This is written by Jennifer Kasten, a pathologist with degrees in epidemiology and infectious diseases. She has also done field work in epidemic control. She just posted an article with good news about covid immunity.
[One paper, two bombshells, and some staggeringly good news about immunity, vaccines, and protection against COVID.
MIND YOUR Bs and Ts
When thinking about immunity to COVID, we’ve mostly discussed antibodies (what do they have against bodies, anyways?), those little Y-shaped guys which are pumped out by B cells.
Antibodies can bind to free virus, circulating around outside cells, but since each infected cell pumps out thousands of copies (and then explodes- goes supernova), it’s sort of a mop-up operation, a little behind the 8-ball.
It would be even better to take out the nest, the queen bee, and kill the infected cell directly. In order to do this, they need a little help from their friends- the T cells.
When a cell is infected with a virus, it sends out little distress signals. Sort of like a panic room button. Essentially, it chops up bits of virus and displays those bits on the outside of its cell membrane, waving like a flag.
Lurking in the shadows, waiting for that distress beacon, are killers. The good kind of killers (not the band, though they’re good too)- sort of like vigilante justice. Cytotoxic (killer) CD8+ T cells bind to the viral panic flag, which activates a death response. The host cell dies, but it would have anyway (supernova viral burst) and hey, to make an omelette you have to break a few eggs. Viral replication stops in its tracks.
Th CELLS: IMMUNOLOGICAL RYAN GOSLING
Till now, we hadn’t proven they existed. But a paper published in Cell last week by Crotty, Sette et al (1) proved that, in 20 recovered COVID patients, 70% showed active circulating killer CD8+ T cells.
Even better- 100% showed active circulating CD4+ T cells*. Those are the “helper” T cells. Kind, caring and considerate, the helper T cells are definitely better boyfriend material than the killers, and in some ways they’re even more important.
Helper T cells help (of course they do) both B cells make antibodies, and killer T cells find their targets. They’re a bit like traffic cops in that they secrete signals- cytokines and chemokines- to tell everyone else what to do. And they can recognize viruses, even if they don’t kill them directly. In SARS-CoV-2’s case, 100% of patients showed helper T cells specific to the spike protein. Since they tell B cells what to do (but in a nice and supportive way, to make the B cells feel really empowered), we SUSPECTED they’d be there, and voila, they are.
This is ultra-important because it shows 1) viral immunity to COVID is straight up, plain vanilla NORMAL and EXPECTED and humming right along. And 2) Helper T cells are very important in the VACCINE immune response (they hang out as immunological memory, just being supportive, sending encouraging notes to the B cells that hey girl! you got this). This gives us great hope that a vaccine will be effective.
KISS THE FEET OF THE TODDLER WHO GAVE YOU THAT FIERCE DAYCARE COLD
The other amazing discovery was HALF OF THE NON-COVID PATIENTS ALSO HAD PROTECTIVE CD4+ T CELLS.
The researchers pulled blood from 2015-2018, before this virus slithered the earth, and from California and Italy, where essentially no one would have been exposed to SARS and MERS. A full 50% of these historical samples showed some CD4+ T cell activity against SARS-CoV-2.
What? How? Infection with the common cold coronaviruses has some protective effect. It could explain, partially, why a novel virus with a R-naught over 2 has shown the relatively muted, dampened epidemic curves we’ve seen the world over. And it could lower the Herd Immunity Threshold a bit, too, if a sizable chunk of the population already has some protection against COVID.
*For fancy types worried about Antibody-Dependent Enhancement: NOT OBSERVED. Classical Th1 response here, folks.
1) Alba Grifoni, Daniela Weiskopf, Shane Crotty, Alessandro Sette et al. Targets of T cell responses to SARS-CoV-2 coronavirus in humans with COVID-19 disease and unexposed individuals. Cell. May 14, 2020.