The Pathophysiology of Hemophagocytic lympho-histiocytosis (HLH) syndrome and insights from animal models.

Abstract

Hemophagocytic lymphohistiocytosis (HLH) is a severe hyperinflammatory syndrome characterized by an aberrant immune response against host tissues. It can arise from diverse triggers like infection, inflammation, malignancy, genetic defects, or therapy-related factors. Cytokine storm, capillary leak syndrome, cytokine release syndrome, and macrophage activation syndrome are the different faces of this chimera, and each of them displays significant clinical variability associated with high mortality. The pathogenesis of both primary and secondary HLH generally follows a similar pattern, involving excessive activation of macrophages and uncontrolled destruction of reticuloendothelial tissues. Environmental triggers cause exaggerated activation of innate immune cells in genetically predisposed individuals. This process is further driven by the release of multiple cytokines and soluble mediators that sustain ongoing inflammation and cause subsequent target organ damage. Biomarkers, including cytokines and inflammatory mediators, are crucial for early detection and monitoring treatment response. Persistent immune activation and inadequate resolution mechanisms result in a destructive inflammatory cascade or "immunological massacre". Animal models of HLH and MAS elucidate the roles of impaired cytotoxicity, IFN-γ, TLR signaling, and inflammatory cytokines in disease pathogenesis. Trigger-specific differences highlight the involvement of CD8+ T cells, NK cells, macrophages, and cytokines. Therapeutic strategies include cytokine neutralization, adoptive T-cell transfer, and mTOR inhibition. Timely diagnosis and prompt initiation of therapy are essential to mitigate the serious consequences of HLH and improve long-term outcomes.