Thymocyte antigens do not induce tolerance in the CD4+ T cell compartment

Area of Science:

• Immunology
• T cell biology
• Self-tolerance

Background:

• Transgenic mice expressing H-2Kb in lymphoid cells reject H-2Kb skin grafts, indicating a failure in T cell tolerance to self MHC class I.
• This study investigates the reasons behind the inability of thymocytes to induce tolerance to self MHC class I antigens.

Purpose of the Study:

• To elucidate the mechanisms by which thymocytes fail to establish T cell tolerance to self MHC class I antigens.
• To determine the role of antigen presentation and T cell subsets in this tolerance failure.

Main Methods:

• Utilized CD2Kb transgenic mice and TCR transgenic mice to model self MHC class I expression in thymocytes.
• Analyzed T cell proliferation, cytotoxic function, and T cell receptor (TCR) interactions in response to H-2Kb.
• Investigated the impact of MHC class I expression solely in thymic microenvironments on T cell precursor selection.

Main Results:

• CD2Kb mice showed age-dependent rejection of H-2Kb skin grafts, with younger mice rejecting grafts while older mice accepted them.
• T cells from younger mice proliferated but lacked cytotoxic function against H-2Kb, with CD4+ T cells dominating the proliferative response.
• CD4+ T cell clones recognized processed H-2Kb presented by MHC class II molecules, indicating MHC II restriction.
• TCR transgenic mice crossed with CD2Kb mice did not eliminate H-2Kb-specific thymocyte precursors when H-2Kb was expressed only in lymphoid lineages.
• Efficient elimination of thymocyte precursors occurred when all thymic cells expressed H-2Kb.

Conclusions:

• Self MHC class I antigens expressed exclusively in thymocytes do not induce T cell tolerance.
• Presentation of processed self MHC class I on self MHC class II molecules by CD4+ T cells fails to induce negative selection of CD4+ T cell precursors.
• This suggests compartmentalization of self antigens can prevent tolerance induction in the T cell repertoire.