MHC class I molecules on CD4 T cells regulate receptor-mediated activation signals

Area of Science:

• Immunology
• Cellular Biology
• Molecular Mechanisms of T cell Activation

Background:

• T cell activation is a complex process involving antigen receptor engagement, leading to distinct cellular outcomes.
• Three primary T cell responses to T cell receptor (TCR) ligation include deletion, proliferation, and surface marker upregulation.
• The role of MHC Class I (MHC-I) in regulating these T cell responses is not fully understood.

Purpose of the Study:

• To investigate the regulatory role of MHC Class I molecules in T cell activation.
• To elucidate the mechanisms by which MHC-I influences T cell proliferation, apoptosis, and cell surface marker expression.
• To examine the interplay between TCR signaling, MHC-I, and Interferon-gamma (IFN-γ) in T cell fate determination.

Main Methods:

• Utilizing monoclonal antibodies (MAbs) and Fab fragments targeting MHC-I to modulate T cell responses.
• Employing mitogen stimulation to induce T cell proliferation and deletion.
• Assessing T cell proliferation, apoptosis (using CD95 expression), and costimulatory molecule (CD28) expression via flow cytometry and other cellular assays.
• Investigating the effects of IFN-γ in combination with MHC-I modulation.

Main Results:

• MHC-I MAbs or Fab fragments inhibit T cell proliferative responses to mitogens but do not prevent mitogen-induced deletion.
• IFN-γ enhances the proportion of T cells that proliferate in response to mitogens.
• In the presence of MHC-I MAbs, IFN-γ-stimulated T cells fail to clonally expand and undergo deletion.
• MHC-I MAb Fab fragments induce CD95 (apoptosis marker) upregulation independently of TCR ligand and block CD28 upregulation.

Conclusions:

• MHC Class I plays a critical regulatory role in controlling T cell proliferation and preventing T cell deletion.
• Interferon-gamma can promote T cell proliferation, but this effect is overridden by MHC-I blockade, leading to deletion.
• MHC-I signaling influences T cell fate by modulating apoptosis pathways (CD95) and costimulatory molecule expression (CD28).