The Immune Response Is Initiated by Dendritic Cells via Interaction with Microorganisms and Interleukin-2 Production
- Francesca Granucci,
- Sonia Feau,
- Ivan Zanoni,
- Norman Pavelka,
- Caterina Vizzardelli,
- Giorgio Raimondi and
- Paola Ricciardi-Castagnoli
- Reprints or correspondence: Dr. Paola Ricciardi-Castagnoli, University of Milano- Bicocca, Dept. of Biotechnology and Bioscience, Piazza della Scienza 2, Milan, Italy (paola.castagnoli{at}unimib.it).
Abstract
The immune system of vertebrate animals is characterized by the capacity to respond to disturbances. This function requires 2 different approaches. First, the immune system responds in a few hours to infectious agents (innate immunity) by recognizing molecular patterns typical of microorganisms (but absent in selftissues). Second, it mounts a late response that differentiates among different microbes, giving rise to memory (adaptive immunity). In this context, dendritic cells (DCs) play a central role, becoming efficient stimulators of both innate and adaptive responses after microbial activation. Recent data generated by global transcriptional profiling of DCs after bacterial encounter are discussed, as are the unique DC functional plasticity and the central role of DC-derived interleukin-2 in priming early and late immune responses.
The recent increased interest in the interaction between dendritic cells (DCs) and pathogens both in vitro and in vivo may lead to answers in the next few years to many questions regarding how an immune response to infectious agents is induced and how to intervene when potentiation or inhibition are required. Questions include the identification of the receptors involved in the internalization of bacteria and/or in the activation of DCs, the intracellular signaling routes in DCs, and the strategies whereby microorganisms can evade or impede DCs function, thus escaping immune recognition.
DCs are professional antigen-presenting cells that can initiate the innate and adaptive immune responses [1, 2]. Immature DCs are strategically located in tissues that represent pathogen entry routes, where they continuously monitor the environment through the uptake of both particulate and soluble products. DC maturation is associated with enhanced production of inflammatory cytokines and chemokines, with reduced endocytic and phagocytic capacity, and with acquisition of migratory functions that allow antigenloaded DCs to move from the marginal zones to the T cell areas or from nonlymphoid to lymphoid tissues. Mature DCs and those that have migrated have high cell surface major histocompatibility complex (MHC) and costimulatory protein expression, the ability to activate both CD8 and CD4 T cell responses [3, 4], and they undergo cytoskeleton rearrangements that lead to the inhibition of the phagocytic activity and are programmed for apoptotic death [5].
The process of DC maturation can be reproduced in vitro [5]. Immature bone marrow—derived mouse DCs can be induced to mature in vitro by using a number of microorganisms, including parasites, live bacteria, bacteria cell products (e.g., lipopolysaccharide [LPS] and lipoteichoic acid, bacterial DNA, or cytokines).
Intracellular Pathways that Lead to DC Activation after Microbial Encounter
Toll-like receptors (TLRs) mediate the intracellular signaling of microbial products, such as LPS [6], the complex LPS-CD14, and LPS-binding protein [7]. A constitutive active form of human TLR4 results in the activation of NF-κB and in the expression of several proinflammatory cytokines.We found that in DCs the maturation process induced by LPS is mediated by NF-κB and inhibition of NF-κB blocks phenotypic maturation of DCs [8]. NF-κB activation is also induced by whole bacteria with the same kinetics but with higher intensity [9]. Three of 5 NF-κB family members are rapidly recruited after DC activation (30 min), whereas RelB is translocated into the nucleus only 4 h after bacterial encounter, which suggests that a role exists for RelB in late events of DC maturation (i.e., antigen presentation and/or DC migration). Of interest, activation of DCs by LPS promotes survival of the cells in a growth-arrested state after deprivation of growth factors [8]. This response is dependent on mitogen-activated protein kinases of the ERK (extracellular signal-regulated kinase) family but the antiapoptotic mediators remain to be identified.
DC maturation has been investigated at the molecular level by use of oligonucleotide microarray approaches. Microarrays have been used to measure gene expression of human monocyte-derived DCs (hMDCs) in response to different types of stimuli, such as gram-negative bacteria (Escherichia coli), yeast (Candida albicans), and viruses (influenza virus). With all 3 pathogens, there is rapid down-regulation of genes associated with phagocytosis and pathogen recognition and a transient increase of transcripts for cytokines, chemokines, and receptors that contribute to the recruitment of leukocytes at the infection site [10]. Together with a common response to the different pathogens, a microbe-specific response is observed in most functional category genes.
Stimulus-specific activation and maturation of hMDCs clearly indicate plasticity in their functions. Plasticity is an important DC feature, since immune response outcome depends on their functional state [11]. Pathogen-specific DCs functional plasticity has been documented in vivo: at the yeast stage, C. albicans stimulates DCs to produce interleukin (IL)-12 and induce Th1 responses, whereas, at the hyphae stage, C. albicans stimulates DCs to produce IL-4 and Th2 responses [12].
IL-2 Mediates Adjuvant Effect of DCS
To identify genes relevant for the DC maturation process, in the mouse system we have done a kinetic study of DC gene expression after gram-negative bacterial and LPS activation. (The entire transcriptome analysis can be found at http://www.btbs.unimib.it/DCgenes.) These studies showed that, during the maturation process, DCs undergo a sequence of precise transitional stages characterized by waves of transcriptional activity [13, 14]. The most unanticipated finding of global gene expression analysis on maturing mouse DCs was that they produce IL-2 in a tightly regulated time frame after bacterial activation [13]. IL-2—deficient DCs were severely impaired in their ability to prime CD4 and CD8 T cells in mixed lymphocyte reactions, compared with IL-2—sufficient DCs [13]. Thus, the adjuvant property of bacteria is explained by inducing in DCs not only the up-regulation of costimulatory surface proteins and the maximization of the efficiency in presenting antigens, as suggested elsewhere [1], but also by inducing the production of costimulatory molecules, such as IL-2. This seems to be a unique feature of DCs as macrophages are unable to produce IL-2 after bacterial activation.
Two waves of IL-2 production by DCs after bacterial encounter have been observed: one 4–8 h after bacterial uptake and the second 14–18 h after activation. This timing is compatible with the appearance of MHC class I and class II peptide complexes at the cell surface [15]. Of note, DCs can present exogenous captured antigens to CD4 T cells in a few hours, whereas ⩾8 h are required to process and present bacterial antigens in association with MHC class I molecules [15]. Thus, early activated DCs are perfectly equipped to prime CD4 T cells, despite their relatively low levels of MHC and membrane-associated costimulatory molecules and expression of T cell inhibitory cytokines, such as IL-10 [16]. At later time points, IL-2 could represent a key costimulatory protein in activating CD8 T cells, even though DCs have not yet reached their terminal maturation stage. These data could explain the ability of activated DCs to prime CD8 T cells in a CD4-independent manner [3]. Moreover, exogenous sources of IL-2 for induction of T cell proliferation may be also required when the frequency of responder antigen-specific T cells or their affinity for MHCpositive peptide complexes is low, because it can frequently happen in vivo during immune responses to microorganisms.
The ability of DCs to rapidly respond to microbial interaction with IL-2 production is also shown with parasites (e.g., Leishmania mexicana) and worms (e.g., Schistosoma species; authors' unpublished data). Of interest, only inflammatory stages of these two organisms (Schistosoma eggs or Leishmania promastigote) can induce IL-2 transcription in DCs, whereas the noninflammatory forms (the Schistosomula and the Leishmania amastigote) do not show this property (figure 1).
Figure 1.
Interleukin-2 mRNA gene chip levels in dendritic cells expressed in arbitrary units after activation with different parasites at the indicated time points. Egg, Schistosoma eggs; LeishA, Leishmania amastigote; LeishP, Leishmania promastigote; Mock, untreated; SLA, Schistosomula.
BActeria Drive DCS Toward a Maturation Stage Appropriate for Immune Response Activation
Genes involved in the antigen presentation function are also regulated in LPS or bacterially activated DCs. As for MHC class II genes, the up-regulation of MHC class II protein synthesis is rapid, peaking as early as 1 h after DC activation; this is followed by a striking down-regulation [15]. The microarray approach validated this observation, showing that MHC class II molecule mRNAs were down-regulated after activation. Moreover, in agreement with the increased stability observed in peptide—MHC class II complexes in activated DCs, the H-2M molecules, which regulate MHC class II loading with antigenic peptides, were down-regulated as early as 6 h after LPS stimulation. Thus, activation stimuli are sufficient to drive DCs toward a maturation stage appropriate for the immune response activation.
We have shown that model antigens expressed in recombinant gram-positive and gram-negative bacteria can be presented on both MHC I and II molecules [15, 17]. The efficiency of presentation on MHC I molecules is 106 times higher than presentation of the soluble protein [15]. Unlike macrophages, this exogenous pathway of MHC I presentation is transporter-associated with antigen presentation (TAP) and is proteasome dependent [15, 18]. This implies that exogenous bacterial antigens introduced by phagocytosis are directed into the classical MHC I pathway. Indeed, transport of whole bacterial proteins from phagolysosome to cytosol occurs after bacterial phagocytosis as it happens after immune-complex internalization [19].
The capacity of DCs to present bacterial antigens with very high efficiency on both MHC I and II molecules can be exploited to induce strong and long-lasting immunity toward bacteria, as well as nominal antigens of interest. Several examples of partial protective immune responses achieved in vivo by injecting DCs loaded with microbes have been described against Chlamydiae trachomatis [20], Borrelia burgdorferi [21], and Mycobacterium tuberculosis [22].
Presentation of recombinant bacterial proteins by DCs leads to induction of cytotoxic T lymphocyte responses in vivo and to self-antigens genetically introduced into bacteria. We also found that the PA28 proteasome activator and the TAP-1 mRNA molecules were clearly induced. The PA28 protein dramatically increases the spectrum of peptides produced and the efficiency of the 20S proteasome, whereas the TAP-1 molecule is required to transfer proteasome-produced peptides from the cytosol to the endoplasmic reticulum. The proteasome activator was up-regulated 6 h after LPS activation, but TAP-1 up-regulation was measurable only at later time points. This pattern of mRNAs up-regulation correlates well with the kinetics of new MHC class I biosynthesis that peaks 18 h after bacteria or LPS stimulation [15]. Thus, DC priming induces a remarkable activation of the entire intracellular apparatus necessary for class I and class II antigen presentation function.
Is IL-2 the Molecular Mediator that Bridges the Innate and the Adaptive Immune Responses?
IL-2 is thought to activate NK cells in vitro. However, this effect has never been considered relevant in vivo during immuno-competent responses. It was commonly believed that IL-2 was exclusively produced by T cells during the acquired immune response, whereas cell activation occurred earlier during the innate response [23]. Because DCs can produce IL-2 early after activation, this assumption should be revised (figure 2). It is well established that DCs can activate NK cell responses in direct NK-DC interactions [24. Thus, IL-2 is likely the relevant and obvious costimulatory factor. In agreement with this assumption, IL-2—deficient DCs were inefficient in activating NK cell responses in vitro and in vivo (authors' unpublished data).
Figure 2.
Temporal regulation of dendritic cell (DC) functions. Early activated DCs produce interleukin-2 (IL-2) that may be required to activate NK cells (innate immune response). From 4 to 8 h after microbial activation, transitional DCs that express low levels of costimulatory molecules and peptide—major histocompatibility complexes (MHC) increase the processing machinery efficiency and still secrete IL-2 that allows them to prime T cells, although they have not reached yet the terminal maturation stage. At later time points, fully mature DCs express high levels of costimulatory molecules and peptide-MHC complexes to prime T cell responses in the absence of IL-2 (adaptive immune response). IL-2 may also act on DCs in an autocrine way. CTL, cytotoxic T lymphocyte. Gram+, gram-positive; gram−, gram-negative.
Footnotes
-
Financial support: Italian Association for Cancer Research; 5th European Community Program (dentritic cell strategies).
- © 2003 by the Infectious Diseases Society of America
References
This Article
- AbstractFree
- » Full Text (HTML)Free
- Full Text (PDF)Free
Classifications
-
- Signal Transduction Pathway
Services
Citing Articles
Google Scholar
PubMed
Related Content
Share
- Email this article
-
CiteULike
Connotea
Delicious
Facebook
Google+
Mendeley
Twitter
Published on behalf of
IDSA Members: For your free access to this journal, log in via the IDSA members area.
The Journal
Impact Factor: 6.288
5-Yr impact factor: 6.103
For The Media
For Authors
- Services for authors
- Self-archiving policy
- IDSA Conflict of Interest policy
- Instructions for Authors
- ICMJE Form
- Submit Now!
Open access options for authors visit Oxford Open
This journal enables compliance with the NIH Public Access Policy
Corporate Services
Alerting Services
Related IDSA Publications
