T cell receptors (TCR) recognize antigenic peptides displayed by MHC molecules. Whereas T-cell recognition of foreign peptides is essential for immune defense against microbial pathogens, recognition of self-peptides can cause autoimmune disease. Structural studies of anti-foreign TCR showed remarkable similarities in the topology of TCR binding to peptide-MHC, which maximize interactions with the ligand. However, recent structures involving autoimmune and tumor-specific TCR have revealed that they engage self-peptide-MHC with different topologies, which are suboptimal for TCR binding. These differences might reflect the distinct selection pressures exerted on anti-microbial versus autoreactive T cells. The structures also provide new insights into TCR cross-reactivity, which can contribute to autoimmunity by increasing the likelihood of self-peptide-MHC recognition.
T-cell recognition of non-self, self and mutant selfThe immune system has evolved in response to the need for distinguishing non-self pathogens from self tissues. Discrimination is mediated by T-cell receptors (TCR), which recognize peptide fragments (epitopes) bound to MHC molecules on the surface of antigen-presenting cells (APC) (Box 1). These peptides are generated by proteolytic degradation of self or foreign proteins within cells expressing MHC class I or II molecules in a process known as antigen presentation [1]. Whereas T-cell recognition of foreign peptides is vital for defense against invading microorganisms, recognition of self-peptides can lead to autoimmune disease. A third category of T-cell epitopes involves self-peptides resulting from mutations accumulated during aging or disease [2,3]. However, in terms of T-cell recognition, the boundaries separating foreign, self, and altered-self epitopes are not necessarily absolute. For example, immunity to cancer can arise from mutations in self-proteins, which render them visible to T cells [2,3], a process that might also induce autoimmunity [4]. Similarly, autoimmunity can be triggered by viral or bacterial peptides that mimic self-peptides [5,6].Although much is known about TCR recognition of foreign antigens [7], the structural and biophysical principles governing TCR recognition of self and mutant-self have only recentlyCorresponding author: Mariuzza, R.A. (mariuzza@carb.nist.gov). Publisher's Disclaimer: This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g.
Sources of autoimmunityThe ability to distinguish between self and non-self is known as immunological tolerance, and involves the removal of autoreactive T cells by negative s...