Familial HLH (FHL) has an incidence of approximately 0.12 to 1 cases per 100,000 children per year, even though it may be more common in areas with high consanguinity due to the autosomal-recessive inheritance 56. Five different forms of FHL have so far been described and four genes, accounting for over 90% of familial cases, have been identified (Table 1) 78910111213. They encode the proteins perforin, MUNC13-4, syntaxin-11 and MUNC18-2, all of which play a key role in lymphocyte cytotoxicity (Figure 1) 14. Most patients with FHL develop HLH within the first months of life. However, up to 20% of patients present at more than 2 years of age, and in rare cases, patients with FHL remain asymptomatic until adulthood 1151617.

Syndromal immunodeficiencies associated with albinism, including Chédiak Higashi syndrome (CHS), Griscelli syndrome type II (GSII), and Hermansky-Pudlak syndrome type II (HPSII) also predispose to HLH. These patients show variable degrees of (partial) albinism, platelet dysfunction, and immunodeficiency in addition to their risk of developing HLH 1819. The genes affected in CHS, GSII, and HPSII are also involved in granule-dependent lymphocyte cytotoxicity (Figure 1) 2021. Onset of HLH in patients with these diseases tends to be later than in patients with FHL.

Other primary immunodeficiencies predisposing to HLH include XLP1 (SAP (signaling lymphocytic activation molecule associated protein)), XLP2 (XIAP (X-linked inactivator of apoptosis deficiency), ITK (IL-2 inducible T-cell kinase) and CD27 deficiency 2223242526. These defects can present with various signs and symptoms of immunodeficiency and immune dysregulation, but can also manifest primarily with HLH, almost exclusively in association with Epstein-Barr virus (EBV) infection 2223242526.

All patients with primary HLH have a high risk of recurrence. Therefore, hematopoietic stem cell transplantation (HSCT) is the only curative option 27.

Patients manifesting with HLH in the absence of a disease-causing mutation in the known genes and without strong indications for a genetic predisposition, such as familial disease or recurrent episodes of HLH, are currently classified as suffering from 'secondary' HLH. There are sparse data on the incidence, but it is probably more frequent than primary HLH 28. One source reports an incidence of 0.36 cases of malignancy-associated HLH per 100,000 adults per year 29. It is highly likely that, due to some overlap with sepsis syndromes, HLH is under-diagnosed, in particular in the adult population. Many patients with 'secondary' HLH manifest beyond infancy, but increasing recognition of late-onset FHL renders the age at onset a poor indicator of disease etiology. Moreover, the genetic basis of HLH remains to be fully defined. Thus, final classification of a patient as suffering from 'secondary' HLH must remain preliminary.

The most common form of 'secondary' HLH is infection-associated HLH. Infectious triggers include viruses (for example, EBV, cytomegalovirus, HHV8, HIV), bacteria (for example, mycobacteria, mycoplasma), parasites (leishmania, plasmodium), and fungi (for example, candida, cryptococcus) 3031. EBV and leishmania infection are the most frequent triggers. Notably, detecting an infectious agent does not help to distinguish between 'secondary' and primary forms of HLH, since also in the latter cases acute episodes are often triggered by infections 1.

Malignancies such as leukemias or lymphomas, particularly T-cell lymphomas and rarely solid tumors, are known to be potential triggers for HLH 1. Moreover, a number of metabolic disorders, including multiple sulfatase deficiency, lysinuric protein intolerance, Wolman's disease, and disorders of proprionate metabolism, have been associated with HLH 28323334. Finally, immunosuppressive therapy for malignancies, after organ trans-plantation, or for autoimmune disorders may predispose to 'secondary' HLH 28353637.

Macrophage activation syndrome (MAS) is a potentially life-threatening complication of autoinflammatory and autoimmune diseases and may be classified as a variant of 'secondary' HLH. When fully developed, the clinical features of MAS are indistinguishable from HLH. How-ever, patients with MAS may show distinct findings like neutrophilia or thrombocytosis in early disease stages, which are unusual in patients with primary HLH 38.

MAS is rare, but it is estimated that up to 7 to 30% of patients with active systemic juvenile idiopathic arthritis (Still's disease) experience some form of MAS, ranging from subclinical or mild to full-blown disease, with a mortality of up to 22% 38. MAS is most frequent in systemic juvenile idiopathic arthritis, but has been observed in Kawasaki's disease, systemic lupus erythematodes, and other rheumatic diseases 38. There are some reports of MAS in patients after treatment with anti-TNF-α antibodies for rheumatological disease 39.

Specific diagnostic criteria for MAS have been suggested. They include falling leukocyte and platelet counts, hyperferritinemia, hypofibrinogenemia, hemophagocytosis in bone marrow, elevated liver enzymes, elevated erythrocyte sedimentation rate, and hypertriglyceridemia 4041. MAS can be the presenting feature of patients with autoinflammatory and autoimmune diseases and features of such diseases (such as arthritis or rash) should there-fore be carefully looked for in HLH patients in the course of their disease.

One of the main immune defense mechanisms against infections with intracellular pathogens is contact-dependent cytotoxicity mediated by CTLs and NK cells 42. After recognition of infected cells and formation of an immunological synapse, cytotoxic granules, containing perforin and granzymes, are polarized to the contact site between effector and target cell and are released into the intercellular space, where they can mediate their cytolytic effector function 43. Importantly, lymphocyte cytotoxicity is not only directed at infected cells, but also against antigen-presenting cells (APCs). Elimination of APCs provides an important negative feedback to limit T-cell-mediated immune responses. There is evidence that CTLs are more important in the pathogenesis of HLH than NK cells, but immune regulation through killing of APCs has also been described for NK cells 444546. In the absence of effective cytotoxicity, APCs continue to stimulate CTL 47, leading to ongoing production of cytokines, in particular IFN-γ, which has a key role in macrophage activation. Activated T cells and macrophages infiltrate tissues such as the liver, bone marrow and central nervous system, secrete cytokines and show excessive phagocytic activity. IFN-γ and TNF-α have toxic effects on hematopoietic cells, contributing to cytopenias. TNF-α also inhibits lipoprotein lipase, causing hypertriglyceridemia 1, and IL-1, IL-6, and TNF-α induce fever 1. Activated macro-phages secrete ferritin as well as plasminogen activator, while activated T and NK cells shed their IL-2 receptor, further contributing to the characteristic laboratory abnormalities of HLH. Data from animal models suggest that IFN-γ is the key cytokine involved in this inflammatory cascade and IFN-γ blockade is an effective treatment for HLH in mice 45.

FHL and the immunodeficiencies associated with albinism are associated with defective lymphocyte cytotoxicity. In FHL2, perforin itself is defective, while the other diseases affect proteins involved in the biogenesis, intracellular transport and exocytosis of perforin-containing lytic granules (Figure 1 and Table 1) 14. The complex syndromic nature of the albinism disorders can be explained by the similarities in the molecular machineries of vesicle trafficking, including pigment transport in skin and hair or degranulation of platelets and mast cells.

It remains to be determined whether the above stated concept of pathogenesis is sufficient to explain the immune dysregulation of HLH or whether an additional immune regulatory role of perforin - directed at immune cells other than APCs - has to be postulated. Another open question is whether an external triggering factor, such as an infection, is always required (but not always detected) or whether presentation of auto-antigens and APC activation by endogenous inflammatory triggers may be sufficient to trigger disease. In any case, the fulminant nature of this highly inflammatory disease indicates a key role for lymphocyte cytotoxicity in limiting physiological immune reactions.

At least four other primary immunodeficiencies predispose to HLH, the two X-linked disorders SAP and XIAP deficiency and the autosomal-recessive ITK deficiency and CD27 deficiency 22232526. In all four diseases, development of HLH is almost exclusively triggered by EBV infection 22232526. SAP deficiency is characterized by an impaired T cell-B cell interaction, which also involves an inability of cytotoxic T cells to lyse B cells, which are the main target cells of EBV 48. ITK and CD27 deficiency are characterized by a poor control of EBV infection, but as in XIAP deficiency, the molecular mechanisms predisposing to HLH so far remain elusive. Of note, NK T-cell development, which may also be relevant for control of EBV infection, is impaired in all four diseases 2425264950.

The pathogenesis of 'secondary' HLH is less well understood. Cytotoxic lymphocyte degranulation and cytotoxicity are not impaired in most cases 51. Nevertheless, the balance between APC activation and CTL-mediated control may be disrupted through increased APC activation. Intracellular pathogens can activate APC directly - for example, via toll-like receptor (TLR) activation. TLRs could also be stimulated by anti-DNA antibodies in systemic lupus erythematosus 4452. Data from a mouse model indicate an important protective role for IL-10 in 'secondary' HLH induced by TLR9 stimulation 44.

The extent of hyperbilirubinemia, thrombocytopenia, hyperferritinemia and cerebrospinal fluid pleocytosis seem to be key risk factors for early death in HLH, as are lack of improvement in hemoglobin or fibrinogen levels, persisting thrombocytopenia and persistent fever after the start of therapy 58. In EBV-induced HLH, a high viral load is associated with a poor outcome 31.

Untreated primary HLH is rapidly fatal within a few weeks 5. Prompt and adequate treatment is of crucial importance for a positive outcome. Therapy should be started in all cases with high suspicion after diagnostic tests have been initiated but regardless of whether the results of all examinations have been obtained. The initial therapy consists of immunosuppressive and/or chemotherapeutic agents and aims at suppressing the hyper-inflammatory component of the disease as well as eliminating activated cytotoxic lymphocytes and macro-phages. Steroids inhibit inflammation by attenuating cytokine responses and inhibiting differentiation of dendritic cells and also have cytotoxic effects on lymphocytes. Cyclosporin A directly affects CTL activation as well as macrophage function. Etoposide induces apoptosis in lymphocytes as well as in APCs. Anti-thymocyte globulin (ATG) directly targets T cells, whereas alemtuzumab, an anti-CD52 antibody, targets lymphocytes in general and APCs. Intrathecal therapy with methotrexate and steroids is targeted at central nervous system disease.

Immunochemotherapy is widely used to induce and maintain remission until HSCT in primary HLH. The HLH-2004 protocol 59 consists of a two-week induction phase including etoposide, cyclosporin A, dexamethasone, intrathecal methotrexate, and intrathecal prednisone followed by a tapering phase of 6 weeks. If HSCT is planned, patients are placed on continuation therapy consisting of cyclosporine A and biweekly pulses of etoposide and dexamethasone 59. Remission could be achieved in 78% of all patients treated with the HLH-94 protocol (which included cyclosporin A treatment only in the continuation phase) 68.

A more targeted immunotherapeutic protocol has mainly been used by a single center with similar survival rates compared to the HLH-94 protocol 69. This regimen consists of ATG (rabbit) and methylprednisolone. There-after, cyclosporin A is given until HSCT, generally allowing tapering of methylprednisolone. Patients with central nervous system disease also receive intrathecal methotrexate and corticosteroids. First experiences with alemtuzumab have also yielded promising results 7071.

There are not sufficient data to indicate if and which patients with 'secondary' HLH need the full treatment protocol. Although initial treatment with steroids alone or in combination with cyclosporin A may be justified in some patients, the timely use of more aggressive therapy is mandatory for a good outcome. In patients with MAS, immunosuppression with corticosteroids with or without cyclosporin A in most cases leads to a dramatic improvement of disease within days 7273. Etoposide may be added if there is no response or highly active disease 38.

Control of the underlying disease is of key importance in the overall treatment concept. For rheumatological disease this in particular involves the use of IL-1 receptor antagonists and IL-6 antibodies in Still's disease and anti-TNF in some other rheumatological diseases 7475. Whether these agents may also trigger MAS is a matter of debate. Of note, they have been shown to be of limited use in the treatment of primary HLH. Control of infectious disease is of similar importance both in primary as well as 'secondary' HLH. EBV-triggered HLH has been treated successfully with rituximab (anti-CD20 antibody) in addition to conventional therapy 27767778. Immunoglobulins, which may act against pathological antigens or cytokines, have been used as an adjunct in infection-triggered HLH 127.

HSCT is recommended in genetic cases, in patients with recurrent HLH and disease progression despite adequate therapy 27. For genetic cases it is the only curative option. To avoid delays in starting HSCT, HLA typing and donor search should be initiated as soon as the diagnosis of primary HLH is established 27. Due to the highly inflammatory nature of HLH, myeloablative conditioning has been the standard regimen for many years. However, venoocclusive disease is a well-known complication when using busulfan-based protocols, occurring in up to 25% of HLH patients 7980. Recently, reduced-intensity conditioning regimes have been used successfully and with a much lower incidence of veno-occlusive disease 8182. Mixed chimerism can be a relevant limitation, but murine studies and some observations in patients indicate that a stable long-term level of about 10 to 15% donor chimerism may be sufficient to maintain remission 8081.

Despite advances in therapy, up to 40 to 60% of children initially do not respond to treatment and die of HLH or die of infections or complications during therapy 2783. Active HLH at the time of HSCT and central nervous system involvement are associated with a worse outcome 8485. The HLH-94 protocol induced remission or allowed the patient to undergo HSCT in 71% of cases 85. Patients with a positive family history and who received HSCT after induction therapy according to the HLH-94 protocol had a 5-year survival of 50% 85. Patients with all forms of HLH who were treated according to the HLH-94 protocol had a 5-year survival of 54%. The patient group that underwent HSCT showed a 5-year survival of 66% 85. The ATG-based therapeutic regimes induced remission of HLH in 73% of patients with FHL in one center, and patients had an overall survival after HSCT of 55% 69. After induction therapy with varying therapeutic regimes, a survival rate of 58.5% 6 years after HSCT has been reported in a single center study 80. Survival rates of 86% have been reported in EBV-associated 'secondary' HLH after HSCT in Japan 86. Malignancy-related HLH had the worst prognosis of all forms of HLH. A Japanese survey on HLH in all age groups showed a 5-year survival of <15% in malignancy-associated HLH 6.

Long-term complications of HLH encompass therapy-related morbidity, especially after HSCT, and neurocognitive deficits. The latter can manifest months to years after HLH, but fortunately most patients can return to normal life 87.