C4a: the third anaphylatoxin of the human complement system

Area of Science:

• Immunology
• Biochemistry
• Complement System

Background:

• The complement system is a crucial part of innate immunity.
• Activation peptides like C3a and C5a are known anaphylatoxins.
• The role of C4a, derived from the fourth complement component, was less understood.

Purpose of the Study:

• To isolate and characterize the C4a peptide.
• To investigate the biological activities of C4a.
• To determine if C4a functions as an anaphylatoxin.

Main Methods:

• Isolation of C4a from C1s-cleaved C4.
• Electrophoresis for homogeneity assessment.
• Molecular weight and electrophoretic mobility determination.
• Carboxypeptidase B digestion to identify the C-terminus.
• Assays for spasmogenic, desensitizing, and vascular permeability activities.
• Radioimmunoassays to rule out contamination with C3a or C5a.

Main Results:

• Homogeneous C4a peptide isolated with specific molecular weight and mobility.
• Carboxypeptidase B released arginine, revealing a Leu-Gln-Arg-COOH sequence.
• C4a demonstrated spasmogenic activity on guinea pig ileum.
• C4a induced tachyphylaxis to C3a but not C5a.
• Intradermal C4a increased vascular permeability in human skin.
• Activities were lost upon arginine removal, similar to C3a and C5a.
• Radioimmunoassays confirmed the absence of C3a or C5a contamination.

Conclusions:

• C4a is a newly identified anaphylatoxin.
• C4a shares biological and chemical properties with C3a and C5a.
• Despite lower potency, C4a contributes to the complement system's anaphylatoxic functions.