Are Camelids the Key to Beating COVID-19?

The normal antibody (immunoglobulin) has two light chains and two heavy chains. Each chain, be it light or heavy, has a variable and a constant fragment. The variable fragment binds to the putative antigen, while the constant fragment carries the Fc receptor that lets cells like NK cells and neutrophils bind to the antibody. The constant portion also binds and activates complement through the classical pathway.

in 1984, Raymond Hamers at the VUB university in Brussels, while analysing the blood of dromedary camels for antibody response to a Trypanosomal species (the dromedary camel is the Arabian camel, with a single hump, as opposed to the double humped Bactrian camel, found in the plains of Central Asia), found to his surprise that the camel antibodies did not look like the human counterparts at all. They lacked the light chain altogether, and contained only the heavy chain, comprised of the variable and heavy fragments. Quite appropriately, these antibodies were named "camelids".

The 1990s saw the establishment of phage display libraries, which allow the manufacture of virtually any antibody in bacteria, by inserting the relevant sequence in bacteriophages, which then infect the bacteria, and uses the bacterial enzymes to make the protein whose sequence has been inserted into the phage. This technique is responsible for producing most monoclonal antibodies these days, having moved on from the days when antibody producing B-cells were immortalised by fusing them with rat myeloma plasmablasts, a technique described as hybridoma.

It is now possible to produce through such phage display techniques, not just whole immunoglobulin molecules, but parts thereof, such as a the Fab fragment (commercially marketed as Certolizumab), a single chain variable fragment, the camelid (commercial application Caplacizumab, used to treat acquired thrombotic thromobocytopenic purpura), or an isolated variable heavy chain fragment, called VHH. Please see Diagram.

It is the VHH, or the variable heavy chain single domain antibody that now provides promise for the treatment of COVID-19. While vaccines can take years, and canonical (standard) monoclonal antibodies around 6 months to prepare, VHH can be prepared very quickly- within weeks, and therefore are ideally suited for dealing with a pandemic. Please see the linked paper in Cell below:

https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(20)30250-X

VHH has several advantages over other techniques. It ways around 15 kDa, around a tenth of the full immunoglobulin molecule. It can reach antigenic epitopes that the full immunoglobulin molecule cannot reach, such as hidden epitopes, a fact that is relevant with COVID-19. And it does not have the foreign antigenicity that full camelids have, thus reducing the risk that they would be rendered ineffective by the human immune system.

Immune Serum and the promise of Monoclonal Antibodies Against COVID19

According to an article in JAMA, immune serum from convalescent subjects has been used successfully against COVID19. This shouldn't come as a surprise. The technique was used for SARS and Ebola with success, and is still used in the treatment of Tetanus, and prophylaxis against Hepatitis B, Rabies and Chicken Pox following exposure.

While the preliminary results are encouraging, I'd caution against regarding it as some kind of cure though. Apart from the difficulties of using it widely (one patient- one recipient), there are several caveats.

Firstly, the antibodies in immune serum may provide protection against the virus temporarily, but will stymie the development of intrinsic antibodies or a T-cell response and therefore leave the subject vulnerable against re-infection if the pandemic continues.

Secondly, under certain conditions, infused antibodies in immune serum can lead to a phenomenon called Antibody Dependant Enhancement (ADE), which leads to increased virulence of the same virus or a related virus.

The immunoglobulin molecule (antibody)- IgG for all practical purposes, has two principal components- the Fab or antigen binding fragment and the Fc, which is the bit that binds to receptors on macrophages, neutrophils, T-cells and antigen presenting cells. The mechanism for ADE goes like this- the infused IgG binds to say, the spike protein in COVID through the Fab portion, and causes a conformational change in the receptor binding domain of the spike protein. It then binds to the host cell- say macrophages in the lungs, through the Fc portion, and actually facilitates the entry of the virus into the cell, thus increasing the cytolytic effect of the virus, before the host immunity has upgeared to deal with this new invader.

Something similar happened while trying to treat MERS cases in the past with immune serum. This increased the vulnerability of some serum recipients to infection with SARS, thought to be due to ADE. Later, a research group showed that a similar thing could happen with SARS itself, i.e immune SARS serum enhancing SARS severity.

Thirdly, serum carries the risk of transmitting other infections. While certain infective agents are screened for routinely- HIV, Hep B, Hep C, Syphilis universally, Malaria, Chaga's disease and HTLV in endemic areas and CMV for transplant recipients, it is impossible to screen for all known infective agents.

Fourth, foreign serum can lead to a Type III immune reaction called serum sickness. Yes, this was first demonstrated with horse serum, but can happen with human to human serum transfer at low levels.

So, what's the solution?

The really encouraging news to emerge from the successful use of immune serum lies in exploiting the promise of monoclonal antibodies. These are already in widespread use for autoimmune diseases- RA, Crohn's, Psoriasis, Lupus, etc, but are increasingly finding their way into infectious diseases.

They are easy to manufacture from scratch- take weeks to months from conception to readiness for use, rather than years. You simply extract the immune serum from a convalescent person, identify the memory B cell that makes the antibody that imparts immunity to your putative antigen by flow cytometry, clone the variable region of the immunoglobulin heavy and light chains and attach it to a Fc fragment. You can even use the Fab fragments themselves without a Fc portion, as with Certolizumab in RA.

This is not just theoretical. Palivizumab, a monoclonal antibody to the F protein of Respiratory Syncytial Virus, is used for prophylaxis in vulnerable infants against RSV induced bronchioloitis.

During the Zaire Ebolavirus outbreak in 2014-6, a combination of 3 mouse-human chimeric antibodies (like infliximab or rituximab) entered RCT. Unfortunately the latter was abandoned because the Ebola epidemic ended, and recruitment petered out (this is one of the biggest barriers to get drug companies to invest in such technology for infectious diseases).

A monoclonal antibody against Zika virus was waiting to enter clinical trial in pregnant women in Zika endemic areas. Again, the epidemic ended, and the enthusiasm waned.

The limiting factor is of course, cost. Monoclonal antibodies are expensive to mass produce and without the certainty of getting their money back, pharmaceutical companies have no incentive to change that.

But COVID is about to change everything. The cost to the world economy is likely to run into hundreds of billions of pounds, dwarfing the cost of production of such drugs. If COVID continues apace, the governments of the world, particularly the richer ones, will have no option but to pool their resources and incentivise the pharmaceutical sector to invest in this. After all, success is guaranteed, this is a mature and well tested technology, several companies have expertise in it, and there is an overwhelming moral imperative to save lives.

If not now, when?