A Review on Macrophage Activation Syndrome

MAS, which is currently grouped under secondary or acquired haemophagocytic lymphohistiocytosis (sHLH), is a rare and fatal disorder that results from excess activation of T-cells and macrophages. Though the pathogenesis of MAS is poorly understood, various proinflammatory cytokines like interleukins (IL-1, IL-6), tumor necrosis factor α (tNF α), interferons are thought to play significant roles. MAS is associated with various clinical features such as non-remitting fever, bleeding, cytopenias, splenomegaly, hepatic dysfunctions, increased levels of triglyceride, ferritin and decreased levels of albumin and fibrinogen. Early diagnosis and interventions are crucial to reduce mortality risk but diagnosis is not often easy due to persistence of wide range of features that overlap with other rheumatic diseases, most commonly sJIA (systemic juvenile idiopathic arthritis). Corticosteroids and cyclosporins are commonly used for MAS treatment. Intravenous immunoglobulins, biologic agents like IL-1 blockers (anakinra, canakinumab), IL-6 blockers (tocilizumab) are also frequently used. Moreover there is still the need of genetic and immunohistological study in order to understand the exact mechanism of the syndrome development and establishment of novel therapies with lesser toxicities.

The laboratory results in MAS are not specific, however the frequently observed changes ( Table 2) are increase in hepatic enzymes, bilirubin, triglycerides, ferritin, D-dimer and lactate dehydrogenase while there is decrease in levels of sodium, hemoglobin, blood cells like leucocytes and platelets, albumin, fibrinogen, clotting factors like II, VII and X. Patients also show elevated partial thrombo-plastin time along with the presence of degradation product of fibrinogen. Levels of various cytokines are also found to be elevated. Some of the examples are interleukins (IL-1, IL-2, IL-6), tumor necrosis factor α (TNF α), interferons (INF α, INF γ) and macrophage colony stimulating factor (MCSF) 17 .
Bone marrow biopsy shows the highly elevated number of well differentiated macrophages that actively phagocytize hematopoietic cells. However hemophagocytosis may not be detectable during the initial stages and may lack sensitivity in 40% of cases 19 . Other tissues like lymph node, spleen and liver also exhibit hemophagocytic pattern but these sites are rarely explored in literatures.

Type of MAS
MAS is basically categorized as subtype of secondary hemophagocytic-lymphohisto-cytosis (sHLH) 21 . HLH is broadly categorized into two distinct types as: Primary HLH ( inherited or familial form) Secondary HLH ( acquired form) 19,22 . Primary HLH is autosomal recessive in nature with high incidence in infants (80% cases) compared to adults 23 . It is characterized by the following features: Defective T-cell activation Defective NK cell mediated cytolytic pathway Excess production of cytokines that result in uncontrolled activation T-cells and macrophages 24 .

Diagnosis
It is often difficult and challenging to properly diagnose MAS as it can mimic infections or malignancies or may be present as the complications of sJIA. Diagnosis is also difficult in ealrly phases due to lack of pathognomic clinical findings. In 1991, the criteria for diagnosis of MAS was first proposed by a scientific team studying histocyte disorders. These criterias were later modified in 2004 25 . According to this modified diagnostic guideline, the patient is supposed to be suffering from MAS if any one of the following characteristics is met: Presence of gene mutations (PRF, UNC13D, STX11) associated with HLH 18 or 5 out of 8 laboratory and clinical diagnostic criterias. These include cytopenia, increased levels of triglyceride/fibrinogen, hemophagocytosis, low or absence of NK cell activity. Increase in ferritin and soluble CD25, fever and hepatospleenomegaly 21 (Table 3).
However, several researches have focused on challenges in applying 2004 diagnostic criteria for MAS, as sJIA, the most frequently associated rheumatologic disorder with MAS 26 is independently associated with hyperferritinemia, anemia 27 and leucocytosis 28 in absence of MAS. Such overlap of clinical features may pose difficulty in early diagnosis of MAS onset 29 . Likewise coagulopathies which are not only common in MAS 30 but also in other disorders can further complicate the sample collection procedure for histological diagnosis 25 . To address these issues, Raveli et al. 7 proposed a preliminary diagnostic criteria for MAS associated with sJIA in 2005. According to this guideline, diagnosis of MAS needs the fulfillment of ≥2 laboratory and ≥2 clinical criterias ( Table 4).
The preliminary diagnostic guideline of Raveli et al were assessed by Davi et al, who compared it with that of 2004 diagnostic guidelines. They concluded that preliminary  diagnostic criteria showed best results with high sensitivity and specificity of 86% 31 . Another diagnostic criteria for MAS associated with sJIA as also approved by European league agains Rheumatism/ American College of Rhematology (EULAR/ACR) in 2016 (Table 5) 32,33 .

Cytokine storm in MAS
Many studies have reported high levels of cytokines, TNF receptors and IL-1R antagonists in MAS 34 . Cytokines present include proinflam-matory cytokines like those derived from lymphocytes (INF γ, IL-2) and cytokines produced by monocytes and macrophages (IL-1β, TNF γ, IL-6 and IL-18). Regulatory cytokines such as IL-10 are also elevated in MAS 34 . Patients with severe MAS may disturb the regulatory pathways of IL-10 which can lead to uncontrolled inflammation. Stimulation of toll like receptor 9 along with blockade of IL-10 receptor can lead to more severe complications 35 .
IL-1α, a proinflammatory cytokine, is considered to be the central one in pathogenesis of disease. It is initially secreted in an inactive form but when cells are activated, caspase-1 protein cleaves pro IL1α to its active form. On activation, it causes further activation of lymphocytes and endothelial cells thereby stimulating production of other inflammatory cytokines like IL-6 36 . Fig. 3. Failure of cytolytic killing mechanism of T cells and NK cells due some genetic defects (such as defect in perforin gene), can also promote MAS, as defective cytotoxicity casuses continuum of antigenic stimulation leading to persistent T and NK cell activation and proliferation. Cytokines are persistently released from activated cells that further stimulate macrophages leading to cytokine storm.

Pathophysiology
The etiology and specific pathogenesis of MAS are yet to be clearly elucidated. However most of the theories derived rely on the study of primary HLH which is clinically quite similar to MAS. It is stated that hemophagocytosis can develop due to dysregulation of immunologic phenomenons resulting in the exaggeration of inflammatory responses which is characterized by excessive activation of macrophages and T lymphocytes that ultimately leads to cytokine storm 37 .
It has been reported that numerous proinflammatory cytokines are released. It is also demonstrated that hyperactive T-cells and macrophages reside in various organs and produce cytokines like INF γ, TNF α, IL-6 and IL-1 which play a major role in pathogenesis of MAS. The condition is further aggravated by the presence of perforin deficiency 38 . Perforin is a type of cytotoxic protein secreted by T-cells and NK cells to destroy virus infected cells. It is also involved in regulation of lymphocyte prolife-ration. Mutation in gene coding for perforin is linked with MAS as it can lead to T-cell and NK cell dysfunction 39,40 and persistent lymphocyte activation 38 .
IL-1β via its receptors signals production of IL-6 41 that plays a central role in inducing acute phase response 40 . Some researchers have shown that imbalance between IL-18 and its inhibitor results in activation of macrophages and Th-1 lymphocytes that have bypassed NK cell mediated cytotoxic control, leading to the manifestation of sHLH  . IL-18 belongs to IL-1 family and stimulates the production of INF γ (by NK cells and T-cells), TNF γ and chemokines (by macrophages) 41 . The major role of INF γ is to cause profound activation of macrophages and monocytes. The increased levels of INF γ is also well correlated with clinical and laboratory features of MAS 43 .

Treatment
There is necessity of urgent management of MAS. The preliminary purpose of the therapy used should be control of hyper inflammatory state with the use of immunosuppressive or cytotoxic drugs. The most commonly used drugs are corticosteroids and cyclosporins. However other therapeutic agents like etanercept, etoposide, immunoglobins in high dose and plasmapheresis are also used in practice 44 .
Steroids such as methylprednisolone and dexamethasone are the first line drugs of choice as the response of patients is usually prompt. However steroids are rarely used as monotherapy for long term due to their side effects 45 . In case of MAS unresponsive to steroids alone, a combination therapy consisting of steroids and cyclosporine is given. Cyclosporin is an inhibitor of T-cell function and mostly administered in early stage of the syndrome 25 . It also has membrane stabilizing effect on macrophages 46 .
Etoposide is a bone marrow suppressant and its use can result in severe infection, therefore caution is to be taken in patients having renal or hepatic impairment 47 . As per Coca et al, antithymocyte globulin (ATG) could be used in place of etoposide 48 but it may be associated with infusion reactions 49 .
In case of MAS associated with sJIA, IL-1 inhibitor (such as anakinra) is being used increasingly 50 . Though the exact role of IL-1 in pathogenesis of MAS is unclear, it is expected that IL-1 inhibition may aid in better control of the disease. According to the several reports, Il-1 inhibitors have proven better treatment choice in  51 . Successful treatment of MAS associated with ASOD using IL-1 blockers have also been documented 46 . However according to the recent report, of 23 patients with sJIA who were treated with anakinra, one patient developed MAS 52 and in another report it was stated that anakinra might have triggered MAS in 5 out of 46 patients of sJIA 53 . IL-6 blockers (like tocilizumab) is also used as the treatment strategy among the patients who could not tolerate IL-1 46 .
Activated macrophages produce IL-6 which may further amplify the macrophage response to the proinflammatory stimuli 54 . However, as per the Japaneses study, tocilizumab when used in the treatment of ASOD gradualy progressed the patient to MAS 55 . Reports regarding development of cytopenias following the use of tocilizumab in patients with sJIA (complicated with MAS) are also available 56

CONCLUSION
MAS, though rare, is a potentially fatal disorder that requires prompt and appropriate treatment to prevent deleterious outcomes. Since there is lack of valid criteria for the diagnosis of MAS, the treatment appears highly variable at different clinical sectors. However the most commonly used treatment is the use of corticosteroids as monotherapy or along with cyclosporins or immunoglobulins. In some cases use of etoposide and IL-1 bockers are also recommended.
As MAS is mostly associated with rheumatic disease, clinicians should be aware of diagnostic principle and management. It is necessary to understand the underlying pathophysiology of MAS in order to identify the pathways involved in early phase of the syndrome so that better ideas could be developed in furnishing newer treatment biomarkers. Additionally larger cohorts regarding immunological and genetic studies must be carried out to formulate more effective and appropriate therapy as well as to improve mortality. Also studies have shown that the cytokine storm is the main driving force for pathology of hyper-inflammatory syndromes. Therefore therapeutic approaches blocking cytokine function might be fruitful with less toxic effects.