To the editor,

Lymphoproliferative disorders represent a heterogeneous group of monoclonal or polyclonal lymphoid neoplasm that occurs in patients with immune dysfunction. Approximately 90% of patients with lymphoproliferative disorders have lung involvement, which is usually diagnosed at presentation [1].

Familial hemophagocytic lymphohistiocytosis 2 (FHL) is an autosomal recessive disorder resulting from biallelic mutation in the gene encoding perforin (PRF1; 170280) gene [2]. According to the Histiocyte Society, five out of the following eight criteria are necessary to reach a diagnosis of hemophagocytic lymphohistiocytosis (HLH): fever, enlarged spleen, pancytopenia, hypertriglyceridemia, or hypofibrinogenemia and hemophagocyte in organ biopsy. Additional diagnostic criteria include decreased or absent natural cell activity, hyperferritinemia, and elevated CD25 [3]. Previously, it was reported that 13–50% of adult or pediatric HLH patients had respiratory symptoms; however, no details of the respiratory involvement were presented [4].

Interstitial lung disease (ILD) is one presentation of a variable HLH pulmonary phenotype; it includes granulomatous disease, lymphoid interstitial pneumonia, organizing pneumonia, and lymphoproliferative disorder. The pathophysiology of these complications in HLH patients remains unclear [4].

Lymphocytic interstitial pneumonia is an uncommon lympho-proliferative disorder characterized by infiltration of the interstitial spaces of the lungs by lymphocytes, plasma cells, and other lymphoreticular cells. Radiologically, it is characterized by diffuse ground-glass opacification and thin-walled cysts distributed in a perivascular fashion. Indeed, lung disease is a common and major cause of morbidity and death in these patients [5].

Herein, we report a fatal case of PRF1 mutation and early manifestations of diffuse parenchymal lung disease, as well as interstitial lymphocyte pneumonia without diagnostic features of FHL2, in a female child.

The patient was born to a consanguineous couple of Saudi descent, and she had three healthy siblings. She was delivered at term with unremarkable prenatal and postnatal course. She had a normal growth and development for her age. Apart from minor upper respiratory tract infections, atopic dermatitis, and allergic rhinitis, she was in a good health. Her vaccinations were up-to-date, and there was no remarkable family history for recurrent infections or immune deficiency.

Since the age of 4 years, the patient started to develop recurrent chest infections that necessitated frequent admission to the pediatric intensive care unit (PICU). She received multiple courses of empiric antibiotics, and was intubated several times, with an average length of stay for each hospitalization of 10–20 days. Each episode was characterized by fever, cough, tachypnea, and labored breathing with the appearance of new infiltrates on a chest X-ray. A tracheal aspirate culture revealed Pseudomonas aeruginosa on different occasions. Viral assays for cytomegalovirus (CMV), Epstein–Barr virus, herpes, and adenovirus were all negative. The repeated chest infections prompted the treating physician to perform a basic immunological workup, including serum immunoglobulins and lymphocyte subsets, which was normal (Table 1). However, the initial lymphocyte proliferation test revealed poor response to phytohemagglutinin (PHA), 13% compared with control value, she was on steroids at that time. The lymphocyte proliferation test was repeated 6 months after stopping the steroid therapy, and the result of the PHA was normalized, 123% compared with the control.

Table 1 Laboratory tests results including CBC, liver enzymes, fibrinogen, and immunological workup during last admission
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Last admission at the age of 7 years was for a chest infection which was complicated by a tension pneumothorax and severe acute respiratory distress syndrome (ARDS) that necessitated the insertion of chest tubes and intubation. Conventional ventilation failed to improve her condition, so high frequency oscillatory ventilation (HFOV) was started but failed to improve her condition. Consequently, a cannula was placed and veno-venous extracorporeal membrane oxygenation (ECMO) was started and maintained for 6 days. She was weaned gradually and ECMO was discontinued. A tracheal aspirate culture was positive for Psudomonas aeruginosa and empiric antibiotics and antifungal medication were prescribed with vancomycin, tazocin, and amphotericin B. The chest X-ray was suggestive of diffuse bilateral air space disease with lower lobe predominance and the CT scan showed interval development of scattered bilateral thin-walled cysts, with no lobe predominance, on a background of extensive airspace disease and signs of bilateral pneumothorax (Fig. 1a, b). Unfortunately, the patient’s condition deteriorated with severe ARDS along with multi-organ failure. In spite of full supportive measures and treatment were delivered to the patient including HFOV and ECMO, she died during the admission.

Fig. 1
figure 1

a Axial chest computed tomography scan in the lung window demonstrates extensive bilateral asymmetric areas of peripheral air space consolidation with associated air bronchograms and b CT scan showed interval development of scattered bilateral thin-walled cysts, with no lobe predominance

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Laboratory investigations are summarized in Table 1. The workup has excluded cystic fibrosis. A broncho-alveolar lavage revealed inflammatory cells, including lymphocytes, alveolar macrophages, and few neutrophils and negative for malignant cells. Also, acid-fast bacilli and fungi staining and viral workup were negative. A lung biopsy was obtained, which showed features suggestive of lymphocytic interstitial pneumonia, with diffuse infiltration of small mature lymphocytes and macrophages associated with alveolar pneumocytes, hyperplasia, and widening of the septa (Fig. 2). The biopsy also showed focally organizing pneumonia but was negative for granulomas, viral inclusions, and necrosis. Immunohistochemistry for CMV, herpes, and adenovirus was negative.

Fig. 2
figure 2

There is diffuse interstitial lymphocytic infiltrate of small lymphocytes with widening of the alveolar septa and hyperplasia of type 2 pneumocytes. The inflammatory infiltrate also includes macrophages and few neutrophils. (hematoxylin and eosin stain; 400x magnification)

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Exome sequence revealed a pathogenic homozygote variant of PRF1 (NM_001083116: c.148G > A; p.V50M), this variant has previously been described as disease causing for FHL2 by Molleran et al. (2004) followed later Sumegi et al., (2014) [6, 7]. This result prompting the investigation of FHL including serum ferritin, triglyceride, and fibrinogen level which were normal.

Because the patient’s critical condition, we were unable to complete the HLH diagnostic workup, upon the identification of the PRF1 gene mutation, by performing NK-cell activity testing or measure soluble interleukin-2 receptor levels. Additionally, a bone marrow biopsy was not requested initially because there were no signs of hematopathology to look for increased hemophagocytosis, which is found in 82% of FHL2 patients.

Okur et al. described a patient carrying homozygous V50M variant, who had a very late-onset with initial presentation of central nervous system involvement in the absence of systemic manifestation of FHL2 [8].

Molleran et al. also described a patient with homozygous V50M variant who presented at 7 years of age with the classical FHL2 including hepatosplenomegaly, liver dysfunction, pancytopenia, and hypofibrinogenemia. However, he had a rapid evolving phenotype. This patient showed normal NK function which is exceptional finding in FHL2 patients. The association of the V50M mutation with normal NK function is a compelling feature that requires further studies [7].

Experiments by Schulert et al. [9] revealed that a disruption of NK cell cytolytic function was probably implicated in the pathophysiology of macrophage activation syndrome (MAS). They identified mutations in PRF1 in approximately 36% of adults with fatal H1N1 influenza [9].

In studies conducted on children with autoinflammatory diseases, polymorphisms in perforin genes were reported in about one-third of the cases of those who developed MAS [9]. According to previous reports, patients with HLH may have variable clinical manifestations with prominent neurologic or hepatic abnormalities [10, 11]. However, lung involvement has rarely been evaluated in these patients. In some reports, ARDS is described as the initial manifestation of HLH [12], and patients are typically admitted to the (PICU) due to acute respiratory failure [13]. Until now, however, the mechanism underlying the pathophysiology of acute respiratory failure or its prognostic significance in HLH patients has not been established.

Currently, no incidence or prevalence estimates for lymphocytic interstitial pneumonia (LIP) in children exist. According to a 2017 report from Europe, only three cases of LIP were reported among 346 children with interstitial lung disease [14]. LIP is considered a form of pulmonary lymphoproliferative disease and should be included in the differential diagnosis for childhood interstitial lung disease (ILD). The condition is usually caused or triggered by an underlying autoimmune disease and immunodeficiencies; however, it has also been reported to occur in familial and idiopathic forms [15, 16]. Additionally, LIP occurs in a considerable proportion of pediatric patients with perinatally acquired HIV who do not receive antiviral therapy [17, 18].

Ours is the first individual reported with LIP diagnosed by lung histopathology associated with homozygous pathogenic PRF1 mutations in the absence of clinical characteristics suggestive of FHL-HLH. This case expands the phenotypic spectrum of FHL2 and provides a pathophysiological link between the PRF1 gene defect and the interstitial lymphocyte pneumonia through impaired NK cell cytotoxicity [19]. We believe that PRF1-related interstitial lung disease is underreported, and it should be suspected in a patient with immune-mediated interstitial lung disease to avoid the delayed diagnosis of this potentially curable condition.