C3d regulates immune checkpoint blockade and enhances antitumor immunity

Area of Science:

• Immunology
• Complement System
• Cancer Biology

Background:

• Tumors evade immune surveillance through mechanisms like T cell checkpoint regulators and regulatory T cells (Tregs).
• The orchestrating factors controlling these tumor immune evasion mechanisms remain largely unknown.
• Understanding these controls is crucial for developing effective cancer immunotherapies.

Purpose of the Study:

• To investigate the role of free C3d, a complement fragment, in modulating antitumor immune responses.
• To determine if C3d can overcome tumor-induced immune suppression.
• To explore C3d's potential as a therapeutic agent for cancer and premalignant conditions.

Main Methods:

• Investigated the effects of free C3d on tumor cells and associated immune cells.
• Assessed C3d's impact on T cell infiltration, Treg populations, and PD-1 expression.
• Evaluated C3d's efficacy in reversing or preventing tumor growth in preclinical models.
• Determined C3d's dependency on complement receptor 2 and its systemic toxicity.

Main Results:

• Free C3d, when localized to tumors, recruits, accelerates, and amplifies antitumor T cell responses.
• C3d enhances CD8+ T cell infiltration into tumors and depletes Tregs.
• C3d suppresses programmed cell death protein 1 (PD-1) expression on T cells, independent of B cells, NK cells, or antibodies.
• C3d's anti-tumor effects are specific to the tumor microenvironment and dependent on complement receptor 2, with no observed systemic toxicity.

Conclusions:

• Free C3d possesses previously unrecognized properties that enhance antitumor immunity.
• C3d may play a critical role in orchestrating immune surveillance against tumors.
• Increasing C3d availability presents a promising therapeutic strategy for cancer and premalignant conditions.