User:Marvin O'Neal/Antibody OspA and OspB

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Contents 1 Introduction 1.1 Borrelia burgdoferi 1.2 Fragment Antigen Binding (fab) 2 OspB and H6831 2.1 Interaction between OspB and H6831 2.2 Structural changes to OspB in the complexed form 2.3 Bactericidal action 3 OspA and LA-2 3.1 OspA 3.2 Interaction between OspA and LA-2 3.3 Structural changes to OspA in the complexed form 4 Medical Application

Introduction

Borrelia burgdoferi

The causative agent of Lyme disease is Borellia burgdorferi, a spirochaete found in the gut of hard bodied ticks of genus Ixodes. A factor contributing to the severity of Lyme disease is its resistance to certain forms of complement-dependent immune response by the evasion of the alternative complement pathway and the blocking of complement C3.^[1] This resistance increases the importance of the complement independent immune response when combating B. burgdorferi. Certain fragment antigen binding regions (fab) of IgG and IgM monoclonal antibodies (mAbs) are bactericidal even in the absence of complement. Binding of these fabs to their corresponding outer surface protein (OspA and OspB) of B. burgdoferi leads to the lysis of the bacteria.^[2]

Fragment Antigen Binding (fab)

Fab consists of a heavy chain and light chain and each chain is composed of a variable and a constant region. The paratope is located in the N terminal of the variable region of the heavy and light chains of the fab. H6831 and CB2 are IgG mAbs that targets the C-terminal of OspB and LA-2 is an IgM mAb that targets the C-terminal of OspA.^[3]

OspB and H6831

	PDB ID 1RJL Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image
	OspB-H6831 Complex () : ·· ·· ·· : ·· ·· : :

Interaction between OspB and H6831

The consist of two components, the outer surface protein and the , which is subdivided into the and the . Most hydrogen bonds and electrostatic interactions that are responsible for the binding of H6831 to OspB are between the at the C-terminal of OspB and some that include tyrosine, tryptophan, glutamate, and histidine.^[4]

The majority of hydrogen bonds and electrostatic interactions are between (residues 250-254) and the fab heavy chain. in loop 2 of OspB has a necessary and major role due to its central position in the exposed loops. A mutation at its position abrogates the binding interaction and causes the resistance of the bacteria to the bactericidal effect of the fab. Lys 253 interacts with the two aromatic residues on the fab heavy chain, tyrosine and tryptophan. It also makes hydrogen bonds with the glutamate 50 in the heavy chain of the fab and forms an ionic bond. Carbonyl in of the OspB interacts with in the fab heavy chain. of OspB interacts with fab light chain.^[4]

Structural changes to OspB in the complexed form

The binding of H6831 to OspB leads to some conformational changes in OspB compared to its . Crystallography has shown that the most significant difference is the loss of the .^[4] The loss of these β sheets may be due to conformational change as a result of the binding or a disorder that could have occurred during a crystallization of the complex. Both small positional shifts near the fab binding site and a few larger structural changes away from the binding site were observed. The largest shifts (7– 8 Å) correspond to the repositioning of a loop opposite the fab-binding site . In the free OspB structure, all regions that exhibit shifts are adjacent to the central sheet; in the OspB-H6831 complex they all shift toward, and slightly overlap the position of the missing sheet.

Bactericidal action

The fab binding destabilizes the outer membrane (OM) of B. burdorferi, with subsequent formation of spheroplasts. It has been observed that the bactericidal action, but not the binding, requires the presence of divalent cations (Mg²⁺ and Ca²⁺), and fab is unable to clear bacteria in the absence of these cations.^[5] It is speculated that OspB-Cb2 (a fab similar to H6831) complexes could lead to the lysis of the cell by creating physical openings in the OM, allowing for rapid infusion of electrolytes and increasing the osmolarity of the periplasm.^[6]

OspA and LA-2

	PDB ID 1FJ1 Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image
	OspA-LA2 Complex () : ··

OspA

OspA is usually undetectable in the early stages of Lyme disease, and is down regulated when OspC is expressed.^[7] OspA is 53% similar to OspB. Despite their similarity, OspB is susceptible to cleavage by exogenous proteases both in vivo and in vitro, whereas OspA is relatively resistant in both cases.^[4]

Interaction between OspA and LA-2

LA-2 is an IgM murine monoclonal antibody that interacts with on the C-terminal of OspA. These interactions include eight direct hydrogen bonds, four solvent-bridged hydrogen bonds, three ion pairs, and numerous van der Waals interactions.^[5]

Structural changes to OspA in the complexed form

Conformational changes upon the binding of OspA and LA-2 show that LA-2 recognition of OspA involves an induced fit mechanism where the conformations of loops 1-3 shift to optimize complementarity to the antigen-combining site.^[5] The overall structure of the C-terminal of OspA is unchanged upon the binding of LA-2 with comparison to the free OspA. The maximum atomic shift is 4.7Å at the site of .^[5]

Medical Application

B. burgdoferi is able to escape human immune response because the outer surface proteins, to which the immune system responds, are variable. The effectiveness of different Osp molecules as vaccines can vary depending on their variability. OspC and OspB are highly polymorphic, with variability of OspC observed between strains collected from within a single geographical area.^[7] Observed variations of OspB in B. burgdoferi (including its absence from the bacteria) are not accounted for by major DNA arrangements or failure in transcription of the OspB gene. This indicates that the OspB gene may code for a variety of proteins, making OspB a poor candidate for use in vaccines.^[8] OspA is the most conserved; of the three exposed loops, only loop 1 is variable while loops 2 and 3 are conserved. This makes OspA a more consistent antigen (compared to OspB and OspC) for the immune system to target and usable as a vaccine to Lyme disease.^[5]

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