What Controls Macrophage Numbers in vivo?
At present, we don’t know the answer to this question. As a first step towards answering it, we have compiled a list of processes which act to increase the tissue macrophage population, and a list of opposing processes which act to reduce macrophage number. These processes are illustrated in the diagram below:
The relative rates of macrophage accumulation and loss will regulate the equilibrium number of macrophages present in a tissue. As a result, to understand macrophage population dynamics it is necessary to estimate the rates of all these different contributory processes.
Unfortunately, estimating the rates of dynamic processes in vivo is notoriously difficult. With current technology it is hard to make repeated measurements over time in a living organism (for example, to watch the rate of monocyte recruitment into a given tissue). Instead, we have used molecular markers of the various dynamic processes to give us a first estimate of their rates. For example, we know that newly recruited monocytes express the M1/70 antigen (the integrin CD11b) at high levels for only about 24 hours after recruitment into tissue, and that as differentiation in tissue macrophages occurs, CD11b expression is lost. Thus, the fraction of monocyte/macrophages (defined as F4/80+ cells) in a tissue which are also M1/70+ gives a first estimate of the current rate of monocyte recruitment into that tissue.
Similarly, we can use markers of cell death (such as TUNEL+ staining or staining for activated caspases) to estimate the rate of macrophage loss by apoptosis or necrosis. The disadvantage of these molecular estimates of the rate of dynamic processes is that lour estimates can be seriously flawed if the duration that a given cell remains TUNEL+ or M1/70+ changes dramatically. Thus, an increase in the fraction of macrophages which are TUNEL+ might indicate an increased rate of macrophage apoptosis, or it might indicate that apoptotic macrophages are remaining TUNEL+ for longer (the apoptotic process has slowed down, or the TUNEL+ cells are being cleared more slowly, for example).
Despite these caveats, we have recently utilised these approaches to investigate the impact of Chemotides on macrophage population dynamics in apoE-deficient mice. We found that macrophage numbers, and macrophage population dynamics in liver lung and brain were essentially unaltered by treatment with NR58-3.14.3 at 30mg/kg/day for six months – a treatment regiment which results in profound suppression of leukocyte recruitment to sites of inflammation. We conclude that the basal trafficking of monocytes into a range of tissues, as well as their differentiation into macrophages, is not dependent on any chemokine signalling pathway inhibited by the Chemotide NR58-3.14.3.