The Thyroid Hormone Inactivating Enzyme Type 3 Deiodinase is Present in Bactericidal Granules and the Cytoplasm of Human Neutrophils

Abstract

Neutrophils are important effector cells of the innate immune system. Thyroid hormone (TH) is thought to play an important role in their function. Intracellular TH levels are regulated by the deiodinating enzymes. The TH-inactivating type 3 deiodinase (D3) is expressed in infiltrating murine neutrophils, and D3 knockout mice show impaired bacterial killing upon infection. This suggests that D3 plays an important role in the bacterial killing capacity of neutrophils. The mechanism behind this effect is unknown. We aimed to assess the presence of D3 in human neutrophils, and determine its subcellular localization using confocal and electron microscopy, because this could give important clues about its function in these cells. D3 appeared to be present in the cytoplasm and in myeloperoxidase containing azurophilic granules and as well as lactoferrin containing specific granules within human neutrophils. This subcellular localization did not change upon activation of the cells. D3 is observed intracellularly during neutrophil extracellular trap formation, followed by a reduction of D3 staining after release of the neutrophil extracellular traps into the extracellular space. At the transcriptional level, human neutrophils expressed additional essential elements of TH metabolism, including TH transporters and TH receptors. Here, we demonstrate the presence and subcellular location of D3 in human neutrophils for the first time and propose a model, in which D3 plays a role in the bacterial killing capacity of neutrophils either through generation of iodide for the myeloperoxidase system or through modulation of intracellular TH bioavailability.

Thyroid hormone metabolism in innate immune cells

“…Murine neutrophils contain the TH transporter MCT8, whereas human neutrophils express MCT10 but not MCT8 mRNA (Boelen et al 2005, van der Spek et al 2016. It has long been known that activated neutrophils are capable of deiodinating both T 3 and T 4 (Woeber 1971, Woeber et al 1972, Klebanoff & Green 1973, Woeber & Ingbar 1973.…”

“…It has since been found that type 3 deiodinase (D3), the TH-inactivating enzyme, is present in both human and murine neutrophils and is located in the cytosol and in bactericidal granules within the cell (Boelen et al 2005, 2008, van der Spek et al 2016. Human neutrophils were also recently shown to express type 1 deiodinase (D1) and TRα1 at the transcriptional level (van der Spek et al 2016).…”

“…Intracellular thyroid hormone metabolism may also play a role in neutrophils during bacterial killing. The thyroid hormone-inactivating type 3 deiodinase (D3) is present in murine and human neutrophils (Boelen et al 2005, 2008, van der Spek et al 2016. Mice that lack this enzyme suffer from impaired bacterial killing (Boelen et al 2009).…”

“…Mice that lack this enzyme suffer from impaired bacterial killing (Boelen et al 2009). D3 is located in the cytoplasm and in granules containing either myeloperoxidase (MPO) or lactoferrin (LF) (van der Spek et al 2016). TH enters the neutrophil via transporters (MCT8 or MCT10) where it is inactivated by D3, which removes an iodine atom from the inner ring of the hormone, converting T 4 to reverse (r)T 3 and T 3 to T 2 .…”

“…It was recently shown to also be present in human neutrophils (van der Spek et al 2016). In a murine model for chronic local inflammation in which mice were injected with turpentine resulting in the formation of a subcutaneous abscess, D3 activity was strongly elevated in inflamed tissue compared to control tissue (Boelen et al 2005).…”