Hemophagocytic Lymphohistiocytosis and Other Cytokine Storm Syndromes in Adults

Hemophagocytic lymphohistiocytosis (HLH) is a rare and highly fatal syndrome of pathological immune activation leading to excessive inflammation, hypercytokinemia, and multi-organ failure. HLH is broadly divided into primary HLH, driven mainly by genetic defects in cytotoxicity and secondary HLH, a heterogeneous group of disorders with similar clinical and laboratory features to primary HLH, but characterized by hyperinflammation rather than defective cytotoxicity. Primary HLH occurs nearly exclusively in children. Most adult HLH is secondary, often in the context of immunomodulatory therapy, infection, malignancy, autoimmune/autoinflammatory diseases, or immunodeficiency. HLH falls under the umbrella concept of cytokine storm syndrome (CSS). In 2020, the coronavirus disease 2019 (COVID-19) pandemic greatly amplified clinical interest and research in CSS, and specifically the concept of a maladaptive immune response to infection. Early on, COVID-19-CSS was compared to HLH. However, HLH is mainly driven by the interferon-γ (IFN-γ)-chemokine ligand 9 (CXCL-9) axis, resulting in profound T cell and macrophage activation, and is characterized by very high ferritin and soluble CD25 (sCD25, synonymous with the alpha chain of the soluble interleukin (IL)-2 receptor), often with only modestly elevated C-reactive protein (CRP). In contrast, COVID-19-CSS is characterized by defective type I/type III interferon responses leading to excessive IL-6 signaling and very high CRP, which can be ameliorated by IL-6 inhibition. The increased interest in CSS spurred by COVID-19 has coincided with significant recent advances in our understanding of other CSS, such as thrombocytopenia, anasarca, fever/(reticulin) fibrosis, organomegaly, renal dysfunction (TAFRO) syndrome (typically associated with idiopathic multicentric Castleman disease, iMCD-TAFRO) and severe or catastrophic Still’s disease. This review will provide practical guidance for clinicians in diagnosing adult HLH, differentiating it from TAFRO syndrome and Still’s disease. Specifically, in section 3 and Table 2, we propose a heuristic (problem-solving strategy or shortcut) to decrease cognitive load when faced with an acutely ill patient with evolving CSS, with a focus on simple and readily available inflammatory markers (CRP, ferritin, sCD25). This heuristic can help clinicians make diagnostic and therapeutic decisions in real time.