What is the Difference Between Positive and Negative Selection of T Cells

The main difference between positive and negative selection of T cells is that in the positive selection of T cells, the TCRs (T cell receptors) of mature T cells bind to the self-antigens presented along with HLA molecules by thymocytes. But, in the negative selection of T cells, the TCRs of the mature T cells interact strongly with self-antigens of thymocytes. Furthermore, the positive selection of T cells results in the maturation of thymocytes either into CD4+ or CD8+ T cells while the negative selection of T cells results in the apoptotic cell death of thymocytes. 

Positive and negative selection of T cells are two processes of the T cell development, which occurs in the thymus. Here, thymocytes are a large population of immature T cells. 

Key Areas Covered 

1. What is the Positive Selection of T Cells
     – Definition, Process, Importance
2. What is the Negative Selection of T Cells
     – Definition, Process, Importance
3. What are the Similarities Between Positive and Negative Selection of T Cells
     – Outline of Common Features
4. What is the Difference Between Positive and Negative Selection of T Cells
     – Comparison of Key Differences

Key Terms 

Apoptosis, Negative Selection of T Cells, Positive Selection of T Cells, T Cell Development, Thymocytes 

What is the Positive Selection of T Cells 

Positive selection of T cells is the maturation process of thymocytes or the immature T cells in the thymus. Hematopoietic stem cells in the bone marrow differentiate into the lymphoid progenitor cells called thymocytes. Significantly, the earliest stages of these thymocytes express neither CD4 or CD8 receptors. Hence, they are called double-negative (CD4-CD8-) thymocytes. However, positive selection is the process through which these thymocytes develop into double-positive (CD4+CD8+) cells. Here, the TCRs of mature T cells bind to the self-antigens presented along with the HLA molecules by thymocytes. Upon this binding, thymocytes develop either CD4 or CD8 receptors on their cell membrane. 

Figure 1: T Cell Development

Subsequently, the positively-selected thymocytes move into the boundary of the cortex and medulla of the thymus for the presentation of self-antigens by the medullary thymic epithelial cells for the second time.  

What is the Negative Selection of T Cells 

The negative selection of T cells is another process of T cell development. More importantly, this process removes T cells that are capable of binding strongly with self-antigens presented along with the MHC complex. The strong binding of mature T cells with self-antigens generates apoptotic signals, inducing cell death. However, some of these thymocytes become Treg cells while the remaining thymocytes exist as immature naive T cells in the thymus. Furthermore, the negative selection of T cells prevents the formation of self-reactive T cells.  

Similarities Between Positive and Negative Selection of T Cells 

• Positive and negative selection of T cells are two processes responsible for T cell development. 
• Both processes occur in the thymus. 
• Also, mature T cells are responsible for both processes. 
• Besides, both of the processes occur at the development stage of CD4+ and CD8+ T cells. 

Difference Between Positive and Negative Selection of T Cells 

Definition 

Positive selection of T cells refers to the process of T cell development in the thymus by the binding of TCRs of mature T cells to the self-antigens presented by MHC complex of thymocytes. Negative selection of T cells refers to another process of T cell development in thymus due to the incapability of TCRs to bind the MCH complexes of thymocytes. Thus, this explains the main difference between positive and negative selection of T cells. 

Results in 

Another difference between positive and negative selection of T cells is in their results. The positive selection of T cells results in the maturation of thymocytes into CD4+ or CD8+ T cells while the negative selection of T cells results in the cell death of thymocytes. 

Importance 

Moreover, positive selection of T cells is responsible for the maturation of thymocytes while the negative selection of T cells prevents the formation of self-reactive T cells. Hence, this is another difference between positive and negative selection of T cells.

Conclusion 

Positive selection of T cells occurs through the interaction of TCRs of mature T cells with the self-antigens presented along with HLA complex by thymocytes. Further, this process is responsible for the development of CD4 and CD8 receptors on thymocytes. On the other hand, negative selection of T cells occurs by the strong interactions of TCRs of mature T cells with the self-antigens on the thymocytes. And, this process induces cell death through apoptosis, preventing the formation of self-reactive T cells. Hence, the main difference between positive and negative selection of T cells is the effect of selection on thymocytes.

References:

1. “Positive and Negative Selection of T Cells.” ImmunoBites, 20 Aug. 2018, Available Here

Image Courtesy:

1. “2217 Differentiation of T Cells Within the Thymus” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia