Pulmonary alveolar proteinosis, a primary immunodeficiency of impaired GM-CSF stimulation of macrophages

Abstract

Pulmonary alveolar proteinosis (PAP) is a rare syndrome characterized by accumulation of pulmonary surfactant, respiratory insufficiency, and increased infections. It occurs in various clinical settings that disrupt surfactant catabolism in alveolar macrophages, including a relatively more common autoimmune disease caused by GM-CSF autoantibodies and a rare congenital disease caused by CSF2RA mutations. Recent results demonstrate that GM-CSF is crucial for alveolar macrophage terminal differentiation and immune functions, pulmonary surfactant homeostasis, and lung host defense. GM-CSF is also required to determine the basal functional capacity of circulating neutrophils, including adhesion, phagocytosis, and microbial killing. PAP research has illuminated the crucial role of GM-CSF in innate immunity and led to novel therapy for PAP and the potential use of anti-GM-CSF therapy in other common disorders.

Figure 1. Mechanisms by which GM-CSF regulates the survival, differentiation, functions and activation of alveolar macrophages

GM-CSF (GM) initiates signaling by first binding to the GM-CSF receptor α chain (α), which then associates with homodimers of the affinity-enhancing GM-CSF receptor β chain (β). Jak2 is bound constitutively to the β chain and signals through an intracytoplasmic β chain motif including residues tyrosine⁵⁷⁷ and serine⁵⁸⁵, which is necessary and sufficient for GM-CSF receptor signaling. At low GM-CSF concentrations (0.01 – 10 pM), phosphorylation of serine⁵⁸⁵ couples signaling via the adapter protein, 14-3-3 through PI3K and Akt, resulting in cell survival without proliferation. At high GM-CSF concentrations (10 – 10,000 pM), phosphorylation of tyrosine⁵⁷⁷ couples signaling via STAT5 or Shc-dependent pathways, stimulating cell survival, cellular activation and proliferation. Pulmonary GM-CSF stimulates expression of PU.1 in alveolar macrophages, which in turn regulates the expression of numerous genes enabling multiple immune and non-immune functions consistent with terminal differentiation of alveolar macrophages in the lungs. Interruption of GM-CSF signaling, either by neutralizing autoantibodies or function-altering amino acid changes in GM-CSF receptor α (G196R) impair GM-CSF receptor signaling and alveolar macrophage maturation. One of the functions affected is the ability to catabolize surfactant lipids internalized into endosomes, thereby reducing surfactant clearance and causing PAP.

Figure 2. Relationship between GM-CSF autoantibody concentration, GM-CSF bioactivity and regulation of GM-CSF-dependent myeloid cell functions

Over a range of low GM-CSF autoantibody levels present in healthy subjects, increasing GM-CSF autoantibody concentrations (abscissa) rheostatically lower GM-CSF bioactivity (right ordinate) thereby reducing in tandem, GM-CSF-dependent myeloid cell functions (left ordinate). Some functions have activity that is GM-CSF-independent (open bar, left ordinate), modulated by physiologic changes in GM-CSF concentration (hatched bar, left ordinate), or stimulated to supranormal levels by exogenous or pathologically increased GM-CSF levels (black bar, left ordinate). Above a concentration sufficient to block GM-CSF completely (the critical threshold), GM-CSF bioactivity is zero and GM-CSF-dependent functions are minimal. GM-CSF autoantibody concentrations between zero and the critical threshold are present in healthy individuals and may serve a physiological role by negatively regulating myeloid cell reactivity. The critical threshold also helps to define a therapeutic window for the potential use of GM-CSF autoantibodies to treat other disorders. GM-CSF antibody levels above the critical threshold are anticipated to increase the risk of iatrogenic PAP. Adapted from reference [8].

Figure 3. Proposed modes by which GM-CSF regulates alveolar macrophage functions and modulation GM-CSF autoantibodies

GM-CSF produced locally in the lung interacts with receptors on nearby alveolar macrophages stimulating terminal differentiation (paracrine mode) thereby enabling the numerous functions and signaling pathways, e.g., TLR4 pathway. Pathological stimuli activate signaling pathways with biologic responses important to host defense of that cell. GM-CSF released by and binding to the cell’s own GM-CSF receptors (autocrine mode) switches them into the tyrosine⁵⁷⁷-mediated, high activity state [15], activating the macrophage, which enhances immune functions and stimulates proliferation. This autocrine mode of action provides a fine control for GM-CSF to modulate host defenses on a microscopic scale in the local microenvironment of the cell (i.e., after encountering a pathogen) independent of other components of regional or systemic immunity. GM-CSF originating from a ‘upstream’ site of inflammation can stimulate macrophages at distal sites (endocrine mode), which may result in unnecessary (pathologic) activation. Low levels of GM-CSF autoantibodies in healthy individuals appear to block endocrine signaling and may modulate autocrine modes of GM-CSF signaling, whereas high levels in PAP patients also block paracrine signaling resulting in maturational arrest of macrophages.