Pulmonary alveolar proteinosis (PAP) is a rare disease of unknown etiology, first described by Rosen in 1958, characterized by the deposition of large amounts of soluble phospholipid-like material in the alveoli and small airways, with a positive periodic acid (PAS) reaction. Schiff (PAS) reaction is positive, and the clinical presentation and course of the disease are varied. The disease can develop from newborns to the elderly and has a worldwide distribution, with more than 90% of cases being idiopathic. In recent years, with the in-depth research on GM-CSF, there is now a further understanding of PAP. This paper mainly reviews the pathogenesis, diagnosis and treatment progress of idiopathic PAP. PAP is an autosomal recessive disorder caused by mutations in the gene encoding the surface active substance protein B or GM-CSF receptor β chain. Secondary PAP is secondary to other systemic diseases or associated with inhalation of certain substances, mainly related to reduced numbers and/or impaired function of alveolar macrophages, mostly associated with immunodeficiency and certain malignancies (especially hematologic), and some patients have a history of chemical and dust exposure such as aluminum dust, titanium, drugs and silica dust inhalation. More than 90% of PAP cases are idiopathic. Current research shows that idiopathic PAP is an autoimmune disease due to the presence of GM-CSF auto-neutralizing antibodies that bind specifically to GM-CSF, blocking the function of GM-CSF and leading to changes in alveolar macrophage function, resulting in decreased clearance of alveolar surface active substances. Pathogenesis 1. GM-CSF is an important regulator of pulmonary surface active substances. Current research shows that PAP is caused by abnormal metabolism of alveolar surface active substances, due to impaired alveolar macrophage function and decreased clearance of alveolar surface active substances. 1.1 Composition and metabolism of normal alveolar surface active substances: surface active substances are complexes of lipids and proteins, which are expressed in a steady state in mammals. Of these, phospholipids account for about 80%, other lipids (cholesterol, triglycerides, free fatty acids) for 8%, and proteins for 12%. In steady-state metabolism, the half-life of the surface-active protein (SP) is about 4-10 h. About 50% of the SP is recycled in alveolar type II epithelial cells, and the major route of clearance is through metabolism by alveolar macrophages and reabsorption by type II alveolar epithelial cells. Traditionally, it is believed that this metabolism is mainly accomplished in type II alveolar epithelial cells. Recent studies in rats using radiographic tracing techniques have shown that approximately 50% of the metabolism of surface phospholipids and SP-A is done within alveolar macrophages. 1.2 GM-CSF and lung surface active substance homeostasis: GM-CSF is an important regulator of lung surface active substances, and the presence of GM-CSF is indispensable for the maintenance of lung surface active homeostasis. The first experimental recognition of the important role of GM-CSF in pulmonary surface active substance homeostasis was made in experimental mice by applying targeted gene knockout techniques. 1994, two independent laboratories successively generated GM-/- mice by targeted gene knockout to observe the effects on the hematological system. In the hematologists’ opinion, GM-/- mice showed no hematopoietic disturbances, and the only organ changes were accumulation of surface-active phospholipids and proteins in the lungs and the presence of large numbers of inclusion-rich, foamy, enlarged lung macrophages in lung sections. To further understand the role of GM-CSF, Robb et al. removed the gene encoding the murine GM-CSF receptor Rβc to obtain an animal model of GM-CSF Rβc -/-, which resulted in lung pathological changes identical to those in GM -/- mice. It was further demonstrated that PAP-like lung pathological features occur when GM-CSF fails to function. To demonstrate the important role of GM-CSF in lung surface active substance homeostasis, Huffman et al. applied the human SP-C promoter to selectively express GM-CSF in GM-/- mouse lung epithelial cells, and the result was that the expression of GM-CSF completely corrected the PAP caused by target gene knockdown of the endogenous GM-CSF gene. In addition, inhalation of GM-CSF or application of adenoviral gene vector to GM-CSF expression corrected the pathological changes of PAP in GM-/- mouse lungs [7,8], thus demonstrating another important role of GM-CSF in the regulation of pulmonary surface active substances, the presence and action of which is essential for the homeostasis of pulmonary surface active substances. Further studies on GM -/- rats showed that the mRNA concentrations of SP-A, SP-B and SP-C did not change significantly from those of the control group, and the secretion of surface active substances did not increase, indicating that the accumulation of surface active substances in the lungs of GM -/- rats was not caused by increased synthetic secretion, but by impaired metabolic clearance of surface active substances by alveolar macrophages. 2. Relationship between GM-CSF and lung macrophage function GM-CSF was closely related to lung macrophage function. The pathological results showed that the alveolar macrophages in the lung tissue of GM -/- and GM Rβc-/- rats were increased in size, foamy and contained a large number of inclusion bodies composed of phospholipids and proteins. In terms of function, in vitro studies showed that newly isolated lung macrophages from GM -/- mice were significantly defective in degrading SP-A and phosphatidylcholine, and that primary lung macrophages obtained from GM -/- mice and passaged cultured macrophages were significantly impaired in immune functions such as chemotaxis, cell adhesion, expression of antigen recognition receptors, phagocytic capacity, and ability to kill microorganisms. GM-CSF receptors were present on the surface of type II alveolar epithelial cells and alveolar macrophages, and the target cells of GM-CSF were lung macrophages, as both types of cells were unresponsive to GM-CSF in GM Rβc-/- rats. Transplantation of normal rat bone marrow was able to correct the metabolism of GM Rβc-/- to lung surface active substances, whereas bone marrow transplantation was only able to restore the function of lung macrophages but not lung type II epithelial cells in recipient rats, and therefore lung macrophages were the main target cells for GM-CSF action. In conclusion, it is clear from animal experiments that GM-CSF plays an important role in lung surface active substance homeostasis, and that the accumulation of GM-/- murine lung surface active substances is the result of impaired metabolism by lung macrophages rather than increased secretion by them. 3. the role of GM-CSF in patients with idiopathic PAP Animal experiments have shown that GM-CSF plays a key role in the maintenance of pulmonary surface active substance homeostasis. deficient or functionally defective GM-CSF is associated with pulmonary pathological features of PAP. To date, however, no mutations in GM-CSF or its receptor have been identified in patients with idiopathic PAP, but instead elevated concentrations of GM-CSF-neutralizing antibodies have been found in lung lavage fluid and plasma from patients with idiopathic PAP. 1999, Japanese scholar Tanaka and colleagues first identified the presence of a GM-CSF-dependent factor, which inhibited the activity of lung macrophages, and further studies demonstrated that this factor was an endogenous anti-GM-CSF auto-neutralizing antibody, polyclonal to IgG (IgG1 and IgG2). In contrast, Bonfield et al. examined GM-CSF neutralizing antibodies in lung lavage fluid and plasma from patients with idiopathic PAP, congenital PAP, secondary PAP, other lung diseases, and normal healthy controls, and showed that GM-CSF neutralizing antibodies were significantly elevated in lung lavage fluid and plasma from patients with idiopathic PAP, but were not detected in the remaining groups. Moreover, the presence of such antibodies correlated significantly with the inhibition of the biological activity of GM-CSF. Thus, GM-CSF-neutralizing antibodies blocked the action of GM-CSF to the extent that they affected the function of alveolar macrophages, resulting in an imbalance in the homeostasis of lung surface active substances and leading to the typical pathological features of PAP. Thus, given the role of GM-CSF-neutralizing antibodies in the pathogenesis of idiopathic PAP, it suggests that idiopathic PAP is an autoimmune disease. Congenital PAP is considered to be an autosomal recessive disorder associated with mutations in the relevant genes. Current studies suggest that congenital PAP is due to mutations in the genes encoding the SP-B gene (located on chromosome 2), the GM-CSF gene (chromosome 5) and the GM-CSF receptor gene (chromosome 22).The SP-B gene has two site-specific mutations, a frameshift mutation in which three bases (GAA) are replaced with nucleotide C at codon 121 and a frameshift mutation in ( 122delT) deletion of one base pair, generating an early occurrence of a stop codon, which in turn leads to the deletion of SP-B in the surface active material. DNA analysis of the βc gene, the common receptor unit of GM-CSF/IL-3/IL-5, revealed a point mutation at codon 603, resulting in a change from proline to threonine, and the inability of GM-CSF to function properly due to GM-CSF deficiency or reduced GM-CSF receptor function, which in turn leads to lung macrophage dysfunction and pulmonary surface active substance homeostasis disorders. However, these mutations account for only a small proportion of the etiology of congenital PAP, and further studies are needed to elucidate the mechanisms involved. Secondary PAP is a rare pulmonary complication due to exposure to minerals such as silica or titanium, hematologic malignancies, and immunosuppressive diseases. The specific pathogenesis is unknown and needs to be elucidated by further studies. Clinical features 1. Epidemiological features: PAP is a rare disease seen in all age groups. Current data show an incidence of 1/2 million people and a prevalence of 3.7 per million people. The majority of patients are male, with a male to female ratio of about 2 to 4:1. 72% of patients have a history of smoking, the average age of onset is 39 years, and the average time to the appearance of significant clinical symptoms is 7 months, with 25% of cases being 2 years or longer. 2, clinical manifestations: PAP clinical manifestations are varied and non-specific. This non-specific clinical feature leads to misdiagnosis for months and sometimes years, and even 30% of patients have no clinical symptoms. In terms of clinical symptoms, dyspnea and cough are the most common symptoms. Dyspnea often occurs after mild exertion and, in a few cases, at rest. Other symptoms such as fever, weight loss, chest pain and hemoptysis are also sometimes present. Physical examination is usually normal, with end-inspiratory bursting sounds being the most common abnormal sign, and pestle and mortar fingers and cyanosis have been reported in isolated cases. 3. Pulmonary function; PAP patients can have normal pulmonary function. Typical pulmonary function abnormalities are restrictive ventilatory dysfunction and diffusion disorders. In a study of 410 patients with PAP, D2CO decreased 47% from the predicted value, and there were mild to moderate restrictive abnormalities in FEV1, VC, and TLC. These abnormalities are partially reversible in patients with PAP who are effectively treated such as total lung lavage or in spontaneous remission. 4. Imaging changes: There are no specific imaging features on PAP chest radiographs. Mostly non-specific solid alveolar shadows, usually bilateral lamellar distribution, sometimes nodules, patches and large shadows. The lesions are mainly in the lower lungs, with a symmetrical or asymmetrical distribution in the lung field centered on the hilum, sometimes forming a “butterfly wing” sign, similar to pulmonary edema. Some of the edges are angular, forming triangles and polygons. High-resolution CT typically shows a “gravel road”-like sign, showing extensive intra-alveolar solidity and thickened alveolar septa, indicating the so-called crazy-paving sign. In some other cases, such as lipoid pneumonia, bronchoalveolar carcinoma, etc., chest CT also has the same sign. 5. Laboratory tests: Hypoxemia is a common phenomenon in patients with symptomatic PAP. In patients with PAP, there are no abnormalities in general routine tests and biochemical indices, and some specific biochemical indices are significantly abnormal. The sensitivity and specificity of serum GM-CSF autoantibodies are significantly increased in patients with idiopathic PAP, and the sensitivity and specificity for the diagnosis of idiopathic PAP reach 100% and 98%, respectively, while the titer in normal subjects is very low. LDH is also increased in patients with PAP and is considered a non-specific serological index. correlated with blood oxygen levels and decreased after lavage treatment. CEA is a serological index of malignancy and was found to be elevated in the serum of PAP patients and correlated with LDH and P(A-a)O2. Xu et al. examined serum CEA in 17 PAP patients and showed that serum CEA concentration was elevated in 52.9% of PAP patients and correlated with the severity of the disease. In addition, the clinical significance of some other indicators in PAP patients such as cytokeratin 19 fragment, glycoprotein KL-6, surface active substances A, B, D and monocyte adhesion protein 1 in alveolar lavage fluid needs to be further explored. IV. Diagnosis According to the patient’s medical history, age, clinical manifestations, lung function, especially chest X-ray and CT can suggest the diagnosis. Idiopathic PAP is mostly seen in adults, while congenital PAP is predominant in children. Secondary PAP can be seen in any age group, but is predominant in adults, mainly secondary to hematologic malignancies, immunosuppressive diseases and exposure to silicon and beryllium. In the past, 70% of cases were diagnosed by open-chest lung biopsy. Recently, with the development of imaging techniques and the use of fibrinoscopy, the diagnosis can be confirmed by alveolar lavage fluid in more than 75% of cases. Typical alveolar lavage fluid is milky or thick, pale yellow fluid that is stratified upon placement. Light microscopy reveals a large amount of eosinophilic granular lipoprotein-like material of irregular morphology and varying size, which stains positive for PAS. Under electron microscopy these substances were composed of a large number of cellular debris of different sizes, surface active substance particles and some other protein-like substances, which had a concentric circular arrangement structure. In recent studies, surface active substances SP-A and SP-D were found to be abnormally elevated in serum and alveolar lavage fluid in most patients with PAP, so clinical measurement of serum and alveolar lavage fluid SP-A and SP-D can help in the diagnosis of PAP. Recent studies have demonstrated the presence of elevated GM-CSF autoantibodies in the serum of patients with idiopathic PAP, and the detection of serum GM-CSF autoantibodies has a sensitivity of 100% and specificity of 98% for the diagnosis of idiopathic PAP. Therefore, it can be used as a serological index for the diagnosis of idiopathic PAP. In conclusion, the definitive diagnosis of PAP is still based on pathological diagnosis. Although trans-fiberoptic lung biopsy and thoracoscopic lung biopsy can diagnose PAP, open lung biopsy is still the gold standard for the diagnosis of PAP. V. Treatment The treatment of PAP is based on its etiology. Treatment for patients with congenital PAP is currently supportive only, although successful treatment with lung transplantation has been reported. Treatment of secondary PAP includes effective control and treatment of the primary condition, such as effective chemotherapy or bone marrow transplantation for patients with secondary hematologic malignancies can improve the patient’s lung condition, while the most effective treatment for idiopathic PAP is whole lung lavage to remove alveolar lipoprotein-like material. This approach is not a radical treatment, and a small number of patients with PAP still have progressive disease and eventually die of respiratory failure. With the continuous research on the pathogenesis of PAP, some new methods have emerged for the treatment of PAP. GM-CSF therapy: GM-CSF is an important regulator of lung surface active substances, and the GM-CSF deficiency mouse model suggests that GM-CSF is closely related to the pathogenesis of PAP. alternative treatment with GM-CSF may replace the traditional whole-lung lavage therapy. A two-stage prospective study abroad illustrates the effectiveness of GM-CSF in the treatment of PAP. In the first phase, 14 patients with PAP underwent subcutaneous GM-CSF (at a dose of 5µg/kg/day) for 6 to 12 weeks from 1995 to 1998, with an average improvement in alveolar arterial oxygen gradient ([A-a]D O2) of 23.2 mmHg in 5 patients and 4 non-responders with PAP continuing to receive 20µg/kg/day. After GM-CSF treatment, one of them responded, resulting in an overall response rate of 43%. in the second phase of the study conducted in 1998, four cases underwent daily subcutaneous GM-CSF injections and the dose was gradually increased until 12 weeks. three of them showed significant efficacy, and in the efficacy evaluation conducted after 16 weeks of treatment, their clinical symptoms, physiological and imaging conditions improved significantly, and [A-a]D O2 increased from the initial The Japanese scholars applied GM-CSF inhalation to treat a 57-year-old female patient with idiopathic PAP also achieved significant efficacy and enhanced diffusion capacity, indicating that GM-CSF treatment for PAP is indeed an effective method despite the different application routes. 2. Plasma replacement: Plasma replacement is often used in the treatment of immune disorders. It is now recognized that patients with idiopathic PAP have relatively high titers of GM-CSF-neutralizing autoantibodies in their plasma, which affects the function of GM-CSF. Bonfield et al. performed plasma exchange in a patient with PAP who had failed whole lung lavage and GM-CSF therapy. The patient’s plasma GM-CSF antibody titer was 1:6400, and after 10 plasma exchange treatments over 2 months, his GM-CSF antibody titer decreased to 1:400, and his clinical symptoms, chest imaging and oxygenation improved significantly, with a PaO2 of 75 mmHg on room air. therefore, plasma exchange is also an effective treatment for PAP. 3. Gene therapy: Gene therapy is mainly applied to patients with specific genetic defects. Patients with congenital PAP lack surface active protein B or have mutations in the β-chain gene of the receptor unit common to GM-CSF/IL-3/IL-5, and gene therapy may be an option for effective treatment in the future. Application of an adenoviral vector to transfect GM-CSF gene into GM-CSF-deficient mice and selectively express GM-CSF resulted in improvement of alveolar protein-like substance and relief of symptoms in mice. In human epithelial cells in vitro and in mice in vivo, DNA with SP-B and SP-A were transfected in vivo, and the corresponding surface active protein was expressed, so the above tests suggest that gene therapy may be one of the promising means for effective treatment of PAP. 4. Others: Some studies have shown that the application of bone marrow transplantation in βc mutant mice can reverse the pathological changes in the lung, and the application of allogeneic hematopoietic stem cell transplantation for the treatment of some secondary PAP has also reported successful cases, but the efficacy of the above methods needs to be further evaluated and explored. PAP is a rare disease. Recent studies have shown that the presence of GM-CSF-neutralizing autoantibodies in patients with idiopathic PAP makes it possible to recognize idiopathic PAP as an autoimmune disease, and the effect of GM-CSF-neutralizing autoantibodies on the signaling pathway of GM-CSF and the effect of signaling molecular dysfunction on the pathogenesis of PAP will be the direction of future research, which will lead to a fundamental change in the treatment strategy of PAP. This will lead to a fundamental change in the treatment strategy of PAP.