A 33-year-old female with a history of encephalomyelitis treated with high-dose methylprednisolone in 2018 presented with neutropenia and mild anaemia in November 2021. A month later, she developed a high fever, fatigue, headache, and pancytopenia with an absolute neutrophil count (ANC) of 0.3 x 109/L, HGB of 93 g/L, and PLT of 37 x 109/L ( Figure 1E, H ). The CRP level was 131 mg/L (normal: 0-8 mg/L) ( Figure 1E ). She was admitted to our hospital. Bone marrow aspiration and biopsy demonstrated haemophagocytosis. Flow cytometric (FC) analysis of bone marrow showed no clonal abnormalities, and chromosome karyotype analysis was normal. The laboratory data revealed abnormal liver function with an ALT of 70 units/L, AST of 182 units/L, and TBil of 24 micromol/L. Coagulation detection showed a decreased level of fibrinogen (0.25 g/L) and prolonged APTT (75.4 s) ( Figure 1F, H ). Her ferritin level was 12459 microg/L and the TG level (7.04 mmol/L) was four times higher than normal ( Figure 1G ). EBV-DNA in blood plasma was 896.68 copies/ml (normal: <400 copies/ml). EBV nuclear antigen IgG (EBNA-IgG) and EBV capsid antigen IgG (EBCA-IgG) antibody titers were 235 U/ml (normal: < 5 U/ml) and 218 U/ml (normal: < 20 U/ml), respectively. Abdominal CT showed splenomegaly. The patient denied that she had repeatedly developed HLH and denied a family history of HLH. According to her clinical manifestations and laboratory parameters, she was diagnosed with HLH ( Table 1 ).
Magnetic resonance imaging (MRI) showed multiple inflammatory demyelinating lesions in the bilateral paraventricular area, bilateral radiation crown area, and bilateral frontoparietal temporal cortex. Cerebrospinal fluid pressure was high, protein content was normal, and no tumour cell infiltrations were found. The recurrence of encephalomyelitis was considered. No clear malignant, autoimmune, or infectious triggers for HLH were identified. EBV infection was suspected as a potential trigger for her HLH. The level of sCD25 and NK-cell activity were unavailable during this time in Shanghai.
The patient was given 90 mg DXM a day for 3 days. At the same time, RUX 10 mg was administered orally twice a day. The DXM dose was reduced by half every three days, finally reaching a reduced dose of 10 mg. After that, the dose was adjusted every two weeks to 1.5 mg according to the patient's laboratory examination results. Then, DXM was replaced with 8 mg methylprednisolone for maintenance treatment. Her fever was controlled on Day 2. The PLT and ANC counts had normalized on Day 7 ( Figure 1E, H ). The ferritin, fibrinogen, and TG level improved on Day 14 ( Figure 1G, H ). She achieved PR according to the response criterion on Day 14.
Treatment with RUX and a low dose of methylprednisolone were continuous. Plasma EBV-DNA load was 423 copies/ml on Day 265. EBNA-IgG and EBCA-IgG antibody titers were 185 U/ml and > 750 U/ml on Day 273, respectively. Two antibodies were still positive with high avidity. The onset of primary HLH in adults may lead clinicians to ignore or even misdiagnose the disease. To further clarify the aetiology of HLH in Case 2, the next-generation sequencing (NGS) of HLH-related genes was performed on Day 275. A mutation of the PRF1 gene, c.1349C > T (p. T450M) was reported. Two mutations of the lysosomal trafficking regulator (LYST) were also detected: c.1183C > T (p. R395C) and c.2183G > T (p. S938I). All of them were heterozygous missense mutations. The NGS results are shown in  Table 2 . Unlike PRF1 mutations, which are known pathogenic variants responsible for FHL, the clinical significance of the two LYST gene variants is unclear. FC analysis revealed the decreased expression of PRF1 in NK cells ( Figure 2A ). NK cells stimulation test showed severe dysregulation of immune response ( Figure 2B ) on Day 352. The final diagnosis of the patient was FHL type 2 from homozygous c.1349C > T (p. T450M) missense variants in the PRF1 gene. The patient's parents were reluctant to test HLH-related genes for financial stress, so it was impossible to determine the location of these variants. She remained well without neurological symptoms at the monthly follow-up. The brain MRI on Day 329 showed stable disease without any new lesions. The ferritin level, liver function, coagulation, and hematologic parameters were normal at the last follow-up on Day 353 ( Figure 1E, F, G, H ). The patient is currently enjoying excellent PS.