Role of GM-CSF in Alveolar Macrophage Self-Renewal

ABSTRACT
Alveolar macrophages (AMs) are believed to be a self-renewing cell population without a requirement of replenishment
from extra-pulmonary sources in healthy adult mice; however, the mechanism(s) involved and whether replenishment
occurs by stimulating the proliferation of progenitors or mature AMs are not known. We reported that lung levels of
granulocyte/macrophage-colony stimulating factor (GM-CSF) control long-term maintenance of macrophages trans-
planted into the lungs (via a reciprocal feedback loop) as well as endogenous AMs. My preliminary data demonstrate that
GM-CSF is a critical regulator of AM mitochondrial turnover, integrity and functions and is required for fatty acid oxidation-
derived energy production, processes vital to cell proliferation. Prior studies in a leukemia cell line (TF-1) suggest the
pleotropic effects of GM-CSF on macrophages may be mediated by biphasic, ligand concentration-dependent receptor
signaling, i.e., low levels of GM-CSF promote survival and differentiation (but not proliferation) while high levels also stim-
ulate proliferation. Our long-term goal is to determine mechanisms responsible for AM self-renewal. The objective here
is to elucidate the mechanism by which GM-CSF regulation of mitochondrial homeostasis controls AM self-renewal. Cen-
tral Hypothesis: AMs are maintained by homeostatic self-renewal driven by GM-CSF threshold-triggered/concentration-
dependent, niche-limited proliferation of mature (long-lived) AMs (not progenitors). Rationale: This hypothesis was de-
veloped based on my reported and preliminary data demonstrating GM-CSF deficiency results in reduced mitochondrial
metabolism and integrity despite increased mitochondrial mass. Approach: I will utilize complementary genetic and phar-
macologic tools, in vivo and ex vivo studies (with isolated AMs), and pathway-specific inhibitors to pursue two Specific
Aims: 1) Ontogeny and transcriptional control of AM renewal, and 2) Role of GM-CSF regulated mitochondrial metabolism
in AM renewal. The expected results will inform cellular and molecular mechanism(s) by which GM-CSF regulates AM
population size and will lay the foundation for developing novel therapeutic strategies to modulate AM population size.
The proposed research is innovative, in my opinion, because it challenges the previously widely-held concept of AMs as
short-lived, non-dividing cells replenished from circulating monocytes (regulated by M-CSF) – and instead posits that AM
population size is maintained by homeostatic self-renewal mediated by GM-CSF threshold-triggered/concentration-de-
pendent, niche-limited proliferation of mature (long-lived) AMs. My novel preliminary data identified GM-CSF dependent
regulation of mitochondrial homeostasis as a molecular mechanism for AM self-renewal. In addition, utilization of novel
mouse models and novel methodologies will enable determination of how GM-CSF regulation of mitochondrial homeosta-
sis regulates AM proliferation and will identify the regulatory genes and their downstream targets responsible for long-
term maintenance of AMs. The proposed research is significant because it will advance our knowledge of a critical pulmo-
nary hormone (GM-CSF) by informing the mechanism(s) by which GM-CSF regulates the long-term maintenance of endog-
enous and transplanted AMs and inform us on the therapeutic mechanism of action of PMT, a novel cell therapy in devel-
opment to treat patients with hereditary pulmonary alveolar proteinosis and potentially other lung diseases.

Grant Number: 5R01HL149743-04
NIH Institute/Center: NIH
Principal Investigator: Paritha Arumugam