Class II MHC molecules can use the endogenous pathway of antigen presentation

Article Abstract:

For an individual to develop an immune response to a foreign agent, such as a bacteria or virus, antigen processing cells (APCs) must process an antigen or protein from the foreign agent to an immunogenic peptide. The processed antigen combines with molecules in immune cells known as histocompatibility antigens. The molecules form a complex which can be recognized by a subset of T lymphocytes known as helper T cells. The helper T cells then react with other cells in the immune system and antibody molecules are produced, leading to the destruction of the foreign agent. It is thought that there are two different pathways for antigen processing, one for exogenous antigens, proteins which are on the outside of foreign organisms, and another for endogenous antigens, proteins that are present on the inside of an organism. Exogenous antigens are thought to be taken up into compartments located in the cytoplasm of the cell and processed to join with class II histocompatibility antigens. Endogenous antigens are present in the cell cytoplasm, but are not associated with a compartment; they are degraded and complexed with class I histocompatibility antigens. Treatment of cells with two different drugs causes the separation of the two pathways, allowing each to be studied individually. It was shown that an endogenous protein from the influenza A virus can be processed and combined with the class II histocompatibility antigen by either of the two pathways. The results suggest that there may not be separate pathways for endogenous and exogenous antigens to be processed and to combine with either class I or class II molecules. These studies further the understanding of how the immune system is able to mount an effective response against foreign organisms that might cause disease. (Consumer Summary produced by Reliance Medical Information, Inc.)

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Defective processing and presentation of exogenous antigens in mutants with normal HLA class II genes

Article Abstract:

For an individual to develop an immune response to a foreign agent, such as a bacteria or virus, antigen processing cells (APC) must process an antigen or protein from the foreign agent to an immunogenic peptide. The processed antigen combines with molecules on immune cells known as histocompatibility molecules. These molecules form a complex which can be recognized by a subset of T lymphocytes known as helper T cells. The helper T cells then react with other cells in the immune system and antibody molecules are produced to act against the foreign agent. The foreign agent is then destroyed. The mechanisms of the processing of the protein and the combination of the two molecules to form a complex is not well understood. Defective APCs which cannot process proteins for presentation to helper T cells, but can present immunogenic peptides from these proteins were used in a study. It was shown that the genes which code for the histocompatibility antigens were normal, but the molecules could not form the proper protein conformation or shape needed for an effective molecule. The mutant APCs can be used for further study of antigen processing and presentation and to understand how the immune system makes an effective antibody response against various foreign organisms which can cause disease. (Consumer Summary produced by Reliance Medical Information, Inc.)

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Quantitation of antigen-presenting cell MHC class II/peptide complexes necessary for T-cell stimulation

Article Abstract:

Antigens, molecules that are recognized by the immune system as foreign, are processed into peptides by antigen-presenting cells. The peptides form a complex with the major histocompatibility antigens on the cell surface, which are then recognized by T cells, resulting in the activation of an immune response. The number of complexes necessary to activate T cells was determined to be as little as 210 to 340. Therefore, it does not take a large number of peptides to bind to histocompatibility antigens to stimulate the immune response. T cells are very sensitive to the presence of antigens, as there are many more class II histocompatibility antigens than are occupied by the peptides, and these could bind to other peptides. This explains why stimulation can occur even if there is competition by other peptides for the histocompatibility antigens. (Consumer Summary produced by Reliance Medical Information, Inc.)

Immunity, Immunity (Physiology), Immune response

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