Abstract
Neutrophils are the most abundant circulating leukocytes and are characterized by a proteome in which granule-associated proteins synthesized during granulopoiesis constitute a major fraction of total cellular protein, reflecting their preloaded effector nature in innate immune defense. A striking feature of neutrophil biology is the unusual abundance of the calcium-binding proteins S100A8 and S100A9, which together form the heterodimeric complex known as calprotectin. Early biochemical studies estimated that S100A8/A9 constitutes a substantial fraction of the soluble cytosolic proteome in neutrophils, with later studies often describing it as one of the most abundant protein complexes in these cells. Despite extensive studies on the antimicrobial and inflammatory activities of calprotectin, the biological rationale for this unusual abundance remains incompletely understood. In this review, we examine the structural, biochemical, and regulatory features of S100A8/A9 and explore the potential explanations for its high abundance in the neutrophil cytosol. We first discuss the unique organization of the neutrophil proteome and the transcriptional programs governing granulopoiesis that lead to large-scale production of neutrophil effector proteins. We then review the structural and biochemical properties of S100A8/A9, including its calcium-dependent conformational dynamics and high-affinity transition metal binding, which contribute to antimicrobial defense through nutritional immunity. Several functional hypotheses are considered to explain calprotectin abundance, including roles as an antimicrobial reservoir, a metal-sequestering molecule, a regulator of oxidative stress, and a source of damage-associated molecular patterns. Finally, we discuss the evolutionary logic of neutrophil protein preloading and the implications of calprotectin biology in inflammatory diseases and the tumor microenvironment. Resolving the abundance paradox of S100A8/A9 may reveal fundamental principles governing the organization of innate immune cell proteomes and provide new insights into the strategies used by neutrophils to achieve rapid and effective host defense.