The Microbiome Modifies Manifestations of Hemophagocytic Lymphohistiocytosis in Perforin-Deficient Mice

Primary hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome caused by inborn errors of cytotoxicity. Patients with biallelic PRF1 null mutations (encoding perforin) usually develop excessive immune cell activation, hypercytokinemia, and life-threatening immunopathology in the first 6 months of life, often without an apparent infectious trigger. In contrast, perforin-deficient (PKO) mice only develop HLH after systemic infection with lymphocytic choriomeningitis virus (LCMV). We hypothesized that restricted microbe-immune cell interactions due to specific pathogen-free (SPF) housing might explain the need for this specific viral trigger in PKO mice. To investigate the influence of a "wild" microbiome in PKO mice, we fostered PKO newborns with Wildling microbiota ('PKO-Wildlings') and monitored them for signs of HLH. PKO-Wildlings survived long-term without spontaneous disease. Also, systemic infection with vaccinia virus did not reach the threshold of immune activation required to trigger HLH in PKO-Wildlings. Interestingly, after infection with LCMV, PKO-Wildlings developed an altered HLH pattern. This included lower IFN-γ serum levels along with improved IFN-γ-driven anemia, but more elevated levels of IL-17 and increased liver inflammation compared with PKO-SPF mice. Thus, wild microbiota alone is not sufficient to trigger HLH in PKO mice, but host-microbe interactions shape inflammatory cytokine patterns, thereby influencing manifestations of HLH immunopathology.