T Lymphocytes

The immune system is made up of two components, innate and adaptive immunity. Innate immunity is a non-specific but very crucial first defense of the body. Adaptive immunity is what we acquire during our lifetime due to exposure of various pathogens. Adaptive immunity is very specific and not only fights and neutralizes the pathogens, but also remembers the pathogen so that re-infection with the same pathogen leads to faster response. Though functionally different, both innate and adaptive immunity works hand in hand for the optimal immune responses. 

1.0T Lymphocytes Introduction

• Adaptive immunity is driven by B- and T-cells which perform distinguished functions during microbial invasions. 
• T-cells play an important role in mounting effective responses against pathogens and cancers whereas B- cells are the main antibody secreting cells. 
• The important structural and functional attributes of T cells are as follows:  T-cells are named so as they mature in thymus.  
• Among several B- and other cells, T cells constitute a major proportion of blood cells.  
• They play an important role in defense as they recognize the antigen presented by class I and II Major Histocompatible Complex (MHC) molecules on Antigen Presenting Cells (APCs).
• The distinguished characteristic of T-cells is that they can recognize antigens displayed on MHC molecules same as that of the host's MHC from which they are isolated. T-cells demonstrate self-MHC restriction.  
• Most T-cells don’t activate in response to the self-antigens of the host from which they are isolated. Hence, T-cells demonstrate self-tolerance.  T-cells have T-cell Receptor (TCR) with the help of which they recognize antigen on self MHC molecules. Along with TCR, they have a CD3 molecule which helps in transducing downstream signaling after antigen binding and leads to activation.  
• Most T-cells have TCR with alpha and beta chains (αβ T-cells), few are present with gamma and delta chains (γδ T-cells). The γδ T-cells differ from αβ T-cells in antigen recognition as they have limited antigen diversity and recognize antigen presented by CD1 molecules (MHC-like molecules).  αβ T-cells also have CD4 or CD8 co-receptors which further helps in antigen recognition. 
• In fact CD4 displaying T-cells (also known as T- helper cells) can interact with antigen presented on MHC Class II molecules. So CD4 T-cells are class II MHC restricted. While CD8 T-cells (also known as T cytotoxic cells) are class I MHC restricted. The γδ T-cells don’t have CD4 as well as CD8 co-receptors.

2.0T-Cell Development 

Origin in Bone Marrow

• T-cells like other blood cells originate from multipotent Hematopoietic Stem Cells (HSCs) in the bone marrow. 
• Hematopoietic cells differentiate into myeloid and lymphoid progenitor cells under the influence of lineage specific transcriptional factors. 
• The lymphoid progenitor cells further differentiate into T-cell progenitor cells among many other cells.

Development in Thymus: Movement and Maturation

• T-cell progenitor cells migrate to thymus at about day 11 of gestation in mice and in the 8th - 9th week of gestation in humans. 
• Further maturation of T-cell progenitor cells happens in the thymus. The developing T-cells are also known as thymocytes. 
• During maturation, thymocytes interact with stromal cells of the thymus such as epithelial cells, dendritic cells and macrophages and their interaction contributes to the growth and maturation of thymocytes. 
• The major changes include:  Arrangement of the germ line TCR genes followed by acquisition of functional TCR genes.  Expression of specific cell surface / membrane markers such as CD4 and CD8.  They get trained by thymic education. 
• This includes recognition of MHC molecules on thymic stromal cells, thus becoming self-MHC restricted and non-reactive to self antigens, thus becoming self-tolerance. 
• This training is popularly known as thymic selection. The maturation steps can be divided into various stages 

1. Stage 1: T-cell Progenitor Cells Proliferation and Differentiation

• Basically T-cell progenitor that arrives at thymus is completely different from mature T-cell and can be stained for the following molecules:
• C-Kit (receptor for stem cell growth factor)  CD44 (adhesion molecule involved in homing)  No rearrangement of TCR genes and lacks TCR and CD3 complex.  No expression of CD4 or CD8 co-receptors (CD4 and CD8 negative or double negative (DN1) T-cell progenitor cells generally enter at the outer cortex of thymus and slowly proliferate for about three weeks. 
• During proliferation it starts expressing CD25 (alpha chain of IL-2 receptor). At this stage, the cell is DN2 (double negative 2). Then it differentiates into DN3 under the influence of thymic stromal cells, during differentiation it shows.
• Reduce expression of c-Kit and CD44.  CD25 (alpha chain of IL-2 receptor) expression.  Switch on the expression of RAG1 and RAG2 gene expression, the product of which is required for rearrangement of TCR genes.  
• Initiate rearrangement for beta chain genes of TCR.  Some shows rearrangement for gamma and delta chain genes. A DN3 cell with successfully completed rearranged beta chain survives and loses CD25+ and progresses to DN4 stage. 
• The cells that fail to rearrange beta chain locus remain at DN3 (CD44- and CD25+ ) stage and soon die by apoptosis .

2. Stage 2: Formation of pre- T-cells 

• The beta chain combines with 33-KDa glycoprotein known as pre- T alpha chain and associates with the CD3 group to form a complex called as pre-T-cell receptor or pre-TCR. Studies indicate that the pre-TCR complex get involved in signal transduction in ligand-independent manner via CD3 which in turn helps in the following process
• Proliferation of pre-T cells  Suppress further rearrangement of beta chain genes  Induce expression of CD4 and CD8 membrane molecules.  Permit for rearrangement of alpha chain genes. 

3. Stage 3: Formation of Double Positive (DP) Pro

• T-cells with Rearranged Alpha ChainThe Pre-T cells proliferate in a rapid manner. Although they show increased expression of RAG1 and RAG2 genes, in proliferating cells RAG 2 proteins degrade rapidly. Only cells with rearranged beta chains proliferate and make clones of themselves. 
• When the proliferative phase ends, the alpha chain genes show rearrangement. In different clones, different alpha chain genes undergo rearrangement which further adds to diversity of rearranged TCR genes. The Pro-T cells with rearranged alpha and beta chains are also both CD4 and CD8 positive hence Double Positive (DP) cells.

3.0Mechanism for Thymic Selection and Single Positive Cells Formation

• During positive selection, the interaction between TCR of thymocytes and peptide-MHC molecules on stromal cells lead to survival of cells with self-MHC restriction. 
• During negative selection, the interaction leads to thymocytes elimination. So both selections would lead to very poor outcomes for T-cells. However, interestingly it is not the case. There is still a way that a significant number of T cells is generated. 
• There are two hypotheses proposed to distinguish interaction between positive and negative selection.
• Differential avidity hypothesis: The interaction between TCR and MHC-peptides complex in thymus determines distinct signal for selection . For example –  No interaction leads to no signal which results in death of thymocytes,  weak interaction leads to less signal required for survival of thymocytes (positive selection)  Very strong interaction leads to more signals which results in elimination or negative selection. 
• Differential signaling hypothesis: the interaction between TCR and MHC-peptides complex involves different sets of peptides. Agonist peptides deliver very strong signals which lead to negative selection. Antagonist peptide delivers a partial signal which favors positive selection.