Autoimmune Polyglandular Syndromes

Introduction

The body’s first line of immune defense, after physiological barriers against invading foreign pathogens, is the innate arm of the immune system. Innate immunity is a nonspecific response mediated by expression of germ line genes, which does not require prior exposure to an antigen and provides immediate defense. However, the innate immune system does not exhibit immunologic memory and does not provide long-lasting protection. The next line of defense against foreign antigens is the adaptive immune system. Although the adaptive immune system assumes all exogenous antigens are potentially harmful, it produces antigen-specific responses. In a normal adaptive immune response; the host organism must differentiate self- from nonself-antigens; mount an immune response; eliminate or remove the inciting antigen; and protect the host from injury, organ dysfunction, and even death. Discrimination of self/nonself is carried out by the adaptive (specific) immune system by a mechanism that uses specific T-and B-cell surface receptors. T-cell and B-cell receptors recognize distinctive antigen peptides and epitopes, respectively, and are the keys to the specificity of the adaptive immune response. Whereas B cells and their receptors recognize soluble antigens on cell surfaces of pathogens, T cells and their receptors (TCRs) perceive short polypeptides only when presented by antigen presenting cells (APCs), using specialized cell-surface molecules encoded by the major histocompatibility complex (MHC). The human MHC is termed the human leukocyte antigen (HLA) complex. Class I MHC (e.g., HLA-A, HLA-B, and HLA-C molecules) is found on all nucleated cells, and presents endogenous peptides derived from the cytoplasm (such as from a pathogen infected cell) to CD8 T cells. Class II MHC (e.g., HLA-DP, HLA-DQ, and HLA-DR) molecules, on the other hand, present foreign antigens (peptides) that have been endocytosed and processed by APCs (such as B cells, dendritic cells, and macrophages) to CD4 T cells.

T cells are initially classified based on their cluster of differentiation (CD) surface proteins that bind differentially to antigens presented on class I (binds to CD8) and class II (binds to CD4) MHC. For example, CD8 T cells, also called cytotoxic T cells when activated, typically recognize antigens presented via class I MHC, and mediate an immune response specific to that antigen. Conversely, CD4 T cells, serving as helper or regulatory T cells, are activated when presented with peptides via class II MHC. Regulation of T-cell self-tolerance occurs at two distinct but interdependent levels: central tolerance and peripheral tolerance (described later) ( Fig. 22.1 ). Central tolerance occurs in the thymus, whereas peripheral tolerance occurs in both lymphoid and nonlymphoid tissues. Although many of the mechanisms involved in self-tolerance remain poorly understood, almost 2 decades of characterizing the autoimmune regulatory gene (AIRE) has improved our understanding of the pathways in the establishment and maintenance of self-tolerance. The AIRE gene encodes a transcription factor in the thymic medulla, which plays a critical role in establishing central tolerance (described in detail in the later part of this chapter). Deletions in the mouse AIRE homologue result in multiorgan autoimmunity, whereas mutations in the human AIRE gene result in APS I.

Tolerance is initially developed in utero with T and B cells playing important roles. T and B cells are produced continuously throughout life by hematopoietic stem cells of the bone marrow, with the T-cell precursors migrating to the thymus for further maturation. Although the thymus atrophies after puberty, residual thymic tissue may provide for T cell development throughout life.

Although T cells require exposure to small doses of antigen to achieve tolerance during their thymic development, larger doses of antigen are required to induce B-cell tolerance, and B-cell tolerance is often short lived. Tolerance is immunologically specific and induced in developing lymphocytes early in life; however, it can also be induced in mature lymphocytes when costimulatory signals are absent at the time of peptide interaction with TCR.

Central T-Cell Tolerance and AIRE

T cells are primarily educated to distinguish self and nonself when they develop in the thymus. In the thymic cortex, CD4 + CD8 + (double-positive) T cells bearing α/β TCRs that are able to bind to self-peptide/MHC complexes are selected to survive, whereas T cells whose TCRs fail to bind undergo apoptosis. As many as 99% of developing thymocytes undergo apoptosis and never reach the periphery (see Fig. 22.1 ). This is referred to as positive selection and is carried out by antigen-presenting cortical thymic nurse epithelial cells (cTECs), bearing MHC I and MHC II. T cells that bind to MHC I commit the developing T cell toward a CD8 T-cell pathway, whereas those that bind to MHC II develop into CD4 T cells. Positively selected cells migrate through the corticomedullary junction into the medulla where a secondary checkpoint occurs. Although T cells must be able to bind to MHC/self-peptide to establish tolerance, cells that bind too tightly to self-antigens are capable of inducing autoreactivity and therefore undergo negative selection and apoptosis. This process of positive selection of T cells for positive selection of MHC binding T cells (in the thymic cortex) and negative selection of T cells tightly bound to self-antigens (in the medulla), accounts for central (thymic) immunological tolerance. The single positive T cells expressing either CD4 or CD8 then migrate to the periphery and secondary lymphoid organs.

Medullary thymic epithelial cells (mTECs) are specifically involved in the process of negative selection of T cells and express an enormously diverse range of peripheral tissue–specific antigens (TSAs) for presentation to the developing T cells. This has been referred to as promiscuous gene expression (PGE) and is mediated by AIRE, a transcriptional regulator. Notably, AIRE is expressed in a small population of mature mTECs, with high levels of CD80 and MHC II. Unlike traditional transcriptional regulators, it does not bind to deoxyribonucleic acid (DNA) segments but activates ribonucleic acid (RNA) polymerases, and elongates TSA RNA transcripts. This process is also mediated by interaction of AIRE with other transcriptional regulators. Another transcriptional regulator that is involved in negative T-cell selection in the thymus is the FEZ family zinc finger protein 2 (FEZF2), which is involved in AIRE-independent TSA expression in mTECs. Studies have shown that AIRE and FEZF2 share complementary and parallel function in establishing and maintaining central tolerance. In addition to regulation of TSA expression in mTECs, AIRE is involved in thymic selection and differentiation of autoreactive CD4 T cells into regulatory T cells (Tregs), and upregulation of chemokines that aid in thymocyte migration. Extrathymic expression of AIRE in tissues, such as the bone marrow, has also been shown to aid in peripheral tolerance by inducing anergy of CD4 and CD8 T cells.

Peripheral T-Cell Tolerance

Once naïve T cells enter the circulation or secondary lymphoid organs (e.g., lymph nodes and spleen) and recognize specific antigens, they require additional cosignals to become activated. The first signal involves the interaction of antigen-peptides bound to the MHC molecules on the surface of APCs with TCRs on the surface of CD4 and CD8 T-cells. CD4 and CD8 molecules on these T-cell subsets serve as antigen-nonspecific coreceptors binding to nonpolymorphic portions of the class II and class I MHC molecules, respectively. The second signal is antigen nonspecific and is provided by the B7.1 (CD80) and B7.2 (CD86) molecules of the APC interacting with the CD28 molecule on the T-cell surface ( Fig. 22.3 ). In addition to CD80/CD86/CD28 interactions, there are other costimulatory signals that have important roles in T-cell development.

When T cells perceive both signals (MHC-antigen with TCR and B7-CD28), a cascade of intracellular signaling events occur, leading to T-cell activation. Activated cytotoxic CD8 T cells mediate direct lysis of target cells, whereas activated CD4 T cells lead to expression of numerous cytokines, cytokine receptors, and cytotoxic T-lymphocyte antigen 4 (CTLA-4). CTLA-4 is homologous to CD28 and competes with CD28 for binding to B7.1/B7.2. CTLA-4 expression by the activated T cell and its interaction with B7.1/B7.2 provide an immunosuppressive/immunoregulatory signal to the T-cell, thereby downregulating the T-cell responses. Thus CTLA-4 and CD28 though homologous, act antithetically: B7.1/B7.2-CD28 turns on T cells, whereas B7.1/B7.2-CTLA-4 downregulates the T cell (see Fig. 22.2 ).

The requirement for two signals to activate naïve T cells mainly accounts for peripheral T-cell tolerance. When the naïve T cell perceives antigen peptide presented by MHC molecules without the necessary costimulatory signal (e.g., B7.1/B7.2-CD28), the T cell becomes unresponsive. This state of unresponsiveness is termed anergy ; anergic T cells are generally not restimulated with antigen peptide displayed by the APCs. T cells may also undergo apoptosis (programmed cell death) to be removed completely from the T-cell repertoire. Tolerance may also exist because the TCR does not encounter the relevant peptide, and this has been termed T-cell ignorance .

Another mechanism whereby T-cell tolerance is mediated is the interaction of programmed death 1 (PD-1) receptor on T cells and its ligands PD-L1 and PD-L2. These interactions bring about inhibition of T-cell effector functions in an antigen-specific manner. PD-1 signaling can also mediate the conversion of naïve T cells to Tregs.

Helper CD4 T cells are classically divided into two distinct lineages: (1) Th1 cells, which activate cell-mediated and some antibody responses, and (2) Th2 cells, which predominantly activate antibody-mediated responses. However, additional T-cell lineages exist (i.e., Th17 cells, T-follicular helper cells, and regulatory T cells) and recent data have described remarkable plasticity in their cytokine expression, suggesting shifting T-cell functionalities, depending on environmental cues. Although overtly simplistic, Th1 subsets secrete predominantly proinflammatory cytokines, such as interleukin (IL)-2, interferon-gamma (IFN-γ), tumor necrosis factor-beta (TNF-β), induce IL-12 secretion from dendritic cells, and activate macrophages and CD8 T cells to eliminate intracellular pathogens. Upon activation, CD8 T cells, often with the help of Th1 cells supplying IFN-γ to upregulate B7 expression on APCs, become functional cytotoxic T killer cells. Conversely, Th2 cells elaborate IL-4, IL-5, IL-6, IL-10, and IL-13, which aids antibody and eosinophil production. There is also crosstalk between Th1 and Th2 cells; for example, IFN-γ from Th1 cells suppresses Th2 cells, and IL-10 from Th2 cells inhibits Th1 cells.

Tregs are a subset of T cells, which play a critical role in suppressing the activity of effector T cells that escape negative selection to self-antigens in the thymus. Functional Tregs are able to anergize previously self-reactive T-effector cells, resulting in improved tolerance to self. Tregs originating from the thymus are termed as central or natural Tregs and carry surface CD4 ⁺ CD25 ⁺ and intracellular forkhead transcription factor (FOXP3 ⁺ ) markers, which are specific to CD4 ⁺ CD25 ⁺ Treg cell population. FOXP3 is a transcription factor that is required for developing α/β TCR-positive T cells to differentiate into Tregs in the thymus. First identified in the Scurfy mouse, a mouse model of immune dysfunction and polyendocrinopathy, abnormal FOXP3 expression is now known to be responsible for failure in immune tolerance in humans affected with a similar polyendocrinopathy, as further discussed later. Abnormal FOXP3 expression in humans leads to an extremely rare, X-linked inherited, and typically fatal autoimmune lymphoproliferative disease known as IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, and X-linked inheritance). Defects in FOXP3 responsible for IPEX map to Xp11.23-Xq13.3.

B-Cell Tolerance

Naïve B cells, during the early stage of development in the bone marrow, express surface immunoglobulin (Ig)M, which serves as B-cell receptors (BCRs). Upon interacting with self-antigens, naïve immature B cells undergo negative selection, either through clonal deletion or anergy, whereby B cells enter a state of unresponsiveness and have a reduced life span. Another process mediating B-cell central tolerance is receptor editing, whereby genetic rearrangement of the Ig chain leads to generation of BCRs with new antigen specificities. B cells with nonautoreactive BCRs are positively selected and continue to the periphery. If self-reactive B cells escape into the periphery, they undergo anergy. Anergized B cells do not die immediately but have a shorter half-life. Naïve mature B cells require T-cell help for realization of their full potential through affinity maturation and class switching. The absence of T-cell help also leads to B-cell tolerance.

Autoimmune Diseases

The organ-specific nature of many autoimmune diseases results from abnormal immune system recognition of tissue-specific self-antigens. In many autoimmune endocrinopathies, the target molecule is either a tissue-specific or tissue-limited (i.e., the protein is not unique to one tissue but is clearly restricted in its distribution) enzyme or cell-surface receptor ( Table 22.2 ).

The criteria for classification of a disease as autoimmune are not universally agreed upon. However, major criteria that are generally accepted as strong evidence of autoimmune disease include: (1) detection of autoantibodies or autoreactive T cells, including lymphocytic infiltration of the targeted tissue or organ; (2) disease transfer with antibodies or T cells; (3) disease recurrence in transplanted tissue; and (4) ability to abrogate the disease process with immunosuppression or immunomodulation. Few, if any, human autoimmune diseases meet all these criteria. Further information that is supportive of, but not diagnostic for an autoimmune disease, include: (1) increased disease frequency in women compared with men, (2) the presence of other organ-specific autoimmune diseases in affected individuals, and (3) increased frequencies of particular HLA alleles in affected individuals.

Defects in Tolerance That Cause Autoimmune Disease

Several different hypotheses explaining defects in tolerance have been proposed. Autoimmunity may develop because (1) tolerance never developed to specific self-antigens or (2) established tolerance was lost ( Fig. 22.3 ). If self-antigen is not efficiently presented in the thymus, tolerance may not be established during T-cell education within the thymic cortex. For example, AIRE mutations lead to lack of expression of ectopic self-antigens by mTECs and their presentation to developing T cells. This leads to escape of autoreactive T cells into the periphery and ultimately multiorgan autoimmunity. Another example is the insulin gene ( INS ) VNTR (variable number of tandem repeats), which lies around 500 basepairs upstream of the INS gene promoter. The INS VNTR influences thymic insulin T-cell expression and education based on its length. Specifically, longer VNTRs are associated with increased thymic expression of insulin, and thus a decreased risk of developing type 1 diabetes, whereas shorter VNTR are associated with decreased thymic expression of insulin, failure to delete specific autoreactive T-cell clones, and an increased risk of developing diabetes.

If tolerance has not developed because of intracellular sequestration of an antigen, and thus not expressed in the thymus during T-cell ontogeny, T-cell reactivity in the periphery will not be abolished. However, several antigens initially thought to be sequestered intracellularly have now been shown to circulate in low concentrations in normal individuals. Thyroglobulin, a self-antigen in autoimmune thyroid disease, is known to circulate in low but appreciable quantities in individuals with no serologic evidence of thyroid autoimmunity. Thyroid follicular cell destruction in Hashimoto thyroiditis is mostly cell mediated and not mediated by humoral factors.

If sequestered antigens do play a role in autoimmune disease, viral infections, trauma, ischemia, or irradiation are all possible mechanisms that could disturb cellular integrity and lead to release of intracellular antigens. Some sequestered self-antigens may never encounter the immune system, unless there is a breakdown of anatomic barriers within the body. An example is the occurrence of autoimmunity to intraocular proteins, following orbital trauma. Although a rare consequence of orbital damage, initiation of an autoimmune response to released sequestered intraorbital proteins in adjacent lymph nodes can generate autoreactive T cells that can invade and damage the contralateral eye (sympathetic ophthalmia). Removal of the inciting damaged tissues and immunosuppression may be required to sustain vision in the undamaged eye. Similarly, transient autoantibody reactivity to cardiac myosin following myocardial infarction has been described albeit with no pathological consequences.

Alteration of self-antigens because of infection or neoplasia is another possible theory explaining some types of autoimmunity. As environmental triggers, viral infections could lead to modification of self-proteins and neoantigen expression. Alternatively, a self-antigen may be partially degraded, leading to a “new” antigenic target for the adaptive immune system. This new antigen is recognized as foreign by the immune system, and the immune response to these new antigens results in autoimmunity. Some cells/tissues may suffer unintended autoimmune damage when substances bind to the cells and elicit an initial immune response. For example, certain drugs bind to red blood cells and result in an immune hemolytic anemia. If an antibody response to the red-cell–bound drug is elicited, the antigen-antibody complex present on the red blood cell can lead to red blood cell destruction. This can occur either through red blood cell phagocytosis by the monocyte-macrophage system or via complement-mediated lysis of the red blood cell. Thus the red blood cell becomes an innocent bystander to the antidrug humoral immune response. Theoretically, this could also occur with viruses that serendipitously attach to tissues.

Molecular mimicry is another mechanism to explain development of autoimmunity. Following exposure to a dietary, viral, or bacterial antigen (e.g., infection) and similarity (molecular mimicry) between the self-antigen and the foreign antigen, the immune response to the foreign antigen leads to cross-reactivity with self-antigen, autoimmunity, and disease. For this theory to work, tolerance must not previously exist to the self-antigen. This might be true if the self-antigen is truly sequestered and the immune system has never developed tolerance to the self-antigen. Alternatively, the self-antigen peptides may be present in very low concentration to elicit an immune response and initial tolerogenicity has not occurred. Only after infection or novel dietary exposure would there be a sufficient degree of immunization to the exogenous antigen (which is similar to a self-antigen) and subsequent immune autoreactivity. If the self-antigen is a cell-surface antigen, the “pathogen-induced” autoantibodies could fix to self and produce disease via complement fixation, or the antibodies could act as opsonins for fixed or circulating phagocytes (antibody-dependent cell cytotoxicity). In rheumatic fever, associated comorbidities such as carditis and Sydenham chorea are thought to be autoimmune manifestations secondary to similarities in the structural components of group A Streptococcus with collagen (I and IV) and fibronectin in human cardiac connective tissues and tubulin in human brain cells, respectively.

Some cases of autoimmunity may result from superantigens, which can be secreted by certain pathogenic bacteria and viruses. Superantigens are polyclonal T-cell stimulators that can cross-link TCR β chains and MHC molecules, and activate as many as a third of T cells in the body. This can initiate a nonspecific T-cell immune response, including against self-antigens. In such cases, systemic disease can develop from massive cytokine release (e.g., systemic inflammatory response syndrome [SIRS]). This is the case in toxic shock syndrome, wherein a staphylococcal exotoxin acts as a superantigen. Mycobacterial antigens have also been proposed as possible superantigens in Crohn disease. This theory requires that T cells bearing antiself TCRs have not been deleted or become permanently anergic. These T cells may be stimulated and proliferate to develop an autoimmune response if they encounter the specific self-antigen.

Similar to polyclonal T-cell activation, polyclonal B-cell stimulation has also been implicated in humoral autoimmunity. Autoreactive B cells arise routinely as part of the naïve B-cell repertoire, and can be found in healthy individuals. If an autoreactive clone of B cells encounters a self-antigen and a costimulator (which might be nonspecific, e.g., a virus, such as Epstein-Barr virus, or a bacterial product, such as lipopolysaccharide), autoantibodies could be produced, bypassing the need for T-cell help.

Autoimmune human disease likely results from an interaction of environmental and genetic factors. Environmental factors implicated include: wheat gliadin ingestion and celiac disease, penicillamine exposure and myasthenia gravis, methimazole and autoimmune hypoglycemia from insulin autoantibodies (reported primarily in Japanese patients), and amiodarone and thyroiditis. Cancer has also been associated with the development of autoimmunity: thymoma and myasthenia gravis, ovarian teratoma and N -methyl- D -aspartate receptor–meditated encephalitis, and breast cancer and stiff-person syndrome. Despite remarkable improvements in our understanding of immunology, the mechanisms whereby the complex interaction of genes, environment, and immune system lead to autoimmunity remain to be fully elucidated.

Checkpoint Inhibitors and Autoimmunity

Tumor cells can manipulate the inherent immune tolerance mechanism to disrupt antitumor immunity. An efficient way of escaping antitumor activity is by increasing checkpoint pathways, which suppress T-cell responses. An example is the interaction of CTLA-4 (present on activated T-cells) with B7.1/B7.2 of APCs that downregulates T cell response. CTLA-4 can also remove B7 molecules from APCs, through a process called transendocytosis and prevent binding of CD28 costimulatory molecules, and thus bring about T-cell anergy. Another example is the interaction of PD-1 on T cells with its ligands PD-L1 and PD-L2. PD-1/PD-L1/PD-L2 interactions inhibit T-cell proliferation and production of proinflammatory cytokines (TNF-α, IFN-γ, and IL-2), allowing immune checkpoint pathways to promote a tolerogenic environment.

Checkpoint inhibitors, now increasingly used in anticancer therapy, include monoclonal antibodies targeting the CTLA-4 and PD-1 pathway (both the PD-1 receptor and PD-L1 ligand) and thus removing the restraint on antitumor activity ( Fig. 22.4 ). Blockade of CTLA-4 enhances costimulatory signals and leads to naïve T cells having increased effector T-cell responses to tumor cells, whereas PD-1 pathway blockade leads to increased T-cell proliferation and a proinflammatory milieu that aids in antitumor activity. In addition, there are other immune pathways impacted by blockade of these pathways. The discovery of immune checkpoint inhibitors was a breakthrough in anticancer therapy, resulting in the 2018 Nobel Prize for Medicine awarded to Drs. James Allison and Tasuku Honzo, two pioneers in this field.

Given that checkpoint inhibitors have the ability to eliminate a tolerogenic environment, it is not surprising that these immunomodulatory agents can bring about immune-related adverse effects (irAEs), and such irAEs have even been referred to as the Achilles heel of cancer immunotherapy . Hypophysitis, hepatitis, dermatitis, colitis, type 1 diabetes, thyroiditis, adrenalitis, and myocarditis have been reported following treatment with ipilimumab (CTLA-4 antibody) and/or PD-1 antibody (nivolumab) therapy. Other potential mechanisms leading to irAES after use of checkpoint inhibitors include: (1) cross-presentation, where tumor antigens released after antitumor activity are picked up by APCs, initiating secondary immune responses, and (2) epitope spreading (after release of tumor antigens), whereby there is continuous acquisition of neo antigens and recruitment of untargeted T cells. Despite the success of checkpoint inhibitors as anticancer drugs, these irAEs remain a concern. There are few studies in cancer patients with and without preexisting autoimmune conditions, and the long-term effects of checkpoint inhibitor use remain unknown. Newer modes of anticancer immunotherapy that have been investigated to lessen irAEs include increasing the efficacy and the use of vaccines against tumor neoantigens.

Having provided a review of basic immunological concepts related to central and peripheral tolerance, we will shift our focus for the remainder of the chapter toward the clinical and pathological aspects of APS.