Why antibody levels do not equal "immunity": Part 2

Epithelial cells and mucus!

We constantly hear about antibodies in the context of “immunity.” In fact, this is how the CDC defines “immunity”:

Immunity to a disease is achieved through the presence of antibodies to that disease in a person’s system.

This cartoon depicts a faceless man getting injected with a vaccine. It’s supposed to show how that leads to the production of antibodies (the green things that look like Ys) against the virus (red spikey circles).

In Part 1, we saw how this was grossly misleading, because it’s possible to recover from SARS-CoV-2 infection without the help of antibodies in the blood. This obviously means that there are other components of the immune system that are relevant to fighting off viruses like SARS-CoV-2.

In this article, we’ll review two underappreciated components of the immune system: the epithelial cells that form the barrier of our respiratory tract, and mucus.

In future articles of this series, we’ll review other components of the immune system.

This background knowledge of the immune system will allows us to compare “natural immunity” with the immune response provoked by the COVID-19 vaccines.

The cells at our barriers

The outer layer of the body’s surface is made of epithelial cells. These cells cover any part of the body that comes into contact with the “outside” world; this includes the skin, airways, gut, and urinary tracts.¹

Fig 13-1 from Kuby Immunology, 8th Edition. Image depicts how different parts of the body have one or more layers of epithelial cells, which interact with immune cells underneath.

Mucus

Specialized epithelial cells secrete mucus, which is viscous and gel-like. The image below shows a layer of epithelial cells with a layer of mucus on top:

Fig. from here. Image depicts epithelial cells (ciliated cells, basal cells, secretory cells, goblet cells) with a gel layer on top.

The mucus forms a mesh-like layer which acts as a “sieve” for pathogens. This can trap pathogens that are larger than the pore size of the “sieve.”

Mucus contains several different types of molecules called “mucins.” These are large molecules with lots of different carbohydrate chains. The chains offer a high degree of resistance to enzymatic breakdown from microbes.

Image from here depicting mucin. Mucins are covered in glycans (carbohydrates), which make them resistant to the enzymes of microbes, and give them the gel-like properties found in mucosal barriers. They may also be associated with membranes, as shown here.

In order to learn what these different mucins do, researchers have created mouse strains that lack a gene for one of the mucins.

Table from here. “Strain” refers to the type of genetically modified mouse. The strain is named after the gene that’s been modified (whether it’s been knocked out or overexpressed). For example, “Muc1-/-” means the strain is a homozygous knockout for the Muc1 gene. This means that both copies of the Muc1 gene (from mother and father) are no longer functioning, so the strain doesn’t make any Muc1 protein.

These experiments revealed, among other things, that a mucin called MUC5B is particularly important for normal airway function. Mice that lack the ability to make this mucin are much more susceptible to infections in the lungs.

Molecules in mucus can act as “receptor decoys” for viruses. Normally viruses need to bind to receptors on the surface of cells in order to infect them, but “receptor decoys” can bind certain viruses and help trap them (more here).

Mucus also contains numerous other compounds involved in defense. These are just some of them:

This table shows some of the products and the cells that make them. Clara cells are the same as club cells.

You’ll see quite a few microbicidals on the list, which are compounds that kill microbes like bacteria or viruses.

Some of these, like surfactant proteins, work as “opsonizers,” which tag foreign proteins for elimination by immune cells. Then there’s lactoferrin, which sequesters iron, making this nutrient unavailable to bacteria. Lactoferrin also has antiviral properties, and may even have some efficacy against coronaviruses (more here).

Lysozyme, cathelicidin, and β-defensin work by perturbing the cell walls or membranes of pathogens. Defensins can be found in the serum (blood) as well. There’s some evidence that physical activity increases the production of serum defensins (more here and here).

There are many other mucosal antimicrobial compounds in mucus that aren’t listed here.

Epithelial cells play an active role in defense

Now let’s dig deeper into the epithelial cells that make up the barriers. The figure below depicts a simplified view of the cells that make up the airway epithelium:

From Fig. 1 here.

These cells not only form a barrier to the outside world, but also play an active role in defense.

Goblet cells secrete mucus, as well as immune signaling molecules (more here). Ciliated cells use their little hairs (cilia), to sweep mucus upwards to help expel trapped foreign particles or pathogens from the respiratory tract. Both basal and club cells can differentiate into other types of epithelial cells, and produce immune signaling molecules. Basal cells also produce antimicrobial molecules (see here).

More recently, we’ve learned of new cells types in the airway epithelium that are involved in immune defense. The image below shows a revised contemporary view that includes newly identified cell types:

From Fig. 1 here.

For example, tuft cells seem to be involved in promoting respiratory reflexes like sneezing. Their numbers expand following inhalation of common allergens. And pulmonary neuroendocrine cells are not well understood, but they seem to sense inhaled inputs in the airway and release neuropeptides that enable lungs to respond with mucosal immune responses (more here and here).

We’re also learning of new immune properties of airway epithelial cells:

From Fig. 3 here.

For example, in the presence of pathogens, or allergens, some cells in the airway will commit apoptosis, which is a form of cell suicide. “Textbook immunology” would tell you that these suicidal cells would get cleared by “professional” phagocytes, which are immune cells that engulf other cells. However, we now have evidence that certain airway epithelial cells also engulf apoptotic cells, and play a role in taming the inflammatory response (see Fig. 3a above).

We’ve also discovered that basal cells seem to have some capacity for “inflammatory memory” (more here, here, or here).²

Epithelial cells modify and transport immune molecules

Epithelial cells also take immune molecules from below the mucosal layer, and transport them to the surface facing the airway.

For example, they transport antibodies like dimeric IgA (dIgA), which are made locally by antibody secreting cells (B cells) underneath the epithelial cell layer.

The image below shows this process:

Image from here. The bottom sides of the epithelial cells have something called “polymeric immunoglobulin receptors” (pIgR) which transport antibodies from the bottom of the epithelial layer to the mucus-facing side. When they do that, they also leave a component of the receptor, called “secretory component” or SC, on the antibody. This antibody, which is composed of the original dimeric IgA plus secretory component, is called secretory IgA, or SigA.

While they transport the dIgA antibody across the epithelial layer, something gets attached to it called “secretory component” (SC). The resultant molecule is called “secretory IgA,” or SigA.

Another type of antibody, IgM can also be transported this way, though the dominant class of antibody in the mucosal immune system is in the form of SigA. SigA can do many things, but one of its most important functions in the mucosa is to bind pathogens and block their ability to gain access to the epithelium (more here, here, and here).

By the way, IgA can also be found in serum (blood), but serum IgA is different from secretory IgA found in mucosa.³ One important difference is that secretory IgA usually leads to clearance of microbes in a way that's less inflammatory than the effect of serum IgA.

This makes sense because the mucosal barriers are constantly exposed to foreign particles and microbes; if the the body always reacted to these particles and microbes with a lot of inflammation, the barriers would be constantly inflamed.⁴

Selective permeability through the barrier

It should also be mentioned that there can be “seepage” from blood into the airway.

This flow of blood fluid through the epithelium not only flushes out invading pathogens, but also brings numerous plasma proteins to the mucosal layer. These include important components of the immune response, including serum antibodies and complement proteins (more here and here).

Figure from here. (E) Pathogens or other inflammatory challenges act directly or indirectly on certain receptors which regulate permeability. (F) Plasma exits through pores and distributes to the lamina propria (the layer underneath the epithelial cells that form the barrier) (G) Plasma moves up between epithelial cells (H) This movement of plasma can occur without compromising the barrier; in other words, without changing the barrier’s ability to keep things out.

Under certain inflammatory conditions, chemical agents can be released that increase vascular permeability and allow even more blood fluids to seep through.

Epithelial cells relay information to immune cells

Since epithelial cells are at the barriers of the body, they are usually the initial targets of pathogens, and therefore serve as “sentinels” for infection. These cells have pattern recognition receptors (PRRs) that recognize patterns or motifs that are conserved across many bacteria and viruses.

Different receptors specialize in recognizing different patterns on pathogens. For example, mannose receptors specialize in picking up on sugars on the surface of pathogens. Toll-like receptors pick up on all different kinds of patterns on pathogens, including components of bacterial or fungal cells walls, or viral RNA.⁵

Image adapted from here showing two different kinds of toll-like receptors on the surface of a cell, picking up on SARS-CoV-2 virus.

By the way, young people seem to express pattern recognition receptors at higher levels on their epithelial cells, compared to older people (see Fig. 2 here). Perhaps this is one reason why they generally manage SARS-CoV-2 infection well.

What happens if these receptors sense the presence of pathogenic microbes?

The epithelial cells release signals that alert other cells to the presence of pathogens, and recruit nearby immune cells.

Image from here. Image depicts different foreign particles (allergens, bacteria, virus) in the airway (lumen) which get picked up by different pattern recognition receptors (MR=mannose receptors, TLR=toll-like receptors, PAR=protease-activated receptors) in the epithelium (layer of epithelial cells). Some TLRs, especially ones that recognize viruses, are inside of cells instead of on the surface of cells. Activation of these receptors lead to a signal cascade that recruits immune cells in the layer below.

In the image above, some of those immune cells are shown below the layer epithelial cells.

In Part 3, we’ll talk about what those immune cells do.

1. By the way, in addition to our own cells there are commensal organisms that inhabit the airway; these are organisms like bacteria, fungi, etc. that aren’t necessarily bad or pathogenic. For a long time it was thought that these microbes only inhabited the upper airway, and the lower airway was believed to be sterile, at least in healthy individuals. Recently, however, there’s been evidence that healthy people also have microbes living in the lower airways. More here.

2. This “memory” is thought to come about from epigenetic changes in the cells, which is when DNA is reversibly modified to alter how genes are expressed without altering the underlying DNA sequence. This kind of “memory” is different from the kind of memory conferred as a result of “memory cells” that are part of the adaptive immune response (this will be discussed in more detail in future articles).

3. Whereas secretory IgA is dimeric, serum IgA is composed mainly of monomers, and also lacks the secretory component that secretory IgA has. This allows serum IgA to bind to certain receptors on myeloid cells. The net result is that serum IgA tends to induce more pro-inflammatory responses, compared to secretory IgA (more here).

4. This is a fascinating aspect of the mucosal immune system; how they maintain a balance between responding to pathogenic microbes, while not overreacting to harmless microbes. The way this is achieved is not completely understood.

5. Toll-like receptors (TLRs) can be found on the surface of cells, as well as inside of cells. TLRs that are inside of cells can pick up on viruses that have already gotten inside and infected a cell.