Dephosphorylation of Carma1 by PP2A negatively regulates T-cell activation

Area of Science:

• Immunology
• Molecular Biology
• Cell Signaling

Background:

• The Carma1-Bcl10-Malt1 (CBM) complex is central to T-cell receptor (TCR) signaling, linking TCR/CD28 co-stimulation to NF-κB activation.
• Protein kinase C theta (PKCθ)-dependent phosphorylation of Carma1 is essential for CBM complex assembly and subsequent NF-κB pathway activation.

Purpose of the Study:

• To identify novel regulators of Carma1 function within the CBM complex.
• To elucidate the role of protein phosphatase 2A (PP2A) in modulating Carma1 phosphorylation and T-cell activation.

Main Methods:

• Identified PPP2R1A, a regulatory subunit of PP2A, as a Carma1 interaction partner using co-immunoprecipitation.
• Utilized siRNA-mediated knockdown of PPP2R1A and in vitro dephosphorylation assays to assess PP2A activity on Carma1.
• Measured NF-κB activation and cytokine production (IL-2, IFN-γ) in stimulated T cells (Jurkat and murine Th1 cells).

Main Results:

• Discovered PPP2R1A as a novel binding partner of Carma1, associating with the CBM complex in T cells.
• Demonstrated that PP2A dephosphorylates Carma1 at Serine 645 (S645), counteracting PKCθ-mediated phosphorylation.
• Showed that impaired PP2A activity leads to increased Carma1 S645 phosphorylation, enhanced CBM complex formation, augmented NF-κB activation, and elevated IL-2/IFN-γ production.

Conclusions:

• PP2A-mediated dephosphorylation of Carma1 at S645 is a critical negative feedback mechanism controlling T-cell activation.
• This phosphatase activity limits the magnitude and duration of T-cell signaling and effector cytokine release.
• Targeting the PP2A-Carma1 interaction could offer therapeutic strategies for immune modulation.