A mechanism for prozone formation in the complement fixation test for bovine brucellosis

Area of Science:

• Immunology
• Veterinary Medicine
• Bacteriology

Background:

• Brucella abortus causes significant economic losses in cattle.
• Complement fixation (CF) is a serological test used to detect antibodies against Brucella.
• Discrepancies in Brucella serological testing, such as prozones and differing results between tests, are clinically relevant.

Purpose of the Study:

• To investigate the mechanism behind the inhibition of complement fixation (CF) in bovine anti-Brucella abortus serum.
• To determine the role of different bovine IgG subclasses in the CF reaction.
• To explain serological test discrepancies observed in bovine brucellosis diagnostics.

Main Methods:

• Bovine serum samples with known antibody profiles were used.
• Complement fixation (CF) assays were performed.
• Titrations of specific antibody subclasses (IgG1, IgG2, IgM) against Brucella abortus were analyzed.
• Inhibition assays were conducted using purified antibody subclasses.

Main Results:

• Bovine IgG1 and IgM antibodies to Brucella abortus effectively mediated complement fixation.
• Specific non-complement-fixing bovine IgG2 antibodies significantly inhibited the CF activity of IgG1 and IgM.
• This IgG2-mediated inhibition was observed in vitro and could explain prozone phenomena in CF titrations.
• The results suggest IgG2 interference can lead to false-negative CF tests despite the presence of Rose Bengal test-positive antibodies.

Conclusions:

• Bovine IgG2 antibodies play a crucial role in regulating complement fixation responses to Brucella abortus.
• The inhibitory effect of IgG2 antibodies can lead to inaccurate interpretations of CF test results.
• Understanding these subclass-specific antibody interactions is vital for improving the accuracy of diagnostic tests for bovine brucellosis.