89. Under which of the following circumstances do T cells develop anergy?
(1) With the expression of CD69 on T cells.
(2) When the CD4/ CD8 molecules present on T cell surfaces do not recognize self MHC II/MHC I molecules.
(3)When the MHCII molecules present on antigen presenting cells bind to the peptides with less avidity.
(4) When co-stimulatory molecules present on the antigen presenting cells fail to interact with T cells.
Introduction
T cell anergy is a crucial mechanism in the immune system that helps maintain immune tolerance and prevent autoimmunity. Anergy refers to a state of functional inactivation or non-responsiveness of T cells to antigenic stimuli, which prevents T cells from attacking the body’s own tissues. Understanding when and why T cells become anergic is essential for unraveling the complex regulation of immune responses.
In this article, we will explore the different scenarios in which T cell anergy occurs, the role of antigen-presenting cells (APCs), and how this mechanism contributes to immune tolerance and the prevention of autoimmune diseases.
What is T Cell Anergy?
T cell anergy is a state of immune unresponsiveness in which T cells are unable to mount an immune response despite being exposed to an antigen. This unresponsiveness is typically induced during T cell activation when certain signals that are essential for activation are missing or defective. The process of anergy is crucial for the maintenance of self-tolerance, meaning that the immune system is trained not to attack the body’s own tissues and organs.
Mechanisms Leading to T Cell Anergy
T cell activation requires two key signals:
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Signal 1: Recognition of a specific antigen by the T cell receptor (TCR) on the T cell, which is presented by major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs).
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Signal 2: Co-stimulatory signals, typically provided by the interaction of co-stimulatory molecules such as CD28 on the T cell with B7 molecules (CD80/86) on APCs.
If either of these signals is insufficient or absent, T cells may enter an anergic state.
Conditions that Induce T Cell Anergy
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Failure of Co-stimulatory Signals: One of the most common conditions under which T cells develop anergy occurs when co-stimulatory molecules on antigen-presenting cells (APCs) fail to interact with the T cell. Co-stimulatory signals are essential for the full activation of T cells. Without proper co-stimulation, T cells do not receive the necessary signals to fully activate, leading to anergy.
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Example: In the absence of CD28 binding to B7 molecules (on APCs), the T cell may recognize the antigen but fail to receive the second necessary signal for activation, resulting in an anergic state.
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Defective MHC Recognition: If CD4 or CD8 molecules on T cells fail to recognize the self-MHC molecules properly, the T cell may not receive the proper signals to activate. This can occur during thymic selection or when there is a lack of appropriate MHC molecules to present the antigen, leading to a state of anergy. T cells are trained to recognize foreign antigens presented by MHC, and failure to recognize these molecules may prevent the activation of T cells.
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Low Affinity Binding to MHC-peptide Complexes: When the interaction between MHC class II molecules on APCs and peptides presented to T cells occurs with low avidity (weak binding), it is a signal that the T cell should not mount a full immune response. This weak interaction is often interpreted by the T cell as a non-dangerous signal, leading to anergy.
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Expression of CD69 on T Cells: CD69 is an early activation marker on T cells. Its expression on T cells can be a sign of immune activation. However, CD69 alone does not induce anergy. The development of anergy is more strongly associated with the lack of co-stimulatory signals rather than the expression of CD69.
The Role of T Cell Anergy in Immune Tolerance
The primary function of T cell anergy is to maintain immune tolerance. By preventing T cells from responding to self-antigens, anergy helps to avoid autoimmune reactions, where the immune system attacks the body’s own tissues. Anergy is a mechanism that contributes to the prevention of autoimmunity, ensuring that T cells only respond to pathogens or foreign antigens and not to self.
Anergic T cells may be removed by clonal deletion or remain in a non-functional state in peripheral tissues. The ability to induce anergy is a key part of how the immune system discriminates between self and non-self, ensuring the body’s defense against infections without harming its own cells.
Answer to the Question:
The correct answer is:
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(4) When co-stimulatory molecules present on the antigen presenting cells fail to interact with T cells.
The lack of co-stimulatory signals from antigen-presenting cells is a critical factor in the induction of T cell anergy. Without these signals, T cells are unable to fully activate, and they enter a state of functional inactivation, preventing them from responding to antigens.
Conclusion
T cell anergy is an essential mechanism for maintaining immune tolerance and preventing autoimmune diseases. By understanding the conditions under which T cells develop anergy—such as the failure of co-stimulatory interactions and weak antigen recognition—researchers can gain insights into the regulation of immune responses and how to potentially manipulate these processes for therapeutic purposes, such as in the treatment of autoimmune disorders or organ transplantation.
Related Keywords: T cell anergy, co-stimulatory signals, immune tolerance, antigen-presenting cells, CD28, B7 molecules, self-tolerance, autoimmune disease, immune regulation.
