After an infection with dengue virus, both human patients and murine models have been shown to developed antibodies against structural and non-structural viral proteins. Proteins E, PrM and NS1 are the main targets of the antibody response while antibodies
38
against C, NS3, NS4b and NS5 have also been described although they are not as common or abundant268-270.
4.3.1.1. The envelope protein
E protein is the most important target of the antibody response during infection. Neutralising epitopes have been described on all three domains displaying varying degrees of cross-reactivity not only between dengue serotypes, but also among other flaviviruses64,260. Several reports indicate that antibodies against DIII, particularly those
that recognise the upper lateral surface of the domain, show the highest neutralizing capacity; coincidently, this is also the most variable domain between serotypes, which means that these antibodies are usually highly specific and explain why DIII has been considered for vaccine development92,271,272.
However, studies have shown that after natural dengue infection, the immune response is dominated by highly cross-reactive, weakly neutralising antibodies, directed mostly against epitopes on DI/DII273,274. While the FL appears to be a major target of the
antibody response275, there are reasons to think that the contribution of DIII is very limited
and does not account for the strong type-specific neutralising response seen in patients: levels of anti-DIII antibodies in human sera are very low276 and depletion of DIII-specific
antibodies has little effect on the viral neutralizing capacity of human sera271. Moreover,
viruses carrying mutations in DIII epitopes recognised by neutralising antibodies were still efficiently neutralised by human immune sera277.
While enhancing cross-reactive epitopes seem to be simpler in structure, recent data indicates that the majority of neutralising antibodies present in sera from infected patients recognise complex quaternary epitopes that are only present on the surface of the intact virion278,279. In fact, the symmetric organisation of E proteins on the viral surface, together
with the curvature imposed on E by restriction of the membrane attachment, creates unique quaternary and conformational epitopes involving two or more E molecules that are targeted by strong neutralising antibodies275. Of note, some of these antibodies bind
quaternary epitopes that are formed on the herring-bone configuration of E and are thus restricted to the viral surface: monoclonal antibody (mAb) HM14c10 binds an epitope present between two adjacent E dimers of DENV1280, while mAb 5J7 footprint involves
critical interaction with 3 adjacent E monomers281. On the other hand, mAb 1F4 epitope is
restricted to a single E monomer of DENV1 but on a particular conformation that exist only within the context of the viral particle and not on the recombinant protein282. Binding of
epitopes located at the DI/DII hinge region is a common feature of these mAbs, indicating that interference with this region has highly neutralising potential283.
Recently, a new group of strongly neutralising antibodies that target complex quaternary epitopes was described. These mAbs target epitopes restricted to the interface
39
between two E proteins in a head-to-tail dimer conformation and therefore do not require higher-order arrangements of E like mAbs HM14c10 and 5J7284. MAb 2D22 for example,
binds to the dimer interface but its epitope involves significant interactions with residues on DIII, which is why it only recognises DENV2285. In contrast, a recent group of mAbs
isolated from previously infected patients target the valley formed between the E monomers; specifically, these antibodies target conserved residues on the DI/DIII-DII dimer interface284,286. This epitope, named EDE (E-dimer epitope), overlaps the highly
conserved region of interaction with PrM which explains why these antibodies are able to induce strong neutralisation of all 4 serotypes286. The binding determinants of these EDE
mAbs involve the FL and the loops in its close vicinity within the context of the E dimer and were further classified in two groups: EDE1 mAbs were shown to improve binding in absence of the Asn-153 glycosylation, while EDE2 mAbs showed better binding in presence of the glycan284. Moreover, a recent study demonstrated that EDE1 antibodies
are also able to neutralise ZIKV, EDE2 antibodies showed less affinity towards ZIKV287-289.
4.3.1.2. PrM protein
Antibodies against this protein seem to be stimulated by the Pr peptide released from the viral surface once the maturation process is complete. In addition, the presence of immature viral particles as a result of incomplete cleavage of the Pr peptide can also stimulate the response against this protein78.
PrM antibodies are highly cross-reactive among the different serotypes and show very little neutralizing capacity78,260,290. Instead, increasing amounts of evidence supports
the notion that anti-Pr antibodies can actually favour the infection process by promoting the internalization and infection of immature viruses102,291,292.
4.3.1.3. NS1 protein
As mentioned before the NS1 glycoprotein can be found attached to the surface of infected cells and is also secreted from them as a hexamer. This explains why, in spite of being a non-structural protein, it’s among the significant targets of the antibody response110. When compared with antibodies induced against the E protein or PrM,
antibodies against NS1 show restricted cross-reactivity between the four serotypes291.
Beside antibodies that cross-react with other dengue serotypes, the antibodies produced during viral infection have the capacity to recognise host proteins. Antibodies against E protein obtained from infected patients were shown to recognise human plasminogen and fibrinogen, while there is evidence suggesting the existence of molecular mimicry between NS1 and several host protein in a way that anti-NS1
40
antibodies are able to cross recognize human plasminogen, coagulation factors, platelets and endothelial cells and elicit responses that might be implicated in the pathogenesis of infection122,293,294.