Foxp3 processing by proprotein convertases and control of regulatory T cell function

Area of Science:

• Immunology
• Molecular Biology
• Cell Biology

Background:

• Foxp3 is a crucial transcription factor for regulatory T cell (Treg) function.
• Tregs are vital for preventing autoimmunity by controlling self-reactive T cells.
• Previous understanding of Foxp3 function did not account for post-activation processing.

Purpose of the Study:

• To investigate the post-translational modification of Foxp3 upon T cell activation.
• To identify the mechanisms and functional consequences of Foxp3 proteolytic cleavage.
• To explore the implications of Foxp3 processing for Treg competency and autoimmunity.

Main Methods:

• Analysis of Foxp3 expression in natural and activated Tregs.
• Bioinformatic searches for potential proteolytic cleavage sites (RXXR motifs).
• Retroviral expression of wild-type and mutated Foxp3 in CD4+ T cells.
• Fractionation studies to determine the cellular localization of Foxp3 species.
• Functional assays assessing Treg gene expression and experimental colitis models.

Main Results:

• A shorter 41-kDa Foxp3 species was identified in activated Tregs, indicating proteolytic cleavage.
• Two RXXR proprotein convertase motifs were identified at the N- and C-termini of Foxp3.
• Mutagenesis of these motifs prevented Foxp3 cleavage, confirming their role.
• Cleaved Foxp3 forms were localized to the chromatin fraction.
• Distinct functional profiles were observed for N-terminally, C-terminally, and doubly cleaved Foxp3 variants.
• C-terminally cleaved Foxp3 enhanced the suppression of experimental colitis.
• Foxp1 and Foxp2 also possess similar N-terminal cleavage motifs, suggesting broader applicability.

Conclusions:

• Foxp3 undergoes proteolytic cleavage at specific RXXR sites upon Treg activation.
• This processing generates distinct Foxp3 isoforms with differential effects on Treg function.
• Foxp3 cleavage is a critical mechanism influencing Treg cytokine production and suppressive capacity.
• The complexity of Foxp3 generation impacts the overall functional competence of Tregs in maintaining immune homeostasis and preventing autoimmunity.