Developing a New Brand of Antibodies to Fight a Widespread Fungal Infection

In reference to a new publication in Nature Communications authored by researchers at the University of Aberdeen and the University of Edinburgh.

When you think of the deadliest illnesses in the world, diseases like HIV, malaria, and cancer jump to mind. Strangely, a top contender with all of these conditions is fungal infection. Clocking in at 1.5 million deaths per year, fungi are some of the most dangerous infectious agents on the planet. The most common of these is Candida albicans (C. albicans), the fungus behind common vaginal yeast infections and oral thrush. In people with weakened immune systems (such as HIV patients), these common infections can quickly progress to be life-threatening.

Candidiasis is generally only diagnosed after a blood test, so the infection often isn’t noticed until symptoms have already developed. To prevent infections from progressing that far in the first place, scientists have been working on building vaccines to fungal pathogens. However, these efforts are relatively new and none have been made available to the public as of yet (although one is in Phase II clinical trials).  A new publication in Nature Communications describes a method for generating a fully human-based, highly specific C. albicans antibody which can prepare the immune system to fight off the fungus before an infection ever occurs.

Before we can understand these findings, we need to understand how B cells and antibodies work. B cells are a part of the adaptive immune system which creates long-lasting immune memory against secondary pathogen infection. This system is what stops the same bug from making you sick twice, and also explains why vaccinating with a small piece of a pathogen preps your immune system to face the real thing. When the immune system is seeing a new type of pathogen, T cells (the other major cell in the adaptive immune system) become activated and bind to B cells. This “T cell help” tells the B cells that there is an infection and causes the B cells to multiply. Some of these new B cells will be plasma cells, which quickly secrete lots of antibodies to fight the infection. Other new B cells become memory B cells, which will remain in the body for years in case the same pathogen infects the body again. If so, the memory B cells are quickly activated and the immune response is much faster and more effective the second time around.

The antibodies themselves are receptor proteins made by B cells. They are highly variable in structure and, therefore, in their ability to bind different antigens. When binding an antigen, antibodies can perform a number of different functions to aid the pathogen’s destruction. Their binding can neutralize the pathogen by physical blocking them from breaking into host cells (Lu et al. 2018). They can recruit other immune cell types (such as natural killer cells) to the infected cells to kill them before the pathogen can spread (Lu et al. 2018). As seen in this study, they can also recruit large cells called macrophages to engulf and digest (AKA phagocytose) the antibody-bound infected cells in a process known as opsonization.

Because they are so specific and so good at binding to their antigen of choice, building antibodies to recognize antigens associated with infectious pathogens is a popular field of research at the moment. This Nature Communications article details a new way to make antibodies against a C. albicans antigen (the protein Hyr1, a component of the fungal cell wall).  The process begins by taking memory B cells from the blood of patients who have experienced a Candida infection in the past year. Some of these cells were involved in the response to the Candida infection and produced anti-Candida antibodies. So, the B cells were tested against multiple Candida antigens and the ones whose antibodies bound an antigen were selected. The chosen cells were broken open and had their DNA extracted, specifically the genes which encode for the antibody proteins (called the VH and VL genes). Using a DNA amplification process called RT-PCR, several copies of the VH and VL genes were generated. The genes were then inserted into new cells which multiplied and expressed the VH and VL genes, thus creating the antibodies. Finally, the researchers purified and extracted these antibodies so they could test them in cell cultures and in live mice.

These new antibodies are exceptional because they are monoclonal and fully-human. So rather than transferring whole blood serum (containing lots and lots of different antibodies) from a past patient into a healthy person, the researchers specifically cloned the one antibody that they were looking for. This approach is much more specific than a whole serum transfer, which may cause some sort of graft rejection. This is also an issue if antibodies were originally taken from another animal such as mice, hence why the “fully-human” aspect of the procedure is vitally important.

Just as antibodies tend to do, the anti-Hyr1 antibodies binds its target Hyr1 very specifically. After attaching the antibodies to a fluorescent tag, the researchers could visualize under a microscope the antibodies specifically binding to the cell wall of C. albicans. When tested against mammalian antigens, the anti-Hyr1 antibodies did not show signs of cross reactivity. This is great news for vaccine development, since binding to mammalian antigens could be a sign that the antibody would cause an autoimmune reaction in humans. Even better, the researchers raised other antibodies against a wide host of C. albicans cell wall antigens (anti-whole cell antibodies) which are able to bind other Candida species as well.

Now comfortable that their more expansive anti-whole cell antibodies were specific at targeting only the pathogenic fungus, the researchers added them to cultures of mouse cells and exposed them to  C. albicans infection. Compared to controls, the antibody-injected cultures showed increased phagocytosis of fungal particles by macrophages. Blocking the antibody-binding proteins on macrophages reversed this effect, indicating that it was indeed the antibodies which were stimulating the increased macrophage activity. In live mouse models, the anti-Hyr1 antibodies didn’t increase resistance to fungal infection. However, using the anti-whole cell showed significantly reduced disease symptoms and a smaller presence of fungal cells in the mouse tissues.

As fungal vaccines are in dramatically short supply given the damage they do across the world, this study could provide a strong basis for developments in this emerging field. Not only are the authors addressing a massively understated medical issue, but they are doing it through a widely applicable method for harvesting new recombinant antibodies. This procedure could be valuable in any number of immunological studies, especially those studying vaccine development.

Reference

Lu, L. L., Suscovich, T. J., Fortune, S. M., & Alter, G. (2018). Beyond binding: antibody effector functions in infectious diseases. Nature Reviews Immunology, 18(1), 46.