The general review of causes and mechanisms that contribute to glucocorticoid induced leukocytosis (increase in white blood cell (WBC) counts) can be found in this EBM Consult article ... click here.1  Of the mechanisms contributing to the increase in WBC counts, the demargination of neutrophils (polymorphonuclear leukocytes; PMN) from the endothelial lining of the blood vessels into the general circulation is most significant.2  In fact, approximately 61% of the increase in WBC counts comes from this biologic effect of glucocorticoid use and can be observed within 5-24 hours of initiation.2-4  

How do glucocorticoids (e.g., dexamethasone, methylprednisolone, prednisone) effect neutrophils (PMN) to cause this demargination?
Neutrophils reside in a number of compartments; the two compartments related to this issue are the marginal compartment (those neutrophils attached to the endothelium of the blood vessel) and the circulating compartment (those circulating in the blood vessels along with other cells).5  This distinction is important as neutrophils traveling on the endothelial surface within the lumen of the blood vessel (i.e., in the marginal compartment) are not reflected in a WBC count.  Only the PMN freely circulating within the circulatory compartment will be found in the venous sample used for analysis.  As such anything that causes the marginal neutrophils to detach from the endothelial surface of the blood vessel wall will result in a greater concentration of neutrophils in the circulatory compartment and thus increase the WBC count.  Glucocorticoids are known to do this.2-4 

Neutrophils (both bands and segs) normally express a cell surface adhesion molecule called L-selectin (CD26L) that is known to contribute to the formation of temporary carbohydrate-protein bonds on the surface of the endothelial cells lining the blood vessel wall.6,7  The selectin adhesion molecules do not interact tightly with the protein molecules on the endothelial surfaces which explains the neutrophil's ability to roll quickly along the surface until alerted to transmigrate into the tissue to fight an infection.6,7  L-selectin is known to undergo rapid turnover as the neutrophils roll along the endothelial surface.8,9  In fact, the L-selectin are constantly being cleaved by membrane-associated cysteine metalloproteinase (also known as sheddase).8,9  The replacement of L-selectin on the surface of the cell membrane is dependent on the neutrophil's ability to undergo gene transcription to make more L-selectin that is then translated, packaged and prepared for transport to the cell surface to replace the shed L-selectin.4  If this does not occur, then the number of  temporary or loosely bound carbohydrate-protein bonds will decrease and eventually not be sufficient to keep the neutrophil attached to the endothelial surface and thus demargination into the circulatory compartment will occur.4  Therefore, it is the inhibition of gene transcription of L-selectin by glucocorticoids that results in L-selectin not being replaced and thus reducing the ability of the neutrophil to interact with the endothelium of the blood vessel.

How do glucocorticoids decrease gene transcription of L-selectin thereby decreasing the neutrophil's ability to replace shed L-selectin to maintain its attachment to the endothelial surface?
The exact details of how glucocorticoids turn off gene transcription for L-selectin is not completely understood. However, glucocorticoids are very well known to bind to glucocorticoid receptors (in particular GR-alpha) found in the cytoplasm of the neutrophil.4  Upon binding, the glucocorticoid/GR complex results in a conformational change that results in the dissociation of other proteins that free the glucocorticoid/GR complex to enter into the nucleus.   After entering the nucleus it can bind to glucocorticoid responsive elements (GRE) in the region just above the target gene (L-selectin in this case) where it can either turn on gene transcription or inhibit it.  It is likely that this is where the inhibition occurs since mRNA levels for L-selectin are decreased with glucocorticoid use.4  This process does not occur immediately which explains why the progressive loss of L-selectin starts 5-24 hours post administration of the glucocorticoid and coincides with the rise in PMNs in the WBC count.

While glucocorticoids are known to increase the concentration of neutrophils present in the circulatory compartment thereby increasing the WBC count, the effect of demargination is also one of the mechanisms involved in their antiinflammatory properties.  If the neutrophil is not attached to the endothelium, transmigration into the tissue is decreased. 

References:

1. Busti AJ, et al.  A general review of the mechanisms for steroid or glucocorticoid (e.g., dexamethasone, methylprednisolone, prednisone) increases the white blood cell (WBC) count.   EBM Consult.  
2. Nakagawa M, Terashima T, D'yachkova Y et al.  Glucocorticoid-induced granulocytosis: contribution of marrow release and demargination of intravascular granulocytes.  Circulation  1998;98:2307-13.  
3. Shoenfeld Y, Gurewich Y, Gallant LA, et al.  Prednisone-induced leukocytosis. Influenced of dosage, method and duration of administration on the degree of leukocytosis.  Am J Med  1981;71:773-8.  
4. Weber PS, Toelboell T, Chang LC et al.  Mechanisms of glucocorticoid-induced down-regulation of neutrophil L-selectin in cattle: evidence for effects at the gene-expression level and primarily on blood neutrophils.  J Leukoc Biol  2004;75:815-27.  
5. Junqueira LC, Carneiro J.  Blood cells.  In: Basic Histology.  11^th ed.  Junqueira LC, Caneiro J eds.  McGraw-Hill Medical Publishing Division.  New York, NY. 2005; 223-237.
6. Tedder TF, Steeber DA, Chen A et al.  The selectins: vascular adhesion molecules.  FASEB J  1995;9:866-73.  
7. Van Eeden S, Miyagashima R, Haley L et al.  L-selectin expression increases on peripheral blood polymorphonuclear leukocytes during active marrow release.  Am J Respir Crit Care Med  1995;151:500-7.  
8. Preece G, Murphy G, Ager A.  Metalloproteinase-mediated regulation of L-selectin levels on leucocytes.  J Biol Chem  1996;271:11634-40.  
9. Peschon JJ, Slack JL, Reddy P et al.  An essential role for ectodomain shedding in mammalian development.  Science 1998;282:1281-4.

• Pharmacology: A General Review of all of the Mechanisms for Steroid Induced Increases in WBC
• Pharmacology: The Average Increase in WBC from Steroids
• Pharmacology: The Average Time of Onset for Steroid Induced Increases in the WBC
• Pharmacology: The Mechanism of Steroid Induced Avascular Necrosis of the Femoral Neck

Steroids, Glucocorticoids, Dexamethasone, Methylprednisolone, Prednisone, Steroid Induced Demargination, Demargination of WBC