Superantigens are presented by and activate thymocytes from infants

Area of Science:

• Immunology
• Cell Biology
• Developmental Biology

Background:

• A significant proportion of human fetal and postnatal thymocytes express MHC class II molecules.
• This expression suggests a potential role for thymocytes in presenting peptides to immature T cells, influencing thymic selection.
• Understanding T-cell interactions within the thymus is vital for comprehending immune system development.

Purpose of the Study:

• To investigate whether human thymocytes can present peptides to other immature T cells.
• To determine if thymocytes can initiate thymic selection through T-cell/T-cell interactions.
• To elucidate the role of MHC class II molecules in these thymocyte-mediated interactions.

Main Methods:

• Infant thymocytes were exposed to superantigen (Sag), which binds to T-cell receptors and MHC class II molecules.
• Responses such as proliferation and apoptosis were measured following Sag exposure.
• Mutations in MHC class II binding sites were used to confirm the necessity of class II molecules.

Main Results:

• Thymocytes successfully presented Sag to other thymocytes, leading to T-cell activation, proliferation, and apoptosis.
• Responses were dependent on functional MHC class II molecules, as evidenced by experiments with mutated binding sites.
• T-cell/T-cell interactions were primarily observed among T-cell receptor-positive CD4+8+, CD4+8-, and CD4-8+ thymocyte subsets.

Conclusions:

• Human thymocytes expressing MHC class II molecules actively participate in T-cell/T-cell interactions.
• These interactions, mediated by MHC class II, are essential for T-cell activation, proliferation, and apoptosis within the infant thymus.
• Class II-positive thymocytes likely play a critical role in the selection of self-restricted T cells during early human immune system development.