2020 has been a prime year for immunology, there is no doubt about that. Though recognized as one of the most complex, yet all-encompassing topics in biology, immunology has squeezed its way into the limelight, thanks to COVID-19.
You may have come across these words recently…
Virus.
mRNA.
CRISPR.
Cytokines.
T cells.
Funny how these words were once part of a private exchange between my dense biology textbooks and I, muttered over and over until the concepts gelled in my brain just in time for Advanced Cell Biology exams in the first year of my PhD.
Now they’ve made their way to celebrity status—gracing social media feeds and TikTok videos.
But as a scientist working in the immunology field myself, I cringe when I see posts that have not been fact-checked, or twisted definitions of basic biological concepts circulating in the mainstream media.
Before diving into all the COVID-19 articles out there (many of which are based on publications that have yet to be formally peer-reviewed), let’s get some things straight.
Like, what’s a T cell anyways?
T cells are born in an organ snugly fit between our lungs, the thymus (hence T cells), and are categorized as players of the “adaptive immune system”, which makes sense since T cells are quite the malleable bunch. They adapt to the surroundings of their biological environment, and play a critical role in maintaining immune homeostasis in the body.
T cells have the capacity to develop specific receptors against foreign particles, signaling other players in the immune system to fight off burgeoning infections, while also having the potential to remain in the body for years, ready to fight back in case those particular “foreign” particles enter the body again.
They are further categorized as “T helper” cells (CD4+) or “cytotoxic T” cells (CD8+). CD4 and CD8 are structures made out of carbohydrate and protein “blocks” and exist on the surface of T cells, giving off their identity. CD4+ and CD8+ T cells differ in how they interact with other cells in the immune system and foreign invaders.
CD4+ T cells rely on the help of other immune cells (like B cells and macrophages) to fight off infections. Their ability to secrete particles called cytokines (imagine a cell sneezing onto another cell) helps to activate these supporting immune cells so that they can go on to kill the infectious source.
CD8+ T cells are more precise in their function, since they are able to kill cancerous cells and virus-infected cells directly. They secrete cytokines as well, two of which are IFNy and TNFa, that can help to destabilize infectious cells and tumors.
Within these two categories, we can break CD4+ and CD8+ T cells down further into three sub-types (though there are more sub-types, the following are the most general).
Naïve T cells are the least differentiated of the three, waiting for the day they can respond to a unique pathogen and develop a specialized functions.
Effector memory T cells (TEM) are rapid-acting and ready to respond to foreign antigens (think, unwanted floating pieces of protein from the “bad guy”), since they are circulating in the blood or housed in non-lymphoid tissues that may be exposed to foreign antigens immediately (like the skin, gut, or lung).
Central memory T cells (TCM) are more stagnant, residing in secondary lymphoid organs, like the spleen or lymph nodes, unless stimulated by a foreign antigen—after which they can proliferate into an army of effector cells to enter battle.
CD44+CD62L- are effector memory T cells.
CD44+CD62L+ are central memory T cells.
CD44-CD62L+ are naïve T cells.
In the lab, we can assess the markers for these T cell sub-types and their cytokine production to determine if a stimulus of our interest (i.e. a potential cancer drug) can help a T cell to be more effective in fighting off infections. The idea has been a prime goal for many immunology-based labs for years.
Faster-acting T cells should also get rid of unwanted, foreign invaders in the body faster, right? Fast is a relative term, and unfortunately in the world of biological science, nothing is ever fast enough.
Still, we do our best to mimic how a T cell functions in real life (or, in vivo as we fondly refer to it) by activating, stimulating, and measuring markers that help further identify a T cell’s function.
In the lab, T cells are often obtained from spleens of mice and grown in culture (a.k.a. in vitro—imagine a large, nutrient-rich suspension full of blob-like shapes, swimming without abandon—those are cells in culture).
T cells can be activated by a number of ways, but activation via CD3 and CD28 is one of the most common ways to do so.
Just like CD4 and CD8, CD3 and CD28 are proteins that are expressed on T cells and are involved directly with activation. CD3 is part of the prime T-cell receptor (TCR) complex and when stimulated with CD28, can lead to the activation and expansion of T cells.
This process normally takes about 2-3 days in the lab, and we can perturb this process by keeping cells in the presence of increasing concentrations of a drug during activation. Depending on the goal of the experiment, T cells can be grown in the presence of this drug for longer periods of time, and we can select different time points to collect cells and measure markers of their present “identity”.
Simply put, we collect these cells at a given time, count them under a microscope, and then proceed to stain them with fluorescent dyes that are bound to the markers we are interested in.
Remember IFNy and TNFa? When we stain cells, we can add antibodies that are bound to a fluorescent probe that targets these cytokines. Same for CD44 and CD62L, which are prime markers for identifying effector or central memory T cells (as you saw earlier😉) .
After staining, we analyze the presence of our markers of interest using a tool called Fluorescent-Activated Cell Sorting (FACS) , which is able to isolate single cells and sort them by the fluorescence they give off.
It can be a tricky thing to configure at first, but once you know what you are looking for, it’s an exciting sight for an immunologist to look forward to:
When it comes to T cell function and optimizing it, T cells can be transduced, or have DNA introduced into their system via a virus. In this way, T cells can be “engineered” to express certain receptors on their surface if they come into contact with a specific antigen.
In the images below, we are measuring how many CD8+ T cells are also expressing the VB9 receptor after the transduction process with the SV40 virus.
I’ll keep it simple here because otherwise I may get into another blog post within a blog post 😅…
In this plot, we expect very few CD8+ T cells to express VB9, since they were untransduced.
But look what happens after a “successful” transduction (look at Q2):
Boom. Plenty more CD8+ T cells expressing VB9 as well!
————————————————– 𝕊𝔾𝔻 ————————————————-
There is absolutely no way all of immunology can be covered in a single blog post, let alone T cells, but having a basic understanding is a perfect place to start. The basics are important when it comes to figuring out if what the media is telling us is sensible versus sensational.
And as a scientist in the throws of it, I also come across the other extreme: the demand to read countless of peer-reviewed papers that are dense, distracting, and rather than furthering the field, make it all the more confusing!
Science doesn’t need to be intimidating or exclusive, but it can certainly feel that way given the immense amount of information out there and figuring out how to sift through it all.
The important thing is to keep an open mind, and don’t be afraid if you are not understanding the story before you—in fact, feel free to question it, because ultimately, that’s what science is.
If you found my tidbits on T cells interesting, I recommend these links for more simple as well as some in-depth reading!
British Society for Immunology
Cells | British Society for Immunology
T-cell activation | British Society for Immunology
Wikipedia
T Cell Activation via Anti-CD3 and Anti-CD28
T Cell Activation via Anti-CD3 and Anti-CD28 | Thermo Fisher Scientific
Scientific Review