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In immunology, a naive T cell (T h 0 cell) is a T cell that has differentiated in the thymus, and successfully undergone the positive and negative processes of central selection in the thymus. Among these are the naive forms of helper T cells ( CD4 + ) and cytotoxic T cells ( CD8 + ).
CD2 was shown to prime naive T cells (T N) even without CD28 or TCR. [2] Also, CD27 is a receptor constitutively expressed on T N (its expression is downregulated upon TCR stimulation) and enhances T cell proliferation. [9] The differentiation of T helper cells (T H) into different subsets also partially depends on their co-stimulatory molecules.
The activation of naive T cells is commonly explained in terms of the 3-signal model, elaborated upon below. [10] Activation (signal 1)
This activation of naive T cell is controlled by a variety of signals: recognition of antigen in the form of a peptide: MHC complex on the surface of a specialized antigen-presenting cell delivers signal 1; interaction of co-stimulatory molecules on antigen-presenting cells with receptors on T cells delivers signal 2 (one notable example ...
Markers of T cell activation include CD69, CD71 and CD25 (also a marker for Treg cells), and HLA-DR (a marker of human T cell activation). CTLA-4 expression is also up-regulated on activated T cells, which in turn outcompetes CD28 for binding to the B7 proteins. This is a checkpoint mechanism to prevent over activation of the T cell.
CD3 (cluster of differentiation 3) is a protein complex and T cell co-receptor that is involved in activating both the cytotoxic T cell (CD8+ naive T cells) and T helper cells (CD4+ naive T cells). [1] It is composed of four distinct chains. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains.
CD28 (Cluster of Differentiation 28) is a protein expressed on T cells that provides essential co-stimulatory signals required for T cell activation and survival. When T cells are stimulated through CD28 in conjunction with the T-cell receptor (), it enhances the production of various interleukins, particularly IL-6.
The activation of cross presenting dendritic cells is dependent on stimulation by CD4+ T helper cells. The co-stimulatory molecule CD40/CD40L along with the danger presence of an exogenous antigen are catalysts for dendritic cell licensing, and thus the cross presentation and activation of naive CD8+ cytotoxic T cells. [11]