The secretory leukocyte protease inhibitor (SLPI) and the secondary granule protein lactoferrin are synthesized in myelocytes, colocalize in subcellular fractions of neutrophils, and are coreleased by activated neutrophils.

The secretory leukocyte protease inhibitor (SLPI) re-establishes homeostasis at sites of infection by virtue of its ability to exert antimicrobial activity, to suppress LPS-induced cellular immune responses, and to reduce tissue damage through inhibition of serine proteases released by polymorphonuclear neutrophil granulocytes (PMNs). Microarray analysis of bone marrow (BM) populations highly enriched in promyelocytes, myelocytes/metamyelocytes (MYs), and BM neutrophils demonstrates a transient, high mRNA expression of SLPI and genuine secondary granule proteins (GPs) in MYs. Consistent with this finding, immunostaining of BM cells showed SLPI and the secondary GP lactoferrin (LF) to be present in cells from the myelocyte stage and throughout neutrophil differentiation. Subcellular fractionation studies demonstrated the colocalization of SLPI and LF in subcellular fractions highly enriched in secondary granules. Finally, exocytosis studies demonstrated a corelease of SLPI and LF within minutes of activation. Collectively, these findings strongly indicate that SLPI is localized in secondary granules of PMNs. However, the amount of SLPI detected in PMNs is low compared with primary keratinocytes stimulated by growth factors involved in wound healing. This implicates that neutrophil-derived SLPI might not contribute essentially to re-establishment of homeostasis at sites of infection but rather, exert physiologically relevant intracellular activities. These might include the protection of secondary GPs against proteolytic activation and/or degradation by proteases, which might be dislocated to secondary granules at minute amounts as a consequence of spillover.