Inhibition of cell surface MHC class II expression by Salmonella

Area of Science:

• Immunology
• Microbiology
• Cell Biology

Background:

• MHC class II molecules present peptides to T cells, crucial for adaptive immunity.
• Pathogens often evade immune responses by interfering with antigen presentation.
• Salmonella is an intracellular pathogen that can manipulate host cell processes.

Purpose of the Study:

• To investigate the effect of intracellular Salmonella infection on MHC class II cell surface expression.
• To elucidate the mechanism by which Salmonella interferes with MHC class II presentation.
• To identify Salmonella factors responsible for modulating MHC class II expression.

Main Methods:

• Analysis of MHC class II cell surface expression in human cells infected with Salmonella.
• Investigation of MHC class II biosynthesis, peptide loading, and intracellular trafficking.
• Utilizing Salmonella mutants deficient in Salmonella pathogenicity island-2 type-III secretion system components, including SifA.

Main Results:

• Salmonella infection significantly reduced MHC class II cell surface expression in human cells.
• MHC class II biosynthesis and peptide loading were unaffected, but mature molecules accumulated intracellularly.
• Intracellular MHC class II localized with LAMP-1 and HLA-DM, not the Salmonella-containing vacuole.
• The Salmonella effector SifA was identified as responsible for the reduction in MHC class II surface expression.

Conclusions:

• Intracellular Salmonella interferes with adaptive immunity by downregulating MHC class II presentation.
• Salmonella SifA plays a critical role in this immune evasion mechanism, distinct from its known role in vacuole biogenesis.
• This study reveals a novel function for SifA in disrupting host antigen presentation pathways.