The Poxviridae are large DNA viruses that replicate in the cytoplasm of vertebrates or invertebrates. The genus Orthopoxvirus includes variola virus, the cause of smallpox, and vaccinia virus (VACV), the prototypal family member used in the licensed smallpox vaccine. Interest in the development of an alternative smallpox vaccine emerged because of complications associated with recent vaccination efforts and the growing number of people excluded from vaccination. Antibody therapies are also of interest for Orthopoxvirus infection treatment instead of vaccinia immune globulin from human donors. Essential to these efforts are studies that elucidate aspects of the immune response required for protection against disease. Two infectious forms of virus exist, intracellular mature virus (IMV), which mediates spread between hosts, and extracellular virus (EV), which is required for efficient spread within a host. IMV and EV each possess an outer membrane with viral proteins targeted by the adaptive immune response.

 I have used soluble baculovirus-expressed forms of VACV proteins from the IMV and EV in order to understand the role of immunity to these particles during infection. Subcutaneous immunization of mice multiple times with the EV proteins A33 and B5 and the IMV protein L1 either individually or in combinations induced specific antibody responses and protected against weight loss and death caused by virus infection, especially following immunization with A33+B5+L1 or A33+L1. Similar patterns of protection were observed by passive immunization of mice with polyclonal or monoclonal antibodies against A33, B5, or L1 prior to or after intranasal challenge. A27 was investigated as an alternative IMV protein to L1, but proved less effective alone or in combination with A33. Potent and more rapid immune responses to the A33 and L1 proteins were stimulated by the use of the adjuvants QS-21, or alum mixed with CpG oligodeoxynucleotides.  Protection against a lethal challenge was observed in a small study with monkeys that were immunized with A33, B5, and L1 and challenged with monkeypox.  My data indicate protection against orthopoxviruses is seen in animal models so long as a good antibody  response is made to both the IMV and EV forms.