Mature T cell seeks antigen for meaningful relationship in lymph node

Potential of mannan or dextrin nanogels as vaccine carrier/adjuvant systems

Polymeric nanogels have been sophisticatedly designed promising a new generation of vaccine delivery/adjuvant systems capable of boosting immune response, a strategic priority in vaccine design. Here, nanogels made of mannan or dextrin were evaluated for their potential as carriers/adjuvants in vaccine formulations. Since lymph nodes are preferential target organs for vaccine delivery systems, nanogels were biotin-labeled, injected in the footpad of rats, and their presence in draining lymph nodes was assessed by immunofluorescence. Nanogels were detected in the popliteal and inguinal lymph nodes by 24 h upon subcutaneous administration, indicating entrapment in lymphatic organs. Moreover, the model antigen ovalbumin was physically encapsulated within nanogels and physicochemically characterized concerning size, zeta potential, ovalbumin loading, and entrapment efficiency. The immunogenicity of these formulations was assessed in mice intradermally immunized with ovalbumin–mannan or ovalbumin–dextrin by determining ovalbumin-specific antibody serum titers. Intradermal vaccination using ovalbumin–mannan elicited a humoral immune response in which ovalbumin-specific IgG1 levels were significantly higher than those obtained with ovalbumin alone, indicating a TH2-type response. In contrast, dextrin nanogel did not show adjuvant potential. Altogether, these results indicate that mannan nanogel is a material that should be explored as a future antigen delivery system.

Unique Transcompartmental Bridge: Antigen-Presenting Cells Sampling across Endothelial and Mucosal Barriers

Potentially harmful pathogens can gain access to tissues and organ systems through body sites that are in direct contact with the outside environment, such as the skin, the gut, and the airway mucosa. Antigen-presenting cells (APCs) represent a bridge between the innate and adaptive immunity, and their capacity for constant immune surveillance and rapid sampling of incoming pathogens and other potentially harmful antigens is central for mounting an effective and robust protective host response. The classical view is that APCs perform this task efficiently within the tissue to sense invading agents intra-compartmentally. However, recent data based on high resolution imaging support an additional transcompartmental surveillance behavior by APC by reaching across intact physical barriers. In this review, we summarize intravital microscopic evidences of APC to sample antigens transcompartmentally at the gut mucosa and other body sites.

Stromal and hematopoietic cells in secondary lymphoid organs: partners in immunity

Secondary lymphoid organs (SLOs), including lymph nodes, Peyer's patches, and the spleen, have evolved to bring cells of the immune system together. In these collaborative environments, lymphocytes scan the surfaces of antigen-presenting cells for cognate antigens, while moving along stromal networks. The cell-cell interactions between stromal and hematopoietic cells in SLOs are therefore integral to the normal functioning of these tissues. Not only do stromal cells physically construct SLO architecture but they are essential for regulating hematopoietic populations within these domains. Stromal cells interact closely with lymphocytes and dendritic cells, providing scaffolds on which these cells migrate, and recruiting them into niches by secreting chemokines. Within lymph nodes, stromal cell-ensheathed conduit networks transport small antigens deep into the SLO parenchyma. More recently, stromal cells have been found to induce peripheral CD8(+) T-cell tolerance and control the extent to which newly activated T cells proliferate within lymph nodes. Thus, stromal-hematopoietic crosstalk has important consequences for regulating immune cell function within SLOs. In addition, stromal cell interactions with hematopoietic cells, other stroma, and the inflammatory milieu have profound effects on key stromal functions. Here, we examine ways in which these interactions within the lymph node environment influence the adaptive immune response.

Life cycle, migration and antigen presenting functions of spleen and lymph node dendritic cells: limitations of the Langerhans cells paradigm

The phenotypic and functional studies carried out in recent years on dendritic cells (DC) purified from spleen and lymph nodes has revealed the existence of heterogeneous populations with distinct life cycles, migratory properties and antigen presenting functions. A major subdivision can be made between "tissue derived" DC that migrate to the lymph nodes from peripheral tissues, both in the steady state and in the course of infections, and "blood-derived" DC, which reside in the spleen and lymph nodes throughout their life cycle. These two groups of DC can be subdivided into smaller subsets. The tissue-derived and the blood-derived DC also show fundamental differences in maturational status and antigen presenting capabilities. In this review, we summarize the roles played by the different DC types in the steady state and during pathogen infections, relating those roles to maintenance of peripheral tolerance and the induction of immunity. We point out the caveats of assuming that the DC that collect antigens are the ones involved in their presentation, emphasizing the phenomenon of antigen transfer as an important component of the immune response.

Dendritic cells in a mature age

A common view supposes that dendritic cells (DCs) exist in two basic functional states: immature DCs induce tolerance to self, whereas mature DCs induce immunity to foreign antigens. However, the term 'mature' is often used not only functionally to designate immunogenic DCs but also as a phenotypic description of DCs expressing high levels of MHC, adhesion and co-stimulatory molecules. The recent realization that DCs can express such markers under non-immunogenic conditions raises the question of whether the two connotations of the term 'mature' should continue to be used interchangeably. Here, I discuss the origins of the maturation model and how terminology is evolving to better accommodate our current understanding of the function of DCs.

Plasmacytoid dendritic cells (PDC) are the major DC subset innately producing cytokines in human lymph nodes

Plasmacytoid dendritic cells (PDC) constitute a distinct subset of DC found in human peripheral lymph nodes (LN), but little is known about their function. Cell suspensions were prepared from tumor draining LN (n=20) and control LN (n=11) of women undergoing surgical resection for primary breast cancer and elective surgery for benign conditions, respectively. Using four-color flow cytometry, human leukocyte antigen-DR+ DC subsets were identified phenotypically. The proportions and numbers of cells innately producing interleukin (IL)-4, IL-10, IL-12, and interferon-gamma (IFN-gamma) were also measured from intracellular accumulation of cytokine after blocking with monensin. All flow cytometry data were collected without compensation and were compensated off-line using the Winlist algorithm (Verity software). This package also provided the subtraction program to calculate percentage positive cells and intensity of staining. PDC (CD11c-, CD123+) expressed more cytokines than did myeloid DC (CD11c+) or CD1a+ putative "migratory" DC (P<0.001). LN PDC from patients with a good prognosis (px; n=11) demonstrated a relative increase in IL-12 and IFN-gamma expression (median IL-10:IL-12 ratio=0.78 and median IL-4:IFN-gamma ratio=0.7), and PDC from LN draining poor px cancer (n=9) showed a relative increase in IL-10 and IL-4 expression (median IL-10:IL-12 ratio=1.31 and median IL-4:IFN-gamma ratio=2.6). The difference in IL-4:IFN-gamma expression between good and poor px cancer groups was significant (P<0.05). Thus, PDC innately producing cytokines were identified in cell suspensions from human LN, and the character of PDC cytokine secretion may differ between two breast cancer prognostic groups. We speculate that a shift towards PDC IL-10 and IL-4 expression could promote tumor tolerance in LN draining poor px breast cancer.

Control of MHC class II antigen presentation in dendritic cells: a balance between creative and destructive forces

The antigen capturing and presenting abilities of dendritic cells (DCs) are developmentally regulated in a process known as maturation. During maturation, DCs increase several fold their surface expression of major histocompatibility complex class II (MHC II) molecules. This increase is accompanied with a dramatic change in localization of MHC II molecules, which are abundant in endosomal structures in immature DCs but located mostly on the plasma membrane in mature DCs. How these changes relate to antigen processing, generation of MHC II-peptide complexes, and trafficking of MHC II molecules, in the immature and mature states of DC development, has been a matter of debate. Here, we discuss the work carried out to characterize the biochemical and cell biological mechanisms that control MHC II antigen presentation in mouse and human DCs, and how these mechanisms relate to the function of the DC network in vivo. We conclude that the control checkpoints operate downstream of MHC II-peptide complex formation and expression on the plasma membrane, acting in accord with control of MHC II synthesis. Therefore, immature and mature DCs present antigens to T cells under steady state and inflammatory conditions. We advocate that the mechanisms regulating MHC II-peptide complex turnover should be emphasized as an important theme for future DC research.

The Conduit System Transports Soluble Antigens from the Afferent Lymph to Resident Dendritic Cells in the T Cell Area of the Lymph Node

Resident dendritic cells (DC) within the T cell area of the lymph node take up soluble antigens that enter via the afferent lymphatics before antigen carrying DC arrive from the periphery. The reticular network within the lymph node is a conduit system forming the infrastructure for the fast delivery of soluble substances from the afferent lymph to the lumen of high endothelial venules (HEVs). Using high-resolution light microscopy and 3D reconstruction, we show here that these conduits are unique basement membrane-like structures ensheathed by fibroblastic reticular cells with occasional resident DC embedded within this cell layer. Conduit-associated DC are capable of taking up and processing soluble antigens transported within the conduits, whereas immigrated mature DC occur remote from the reticular fibers. The conduit system is, therefore, not a closed compartment that shuttles substances through the lymph node but represents the morphological equivalent to the filtering function of the lymph node.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

Challenges facing adjuvants for cancer immunotherapy

An adjuvant is defined as a product that increases or modulates the immune response against an antigen (Ag). Based on this general definition many authors have postulated that the ideal adjuvant should increase the potency of the immune response, while being non-toxic and safe. Although dozens of different adjuvants have been shown to be effective in preclinical and clinical studies, only aluminium-based salts (Alum) and squalene-oil-water emulsion (MF59) have been approved for human use. However, for the development of therapeutic vaccines to treat cancer patients, the prerequisites for an ideal cancer adjuvant differ from conventional adjuvants for many reasons. First, the patients that will receive the vaccines are immuno-compromised because of, for example, impaired mechanisms of antigen presentation, non-responsiveness of activated T cells and enhanced inhibition of self-reactivity by regulatory T cells. Second, the tumour Ag are usually self-derived and are, therefore, poorly immunogenic. Third, tumours develop escape mechanisms to avoid the immune system, such as tumour editing, low or non-expression of MHC class I molecules or secretion of suppressive cytokines. Thus, adjuvants for cancer vaccines need to be more potent than for prophylactic vaccines and consequently may be more toxic and may even induce autoimmune reactions. In summary, the ideal cancer adjuvant should rescue and increase the immune response against tumours in immuno-compromised patients, with acceptable profiles of toxicity and safety. The present review discusses the role of cancer adjuvants at the different phases of the generation of antitumour immunity following vaccination.

Dynamic populations of dendritic cell-specific ICAM-3 grabbing nonintegrin-positive immature dendritic cells and liver/lymph node-specific ICAM-3 grabbing nonintegrin-positive endothelial cells in the outer zones of the paracortex of human lymph nodes

In the paracortex of lymph nodes, cellular immune responses are generated against antigens captured in peripheral tissues by dendritic cells (DCs). DC-SIGN (dendritic cell-specific ICAM-3 grabbing nonintegrin), a C-type lectin exclusively expressed by DCs, functions as an antigen receptor as well as an adhesion receptor. A functional homologue of DC-SIGN, L-SIGN (liver/lymph node-SIGN, also called DC-SIGN-related), is expressed by liver sinus endothelial cells. In lymph nodes, both DC-SIGN and L-SIGN are expressed. In this study, we analyzed the distribution of these two SIGN molecules in detail in both normal and immunoreactive lymph nodes. DC-SIGN is expressed by mature DCs in paracortical areas and in addition by DCs with an immature phenotype in the outer zones of the paracortex. L-SIGN expression was also detected in the outer zones on sinus endothelial cells characterized by their expression of the lymphatic endothelial markers LYVE-1 and CLEVER-1. During both cellular and humoral immune responses changes in the amount of DC-SIGN+ immature and mature DCs and L-SIGN+ endothelial cells were observed, indicating that the influx or proliferation of these cells is dynamically regulated.

Most lymphoid organ dendritic cell types are phenotypically and functionally immature

Dendritic cells (DCs) have been thought to follow a life history, typified by Langerhans cells (LCs), with 2 major developmental stages: an immature stage that captures antigens in the periphery and a mature stage that presents those antigens in the lymphoid organs. However, a systematic assessment of the maturity of lymphoid organ DCs has been lacking. We have analyzed the maturity of the DC types found in the steady state in the spleen, lymph nodes (LNs), and thymus. The DCs that migrate into the iliac, mesenteric, mediastinal, or subcutaneous LNs from peripheral tissues were mature and therefore could not process and present newly encountered antigens. However, all the other DC types were phenotypically and functionally immature: they expressed low levels of surface major histocompatibility complex class II (MHC II) and CD86, accumulated MHC II in their endosomes, and could present newly encountered antigens. These immature DCs could be induced to mature by culture in vitro or by inoculation of inflammatory stimuli in vivo. Therefore, the lymphoid organs contain a large cohort of immature DCs, most likely for the maintenance of peripheral tolerance, which can respond to infections reaching those organs and mature in situ.

Considering the immune response to botulinum toxin

The immune system is unable to determine whether material it encounters is deleterious, benign, or even beneficial to the organism. This presents a significant challenge when protein-based biological therapies, such as botulinum toxin, are administered to patients. Many factors combine to influence the likelihood and the magnitude of an immune response if a response is elicited. Those factors intrinsic to antigens that heighten their immunogenicity include nonhuman origin, larger molecules, and aggregated forms of the protein. Extrinsic factors also must be considered, such as the presence of adjuvants in the formulation, either intended or unintended; increasing amounts of antigen within specific dosing ranges; frequent dosing; and, finally, the genetic predisposition of the patient. Once present, not all immune responses preclude the biological therapy from being clinically effective. Only antibodies that bind botulinum toxin in a manner that neutralizes its biological activity will attenuate its effect on the neuromuscular junction. The majority of anti-toxin antibodies do not affect its function. Finally, although crossreactivity has been reported among the seven botulinum toxin serotypes, non-neutralizing antibodies are present that recognize regions of similarity among the serotypes. No cross-neutralizing antibodies have been described in patients administered any of the toxin serotypes.