Perforin lytic activity is controlled by calreticulin.

Area of Science:

• Immunology
• Cell Biology
• Protein Biochemistry

Background:

• Cytotoxic lymphocytes utilize granule components for cell-mediated killing.
• Perforin and calreticulin are co-stored in cytotoxic granules.
• Calreticulin, an endoplasmic reticulum chaperone, is uniquely found in these granules.

Purpose of the Study:

• To investigate the role of calreticulin in cytotoxic lymphocyte-mediated cell death.
• To determine if calreticulin regulates perforin-dependent osmotic lysis.
• To elucidate the mechanism by which calreticulin affects perforin activity.

Main Methods:

• Perforin-mediated osmotic lysis assays using purified perforin.
• Inhibition studies with native and recombinant calreticulin.
• Analysis of calreticulin domain fragments (GST fusion proteins) for inhibitory activity.
• Assessment of inhibition independence from granzyme inactivation or calcium sequestration.

Main Results:

• Calreticulin significantly inhibited perforin-mediated osmotic lysis in a dose-dependent manner.
• Inhibitory activity was localized to the C-domain of calreticulin, which does not bind perforin.
• Inhibition was stable over 5 hours and independent of granzyme inactivation or calcium binding.
• Recombinant calreticulin confirmed the inhibitory effect.

Conclusions:

• Calreticulin plays a regulatory role in perforin-mediated cell lysis.
• The inhibitory mechanism likely involves membrane stabilization, preventing polyperforin pore formation.
• Calreticulin regulates lysis independently of direct interaction with perforin or calcium sequestration.