We report the largest series of cases of PAP in children from La Réunion Island. This cohort had particular characteristics: early onset, within the first months of life, a severe course with frequent progression to pulmonary fibrosis, associated liver involvement and more severe disease in boys than in girls.
The early onset of the disease seems to be highly specific to our series. PAP is a very rare cause of chronic lung disease in children, and is mostly diagnosed during adulthood. A pediatric onset has been reported for the forms due to CSF2RA and CSF2RB mutations, but at a wide range of ages. For CSF2RA mutations, reported ages at the onset of symptoms range from 1.5 to 9 years . For CSF2RB mutations, onset was neonatal in two cases, occurred at the ages of one month and at nine years in two other cases and during adulthood for the last case [11–13].
The second characteristic feature of our series was the severity of the disease, with a high rate of mortality and frequent progression to pulmonary fibrosis. Published mortality data relate to adult forms and report a better prognosis, with a five-year survival rate from diagnosis of 88% . Moreover, in adult PAP, and for autoimmune forms in particular, lung fibrosis is rarely described. Several case reports have mentioned focal interstitial fibrosis associated with adult PAP [28–32], but diffuse fibrosis is actually a very rare occurrence, with only a few cases reported [28, 33–38]. In a cohort of 223 patients with autoimmune PAP, lung fibrosis occurred in only two cases (0.9%) . In a retrospective radiologic study of 139 thoracic CT scans from 27 patients, substantial regions of fibrosis were observed in only two patients (7%) . In the adult with a CSF2RB mutation, thoracic CT scans showed the progressive appearance of traction bronchiectasis and cystic lesions suggestive of fibrosis , but no lung fibrosis was described in children bearing CSF2RA mutations [9, 10].
Given the small size of our population and the retrospective nature of the study, it is difficult to evaluate the efficacy of WLL in this cohort. Indeed, our clinical experience and management of patients diagnosed in the last 10 years suggest that WLL are effective in the short term, whereas statistical analyses on the entire cohort showed that there was no difference in survival between treated and untreated children. There are several possible reasons for this apparent discrepancy. First, the treated and untreated groups differed in terms of age at diagnosis, clinical presentation and severity. Second, this study covered a long time period (42 years), during which substantial changes occurred in both the WLL technique and the overall management of these patients. Prospective data collection would be required for the characterization of phenotypic groups and to identify the patients most likely to benefit from WLL.
Liver disease was also a distinctive feature in these patients, with no other organ involvement. As a whole, 31 patients (91%) had a liver involvement, and the absence of liver abnormalities in 3 patients could be attributable to a variable penetrance of this trait in this very likely genetic syndrome. Hepatic involvement can be observed in association with PAP in lysinuric protein intolerance  and adenosine deaminase deficiency [40, 41], but has never been reported in other forms, particularly those of the autoimmune type that is the most reported type of PAP. In our cohort, it is debatable whether the steatosis observed arose due to the severe malnutrition often present at diagnosis. However, the ultrasound findings were at all ages and in all states of nutrition suggestive of fat overload in the liver. Liver disease progressed to extensive fibrosis or cirrhosis in seven children in the absence of severe complications of portal hypertension or liver failure.
The pathophysiological mechanism underlying PAP in these children remains unknown. The clustering of this disease in a specific geographic area, the existence of cases among closely related individuals and the results of our thorough genealogical study (94% of patients are family connected) strongly suggest a genetic origin. Moreover, the frequency of inbreeding in this geographic area during the 18th and 19th centuries , the absence of vertical transmission (i.e. parent-to-child transmission) with all parents being healthy, and the recurrence among siblings are highly suggestive of an autosomal recessive mode of inheritance.
As the GM-CSF pathway has been implicated in the autoimmune form and in known genetic forms, genes encoding targets or partners of GM-CSF (reviewed in  and ) are potentially good candidates. However, the available animal models of disrupted GM-CSF signalling [44, 45] inducing a PAP phenotype have no liver phenotype and display no progression to pulmonary fibrosis. Furthermore, a dysfunction of GM-CSF signalling would led to alveolar macrophages’ dysfunction. In this hypothesis, one could expect that the disease would recur in transplanted patients. The absence of recurrence of the disease in the alive patient who underwent lung transplantation one year ago is one argument against the pivotal role of macrophages in the pathophysiology of this disorder. Nevertheless, it is impossible to draw definitive conclusions from the analysis of a single case with a relatively short follow-up since lung transplantation.
The associated liver involvement observed in our series raised questions about possible abnormal phospholipid metabolism or transport in both type II alveolar epithelial cells (type II AEC) and hepatocytes, potentially leading to abnormal surfactant turnover and fat overload in the liver. The fibrotic progression of the disease also suggests a potential role of type II AEC in this disease. Pathological review showed that inflammatory lesions were present at diagnosis in 16 children, seven of whom were investigated before the age of two years, suggesting that the inflammatory and subsequent fibrotic processes may occur at the same time as the accumulation of abnormal lipoproteinaceous material, rather than after this accumulation. PAP is classically characterized by preservation of the architecture of the alveoli, with normal and thin alveolar walls and rare descriptions of lymphocytic infiltrates [3, 5]. In interstitial lung diseases caused by SFTPC mutations, mutant surfactant protein C has been shown to accumulate in the endoplasmic reticulum (ER), leading to ER stress, type II AEC injury, inflammation and the activation of apoptosis [46, 47]. All these mechanisms seem to be involved in the fibrotic progression associated with SFTPC mutations . In the disease described here, similar mechanisms could be suggested, with a genetic mutation leading to an abnormal protein that is misfolded in the ER, leading to inflammation and fibrosis. The defective functioning of this abnormal protein would also lead to abnormal surfactant turnover and accumulation in the alveoli. The important systemic inflammation in those children could also play a role in the development of lung fibrosis.
The reasons for the greater severity of disease in boys remain to be determined, and will probably become clearer when genetic analyses have determined the etiology of the disease. X-linked inheritance is very unlikely because both boys and girls can be affected by the disease which exclude a recessive X-linked disorder. As mentioned above, there is no case of parent-to-child transmission and this element a priori excludes a dominant X-linked mode of inheritance. Instead, modifier genes or hormonal factors may be involved.
In conclusion, we describe a cluster of 34 cases of primary PAP in children from La Réunion Island with very particular features, including a high rate of mortality and frequent progression to cholesterol granulomas and pulmonary fibrosis, despite WLL treatment. In this setting, the cause is very likely to be genetic, with an autosomal recessive mode of inheritance. Genetic studies should make it possible to determine the cause and mechanism of PAP and fibrosis in this population.