Efferocytosis perpetuates substance accumulation inside macrophage populations

Abstract

In both cells and animals, cannibalism can transfer harmful substances from the consumed to the consumer. Macrophages are immune cells that consume their own dead via a process called cannibalistic efferocytosis. Macrophages that contain harmful substances are found at sites of chronic inflammation, yet the role of cannibalism in this context remains unexplored. Here we take mathematical and experimental approaches to study the relationship between cannibalistic efferocytosis and substance accumulation in macrophages. Through mathematical modelling, we deduce that substances which transfer between individuals through cannibalism will concentrate inside the population via a coalescence process. This prediction was confirmed for macrophage populations inside a closed system. We used image analysis of whole slide photomicrographs to measure both latex microbead and neutral lipid accumulation inside murine bone marrow-derived macrophages (10 4 – 10 5 cells ) following their stimulation into an inflammatory state ex vivo . While the total number of phagocytosed beads remained constant, cell death reduced cell numbers and efferocytosis concentrated the beads among the surviving macrophages. As lipids are also conserved during efferocytosis, these cells accumulated lipid derived from the membranes of dead and consumed macrophages (becoming macrophage foam cells). Consequently, enhanced macrophage cell death increased the rate and extent of foam cell formation. Our results demonstrate that cannibalistic efferocytosis perpetuates exogenous (e.g. beads) and endogenous (e.g. lipids) substance accumulation inside macrophage populations. As such, cannibalism has similar detrimental consequences in both cells and animals.

A lipid-structured model for macrophage populations in atherosclerotic plaques

“…Rates of LDL ingestion occur on the scale of minutes-to-hours whereas rates of macrophage apoptosis occur on the scale of days-to-weeks [8,20,39]. We assume that changes in occur over a much shorter timescale than the timescale of the cell dynamics such that changes in cell numbers M produce rapid changes in LDL particle numbers .…”

“…We assume that apoptotic macrophages are typically consumed whole by live macrophage via efferocytosis. During efferocytosis, the lipid content of an apoptotic cell, including its endogenous lipid, is transferred to the live macrophage that ingests it [20,42,43]. In equations (3) and 4we account for the effects of efferocytosis on the live and apoptotic macrophages via the three terms E, E and E .…”

“…The number of monocyte-derived macrophages inside the plaque (of the order of 10 6 cells) is determined by the balance [9] between rapid monocyte recruitment (of the order of 10 5 cells per day) [6][7][8], macrophage proliferation [16,17], rapid programmed cell death (apoptosis) (of the order of a day to a week) [14] and/or emigration of macrophages out of the artery wall [18,19]. Macrophages accumulate lipid via phagocytosis of both LDL and apoptotic cells [20]. (Efferocytosis refers to phagocytosis of apoptotic cells.)…”

“…(Efferocytosis refers to phagocytosis of apoptotic cells.) LDL phagocytosis introduces lipid into the macrophage population whereas efferocytosis recycles and concentrates lipid that is already present in the macrophage population [20]. Macrophages can offload cholesterol to high-density lipoproteins (HDL) [21,22] and emigrating macrophages ferry their accumulated lipid out of the plaque [18].…”

“…This is true in ODE models [29], spatially resolved PDE models [30][31][32] and other computational models [33]. Yet it is clear that there is a continuous distribution of lipid loads across macrophage populations, stretching from monocytes with low lipid loads to macrophages with high lipid loads that can be labelled as foam cells in atherosclerotic plaques [20]. The variation in lipid load across a population of macrophages suggests that it may be instructive to describe them using structured models in which the macrophages are characterised by their intracellular lipid load similar to adipocytes [20,34].…”