A3 Autoimmunity

Topic

Autoimmunity arises when the immune system loses tolerance to self-components and begins attacking the body's tissues. This tolerance is established through two successive mechanisms: a central one, occurring during T-cell maturation in the thymus, and a peripheral one, responsible for controlling autoreactive cells that manage to escape that initial selection process.

In the thymus, developing T cells are exposed to self-components. The higher their affinity for these components, the greater the likelihood that they will be eliminated before entering circulation. Thus, there is a selection threshold above which strongly autoreactive lymphocytes are effectively eliminated. However, some cells with lower affinity may pass this selection process and leave the thymus while retaining some capacity to recognize self-tissues.

Peripheral tolerance acts as a second barrier against these cells. Regulatory T cells play a key role in this process, inhibiting the activity of autoreactive lymphocytes through inhibitory signals and by competing for signals required for their activation. The effectiveness of this regulation depends on the balance between regulatory T cells and autoreactive cells within each tissue. If the proportion of regulatory cells drops too low, autoreactive lymphocytes can proliferate and attack the tissue. Disruptions to this balance are observed in diseases such as rheumatoid arthritis, lupus, and type 1 diabetes.

Autoreactive lymphocytes can also promote the production of autoantibodies, which are capable of causing damage through three mechanisms. First, they can form complexes with their targets and deposit in tissues, where they activate the complement system and trigger inflammation—as seen in certain renal lesions associated with lupus and in vasculitis. Second, they can bind directly to cells and activate the complement system on their surface, leading to cell destruction; this can occur with red blood cells or platelets. Third, they can alter receptor function without destroying the cell by either blocking the receptors or activating them abnormally. Complement amplifies damage through a cascade: initial activation generates products that recruit and activate additional proteins, significantly intensifying the inflammatory response beyond that initially triggered by the autoantibody.

In rheumatoid arthritis, an inflammatory circuit also exists within the joint. A signal promotes the expansion of T lymphocytes that produce another inflammatory signal; this signal acts on joint membrane cells and on cells responsible for bone resorption, thereby contributing to the destruction of cartilage and bone tissue. Certain biological treatments selectively interrupt these signals. When the drug concentration far exceeds the level required to block its target, the inflammatory signal is almost completely inhibited, and further increasing the dose yields little additional effect.

Conventional immunosuppressants act more broadly, reducing T-lymphocyte activation and differentiation without fully distinguishing between those that drive autoimmunity and those needed to combat infections or tumor cells. Biological treatments, in contrast, block more specific components of the immune response, aiming to control the disease while having less impact on the body's remaining defenses.

However, any form of immunosuppression can tip the balance too far toward inhibition. Excessive regulation may end up suppressing the very lymphocytes needed to control opportunistic infections and detect nascent tumor cells. Treatment must therefore strike a balance between controlling autoimmunity and maintaining adequate immune surveillance.

Overall, autoimmunity reflects a disruption of the balance between recognition and tolerance. Thymic selection eliminates the most autoreactive T lymphocytes; peripheral tolerance keeps those that escape in check; autoantibodies and complement can amplify damage; and specific inflammatory circuits sustain certain diseases. Therapy seeks to restore a middle ground: sufficient regulation to prevent attacks on the body's own tissues, yet not so much as to compromise defenses against infections and tumor cells.

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