Hemophagocytic lymphohistiocytosis: A disorder of T cell activation, immune regulation, and distinctive immunopathology

Summary Hemophagocytic lymphohistiocytosis (HLH) is a disorder that has been recognized since the middle of the last century. In recent decades, increasing understanding of the genetic roots and pathophysiology of HLH has led to improved diagnosis and treatment of this once universally fatal disorder. HLH is best conceptualized as a maladaptive state of excessive T cell activation driving life‐threatening myeloid cell activation, largely via interferon‐gamma (IFN‐γ). In familial forms of HLH (F‐HLH), inherited defects of lymphocyte cytotoxic biology underlie excessive T cell activation, demonstrating the importance of the perforin/granzyme pathway as a negative feedback loop limiting acute T cell activation in response to environmental factors. HLH occurring in other contexts and without apparent inherited genetic predisposition remains poorly understood, though it may share some downstream aspects of pathophysiology including excessive IFN‐γ action and activation of innate immune effectors. Iatrogenic forms of HLH occurring after immune‐activating therapies for cancer are providing new insights into the potential toxicities of inadequately controlled T cell activation. Diagnosing HLH increasingly relies on context‐specific measures of T cell activation, IFN‐γ activity, and inflammation. Treatment of HLH largely relies on cytotoxic chemotherapy, though targeted therapies against T cells, IFN‐γ, and other cytokines are increasingly utilized.