HOW DO T CELLS RECOGNIZE ANTIGEN - PART 1

DO T CELLS RECOGNIZE ANTIGEN - PART 1 (VIDEO 7)

Now, let us move on to the subject of antigen recognition by T cells.

How do T cells recognize antigens and how is it different from antigen recognition by B cells?

As you see here, T cells require dendritic cells for antigen recognition.

When microbes are coming into the tissue, they are captured by dendritic cells, they are

degraded into peptides in endocytic vesicles and transported to the cell surface.

This is antigen uptake and degradation by dendritic cells.

The microbial products enter the cleft of major histocompatibility complex or MHC molecules

and displayed as peptides to T cells.

This is antigen presentation.

When a T cell has an appropriate T-cell receptor that can recognize the peptide, the T cell

makes a conjugate with the antigen-displaying dendritic cells.

However, this alone is not sufficient for T cells to recognize an antigen successfully.

At the same time, costimulatory molecules that are expressed by dendritic cells and T cells must

bind to each other.

So, when both T-cell receptor-peptide-MHC interactions and costimulatory molecule interactions

take place, T cells can recognize antigens successfully, and as a result, they become activated.

Let us look at this issue a bit more closely.

The binding of MHC-peptide complex with T-cell receptor alone provides only one of the signals

required for T cell activation.

When antigen-presenting dendritic cells are unstimulated, they are lacking in costimulatory

molecule expression, although T cells always express costimulatory molecules such as CD28.

In the absence of costimulatory molecules binding between dendritic cells and T cells, binding

of MHC-peptide complex with T-cell receptor alone can provide only signal 1, which alone is not

sufficient for T cell activation.

So, no effective T cell responses occur.

However, when dendritic cells have been activated by microbes and/or by their products

sufficiently, they show increased expression of costimulatory molecules, such as B7, now B7 is

appearing on the surface of dendritic cells, which can provide strong binding between dendritic

cells and T cells.

The activated dendritic cells can also stimulate T cells by secreting cytokines, thus providing

another signal, which is called signal 2, that is essential for proper T cell activation.

So, T-cell receptor-MHC-peptide interactions provide only signal 1, whereas interactions among

costimulatory molecules provide signal 2 to T cells.

In order for a T cell to respond to an antigen properly, signal 1, which is antigen-specific, and

signal 2, which is not antigen-specific are both required.

So, T cell activation requires not only T cell receptor ligation with antigen, but also costimulation.

Costimulation is absolutely essential for T cell recognition.

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