Clara cells are non-mucous and non-ciliated secretory cells found in the primary bronchioles of the lungs, and were originally described by their namesake, Max Clara in 1937.

Clara cells are dome-shaped and have short microvilli. One of the main functions of Clara cells is to protect the bronchiolar epithelium. They do this by secreting a small variety of products, including Clara cell secretory protein (CCSP) and a component of the lung surfactant. They are also responsible for detoxifying harmful substances inhaled into the lungs. Clara cells accomplish this with cytochrome P450 enzymes found in their smooth endoplasmic reticulum. Clara cells also multiply and differentiate into ciliated cells to regenerate the bronchiolar epithelium.

CC10 (Clara cell secretory protein)

• cell-specific marker of airway Clara cells. PMID: 9070353, PMID: 9564884
• murine Clara cell-specific protein. PMID: 11405061
• an inhibitor of phospholipase A2 and binds to phosphatidylinositol, may play a role in the regulation of epithelial repair. PMID: 11193477
• has been thought to be fairly specific to Clara cells and a major secretory protein that is both synthesized and released from Clara cells. PMID: 9880692
• a secretory protein mainly synthesized in the pulmonary Clara cells. PMID: 9473294
• monkey CC10 plays a role in the anti-inflammatory process at the air-liquid interface over the bronchio-bronchiolar epithelium. PMID: 8810640
• a major secretory protein of lung Clara cells. PMID: 9635144

CC16 (Clara cell secretory protein)

• small-size and readily diffusible protein of 16 kDa secreted by bronchiolar Clara cells. PMID: 9517618
• a marker of Clara cell damage and bronchoalveolar blood barrier permeability. PMID: 10414398
• a valid marker of the distal airway damages caused by air pollutants. PMID: 9389087
• a 16-kD protein secreted at the surface of respiratory airways by nonciliated bronchial and bronchiolar cells, including Clara cells, might serve as a marker of bronchial epithelium growth. PMID: 7898986, PMID: 7532741
• a protein with anti-inflammatory and immunomodulatory properties. PMID: 17338426
• a major immunomodulatory protein produced in the fetal lung. PMID: 16236364
• CC16 in serum might provide a useful tool to noninvasively evaluate the damage and increased permeability to proteins of the air-blood barrier associated with sarcoidosis, is a marker for lung damage. PMID: 15477284, PMID: 11589348
• the predominant protein secreted by bronchiolar Clara cells, increasingly appears to protect the respiratory tract against oxidative stress and inflammation. PMID: 14515223
• a small and readily diffusible protein of 16 kDa secreted by bronchiolar Clara cells in the distal airspaces. PMID: 12101785
• a 15.8-kDa homodimeric protein secreted in large amounts in airways by the non-ciliated bronchiolar Clara cells. This protein increasingly appears to protect the respiratory tract against oxidative stress and inflammation. PMID: 10718843  
• related to stress-induced anxiety, psychoses and major depression. PMID: 10579545
• a 16 kiloDalton protein secreted by Clara cells in the lining fluid of bronchiolar and bronchial epithelium. PMID: 9641352
• a small-size and readily diffusible protein of 16 kDa secreted by bronchiolar Clara cells. PMID: 9517618
• Serum CC16 seems potentially to be a new biomarker for the early detection of acute airways injury caused by smoke. PMID: 9072037
• a new valid marker of the distal airway damages caused by air pollutants in rats. PMID: 9389087

CC26

• localized to the dense secretory bodies and endoplasmic reticulum of the Clara cell and secretory granules of tracheal nonciliated serous and goblet cells. PMID: 10483522, PMID: 12151308

CCSP (Clara cellspan> s secretory protein)

• one of the most abundant proteins present in airway lining fluid of mammals. PMID: 11704549, PMID: 11193758
• also known as uteroglobin (Ug), is a 16-kDa homodimeric protein of unknown function. Within rodent species, CCSP is expressed predominantly by nonciliated Clara cells that line conducting airways of the lung. PMID: 10070919

CYP2F2/CYP2B4

• Marker proteins for differentiated Clara cells. PMID: 8652187

Cytochrome P-450 (CYP450)

• Clara cell marker of differentiation. PMID: 9575866
• localized abundantly in endoplasmic reticulum of Clara cells. Morphological alterations in Clara cells appear to be closely related with changes in the cytochrome P-450b content of the lung. PMID: 1736414
• observed in nonciliated bronchial and bronchiolar epithelial cells (Clara cells) and type I pneumocytes. PMID: 1485004
• the cytochrome P-450 monooxygenase system is distributed throughout the tracheobronchial airways of the rabbit and (2) the Clara cells of the trachea and bronchi are functionally, as well as structurally, similar to those of the bronchioles. PMID: 3653045
• Two forms of cytochrome P-450 (P-450I and P-450II) have been shown by several techniques to be present in both nonciliated bronchiolar cells (Clara) and alveolar type II cells isolated from rabbit lung. PMID: 7028473
• we have demonstrated for the first time that components of the pulmonary cytochrome P-450 monooxygenase, although localized in the AER-rich regions of the Clara cells and type II pneumocytes, are not restricted to these cell types or to the endoplasmic reticulum. PMID: 3352593

NADPH reductase

• Clara cell marker of differentiation. PMID: 9575866
• NADPH-cytochrome P450 reductase was localized by ultrastructural immunocytochemistry in the cytoplasm of Clara cells, ciliated cells and type II pneumocytes of young female Wistar rats and MF1 mice. Immunogold labelling was particularly dense over the apical cytoplasm of Clara cells. PMID: 7763281
• In the lung, reductase was detected in bronchiolar and bronchial epithelial cells, Clara cells and alveolar lining cells. PMID: 2493999

SP-A

• produced by alveolar type II cells and Clara cells. PMID: 16540737

SP-B

• Surfactant protein B (SP-B) mRNA and protein are restricted to alveolar Type II and Clara cells in the respiratory epithelium. PMID: 10383422
• the SP-B promoter may be useful in targeting type II and Clara cells for gene therapy of conditions such as inherited deficiency of SP-B. PMID: 9448040

SP-C

• rat Clara cells do not express SP-C mRNA, and thus SP-C can be regarded as a specific marker for rat type II cells. PMID: 1591008

SP-D

• Specific gold labeling for SP-D was found in alveolar Type II cells, Clara cells, and alveolar macrophages. PMID: 1527377

Urinary protein 1

• a low mol mass urinary protein of unknown function. The molecular size, subunit composition and partial amino acid sequence of P1 are similar to those of the 10 kDa Clara cell protein (CC10). PMID: 1395029
• a recently discovered urinary protein identical to Clara cell protein. PMID: 1488625

Uteroglobin

• the main secretory product of bronchiolar Clara cells, plays an important protective role in the respiratory tract against inflammatory processes. PMID: 12838427
• a nonglycoprotein with a molecular mass of 16 kilodaltons, which is produced by mucosal epithelial cells in the lung (Clara cells), uterus and prostate. PMID: 12769755
• a secretory protein mainly synthesized in the pulmonary Clara cells. PMID: 9473294

UGRP1

• a uteroglobin/Clara cell secretory protein-related protein, is a novel lung-enriched downstream target gene for the T/EBP/NKX2.1 homeodomain transcription factor. PMID: 11682631