CTLA-4

Discovery

In 1987, the immune checkpoint cytotoxic T lymphocyte associated protein (CTLA-4) was discovered (1). It is in many ways, including structurally and genetically, homologous with CD28 (2). Its function however, is entirely opposite. CD28 acts as a co-stimulatory molecule to the T cell receptor:peptide-Major Histocompatibility Complex (TCR:p-MHC), interaction, allowing the development of an effective immune response. TCR:p-MHC complex without CD28 co-stimulation may lead to T cell anergy or immune tolerance. On the other hand, CTLA-4 acts as a 'co-inhibitory' molecule to the TCR:p-MHC interaction and is therefore important in preventing autoimmunity and immunopathology (3).

Signalling

CTLA-4 is present on CD4+ and CD8+ T cells after TCR activation, and on regulatory T cells. As the TCR:p-MHC interaction occurs, CTLA-4 migrates to the area of contact (i.e the immune synapse). The stronger the interaction is, the greater the recruitment of CTLA-4, this functions as an effective negative feedback mechanism in order to prevent overstimulation of the immune response (4). CTLA-4 then downregulates the immune response by acting as an ‘off switch’ to immune signals when bound to B7-1 or B7-2 on the antigen presenting cell (APCs). It does this intrinsically by outcompeting CD28 for the B7-1 or B7-2 by binding with greater avidity and affinity.

Schematic diagram of interaction between CTLA-4 and B7-1 on an APC, taken from reference 3, available at: http://www.nature.com/doifinder/10.1038/35069118

There are also some pretty whacky extrinsic mechanisms that bring about the immunosuppressive effects of CTLA-4:

Firstly, CTLA-4 signalling stimulates production of immunosuppressive cytokines such as transforming growth factor beta (TGF-Beta). In the context of cancer, this would be in addition to the immunosuppressive microenvironment that the tumour cells have created for themselves using TGF-Beta, IL-10 and regulatory T cell recruitment (5).

Post TCR engagement, the B7 of the APC may be acquired by the T cell, a study found that in order to do this, CTLA-4 actually "captures" B7-1 or B7-2 from the membrane of the APC. This is described as ‘trans-endocytosis’ and makes co-stimulation unavailable, which could lead to T cell deactivation (anergy). The B7 molecule is then degraded within the cytoplasm of the T cell, rendering it docile until it is stimulated by another APC (6).

Perhaps strangest and craftiest of all, is the "reverse signalling" mechanism through B7-1 or B7-2 on APCs (mainly dendritic cells). In forward signalling, the APC stimulates the TCR through engagement with the TCR and its associated co-receptors. However in this reverse signal, CTLA-4 outcompetes CD28 for its interaction with the B7 molecule preventing TCR stimulation. Then, to rub salt into the wounds of the APC’s efforts, CTLA-4 signals to the APC (through B-7) to convert more naive T cells to regulatory T cells. This results in even more immunosuppression (7).

It should be noted that all of these extrinsic mechanisms were observed in murine models and we cannot infer that they work similarly across different species of mammals such as humans. Furthermore, immune and cellular responses are the pinnacle in reflecting the importance of context, with the incredible complexity and redundancy of cytokines and immune signals, making predictions about untested environments are difficult. However, understanding animal models is an important (and ethical) step towards understanding human physiology and towards the development of interventions.

References

1. Brunet J, Denizot F, Luciani M, Roux-Dosseto M, Suzan M, Mattei M et al. A new member of the immunoglobulin superfamily—CTLA-4. Nature. 1987;328(6127):267-270.

2. Harper, Katherine, et al. "CTLA-4 and CD28 activated lymphocyte molecules are closely related in both mouse and human as to sequence, message expression, gene structure, and chromosomal location." the Journal of Immunology 147.3 (1991): 1037-1044.

3. Stamper CC, Zhang Y, Tobin JF, Erbe DV, Ikemizu S, Davis SJ, Stahl ML, Seehra J, Somers WS, Mosyak L (March 2001). "Crystal structure of the B7-1/CTLA-4 complex that inhibits human immune responses". Nature 410(6828): 608–11. doi:10.1038/35069118

4. Egen J, Kuhns M, Allison J. CTLA-4: new insights into its biological function and use in tumor immunotherapy. Nature Immunology. 2002;3(7):611-618.

5. Chen W, Jin W, Wahl S. Engagement of Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA-4) Induces Transforming Growth Factor Beta Production by Murine CD4+ T Cells. Journal of Experimental Medicine. 1998;188(10):1849-1857.

6. Qureshi O, Zheng Y, Nakamura K, Attridge K, Manzotti C, Schmidt E et al. Trans-Endocytosis of CD80 and CD86: A Molecular Basis for the Cell-Extrinsic Function of CTLA-4. Science. 2011;332(6029):600-603.

7. Grohmann U, Orabona C, Fallarino F, Vacca C, Calcinaro F, Falorni A et al. CTLA-4–Ig regulates tryptophan catabolism in vivo. Nature Immunology. 2002;3(11):1097-1101.