Alveolar proteinosis is a rare lung condition where the tiny air sacs in your lungs become filled with a fatty, protein-rich substance that normally helps keep your lungs working properly. When this substance builds up instead of being cleared away, it makes breathing difficult and prevents your blood from getting enough oxygen.
Epidemiology
Pulmonary alveolar proteinosis, often shortened to PAP, is an extremely rare disease that affects only a small number of people worldwide. According to current research, it affects somewhere between 3 and 40 people per million across the globe[1]. Some studies have found the number to be around 6 to 7 cases per million in the general population[6][13]. Because the disease is so uncommon and symptoms can be mistaken for other lung problems, these numbers may not capture everyone who has the condition.
The disease most commonly appears in adults between the ages of 30 and 60 years old[1][7]. While earlier research suggested that men were affected more often than women, more recent studies indicate there may be no significant difference between genders[13]. The most common form of the disease, called autoimmune PAP, accounts for approximately 90 percent of all cases in adults[1][2]. This means that the vast majority of people diagnosed with PAP have the autoimmune type, where the body’s own immune system is involved in causing the problem.
There is also a form of PAP that can affect children, particularly those under age 10. This congenital form, which is present from birth or appears early in life, results from genetic changes passed down from parents[1]. However, this form is the least common type of PAP. A third category, called secondary PAP, develops as a complication of other medical conditions and can occur at various ages depending on the underlying disease.
Causes
To understand what causes alveolar proteinosis, it helps to know a bit about how healthy lungs normally work. The tiny air sacs in your lungs, called alveoli, have thin walls that are coated with a slippery, oily substance made of proteins and fats. This coating is called surfactant, and it serves an important purpose. It keeps the air sacs open and flexible so that oxygen can pass easily from the air you breathe into your bloodstream[1].
Your lungs have specialized cleaning cells called alveolar macrophages that regularly remove old surfactant from the air sacs. This prevents surfactant from accumulating and clogging up the alveoli. Think of these cells as tiny maintenance workers that sweep away debris to keep your lungs functioning smoothly[1].
In people with PAP, something goes wrong with this cleaning process. The macrophages do not receive the signal they need to do their job properly. As a result, surfactant builds up in the alveoli instead of being removed. When too much surfactant accumulates, it blocks the thin walls of the air sacs, making it difficult for oxygen to pass through into the blood[1][2]. Importantly, current understanding shows that PAP is caused by decreased clearance of surfactant rather than overproduction of it[2][9].
The underlying reason why the macrophages fail varies depending on which type of PAP a person has. In autoimmune PAP, the most common form, the body produces abnormal antibodies against a substance called granulocyte-macrophage colony stimulating factor, or GM-CSF for short. GM-CSF is crucial for helping alveolar macrophages develop properly and function correctly. When antibodies attack GM-CSF, the macrophages cannot mature or work as they should, leading to surfactant buildup[2][4].
Scientists are still investigating why some people develop these harmful antibodies. There has been speculation that cigarette smoke or certain infections might trigger the immune system to create these antibodies, particularly because smoking and infections are commonly found in patients with PAP. However, no direct causal link has been proven between smoking and autoimmune PAP, nor between infections and the development of this form of the disease[2][9].
Secondary PAP develops when another disease or condition reduces the number of functional macrophages in the lungs. Any illness that impairs these cleaning cells can lead to surfactant accumulation[2][9]. This can happen with certain blood cancers like myelodysplastic syndrome or chronic myelogenous leukemia, immune deficiency disorders, or exposure to toxic substances such as nickel, aluminum, titanium, or certain dusts[2][4][7].
Congenital PAP is caused by genetic mutations that affect either the GM-CSF receptor proteins or the surfactant proteins themselves. These genetic changes can be inherited from parents and result in abnormal surfactant regulation from birth or early childhood[2][9].
Risk Factors
Several factors appear to increase the likelihood of developing PAP, although for many people with the disease, no clear risk factors are identified. Smoking is associated with a higher risk, particularly for autoimmune and secondary forms of the disease[1][7]. While smoking does not directly cause autoimmune PAP, many patients with the condition have a history of tobacco use.
Workplace or environmental exposure to certain dusts and chemicals can contribute to secondary PAP. People who work with substances like silica dust, aluminum dust, titanium, cement, or cellulose may be at increased risk[1][7]. Acute silicosis, a lung disease caused by breathing in large amounts of silica dust, has been specifically linked to secondary PAP[7].
Having certain underlying medical conditions also raises the risk of developing secondary PAP. These include blood disorders such as myelodysplastic syndrome or chronic myelogenous leukemia, immune system diseases like common variable immunodeficiency or DiGeorge syndrome, and infections such as Pneumocystis jirovecii pneumonia[2][7]. People who are on medications that suppress the immune system, or those who have had organ or bone marrow transplants, may also face increased risk[7].
For congenital PAP, the main risk factor is having parents who carry genetic mutations related to surfactant production or GM-CSF receptor function. If both parents carry a copy of certain recessive genes, their children may inherit the condition[1].
Symptoms
The symptoms of pulmonary alveolar proteinosis typically develop slowly over time. Many people notice that they are gradually becoming more tired and finding it harder to catch their breath, especially when they exert themselves. The most common symptom is shortness of breath, medically known as dyspnea[1][8]. At first, you might only feel breathless when exercising or climbing stairs, but as the disease progresses, breathing difficulties can occur even when you are resting.
Other symptoms that people with PAP may experience include a persistent cough that sometimes produces phlegm or, in some cases, blood. Chest pain or discomfort can occur, making it uncomfortable to breathe deeply[1][5]. Fatigue is very common, and you might feel unusually tired even when you have not been particularly active. Some people develop a low-grade fever, which may come and go[1].
As PAP affects your lungs’ ability to transfer oxygen into your blood, you might notice physical signs of low oxygen levels. Your skin and fingernails can take on a bluish tint, a condition called cyanosis[1][8]. This happens when there is not enough oxygen in the blood circulating through your body. Some people develop a condition called clubbed fingers, where the fingertips become enlarged and the nails curve downward more than usual[1][8]. Weight loss may also occur[1][7].
People with PAP are more vulnerable to lung infections because the accumulated surfactant in the alveoli creates an environment where bacteria and fungi can grow more easily. Infections such as nocardiosis, Mycobacterium avium-intracellulare infection, or fungal infections may develop[4]. When infections occur, symptoms might worsen suddenly or new symptoms like increased fever or cough might appear.
It is important to note that the severity of symptoms varies widely among people with PAP. Some individuals have very mild disease and may not experience any noticeable symptoms at all. Their condition might only be discovered incidentally during medical tests done for other reasons[4][5]. Others may have symptoms that remain stable for long periods without getting worse. However, in severe cases, PAP can progress and lead to life-threatening respiratory failure[1][5].
Because the symptoms develop gradually and can resemble other more common lung conditions, many patients are initially misdiagnosed. Doctors may first think someone has asthma or recurring pneumonia before the correct diagnosis of PAP is made[5].
Prevention
Because the exact cause of autoimmune PAP remains unclear and genetic forms are inherited, there are no guaranteed ways to prevent the disease from developing. However, there are steps you can take that may reduce your risk or help protect your lungs if you have been diagnosed with PAP.
If you smoke, quitting is one of the most important things you can do for your lung health. While smoking has not been proven to directly cause autoimmune PAP, it is associated with the disease and can worsen lung function overall[1]. Avoiding tobacco smoke can help your lungs work as well as possible.
For secondary PAP related to workplace exposures, taking proper precautions is essential. If your job involves working with dusts, chemicals, or other substances that can harm your lungs, always use recommended protective equipment such as respirators or masks. Following workplace safety guidelines can help prevent toxic exposures that might lead to lung disease[1].
If you have been diagnosed with PAP, protecting yourself from infections is important because your lungs are more vulnerable. Getting vaccinated can help prevent serious respiratory infections. You should receive a yearly flu vaccine, as well as vaccines for pneumonia and COVID-19 as recommended by your healthcare provider[5][16]. Washing your hands frequently and asking family and friends to avoid visiting when they are sick can also reduce your exposure to infections.
Regular medical follow-up is crucial if you have PAP or are at risk for the disease. Routine monitoring allows your healthcare provider to detect changes in your lung function early and adjust treatment if needed. If you have risk factors such as exposure to certain chemicals or an underlying condition that predisposes you to PAP, discuss monitoring strategies with your doctor.
Pathophysiology
The pathophysiology of pulmonary alveolar proteinosis involves disruption of the normal balance between surfactant production and clearance in the lungs. In healthy lungs, type II alveolar epithelial cells produce surfactant, which then spreads across the surface of the alveoli. Alveolar macrophages continuously remove old surfactant through a process of ingestion and breakdown[13]. This careful balance ensures that just the right amount of surfactant is present to keep the air sacs open without clogging them.
In PAP, this balance is disrupted due to impaired macrophage function. The fundamental problem in most cases is that alveolar macrophages cannot properly clear surfactant from the alveoli[2][9]. Rather than being broken down and removed, surfactant accumulates in increasing amounts. This accumulated material is made up of lipoproteins derived from surfactant and appears as a milky or gummy substance in the air sacs.
The key player in maintaining normal macrophage function is granulocyte-macrophage colony stimulating factor, or GM-CSF. This growth factor is critical for the development, maturation, and proper functioning of alveolar macrophages[13]. In autoimmune PAP, the body produces antibodies that bind to and neutralize GM-CSF, preventing it from doing its job. Without functioning GM-CSF, macrophages cannot develop normally or effectively process surfactant[2][4].
This understanding came from important research involving mice. Scientists discovered that mice bred without the ability to produce GM-CSF developed a lung condition very similar to human PAP, with abnormal surfactant accumulation in their alveoli[4]. Further research showed that patients with autoimmune PAP have anti-GM-CSF antibodies in their blood and lung fluid, and that these antibodies could reproduce the disease when given to healthy mice[4].
In secondary PAP, the mechanism is somewhat different. Here, other diseases or exposures damage or reduce the number of functional alveolar macrophages, even though anti-GM-CSF antibodies are not present[2][9]. For example, blood cancers or immune deficiency diseases can impair the production or function of macrophages. Toxic inhalation exposures may directly damage these cells. The end result is the same: without enough working macrophages, surfactant builds up.
In congenital PAP, genetic mutations affect either the receptors that respond to GM-CSF or the surfactant proteins themselves. Some genetic mutations cause defects in the common beta chain of the GM-CSF receptor, which prevents the signal from GM-CSF from reaching the macrophages[7]. Other mutations affect the production of normal surfactant, leading to abnormal material accumulating in the alveoli[2].
As surfactant accumulates in the alveoli, it physically blocks the air spaces and prevents oxygen from crossing the alveolar walls into the blood vessels. The result is impaired gas exchange. Your blood cannot pick up enough oxygen from the air you breathe, leading to hypoxemia, which means low blood oxygen levels[5][7]. Carbon dioxide removal may also be affected, though oxygen deficiency is typically the more significant problem.
The accumulated lipoproteinaceous material stains positive with a laboratory test called periodic acid-Schiff staining, which is one way pathologists can identify PAP when examining lung tissue or fluid under a microscope[7]. Despite the extensive filling of alveoli with this material, the structure of the alveolar walls and surrounding tissue usually remains normal in PAP, at least early in the disease[7].