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Dendritic Cells – Future Hope for New Vaccine Development
Posted on: March 30, 2004

Aging is not only associated with decreased metabolic rate, miss regulated hormone secretion, common age related diseases, other annoying disorders. The decrease in immune system in general is observed here too. For example amount of basic antibodies responsible for immune response, in the person of 50 and 70 is lower at least in 2 fold. A depletion of immunocompetent cells in the blood of elderly persons is also a rule. Because of this elderly people are more prone to acquire chronic diseases, which are not common in young ones. There are evidence that Alzheimer's disease is caused by amyloid fibrils, which are toxic to neurons, nevertheless these fibrils are immunogenic and should be treated in organism as antigens. In other words here we should have an immune response against amyloid fibrils, and something like that has already been found. But could it be that Alzheimer's and other protein fibrillation linked disorders progressions are closely connected with ablated immune system in elderly stage? Maybe it could be so, but before we can prove this statement lets get more acquainted with basic role players and common antigen presenting mechanisms in immune response and their practical use.

Dendritic cells (DCs) comprise an essential component of the immune system. These cells, as antigen presenting cells (APCs) to naive T cells, are crucial in the initiation of antigen specific immune responses. Dendritic cells (DCs) are a complex, heterogeneous group of multifunctional APCs. DCs are leukocytes, distributed throughout lymphoid and non-lymphoid tissues, in peripheral blood and afferent lymph vessels. It has been shown that DCs after activation with different stimuli achieve maturation, where they express high levels of several molecules on the cell surface such as MHC class I and II, accessory molecules CD (Cluster Density) 40, CD80, CD86 and early activation markers such as CD83. These cells do not proliferate and after a certain time course they undergo apoptosis and will be replaced by a new pool of cells. Functionally, DCs exert various effects on other immune cells, particularly in secondary lymphoid organs; DCs present non-self peptide-MHC complexes to naive and memory T lymphocytes to mobilize specific immunity. The capacity of DCs to initiate primary immune responses is due to their ability to deliver specific co-stimulatory signals, which are essential for T cell activation from the resting or naive state into distinct classes of effector cells. These immunogen-specific immune responses are critical for example, to tumor resistance, prevention of metastasis, and blocking infections. DCs also can alter the function of regulatory T cells that control activated T cells through their suppressive signals. In addition, DCs play an important role in innate immunity by secreting cytokines, e.g. IL-12 (Interleukin) and Interferon classes I and II, involved in host defense. Moreover, DCs activate Natural killer cells (NK) and NKT cells that rapidly eradicate select targets. Such diverse functions of DCs have begun to shed light on their pre-eminent role in immunological events.

Origin and Developmental Processes of Dendritic Cells
DCs originate from hematopoietic stem cells in the bone marrow. Recently, there have been great insights into the origins of DC subsets and their modulation by distinct cytokines of neighboring cells. Progenitors of DCs in bone marrow migrate via the blood stream and home to peripheral tissues where they encounter several essential growth factors such as GM-CSF (Granulocyte macrophage colony stimulating factor), IL-4, IL-15, Tumor necrosis factor TNF-α, Transforming growth factor TGF-β, and IL-3 secreted by various cell types including endothelial cells, Mast cells, keratinocytes and fibroblasts in the microenvironment. Such growth factors determine the fate of the progenitors to differentiate into immature Langerhans DCs, interstitial DCs or plasmacytoid DCs.
One of the hallmarks of DC progenitors is their capacity to migrate. Skin is surrounded by various immunogen antigens that can penetrate the epidermis. These antigens can be captured by immature Langerhans DCs, and processed. Cutaneous DCs will then be activated, migrate, and home to the lymph nodes. Matured DCs present processed antigen-to-antigen specific T cells inducing specific immunity. For example, cutaneous interstitial DCs enter mesenteric lymph nodes. Liver DCs, which reside in the portal triads and along the sinusoids, migrate into hepatic lymph and subsequently to the celiac lymph nodes. In addition, DC subsets are ready to confront invading pathogens. In such environments DCs ingest antigens via several mechanisms including phagocytosis and receptor-mediated endocytosis.
Antigenic infectious agents including vaccines induce proinflammatory cytokines (e.g., TNF-α). These cytokines promote Langerhans DC maturation in lymphoid organs where they home to the T cell rich area. Langerhans DCs undergo phenotypic and functional changes during their maturation and migration. These cells, which are now loaded with antigenic peptides, lose the capacity to capture foreign antigens. Mature DCs are an end stage of differentiation, and they cannot be converted into either macrophages or lymphocytes.
DCs in general present marked heterogeneity in phenotype and function, which relate to their precise localizations within different tissues in the body.

The Role of Dendritic Cells in Clinical Diseases
Recent studies shed light on the role of DC involvement in various diseases such as autoimmunity, allergy, transplantation, infection and cancer. For example, studies showed that DCs differentiated in vitro express very important co-stimulatory molecules, e.g. CD40, which allow these cells to approach T cells and deliver signals to them. With respect to that phenomenon, cytokines (e.g., GM-CSF, TNF-α) produced by keratinocytes affect DC differentiation dramatically. Moreover, DCs alone produce essential cytokines (e.g. IL-1β, TNF-α, IL-6), and chemokines MIP-1α, MIP-1γ, IL-15 and IL-8. Some of these cytokines contribute directly to the DCs ability to attract and recruit T cells in sites of inflammation. A number of autoimmune diseases (rheumatoid arthritis) or skin psoriasis demonstrates the accumulation of DCs in diseased tissues. This evidence suggests that DC enrichment within the cytokine-rich synosium or epidermis undergo phenotypic and functional maturation in vivo. Moreover, DCs in transplanted organs are involved and they represent potent "passenger leukocytes" that sensitize host graft antigens and trigger rejection. Studies have shown that the depletion of DCs from mouse islets or thyroid tissue prolonged survival in allogeneic recipients. Other studies on the function of DCs after transplantation of skin and heart tissues to allogeneic recipients have shown that soon after grafting, DCs enter the recipient's lymphoid tissues. Thus, there appears to be a sensitization of host T cells, which occurs primarily in these tissues when they encounter the graft-derived, allogeneic DCs.
It is clear now, that cancer cells can express tumor associated antigens, which are recognized by host T cells. These T cells may not be able to reject tumor cells. These molecules, then, are not immunogenic. In order to become immunogenic they must be processed and presented by professional antigen presenting cells (APC). Since DCs possess relevant features, e.g. a) internalizing of immunogenic antigen through endocytosis, b) phagocytosis for subsequent processing and presentation of several antigens to T cells, and c) migration capability, they could acquire tumor antigen.
In the past few years the role of DCs in cancer has been suggested. There is evidence that DCs can induce immunity to tumors if they are administrated to animals or exposed to tumor-associated antigen before or when the tumor is inoculated into animals.
Although prior investigations have established that targeting immune cells to tumors may improve immunity, in the case of DCs, however, it has been shown that the tumor microenvironment is detrimental to DC function, and in fact may condition DCs to induce a T cell response that energizes or suppresses tumor-specific immunity. Thus, targeting DCs directly to tumors, as demonstrated by several studies, may be inefficient. Therefore, methods should be developed in order to target DCs by immunogenic TAAs outside the tumor microenvironment to improve immunity.

<>Vaccine design by targeting dendritic cells
Given the central role of DCs in controlling immunity, has brought a scientific focus to the critical role of DCs as an efficient vector in vaccine technology. Several approaches to target DCs efficiently have been designed. There is a large body of literature involving experimental animal models and for tumors and infection in which DC subsets pulsed with tumor-associated antigen or subunits of the pathogens such as HCV or HIV are to induce protective immunity against tumors. However, it is even more important to create novel strategies by targeting immunogenic antigens or immune regulatory agents specifically to DCs without impairing the functional properties of DC subsets and in this way modulate the immune responses in vivo.
These novel strategies must be not immunopathogenic, but specific in order to overcome energy established through negative signals, which may be provided by immune component cells including DCs to the microenvironment.
One possible strategy is to target novel molecules expressed on the cell surface of DCs. It is important to use small peptides, which solely bind to DC subsets and not other cells. DNA sequences encoding DC-peptides can then be fused genetically with tumor-associated antigen coding regions or with the subunit of the pathogen of interest. Immunogenic fusion proteins can be then expressed by probiotic microorganisms such as Lactobacilli or attenuated strains of Salmonella in vivo (Figure 1).


Fig. 1. Delivery of immunogenic antigen to DCs by probiotic microorganisms. DNA encoding sequences of DC-binding peptides and immunogenic subunit of any pathogen will be expressed in Gram-positive bacteria including Lactobacillus. Lactobacillus will be orally administrated. These bacteria colonize the gut and express and release the immunogen in the intestine. DCs in the mucosal site will then capture the immunogen via DC-binding peptide motifs. They internalize the immunogen, process and present it to T cells inducing specific responses against released immunogen.

Such novel vaccine strategies should take advantage of mucosal sites in the body, as well as the skin in order to be delivered specifically to DC subsets in vivo.
More specifically, in order to target any vaccine to DCs, recently, a novel strategy was proposed. Investigators fused a subunit of hepatitis C virus with a DC-binding peptide. Studies are ongoing to express such immunogenic fusion proteins by a strain of Lactobacilli. Such a Lactobacillus strain will express and secretes the immunogenic protein in the intestinal region. DCs will be able to capture such immunogen via the motifs of DC-binding peptides. Such binding of an immunogen to DCs will facilitate rapid internalization of the immunogen into DCs. DCs will then process and present it to T cells residing in the gut. These cells will be activated and will circulate through the body in order to elicit specific T cell immune responses against the pathogen of interest.
A transdermal delivery system also offers an interesting route to approach DC subsets in order to enhance immunity against cancer or pathogens (Figure 2).
Accordingly, the immune system of the skin harbors two very potent antigen-presenting DC subsets, which induce primary antigen specific T cell immune responses. Furthermore, careful experimentation of various vaccine delivery routes has shed light on the skin and its immune mechanisms. It has previously been shown that cutaneous DC subsets can be targeted and activated in situ in order to achieve specific T cell mediated immune responses. Thus, the feasibility of using immunogenic DC-peptide fusion proteins should be tested to determine whether administration of such immunogenic fusion proteins would induce the activation of cutaneous DC subsets that in turn prime antigen-specific T cells in situ.


Figure 2. Transdermal delivery of immunogenic fusion protein by cutaneous DCs. Genetically engineered immunogenic fusion protein can be transdermally administrated into the skin whereby cutaneous DC subsets can capture it via DC-peptide motifs fused to immunogen subunits. Loaded cutaneous DC subsets can be activated, leave the skin and enter the lymph nodes where they can present processed antigen as immunogenic peptides to T cells eliciting specific T cell immune responses.

DCs play a crucial role in host-pathogen interactions. A recent example involves the report in human papilloma virus 16, which is strongly associated with the development of cervical cancer, that in infected cells the E6 oncogenic protein limits the numbers of LC in infected epidermis. This appears to decrease the host's ability to mount an effective immunological response to HPV 16. We anticipate that future studies will be focused on enhancing functional aspects of DCs to prevent such events and establish novel vaccine strategies to efficiently target immunogenic antigens or inhibitory agents to DCs in order to elicit or suppress specific immune responses in vivo.

Source: Mansour Mohamadzadeh and Ronald Luftig; Dendritic cells: In the forefront of immunopathogenesis and vaccine development; Journal of Immune Based Therapies and Vaccines 2004, 2:1-11.
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