What is Pulmonary Alveolar Proteinosis?

Pulmonary Alveolar Proteinosis, commonly known by the acronym PAP, is a rare lung disease characterized by the build-up of grainy material in the alveoli (air sacs) of the lungs. This grainy material is composed mainly of phospholipid (a fat-like substance) and protein. Phospholipid and protein are the key components of lung surfactant, an important substance that coats the alveoli to prevent lung collapse and which promotes oxygen absorption by the lungs.

In normal lungs, specialized immune cells called alveolar macrophages swallow and remove inhaled particles and excess surfactant from the alveoli. In the most common form of PAP, it has been suggested that the alveolar macrophages do not function properly and consequently cannot break-down and remove the material they swallow. They become inefficient at clearing material from the lungs.

In PAP patients, the accumulation of excess surfactant material in the alveoli makes it increasingly difficult for the lungs to absorb oxygen from inhaled air. This leads to breathing difficulties such as shortness of breath and cough. Respiratory failure may occur in severe cases. In addition to breathing problems, many PAP patients acquire secondary infections, frequently caused by opportunistic pathogens. Interestingly, some patients with the disorder exhibit no symptoms.

PAP may progress in severity, remain stable, or spontaneously clear. The lower and rear lung regions are most commonly affected. Occasionally only the front segments of the lungs may be involved.

• Three forms of PAP are currently recognized: congenital, secondary, and acquired.
  The congenital disease is most frequently diagnosed in infants and is considered a genetic disorder that has been linked to mutations in a gene encoding for a type of surfactant protein. In a limited number of cases it has been linked to a defect in a growth factor receptor.
• Secondary PAP is diagnosed in individuals who may exhibit a primary disease that causes the alveolar macrophages to malfunction or reduces the number of alveolar macrophages in the lungs.
• Acquired PAP (also known as idiopathic PAP) is the most common form of PAP, accounting for 90% of all cases. It is considered to be an autoimmune disease in which an individual produces antibodies that target an important growth factor for destruction by the immune system.

Effective treatment of PAP may depend on the form of the disease that an individual has.