Mechanism of action of T-lymphocytes- Part II: T-Cytotoxic Cells

Introduction

As seen in the earlier blog, T-Helper cells just act to evoke the response by other cells which actually kill the microbe responsible for delivering their antigens; T-Helper cells aren’t cytotoxic themselves. This role is played by another sub-class of T-lymphocytes called as T-Cytotoxic cells.  

Structure of T-Cytotoxic cells

The T-Cytotoxic cells has a structure resembling the picturized diagram given below:

It consists of a regular eukaryotic leukocyte cells; the difference being primarily in the cell membrane molecules. It has 2 main receptors: 1) TCR- T-Cell Receptor is common to both T-Helper cells and T-Cytotoxic cells and functions to bind the MHC classes in the body and thereby getting evoked to play a role lethal for microbial (and even body’s own cells, sometimes) cells. 2) CD8 – This is present only on T-Cytotoxic cells and characterizes this sub-class of lymphocytes.

The initial Stimulus

The initial impulse depends much upon the antigens against which the body has to act, as similarly shown for T-Helper cells. But in this case, these antigens are usually viral antigens (viral peptides). These peptides enter the eukaryotic cells (host’s cells), enter the Rough Endoplasmic Reticulum and get processed to act as an effective antigen. Later, it enters the Golgi Body gets processed further and integrate therein with MHC-I class of membrane proteins and are then expressed over cell membrane. A 2nd way of introduction of viral peptide is the synthesis (by transcription and translation) of viral peptide itself by the viral DNA and from there on similar cascade follows.

The diagram shown below illustrates this phenomenon well.

Now, the eukaryotic cell is ready to evoke the T-Cytotoxic cells and is discussed in the subsequent section.

Response by T-Cytotoxic cells

Now, the eukaryotic cell is ready with the processed antigens. The next task that is performed is to summon the T-Cytotoxic cells. The host cell plays this role probably by secreting various types of cytokines and even interferons, which serve to inhibit the growth of virus. Now, T-Cytotoxic cells approach the infected host cell and kill them after they bind to them because they possess a TCR and CD8 which is specific for MHC-I found over host cells.

Reference Texts

1) Kuby Immunology

2) Abbas, Lichtman basic immunology