeric form as well as of peptide/SC-Kd-loaded beads to generate in vitro specific CTL responses from naive DBA/2 spleen cells. Peptide/SC-Kd-coated beads were consistently more efficient. We evaluated the role of costimulatory molecules, using monoclonal antibodies anti-CD80 or anti-CD86. In addition, the capacity of peptide/SC-Kd-coated beads to generate a CTL response from purified naive CD8+ T cells was ascertained. Taken together, the results indicate that, under our conditions, CTL priming does not require the participation of co-stimulatory molecules and is the consequence of a direct interaction between the cognate TCR on peptide-specific CTL precursors and the peptide/SC-Kd-loaded beads. Titration of the amount of preformed complexes of SC-Kd and peptide 170-179 of HLA-CW3 that need to be coated onto the beads to prime CTL precursors shows an activation threshold which can be calculated to be between 25000 and 50000 complexes. In effect, in cultures stimulated with specific peptide CW3/SC-Kd complexes representing less than 50% occupancy of the total (10(5)) complexes on the beads, no peptide-specific cytolytic activity was observed. These results suggest that the efficiency of the primary CTL induction depends on the density of specific peptide/SC-Kd complexes present on the beads.

Reduced IL-1 production in adolescents with mite antigen asthma in remission

To determine the immunological mechanisms associated with outgrowing mite antigen-induced bronchial asthma during adolescence, we studied the relationship between clinical status and Dermatophagoides farinae (Df) antigen-induced peripheral cell activation by measuring IL-1alpha and IL-1beta production in patients with bronchial asthma. After antigen-driven restimulation in vitro, there was increased IL-1alpha, IL-1beta production by peripheral blood mononuclear cells (PBMC) from patients with active bronchial asthma, while cellular IL-1alpha, IL-1beta production was reduced in patients with asthma in remission. IL-1alpha and IL-1beta production by PBMC (possibly reflecting airway inflammation) after exposure to Df antigen might be down-regulated in patients outgrowing mite antigen-induced asthma, because lipopolysaccharide-induced IL-1alpha, IL-1beta production (seen in both normal individuals and patients with active asthma) was also reduced when patients were in remission.

Dissociation of intracellular signaling pathways in response to partial agonist ligands of the T cell receptor

The T cell receptor (TCR) is a versatile receptor able to generate different signals that result in distinct T cell responses. The pattern of early signals is determined by the TCR binding kinetics that control the ability of the ligand to coengage TCR and coreceptor. Coengagement of TCR and CD4 results in an agonist signaling pattern with complete tyrosine phosphorylation of TCR subunits, and recruitment and activation of ZAP-70. In contrast, TCR engagement without CD4 coengagement causes a partial agonist type of signaling, characterized by distinct phosphorylation of TCR subunits and recruitment but no activation of ZAP-70. The pathways triggered by partial agonist signaling are unknown. Here, we show that agonists cause association of active lck and active ZAP-70 with p120-GTPase-activating protein (p120-GAP). These associations follow engagement of CD4 or CD3, respectively. In contrast, partial agonists do not activate lck or ZAP-70, but induce association of p120-GAP with inactive ZAP-70. Despite these differences, both agonist and partial agonist signals activate the mitogen-activated protein kinase (MAPK) pathway. However, MAPK activation by partial agonists is transient, supporting a kinetic, CD4-dependent model for the mechanism of action of variant TCR ligands. Transient MAPK activation may explain some of the responses to TCR partial agonists and antagonists.

Effect of ethanol on B cell expression of major histocompatibility class II proteins in immunized mice

Acute or chronic exposure to ethanol (EtOH), as well as other stimuli that induce a neuroendocrine stress response, can decrease the expression of MHC class II proteins (immune-associated antigens, Ia) on B cells and macrophages. In a mouse model for binge drinking, it has been shown that this decrease is caused by EtOH-induced increases in endogenous glucocorticoids. Decreased Ia expression would be expected to suppress T-dependent humoral responses, and such suppression has been noted in our model. However, it has been reported that activated B cells are much less susceptible to glucocorticoid-induced decreases in Ia expression than are resting B cells. Thus, it is not clear that the decreased Ia observed in our previous studies with non-immunized mice could account for decreased humoral responses, because it has not been directly determined that decreased Ia expression occurs in immunized mice. To examine this issue, splenocytes from mice immunized with sheep erythrocytes were studied by flow cytometry. Mice were treated with EtOH by gavage and immunized 12 h later, because our previous results indicate that this produces maximal suppression of the humoral response. In immunized mice, EtOH decreased Ia expression on B cells at 6 and 12 h after immunization, but not at 24 or 74 h. In a dose-response study, a substantial decrease in Ia expression on B cells was observed at an EtOH dosage of 6.0 or 7.0 g/kg. Thus, decreased Ia expression is a potential mechanism for EtOH-induced suppression of the humoral response. A glucocorticoid antagonist (RU 486) partially blocked the EtOH-induced decrease in Ia expression, suggesting that glucocorticoids are involved in the reduction of Ia expression in immunized mice. Direct administration of corticosterone to produce blood levels comparable to those noted in EtOH-treated mice did not significantly decrease Ia expression, but Ia expression tended to be lower in mice treated with corticosterone. Taken together, these results indicate that glucocorticoids play some role in decreasing Ia expression in immunized mice, but they are less important than in non-immunized mice.

In vivo class II transactivator expression in mice is induced by a non-interferon-gamma mechanism in response to local injury

BACKGROUND

Tissue injury induces MHC class II expression, which could be important in the recognition of that tissue as an allograft. The class II transcriptional activator (CIITA) is the major regulator of basal and induced MHC class II expression and is essential for antigen presentation. The role of CIITA in the induction of class II by tissue injury is unknown. In this study, we examined CIITA induction in the course of acute ischemic or toxic renal injury in mice, including the role of interferon (IFN)-gamma and of the transcription factor, interferon regulatory factor (IRF)-1.

METHODS

Kidneys were injured by ischemia or by gentamicin toxicity and were then studied for changes in gene expression using Northern blot, reverse transcriptase-polymerase chain reaction, radioimmunoassay, and tissue staining. We compared wild-type (WT) mice to IFN-gamma knockout (GKO) or IRF-1 knockout mice.

RESULTS

Ischemic injury induced CIITA and class II expression in the kidney, in WT and GKO mice. Gentamicin injury also induced both CIITA and class II expression, independent of IFN-gamma, in WT and GKO mice. After ischemic injury, the induction of class II protein levels and CIITA and class II mRNA levels were induced, to a lesser degree, in IRF-1 knockout mice.

CONCLUSIONS

These data indicate that CIITA is induced by tissue injury, and probably accounts for class II induction during tissue injury. CIITA induction by injury is largely IFN-gamma independent but requires IRF-1. The similarities of the pattern of CIITA and class II induction in ischemic and toxic injury suggest that this is a stereotyped response of injured tissue and not a consequence of a particular mechanism of injury.

HMG-I(Y) phosphorylation status as a nuclear target regulated through insulin receptor substrate-1 and the I4R motif of the interleukin-4 receptor

Interleukin (IL)-4 is a cytokine that regulates both the growth and differentiation of hematopoietic cells. Its ligand binding specificity and important signal transduction mechanisms are conferred by the IL-4 receptor alpha chain (IL-4Ralpha). The I4R is a tyrosine-containing motif within IL-4Ralpha that is critical for proliferative responses to IL-4. Although the I4R also contributes to gene regulation, nuclear targets directly regulated by this motif have not been described. It is shown here that the tyrosine at position 497 in the I4R is critical for regulation of the phosphorylation status of a set of nuclear proteins that includes HMG-I(Y), small non-histone chromosomal proteins involved in the control of gene expression in hematopoietic cell lines. Moreover, IL-4 is unable to induce HMG-I(Y) phosphorylation in insulin receptor substrate-1-deficient cells, and the inhibitor wortmannin completely blocks IL-4 regulation of HMG-I(Y) phosphorylation status but not activation of an IL-4 Stat protein. Taken together, these data indicate that HMG-I(Y) is a nuclear target whose phosphorylation status is regulated through the I4R motif via insulin receptor substrate proteins, independent of activation of the Stat pathway.

Cholera toxin stimulates human B-cell DR antigen biosynthesis at the level of translation

Cholera toxin (CT) exerts many diverse regulatory effects on cells of the immune system and is considered a potent adjuvant on gut mucosal immune responses to orally presented antigens. It has been previously described that CT induces surface DR expression in human resting B-cells. As a further step toward understanding this phenomenon, the molecular mechanisms underlying the regulation of DR expression were investigated. By the use of Western analysis, it is shown that CT increases the total levels of DR protein in highly purified human tonsillar cells. [35S]Methionine incorporation studies show that the aforementioned increase is due to de novo biosynthesis of DR protein at as early as 6 hr after CT stimulation and is completed by 24 hr. [3H]Uridine uptake experiments, nuclear transcription runoff assays, and Northern analysis show that CT does not exert its effect at a transcriptional level, indicating translational regulation. Anti-IgM, which mimics B-cell antigen binding, behaves in a manner similar to CT. The B subunit of CT (B-CT) and prostaglandin E2, either alone or in combination, do not induce DR protein biosynthesis nor do they exert any effect on the transcription of DR beta 1 gene. These results show that cAMP elevation as well as binding of B-CT to GM-1 ganglioside, by themselves, do not lead to DR protein biosynthesis suggesting that other activation pathways may be involved.

In vitro production of antigen-specific T cells from unprimed mice: role of dexamethasone and anti-IL-10 antibodies

We describe here a culture system for studying the development, in vitro, of antigen-specific CD4 T cells from unprimed mice. T cells from young mice are initially exposed to antigen, such as pigeon cytochrome C or keyhole limpet hemocyanin, in the presence of adherent accessory cells and then allowed to proliferate in the absence of antigen but in the presence of IL-2, 10(-8) M dexamethasone, and antibodies to IL-10. Proliferation and IL-2 production by T cells harvested from such expansion cultures are antigen-dependent but not antigen-specific and at different doses can be either stimulated or inhibited both by the priming antigen and by irrelevant proteins. Antigen-specific T cell reactions can be elicited by any of three modifications of the culture protocol: (a) absorption of nonspecific cells on accessory cell monolayers bearing irrelevant proteins; (b) increased doses of dexamethasone during the expansion phase; or (c) a second cycle of antigen activation and antigen-free expansion. These observations provide a foundation for further analysis of in vitro maturation of primary immune responses and suggest an important role for IL-10 and glucocorticoids in regulating the early stages of activation and proliferation by naive T cells.

The regulation of immunological responses to parasitic infections and the development of tolerance

It is well established that specific unresponsiveness to immunization can be induced by prolonged exposure to antigenic proteins. More generally, many parasitic infections, such as the helminth worms and the Leishmania parasites, appear to be able to persist in some of their human hosts over long periods of time, via what appears to be an ability to induce defective or inappropriate T-cell responses (= tolerance). Recent research has suggested that cytokines, produced by specific subsets of CD4+ T-cells (characterized by cytokine secretory profiles and growth properties), have an important, and often complex, role in promoting or inhibiting host protective immunity to parasitic infections. By examination of the population dynamics of the stimulation and regulation of cellular responses to infection, via the use of simple mathematical models, we show that nonlinear interactions between CD4+ T-cell subsets and their secreted cytokines can result in either host protection or immunological unresponsiveness, depending on the magnitude and duration of exposure to parasitic infection. Analyses also identify a possible mechanism to explain the stimulation of two separate peaks of enhanced T-cell-mediated responses over a wide range of levels of antigenic exposure.