A recent study published (Science Immunology 2018) by Maziar Divangahi’s research group at McGill University reveals a novel and important way that the immune system regulates itself during chronic infection.
Tuberculosis is a devastating disease caused by Mycobacterium tuberculosis (Mtb) that kills over one million people per year. It is estimated that one-quarter to one-third of the world’s population is chronically infected with Mtb. Despite these unnervingly high infection rates, most people never actually become sick with tuberculosis. This presents an interesting conundrum: If disease is not caused by infection status alone, what else could be causing it?. In fact, many symptoms of tubercular disease are caused by our own immune systems.
The immune system is a specially evolved set of defenses that together prevent, or restrict, the growth and spread of invading substances that could be pathogenic. (Metaphorical images of walled fortresses, special ops soldiers, and heavy artillery may be coming to mind right now). In a world teeming with countless species of potentially infectious microbes, a bigger, stronger immune response should always be the ideal, right?
Well…not really. Just as with the use of heavy artillery, traditional restriction mechanisms can cause devastating collateral damage to host tissues if the body has not devised ways to withstand, or “tolerate” the damage. Although tolerance mechanisms do not contribute to pathogen clearance, they can mean the difference between life or death, as Divagahi’s group discovered using a mouse model of Mtb infection.
>>>A side note here, “immune tolerance” is often used to describe a state of inactivity or low (hypo) reactivity to an antigen. Most T cell reactivity is set and adjusted during positive and negative selection (see our Immunology 101 on T cell selection). Further tolerances are established in other processes (see our Immunology 101 on tolerance). These antigens are usually self-antigens or those coming from innocuous sources such as food, environmental non-pathogens, or commensal microbes. In this capsule review, “tolerance mechanisms” refer to pathways that limit the amount of tissue damage caused by the immune system.<<<<
The team began by examining the role of a protein called cyclophilin D (CypD) in the context of Mtb infection. CypD is a mitochondrial matrix protein that regulates the mitochondrial permeability transition pore (MPTP). Normally, CypD is able to keep the MPTP closed; however, certain triggers like increased mitochondrial calcium concentrations or increased reactive oxygen species (ROS) cause CypD to open the MPTP, allowing mitochondria to become permeable to molecules smaller than 1.5 kD and often expediting cell death. When macrophages become experimentally infected with Mtb in tissue culture and zebrafish models, the mycobacterium activates CypD, which causes macrophage necrosis and enhanced bacterial spread. Accordingly, blocking or deleting CypD in these systems reduces mycobacterial growth and limits the spread of Mtb in the host.
Divangahi’s group infected CypD–deficient mice with Mtb. Surprisingly, there was no difference in bacterial burden compared to wild-type mice; however,the CypD-deficient mice had drastically increased morbidity and mortality, which was caused by severe inflammation and tissue destruction in the lungs. This unexpected finding lead the researchers to hypothesize that CypD may be playing an important role in controlling tolerance to Mtb infection and in its absence, mice succumb to immune-mediated tissue destruction.
To understand how CypD may be promoting tissue tolerance, the team looked a little closer at the immune players in the lungs. Interestingly, the exacerbated lung pathology wasn’t caused by a generally stronger immune response. There was just one primary culprit–the CD8+ “killer” T cell. CD8+ cells are critical for “search and destroy” missions when pathogens like Mtb hide inside cells. They can recognize cells that are infected with a specific pathogen and kill the cell, destroying the pathogen in the process. In this case, the CypD-deficient killer T cells didn’t seem to be killing more Mtb compared to wild-type cells, but they were hyperactive: dividing more quickly, and producing more pro-inflammatory cytokines like interferon-γ and TNF-α, allowing them to cause more collateral damage.
CypD-deficient CD8+ T cells alone caused increased mortality and pathology during Mtb infection when transferred into other mice, and depleting T cells in CypD-deficient mice was sufficient to restore survival to wild-type (CypD sufficient) levels, confirming that in the absence of CypD, these T cells became pathological.
The puzzle is this: CypD is a mitochondrial matrix protein–physically separated from the cell membrane-based signaling cascades and transcriptional control centers that dictate so much of T cell behavior. How is it so effectively regulating T cell activity? The answer is metabolism.
A T cell’s activity is intimately linked to how it metabolizes nutrients. You may remember that although glycolysis (the breaking down of glucose) is important for generating pyruvate, it’s not a very efficient process for generating ATP. Most mammalian cells rely on oxidative phosphorylation in the mitochondria to generate the amount of ATP necessary to sustain cellular activity. Glycolysis is typically only relied upon as an energy source in low oxygen conditions, such as during bursts of intense exercise. Rapidly proliferating cells like tumors and T cells are capable of undergoing a process known as “the Warburg effect.” During the Warburg effect, a cell will preferentially undergo glycolysis to generate ATP, even in the presence of sufficient oxygen. Although this may seem counterintuitive, the process of glycolysis generates the necessary biomass to rapidly proliferate and secrete effector proteins.
To examine T cell metabolism, Divangahi et al employed a technology called Seahorse. Seahorse can distinguish between different metabolic pathways, determining which are most active by measuring pH changes (glycolytic activity produces lactic acid) and oxygen consumption rate. Adding various mitochondrial inhibitors allowed them to measure the metabolic differences between normal and CypD-deficient T cells.
Sure enough, they found that the loss of CypD pushed the cells into a more glycolytic state, engaging the Warburg effect, thus sending them into T cell overdrive. The researchers could actually block excess cytokine production by blocking glycolysis, supporting the hypothesis that CypD controls T cell activation by keeping glycolytic activity in check.
Further experiments demonstrated that CypD-deficient mice had higher levels of reactive oxygen species (ROS), and that blocking ROS production restored T cell proliferation to wild-type levels, indicating that under wild-type conditions, CypD prevents ROS production.
All in all, they concluded that CypD metabolically restrains CD8+ T cell activity during Mtb infection sufficiently to allow the animal to tolerate the infection and minimize bystander damage.
Beyond broadening our basic understanding of the importance of tolerance to preserving health and homeostasis, the implications of this work are important for clinical reasons as well. The current tuberculosis vaccine isn’t great at preventing disease in adolescents and adults, and researchers are struggling to develop a more effective vaccine. Typically, a great vaccine is one that elicits a strong and decisive immune response against an infectious agent. Divangahi’s work warns that empowering the immune system (particularly the T cells) too much could override the body’s tolerance mechanisms, resulting in greater manifestation of disease. An ideal vaccine strategy would thus be able to strike the balance between controlling Mtb spread and mitigating host-driven tissue destruction.
Tzelepis, et al. Mitochondrial cyclophilin D regulates T cell metabolic responses and disease tolerance to tuberculosis. Science Immunology. 2018 May 11; 3, eaar4135 (2018)
Stephanie Melchor is a fifth year graduate student in the Biomedical Sciences PhD program at the University of Virginia. She currently spends her days studying the innate immune response to Toxoplasma gondii infection, but dreams of becoming a science writer.