Hemophagocytic Lymphohistiocytosis and Macrophage Activation Syndrome

Familial Hemophagocytic Lymphohistiocytosis: Cytotoxic Granule Pathway Defects

Familial (inherited) HLH that has an early onset and is fulminant usually is due to impaired T- and NK-cell-mediated cytotoxicity as a result of biallelic mutations in one of six genes that encode proteins with nonredundant functions for cytotoxicity mediated by the perforin/granzyme pathway. These include familial hemophagocytic lymphohistiocytoses (FHLs), Chediak-Higashi syndrome, and Griscelli syndrome type 2 ( Table 12.1 ). ^, Impairment of cell-mediated cytotoxicity prevents the elimination of a stimulus for T-cell activation, such as target cells infected with virus, which results in T-cell hypersecretion of cytokines, such as interferon-gamma (IFN-γ), that, in turn, activate macrophages as a side effect. The combined profound T-cell and/or macrophage activation result in systemic pathologic effects characteristic of HLH. Infection, particularly with viruses, may be a precipitant of HLH, with the failure of lymphocyte-mediated cytotoxicity in eliminating infected target cells leading to ongoing hyperstimulation of microbial peptide-specific CD8 ⁺ T cells. ^{, , ,} However, infection is not demonstrable in many cases of HLH in neonates and young infants, and HLH can occur in utero in the absence of demonstrable congenital infection. ^, These observations suggest that lymphocyte-mediated cytotoxicity by the perforin/granzyme pathway also plays an important physiologic regulatory role in normally limiting T-cell activation.

Familial HLH (FHL), which was first reported in 1952, accounts for approximately 25% of all HLH cases in children worldwide and consists of five types that predominantly have an autosomal recessive inheritance pattern with biallelic mutations. ^, FHL1, which maps to the chromosome 9q21.3-q22 region, is the only type for which a monogenic cause has not been determined. In FHL2, mutations in the PRF1 gene result in deficiency of perforin, which plays an essential role in lymphocyte-mediated cytotoxicity by forming pores in the target cell through which granzymes enter to trigger target cell death by apoptosis. FHL3 is due to deficiency of Munc13-4, encoded by the UNC13D gene, which is required for priming cytotoxic granules for their fusion with the plasma membrane, which in turn prompts the extracellular release of the cytotoxic granule contents. FHL4 is due to mutations of the STX11 gene, which encodes syntaxin 11, a protein that is involved in the formation of a SNARE complex that regulates cytotoxic granule membrane fusion. FHL5 is due to mutations in the STXB2 gene that encodes syntaxin-binding protein 2, which is also involved in the regulation of the SNARE complex and serves as chaperone for syntaxin 11. Unlike FHL1-4, in which early onset HLH is the only major phenotype, FHL5 also includes severe enteropathy reflecting a host defense role for syntaxin-binding protein 2 in gut epithelial cells as well as in neutrophil degranulation ( Table 12.1 ).

Chediak-Higashi syndrome, due to mutations in the gene encoding the LYST protein, can manifest similarly to the FHLs with early onset and fulminant HLH. The LYST protein may play a role at the later steps of cytotoxic granule maturation, and it is essential for normal melanocyte, neutrophil, and platelet function. Thus, Chédiak-Higashi syndrome is readily distinguished from other forms of familial HLH by partial oculocutaneous albinism, dysfunctional neutrophils with giant inclusions visible on the peripheral blood smear, and platelet dysfunction associated with a mild bleeding tendency.

Griscelli syndrome type 2, in which there are biallelic mutations in the RAB27A gene that encodes the Rab27a protein, also can manifest similarly to FHLs. Rab27a interacts with MUNC13-4 and is involved in the terminal intracellular transport of the cytotoxic granules to the cell membrane. Rab27a also is involved in melanosome transport such that a complete loss of Rab27 function results in partial albinism and the hair having a characteristic silvery sheen. Although these physical signs of albinism are helpful in suspecting the diagnosis, they may be absent in patients with certain Rab27a missense mutations that preserve its function in melanosome transport but still impair lymphocyte-mediated cytotoxicity and predispose to HLH. Partial albinism with HLH has also rarely been associated with the Hermansky-Pudlak type 2 (HPS2) syndrome, which is due to biallelic mutations of the ADTB3A gene that encodes the of the β-subunit of the adaptor protein-3 (AP3) complex. In addition to partial oculocutaneous albinism, HPS2 patients have congenital neutropenia, impaired neutrophil killing and a bleeding diathesis, reflecting important roles of the AP3 complex in granule-dependent neutrophil and platelet function. Although HPS2 patients appear to have consistent defects in NK-cell cytotoxicity or degranulation in vitro, their risk for HLH appears to be relatively low, with only 1 of a cohort of 22 patients with HPS2 developing frank HLH and another 2 patients having incomplete and transient HLH-like episodes.

In FHL2 with biallelic mutations that are highly damaging for perforin expression, HLH often occurs in the first few months of life (mean age of onset 3.4 months) with an earlier age of onset associated with greater disease severity. In contrast, FHL4 with biallelic damaging STX11 mutations on average results in a later onset of HLH (mean age of onset 27.3 months). These differences in age of onset and severity are not reflected in obvious differences between FHL2 and FHL4 in the impairment of lymphocyte-mediated cytotoxicity in vitro, suggesting that this clinical difference reflects other poorly defined roles of these proteins in lymphocyte regulation. However, even children of a single family having the same mutation genotype can differ in their age of presentation in the absence of a demonstrable infection trigger, again suggesting the importance of noninfectious environmental factors that impact lymphocyte regulation by the perforin/granzyme pathway. ^,

Familial HLH due to impaired lymphocyte-mediated cytotoxicity can manifest later in childhood or even adulthood in cases for which there is inheritance of a gene of two mutated alleles for which at least one has reduced but substantial residual (i.e., hypomorphic) function. Other modes of inheritance associated with reduced but not absent lymphocyte-mediated cell cytotoxicity and delayed presentation of HLH include heterozygosity for mutations that act in a dominant negative manner to reduce gene function, which has been demonstrated for the STXBP2 and RAB27A genes, or heterozygous inheritance of mutated alleles for two different genes that act together in the lymphocyte-mediated cytotoxicity pathway (i.e., digenic inheritance).

Familial Hemophagocytic Lymphohistiocytosis: Gene Defects Impairing Epstein-Barr Virus Host Defense

Familial HLH can also result from genetic disorders in which specific susceptibility to severe Epstein-Barr virus (EBV) infection results in HLH following primary EBV infection. A well-described example is X-linked lymphoproliferative (XLP) syndrome 1 due to mutations in the SH2D1A gene that encodes SAP protein. ^, The SAP protein is a signal adapter molecule that is important for CD8 T-cell and NK-cell activation and cytotoxicity. Except for very rare cases in females due to extreme X-chromosome lionization, XLP syndrome 1 affects only males. It usually manifests in association with primary EBV infection, with about 60% of cases developing fulminant HLH. In contrast to FHL, XLP syndrome frequently is characterized by hypogammaglobulinemia, often present before EBV infection, as well as the rapid development of EBV-related lymphoproliferative disease or lymphomas.

Other inherited defects in EBV host defense that predispose to HLH include biallelic mutations in the interleukin-2−inducible T-cell kinase (ITK) gene that results in susceptibility to EBV disease that frequently progresses to lymphoma and that can also result in HLH. A biallelic gene defects in CD27, a member of the tumor necrosis factor (TNF) receptor family, results in severe EBV disease that has been complicated by HLH in several patients. Biallelic mutations of TNFRSF9 , which encodes 4-1BB, another TNF receptor family member, also results in immunodeficiency predisposing to EBV-induced lymphoproliferation and HLH. The X-linked immunodeficiency with magnesium defect. EBV infection and neoplasia (XMEN) syndrome is a T-cell immunodeficiency due to impaired magnesium transport as a result of MAGT1 gene defects. EBV infection is chronic and frequently progresses to lymphoproliferation but for unclear reasons, XMEN patients only rarely develop HLH. It is likely that additional causes of EBV-induced HLH will be recognized among the growing number of patients with identifiable primary immunodeficiencies (PIDs) that predispose to severe impairment of the immune control of EBV, such as genetic deficiencies of CTPS1, RASGRP1, and CD70.

Familial Hemophagocytic Lymphohistiocytosis: Autoinflammatory and Other Complex Immune Dysregulatory Disorders

XLP syndrome 2, also known as X-linked familial hemophagocytic lymphohistiocytosis, is an important cause of recurrent HLH in males and in females with skewed X-chromosome lyonization, due to a mutation of the BIRC4 gene, which encodes the XIAP protein. ^, The deficiency of XIAP results in dysregulated activity of the NLRP3 inflammasome, resulting in increased secretion of IL-1β and IL-18, which promote HLH, at least in part, by activating macrophages. XIAP-deficient cells, including T cells, have dysregulated caspase activity and an increased tendency to undergo cell death, which may impair of T-cell immunity and also predispose to HLH. Although the XLP syndrome 2 rarely can manifest similarly to FHL, XLP syndrome 2 usually has a more delayed, mild, or recurrent course, with recovery without cytotoxic chemotherapy, such as etoposide. In contrast to XLP syndrome 1 disease, EBV-related lymphoproliferative disease in XLP syndrome 2 is relatively rare and there is no increased risk of lymphoma. XIAP-deficient patients also commonly develop inflammatory bowel disease, hypogammaglobulinemia with recurrent infections, and are also at risk for a host of other less frequent complications.

Familial HLH may also result from genetic disorders in which autoinflammation involving macrophages and inflammasomes play a prominent role. For example, gain-of-function mutations in the inflammasome NLR-family CARD-domain-containing protein 4 (NLRC4) are associated with hypersecretion of pro-inflammatory cytokines IL-1β and IL-18 from leukocytes and intestinal epithelial sources, resulting in enterocolitis and HLH/MAS. Neonatal-onset cytopenia with dyshematopoiesis, autoinflammation, rash, and HLH (NOCARH syndrome) is due to heterozygous missense mutations in the cell division control protein 42 homolog (encoded by CDC42 ). Patients with NOCARH syndrome have both NK cell-mediated cytolytic defects reminiscent of FHL and also markedly elevated IL-18 production suggesting profound innate immune activation. CDC42 is an intracellular protein involved in regulating the assembly of actin cytoskeletal structures, but it remains to be determined how these mutations result in immune dysregulation. Relapsing HLH has also been reported in a patient with a homozygous null mutation of the RC3H1 gene, which encodes Roquin-1, a post-transcriptional repressor of immune regulatory proteins, including TNF. Roquin-1 normally downregulates mRNA levels by its effect on polyadenylation; impairment of its function results in abnormally increased expression of a number of pro-inflammatory proteins. Finally, HLH has been reported in two unrelated patients with autosomal recessive null mutations of the NCKAP1L gene, which encodes a Nck-associated protein 1-like product. As for CDC42, the NCKAPIL protein is involved in regulation of the actin cytoskeleton; deficient patients have substantial elevations of IL-18, suggesting that both deficiencies share pathogenic mechanisms in the induction of HLH.

Macrophage Activation Syndrome Associate With Systemic-Onset Juvenile Idiopathic Arthritis

Macrophage activation syndrome (MAS), also known as rheumatic-HLH, is a severe potentially life-threatening hyperinflammatory complication that affects approximately 10% of patients with systemic-onset juvenile idiopathic arthritis (sJIA). It has also been observed in other rheumatologic disorders, including systemic lupus erythematosus, Kawasaki disease, scleroderma, and histiocytic necrotizing lymphadenitis ( Table 12.1 ). The pathophysiology of MAS substantially overlaps with HLH, including evidence of excessive mononuclear phagocyte activation, increased expression of multiple cytokines (e.g., IL-1β, IL-6, IL-18, and IFN-γ), and impaired lymphocyte-mediated cytotoxicity, ^, which may be due, in part, to an effect of elevated IL-6. A whole exome sequencing analysis study found that approximately 35% of the MAS/sJIA patients were heterozygous for protein-altering mutations in genes known to cause lymphocyte-mediated cytolytic defects in familial HLH ( MUNC13-4 , STXBP2 , and LYST ). Moreover, MAS/sJIA patients were also enriched for rare protein-altering variants for other genes known or predicted to be involved with actin and microtubule reorganization and vesicle-mediated transport. In some cases, the protein encoded by these genes directly interact with those encoded by known familial HLH genes, such as RAB27A and CDC42 . Together, these findings strongly suggest the importance of the granule dependent cytolytic pathway and the regulation of the actin cytoskeleton in the pathogenesis of both primary HLH as well as in secondary HLH cases, such as those associated with MAS/sJIA.

Hemophagocytic Lymphohistiocytosis Due to Infections in Immunocompromised Hosts

Nonfamilial HLH most often is triggered by severe or prolonged infections in immunocompromised patients ( Table 12.1 ), particularly in patients with severe acquired immunodeficiency, for example, due to HIV/AIDS, treatment with cancer chemotherapy, or after the receipt of potent immunosuppression, for example, for treating allogeneic graft rejection or severe autoimmune disease. A wide variety of bacterial, fungal, viral and parasitic infections can be involved.

Importantly, HLH can be a complication of severe infection in patients with PIDs other than the genetic causes of primary HLH. These PIDs are diverse and include chronic granulomatous disease, severe combined immunodeficiencies, combined immunodeficiencies, DiGeorge syndrome (e.g., 22q11.2 deletion), Wiskott-Aldrich syndrome, X-linked agammaglobulinemia, signal transcription and activator of transcription (STAT)1 gain-of-function, STAT2 loss-of-function, autosomal dominant STAT3 deficiency/hyperimmunoglobulin E syndrome, IFN-α receptor 1 and 2 deficiencies, ^, Mendelian susceptibility to mycobacterial disease (MSMD) disorders (STAT1 loss-of-function, IL-12 receptor β1 chain deficiency, partial dominant IFN-γ receptor 1 deficiency ), and GATA2 deficiency. Importantly, in some cases, an infection complicated by HLH may be the first presentation of the PID.

Hemophagocytic Lymphohistiocytosis Associated With Primary Epstein-Barr Virus Infection

Primary EBV infection is the leading infectious trigger of HLH in children who are apparently immunocompetent, i.e., who lack a family history of HLH or gene mutations indicative of a lymphocyte-mediated cytotoxicity disorder or other form of primary HLH, such as XLP syndrome 1 and/or 2. ^, EBV-associated HLH is more common in Asian countries, for example, accounting for up to 70% of pediatric HLH cases in China, and in Central and South American indigenous populations. ^, A frequent finding is that EBV infects CD8 T cells or NK cells rather than B cells, ^{, ,} as is typical for uncomplicated primary EBV infectious mononucleosis. Studies of EBV-associated HLH in Asia suggest that there is a high rate of T-cell receptor clonality and that, in some cases, the disorder may be a precursor of a frank lymphoproliferative/malignant process. ^, Most patients have hypercytokinemia, including elevated production of IFN-γ, and its downstream effects on macrophage activation similar to familial HLH ^, and although the course is variable it can be fulminant and rapidly fatal. The pathogenesis of this form of HLH remains poorly understood but defects in lymphocyte-mediated cytotoxicity by the perforin/granzyme pathway are not typical.

Hemophagocytic Lymphohistiocytosis Associated With Infections Other Than Epstein-Barr Virus in the Apparently Immunocompetent Host

In the immunocompetent host, HLH associated with infection originally was considered to be mainly a complication of primary EBV infection in previously healthy older children; however, it is now appreciated that HLH can occur in all ages in association with severe infections with a wide variety of pathogens ( Table 12.1 ). ^{, ,} Common non-EBV viral triggers include other herpesviruses (e.g., cytomegalovirus, herpes simplex virus, human herpesvirus type 6 [HHV-6], HHV-8), adenovirus, dengue, enterovirus, parvovirus, and influenza A. In fatal H1N1 infection complicated by HLH, there is an enrichment of inheritance of familial HLH gene mutations that compromise lymphocyte-mediated cellular cytotoxicity, suggesting that these genetic variants are a risk factor for a fatal outcome. Thus, it will of interest to determine for other infectious etiologies resulting in severe HLH the role of inheritance of heterozygous mutations in genes associated with familial HLH.

There are also case reports of many other types of viral infections associated with HLH, for example, viral hemorrhagic fevers, suggesting that HLH potentially can complicate any severe systemic viral infection. However, SARS-CoV-2 infection only rarely causes HLH in either children or adults, even in cases associated with severe cytokine storm. These observations suggest that there are particular cytokine pathways or other mechanisms that promote HLH that are not universally active in all types of severe viral infections.

Severe bacterial infections, particularly those involving the reticuloendothelial system (e.g., typhoid fever, rickettsial diseases, brucellosis, and disseminated Mycobacterium tuberculosis ), are frequent precipitants of HLH, and systemic fungal infections (e.g., Histoplasma and Penicillium spp. infection) are recognized triggers. Visceral leishmaniasis, which has been misdiagnosed as FHL, and malaria are frequent protozoal triggers.