By the time of his 1891 promotion, Ehrlich had already begun a series of studies in mice with poisons such as ricin. As a brief aside, ricin is a powerful toxin found in the seeds of the castor oil plant. While it is by no means the most powerful natural toxin (far eclipsed by the lethality of botulinum or tetanus toxins), ricin has garnered considerable notoriety in the real and fictitious worlds of spy craft. The deserved fame is derived from KGB-sponsored assassination attempts using ricin against Bulgarian dissidents in 1978 London.57 In the midst of the Cold War, Georgi Markov was a Bulgarian novelist who had defected to the West and frequently broadcast critical reports on the Bulgarian government on the BBC World Service and Radio Free Europe. Determine to rid themselves of the problem, the Bulgarian secret service enlisted a technology from the KGB and reportedly recruited an Italian-born smuggler by the name of Francesco Gullino (code name Picadilly) to successfully assassinate Markov. Gullino’s weapon of choice was an umbrella with a hidden pneumatic device brilliantly designed to inject ricin into an unwitting victim. On September 7, the birthday of the Bulgarian leader and frequent target of Markov’s vitriol, Todor Zhikov, Georgi was strolling across the Waterloo Bridge in London when Gullino briefly bumped him in the leg with a lethal dose of ricin. Within three days, Markov was dead. An investigation of his suspicious death began in earnest and revealed that just over a week prior, another Bulgarian dissident by the name of Vladimir Kostov had experienced a similar encounter but survived. In his memoir, Kostov writes,
There were crowds of people in the Metro corridors. A few seconds before stepping off the escalator, I felt a sharp pain in the small of my back, just above my waist. At the same moment, I heard a sound like the rattle of a stone hitting the ground. Natalya heard it, too, without suspecting that anything had happened to me. My first thought was that I had been struck by a stone slung with great force, as though from a catapult.58
An autopsy on Markov revealed a small, ricin-coated pellet lodged in his thigh; a similar pellet was retrieved from Kostov’s back. Moreover, the surviving Kostov was later found to have developed antibodies specific for anthrax, a coincidence that will return us to the story of Paul Ehrlich.
Just before Ehrlich’s 1891 move to Berlin, he had begun a series of experiments to investigate if he could make mice resistant to poisons such as ricin. Starting at a low level of exposure, Ehrlich slowly increased the amount of toxin and found that within a month or two, the mice had become entirely tolerant to ricin. Ehrlich realized that this outcome represented an acquired immunity that resembled the situation in which vaccinated individuals could resist subsequent exposure to smallpox. He reasoned the body must respond to toxins (or microbial pathogens) by recognizing portions of the offending substance. He described these residues as “antigens” and the protective substances generated in the animals as “antibodies.” In doing so, Ehrlich single-handedly initiated a science, today known as immunology, that objectively debunked superstitious and further mention of humoral factors—though not entirely.
Sera & Antisera
We will return to Ehrlich when we delve into the history of antibodies later in our story. For now, we will introduce some basic concepts that provide an overview of the system responsible for the acquired immunity as described by Ehrlich. Lymphocytes are a specialized subset of the larger group of leukocytes. Overall, lymphocytes come in two major flavors (and a few minor ones that are not essential to our story), popularly known as T cells and B cells.
Anatomists investigating a mysterious organ that rests atop the heart discovered it to be full of a type of cell rarely seen elsewhere in the body. Given their location in this organ, known as the thymus, the cells were thereafter known as T-cells. Throughout most of history, the butterfly-shaped thymus had developed a rather bad reputation. Returning to the venerable OED, the etymology of the name itself connotes a negative character. The appellation derives from the Greek word thumos, which translates as ‘anger.’ This may reflect the idea that palpitations of the heart arise from strong emotions originating from the organ overlaying the cardiac muscle. To some degree, the negative connotations associated with the thymus were somewhat prescient, as the thymus is the site of massive death, though the overall outcome of this carnage is unquestionably necessary for a healthy life.
A rather odd fact had perplexed physicians and scientists since the first identification of the thymus. Strangely for a human organ, the thymus is largest in babies and atrophies down to virtually nothing by early adulthood. This behavior was first noted by Galen, who, you may recall, was among the first to record the Antonine (smallpox) Plague in the 2nd century, and arguably was the most accomplished physician-scientist in the ancient Greco-Roman world. In a newborn, the thymus can be larger than the heart, enveloping the vascular organ such that when an operation is needed to assist cardiac function in infants, most of the thymus must be cut away to allow the surgeons to access the heart. Whereas the heart continues to grow as the child matures, the thymus shrinks, relegated to the state of a “vestigial organ” (a seemingly nonfunctional structure such as the appendix) by the time a child reaches puberty.
The reason for this progressive loss of size is that the thymus is essentially a schoolhouse for the immune system. By the time a child reaches puberty, most, if not all, of the instruction has been completed. Within this strange and complex organ, immature T cells undergo a series of on-the-job learning activities designed to create an immune system that can distinguish self from others (described by immunologists as “non-self”).
This instruction is essential because the human immune system is intensively powerful and must be tightly controlled. Thus, there are many safeguards to minimize the potential for unleashing the vast firepower of an uncontrolled immune system, as misdirected mobilization of the immune system can be, and often is, fatal. Examples of the consequences of not properly limiting the immune response range from the acute effects of something known as a cytokine storm (which can cause death in circumstances such as an allergic reaction to a bee sting or infection with the Ebola virus) to more chronic but no less deadly autoimmune diseases such as lupus or multiple sclerosis.59 In each case, the pathology arises because the immune system is inappropriate or over-vigorous in attacking what is perceived as a hostile foreign invasion. To minimize such diseases, the thymus oversees a massive culling of all potential T cells, known as “central tolerance,” that might otherwise turn against the body.
When these remarkable T cell receptors “sense” an invader, they can send a signal to the cell interior that causes a general mobilization of the body’s host defense. The so-called activated T cells produce powerful cytokines that cause the cell (and others around it) to proliferate and go into “attack mode.” Arguably, the most well-known of these T cells are those that display on their surface a molecule known as CD4. These CD4 cells, also known as “helper T cells” can be considered the “generals” of the immune system, which alert the body that it is under attack and then direct the counterattack until the pathogen has been eliminated. Once activated, these helper cells dump enormous amounts of powerful cytokines, known as interleukins, that can stimulate or synergize with other signals to amplify the power of an immune response. Without this help from CD4 T cells, the immune system is largely defanged.
The essential function of CD4 helper cells is now widely understood, since CD4 happens to be the target for the human immunodeficiency virus (HIV), the cause of acquired immune deficiency syndrome (AIDS). Indeed, the lethality of HIV/AIDS is based upon the fact that the virus seeks out and destroys CD4 cells. The brilliance of this strategy, at least from the perspective of the virus, is that HIV infection cripples the key regulator of the immune response, which might otherwise serve to detect and fight the invader. HIV is particularly crafty because it does not shut down the immune system all at once, as this could jeopardize its ability to jump to another unwitting victim before it uses up its host. Instead, HIV infection triggers a slow deterioration of CD4 T cell
s, and eventually the system collapses. However, the virus itself is usually not the direct cause of death but rather weakens the immune system such that other pathogens (often exotic fungal or bacterial diseases that are easily addressed by the immune system) convey the coup de grace.
Two other sets of T cell functions are also worth noting. First, some T cells can directly execute foreign invaders. These “killer T cells” are particularly useful for hunting down and eliminating tumors and virus-infected cells (as we will see in the next chapter), both of which had been normal but because of a diseased condition or infection are consequently perceived as “non-self.” Such T-cells are referred to as cytotoxic, a modification of a Greek term meaning “cell-killing” and these cells evolved novel means to puncture the membrane of the disease-bearing cells (the biological equivalent of popping a balloon or puncturing a tire). Specifically, these cytotoxic T-cells deliver a complex and extremely lethal hit that simultaneously attacks and destroys both the DNA and proteins inside the targeted cells. Few cells, human or otherwise, can survive such devastation.
A second set of specialized T cells are known as memory cells. As the name implies, these cells arise during the resolution phase after a foreign threat has been eliminated. Carrying forward billions or trillions of cells that target one pathogen would be wildly inefficient, particularly as a countless number and breadth of invaders are encountered over a typical lifetime. Instead of expending the energy to keep such a system surging at full speed, a subset of the most efficient and effective cells are placed in a sort of long-term storage. The “memory” cells are ready to pounce but kept in check until the antigen they recognized is encountered again. As an example of the efficiency of memory, the first time that a pathogen is encountered, the immune system may require days or weeks to recognize, mobilize, and eliminate the complex of “foreign” antigens that distinguish the potentially-harmful pathogen from the “self” molecules of the host. The presence of memory T cells that target the foreign antigens can allow the same disease challenge to be eliminated in days, if not hours. Consequently, memory cells convey the most important aspect of the human immune system: the ability to recognize and rapidly respond to repeat challenge by an antigen. This is a very constructive achievement if you risk becoming infected by a pathogenic microorganism but less appreciated by sufferers forced to endure annual allergic responses to tree pollen or molds.
A key to understanding the spectacular specificity of T cells and all immune function resides in one of the most bizarre and fascinating aspects of the human body: you were born with billions of T cells, and each one was subtly different from its siblings. The variations reside in a molecule known as the T cell receptor (TCR) complex. T cells recognize foreign invaders through a complex of molecules that comprise a TCR, which spans the cell membrane so that a sensing portion of the complex is localized on the outside of the cell. When stimulated by a foreign antigen, the TCR triggers a powerful signal and thereby prepares the T cell for combat. Importantly, this system is tightly regulated so that T cells often cannot be activated unintentionally. Instead, a series of specialized defense cells known as antigen presenting cells (which includes the neutrophils, eosinophils, and basophils described above, as well as an aggressive patrolling phagocyte cell known as a macrophage; a Greek term meaning ‘large eater’) possess specialized abilities to acutely stimulate T cells following an encounter with a foreign invader. As the name suggests, an antigen presenting cell is generally a cell that obsessively gropes its way around surrounding tissues, blood, lymph, and nodes of the body, always sampling its environment and gobbling up any debris it encounters. Consequently, these cells are known as phagocytes, a modification of a Greek term meaning “cells that eat.” Any proteins ingested during this surveillance are broken into small bite-sized chunks (known as peptides) and nestled within a specialized protein complex on the cell surface. This complex is known as a major histocompatibility complex (MHC) molecule. High-resolution analyses of MHC molecules reveal their structure to look remarkably like a catcher’s mitt, with the foreign peptide cushioned in the pocket.60 This antigen-loaded catcher’s mitt is the structure that interacts with the T cell receptors on T cells.
While the antigen presenting cell is sampling its environment, it is also performing an intricate ballet with T cells in its proximity. Usually the dance is quite quick and uneventful, only a unique combination of T cell, MHC, and “foreign” antigen will trigger a response. If the TCR and foreign peptide engage like two complementary pieces of a jigsaw puzzle, the T cell becomes activated. Otherwise, the T cell releases the antigen presenting cells and the sequence begins anew with a different pair of cells. On those rare occasions when the proteins “displayed” on MHC include bacteria or bits of material believed to be foreign and matching its specific tastes, the activated T cell enters a veritable frenzy and gains a license to kill and kill again.
The activation of a T helper cell initiates a cascade of events to rapidly and vigorously destroy perceived enemies. The activated T cell will immediately begin a process of rapid proliferation, increasing its numbers exponentially. In parallel, these activated T cells begin pumping out vast amounts of highly active interleukins meant to increase the growth and power of the T cell response as well as to elicit assistance from other leukocytes all throughout the body. A familiar sign that this is happening is the lethargy and fever often associated with infections such as influenza. These frenzied activities require an enormous investment of energy. For example, when the infection is localized (such as a wound), the site of infection can be particularly warm to the touch. Once the infection has been stemmed, it is essential for the body to halt the considerable energy investment, and the process is slowed. Most of the cells that participated in the fight are decommissioned. Many are killed, with their component proteins, sugars, and fats recycled for use in manufacturing future cells.
In a remarkable mechanism that balances such unsustainable energy usage with the need to avoid future dangers from encounters with the same pathogen, a subset of the T cells that successfully responded to the infection are put into a storage hall of fame. These memory T cells, which we met above, are equipped with the ability to respond even more vigorously and efficiently to the danger should it ever be encountered again. These memory cells provide a means to deploy learning of the past to ensure future recall. However, over the years, encounters with more and different types of antigens can lessen the readiness of some of these memory cells.
All of these interactions are based on an exquisite recognition of foreign antigens expressed within the groove of an MHC molecule to interact with and trigger a T cell that displays a particular T cell receptor targeting that antigen. However, the diversity of molecules that exist in nature is staggering and this presents a challenge to the T cell. The question boils down to this: How can T cells anticipate “foreign” antigens that the body has never encountered and thereby provide protection for new threats that may arise or evolve in the future?
Through a fascinating process that is unique to T cells during their time in the thymus (and B cells as they mature in the bone marrow through an analogous process), the genes for T cells undergo a spontaneous transformation that cuts out, rearranges and mutates multiple small pieces of DNA. These pieces eventually come back together in different combinations to create the unique region on the T cell receptor (TCR) that binds antigens. This complex process can be thought of as being analogous to having dozens of colors of Lego blocks in different piles. One or more block from each pile can be brought together to generate a final structure (e.g., a Lego building) with the colors arranged in an almost infinite number of combinations. Even more diversity derives from the fact these genes are rendered hyper-susceptible to mutation as this process of building takes place. Extending the Lego analogy, a red block might mutate to become pink or orange. The result of this bizarre remodeling of T cell receptor DNA is that the resulting T cell receptor protein found on each T cell in the thymus d
iffers slightly from the TCR on neighboring cells.
These subtle differences can allow a full complement of T cells in our body to detect, in theory, any molecule that might ever be encountered in a lifetime. Although such diversity is crucial for protecting our bodies against the wide variety of threats posed by an environment replete with bacteria, viruses and parasites, one can imagine that such extreme variability might also create opportunities to trigger “friendly fire” casualties were a T cell to recognize and become stimulated by “self” molecules or protein conformations. This potential hazard in turn explains the need for the massive culling associated with “central tolerance,” a mechanism by which certain cells of the thymus can identify and eliminate any T cells with the potential to target “self” tissues before they can cause harm.
Between Hope and Fear Page 12