Eosinophils and secretory leukocyte protease inhibitor (SLPI) are both associated with Th2 immune responses and allergic diseases, but whether the fact that they are both implicated in these conditions is pathophysiologically related remains unknown. Here we demonstrate that human eosinophils derived from normal individuals are one of the major sources of SLPI among circulating leukocytes. SLPI was found to be stored in the crystalline core of eosinophil granules, and its dislocation/rearrangement in the crystalline core likely resulted in changes in immunostaining for SLPI in these cells. High levels of SLPI were also detected in blood eosinophils from patients with allergy-associated diseases marked by eosinophilia. These include individuals with eosinophilic granulomatosis with polyangiitis (EGPA) and atopic dermatitis (AD), who were also found to have elevated SLPI levels in their plasma. In addition to the circulating eosinophils, diseased skin of AD patients also contained SLPI-positive eosinophils. Exogenous, recombinant SLPI increased numbers of migratory eosinophils and supported their chemotactic response to CCL11, one of the key chemokines that regulate eosinophil migratory cues. Together, these findings suggest a role for SLPI in controlling Th2 pathophysiologic processes via its impact on and/or from eosinophils.

Congenital neutropenia, which refers to an inherited deficiency in neutrophils, is a rare pathologic condition that affects approximately 0.0001-0.0009% of the general population. While congenital neutropenia can result from mutations in approximately 30 genes, its leading cause is gain-of-function mutations in the ELANE gene, which encodes the neutrophil granule serine protease, neutrophil elastase. This review focuses on established and novel concepts in the genetic, molecular and cellular mechanisms underlying neutrophil elastase-dependent neutropenia, and discusses possible new avenues for neutropenia research as well as potential novel treatment options that target pathogenic elastase variants.