Influence of Mycobacterium bovis bacillus Calmette Guérin on in vitro induction of CD1 molecules in human adherent mononuclear cells

CD1a-positive dendritic cell density predicts disease-free survival in papillary thyroid carcinoma

OBJECTIVES

Dendritic cells are common in inflammatory processes and in papillary thyroid carcinoma (PTC). Previous studies of the predictive value of S100-positive dendritic cell density for PTC outcome yielded inconsistent results. This study investigated the association of dendritic cell density and PTC recurrence based on CD1a expression.

METHODS

Representative slides from 56 consecutive specimens of PTC were immunostained with anti-CD1a antibodies, and dendritic cell density was analyzed by disease-free survival.

RESULTS

Dendritic cells were abundant in the tumoral tissue and sparse in the normal peritumoral tissue. Peritumoral dendritic cell density >1.1 cells/HPF was inversely associated with the risk of recurrence. Similar results were obtained with tumoral dendritic cell density (>12 cells/HPF), although the statistical significance was marginal.

CONCLUSIONS

High CD1a-positive dendritic cell density is associated with improved disease-free survival in PTC. The specificity of anti-CD1a immunostain for activated dendritic cells may explain the better outcome prediction in this study than in studies using S100 protein.

Role of host- and pathogen-associated lipids in directing the immune response in mycobacterial infections, with emphasis on Mycobacterium avium subsp. paratuberculosis

Mycobacteria have a complex cell wall with a high lipid content that confers unique advantages for bacterial survival in the hostile host environment, leading to long-term infection. There is a wealth of evidence suggesting the role cell wall-associated lipid antigens play at the host-pathogen interface by contributing to bacterial virulence. One pathway that pathogenic mycobacteria use to subvert host immune pathways to their advantage is host cholesterol/lipid homeostasis. This review focuses on the possible role of pathogen- and host-associated lipids in the survival and persistence of pathogenic mycobacteria with emphasis on Mycobacterium avium subsp. paratuberculosis. We draw upon literature in diverse areas of infectious and metabolic diseases and explain a mechanism by which mycobacterial-induced changes in the host cellular energy state could account for phenomena that are a hallmark of chronic mycobacterial diseases.

Exogenous control of the expression of Group I CD1 molecules competent for presentation of microbial nonpeptide antigens to human T lymphocytes

Group I CD1 (CD1a, CD1b, and CD1c) glycoproteins expressed on immature and mature dendritic cells present nonpeptide antigens (i.e., lipid or glycolipid molecules mainly of microbial origin) to T cells. Cytotoxic CD1-restricted T lymphocytes recognizing mycobacterial lipid antigens were found in tuberculosis patients. However, thanks to a complex interplay between mycobacteria and CD1 system, M. tuberculosis possesses a successful tactic based, at least in part, on CD1 downregulation to evade CD1-dependent immunity. On the ground of these findings, it is reasonable to hypothesize that modulation of CD1 protein expression by chemical, biological, or infectious agents could influence host's immune reactivity against M. tuberculosis-associated lipids, possibly affecting antitubercular resistance. This scenario prompted us to perform a detailed analysis of the literature concerning the effect of external agents on Group I CD1 expression in order to obtain valuable information on the possible strategies to be adopted for driving properly CD1-dependent immune functions in human pathology and in particular, in human tuberculosis.

CD1-restricted adaptive immune responses to Mycobacteria in human group 1 CD1 transgenic mice

Group 1 CD1 (CD1a, CD1b, and CD1c)–restricted T cells recognize mycobacterial lipid antigens and are found at higher frequencies in Mycobacterium tuberculosis (Mtb)–infected individuals. However, their role and dynamics during infection remain unknown because of the lack of a suitable small animal model. We have generated human group 1 CD1 transgenic (hCD1Tg) mice that express all three human group 1 CD1 isoforms and support the development of group 1 CD1–restricted T cells with diverse T cell receptor usage. Both mycobacterial infection and immunization with Mtb lipids elicit group 1 CD1–restricted Mtb lipid–specific T cell responses in hCD1Tg mice. In contrast to CD1d-restricted NKT cells, which rapidly respond to initial stimulation but exhibit anergy upon reexposure, group 1 CD1–restricted T cells exhibit delayed primary responses and more rapid secondary responses, similar to conventional T cells. Collectively, our data demonstrate that group 1 CD1–restricted T cells participate in adaptive immune responses upon mycobacterial infection and could serve as targets for the development of novel Mtb vaccines.

The surprising diversity of lipid antigens for CD1-restricted T cells

CD1 proteins have been conserved throughout mammalian evolution and function to present lipid antigens to T cells. Crystal structures of CD1-lipid complexes show that CD1 antigen-binding grooves are composed of four pockets and two antigen entry portals. This structural information now provides a detailed understanding of how CD1-binding grooves capture a surprisingly diverse array of lipid ligands. CD1-expressing APCs are able to acquire lipid antigens from their own pool of lipids and from exogenous sources, including microbial pathogens, bystander cells, or even the systemic circulation. CD1 proteins bind to certain antigens using high stringency loading reactions within endosomes that involve low pH, glycosidases, and lipid transfer proteins. Other antigens can directly load onto CD1 proteins using low stringency mechanisms that are independent of cellular factors. New evidence from in vivo systems shows that CD1-restricted T cells influence outcomes in infectious, autoimmune, and allergic diseases. These studies lead to a broader view of the natural function of alphabeta T cells, which involves recognition of both cellular proteins and lipids.

Host-derived oxidized phospholipids and HDL regulate innate immunity in human leprosy

Intracellular pathogens survive by evading the host immune system and accessing host metabolic pathways to obtain nutrients for their growth. Mycobacterium leprae, the causative agent of leprosy, is thought to be the mycobacterium most dependent on host metabolic pathways, including host-derived lipids. Although fatty acids and phospholipids accumulate in the lesions of individuals with the lepromatous (also known as disseminated) form of human leprosy (L-lep), the origin and significance of these lipids remains unclear. Here we show that in human L-lep lesions, there was preferential expression of host lipid metabolism genes, including a group of phospholipases, and that these genes were virtually absent from the mycobacterial genome. Host-derived oxidized phospholipids were detected in macrophages within L-lep lesions, and 1 specific oxidized phospholipid, 1-palmitoyl-2-(5,6-epoxyisoprostane E2)-sn-glycero-3-phosphorylcholine (PEIPC), accumulated in macrophages infected with live mycobacteria. Mycobacterial infection and host-derived oxidized phospholipids both inhibited innate immune responses, and this inhibition was reversed by the addition of normal HDL, a scavenger of oxidized phospholipids, but not by HDL from patients with L-lep. The accumulation of host-derived oxidized phospholipids in L-lep lesions is strikingly similar to observations in atherosclerosis, which suggests that the link between host lipid metabolism and innate immunity contributes to the pathogenesis of both microbial infection and metabolic disease.

CD1-restricted T cells in host defense to infectious diseases

CD1 has been clearly shown to function as a microbial recognition system for activation of T cell responses, but its importance for mammalian protective responses against infections is still uncertain. The function of the group 1 CD1 isoforms, including human CD1a, CDlb, and CDLc, seems closely linked to adaptive immunity. These CD1 molecules control the responses of T cells that are highly specific for particular lipid antigens, the best known of which are abundantly expressed by pathogenic mycobacteria such as Mycobacterium tuberculosis and Mycobacterium leprae. Studies done mainly on human circulating T cells ex vivo support a significant role for group I CD1-restricted T cells in protective immunity to mycobacteria and potentially other pathogens, although supportive data from animal models is currently limited. In contrast, group 2 CD1 molecules, which include human CD1d and its orthologs, have been predominantly associated with the activation of CD1d-restricted NKT cells, which appear to be more appropriately viewed as a facet of the innate immune system. Whereas the recognition of certain self-lipid ligands by CD d-restricted NKT cells is well accepted, the importance of these T cells in mediating adaptive immune recognition of specific microbial lipid antigens remains controversial. Despite continuing uncertainty about the role of CD 1d-restricted NKT cells in natural infections, studies in mouse models demonstrate the potential of these T cells to exert various effects on a wide spectrum of infectious diseases, most likely by serving as a bridge between innate and adaptive immune responses.

BCG-infected adherent mononuclear cells release cytokines that regulate group 1 CD1 molecule expression

Increasing evidence is now available showing that CD1-restricted T cell responses against non-peptide mycobacterial antigens could play a role in the immune resistance against tuberculosis. BCG, widely used in anti-tubercular vaccination, shares various constituents with Mycobacterium tuberculosis, but does not provide full protection. In the present study we have investigated the pattern of group 1 CD1 molecule expression in adherent mononuclear cells (AMNC) of human peripheral blood, infected in vitro with BCG. Shortly after exposure to BCG, both BCG-positive and BCG-negative AMNC showed a moderate CD1 expression elicited by BCG-induced release of GM-CSF presumably acting through an autocrine and a paracrine mechanism. This was demonstrated using two-color flow cytometry with green fluorescent BCG and anti-CD1 PE-labeled antibodies. However, high CD1 expression induced by exogenously added GM-CSF in AMNC was reduced if target cells were cocultivated with BCG. Monoclonal antibodies against IL-10 partially restored CD1 expression, thus showing that IL-10, released from infected AMNC, is involved, at least in part, in CD1 negative modulation. Therefore, through a complex cytokine network, including not yet identified factor(s), BCG triggers but does not allow full expression of CD1 on AMNC. It cannot be excluded that this mechanism could play a role in the limited efficiency of BCG vaccination.

Inhaled allergen-driven CD1c up-regulation and enhanced antigen uptake by activated human respiratory-tract dendritic cells in atopic asthma

BACKGROUND

Dendritic cells (DC) mediate inflammation in rodent models of allergic airway disease, but the role played by human respiratory-tract DC (hRTDC) in atopic asthma remains poorly defined. Recent data suggest that CD1 antigen presentation by hRTDC may contribute to asthma pathogenesis.

OBJECTIVE

To investigate the influence of hRTDC on the balance between atopy and allergic asthma in human subjects and to determine whether CD1 expression by hRTDC is modulated during asthmatic inflammation.

METHODS

Sputum cells were induced from steroid-naïve, allergen-challenged and allergen-naïve subjects (atopic asthmatics, atopic non-asthmatics and non-atopic controls). hRTDC were identified using monoclonal antibody labelling and analysis by flow cytometry.

RESULTS

hRTDC stained HLA-DR(+) (negative for markers of other cell lineages) were predominantly myeloid and comprised approximately 0.5% of viable sputum cells. Sputum cells were potent stimulators of allogeneic CD4(+) naïve T cells and enrichment/depletion experiments correlated stimulatory potency with DC numbers. Sputum contained cells that exhibited typical dendritic morphology when analysed by electron microscopy. Myeloid hRTDC were endocytically active, but uptake of FITC-dextran was enhanced in cells from asthmatics (P<0.001). Despite their increased endocytic capacity, asthmatic myeloid hRTDC appeared mature and expressed increased levels of maturation markers (P<0.05-P<0.001), CD1c, CD1d and langerin (P<0.05). CD1c expression by asthmatic myeloid hRTDC was enhanced upon in vivo allergen challenge (three to ninefold within 24 h; P<0.05). CD11c(-)CD123(high) hRTDC were only detected in asthmatic sputum and were increased in number following allergen challenge.

CONCLUSION

Despite limited cell numbers, it proved possible to analyse human RTDC in induced sputum, providing evidence that increased antigen uptake and enhanced CD1 presentation by activated hRTDC may contribute to allergic airway disease. CD1 presentation by hRTDC in atopic asthma may therefore constitute a novel target for future intervention strategies.

Activating Transcription Factor/cAMP Response Element Binding Protein Family Member Regulated Transcription of CD1A

CD1a has a unique expression pattern among Ag-presenting molecules, expressed specifically on cortical thymocytes and APCs. As autoimmune disease, infection, and tumors can all result in alteration of CD1a expression, we are attempting to characterize the transcriptional regulation, and thus shed some light on specific expression, of CD1A. In this study, we have identified a minimal proximal promoter region required for CD1A transcription. Computer searches within this region identified numerous potential binding sites for lymphoid-specific transcription factors, including the ETS transcription factors, C/EBP, GATA, and CREB. Deletion and site-specific mutant analysis revealed a critical role of a potential cAMP response element (CRE) 965 bp upstream of the CD1A translation start site. Two activating transcription factor (ATF)/CREB family members, CREB-1 and ATF-2, are able to bind this site in vitro and in vivo. Notably, activation of ATF/CREB family members decreases CD1A transcription, while decrease in ATF-2 expression results in increased CD1A RNA level. The fact that these factors also bind the CD1A promoter in human monocytes strongly suggests a role for ATF/CREB family members in regulation of CD1A expression.

TLR7/8 agonists impair monocyte-derived dendritic cell differentiation and maturation

Pathogen recognition by TLR activates the innate immune response and is typically followed by the development of an adaptive immune response initiated by antigen presentation. Dendritic cells (DC) are the most efficient APC and express diverse TLRs, including TLR7 and -8, which have been recently identified as targets for ssRNA recognition during viral infection. We have studied the effect of TLR7/8 agonists on DC differentiation and maturation from human monocytes. The synthetic agonist Resiquimod (R-848) or the physiological agonist ssRNA impaired monocyte differentiation to DC phenotypically and functionally. Induced expression of the nonclassical MHC molecules of the CD1 family in DC was inhibited at the protein and mRNA levels, and antigen acquisition was inhibited. Proinflammatory cytokine (including IL-6, IL-8, TNF-alpha, IL-1beta) and IL-10 production were induced during DC differentiation. Cross-talk between TLR4 and TLR7/8 was revealed as immature DC, which had been differentiated in the presence of R-848 were insensitive to LPS-mediated maturation and cytokine production but still induced allostimulation. These data lead us to suggest that ongoing viral activation of TLR7/8 could alter the adaptive immune response by modifying DC differentiation and by down-regulating DC responsiveness to a subsequent bacterial TLR4-mediated signal.

Mycobacterium tuberculosis Regulates CD1 Antigen Presentation Pathways through TLR-2

Mycobacterium tuberculosis remains a major pathogen of worldwide importance, which releases lipid Ags that are presented to human T cells during the course of tuberculosis infections. Here we report that cellular infection with live M. tuberculosis or exposure to mycobacterial cell wall products converted CD1- myeloid precursors into competent APCs that expressed group 1 CD1 proteins (CD1a, CD1b, and CD1c). The appearance of group 1 CD1 proteins at the surface of infected or activated cells occurred via transcriptional regulation, and new CD1 protein synthesis and was accompanied by down-regulation of CD1d transcripts and protein. Isolation of CD1-inducing factors from M. tuberculosis using normal phase chromatography, as well as the use of purified natural and synthetic compounds, showed that this process involved polar lipids that signaled through TLR-2, and we found that TLR-2 was necessary for the up-regulation of CD1 protein expression. Thus, mycobacterial cell wall lipids provide two distinct signals for the activation of lipid-reactive T cells: lipid Ags that activate T cell receptors and lipid adjuvants that activate APCs through TLR-2. These dual activation signals may represent a system for selectively promoting the presentation of exogenous foreign lipids by those myeloid APCs, which come into direct contact with pathogens.

TLR activation triggers the rapid differentiation of monocytes into macrophages and dendritic cells

Leprosy enables investigation of mechanisms by which the innate immune system contributes to host defense against infection, because in one form, the disease progresses, and in the other, the infection is limited. We report that Toll-like receptor (TLR) activation of human monocytes induces rapid differentiation into two distinct subsets: DC-SIGN+ CD16+ macrophages and CD1b+ DC-SIGN- dendritic cells. DC-SIGN+ phagocytic macrophages were expanded by TLR-mediated upregulation of interleukin (IL)-15 and IL-15 receptor. CD1b+ dendritic cells were expanded by TLR-mediated upregulation of granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptor, promoted T cell activation and secreted proinflammatory cytokines. Whereas DC-SIGN+ macrophages were detected in lesions and after TLR activation in all leprosy patients, CD1b+ dendritic cells were not detected in lesions or after TLR activation of peripheral monocytes in individuals with the progressive lepromatous form, except during reversal reactions in which bacilli were cleared by T helper type 1 (TH1) responses. In tuberculoid lepromatous lesions, DC-SIGN+ cells were positive for macrophage markers, but negative for dendritic cell markers. Thus, TLR-induced differentiation of monocytes into either macrophages or dendritic cells seems to crucially influence effective host defenses in human infectious disease.

CD1: antigen presentation and T cell function

This review summarizes the major features of CD1 genes and proteins, the patterns of intracellular trafficking of CD1 molecules, and how they sample different intracellular compartments for self- and foreign lipids. We describe how lipid antigens bind to CD1 molecules with their alkyl chains buried in hydrophobic pockets and expose their polar lipid headgroup whose fine structure is recognized by the TCR of CD1-restricted T cells. CD1-restricted T cells carry out effector, helper, and adjuvant-like functions and interact with other cell types including macrophages, dendritic cells, NK cells, T cells, and B cells, thereby contributing to both innate and adaptive immune responses. Insights gained from mice and humans now delineate the extensive range of diseases in which CD1-restricted T cells play important roles and reveal differences in the role of CD1a, CD1b, and CD1c in contrast to CD1d. Invariant TCR alpha chains, self-lipid reactivity, and rapid effector responses empower a subset of CD1d-restricted T cells (NKT cells) to have unique effector functions without counterpart among MHC-restricted T cells. This review describes the function of CD1-restricted T cells in antimicrobial responses, antitumor immunity, and in regulating the balance between tolerance and autoimmunity.

Expansion of human gamma/delta T cells in vitro is differentially regulated by the measles virus glycoproteins

Impaired proliferative response of lymphocytes after mitogenic stimulation ex vivo is a key feature of the generalized immunosuppression induced by measles virus (MV). Compelling evidence suggests that negative signalling by the MV glycoprotein (gp) complex and the surface of uninfected lymphocytes is essential for this effect. So far, the inhibitory activity of this complex applied to all lymphocyte subpopulations irrespective of the mode of stimulation and could not be overcome by external stimulation. This study shows that the isopentenyl pyrophosphate (IPP)/IL-2-stimulated expansion of human gamma/delta T cell receptor (TCR) T cells from peripheral blood mononuclear cells (PBMCs) is inhibited efficiently when the MV gp complex is expressed on the surface of persistently MV-infected T or monocytic cells. In contrast, persistently infected B cells or infected human dendritic cells (DCs) do not interfere with expansion of gamma/delta TCR T cells from PBMCs. These particular two cell populations, however, efficiently inhibit IPP/IL-2-stimulated expansion of gamma/delta TCR T cells from purified T cells and this is reverted by resubstitution with monocytes. As revealed by filter experiments, cocultivation with B cells and DCs empower monocytes, at least partially by soluble mediators, to provide membrane contact-dependent costimulatory signals that neutralize the inhibitory effect of the MV gp complex. Thus, gamma/delta TCR T cells are sensitive to MV gp-mediated inhibition; however, this is overcome efficiently by signals delivered from monocytes conditioned by B cells and DCs.

Mycobacterium and the coat of many lipids

Pathogenic Mycobacterium reside inside vacuoles in their host macrophages. These vacuoles fail to fuse with lysosomes yet interact with early endosomes. Glycoconjugates released by the intracellular bacilli traffic through the host cell and are released through exocytosis. These molecules represent both antigens for immune recognition and modulators of immune function. The molecules play key roles in the induction and maintenance of the granuloma, a tissue response that limits bacterial spread yet ensures persistence of the infection.