Our environments are bustling with pathogens, and we rely on our antibodies to fight off these invaders. But what if, under some particular sets of circumstances, our antibodies switched sides? Sometimes, our antibodies defect, helping (rather than hindering) those viruses attempting to gain entry. We call this turncoat scenario antibody-dependent enhancement (ADE). In ADE, our antibodies help invading viruses internalise into cells, resulting in more severe infection and illness.
Antibody-triggered diseases like ADE can be difficult to study. And while we’re still uncertain about how much these diseases contribute to human illness, the available data suggests a greater-than-previously-though role for humoral immunity (the antibody defence system) in disease.
What is ADE?
When everything is working as it should, a circulating army of antibodies will identify and neutralize invading viruses. Multiple antibodies will grab hold of a virus and not let go. This grabbing is called opsonisation, and the antibodies use their Fab domain to do this. Next, having grabbed hold of the virus, the antibodies will use their Fc domain to call for backup. The antibodies Fc domain recruits phagocytic cells [via the Fc receptor (FcR)]. We can think of these phagocytic cells as the clean-up crew that come along and collect (internalise) and destroy the virus. Unable to escape from this antibody coating, opsonised viruses remain trapped in the endo-lysosomal network, where they are destroyed.
But what if we have antibodies that aren’t doing a good job; they’re sloppy? While they grab hold of the virus, they’re not as committed as they should be. Or perhaps there are too few antibodies to really get the virus under control (it’s not effectively opsonised). In this case, the antibody Fc domains will still recruit the clean-up crew phagocytic cells [via the Fc receptor (FcR)]. But now, the phagocytic cells will internalise a virus that isn’t sufficiently bound-up by antibodies. This is a dangerous situation because the virus can now escape destruction. But worse than that, the virus can now gain control of the clean-up crew phagocytic cells (the virus fuses with the endosomal membrane of the phagocytic cells and gains access to these cells’ cytoplasm). We call this antibody-dependent enhancement (ADE).
This spells big trouble for the host for two reasons. First, the phagocytic cells get taken over by the virus, with these cells now being used to replicate more virus, increasing the viral load and enhancing disease. Second, these phagocytic cells – an important arm of defence – are now no longer contributing to the fight. A key player in our immune response is now off the field of play.
Dengue Virus and ADE
Dengue virus is a good example to understand ADE. When someone gets infected with one type of dengue virus, the body creates antibodies to fight it off. However, if that person later gets infected again with a different type of dengue virus, instead of helping, the previously formed antibodies might make the infection worse. This is due to ADE, and it’s why some people experience severe dengue symptoms upon a second infection.
Even young children can face this risk. If a mother has had dengue before, she can pass some of her dengue-fighting antibodies to her child. If that child later gets infected with a different type of dengue virus, ADE might come into play, making the disease worse.
Dengvaxia – The Dengue Vaccine Challenge
ADE is also the reason behind the failure of Sanofi’s Dengvaxia, the first authorised dengue vaccine as a prophylactic measure for people naïve to dengue infection. If someone who had never had dengue before was vaccinated and then later encountered the virus, there was a risk that the vaccine could make the infection more severe because of ADE. Thus, now the vaccine is mainly recommended for those who’ve had dengue before.
ADE’s Complexity
Importantly, ADE can be caused by various types of antibodies, including those that are not effectively fighting off the virus or those that are present in low numbers. While it’s challenging to pinpoint an exact “danger zone”, a study showed that there’s a higher risk of severe dengue when specific antibody levels are within a certain range (when antibody-neutralising titres are between 1:21-1:80) (Katzelnick et al. 2017).
The Role of Fucosylation
Recent research hints that a process called ‘fucosylation’ (where fucose sugar molecules are called is added) might be involved in ADE. Without getting too technical, variations in fucosylation levels in our antibodies might influence the severity of an infection (Bournazos et al. 2021). Fucosylation has been suggested to play a role in the infection processes of multiple viruses and deserves further study.
A recipe for ADE
As discussed above, for ADE to happen, at least two elements must be present: 1) poorly neutralising or low titre antibodies, and 2) the successful virus highjacking of those cells that would normally internalise and destroy the antibody-virus complexes.
This is the case for dengue, which infects monocytic cells (white blood cell that acts as our body’s defense against pathogens) like dendritic cells, monocytes, and macrophages. It has also been suggested to be the case for Ebola and HIV, both viruses with a tropism for macrophages, but for these viruses, the likelihood and impact of ADE in vivo is still a matter of debate.
ADE, Ebola, and HIV
In humans, the administration of anti-HIV antibodies has mostly shown a protective role, although contradicting in vitro and in vivo data have been reported (https://www.frontiersin.org/articles/10.3389/fmicb.2022.932408/full).
For the Ebola virus, sub-neutralising antibody concentrations have been shown to enhance infection in rhesus macaque monkeys, where passive immunotherapy (supplying antibodies in an attempt to provide immediate protection) led to increased infectivity and viral titre over 100-fold higher than in the non-immunotherapy controls (Jahrling et al. 2007).
Enhancement of monocyte infection has also been reported in an in vitro study using low concentrations of convalescent human antibodies (Kuzmina et al. 2019). However, there is no clear evidence of ADE for Ebola infection in humans, and high concentrations of neutralising antibodies appear protective.
ADE and Coronaviruses
Concerns over ADE surfaced also at the beginning of the SARS-CoV-2 pandemic. Years before, the development of a vaccine against feline peritonitis coronavirus had failed due to severe enhancement of disease and death upon immunisation followed by virus challenge (Takano et al. 2008).
Also, reports of antibodies against SARS-CoV conferring more severe lung damage (Tseng et al. 2012) suggested that a similar risk could be present for SARS-CoV-2.
ADE and SARS-CoV-2
Fortunately, although some patient-isolated non-neutralising antibodies have shown increased virus internalisation in FcR-bearing cells in vitro (Liu Yet al. 2021), SARS-CoV-2 does not replicate in these cells, and no reports of traditional ADE have been described in vivo (García-Nicolás et al. 2021). Release of cytokines from the same cells upon virus entry (even in the absence of successful replication) has also been reported, but once again, results from different studies are conflicting and likely to depend on the in vitro system used.
The Difficulties of Studying ADE
These discrepancies highlight the difficulties of studying ADE in vitro and extrapolating conclusions from both in vitro and animal experiments. However, they also call for caution in developing vaccines and antibody therapies. In particular, human studies correlating antibody titres with disease severity should be performed in large populations to identify the frequency, extent, and cause of enhancement. As was the case for Dengvaxia, it is not uncommon for these relatively infrequent phenomena to be missed in Phase II and III of clinical trials due to the comparatively small number of participants.
Other Antibody-Mediated Disease Enhancement
While ADE is the best-known antibody-mediated disease enhancement, several other phenomena have been described, and the frequency and extent of these occurrences for different viruses and vaccines are unclear and deserve further investigation.
Vaccine-associated enhanced diseases (VAEDs) are defined as “immune-mediated aggravation of the clinical course of infection following immunisation, relative to that in the absence of previous vaccination” (Bigay et al. 2022).
Antibody-mediated causes of VAEDs include virus-antibody complexes deposition in the lung, blood vessels, or organs; recruitment of inflammatory cells, eosinophils, and TH2-cells; and infiltration of complement. The latter has been associated with increased disease severity upon immunisation with formalin-inactivated RSV (Polack et al. 2002), and measle vaccines (Fulginiti et al 1967), possibly due to antibodies being raised against antigens structurally altered by the inactivation process. Other formalin-inactivated vaccines do not seem to cause the same issue.
Given the difficulties in predicting and mitigating the impact of ADE and other VAEDs on infection and disease, developing novel models and assays able to anticipate and describe antibody-mediated disease enhancement appears a critically important – albeit extremely challenging – new avenue of research.
Virology Research Services and ADE
At VRS, we have set up an immunofluorescence-based ADE assay to measure neutralising and enhancing titres of antibodies and sera against the dengue virus.
In this assay, a serial dilution of antibody or serum is pre-incubated with the virus before infection of FcR-bearing cells. Twenty-four hours later, cells are fixed and immunostained for a viral antigen, and the percentage of infection is quantified and compared with untreated virus.
Contact us to learn more about how we can help with further characterising your therapeutic antibodies or the response to a dengue vaccine using our ADE assay.
