Costimulatory activity of dendritic cells - Essay Example

T cell activation is a highly regulated event involving complex receptor-ligand interactions, ultimately leading to downstream signaling events (Annu Rev Immunol. 2002). Optimal activation of naïve T cells requires at least two signals, antigen recognition and co-stimulation (Bretscher and Cohn 1970).The first signal requires engagement of an antigen receptor by foreign antigen. Professional antigen presenting cells (APCs) such as dentritic cells, macrophages and B-lymphocytes play a major role in this.

They present different sets of antigens and serve to activate T cells at different points during immune response. These cells on encountering foreign antigens such as bacteria, engulf and destroy them. The major histocompatible cell (MHC) present in the APCs binds to a piece of this antigen and displays the antigen to the cell surface. This MHC molecule that displays the antigen is recognized by a compatible T cell receptor (TCR). Thus, these APCs communicate with a T cell (Understanding Autoimmune Diseases.

How Does the Immune System Work? 2007).For a T cell to respond to a foreign antigen on the MHC, a second signal is required. (P.Anton van der Merwe.2000).This second signal or co-stimulation is an antigen-independent signal required for sustained cell proliferation, effector/memory cell generation and prevention of energy or apoptosis. APC participation is required in the second signaling also. In addition to antigen presentation, these cells provide co stimulatory signals. . Provide the co-stimulatory signal necessary for T cell activation.

When anaive T cell binds to its specific peptide: MHC complex, the CD28 on the T cell's surface binds to the B7 molecules on the APC (Fig.1). These two signals together lead to the production of several cytokines.Fig.1: T cell activation by two-signal pathway. [Online] Available at http://www.clinsci.org. Dendritic cells are the most effective stimulators of T cell activation. They continuously express high levels of co stimulatory B7 (Fig 2). Upon recognition of infectious particles, these cells migrate through the lymphatics to the nearest lymph node.

In the follicles of the lymph node, theycome into close contact with the nave T cells where it begins expression of the B7 molecules. Once the T cells are activated, they will leave the lymph node and travel to the sites of inflammation. Macropohages, in the absence of infection, express low levels of MHC II and almost no co stimulators (B7). In the case of infection, however macrophages poses certain types of receptors that recognize differential carbohydrate patterns on foreign cells. They also have receptors for specific bacterial products such as lippolysaccharide endotoxin.

When these molecules bind with their ligands, they stimulate the macrophages to up regulate MHC II and B& providing these cells with strong antigen presenting properties. They also start to secrete cytokines that aid in their functions. It is at this point that antigen presentation by MHC II will activate T cells. MacrophageDendritic CellB cellMHC-II ExpressionLow levels. Induced by Bacteria and/or CytokinesAlways Expressed.Always Expressed. Inducible upon