Smallpox, an epidemic disease caused by Orthopoxvirus
variola, was eradicated worldwide through immunization. The immunization against
smallpox was discontinued in 1980. However, incidences of monkeypox virus
infection in humans have occurred sporadically, and there is also great fear that engineered forms of poxvirus could be used as
biological weapons. Therefore,
monoclonal antibodies against poxvirus are urgently needed for the detection and treatment of
poxvirus infection. The vaccinia virus' extracellular envelope
protein A33 is a potential candidate for a
subunit vaccine. We used multi-fluorescence-labeled tetrameric A33
antigen to identify rare poxvirus-specific memory B cells from the PBMC of volunteers with vaccinia virus immunization more than 40 years ago. Despite extremely low frequencies of the poxvirus-specific memory B cells, we successfully sorted A33 tetramer-labeled single memory B cells and reconstructed the
antibodies with the single-cell RT-PCR of the B-cell receptor. Among the
monoclonal antibodies, one clone H2 exhibited high specificity and affinity with A33. H2 efficiently inhibited
viral infection and spread in cells. Passive immunotherapy of H2 in mice protected mice from lethal
infection when administered either prophylactically or therapeutically. These results suggest the potential of anti-A33 human-antibody-based detection and
therapeutics for
poxvirus infection.