Autoimmune Components of Pathogenesis

 

The severe symptoms of Chagas’ disease come in large part from autoimmunity responses (see Andrade 1994, Brener 1994, Iosa 1994). Thus, most of the pathology associated with T. cruzi infection is referred to as immunopathology. Throughout the chronic phase, a tremendous inflammatory process accompanies cells invaded by T. cruzi amastigotes, especially the amastigotes that live for a long period of time. This inflammatory process most frequently focuses in the heart, causing chronic myocarditis, but it also occurs within the esophagus and colon. This inflammatory response is directed against amastigotes that, over a period of time, slowly release themselves from the cells that they have invaded. This begins an inflammatory process against the escaped amastigotes and destroyed cells left behind. Monocyte or mononuclear infiltrates focus within specific locations where the parasites emerge and eventually invade other tissues, including the heart and neuron tissue. These infiltrates also enter myocardial plexes and plexes that serve the digestive tract. Plexes are nerve nets that serve various organs. Neuronal degeneration can occur during the acute phase in those who are experiencing severe attacks, especially children under the age of five, but this degeneration most frequently occurs during the chronic phase of the infection.

During the acute phases, polyclonal activation incites T and B cells, which produce antibodies not directed against the parasite in ways that will protect the host but rather against epitopes the parasite possesses and may share with host cells. These CSAs or nonproductive antibodies damage heart and nerve cells, frequently leading to death.

During the chronic phase, T. cruzi protects itself by mimicking host antigens which are shared with heart and nerve cells. Molecular mimicry between T. cruzi and host nervous tissue is the most current and acceptable theory of how the immune system is responsible for tissue lesions on infected organs (see Avila 1994 and PAHO 1994 for a discussion of this theory). T. cruzi have on their surfaces various host plasma proteins and immunoglobulins (antibodies), as well as shared heart‑ and nerve‑cell antigens. During the chronic phase, damage of the cardiac system leads to conduction abnormalities, which can be detected through the measurement of cardiac function. Gastrointestinal damage begins with the reduction of muscle tone and ends with muscular atrophy of the smooth muscles of the gut, esophagus, and colon so that these organs dilate and cannot contract to pass food through, resulting in blockage. Patients suffer chronic dysphagia (difficulty in swallowing) and constipation, which can include an enormously enlarged colon (megacolon) and esophagus (megaesophagus).

These severe clinical manifestations, however, cannot adequately be explained by inflammatory responses of the immune system to a few parasites being attacked with a focal inflammation. During the chronic phase, T. cruzi reproduces in numbers, and the immune system attempts to keep them in check; but, at some point, the inflammation process rapidly spreads out from focal areas where parasites are emerging, to attack human cells. This diffusion heralds the critical point for chronically infected patients.

This is not just a matter of parasite antigens attaching to self cells and rendering them nonself cells so that human antibodies mistake human cells for parasite cells, but rather it is a situation where human antibodies attack self cells simply for what they possess. T. cruzi immunizes people to their own antigens: it causes the human immune system to create antibodies that target antigens belonging to cardiac and neuronal cells and which then lyse them with complement. The immune response attacks self cells in two ways: parasites frequently alter the normal structure of host cells or attach parasite antigens to these cells; or, using an even more effective method, these parasites can get the immune system to think that it is attacking the parasite while it is in fact attacking itself. T. cruzi can do this because it has surface molecules that mimic those on the surface of host cells.

This has been demonstrated in experiments with rabbits. When lymphocytes from chronic chagasic rabbits were injected into healthy rabbits, these lymphocytes bound to and destroyed heart cells. This provided evidence that a cell‑mediated autoimmune response is involved in the generation of cardiomyopathy and other pathogenic events that occur with the disease. If spleen cells from T. cruzi ‑infected rabbits are placed in vitro with normal rabbit heart cells, the heart cells are killed. Spleen cells from the infected rabbit contain both lymphocytes and monocytes sensitized to the parasite antigen.

Further proof is found in the fact that serum from chagasic patients invariably contains autoantibodies that attack heart and neuronal cells in vitro. These autoantibodies attach and bind to cardiac epithelium and interstitial tissue, blood vessels, and muscle tissue, and are technically referred to as EVI antibodies (endothelial, vessel, and interstitial). EVI antibodies are detected at high levels in the serum of chronically ill chagasic patients and at extremely high levels in the serum of those with cardiac abnormalities.

The question is, how does T cruzi make human antigens look like T. cruzi antigens? When the host immune system attacks T. cruzi, it recognizes many epitopes on the parasite’s surface that are nonself and belong to the parasite. The surface of T. cruzi has many molecules on its surface which have epitopes that allow macrophages to attach to the surface and phagocytize the parasite. Macrophages pick up epitopes unique to T. cruzi as well as others the parasite shares with the host; so, when T cells develop and receive this information, they confuse parasite and human epitopes. Human cells can also be rendered nonself by attaching to them something that is nonself, such as a parasite antigen. The human immune system then attacks its own cells. When the immune system begins to attack its own cells, it also processes information about this system of antigens and includes all those that are related to it macromolecularly. It is clueing‑in on some antigens that are not nonself and develops a response against its own cells. Therefore, when the immune system goes after T. cruzi, which has on its surface some antigens that are similar to those of the host, the immune system picks up epitopes of self cells that have been disarranged because they are in close association with these nonself antigens and produces antibodies against the human cells. The immune system response is not specific enough to determine which molecules belong to the parasite and which to itself.

The antigenic epitope involved in this autoimmune reaction against cardiac cells contains laminin (a basement membrane glycoprotein). Tested sera from infected humans and monkeys contained high levels of EVI antibodies that reacted with laminin. EVI antibodies are anti‑laminin antibodies. This was detected by pouring serum down a sepharose (gel) column with laminin bound to it so that only anti‑laminin antibodies would stick to it. This affinity chromatography test is called a laminin sepharose affinity column. Laminin is a very important glycoprotein on the surface of many cells, especially those of the heart. If rabbits are immunized with laminin, they produce antibodies that cross‑react with T. cruzi and EVI cells. Conclusions are that laminin may be the major antigenic component inducing EVI antibodies and that laminin is found on the surface of T. cruzi and cardiac cells.

However well laminin explains myocardiopathy, it is not found in large amounts on the surfaces of neuronal tissue, so there must be another cross‑reacting antibody to explain the degeneration of neuronal tissue and muscles of the gut.

In conclusion, T. cruzi is a complex organism that has elaborate mechanisms for survival in its host. Within its millions of years of existence, T. cruzi has evolved a number of strategies for evading and manipulating mechanisms of the human immune response. In an elaborate chess game of moves and countermoves, T. cruzi has literally checked the king, although the game is not over, as scientists rapidly are uncovering complexities of the relationship between T. cruzi and the vertebrate immune system. The second theme implicates the human immune system for sometimes doing more damage than good: the fact that a protozoan such as T. cruzi can turn this system around so that it attacks itself is a strategy any warrior could learn from, but the most important lesson from T. cruzi is that it provides us with information and the stimulus to do more research on this process. Parasitology and immunology are extremely complex disciplines needing more study, and Chagas’ disease has proven to be an important and interesting model in both areas of study.

 

 








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