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From Proteopedia

VlsE

The causative agent of Lyme Disease, a multi-stage infection, is a family of spirochetes found in the guts of Ixodes ticks. The Variable Major Protein (VMP)-like sequence Expressed (VlsE) is the specific outer-surface lipoprotein that enables the spirochete to evade immune response through mechanisms of antigenic variation. In addition to inducing a strong immune response in the host, VlsE is useful in the serodiagnosis of Lyme disease.^[1]

Structural Overview

The relationship between VlsE structure and its possible functions were determined by the three dimensional structure of VlsE from the Borrelia burgdorferi
B31 strain. ^[2] VlsE is shown to be composed of four sub-units, each consisting of a variable domain flanked by two invariable domains (Liang). The variable domains themselves contain six (VR) interspersed amongst six invariable regions (IR). The IRs are embedded deep within the protein covered by alpha helical loops. The IRs are embedded deep within the protein being essentially shielded by the VRs which represent 37% of the total surface area while containing less than 26% of the primary protein sequence. ^[3] The six VRs entirely cover the distal portion of the protein with about 50% of their theoretical surface area exposed to the surface for immune response. ^[4]. In contrast, the portion of the sub-unit responsible for the antigenic variation in the VlsE as well as inducing the host’s immune response only exposes about 13.7% of its surface. Being shielded by the IRs, the host’s antibodies are restricted to interact with a very limited number of residues rendering any antigenic response futile.

Direct Repeat

The vlsE region containing the invariable and variable regions are bound by the 17 base pair direct repeats (DR). DR1, demarcating the beginning of the cassette region is part of helix alpha-3 at the membrane proximal end of VlsE1 and is exposed on the surface of the protein. DR2 is found at the end of the cassette region and is surface exposed. However, DR2 is located at the membrane distal surface and a slight portion of the lateral VlsE1 surface. ^[5]

Variable Region

The outer surface lipoprotein of Borrelia burgdorferi uses antigenic variation via gene conversion to evade host immune response. Crystal structure of VlsE1, a recombinant variant protein of VlsE, reveals six that form loop structures that entirely cover the membrane distal end of the protein. ^[6] Almost 50% of the variable region’s surface area is exposed on the surface of . There are three major areas of the membrane distal portion on the protein surface that undergo antigenic variation.
^[7]It is likely that amino acid mutations of these residues change conformation of the looped structures and therefore vary the epitope
throughout each variable region. Immune systems are overwhelmed with an astronomical number of different antigens. The loop structures cover predominantly the α-helical invariant regions of the protein. ^[8] It is speculated that the locations of these variable regions on the membrane distal end shield the conserved regions of VlsE from antibody interaction and thereby contribute to immune evasion. ^[9]

In the experimental infection of mice, segments of the silent cassette recombines via gene conversion mechanism into the vlsE cassette region. The changes in sequence were detected after 4 days after experimental infection in mice. 28 days later, every skin isolate and other tissues were unique in that they contained about 9-13 recombination events. It is hypothesized that the humoral immune response cannot cope to a seemingly continuous generation of VlsE variants. This permits immune evasion and persistent infection.^[10]

Invariable

Invariable Region: The small invariable regions within the variable region as well as the larger invariable domains C & N terminals are not antigenic in natural infections and tend to be conserved among different isolates of the organism.^[11] Conserved portions are probably responsible for the maintenance of the functional and molecular conformations of the VlsE. They maintain the secondary structure of the protein, the beta sheets and alpha helix. This allows the protein to maintain its functional properties because the alpha and beta sheets allow the secondary structure to be stable enough to allow the side chains of the VlsE protein to construct a biologically active tertiary structure.

Antigenicity

Antigenic diversity enables the spirochete to escape host defense mechanisms and maintain infection. The variable regions of the lp28 -1 gene cassette extend this ability to VlsE. It has been postulated that infection induces sequence changes and thus alters the antigenic properties’ of VlsE. The generation of new antigens is thought to occur through the exchange of DNA cassettes by the process of recombination. This recombination could potentially help spirochetes escape anti-body mediated attack. The Vlse gene cluster consists of the single VlsE and 15 silent cassettes upstream. Unidirectional gene conversion events between the silent cassettes and VlsE gene generate new Vls variants. By producing a myriad of VlsE varaints that do not effectively bind to ant-VlsE antibodies elicited by previous versions of the protein VlsE evades immune destruction.^[11]

One of the invariable regions IR6, showed both the highest levels of antigenicity by the Hopps-Woods algorithm and amino acid seqence identify among Borrelia genospecies and strains. ^[11] Thus it is the most immunodominate region and what allows for the persistent humoral response. Examination of IR6 by the immunoprecipitation with the rabbit anti-C6 antiserum and revealed that IR6 is exposed on VlsE surface. However, IR6 is not accessible to Ab on the outer membrane of the spirochete.^[12]

Function in Immune System Evasion

The contributions that VlsE brings to immune evasion of Lyme Disease in B. Burgderfori is brought on by the property of antigenic variation. The variable domain of VlsE keeps the immune system guessing by constantly changing its side chains. The predicted amount of variations due to VlsE sequence is around 10^30 combinations providing ample evasion against immune responses. The variable region also protects the invariable region by covering the latter region, providing antibodies with a small area to attach to the antigen. There are only 4 amino residues on IR6 that are exposed on the surface of the protein and allowed for binding by antibody. They are Lysine-276, Glutamine-279, Lysine-291, and Lysine-294. The IR6 region exposed is only 13.7% of the entire surface area of VlsE, making immune evasion even easier.^[13]

Clinical Application

Although VlsE is able to evade immune response and maintain persistent infection through antigenic variation, a peptide associated with IR6 has been found to have seriodiagnostic properties in Lyme disease^[14]. In a comparative study of human sera using recombinant antigens of B. Burgdorferi, the samples displayed 80% seropositivity when studied for IgG antibodies when examined by ELISAs^[15] . It suggests that the invariable regions of VlsE may serve as useful universal probes for Lyme disease. The sera sampled by the study were from the archives in Connecticut of patients who resided in the United States^[16]. However, Lyme disease is not exclusive to the United States therefore further analysis of VlsE reactivity with Lyme disease borreliosis from broad geographical regions is required^[17].

Future Research

vlsE recombination is rapid during the infection process in immunocompetent or immunodeficient mice. However, it has not been detected during in vitro culture. Ohnishi J, P. J. and de Silva A. M. reported that ticks inoculated via feeding on infected mice containing vlsE sequence variants that either arose within the ticks or selected during the second feeding.^[13] However, vlsE recombination is not detected by recent studies conducted by Indest, K.J. that inoculated ticks with B. burgdorferi clone 5A3 via capillary feeding.^[13]Present research explored the recombination mechanism and whether it is up-regulated in mammalian tissues in comparison with either the arthropod of in vitro culture environments.

See Also

Additional Links

Antigenic Variation
Borrelia burgdorferi
Lyme Disease

References