Residency · Residency · Allergy Immunology

Immune Dysregulation Syndromes

Overview

Immune dysregulation syndromes represent a group of inborn errors of immunity (IEI) that present primarily with autoimmunity, lymphoproliferation, and hyperinflammation rather than with the recurrent infections traditionally associated with primary immunodeficiency. These conditions often pose diagnostic challenges, as the autoimmune manifestations may precede recognition of the underlying genetic disorder by years, leading to misdiagnosis as isolated autoimmune disease. The International Union of Immunological Societies (IUIS) recognizes "Diseases of immune dysregulation" as a major category within the classification of primary immunodeficiencies. The unifying concept across these disorders is that loss of immune tolerance -- whether through failure of central tolerance mechanisms, defective peripheral tolerance checkpoints, or impaired cell death pathways -- allows autoreactive lymphocytes to persist and mediate tissue damage, while defects in programmed cell death pathways lead to the accumulation of lymphocytes and lymphoproliferation.

Hemophagocytic Lymphohistiocytosis (HLH)

Pathophysiology

Hemophagocytic lymphohistiocytosis is a life-threatening syndrome of uncontrolled activation of T cells and macrophages, accompanied by excessive production of pro-inflammatory cytokines that create a self-amplifying "cytokine storm." The fundamental pathogenic defect lies in defective cytotoxic function: the inability of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) to effectively kill infected, activated, or transformed target cells. Under normal circumstances, cytotoxic killing terminates immune responses by eliminating antigen-presenting cells and activated lymphocytes. When this killing mechanism fails, persistent antigenic stimulation drives unchecked T cell and macrophage activation, resulting in the clinical syndrome of HLH.

Familial HLH (fHLH) is caused by monogenic defects in the perforin-granzyme cytotoxic pathway. Each genetic subtype disrupts a specific step in the process by which cytotoxic granules deliver their lethal cargo to target cells. FHL2, caused by mutations in PRF1 encoding perforin, is the most common form of familial HLH; perforin is the pore-forming protein that creates channels in the target cell membrane through which granzymes enter to trigger apoptosis. FHL3 results from mutations in UNC13D encoding Munc13-4, which is required for the priming of cytotoxic granules before they can fuse with the plasma membrane. FHL4 involves mutations in STX11 encoding syntaxin-11, which mediates granule docking at the membrane. FHL5 is caused by mutations in STXBP2 encoding Munc18-2, which facilitates the final fusion event.

Several syndromic forms of HLH exist. Griscelli syndrome type 2, caused by RAB27A mutations, disrupts granule transport along microtubules and is associated with partial albinism. Chediak-Higashi syndrome, caused by LYST mutations, impairs granule trafficking and presents with partial albinism and the characteristic giant granules in leukocytes. X-linked lymphoproliferative disease type 1 (XLP1), caused by mutations in SH2D1A encoding SAP, results in impaired NK cell and T cell regulation, with particular vulnerability to EBV-triggered HLH. XLP2, caused by XIAP (BIRC4) mutations, disrupts inflammasome regulation and also predisposes to EBV-driven HLH.

HLH-2004 Diagnostic Criteria (>=5 of 8, or Molecular Diagnosis)

The HLH-2004 diagnostic protocol requires either the identification of a molecular diagnosis consistent with HLH or the fulfillment of at least five of eight clinical and laboratory criteria. These criteria are: fever of 38.5 degrees Celsius or higher; splenomegaly; cytopenias affecting two or more lineages (hemoglobin below 9 g/dL, platelets below 100,000, or absolute neutrophil count below 1,000); hypertriglyceridemia (fasting triglycerides 265 mg/dL or higher) and/or hypofibrinogenemia (fibrinogen 150 mg/dL or lower); hemophagocytosis identified on bone marrow, splenic, or lymph node biopsy; low or absent NK cell cytotoxic activity; ferritin of 500 ng/mL or higher (with levels exceeding 10,000 ng/mL carrying approximately 90% sensitivity for HLH); and elevated soluble interleukin-2 receptor (sCD25) of 2,400 U/mL or higher.

Additional markers that support the diagnosis but are not part of the formal criteria include elevated lactate dehydrogenase, elevated hepatic transaminases, and evidence of disseminated intravascular coagulation. The H-Score is a clinical prediction tool specifically designed for reactive (secondary) HLH that assigns points based on clinical and laboratory parameters; a score exceeding 169 points corresponds to a greater than 93% probability of HLH.

Treatment

The treatment of HLH must be initiated urgently, as untreated disease is rapidly fatal. The HLH-94 and HLH-2004 protocols form the backbone of therapy, employing etoposide combined with dexamethasone, with or without cyclosporine A. Etoposide is a critical component because it effectively eliminates the activated cytotoxic T cells and macrophages driving the inflammatory cascade. Intrathecal methotrexate is added when central nervous system involvement is documented.

Emapalumab (Gamifant), a monoclonal antibody targeting interferon-gamma, has received FDA approval for the treatment of primary HLH that is refractory to or has relapsed after conventional therapy. By neutralizing the dominant pro-inflammatory cytokine in the HLH cascade, emapalumab can break the self-amplifying cycle of immune activation. Ruxolitinib, a JAK1/2 inhibitor, is used in refractory HLH to broadly block cytokine signaling. Anakinra, an interleukin-1 receptor antagonist, is particularly useful in macrophage activation syndrome (MAS), the HLH variant that occurs in the setting of autoimmune disease, especially systemic juvenile idiopathic arthritis.

Hematopoietic stem cell transplantation is the only definitive cure for familial HLH and is required for long-term survival in all fHLH patients, as the underlying cytotoxic defect ensures disease recurrence without transplantation. Trigger-directed therapy must be pursued concurrently: underlying infections (particularly EBV and CMV) should be treated with appropriate antiviral agents, and associated malignancies require specific oncologic management.

<image>A diagnostic and pathophysiologic diagram of HLH. Upper panel: Normal cytotoxic killing pathway - NK cell or CTL recognizing target cell, with perforin pore formation, granzyme delivery, and target cell apoptosis. Proteins labeled at each step: Rab27a (transport), Munc13-4 (priming), syntaxin-11 (docking), Munc18-2 (fusion), perforin (pore). Lower panel: HLH - defective killing leads to persistent antigen stimulation, uncontrolled T cell and macrophage activation. Macrophage shown engulfing blood cells (hemophagocytosis). Cytokine storm illustrated with arrows showing IFN-gamma, TNF-alpha, IL-6, IL-18, IL-1beta, sCD25 in a positive feedback loop. Each familial HLH subtype mapped to its defective protein along the killing pathway. Clinical consequences labeled: fever, cytopenias, hepatosplenomegaly, coagulopathy, hyperferritinemia. Drug targets shown: emapalumab blocking IFN-gamma, etoposide targeting activated T cells, ruxolitinib blocking JAK signaling.</image>

Autoimmune Lymphoproliferative Syndrome (ALPS)

Pathophysiology

Autoimmune lymphoproliferative syndrome arises from defective lymphocyte apoptosis through the Fas (CD95)/Fas ligand signaling pathway. Under normal circumstances, this pathway serves as a critical checkpoint for eliminating autoreactive lymphocytes in germinal centers and clearing activated lymphocytes that have completed their effector function. When Fas-mediated apoptosis is impaired, these cells escape deletion and accumulate, producing the characteristic triad of chronic non-malignant lymphoproliferation, autoimmune cytopenias, and an elevated risk of lymphoma.

Genetic Subtypes

ALPS-FAS is the most common subtype, accounting for approximately 65% of cases, and results from germline heterozygous mutations in the FAS gene (TNFRSF6, encoding CD95). Inheritance follows an autosomal dominant pattern with variable penetrance. An important diagnostic consideration is that somatic FAS mutations confined to the double-negative T cell (DNT) population have been identified in patients with the ALPS phenotype but normal germline FAS sequencing; these somatic mutations can only be detected by sequencing DNA specifically from sorted DNT cells. Less common subtypes include ALPS-FASLG (FAS ligand mutations), ALPS-CASP10 (caspase-10 mutations), and ALPS-related disorders involving FADD or caspase-8 mutations (with caspase-8 deficiency, termed CEDS, presenting as a combined immunodeficiency with ALPS-like features). RAS-associated ALPS-like disease (RALD) is caused by somatic gain-of-function mutations in NRAS or KRAS, producing an ALPS-like phenotype without defects in the Fas pathway.

Diagnostic Criteria (NIH Revised)

The revised NIH diagnostic criteria require chronic (persisting for more than six months) non-malignant, non-infectious lymphoproliferation, manifesting as splenomegaly and/or lymphadenopathy. The primary diagnostic criterion is elevation of double-negative T cells (DNTs), defined as CD3-positive, TCR-alpha/beta-positive lymphocytes that are both CD4-negative and CD8-negative, reaching 1.5% or more of total lymphocytes or 2.5% or more of CD3-positive cells. This unusual T cell subset, which is normally present in only trace quantities in peripheral blood, accumulates specifically as a consequence of failed Fas-mediated apoptosis and serves as a highly sensitive and specific biomarker for ALPS. Secondary criteria that provide supportive evidence include defective Fas-mediated apoptosis demonstrated in vitro using activated lymphocytes, and identification of a pathogenic mutation in FAS, FASLG, or CASP10. Additional biomarkers that support the diagnosis include elevated levels of soluble Fas ligand, interleukin-10, interleukin-18, and vitamin B12.

Clinical Features

The clinical presentation of ALPS is dominated by chronic lymphadenopathy and hepatosplenomegaly, which can be massive and may raise initial concern for lymphoma. Autoimmune cytopenias are the most common complication, with autoimmune hemolytic anemia (AIHA) occurring in approximately 50% of patients, immune thrombocytopenia (ITP) in approximately 45%, and autoimmune neutropenia at a lower frequency. Evans syndrome, the simultaneous occurrence of AIHA and ITP, is particularly characteristic. Laboratory evaluation typically reveals polyclonal hypergammaglobulinemia with elevated IgG. Vitamin B12 levels are characteristically elevated, often exceeding 1,500 pg/mL, and elevated levels of interleukin-10, soluble Fas ligand, and interleukin-18 serve as useful biomarkers.

The risk of lymphoma is a major long-term concern, with a lifetime risk estimated at 10 to 20%. Both Hodgkin and non-Hodgkin lymphomas occur, with B cell lymphomas being the most common type. Clinical surveillance with regular monitoring and consideration of annual imaging in high-risk patients is important for early detection.

Treatment

Management of autoimmune cytopenias in ALPS begins with short-term corticosteroids for acute episodes. Sirolimus (rapamycin) has emerged as the first-line steroid-sparing agent, based on its ability to inhibit mTOR signaling and thereby selectively induce apoptosis in DNTs and other lymphocytes that have escaped Fas-mediated deletion. Sirolimus is effective in more than 80% of ALPS patients with cytopenias, with a target trough level of 5 to 15 ng/mL. Beyond controlling cytopenias, sirolimus also reduces lymphoproliferation, producing measurable decreases in spleen and lymph node size. Mycophenolate mofetil serves as a second-line agent. Rituximab is effective for refractory cytopenias but carries risks of persistent B cell depletion and hypogammaglobulinemia in this patient population. Splenectomy should be avoided whenever possible in ALPS patients, as it not only increases the risk of post-splenectomy sepsis but also appears to increase the risk of lymphoma development. HSCT is curative and is considered for severe, refractory cases.

IPEX Syndrome (Immune Dysregulation, Polyendocrinopathy, Enteropathy, X-Linked)

Pathophysiology

IPEX syndrome results from mutations in FOXP3, the master transcription factor that governs the development and function of CD4-positive CD25-positive regulatory T cells (Tregs). FoxP3 expression is both necessary and sufficient for the Treg lineage commitment, and in its absence, the regulatory T cell compartment is functionally ablated. Without effective Treg-mediated peripheral tolerance, self-reactive T cells that escape central thymic deletion are unopposed, leading to devastating multi-organ autoimmunity. IPEX follows X-linked inheritance, affecting males exclusively or predominantly.

Clinical Features

IPEX syndrome typically presents in the first months of life in affected males with a classic triad of autoimmune manifestations. Autoimmune enteropathy is the most consistent feature, presenting as severe watery diarrhea with villous atrophy and failure to thrive that can be life-threatening. Type 1 diabetes mellitus, often with neonatal onset far earlier than typical autoimmune diabetes, is the second element of the triad. An eczematous dermatitis that may be severe and closely resembles atopic dermatitis constitutes the third component. Beyond this classic triad, additional autoimmune manifestations commonly develop, including autoimmune hemolytic anemia, thyroiditis (either hypothyroidism or hyperthyroidism), nephropathy, hepatitis, and various cytopenias. Laboratory evaluation typically reveals markedly elevated IgE and eosinophilia. Without treatment, IPEX is fatal, and HSCT is the only curative therapy.

Comparison of Immune Dysregulation Syndromes

IPEX-Like Syndromes

Several genetic disorders produce clinical phenotypes that overlap substantially with IPEX but are caused by defects in different genes. CTLA-4 haploinsufficiency, an autosomal dominant condition, results from heterozygous loss-of-function mutations in the CTLA-4 gene, which encodes a critical immune checkpoint receptor. The clinical phenotype is variable even within families due to incomplete penetrance and includes multi-organ lymphocytic infiltration, autoimmune cytopenias, and hypogammaglobulinemia. Treatment with abatacept (CTLA-4-Ig fusion protein) provides targeted replacement of the deficient checkpoint molecule.

LRBA deficiency, inherited in autosomal recessive fashion, produces a phenotype that closely mimics CTLA-4 haploinsufficiency because LRBA (LPS-responsive beige-like anchor protein) is responsible for stabilizing CTLA-4 within the cell, preventing its lysosomal degradation. Loss of LRBA leads to decreased CTLA-4 surface expression and function, effectively phenocopying CTLA-4 haploinsufficiency. Abatacept is similarly effective in LRBA deficiency.

STAT3 gain-of-function mutations produce multi-system autoimmunity, lymphoproliferation, and growth failure through constitutive activation of STAT3-dependent signaling pathways. In contrast to the loss-of-function STAT3 mutations that cause hyper-IgE syndrome, gain-of-function mutations result in immune dysregulation rather than immunodeficiency. Treatment with JAK inhibitors (ruxolitinib or tofacitinib) targets the overactive STAT3 signaling. STAT1 gain-of-function mutations produce chronic mucocutaneous candidiasis alongside autoimmune thyroiditis and cytopenias, and similarly respond to JAK inhibitor therapy. CD25 (IL-2 receptor alpha) deficiency produces an IPEX-like syndrome with severe autoimmunity in the absence of FoxP3 mutations.

<image>A comparative clinical features matrix of immune dysregulation syndromes. Organized as a table with columns for IPEX (FOXP3), CTLA-4 haploinsufficiency, LRBA deficiency, ALPS (FAS), STAT3 GOF, and STAT1 GOF. Rows comparing: (1) Gene/protein, (2) Inheritance pattern, (3) Mechanism of immune dysregulation (Treg deficiency, checkpoint deficiency, apoptosis defect), (4) Key clinical features (autoimmune enteropathy, cytopenias, endocrinopathy, lymphoproliferation, infections), (5) Characteristic laboratory findings (elevated IgE, DNTs, vitamin B12, specific autoantibodies), (6) Targeted therapy (HSCT for IPEX, abatacept for CTLA-4/LRBA, sirolimus for ALPS, JAK inhibitors for STAT GOF). Color-coded cells: red for present/severe, yellow for variable, green for absent. Highlight the therapeutic targets with molecular diagrams.</image>

Eosinophilic Granulomatosis with Polyangiitis (EGPA/Churg-Strauss)

Overview

Eosinophilic granulomatosis with polyangiitis (EGPA), formerly known as Churg-Strauss syndrome, is an ANCA-associated small-vessel vasculitis distinguished from other vasculitides by its characteristic association with prominent peripheral eosinophilia and asthma. The clinical triad comprises asthma, peripheral eosinophilia exceeding 10% or 1,500 cells per microliter, and small-vessel vasculitis with granulomatous inflammation on biopsy. Approximately 40% of patients are ANCA-positive, typically with antibodies directed against myeloperoxidase (MPO-ANCA, corresponding to the p-ANCA pattern), and these patients tend to manifest more prominent vasculitic features including glomerulonephritis and mononeuritis multiplex. The remaining approximately 60% are ANCA-negative, and their disease is dominated by eosinophilic tissue infiltration, with cardiac involvement being more common and a major cause of morbidity and mortality in this subgroup.

Phases

The natural history of EGPA classically evolves through three sequential phases, though in practice these may overlap. The prodromal phase, which may persist for years, is characterized by adult-onset asthma, allergic rhinitis, and nasal polyposis. The eosinophilic phase features tissue eosinophilia with pulmonary infiltrates and eosinophilic gastroenteritis. The vasculitic phase represents the most dangerous stage, with the development of systemic vasculitis accompanied by constitutional symptoms, weight loss, and organ-specific damage.

Diagnosis (ACR/EULAR 2022 Criteria)

The 2022 ACR/EULAR classification criteria require the presence of asthma, eosinophilia exceeding 1,000 cells per microliter, and at least one additional feature. Additional features include histologic evidence of extravascular eosinophilic inflammation, necrotizing vasculitis, or granulomas; organ involvement such as mononeuritis multiplex, pulmonary infiltrates, cardiac involvement including cardiomyopathy, glomerulonephritis, or nasal polyps; and ANCA positivity for MPO.

Treatment

Glucocorticoids are the first-line agents for induction of remission. Mepolizumab has received FDA approval for EGPA based on the MIRRA trial, which demonstrated that 300 mg administered subcutaneously every four weeks achieved remission in 50% of patients compared to 32% with placebo, with significant reduction in oral corticosteroid requirements. Benralizumab has also received FDA approval for EGPA based on the MANDARA trial, administered at 30 mg subcutaneously every four weeks after an initial loading period. For severe vasculitic manifestations affecting the kidneys, nervous system, or heart, cyclophosphamide remains the standard induction agent. Rituximab is emerging as an alternative to cyclophosphamide with a growing evidence base. Maintenance therapy may consist of azathioprine or continued mepolizumab.

Key Clinical Pearls

• HLH should be considered in any critically ill patient with fever, cytopenias, hyperferritinemia, and hepatosplenomegaly; ferritin >10,000 has 90% sensitivity for HLH
• ALPS is characterized by elevated double-negative T cells (CD3+CD4-CD8-), massive lymphoproliferation, and autoimmune cytopenias; sirolimus is first-line steroid-sparing therapy
• AVOID splenectomy in ALPS: it increases both infection risk and lymphoma risk
• IPEX presents in male infants with enteropathy, neonatal diabetes, and eczema; HSCT is curative
• CTLA-4 haploinsufficiency and LRBA deficiency are treatable with abatacept (CTLA-4-Ig)
• STAT3 GOF and STAT1 GOF cause multi-system autoimmunity treatable with JAK inhibitors
• EGPA has two phenotypes: ANCA-positive (vasculitis-predominant) and ANCA-negative (eosinophilic/cardiac-predominant); mepolizumab and benralizumab are both FDA-approved
• Elevated vitamin B12 and sFasL are useful biomarkers for ALPS that can be checked from peripheral blood

References

1. Henter JI, et al. HLH-2004: diagnostic and therapeutic guidelines for hemophagocytic lymphohistiocytosis. Pediatr Blood Cancer. 2007;48(2):124-131.
2. Oliveira JB, et al. Revised diagnostic criteria and classification for the autoimmune lymphoproliferative syndrome (ALPS): report from the 2009 NIH International Workshop. Blood. 2010;116(14):e35-e40.
3. Barzaghi F, et al. IPEX syndrome: improved knowledge of immune pathogenesis empowers diagnosis and offers new therapeutic strategies. Front Immunol. 2022;12:784630.
4. Wechsler ME, et al. Mepolizumab or placebo for eosinophilic granulomatosis with polyangiitis (MIRRA). N Engl J Med. 2017;376(20):1921-1932.
5. Lo B, et al. AUTOIMMUNE DISEASE. Patients with LRBA deficiency show CTLA4 loss and immune dysregulation responsive to abatacept therapy. Science. 2015;349(6246):436-440.

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