Question

Describe the process of T cell activation by tissue-resident Dendritic cells following infection, focusing on the maturation process of the DCs and how this process impacts the DC function during this process (Hint: Immature DCs and Mature DCs). Why does this process make DCs the primary antigen presenting cells for naive T cells compared to macrophages and B cells? (10 points)

Answer 1

Dentritic cells are antigen- presenting cells of the mammalian immune system. The main function is to process an antigen and then present it to the T cells. These cells are present in those tissues that are in contact with the external environment like skin and the inner linings of the nose, lungs, stomach and intestines. Once activated they migrate to the lymph nodes where they interact with the T cells.

Dentritic cells are formed from hematopoietic bone marrow progenitor cells. These initially transform into immature dendritic cells characterized by high endocytic activity and low T-cell activation potential. They move around sampling the environment for pathogens. This is done by Pattern Recognition Receptors (PRRs). They identify specific chemical signals on the pathogens. They may also phagocytose small quantities of membrane from live own cells, a process called nibbling. Once they come into contact with an antigen they become mature Dendritic cells and begin to migrate to the lymph nodes. Immature cells phagocytose pathogens and degrade their proteins into small pieces. On maturing they present these fragments at the cell surface using MHC. Meanwhile they also upregulate cell surface receptors that act as co-receptors in T-cell actibvation. Once they reach the lymph node they act as Antigen Presenting Cells by activating helper T-cells and Killer T-cells. The T-cell thus activated by the mature dendritic cell is thus specific to that antigen.

Macrophages, B lymphocytes and dendritic cells (Antigen presenting cells) are the only cells capable of activating a resting helper T-cell when the corresponding antigen is presented. However in non-lymphoid organs, Macrophages and B cells can activate only memory T cells. Dendritic cells can activate both memory T cells and also naive T cells. Naive T cells are those that have not encountered an antigen before. Macrophages and B cells can activate naive T cells only if they are in the lymphoid or secondary lymphoid organs. This is the reason why dendritic cells can be primary antigen presenting cells for naive T cells as compared to B cells and macrophages.

DC Progenitor Mature DC High MHC II, CD80, CD86, CD40, CD25, IL-12, CD83, p55, Low FCR. CD54, CD58 GM-CSF FLt-3L Maturation Signal Pathogens. cytokines Immature DC High intracellular MNC II, FCR, Low CD24, CD80, CD86, CD40, CD25, IL-12, CD83, p55