Capture, Processing, and Presentation of Exogenous Antigen by Dendritic Cells - Essay Example

Antigen Capture The immune system is a complex made up of proteins, cells and other molecular components that cooperatively work together to destroy foreign invaders of the body, also known as antigens. Antigens are proteins or part of a foreign body or organism (examples are viruses, bacteria, proteins and other particulate material) that enter the body that are ultimately recognized by the immune system’s B and T cells through innate immune response. This recognition results in the production of new B and T cells that have the capacity to recognize (memory) and subsequently kill or disable the antigen. B and T cells differ in the manner they recognize antigens. B cells can recognize the antigen through its structure while T cells recognize the same protein only when it has been degraded and presented on the surface of the antigen presenting cell (APC). T cells do not attack free antigens that can be found in the cytosol. In the lymphoid tissues, dendritic cells are considered professional antigen presenting cells (APC) because they are strongest known stimulators of T cells in vivo and in vitro. Thus far, dendritic cells (DC) have only one known function, and that is to present antigens to T cells. Their name was derived dendron, Greek for tree, because of their morphological structure which resembles a tree with many branches or dendrites. The DC arise from myeloid cells in the bone marrow and migrate to peripheral tissues like the skin and mucosa. In these tissues, the immature phenotype of dendritic cells prevail. Immature DC are not yet capable of stimulating T cells. However, the immature DC have receptors that allow them to recognize factors on microbial surfaces allowing them to take up or ingest exogenous antigens through macropinocytosis, endocytosis, and phagocytosis. During macropinocytosis, the dendritic cell membrane forms curved ruffles which fold in to form a pocket enclosing the antigens. The pocket forms a vesicle, called a macropinosome, with 1-5 um diameter, that is filled with extracellular fluid and other molecules including the antigens. The macropinosome then travels into the cytoplasm were it fuses with endosomes and lysosomes. This process occurs constitutively in immature DC and requires the presence of cholesterol (Grimmer, van Deurs, & Sandvig, 2002). Macropinocytosis occurs in other cells but is only induced by the presence of growth factors. A study of early membrane traffic events occurring during micropinocytosis show that the process increases in a regulated fashion that controls overloading of the cell by exocytosis (Falcone, Cocucci, Podini, Kirchhausen, Clementi, & Meldolesi, 2006). Another method of antigen capture is through endocytosis. Specific receptors on the dendritic cells recognize certain chemical and structural components on the cell surface of the antigen. The most commonly known receptors are mannose and DEC205 which recognize glycosylated antigens like the C-type lectins; scavenger receptors that bind apoptopic (dying) bodies and heat shock proteins receptors that recognize antigens form tumour or infected cells. Other ligands are the lipopolysaccharide coating on bacteria, TNF (tumor necrosis factors on cancer cells), and interleukin. The DC with the ligand-receptor complex is engulfed in clathrin-coated pits on the cell surface, which later separate from the peripheral tissues and enter the lymphoid organs. Phagocytosis is best described by the example of the Langerhans' cells of the skin. These cells contain large granules, Birbeck granules, which are actively phagocytic. In phagocytosis, receptors bind with specific ligands ultimately resulting in uptake of large particulate antigens including bacterium, yeast cells, fungal hyphae, and necrotic bodies. Danger signals Aside from taking up, processing, and presenting antigens, receptors perceive different danger signals. To initiate T cell responses, DCs modify their function based on the ‘danger’ signals received. Upon perception of these danger signals, the DCs mature and migrate to the lymphoid tissues to where they initiate immune responses. These receptors cause the DC to utilize intracellular machinery that allow the MHC class II to associate with the immunogenic peptides in the proper endosomal compartments; accumulation of MHC class II-peptide complexes and costimulatory elements in vacuoles and their coexpression on cell surfaces; and further modulation of the immune response through the release of cytokines (Lipscomb & Masten, 2002). Direct danger signals that arise from foreign matter or the extracellular environment are processed by toll-like receptors (TLRs), a highly conserved family of integral membrane receptors that have extracellular leucine rich regions. Specific ligands have been identified for vertebrate TLR 2 (Gram-positive bacteria components, bacterial lipoproteins), TLR 4 (lipopolysaccharides in Gram-positive bacteria), and TLR 9 (specific unmethylated DNA sequences). Studies have shown that only immature DCs have the full capability to respond to microbial products (Lipscomb & Masten, 2002). Indirect danger signals arise from new compounds that are formed because of tissue damage and cell necrosis like cellular HSPs, cytokines, and cell surface ligands. HSPs bind to DC receptors, induce DC maturation, and stimulate migration of DCs into secondary lymphoid tissue. Antigen processing and presentation In the phagosome, endosome, or macropinosomes, antigens are degraded into peptides by proteases such as cathepsin-L and cathepsin-S. The level of the protease inhibitor, cystatin C, in dendritic cells have been used to differentiate immature DC from mature DC. In immature DC, high cystatin C levels indicate that protease activity is very low, while mature DC have low cystatin C concentration (Zavašnik-Bergant, Bergant, Jeras, & Griffiths, 2006). This shows that antigen processing occurs in mature DC. However, the capacity to capture antigens is lost. Instead, the mature DC express high levels of MHC (major histocompatibility complex) class I and MHC class II molecules; proteins upon which peptides are presented. Very high levels of adhesion molecules and B7 molecules (T-cells stimulators) are also expressed. The chemokine DC-CK, expressed in DC only, is secreted to attract naive T cells. After proteolysis, peptide fragments that are produced bind to preformed MHC class II molecules. The MHC class II is a membrane spanning protein heterodimer (Figure 1). It has two peptide chains, α and β, that associate without covalent bonds but possibly through H-bonding. The bound peptide fragment sits within the groove between the two chains. Figure 1. Ribbon structure of the MHC class II complex. MHC Class II molecule is composed of two heterodimers. The domains are named α-1 (blue-green), α-2 (green), β-1 (purple), and β-2 (magenta). The bound peptide (cream) sits within the groove. From http://www.cryst.bbk.ac.uk/pps97/assignments/projects/coadwell/006.htm The peptide chains are synthesized in the endoplasmic reticulum and transported to the MIIC (a specialized late, acidic liposome) where further processing takes place to enhance the availability of the peptide binding site. This entails the removal of a small peptide called the class II-associated invariant- chain peptide (CLIP) which binds to the MHC class II peptide-binding groove by HLADM in humans or H-2M in mouse. The removal of CLIP will allow the antigenic peptide to bind to the MHC class II molecule. Generally, peptides that bind the MHC class II groove are longer, ranging from 14 to 18 amino acids, than those that bind class I molecules. With the antigenic peptide bound, the MHC class II heterodimer traverses the cytosol in exocytic vacuoles to the cell surface. The B7 family, costimulators of T cells, have been found to associate with the MHC class II/peptide complexes (Turley, et al., 2000) during transport to the cell surface. MHC class I molecules are also cotransported with the complexes. MHC class I molecules are mainly utilized in the presentation of endogenous proteins. However, exogenous antigens can be presented also in MHC class I molecules in a process known as cross presentation (Figure 2). By some unknown mechanisms, antigens like virus particles escape endocytic pathways and are degraded by proteosome pathway. The fragments enter the endoplasmic reticulum through the transporter associated with antigen processing (TAP) to be presented in the MHC class I molecules (Figure 3). Surprisingly, loading of exogenous antigens on MHC class I molecules was found to be only marginally less efficient as that observed for MHC class II molecules under in vivo and in vitro conditions (Storni & Bachmann, 2004). T cell activation T cells develop from the bone marrow stem cells that have migrated to the thymus. T cells that can recognize nonself antigens bound to self- MHC molecules are rigorously selected to leave the thymus for the peripheral and lymphoid tissues. Mature T cells express either CD4 or CD8, and have antigen-binding, immunoglobulin -like surface receptors called T-cell receptors (TCRs) that are associated with invariable CD3 and ζ chains in a unit called the TCR-CD3 complex. Genes that encode the TCR are rearranged, resulting in limitless number of T-cell specificities for the antigen peptide presented in the MHC molecule of an APC. Figure 2. Schematic diagram for the cross presentation of exogenous antigens in dendritic cells Figure 3. DC antigen processing pathways (Figure from Lipscomb and Masten, 2002). The activation of the T cells requires the engagement of both the TCR and the peptide-MHC complex (signal 1) and accessory molecules (e.g., CD28 with CD80 or CD86) (signal 2) (Figure 4). If binding is only by the former engagement, the immune response is weak and the T cell dies by apoptosis or become anergic. Accessory binding is necessary to ensure that the T cells will differentiate and divide, therefore multiplying the immune response. Other factors that drive the immune response are concentration of antigen and cytokines within the APC- T cell environment which can determine if the T cells can differentiate into Type I or Type 2 effector molecules (Lipscomb & Masten, 2002). Figure 4. T cell and antigen-MHC binding to produce the immune response. Figure from http://www.merck.com/mmpe/sec13/ch163/ch163b.html# References Falcone, S, Cocucci, E, Podini, P, Kirchhausen, T, Clementi, E, & Meldolesi, J 2006, ‘Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events’, Journal of Cell Science, vol. 119, pp. 4758-4769. Grimmer, S, van Deurs, B, & Sandvig, K 2002, ‘Membrane ruffling and macropinocytosis in A431 cells require cholesterol’, Journal of Cell Science, vol. 115, no.14, pp. 2953-3962. Lipscomb, MF & Masten, BJ 2002, ‘Dendritic cells: immune regulators in health and disease’, Physiological Reviews,vol. 82, pp. 97-130. Palucka, AK 2000, ‘Dengue virus and dendritic cells’, Nature Medicine, vol. 9, pp. 748-749. Storni, T & Bachmann, MF 2004, ‘Loading of MHC class I and II presentation pathways by exogenous antigens: a quantitative in vivo comparison’, The Journal of Immunology, pp. 6129-6135. Turley, SJ, Inaba, K, Garrett, WS, Ebersold, M, Unternaehrer, J, Steinman, RM, et al. 2000, ‘Transport of peptide-MHC class II complexes in developing dendritic cells’, Science, vol. 288 (5465), pp.522 - 527. Zavašnik-Bergant, T, Bergant, M, Jeras, M, & Griffiths, G 2006, ‘Different localisation of cystatin C in immature and mature dendritic cells’, Radiolology and Oncology ,vol. 40, no. 3, pp. 183-188. Read More