C5L2: a controversial receptor of complement anaphylatoxin, C5a

Area of Science:

• Immunology
• Complement System Biology
• Receptor Signaling

Background:

• C5a is a key proinflammatory mediator in the complement cascade, primarily acting through the C5a receptor (C5aR).
• A second receptor, C5L2, was identified but its function remains enigmatic and debated.
• Early hypotheses suggested C5L2 acted as a decoy receptor for C5a.

Purpose of the Study:

• To review the current understanding of C5L2 structure, expression, and controversial functions.
• To explore the conflicting data regarding C5L2's role in inflammatory and pathophysiological processes.

Main Methods:

• Review of existing literature on C5L2.
• Analysis of studies investigating C5L2 interactions with C5aR and beta-arrestin.
• Examination of in vitro and in vivo experimental data from various disease models.

Main Results:

• Conflicting evidence exists: some studies show C5L2 negatively regulates C5aR signaling, reducing inflammation.
• Other studies demonstrate C5L2 promotes inflammation via HMGB1 release, exacerbating pathology in sepsis models.
• C5L2's function appears context-dependent, varying with cell type, species, and disease.

Conclusions:

• C5L2 exhibits complex and contradictory roles in complement biology, potentially acting as a "great masquerader".
• Its function is highly dependent on the specific biological context, challenging therapeutic targeting.
• Further research is needed to elucidate the precise mechanisms and contextual dependencies of C5L2 signaling.