This is the second in a series of article exploring the science and politics behind the anthrax vaccine immunization program. The first article appeared in ASA 98-4.

A similar article by Dr. Nass has appeared in Infectious Disease Clinics of North America, volume 13, Number 1, March 1999.

Biological Warfare and Vaccines: Anthrax
by Meryl Nass, M.D.

A Brief History of Anthrax Vaccine Development

Pasteur, Toussaint and Greenfield developed the first animal anthrax vaccines about 1880 (28, 87). Sterne developed an attenuated live animal vaccine in 1935 that is still employed, and derivatives of this strain account for almost all vaccines used in the world today ( 87). The Sterne vaccine retains some virulence: goats, llamas and occasionally other animals may die following vaccination (22, 40, 41, 43, 88, 95). This vaccine, along with improvements in animal husbandry and industrial hygiene, has made anthrax an almost negligible problem in the developed world. There are rare animal outbreaks in the US, and less than one human case per year (99).

The Sterne vaccine strain lacks the plasmid pX02, which codes for the polypeptide capsule and inhibits phagocytosis and opsonization. The Sterne strain retains the toxin plasmid pX01, which codes for the three toxin proteins: PA (protective antigen), LF (lethal factor) and EF (edema factor) (22, 55). PA, an 82kd (kilo dalton) protein that binds to receptors present on most mammalian cells, is cleaved by a cell surface protease to a 63 kd fragment. This fragment remains on the cell surface, but exposes a site that binds competitively to either EF or LF. The PA-LF or PA-EF complex is believed to enter cells by endocytosis ( 51, 52). A heptamer of the PA-LF or PA-EF complex creates a pore in the cell membrane, through which the toxic protein is inserted into the cell (Collier RJ and Liddington R. Model presented at the 3d International Conference on Anthrax, Plymouth UK, 9/98).

As noted in ASA 98-4, the presence of PA is critical because it is the combination of PA with either LF or EF that is necessary to produce toxicity. Therefore, if an immune response to PA can be induced by vaccination with an anthrax strain that does not contain the capsule plasmid, a nonvirulent strain, immunity should theoretically carry over to other, virulent strains containing PA. However, such broad immunity is not necessarily induced.

Current Human Vaccines

Human vaccines were developed in the Soviet Union by 1940 (1, 80), and in the US and Great Britain in the 1950s (87). The current US vaccine was formulated in the 1960s and licensed in 1970, two years before efficacy data were required for licensing (3, 87).

Russia and China use live attenuated strains for their human vaccines. The Chinese and Russian vaccines may be given by aerosol, scarification, or subcutaneous injection (80, 81). The Russian vaccine was manufactured at the George Eliava Institute of Bacteriophage, Microbiology and Virology in Tblisi, Georgia until 1991 (Nina Chanishvili, PhD, personal communication, June 1998). The efficacy of the live Russian vaccine is reported to be greater than that of the killed US or British vaccines. (31, 53, 80, 89)

The US and British vaccines are filtrates from two different anthrax strains. As with the Sterne vaccine strain, each lacks the capsule plasmid pX02. These strains are composed chiefly of PA (52) with small amounts of EF and LF that may vary from lot to lot. Whether or not the EF and LF contribute to the vaccine's efficacy is not known. The British vaccine consists of alum-precipitated toxin proteins and has larger amounts of EF and LF than the US vaccine (87). The US vaccine uses aluminum hydroxide (alhydrogel) to adsorb PA, and to serve as an adjuvant that is believed to stimulate humoral but not cell-mediated immunity (95). The mechanism of action of the adjuvant is not entirely understood. According to Hambleton and Turnbull, "Such [anthrax] vaccines can produce some protective activity in experimental animals and may be effective in humans (31)."

The US vaccine, termed MDPH-PA or MDPH-AVA (anthrax vaccine adsorbed) consists of a culture filtrate from the toxigenic, nonencapsulated strain of B. anthracis V770-NP1-R ( 74). In addition to PA, aluminum hydroxide, small amounts of EF and LF, and other uncharacterized bacterial byproducts, the vaccine contains up to 0.02% formaldehyde and 0.0025% benzethonium chloride. The potency of vaccine lots is determined both by the survival rates of parenterally challenged guinea pigs, and their anti-PA antibody titres by ELISA.

The US vaccine was used only by several thousand people until 1990. "There have been no controlled clinical trials in humans of the efficacy of the currently licensed US vaccine" ( 7) and no published studies of its safety exist.

Direction of Future Vaccines

Three problems with the current US vaccine have stimulated interest in an improved human anthrax vaccine (17, 31, 33, 36, 37, 38, 39, 40, 41, 42, 43, 56) : 1) the immunization schedule involves 6 initial doses over 18 months, and annual boosters, 2) immunity is not protective against all naturally occurring anthrax strains in guinea pigs and other experimental animals, and 3) there is a high incidence of local reactions (30% according to the package insert). There is also no data available in the open literature of long-term adverse effects. The vaccine is an undefined mix of bacterial products (7, 33, 46). Furthermore, the potency of both the UK and MDPH-PA vaccines is found to vary significantly between lots (37, 40, 68). Therefore, attempts have been ongoing since the early 1980s to develop an improved human vaccine. It has been proposed that better vaccines would generate cell-mediated, as well as humoral immunity, inhibit spore germination and possibly other factors, and be well defined chemically.

Although virulence factors other than the toxin proteins and capsule have been identified, their roles are only beginning to be defined (85). These include a type 1 DNA topoisomerase on pX01 (24), and chromosomally-encoded factors including extracellular proteases (82, 83, 86). Vaccine development has been hampered by limited understanding of anthrax pathogenicity, and lack of knowledge of those epitopes which contribute to the improved immunity conferred by live vaccines (40, 43, 81, 84, 86).

In the US, two general approaches toward an improved vaccine have been taken (22). First, a chemically pure PA vaccine has been sought (40, 42, 89). One candidate has been derived from a recombinant anthrax strain that lacks the capsule, LF, EF and spore, achieving 98% purity and retaining PA's biologic activity (21). Whether it will stimulate adequate immunity is not yet known. The second approach seeks a live vaccine that is safe, contains PA but also contains other immunogenic epitopes, and presents these antigens more effectively than a chemical (killed) vaccine (42). Although a variety of live vaccine candidates have been tested, none has yet been found ideal (17, 20, 40, 41, 95).

Part III of this series will examine the anthrax vaccines' efficacy and the role of vaccines generally as a defense against bioterrorism and BW.

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99-2, issue no. 71
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