Strategies of neutrophil diversification

Abstract

Neutrophils are formidable defenders. Their vast numbers, constant production, high cytotoxicity and capacity to produce extracellular traps, underlie their ability to efficiently protect in a microorganism-rich world. However, neutrophils are much more than immune sentinels, as evidenced by the expanding repertoire of functions discovered in the context of tissue homeostasis, regeneration or chronic pathologies. In this Perspective, we discuss general functional features of the neutrophil compartment that may be relevant in most, if not all, physiological scenarios in which they participate, including specialization in naïve tissues, transcriptional noise in the bloodstream as a potential strategy for diversification and functional bias in inflammatory sites. We intentionally present the reader with more questions than answers and propose models and approaches that we hope will shed new light onto the biology of these fascinating cells and spark new directions of research.

Figure 1.. Modeling the architecture of the neutrophil compartment.

(A) During development in the bone marrow, myeloid progenitors undergo sequential maturation to generate mature neutrophils that are released to the circulation. This process can be modelled as a single developmental continuum under steady state conditions. (B) Extending analyses to peripheral tissues reveals additional degrees of heterogeneity as neutrophils adapt to the different environments, and acquire distinct phenotypic and functional properties. These changes likely rely on tissue-derived signals and localization in specific anatomic niches (hubs), such as perivascular areas in the lungs (Environmental model), or alternatively these are already imprinted in circulating clones of neutrophils with pre-defined programs (Pre-committed model). (C) During emergency granulopiesis, increased neutrophil production driven by a stress (e.g., infection or cancer) may use existing developmental trajectories (1) or create new ones (2) to increase population size, without altering the overall architecture of the neutrophil compartment. Alternatively, the stress could alter the normal architecture of granulopiesis to generate entirely new populations (trajectory 3).

Figure 2.. Mechanisms of diversification in blood.

(A) Profiling of blood neutrophils by single cell transcriptomics fails to find diversity using standard computational approaches, but functionally distinct populations could exist inside this transcriptional cloud (red, green, blue and yellow colors). (B) Genetic and phenotypic heterogeneity of blood neutrophils could be imprinted through a series of orthogonal mechanisms that act during granulopoiesis (vertical axis) and in the circulation (horizontal axis). (C) Different sources of heterogeneity, including somatic mutations, circadian ageing, migration across vessels or circulating metabolites influence neutrophils at different stages of maturation to generate diverse transcriptional and functional outputs (cells of different colors).

Figure 3.. Transcriptional noise of mature neutrophils.

(A) Trajectory of neutrophil maturation with different stages shown over a UMAP plot, with the abundance of transcripts and transcriptional noise at each stage shown at bottom (B). Transcriptional noise of a group refers to the compound Euclidean distance of each cell to the transcriptional centroid of the corresponding group/cluster. (C) Transcriptional rates and noise across immature neutrophils in the marrow, and mature neutrophils in blood and peripheral tissues, shown in a linear bubble plot. Note that transcriptional noise increases as rates decrease as cells mature to reach the blood. These properties appear to be maintained in tissues.

Figure 4.. Origin of behavioral states in blood.

Inflammatory neutrophils display at least 3 behavioral states (B1-B3), as identified by live imaging inside inflamed vessels. This functional diversification may be the result of different environmental signals (flat cells of different colors) acting on a unique neutrophil cell state at the inflamed site (A), or alternatively be the result of already biased neutrophil populations exposed similar inflammatory signals inside the vessels (B). These pre-existing states are predicted to have distinct protein composition and/or genetic backgrounds (genetic modifications or epigenetic marks). In all cases, individual neutrophils manifest certain capacity to transition among the different behavioral states.