Immune cartography of macrophage activation syndrome in the COVID-19 era

Abstract

A hyperinflammatory 'cytokine storm' state termed macrophage activation syndrome (MAS), culminating from a complex interplay of genetics, immunodeficiency, infectious triggers and dominant innate immune effector responses, can develop across disparate entities including systemic juvenile idiopathic arthritis (sJIA) and its counterpart adult-onset Still disease (AOSD), connective tissue diseases, sepsis, infection, cancers and cancer immunotherapy. Classifying MAS using the immunological disease continuum model, with strict boundaries that define the limits of innate and adaptive immunity, at one boundary is MAS with loss of immune function, as occurs in the 'perforinopathies' and some cases of sJIA-AOSD. Conversely, at the other boundary, immune hypersensitivity with gain of immune function in MHC class II-associated sJIA-AOSD and with chimeric antigen receptor (CAR) T cell therapy also triggers MAS. This provides a benchmark for evaluating severe inflammation in some patients with COVID-19 pneumonia, which cripples primary type I interferon immunity and usually culminates in a lung-centric 'second wave' cytokine-driven alveolitis with associated immunothrombosis; this phenomenon is generally distinct from MAS but can share features with the proposed 'loss of immune function' MAS variant. This loss and gain of function MAS model offers immune cartography for a novel mechanistic classification of MAS with therapeutic implications.

Fig. 1. The cellular basis for MAS and allied disorders.

Cytokine storm can manifest in different ways. It is important to point out that these scenarios can rapidly and simultaneously evolve in some settings in which scenarios division is artificial. a | Where the monocyte system comes into direct contact with the circulation in the liver, spleen and bone marrow, a physiological mechanism exists for the removal of worn red blood cells (RBCs). A predominant dysregulation of this mechanism in the context of a cytokine storm leads to the ‘classical’ macrophage activation syndrome (MAS) phenotype. This phenotype is associated with severe systemic inflammation and can trigger a disseminated intravascular coagulation (DIC). b | Cytokines predominantly affecting the endothelial system give rise to a different cytokine storm scenario, characterized by a capillary leak syndrome (CLS) that manifests with hypotension, diffuse tissue oedema and hypoalbuminaemia as key features. c | Organ-specific bacterial or viral infection and/or septicaemia can trigger a cytokine storm that can lead to specific dysfunction of that organ (for example, the lung with SARS-Cov-2 infection) and contribute to progressive multi-organ dysfunction without specific features of MAS or CLS. The influence of adjuvant and cytokine release on pattern recognition and cytokine receptors and the site of organ infection can trigger local or systemic inflammation. Unlike the classical MAS pathology, in which lymphoid dysregulation and high levels of IFNγ can promote pathology, this scenario might be more strongly linked to tissue-specific dysfunction with comparatively low levels of IFNγ, as the pathology is more innate immune-driven. The inflammatory state triggered by COVID-19 better fits this scenario of organ-specific dysfunction related to infection. d | A MAS-like state dominated by coagulopathy due to intravascular viral infection of monocyte lineage cells with cytokine release and prominent clotting cascade activation can exhibit features of DIC. Unlike the DIC of classical MAS, however, this DIC reflects a viral tropism for macrophages. This cytokine storm scenario is typically seen with haemorrhagic viral infection that usually involves circulatory myeloid cells and manifests as diffuse bleeding in addition to MAS-like features. GM-CSF, granulocyte–macrophage colony-stimulating factor.

Fig. 2. The MAS phenotype in relation to physiological erythrocyte disposal.

a | Erythrocytes, which lack a nucleus and cannot undergo apoptosis, are physiologically removed by the reticuloendothelial system by macrophages that are strategically juxta-positioned in sinusoids in the bone marrow, liver and splenic red pulp so that they have direct access to circulatory red blood cells (RBCs). The precise basis for why cytokine storm scenarios trigger macrophage activation syndrome (MAS) rather than other scenarios such as capillary leak syndrome or multi-organ dysfunction syndrome remains to be better defined. b | Experimental models show how the erythrocyte removal system is finely tuned. The normal physiological state of erythrocyte disposal involves M2 macrophage polarization. In experimental settings, perturbations towards T helper 1 (T_H1) cytokines (including IFNγ) or T_H2 cytokines (via excessive IL-4 production) lead to haemophagocytosis, in an exaggeration of the normal physiology. Experimentally, IL-10 production by haemophagocytosing macrophages exerts an anti-inflammatory effect in the local environment, thus representing counterbalancing of haemophagocytosis. GM-CSF, granulocyte–macrophage colony-stimulating factor.

Fig. 3. MAS spectrum from immunodeficiency to immune hypersensitivity.

In classical monogenic primary haemophagocytic lymphohistiocytosis, the failure of the first-line of defence, especially natural killer cells, and also CD8⁺ T cells, leads to persistence of viral antigens. Inability to remove infected immune cells leads to persistent activation of dendritic cells and other antigen-presenting cells and widespread priming of CD8⁺ T cells (which again can be ineffective at killing) and also expansion of T helper 1 (T_H1) CD4⁺ T cell clones. An IFNγ-driven pathology via hyper-activation of macrophages resulting in collateral damage including haemophagocytosis, liver pathology, coagulopathy and other manifestations; other T_H1 cytokines, including GM-CSF, could also contribute to macrophage activation. Engineered gain of function as part of chimeric antigen receptor (CAR) T cell therapy leads to a similar but monophasic phenotype that may include macrophage activation syndrome (MAS) phenotype features and that subsides with elimination of the target antigen (cell population). Also, given that immunodeficiency in the perforin pathway is evident in <40% of systemic juvenile idiopathic arthritis (sJIA) cases and that sJIA is often associated with MHC-II, it is credible that the sore throat at the onset of sJIA and adult-onset Still disease (AOSD) results from a CD4⁺ helper T-cell hypersensitivity reaction following a quickly eliminated non-specific viral trigger; putative antigens await definition. Overlapping immunopathogenic mechanisms could exist. Despite compelling evidence for adaptive T cell responses in both the gain and loss of function settings, sJIA-related and AOSD-related MAS have been placed into the autoinflammatory or innate immunopathology category of disease, rather than along the immunological disease continuum.

Fig. 4. Classification of the MAS spectrum.

The autoinflammatory presentation of macrophage activation syndrome (MAS) can be seen in the context of an integrated immune response. Accordingly, the immunological disease continuum of inflammation against self, modified according to loss and gain of function, can be applied to MAS spectrum disorders. The systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still disease (AOSD) phenotypes span both loss of function and gain of function in innate and adaptive immunity. Likewise, severe inflammation in systemic lupus erythematosus (SLE), another genetically heterogeneous disease, can trigger MAS by mechanisms including both loss and gain of immune function. Rare monogenic disorders leading to MAS can also exhibit loss or gain of immune function. MAS triggered by cancer immunotherapy, including chimeric antigen receptor (CAR) T and bispecific T cell engager therapy, represents a type of gain of function in adaptive immunity. Myeloid malignancy falls in the predominantly innate immune gain of function section. Occasionally, MAS can be triggered in COVID-19, but the dominant mechanism awaits full description. EBV, Epstein–Barr virus; HLH, haemophagocytic lymphohistiocytosis; ICI, immune checkpoint inhibitor; NK, natural killer; XLP, X-linked lymphoproliferative disease.

Fig. 5. MAS in the context of experimental therapeutics.

In a ‘classical’ macrophage activation syndrome (MAS) reaction (upper right), antigen-presenting cells and T cells contribute to macrophage activation and hypercytokinaemia. Severe viral infection (by SARS-Cov-2 and other viruses) not controlled by first-wave type I interferon responses (lower right) is associated with strong myeloid infiltration and a second wave of potent pro-inflammatory cytokines, and with strong local immune activation within the lung or other tissues. Many myeloid-related and tissue-derived cytokines, including IL-1, IL-6, TNF and GM-CSF, granulocyte–macrophage colony-stimulating factor (GM-CSF), are elevated in this second wave, but IFNγ is not particularly high. Unlike MAS, concentrations of cytokines and ferritin are not particularly elevated, but in some cases of viral infection classical MAS can develop. Treatment of MAS is being empirically defined by experimental medicine. Currently, anti-IL-6 therapy is effective for MAS associated with cancer immunotherapy but not for MAS associated with systemic juvenile idiopathic arthritis (sJIA) or adult-onset Still disease (AOSD). IL-1 receptor antagonist (IL-1Ra) is effective for MAS in sJIA and AOSD and in experimental models of cytokine release syndrome in oncology. IL-1β blockade is effective for sJIA but not for sJIA-associated MAS. IFNγ antagonism has shown some efficacy in sJIA-associated MAS and primary haemophagocytic lymphohistiocytosis, but according to this scheme would not be predicted to affect viral pneumonia in most instances. Corticosteroids affect the full spectrum of inflammation associated with sJIA–AOSD to MAS and, given the multi-cytokine and multi-cell inhibition of Janus kinase inhibitors (JAKi) and preliminary emerging data, these agents will also probably have a broad impact. CAR, chimeric antigen receptor.

Fig. 6. Tissue basis for cytokine storm phenotypes encompassing COVID-19.

a | Dysregulation of inflammasomes and particularly NLRC4 leads to excessive production of IL-1β and IL-18 by intestinal epithelial cells and myeloid cells, causing generalized antigen-independent T cell expansion with production of IFNγ and other cytokines and a macrophage activation syndrome (MAS)-like pattern as well as intestinal pathology reflecting dysregulated intestinal mucosal cytokine production. b | Myeloid or lymphoid cell transformation can lead to MAS. Myeloid malignancy with associated gain of function can contribute to a MAS-like picture. Likewise, gain of function in the lymphoid system, as in lymphoid malignancy or lymphoid viral infection associated with immunodeficiency and transformation, can also activate macrophages. c | A cytokine release syndrome leading to MAS can occur in the setting of perforinopathy-associated immunodeficiency. Likewise, the immune activation that occurs with a hypersensitivity reaction to an infectious trigger has the same effect, with excessive production of IFNγ and other cytokines with activation of macrophages. Based on lessons from the monophasic disease linked with chimeric antigen receptor T cell therapy and from the associations of MHC II with systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still disease (AOSD), some of the MAS phenotypes encountered in rheumatology, including cases of MAS associated with sJIA, AOSD and systemic lupus erythematosus, fit with this lymphoid tissue-originating disease. d | A cytokine release syndrome with MAS can occur in the setting of excessive stimulation of Toll-like receptors (TLRs), including TLR9 and TLR4 stimulation in IL-6 overexpression models. The experimental phenotype may correlate with severe sepsis in humans, including viral and bacterial sepsis, which strongly and directly stimulates myeloid cells. Gain-of-function mutations related to innate immunity and factors such as inflammaging could drive this pattern of cytokine storm, which is more organ-centric. Viral infections with a tropism for mononuclear cells or macrophages, such as haemorrhagic virus, could lead to this scenario. e | Immune cell activation, as shown in part d, in a setting such as severe COVID-19, could drive organ- or tissue-specific damage rather than lymphoid or myeloid pathology changes seen in MAS. Severe localized organ-specific inflammation could be responsive to anti-inflammatory strategies, even though the compartmentalized inflammatory response might not trigger marked hypercytokinaemia. Parts b, d and e also illustrate how viral, pathogenic cell or organ tropism profoundly influences the disease phenotype. The tropism of the SARS-CoV-2 virus for the alveolus determines what is generally a very distinct disease phenotype that may occasionally be associated with a more typical MAS phenotype evolution.