Coronaviruses, Colds And COVID: And Cool Immunology

“The most exciting phrase to hear in science…is not ‘Eureka!’ but ‘That’s funny…’”

–Issac Asimov

 

Background. Your run-of-the-mill common cold virus is sometimes related to its more infamous relative that caused the world all sorts of consternation between 2020-2023, and still demands respect like an aging rock star who might still have some chops left. I, of course, allude to SARS-CoV-2.

Yup, the now infamous family of deadly human coronaviruses, which includes the original bat-borne SARS-CoV-1 (which caused the first SARS pandemic in late 2002), its Middle-Eastern camel-riding cousin (that caused MERS in June 2012), and the recent, much more traveled, durable, and concerning SARS-CoV-2 (origins so far unknown and the cause of COVID-19), have some lesser known, ne’er-do-well cousins that have long traveled among us. I refer to certain viruses that visit us often and are as unwelcome as a distant cousin who arrives unannounced needing a place to crash for a few days. This is the “common cold virus” which actually is several different kinds of viruses. Cold viruses are all as irritating and inconvenient as said uninvited distant cousin, and about as enjoyable as a hangover; but seriously debilitating or life threatening? They are not.

The common cold is mostly caused by one of three families of viruses; rhinovirus (not related to any large mammal), adenovirus, or a coronavirus. Yup, a distant cousin to that bug that caused so much serious illness and death across this blue orb during the COVID pandemic also is one of the causes of the mostly benign, but very annoying common cold. In fact, there are four different types of coronavirus cousins that cause 15-30% of the “common colds” in adults. Isn’t it interesting that one coronavirus, like SARS-CoV-2, can kill you, but its cousins just make you sneeze and your nose run like a leaky faucet, but that is all. Aren’t viruses fascinating?

Facts. Just as between unwelcome distant cousins, there are genetic similarities between the dangerous CoV-2 and its nettlesome coronavirus kin that just cause colds. And recent studies found that infection with one of these coronavirus cousins can indeed confer some immune protection to the other distant cousins. In other words, if you were infected with CoV-2, you likely had a much milder cold, if you caught one at all. And vice versa! But the funny thing is that vaccination against COVID did not also protect you against a cold like an infection would. What??

This stuff makes viral immunology so much fun.

To confirm all this, one study showed that this cross protection only occurred in people who had a definite bout of COVID caused by the coronavirus, and the reduced incidence of colds only occurred for colds also caused by a coronavirus, and not for a cold caused by unrelated rhino or adenoviruses. Clearly prior exposure to a different member of the coronavirus family conferred some immunity to other members of that family, even to distant cousins. Also, just being vaccinated to the CoV-2 spike protein did not confer this sort of protection to future coronavirus-caused colds. Wow! This kind of discrimination and specificity gets immunologists salivating like a Pavlovian dog to a ringing bell. I know—I am wiping secretions off my keyboard as I type.

Vaccines to just the spike protein quickly generates antibodies that neutralize the virus and thus prevent serious disease. But, that only offers short term protection to just that coronavirus from whence the spike protein sequence came. The viruses quickly mutate their spike surface proteins so the viral cousins cannot be recognized by the spike protein alone. That is why anti-spike immunity and the vaccines are not very good at protecting against re-infection for very long and why the vaccines don’t confer immunity to distant coronavirus cousins.

However, the immune system is a multi-layered security system. Besides these short-lived neutralizing antibodies that target the coronavirus spike protein (or similar surface proteins in other viruses), other layers of the immune security system can also be generated to other molecules across the SARS-CoV-2 genome following infection with the whole virus (see here and here). These other genome sequences are often more conserved and less likely to change between distant coronavirus cousins, than the highly variable spike protein sequence. This means that any immune response generated to one of these more boring, unchangable sites on a given coronavirus, can also recognize similar sequences on distant cousin coronaviruses.

But who, other than an immunology nerd really cares if having COVID protects you against a future cold? What about the reverse? Can having a cold caused by a coronavirus cousin generate some protective immunity to the nastier SARS-CoV-2 and protection from COVID and future coronaviruses that will emerge? Some, but not all research has indeed shown that people without prior exposure to CoV-2 do indeed show immune reactivity to the virus (see here and here). This means that folks who haven’t been infected with SARS-CoV-2 must have been exposed to another coronavirus that gave them a bit of cross protective immunity to the COVID virus. Other studies confirmed that prior infection with cold-causing coronaviruses can reduce COVID severity following infection with CoV-2 (here and here).

Bottom line.  What this means is that if you have been infected with some sort of mild coronavirus in the past, you just might be able to show some immunity to future infections with distant coronavirus cousins. Vaccination with the spike protein mRNA just doesn’t do the same. You need to be exposed to the whole kit and caboodle to enjoy all this immune goodness.

The responsible part of the immune system for this cross-over immune response is CD8⁺ T cells, also known as cytotoxic T lymphocytes, or CTLs. These immune cells are assassins that seek out other cells infected with a virus and they kill those cells. So, immunologists get all atwitter and think, “Hellz bellz, why don’t we make vaccines using parts of these boring, but conserved virus pieces that generate CTLs to different viral cousins, instead of the ever changing spike proteins to make vaccines? We could make one vaccine for all coronaviruses! Or flu, or whatever virus….”

It is a great idea and that research is well underway. The goal is to make a single coronavirus vaccine that would be long lasting and target many coronavirus cousins to prevent any future pandemic (believe me, another one is sure to come).

Back to earth. As interesting and hopeful as this sounds for making a single vaccine against multiple coronaviruses so we don’t have to continually try different boosters each year, don’t get your hopes up just yet. Similar immuno-optimism has been going on with influenza for decades and what do we have to show for that? We still have the annual guessing game of which flu strain will pester us each winter and then feverishly roll out millions of vaccines to try to nip that particular one in the bud. Meanwhile its flu cousins chortle and conspire in the Southern Hemisphere on how to mix and mutate their genes so they can surprise us again in the Northern Hemisphere the following year with a sufficiently new variation to vex us again.

But, flu, like coronaviruses also has important proteins that are not changeable, and very constant between distant flu cousins. These too can be seen by the immune system’s T cells. Flu immunology’s Holy Grail has long been to make a vaccine to a conserved flu virus genomic sequence so we can use just one vaccine to immunize against all flu strains once and for all for all time. A pan-flu vaccine.

Well, we are still trying to do that. This makes the idea of finding a pan-coronavirus vaccine using similar immunology daunting. Still, these recent studies showing that cross-reactive immunity between distant cousin coronaviruses does exist, just stokes an Immunologist’s stubborn resolve to solve the problem. As I have written before in these pages, amazing science advances have often come from the long, dogged pursuit of goals that very stubborn scientists believe they can see right in front of them, even when others cannot. It often takes a long time to prove what is so clearly obvious to one or two science visionaries yet so oblivious to the rest of us. That often is how science progresses. Thank goodness for these obstinate scientists who see things the rest of us cannot.

Once again, We will see.

Personal note. These anti-viral CD8⁺ or cytotoxic T lymphocytes are near and dear to this correspondent’s heart since I got my PhD in Immunology studying how these immune cells in mice recognize cells infected with viruses. It is a lot more complicated than you would think. In fact, in 1996 two immunologists, Peter Doherty and Rolf Zinkernagel were awarded the Nobel Prize for work they did on this problem in the early 70s, and that work drove my PhD research (and a lot more!).

Doherty and Zinkernagel discovered that T cells have to simultaneously identify two different molecules on an infected cell surface before they actually know a cell is infected with a virus. They made a head-scratching observation that turned viral immunology upside down. It was one of those observations that I bet made them say, “That is funny.” Basically, they found that your T cells that can recognize flu infecting your cells would not recognize flu infecting my cells or anyone else’s cells. And vice versa. You would think flu is flu and that a T cell that can see flu in an infected cell would not care whose cell it came from. But it does care. It turns out that T cells can only see virus within the genetic background from whence they came. They cannot see the same virus on a cell from a different genetic background! How strange is that? An antibody does not care where it sees a virus. T cells do. Picky little suckers.

It gets even crazier. Doherty and Zinkernagel, mapped this genetic restriction in virus recognition to the same genes that the immune system also use to determine whether a tissue or organ is its own or is foreign! For example, the genes your immune cells recognize as a password to determine friend vs foe in a skin graft (do we accept it or reject it?) are the same genes the immune cells use to help them know if your cells are infected with a virus! Tell me that doesn’t make you scratch your head and mutter, “That’s funny?” That is exactly how the world of immunology reacted to Doherty and Zinkernagel’s findings. It was a beautiful time for immunology science. That launched a tsunami of research, my PhD effort included.

This is personal note because I earned my PhD further probing the mechanism of what Doherty and Zinkernagel stumbled on. I used a large panel of mice that had been engineered to carry single point mutations in different parts of these genes that immune system used to ascertain tissue compatibility, and detect viral invasion. This helped us learn what part of these molecules the T cells recognized and how their folding was important for this recognition. It was a grand time!

Immunology is so doggone interesting!

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