Rickettsia conorii O antigen is the target of bactericidal Weil-Felix antibodies

Area of Science:

• Microbiology
• Immunology
• Infectious Diseases

Background:

• Rickettsial diseases are traditionally diagnosed using the Weil-Felix reaction, which detects antibodies cross-reactive with *Proteus vulgaris*.
• The precise rickettsial target of these antibodies and their role in disease pathogenesis remain unclear.

Purpose of the Study:

• To investigate the role of specific genes in *Rickettsia conorii* pathogenesis and serological cross-reactivity.
• To identify the molecular basis of the Weil-Felix reaction and its implications for immunity.

Main Methods:

• Development of a transposon system for insertional mutagenesis in *Rickettsia conorii*.
• Isolation and characterization of replication-defective mutants.
• Analysis of lipopolysaccharide (LPS) O-antigen synthesis, outer-membrane protein assembly, and antibody responses.

Main Results:

• Mutations in the *polysaccharide synthesis operon* (*pso*) abolished LPS O-antigen synthesis and Weil-Felix serology.
• *pso* mutants exhibited altered outer-membrane protein assembly and failed to elicit bactericidal antibodies targeting O antigen.
• The *pso* operon is conserved across various rickettsial pathogens.

Conclusions:

• The *pso* operon is crucial for LPS O-antigen synthesis in *Rickettsia conorii* and is the target of Weil-Felix antibodies.
• Bactericidal antibodies targeting the O antigen may confer universal immunity against rickettsial infections.
• Targeting the O antigen offers a potential strategy for developing broad-spectrum vaccines against rickettsial diseases.