Two monoclonal antibodies with defined epitopes of P44 major surface proteins neutralize Anaplasma phagocytophilum by distinct mechanisms.

The research is about two monoclonal antibodies that have been identified to neutralize Anaplasma phagocytophilum, a bacterium causing human granulocytic anaplasmosis, by different mechanisms.

Overview of the Research

• The research focuses on Anaplasma phagocytophilum, an obligatory intracellular bacterium causing the disease human granulocytic anaplasmosis. The bacterium possesses 44-kDa major outer membrane proteins, P44, known to be dominant antigens which are recognized by infected patients and animals.
• The study aimed to understand the neutralization ability of anti-P44 antibody, which has been unclear due to the diversity of amino acid sequences present in the P44 proteins expressed by given bacterial populations.

Monoclonal Antibodies (MAbs)

• The research utilized two specific types of monoclonal antibodies (MAbs), named 5C11 and 3E65. These were directed to different domains of P44 proteins – the N-terminal conserved region and the P44-18 central hypervariable region respectively.
• It was found that passive immunization using either of these antibodies could partially protect mice from A. phagocytophilum infection.

Specific Findings

• The research found that these two antibodies recognize specific bacterial surface-exposed epitopes of naturally folded P44 proteins and these epitopes were mapped to specific peptide sequences.
• They almost completely blocked the infection of A. phagocytophilum populations that mainly expressed P44-18 in a particular type of white blood cells (HL-60).
• However, the mechanisms by which the two antibodies blocked the infection were different: MAb 5C11 blocked the binding of the bacterium, while MAb 3E65 didn’t block the binding or internalization but inhibited the bacterium from developing into micro colonies called morulae inside the host cells.
• Moreover, the study found plasma from infected horses and mice reacting with these two specific epitopes on P44 proteins.

Conclusion

• The research concluded that the P44 proteins of A. phagocytophilum have at least two distinct bacterial surface-exposed neutralizing epitopes recognized by the two MAbs.
• These results have clinical importance as antibodies targeting these epitopes have a critical role in inhibiting A. phagocytophilum infection in host cells, demonstrating their potential in the development of effective vaccines.