Transmission Research
Researchers do not completely understand the molecular basis of how B. burgdorferi maintains itself in nature via a complex life cycle that involves passage through ticks and various intermediate hosts, such as mice and deer, before infecting humans. The outer surface protein A (OspA) of B. burgdorferi has been well studied, and there is much speculation about its role—in conjunction with other cell surface proteins (OspB and OspC)—in transmitting Lyme disease (J Clin Invest 113: 1093, 2004).
Gene BptA
Although B. burgdorferi depends on Ixodes ticks and mammalian (rodent) hosts for its persistence in nature (J Clin Microbiol 38: 382, 2000), the search for borrelial genes responsible for its parasitic dependence on these types of diverse hosts has been hampered by limitations in the ability to genetically manipulate virulent strains of Borrelia. Despite this constraint, there is evidence to indicate that the inactivation and complementation of a gene (BBE16) encoded by a linear plasmid (lp25) plays a major role in the virulence, pathogenesis, and survival of B. burgdorferi during its natural life cycle (Mol Microbiol 48: 753, 2003). This gene, which has been renamed BptA (for borrelial persistence in ticks-gene A), potentiates virulence in mice and is essential for the persistence of B. burgdorferi in Ixodes scapularis ticks.
Although BptA appears to be a lipoprotein expressed on the outer surface membrane of B. burgdorferi, the molecular mechanism(s) by which BptA promotes persistence within its tick vector remains to be elucidated. Since BptA appears to be highly conserved (>88 percent similarity and >74 percent identity in amino acid sequence) in all B. burgdorferi sensu lato strains examined, it may be widely used to promote persistence in nature. Given the absolute dependence on—and intimate association with—its tick and rodent hosts, BptA must be considered to be a major virulence factor that is critical for B. burgdorferi's overall infectious strategy (Proc Natl Acad Sci 102: 6972, 2005). Strategies designed to block the synthesis or expression of BptA could be of great value in preventing the transmission of Lyme disease.
Outer Surface Proteins
Given the potential role that differentially up-regulated surface proteins play in the transmission of borreliosis and Lyme disease pathogenesis, other investigators have conducted a comprehensive gene expression profiling analysis of temperature-shifted and mammalian host-adapted B. burgdorferi. The combined microarray analyses revealed that many genes encoding known and putative outer surface proteins are down-regulated in mammalian host-adapted B. burgdorferi. However, at the same time, several different genes encoding at least seven putative outer surface proteins were found to be up-regulated during the transmission and infection process. All seven are immunogenic and generate the production of bactericidal antibodies in infected baboons (Infect Imm 74, 296, 2006). This suggests that these outer surface proteins might be excellent second-generation vaccine candidates.
The above findings are consistent with the results of other published studies (J Infect Dis 186:1430, 2002) in which a novel experimental technique (xenodiagnosis by ticks) was used to determine whether B. burgdorferi can persist in mice long after antibiotic therapy. Here, an immunoflourescence assay and the polymerase chain reaction (PCR) assay were used to demonstrate that B. burgdorferi could be detected in doxycycline- and ceftriaxone-treated mice for at least three months—if not longer—after antibiotic therapy. However, the resulting surviving spirochetes are unable to infect other naive mice because they lack those linear plasmids (lp25 and lp28) that are essential for their ability to transmit infection (J Infect Dis 186:1430, 2002). It is noteworthy that lp25 also encodes for a gene product (PncA or BBE22) that is essential for the survival of B. burgdorferi in a mammalian host (Mol Microbiol 48:753, 2003).
OspA and OspB
NIAID-supported investigators have now been able to create various mutant strains of B. burgdorferi and show that although OspA and OspB are not required for infection of mice, they are essential for the colonization and survival of B. burgdorferi in ticks. Ixodes scapularis ticks have a receptor on the inner wall of their intestines to which B. burgdorferi are able to bind tenaciously by means of OspA, a cell surface protein. This receptor is called the “tick receptor for OspA” or TROSPA. Attachment to TROSPA enables B. burgdorferi to persist in the gut from the time they are ingested by ticks through a subsequent molt, thereby avoiding elimination; this enables Borrelia to be injected into a new host when ticks take their next blood meal (Cell 119: 457, 2004). When ticks take a blood meal, the production of OspA is down-regulated in favor of the increased production of OspC. This causes gut-bound spirochetes to become detached, which enables ticks then to migrate to the salivary glands where they can be injected into mammalian hosts. Thus, TROSPA, in addition to other bacterial cell surface components such as OspA, appear to play a key role in the transmission of Lyme disease to humans.
Other studies have shown that if ticks are permitted to feed on mice that have been immunized previously with OspA, or have been treated with antibody specific for OspA, the attachment and subsequent colonization of ticks by B. burgdorferi is significantly impaired if not prevented. This suggests the feasibilty of developing oral or vector expressed transmission-blocking vaccines, that involve the immunization of intermediate hosts upon which ticks feed (Proc Natl Acad Sci 101: 18159, 2004). Several NIAID-supported investigators are now examining and testing this approach under controlled, laboratory conditions.
Salivary Proteins
Other studies conducted by NIAID-supported investigators (Nature 436: 573, 2005) demonstrate that B. burgdorferi utilizes an immunosuppressive tick salivary protein (Salp 15) to facilitate the transmission of infection to mammalian hosts. This is based on observations that: (a) the level of Salp 15 expression is enhanced by the presence of B. burgdorferi in infected ticks; (b) Salp 15 adheres specifically to spirochete surface OspC both in vivo and in vitro, thereby increasing the ability of B. burgdorferi to infect mice; and (c) the binding of Salp 15 protects B. burgdorferi from antibody-mediated killing in vitro, a factor that confers marked survival advantage. All of these findings suggest that Salp 15 and/or other tick salivary proteins might be excellent candidates for vaccines to block the transmission of Lyme disease (Parasitol 129: S161, 2004). In this context, prior and repeated exposure of experimental animals to uninfected ticks—and presumably their salivary proteins—has been shown to limit the capacity of infected ticks to transmit Lyme disease (J Emerg Infect Dis 11: 36, 2005).
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