All 16 patients with fatal influenza A (H1N1) infection at the University of Michigan Hospitals from 2009-2014 who underwent post-mortem examination were evaluated for the presence of hemophagocytic lymphohistiocytosis (HLH). All of these patients met at least two criteria for clinical diagnosis of HLH and 14 met criteria for macrophage activation syndrome (MAS). Thirteen (81%) had histopathologic evidence of hemophagocytosis at autopsy and by record review; all of these patients had clinical and laboratory features of HLH using modified HLH-2004 and macrophage activation syndrome criteria. Fourteen specimens underwent whole-exome sequencing. Known familial HLH gene mutations were identified in PRF1 (two patients) and LYST (four patients). In addition, other HLH variant genes of interest were identified in five patients. The patient-derived PRF1 mutant cDNA was generated from wild type PRF1 cDNA by site-directed mutagenesis and cloned into a lentiviral expression vector, then transfected into HEK293T NK cells. The NK cells expressing lentiviral-transduced mutant PRF1 demonstrated reduced cytotoxicity against K562 erythroleukemia target cells.

COMMENTARY

HLH remains a partially understood disease, but interesting insights into its pathogenesis have been gleaned over the past decade. Familial HLH (fHLH) is a rare autosomal recessive disorder manifested by severe multisystem inflammation, which is generally fatal without hematopoietic stem cell transplantation. fHLH has been shown to be caused by mutations affecting the cytolytic pathways of NK and CD8+ T-cells, especially the genes encoding perforin and proteins essential for the trafficking and fusion of perforin-containing granules. Reactive HLH (rHLH) tends to occur in older children and adults in response to a variety of infections. Its pathogenesis is less well understood than is fHLH, but recent reports show that many of the same (or closely related) gene variants associated with fHLH are present in cases of rHLH.

It has been known for many years that patients with severe, life-threatening H1N1 influenza often manifest features of HLH/MAS, including hyperinflammation, pancytopenia, coagulopathy, and liver dysfunction. The demonstration in this study that the majority of patients with fatal influenza who underwent autopsy were found to have histopathologic evidence of hemophagocytosis sheds light on the pathogenesis of severe influenza. The additional demonstration of gene variants known to be associated with familial HLH is fascinating and certainly points to the need for further study of the pathogenesis of severe influenza with an eye toward eventual exploration of potential interventions to inhibit the overwhelming inflammatory response seen in severe influenza infection.