Natural killer T cells mediate inflammation in the bile ducts

Cholangiocytes function as antigen-presenting cells with CD1d-dependent activation of natural killer T (NKT) cells in vitro. NKT cells may act both pro- and anti-inflammatory in liver immunopathology. We explored this immune pathway and the antigen-presenting potential of NKT cells in the bile ducts by challenging wild-type and Cd1d^−/− mice with intrabiliary injection of the NKT cell activating agent oxazolone. Pharmacological blocking of CD1d-mediated activation was performed with a monoclonal antibody. Intrabiliary oxazolone injection in wild-type mice caused acute cholangitis with significant weight loss, elevated serum levels of alanine transaminase, aspartate transaminase, alkaline phosphatase and bilirubin, increased histologic grade of cholangitis and number of T cells, macrophages, neutrophils and myofibroblasts per portal tract after 7 days. NKT cells were activated after intrabiliary injection of oxazolone with upregulation of activation markers. Cd1d^−/− and wild-type mice pretreated with antibody blocking of CD1d were protected from disease. These findings implicate that cells in the bile ducts function as antigen-presenting cells in vivo and activate NKT cells in a CD1d-restricted manner. The elucidation of this biliary immune pathway opens up for potentially new therapeutic approaches for cholangiopathies.     

Myeloid blood dendritic cells and monocyte-derived dendritic cells differ in their endocytosing capability

Human dendritic cells (DCs) constitute a heterogeneous population of antigen-presenting cells characterized by a unique capacity to stimulate naïve T cells. The functions of DCs depend on the particular subset and in this study we compare two types of myeloid DCs: freshly isolated blood mDCs and in vitro generated monocyte-derived DCs (MoDCs), in their ability to accomplish endocytosis.     

Astragalus-mediated stimulation on antigen-presenting cells could result in higher IL-21 production from CXCR5+ Tfh-like cells and better IL-21-mediated effector functions

T cells in renal cell carcinoma (RCC) patients display multiple features of impairment and exhaustion. Here, we hypothesize that Astragalus membranaceus, a herbal medicine commonly used to accompany chemotherapy, might have adjuvating effects on T cells from RCC patients. To investigate this, circulating T cells from healthy individuals and RCC patients were cocultured ex vivo with aqueous extract from Astragalus. Functional characteristics of T cells in the absence and presence of Astragalus extract were then compared. We first identified a downregulation of IL-21 expression in RCC patients in association with a functional dysregulation of CXCR5⁺ Tfh-like cells. Astragalus extract could significantly increase IL-21 expression in a dose-dependent manner. This Astragalus-mediated effect depended on the presence of antigen-presenting cells (APCs), as purified CXCR5⁺ Tfh-like cells presented little IL-21 upregulation following Astragalus stimulation. APCs primed by Astragalus extract also promoted IL-21 expression from Tfh-like cells. Interestingly, Astragalus-stimulated Tfh-like cells presented enhanced helper function and resulted in higher humoral responses and better CD8 T cell survival. This effect was dependent on the presence of IL-21. Overall, these data indicated that Astragalus could enhance IL-21 production and effector function from CXCR5⁺ Tfh-like cells in a manner that depended on the presence of APCs.     

Low-dose antigen-experienced CD4+ T cells display reduced clonal expansion but facilitate an effective memory pool in response to secondary exposure

The strength and duration of an antigenic signal at the time of initial stimulation were assumed to affect the development and response of effectors and memory cells to secondary stimulation with the same antigen. To test this assumption, we used T-cell receptor (TCR)-transgenic CD4⁺ T cells that were stimulated in vitro with various antigen doses. The primary effector CD4⁺ T cells generated in response to low-dose antigen in vitro exhibited reduced clonal expansion upon secondary antigenic exposure after adoptive transfer to hosts. However, the magnitude of their contraction was much smaller than both those generated by high-dose antigen stimulation and by naïve CD4⁺ T cells, resulting in higher numbers of antigen-specific CD4⁺ T cells remaining until the memory stage. Moreover, secondary effectors and memory cells developed by secondary antigen exposure were not functionally impaired. In hosts given the low-dose antigen-experienced CD4⁺ T cells, we also observed accelerated recall responses upon injection of antigen-bearing antigen-presenting cells. These results suggest that primary TCR stimulation is important for developing optimal effectors during initial antigen exposure to confer long-lasting memory CD4⁺ T cells in response to secondary exposure.     

Temperature-induced stress abrogates co-stimulatory function in antigen-presenting cells

This study examines the effect of heat-induced stress on the function of splenic antigen-presenting cells. Heat shock (at 41°C, 45°C) profoundly inhibits the ability of this population to stimulate antigen-specific proliferation in ex vivo, or short-term cultures of T cells. This inhibition is not due to the release of suppressor factors from the antigen-presenting cells, nor to a lack of interleukin 1. Comparison of the effect of heat shock on presentation to a T cell hybridoma suggest that heat shock may affect the ability of antigen-presenting cells to deliver a co-stimulatory signal to the T cell. The implication of these findings in terms of the regulation of autoimmune phenomena is discussed.     

Comparison of Monocytes and B Cells for Activation of Human T Helper Cell Subsets

Human rye grass allergenLol pI-specific T helper cell clones of Thp, Th0, Th1 and Th2 subtype were activated withLol pI and monocytes or B cells as antigen-presenting cells, and cell proliferation, interleukin (IL)-2, interferon-γ, and IL-4 secretion were measured. Monocytes induced activation of T cell clones of all four T helper cell subsets and were usually more potent antigen-presenting cells than B cells. B cells and monocytes similarly induced proliferation and IL-4 secretion by Th2 clones, whereas B cells, in contrast to monocytes, only weakly activated Th1 clones. However, exceptions to this rule existed within each T helper cell subset suggesting that individual T cell clones, regardless of the subset to which they belong, may have quantitatively and/or qualitatively different requirements for secondary activation signals which are provided by the antigen-presenting cells. The data demonstrate that, in general, monocytes are more effective than B cells in activating human T cell clones of all subtypes and that B cells were efficient antigen-presenting cells only for Th2 cells. However, individual T cell clones of any given T helper cell subset vary with respect to their activation by monocytes or B cells.     

Role of gastric epithelial cell-derived transforming growth factor beta in reduced CD4+ T cell proliferation and development of regulatory T cells during Helicobacter pylori infection

Gastric epithelial cells (GECs) express the class II major histocompatibility complex (MHC) and costimulatory molecules, enabling them to act as antigen-presenting cells (APCs) and affect local T cell responses. During Helicobacter pylori infection, GECs respond by releasing proinflammatory cytokines and by increasing the surface expression of immunologically relevant receptors, including class II MHC. The CD4⁺ T cell response during H. pylori infection is skewed toward a Th1 response, but these cells remain hyporesponsive. Activated T cells show decreased proliferation during H. pylori infection, and CD4⁺ CD25⁺ FoxP3⁺ regulatory T cells (Tregs) are present at the site of infection. In this study, we examined the mechanisms surrounding the CD4⁺ T cell responses during H. pylori infection and found that transforming growth factor β (TGF-β) plays a major role in these responses. GECs produced TGF-β1 and TGF-β2 in response to infection. Activated CD4⁺ T cells in culture with H. pylori-treated GECs were decreased in proliferation but increased upon neutralization of TGF-β. Naïve CD4⁺ T cell development into Tregs was also enhanced in the presence of GEC-derived TGF-β. Herein, we demonstrate a role for GEC-produced TGF-β in the inhibition of CD4⁺ T cell responses seen during H. pylori infection.     

Artificial antigen-presenting cells expressing CD80, CD70, and 4-1BB ligand efficiently expand functional T cells specific to tumor-associated antigens

Professional antigen-presenting cells (APCs), notably dendritic cells (DCs), are the most potent for expanding antigen-specific T cells ex vivo. However, the labor-intensive and expensive procedure for customized preparation of autologous APCs has hampered their broad clinical application. Artificial APC (aAPC) systems, which can be readily prepared from off-the-shelf components, have been proposed as a promising alternative to custom-made professional APCs. Here, in order to develop a novel aAPC system, we established K562 erythroleukemia cells expressing different combinations of co-stimulatory molecule ligands, CD80, CD70, and/or 4-1BB ligand (4-1BBL). When nucleofected with in vitro-generated mRNA encoding a tumor-associated antigen, MART-1, the K562 cells expressing all of CD80, CD70, and 4-1BBL were the most efficient for expansion of functional T cells specific to an HLA-A2-restricted immunodominant epitope, MART-1_26–35. In addition, only the K562 cells expressing all three of these co-stimulatory molecule ligands could clearly expand T cells specific to other less immunogenic antigen epitopes, gp100_154–162 and Cyp1B1_239–247, through transfection with in vitro generated gp100 and Cyp1B1 mRNA, respectively. These results indicated that non-redundant and synergistic effects of co-stimulation via CD28/CD80, CD27/CD70, and 4-1BB/4-1BBL might be critical for eliciting efficient expansion of T cells; co-stimulation via the 4-1BB/4-1BBL interaction might expand antigen-specific T cells by preventing apoptotic cell death triggered by specific antigens in the presence of the CD28/CD80 and CD27/CD70 signaling. Taken together, our findings suggested that this K562-based aAPC system expressing CD80, CD70, and 4-1BBL would be useful for efficiently stimulating functional antigen-specific T cells ex vivo, in particular when detailed information on the epitope specificities is unavailable.     

Costimulation of T cells for tumor immunity

Tumor specific antigens can be demonstrated on many neoplasms by immunization and challenge experiments; however, these antigens do not normally elicit a sufficiently strong immune response to prevent tumor growth in immunocompetent hosts. Recent studies have demonstrated that efficient activation of T cells requires costimulation of the CD28 receptor via the B7 molecule on antigen-presenting cells. Inadequate costimulation of tumor-reactive T cells may contribute to the fact that antigenic tumors are not normally rejected by the immune system, and weak anti-tumor immune responses may be amplified by upregulation of CD28 triggering.     

Peripheral dendritic cells are essential for both the innate and adaptive antiviral immune responses in the central nervous system

Intranasal application of vesicular stomatitis virus (VSV) causes acute infection of the central nervous system (CNS). However, VSV encephalitis is not invariably fatal, suggesting that the CNS may contain a professional antigen-presenting cell (APC) capable of inducing or propagating a protective antiviral immune response. To examine this possibility, we first characterized the cellular elements that infiltrate the brain as well as the activation status of resident microglia in the brains of normal and transgenic mice acutely ablated of peripheral dendritic cells (DCs) in vivo. VSV encephalitis was characterized by a pronounced infiltrate of myeloid cells (CD45^highCD11b⁺) and CD8⁺ T cells containing a subset that was specific for the immunodominant VSV nuclear protein epitope. This T cell response correlated temporally with a rapid and sustained upregulation of MHC class I expression on microglia, whereas class II expression was markedly delayed. Ablation of peripheral DCs profoundly inhibited the inflammatory response as well as infiltration of virus-specific CD8⁺ T cells. Unexpectedly, the VSV-induced interferon-gamma (IFN-γ) response in the CNS remained intact in DC-deficient mice. Thus, both the inflammatory and certain components of the adaptive primary antiviral immune response in the CNS are dependent on peripheral DCs in vivo.     

Antigen-induced regulation of T-cell motility,interaction with antigen-presenting cells and activation through endogenous thrombospondin-1 and its receptors

Antigen recognition reduces T-cell motility, and induces prolonged contact with antigen-presenting cells and activation through mechanisms that remain unclear. Here we show that the T-cell receptor (TCR) and CD28 regulate T-cell motility, contact with antigen-presenting cells and activation through endogenous thrombospondin-1 (TSP-1) and its receptors low-density lipoprotein receptor-related protein 1 (LRP1), calreticulin and CD47. Antigen stimulation induced a prominent up-regulation of TSP-1 expression, and transiently increased and subsequently decreased LRP1 expression whereas calreticulin was unaffected. This antigen-induced TSP-1/LRP1 response down-regulated a motogenic mechanism directed by LRP1-mediated processing of TSP-1 in cis within the same plasma membrane while promoting contact with antigen-presenting cells and activation through cis interaction of the C-terminal domain of TSP-1 with CD47 in response to N-terminal TSP-1 triggering by calreticulin. The antigen-induced TSP-1/LRP1 response maintained a reduced but significant motility level in activated cells. Blocking CD28 co-stimulation abrogated LRP1 and TSP-1 expression and motility. TCR/CD3 ligation alone enhanced TSP-1 expression whereas CD28 ligation alone enhanced LRP1 expression. Silencing of TSP-1 inhibited T-cell conjugation to antigen-presenting cells and T helper type 1 (Th1) and Th2 cytokine responses. The Th1 response enhanced motility and increased TSP-1 expression through interleukin-2, whereas the Th2 response weakened motility and reduced LRP1 expression through interleukin-4. Ligation of the TCR and CD28 therefore elicits a TSP-1/LRP1 response that stimulates prolonged contact with antigen-presenting cells and, although down-regulating motility, maintains a significant motility level to allow serial contacts and activation. Th1 and Th2 cytokine responses differentially regulate T-cell expression of TSP-1 and LRP1 and motility.     

Characteristics of antigen-independent and antigen-dependent interaction of dendritic cells with CD4+ T cells

Dendritic cells (DC) are the main antigen-presenting cells for the initiation of primary T cell-mediated immune responses. In the first stage of activation, T cells bind to DC in an antigen-independent manner. We studied the adhesion characteristics of human CD4⁺ T cells to DC generated from CD34⁺ hematopoietic progenitors following 12 to 13 days of culture in the presence of granulo-cyte/macrophage colony-stimulating factor and tumor necrosis factor-α. A majority of these cells had the morphology, phenotype and functions of DC. CD4⁺ T/DC adhesion was measured by means of fluorescence microscopy and flow cytometry. Four independent receptor/ligand pathways, LFA-1/ICAM, ICAM/LFA-1, CD2/LFA-3 and CD28/CD80, were involved in the transient adhesion of DC to CD4⁺ T cells in antigen-independent and specific alloantigen-dependent situations, as shown by blocking experiments using monoclonal antibodies. The antibodies also blocked a primary mixed lymphocyte reaction (MLR) in which DC were used as stimulatory cells. Adhesion of alloreactive CD4⁺ T cells to antigen-presenting DC was stronger than that of resting CD4⁺ T cells, while peak adhesion occurred after 5 and 20 min, respectively. The LFA-1 ligands involved in adhesion of resting CD4 T cells to DC and alloreactive CD4⁺ T cells to specific DC differed in part, since ICAM-3 on resting T cells and ICAM-1 on alloreactive T lymphocytes preferentially bound LFA-1. Studies of interactions between DC and phorbol ester-activated T cells expressing the CD40 ligand revealed a fifth independent adhesion pathway, CD40/CD40 ligand. CD4-mediated regulation of CD4⁺ T/DC adhesion was suggested by the observation that preincubation of CD4⁺ T cells and DC individually with anti-CD4 antibodies inhibited adhesion. In addition, antibodies specific for HLA class II molecules inhibited adhesion when used to pretreat DC but not alloactivated CD4⁺ T cells.     

Dendritic cells,T cells and their interaction in rheumatoid arthritis

Dendritic cells (DCs) are the key professional antigen-presenting cells which bridge innate and adaptive immune responses, inducing the priming and differentiation of naive to effector CD4⁺ T cells, the cross-priming of CD8⁺ T cells and the promotion of B cell antibody responses. DCs also play a critical role in the maintenance of immune homeostasis and tolerance. DC–T cell interactions underpin the generation of an autoimmune response in rheumatoid arthritis (RA). Here we describe the function of DCs and review evidence for DC and T cell involvement in RA pathogenesis, in particular through the presentation of self-peptide by DCs that triggers differentiation and activation of autoreactive T cells. Finally, we discuss the emerging field of targeting the DC–T cell interaction for antigen-specific immunotherapy of RA.     

Induction of contact-dependent CD8(+) regulatory T cells through stimulation with staphylococcal and streptococcal superantigens

The bacterial superantigen exotoxins of Staphylococcus aureus and Streptococcus pyogenes are potent stimulators of polyclonal T-cell proliferation. They are the causes of toxic shock syndrome but also induce CD25⁺ FOXP3⁺ regulatory cells in the CD4 compartment. Several studies have recently described different forms of antigen-induced regulatory CD8⁺ T cells in the context of inflammatory diseases and chronic viral infections. In this paper we show that bacterial superantigens are potent inducers of human regulatory CD8⁺ T cells. We used four prototypic superantigens of S. aureus (toxic shock syndrome toxin-1 and staphylococcal enterotoxin A) and Str. pyogenes (streptococcal pyrogenic exotoxins A and K/L). At concentrations below 1 ng/ml each toxin triggers concentration-dependent T-cell receptor Vβ-specific expression of CD25 and FOXP3 on CD8⁺ T cells. This effect is independent of CD4⁺ T-cell help but requires antigen-presenting cells for maximum effect. The cells also express the activation/regulatory markers cytotoxic T-lymphocyte antigen-4 and glucocorticoid-induced tumour necrosis factor receptor-related protein and skin homing adhesins CD103 and cutaneous lymphocyte-associated antigen. Superantigen-induced CD25⁺ FOXP3⁺ CD8⁺ T cells were as potent as freshly prepared naturally occurring CD4⁺ regulatory T cells in suppressing proliferation of CD4⁺ CD25⁻ T cells in response to anti-CD3 stimulation. Although superantigen-induced CD8⁺ CD25⁺ FOXP3⁺ express interleukin-10 and interferon-γ their suppressive function is cell contact dependent. Our findings indicate that regulatory CD8⁺ T cells may be a feature of acute bacterial infections contributing to immune evasion by the microbe and disease pathogenesis. The presence and magnitude of regulatory CD8⁺ T-cell responses may represent a novel biomarker in such infections. Superantigen-induced regulatory CD8⁺ T cells also have therapeutic potential.     

Yersinia-hsp60-reactive T cells are efficiently stimulated by peptides of 12 and 13 amino acid residues in a MHC class II (I-Ab)-restricted manner

Heat shock proteins (hsp) are immunodominant antigens in microbial infections. Previous work from this laboratory demonstrated that Yersinia-hsp60 (Y-hsp60)-reactive CD4⁺αβ T cells play an important role for resolution of Y. enterocolitica infections in mice. In the present study we identified two epitopes of Y-hsp60 recognized by CD4⁺ Th1 cell clones. The epitopes comprise 12 (214–225) and 13 (74–86) amino acid (aa) residues of Y-hsp60, and are the first described for MHC class II (I-A^b) molecules. Both epitopes are also recognized by T cells isolated from mesenteric lymph nodes from mice orogastrically infected with yersiniae. Stimulation of T cells with peptides of 12 and 13 aa residues of Y-hsp60 caused highly efficient proliferation compared with longer peptides, full-length recombinant Y-hsp60, or heat-killed Yersinia (HKY). Incubation of antigen-presenting cells with chloroquine blocked both peptide and HKY-triggered T cell proliferation, whereas cytochalasin B only blocked HKY-induced proliferation and to a lesser extent peptide-induced proliferation. The identified epitopes reside in a region of Y-hsp60 that is conserved between Enterobacteriaceae but highly variable when compared with murine or human hsp60. Although both epitopes are identical to the related sequence of hsp60 (GroEL) of Escherichia coli, only weak T cell responses were observed upon stimulation with GroEL of E. coli, suggesting that other factors, e.g. flanking amino acid residues, might be important for antigen processing and T cell stimulation in a class II-restricted manner. Furthermore, these observations might be of significance for the rational design of subunit vaccines.     

Ex vivo expanded telomerase-specific T cells are effective in an orthotopic mouse model for pancreatic adenocarcinoma

Telomerase activity is over-expressed in nearly all pancreatic carcinomas, but not in chronic pancreatitis. Here, we investigated various protocols for expansion of telomerase-specific T cells for adoptive cell transfer and their use in a syngeneic pancreatic carcinoma mouse model. Telomerase-specific T cells were generated by stimulation of splenocytes from peptide-immunized donor mice with either interleukin (IL)-2, IL-15, artificial antigen-presenting cells, anti-signalling lymphocyte activation molecule (SLAM) microbeads or allogeneic dendritic cells in combination with a limited dilution assay. T cells were tested for antigen specificity in vitro and for anti-tumour activity in syngeneic mice with orthotopically implanted tumours pretreated with cyclophosphamide. The immune cells from recipients were immunophenotyped. During a period of 2 weeks, the expansion approach using IL-2 was very successful in generating a high number of telomerase-specific CD8⁺ T cells without losing their function after adoptive cell transfer. Significantly slower tumour growth rate and less metastasis were observed after adoptively transferring telomerase specific CD8⁺ T cells, expanded using IL-2. Further investigations showed that anti-tumour efficacy was associated with a significant shift from naive CD8⁺ T cells to CD8⁺ central memory T cells, as well as recruitment of a high number of dendritic cells. Remarkable amounts of telomerase-specific T cells were detectable in the tumour. Generation of telomerase-specific T cells is feasible, whereat IL-2-based protocols seemed to be most effective and efficient. Antigen-specific T cells showed significant cytotoxic activity in a syngeneic, orthotopic mouse model, whereas central memory T cells but not effector memory T cells appear to be of high importance.     

CD40 Ligand-activated, antigen-specific B cells are comparable to mature dendritic cells in presenting protein antigens and major histocompatibility complex class I- and class II-binding peptides

Dendritic cells (DC) are increasingly exploited for cell-based immunotherapy. However, limitations in ex vivo DC growth and DC functional heterogeneity have motivated development of complementary antigen-presenting cell sources. Here, the ability of CD40 ligand (CD40L)-activated B cells to fulfil that role was investigated. We demonstrate for the first time that non-specific or antigen-specific murine B cells can be grown for extended periods of time by stimulation with CD40L. These cells rapidly up-regulate and maintain high levels of co-stimulatory molecules. In a head-to-head comparison with DC, CD40L-expanded B cells were comparable to DC in the presentation of peptides to CD4⁺ and CD8⁺ T cells. While DC were superior to antigen non-specific CD40L-activated B cells with regard to whole protein (NP-BSA) processing and presentation, CD40L-expanded B cells from NP-BSA-immunized mice were comparable to DC when presenting BSA or NP-BSA to primed primary T cells or when presenting NP linked to an unrelated carrier, CGG, to naïve T cells. Thus, the combination of CD40L activation, which supports B-cell growth and augments intracellular protein processing, and antigen uptake via the B-cell receptor, allows for efficient uptake, processing, and presentation of whole protein antigens in a fashion comparable to that observed with mature DC. Like DC, CD40L-activated B cells efficiently home to secondary lymphoid organs in vivo. This system represents a unique tool for studying primary antigen-specific B cells and the results suggest that the outgrowth of large numbers of highly activated B cells represents a viable and practical complement to DC for cell-based immunotherapy.     

CD4+ Th2 cells are directly regulated by IL-10 during allergic airway inflammation

Interleukin-10 (IL-10) is an important regulatory cytokine required to control allergy and asthma. IL-10-mediated regulation of T cell-mediated responses was previously thought to occur indirectly via antigen-presenting cells. However, IL-10 can act directly on regulatory T cells and T helper type 17 (Th17) cells. In the context of allergy, it is therefore unclear whether IL-10 can directly regulate T helper type 2 (Th2) cells and whether this is an important regulatory axis during allergic responses. We sought to determine whether IL-10 signaling in CD4⁺ Th2 cells was an important mechanism of immune regulation during airway allergy. We demonstrate that IL-10 directly limits Th2 cell differentiation and survival in vitro and in vivo. Ablation of IL-10 signaling in Th2 cells led to enhanced Th2 cell survival and exacerbated pulmonary inflammation in a murine model of house dust mite allergy. Mechanistically, IL-10R signaling regulated the expression of several genes in Th2 cells, including granzyme B. Indeed, IL-10 increased granzyme B expression in Th2 cells and led to increased Th2 cell death, identifying an IL-10-regulated granzyme B axis in Th2 cells controlling Th2 cell survival. This study provides clear evidence that IL-10 exerts direct effects on Th2 cells, regulating the survival of Th2 cells and severity of Th2-mediated allergic airway inflammation.     

Unique functions of splenic CD8alpha+ dendritic cells during infection with intracellular pathogens

Deciphering the prerequisites for the induction of protective cytotoxic T cell responses is essential for future development of more effective CD8⁺ T cell-based vaccines against infectious diseases and cancer. Since crucial events for CD8⁺ T cell priming and differentiation occur during the first contacts of naïve T cells with distinct antigen-presenting cells (APCs), the identification and therapeutic targeting of these ‘master’ APCs has become a major quest in the field.A decade ago, dendritic cells (DCs) were discovered as potent APCs, as they combine all major features for the initiation of T cell responses: (1) naïve DCs demonstrate high endocytic activity and scan continuously their environment in strategic positions throughout the whole body; (2) after activation (e.g. during pathogen invasion), DCs migrate into T cell zones of their draining lymphatic compartments, meanwhile processing captured antigen and maturing in order to stimulate encountered antigen-specific T cells.During the last years, different subsets of DCs that can be distinguished by specific surface marker expression and effector functions have been identified in mice. Their distinct functional capabilities have led to the concept of work-sharing; “migrating” DCs primarily transport antigens to the lymph node, where a specialized subset of “resident” DCs, defined by the expression of the CD8αα homodimer (CD8α⁺ DCs), primes CD8⁺ T cells upon antigen cross-presentation. Accordingly, CD8α⁺ DCs have been found to prime CD8⁺ T cells against different viruses as well as intracellular bacteria such as Listeria monocytogenes (L.m.).Recently, L.m. was found to survive specifically in splenic CD8α⁺ DCs shortly after intravenous infection. Further experiments revealed a more generalized sampling activity of CD8α⁺ DCs for blood-borne particles. These findings indicate that splenic CD8α⁺ DCs might unite efficacious antigen-trapping with the licence to prime CD8⁺ T cells. This new aspect of DC function could have evolved to guarantee a more rapid antigen-specific response against generalized infections.     

Macrophages,but not dendritic cells,present collagen to T cells

Dendritic cells, such as epidermal Langerhans cells, play a crucial role for the antigen-specific priming of T cells. We have addressed the question whether dendritic cells present collagen, a major protein component in tissues through which dendritic cells migrate, i.e. the basement membrane, dermis, and synovial tissue. Langerhans cells, spleen cells and peritoneal macrophages were compared for antigen-presenting capacity using a panel of mouse T cell hybridomas reactive with different determinants on type II collagen, myelin basic protein, ovalbumin and pepsin. Langerhans cells did not present any of the type II collagen determinants, unless the antigen was administered as a 15-mer peptide, but did present myelin basic protein, ovalbumin and pepsin. Spleen cells and peritoneal macrophages, in contrast, presented all type II collagen determinants. This biased antigen presentation was also observed when Langerhans cells were pulsed with antigen in vivo. The inability to present type II collagen is related to the collagen sequence as such, since both native type II collagen, type II collagen α chains, as well as a type II collagen determinant incorporated in type I collagen, were not presented by Langerhans cells. In addition, granulocyte/macrophage colony-stimulating factor-expanded blood dendritic cells displayed the same biased antigen presentation, suggesting that the inability to present collagen is not restricted to dendritic cells localized in epidermis. B cell-deficient mice could prime a type II collagen-reactive T cell response, thus excluding B cells as obligatory antigen-presenting cells for the priming of collagen-reactive T cells. We suggest that neither Langerhans cells nor B cells, but macrophages are the primary antigen-presenting cells in the immune response towards type II collagen.