[Foreign and self--the challenge for the immune system]

Area of Science:

• Immunology
• Cell Biology
• Autoimmunity

Context:

• The thymus is essential for T-cell development and central tolerance.
• Peripheral tolerance mechanisms were traditionally considered primary for tissue-specific self-antigens.
• The thymic microenvironment's role in shaping T-cell repertoire specificity is critical.

Purpose:

• To review evidence highlighting the thymus's expanded role in tissue-specific tolerance.
• To explain the mechanisms underlying this revised understanding of thymic function.
• To discuss implications for autoimmune disease research.

Summary:

• Recent discoveries reveal thymic medullary epithelial cells (TECs) ectopically express tissue-restricted self-antigens, creating a 'molecular mirror of the peripheral self'.
• This allows for central tolerance induction to a broader range of self-antigens within the thymus.
• The thymus also generates regulatory T-cells (CD4+CD25+Foxp3+) selected by TECs, which actively suppress self-reactive T-cells in the periphery.

Impact:

• Shifts the paradigm of self-tolerance induction, assigning a greater role to the thymus.
• Provides new insights into the breakdown of self-tolerance in autoimmune diseases.
• Suggests potential therapeutic targets for autoimmune conditions by modulating thymic function.