CO-STIMULATORY MOLECULES ARE ESSENTIAL FOR T CELL ACTIVATION

The process of activating T cells generally takes place in the lymph node nearest to the infection. The TCR recognizes a specific peptide lodged in the peptide- binding groove of the MHC molecule. This interaction dictates immunological specificity because a peptide asso- ciated with an MHC molecule forms a unique structure to be recognized by the TCR. Other molecules have a complementary role in this interaction.

The initial encounter of T cells with APCs is by non- specific binding through adhesion molecules. This transient binding by adhesion molecules permits the T cell to encounter a large number of different MHC molecule–peptide combinations on different APCs. In the absence of a specific interaction, the APC and T cell rapidly dissociate.

Absence of HLA-DM leads to failure of MHC class II molecules to bind appropriate peptides

Fig. 7.14 In normal APCs (left) peptides are loaded onto class II molecules, as shown in Fig. 7.13. In the absence of HLA-DM (right), the CLIP fragment is not removed, and the blocked MHC molecule is unable to activate T cells.

CLIP T cell activation No T cell activation

DM+ presenting cell DM– cell TCR peptide class II class II DM T cell T cell

Cross-presentation when exogenous peptides are presented by MHC class I molecules

Fig. 7.15 DCs can present internalized antigen via both the MHC class I and class II pathways. This is atypical – external antigen is usually presented via the class II pathway. Peptides are transferred from the endosome to the cytosol where they are processed by the proteasome to enter the class I pathway.

peptide MHC class I MHC class II processing endocytosis antigen dendritic cell TCR CTL

Crucially, co-stimulatory molecules act together with the antigen-specific signals before the T cell is sanctioned for proliferation. Co-stimulatory and antigen-specific signals must be present simultaneously on the same cell.

Overall, antigen presentation through MHC class I or class II molecules can be split into four stages – adhesion, antigen-specific activation, co-stimulation, and cytokine signaling (see Fig. 7.1).

Multiple cell surface molecules interact during antigen presentation to T cells

Intercellular adhesion molecules (ICAMs), particularly ICAM-1 (CD54), interact with the integrin, lymphocyte functional antigen-1 (LFA-1 or CD11a/CD18), present on all immune cells.

If mouse cells are transfected with both human MHC and human ICAM-1, their capacity to act as human APCs is augmented.

When the T cell encounters the appropriate MHC molecule–peptide, which happens rarely except during an ongoing infection, a conformational change in LFA-1 on the T cell, signaled via the TCR, results in tighter binding to ICAM-1, which results in prolonged cell–cell contact. The joined cells can exist as a pair for long periods, allowing time for the T cell to proliferate and differentiate.

The specific MHC molecule–peptide–TCR interac- tion, though necessary, is not sufficient to fully activate the T cell. A second signal is required, otherwise the T cell will become unresponsive. This second signal, also referred to as co-stimulation, is of crucial importance.

Some co-stimulatory molecules that interact with ligands on the T cell’s surface are shown in Fig. 7.16.

The most potent co-stimulatory molecules known are B7s, which are members of the immunoglobulin super- family molecules; they include B7-1 (CD80) and B7-2 (CD86).

Several other B7-related molecules are beginning to emerge.

B7s exist as homodimers on the cell surface. These proteins are constitutively expressed on DCs, but can be upregulated on monocytes, B cells, and probably other APCs.

Upregulation of co-receptors is stimulated by inflam- mation and by interaction of microbial products with Toll- like receptors (TLRs, see Fig. 6.24) on the APC.

Co-receptors are the ligands for other immunoglobulin superfamily molecules – CD28 and its homolog CTLA-4 (CD152), which is expressed after T cell activation. CD28 is the main co-stimulatory ligand expressed on naive T cells. CD28 stimulation:

• has been shown to prolong and augment the pro- duction of IL-2 and other cytokines; and

• is probably important in preventing the induction of tolerance.

Although the CD28–B7 interaction is extremely impor- tant, CD28 knockout mice do respond to antigen, but require higher doses, so CD28 triggering is not oblig- atory, even for naive T cells. In CD28 knockout mice other co-stimulatory signals probably replace that deliv- ered by CD28–B7.

CTLA-4, the alternative ligand for B7, is an inhibitory receptor limiting T cell activation, resulting in less IL-2 production. Thus CD28, constitutively expressed, initially interacts with B7, leading to T cell activation. Once this has peaked, the upregulation of CTLA-4 with its higher affinity limits the degree of activation because available B7 will interact with CTLA-4 (Fig. 7.17).

Q. What effect would you expect to see in mice that have the CTLA-4 gene knocked out?

A. They suffer from an aggressive lymphoproliferative disorder, because they do not inactivate dividing T cells efficiently (see Fig. 7.17).

CO-STIMULATORY MOLECULES ARE ESSENTIAL FOR T CELL ACTIVATION

Critical molecules involved in antigen presentation

Fig. 7.16 The molecules involved in the interaction between T cells and APCs. The various cytokines and their direction of action are also shown. In humans LFA-3 (CD58) acts as a ligand for CD2, but in rodents CD48 performs this function.

T cell IL-1 IL-6 TNF IL-12 IL-15 IFN GM-CSF IL-4 TNF LFA-1 _TCR CD28 CD2 CD4 p56Lck APC ICAM-1 class II LFA-3 (CD48) B7-1/B7-2 (CD80/CD86)

Role of CTLA-4 in controlling T cell activation

Fig. 7.17 Before activation, T cells express CD28, which ligates B7-1 and B7-2 on APCs (e.g. B cells). After activation, CTLA-4 is expressed, which is an alternative high-affinity ligand for B7. CTLA-4 ligates B7, so the T cells no longer receive an activation signal.

T T T APC APC inactivation activation division CD28 B7 CTLA-4 B7

The CD2 molecule on T cells is also involved in T cell activation, in conjunction with the TCR. CD2 is a receptor for LFA-3 (CD58), which is widely distributed on cells and is present on all APCs. In rodents CD48 binds to CD2 and appears to be functionally equivalent to LFA- 3 in humans. Both CD2 and LFA-3 are members of the immunoglobulin superfamily.

Co-stimulation via B7 is a necessary signal for T cell activation

Resting T cells cannot respond optimally without co- stimulation via B7, and if they recognize their antigen in a non-stimulating manner they become inactivated, pro- ducing a state of immunological tolerance (Fig. 7.18). This tolerance is specific, only affecting TH cells that

respond to a particular antigen. Persistent tolerance without cell death is known as clonal anergy.

As discussed in Chapters 2 and 12, many T cells are made tolerant to self molecules during their development in the thymus. However, the process is not perfect and many autoreactive T cells are present in the circulation. These cells may still be tolerized outside the thymus because, although they are specific for self MHC–peptide, this is often first encountered on cells that lack the appropriate co-stimulatory signals.