Carla Rothlin: Taking immunology to the next level while leveling the learning field
The human body is a marvel of remake and renewal. We regularly replace some of our estimated 37 trillion cells – about 300 billion or 1 percent of our cells each day -- with healthy substitutes. We remove damaged or aging, senescent cells and cellular debris from our organs and tissues every second of the day. How important is it that cellular waste be properly removed?
Last summer, University geneticist Kate Adamala and her team made international headlines by reporting that they had made simple cells from chemicals alone, “cells that feed, grow, reproduce and compete with one another for food,” according to a story in the New York Times. “SpudCells,” as they are called, constitute a stunning breakthrough in the field of synthetic biology. But they have one big limitation. After about five cell divisions, SpudCells stop working because they possess no means to clear their metabolic waste. It builds up. And they seize up.
Our cells do possess mechanisms for cellular waste removal from our organs and tissues. Indeed, we have a suite of them. When they are functioning appropriately, our cellular self-renewal proceeds apace. When they fail, the risk of inflammation and systemic disease rises. Perhaps no one knows that better than immunologist and LMP professor Carla Rothlin.
Rothlin joined LMP from Yale University a year ago. She is Director of the Center for Immunology (CFI), Co-Leader of the Masonic Cancer Center (MCC) Immunology Program, and Andersen Chair in Immunology at MCC. Joining her from Yale is Sourav Ghosh, a professor of pharmacology and Program Director in the Immunology of Tissue Resolution, Repair, and Regeneration at CFI.
The July edition of the Medical School’s monthly newsletter The Scope highlighted Rothlin and Ghosh in “Welcoming a New Era in Immunology.” And with good reason. Over the years, Rothlin and Ghosh and their colleagues have been “Cracking the Cell Death Code” by identifying and defining the molecular and biochemical pathways of cell death. They have been delineating the complex processes of dead cell removal through the immune system’s “effector response” and relating disturbances in these processes to pathologies. Chronic inflammation, autoimmunity, neurodevelopment and neurodegenerative disorders, cardiovascular disease, and cancer are among the pathologies that may arise when the process of dead cell removal goes awry.
The body’s response to cell death: A game of yin and yang
Rothlin and Ghosh outline the future direction of their emerging research field in a 2019 paper entitled: “Funerals and feasts: The immunological rites of cell death.” The response of the body to cell death is “purposeful and calibrated,” they write. In brief, cell death and the body’s response to it are inextricably bound with life itself, all life. From early development to adult tissue and organ homeostasis -- the dynamic steady-state balance that maintains tissue structural and functional integrity during cellular turnover -- to tissue repair following injury or infection, “a game of yin and yang is constantly in play between various forms of cell death and the response of the live organism – its balance is crucial for life and its disbalance manifesting in pathology.”
Rothlin observed at the outset of her LMP Grand Rounds talk in May entitled "Dying cells in health and disease: From resolution to chronic inflammation” that cell death “is prevalent across every stage of life beginning with our early development. We think it’s important to try to understand what happens when cells die because how a cell dies matters when it comes to the effector response."
Rothlin’s research team decided to focus on intestinal epithelial cells that undergo rapid turnover. Epithelial cells in the intestinal villi, the tiny, finger-like projections that line the inside of the small intestine to help absorb nutrients, die by apoptosis, a program of regulated cell death in the gut. They are extruded or pushed out of the gut’s inner lining into the intestinal cavity. Investigators have found that their removal is done by two types of immune system cells: macrophages and dendritic cells. Though they operate in quite a different manner, both are phagocytes, specialized white blood cells that protect the body by engulfing and removing harmful foreign particles, bacteria, dead cells, and cellular debris.
Modeling the “effector response” in health and disease
In her Grand Rounds lecture and a keynote talk she gave in April for the Robert P. Hebbel Research Day entitled “The Afterlife: Decoding the response when cells die,” Rothlin described in detail findings from her laboratory that show fibroblasts, one of the most abundant cell types in the human body, also can have an “effector response” to cell corpses in the intestine. Her group initially employed a widely-used chemical mouse model for colitis, a chronic inflammation of the colon lining, then created a chemogenetic mouse model that exclusively induces necroptosis in intestinal epithelial cells. They tested it with a battery of assays and showed that some fibroblasts forgo their traditional role in wound healing and instead respond to the presence of necroptotic cells, cells undergoing a regulated, programmed cell death that can end in local inflammation. Using their chemogenetic model, they showed that subsets of fibroblasts respond to the presence of “necroptotic corpses.” These fibroblasts proliferate and become inflammed.
“A key function of fibroblasts is to transition to myofibroblasts during wound healing, which is driven by environmental cues such as the cytokine or growth factor TGF beta,” Rothlin said. “We modified our colitis assay and asked, ‘Can the fibroblast response to TGF beta be changed in the presence of necroptotic corpses in cell culture?’”
In in vitro experiments, the investigators exposed fibroblasts to TGF beta alone and then in the context of necroptotic corpses. “We were able to observe that the addition of necroptotic corpses significantly blunts the response of fibroblasts to TGF beta, suppressing wound healing in our colitis model.” The experiments showed that necroptotic corpses elevate gene expression for proliferation and inflammation in these fibroblasts. Rothlin proposes that necroptosis drives this type of inflammatory response and that it engages not only immune system cells like macrophages but also the stroma, the supportive, connective framework of tissues and organs of which fibroblasts are the leading cell type. “That would inhibit the ability of the wound to heal,” she said.
Because fibroblasts are broadly involved in chronic inflammatory diseases, identifying the genetic and biochemical pathways that regulate their response is crucial. Inflammation’s profile as a threat to human health and wellbeing has been growing steadily in recent years. Welcome to the new era of immunology, one that looks more holistically at cell-cell interactions and signaling pathways in ridding the body of pro-inflammatory cellular debris.
Above, an abstract image of homeostasis, the body's persistent balancing act. Homeostasis is a fundamental biological principle that ensures stable internal conditions regardless of external changes. From www.meetnlearn.si, with permission.
MERTK: from the eye to the brain
In May, pharmacologist Sourav Ghosh gave an LMP Research Forum entitled “DAMned if I know: Conjectures about macrophage/microglial cell states.” DAM stands for disease-associated microglia, a distinct activation state of the resident immune cells of the central nervous system. Microglia are the brain's first line of defense. They are phagocytic patrols of their environment, clearing out cellular debris, combating infectious agents, repairing damaged tissue, and otherwise helping to maintain healthy neural networks.
“We are very interested in microglia,” Rothlin said in an interview, stressing the synergy that immunology joined with pharmacology can produce. “We are actively working on microglia. It’s a cell that gets into the brain so early. It’s a cell that’s important as we age and in neurodevelopmental and neurodegenerative diseases. We’re fascinated by that.”
Studying how the immune system affects the nervous system from early development to neurodegenerative diseases and, vice versa, how the nervous system affects the immune response “is going to be an area of tremendous growth,” she said.
Over the years, Rothlin, Ghosh and their fellow investigators have done extensive research on the TAM family of receptor tyrosine kinases (RTKs). These molecules regulate tissue homeostatis or steady-state balance by clearing dead cells, dampening immune and inflammatory responses, and supporting cell survival more broadly.
Through gene knockout studies in mice, they’ve learned that one of the receptors, called MERTK (pronounced Mer-TK), figures centrally in retinal disease. They are currently being funded by the National Eye Institute to test the hypothesis that loss of MERTK anti-inflammatory signaling in retinal pigment epithelial cells, rather than defective phagocytosis, is the critical event leading to retinitis pigmentosa, a progressive eye disorder than can lead to blindness. With the loss of MERTK, retinal inflammation sets in, triggering microglia and setting off a tissue-damaging inflammatory cascade.
But it’s the potential role of MERTK in conjunction with AXL, another TAM family RTK, in Alzheimer’s disease (AD) and related dementias that has her most excited. The National Institute on Aging recently awarded Rothlin a research grant for “Augmenting AXL and MERTK function to restrain cognitive decline and improve health span in mouse models of Alzheimer's Disease.”
“We have really beautiful data from MERTK and the AXL receptor, its brother in the tyrosine kinase family,” she said. “We have really beautiful data from mouse models.” The primary model her lab is using is 5xFAD, which involves familial Alzheimer's disease mutations and features rapid amyloid-beta plaque accumulation characteristic of AD. Investigators are homing in on how AXL is upregulated in the presence of damaging plaques and protects against cognitive decline in AD.
The MERTK and AXL genetic and biochemical signaling pathways in microglia, macrophages, and other phagocytic cells, may have much to tell us about AD. Rothlin and her team aim to find out whether augmenting the function of these receptors is a drug-candidate pathway to preventing cognitive decline and preserving memory. They are also raising the odds of successful pathway elucidation by creating SysTAMatic, a mouse model toolbox specifically for the TAM RTK receptors and the vital role they play in tissue homeostasis in both health and disease.
“Global Immunotalks”: A multi-continental learning platform
It is generally agreed that academic immunology emerge during the last quarter of the 19th century following the studies of the French chemist and microbiologist Louis Pasteur, German bacteriologist Robert Koch, and Russian zoologist Élie Metchnikoff, who first identified phagocytosis as a function of the immune response. The Journal of Immunology saw its first issue in 1916, the same year that Johns Hopkins University medical school dean William Henry Welch founded the first academic department of immunology.
More than a century later, immunology is surging. Though its tools of investigation are unsurpassed, its workforce can barely keep up with the demands being made on it by infectious and chronic disease, environmental degradation, mass migration, the consequences of a changing climate, and an aging population. The authors of a bibliometric study on global research trends in immunosenescence and immunotherapy note the growing international reach of these immunology subfields, both of them closely tied to Rothlin and Ghosh’s research on efferocytosis, effector response, and inflammation.
Last September, Rothlin was elected Vice-President of the International Union of Immunological Societies (IUIS), a prestigious and influential role within the global immunology community. After her term as Vice-President, she will serve a three-year term as IUIS President beginning in 2028. Her international profile has been elevated by her own creativity and imagination, and her desire to “level the field” when it comes to learning, as Global Immunotalks demonstrates.
One evening in March 2020, Rothlin said she received a call from her research colleague and friend Elina Zuniga, an immunologist at the University of California San Diego (UCSD) and a fellow Argentine. “Elina said there was a seminar series starting in immune metabolism,” Rothlin said. “We were all at home, super worried about the COVID pandemic. We were trying to advance our research as much as we could, writing papers, things we could do from home. We had this very nice discussion about the possibility of doing something in immunology. We thought this would be an incredible thing to do.”
A month later, Zuniga and Rothlin launched Global Immunotalks, a virtual initiative “to increase opportunities for scientific learning without traveling. It stands to benefit and inspire immunologists across the world in an egalitarian manner,” according to its mission statement. “More than a thousand people logged in,” Rothlin said. “The system almost crashed!”
Now in its sixth year, Global Immunotalks draws more than a thousand views per week from around the world with some 800,000 views in total registered on its YouTube channel, Rothlin said. “It’s been a humbling experience. Many places where I’ve been invited to give a talk, people come up and thank me for this initiative.”
To ensure the series is truly “global,” Rothlin and Zuniga invite organizers from around the world to invite speakers. “We are now in our fourth iteration of organizers – people from Australia, China, Israel, various places in Europe, North America, Latin America,” Rothlin said. “We don’t yet have an organizer from Africa, but we try to make sure we have speakers from all over the world for this global initiative. It’s important to have breadth in the organizers and speakers from everywhere. We want to level the field.”
Their initiative to “level the field” in immunology communication recently expanded to education. “If you want to get into immunology, it can be very daunting in many places,” Rothlin said. “You don’t know how to start. You might not have the foundational knowledge. So, we started Global ImmunoCourses. Sourav and I are very involved in this effort. We’ve run it already three times in India. We want to expand it in the global south because many of our seminar viewers come from certain demographic groups there. So, we will give them an opportunity by providing introductory courses in immunology. We cover the fundamentals in an engaging way,” she said, adding that former and current members from their lab who are committed to education, notably postdoctoral research fellows Lindsey Hughes and Aicha El Allam, also participate. “I think the combination of introductory courses and the seminar series can really provide an opportunity that not everybody has at a global scale.”
The paradox of immunology, as in all fields of scientific investigation, is that what is learned opens a window revealing how much more there is to learn, in immunology about the part the immune system plays in the human body’s persistent balancing act, homeostasis.
“That’s being a scientist,” Rothlin said of the paradox. “I tell people in the lab just do the experiment well. It will be exciting.”
