At high concentrations of GM-CSF ( 300 pg/ml), signaling via Ser585 is extinguished and signaling occurs exclusively via residue Tyr577, activating STAT5 signaling, which enhances NFB activity and the LPS response. pulmonary surfactant clearance. A similar phenotype occurs in mice deficient in the production of GM-CSF or GM-CSF receptors. PAP and related research has uncovered a critical and emerging role for GM-CSF in the regulation of pulmonary surfactant homeostasis, lung host defense, and systemic immunity. [44; 45; 46; 47; 48; 49]. The disruption of surfactant homeostasis, increased microbial susceptibility, and abnormal alveolar macrophage morphology and function in GM-CSF-deficient mice were corrected by replacement [50; 51; 52]. The effects of GM-CSF on surfactant homeostasis and alveolar macrophage function were found to be mediated primarily by PU.1, a transcription factor required for terminal differentiation, surfactant catabolism and multiple immune functions of alveolar macrophages . GM-CSF Receptor Deficient Mice Mice deficient in the GM-CSF receptor chain develop a lung phenotype similar to that of GM-CSF deficient mice . GM-CSF levels are increased in serum and lung in these mice due to reduced clearance , a molecular observation of translational significance (see below) . The disruption of surfactant homeostasis were corrected by bone marrow transplantation, demonstrating that the site Cichoric Acid of the molecular pathogenesis was cells of hematologic and not pulmonary origin, i.e., alveolar macrophages not pulmonary epithelial cells  Autoimmune PAP in Humans A second major clue regarding the pathogenesis of PAP was the strong association of high levels of neutralizing GM-CSF autoantibodies with idiopathic PAP , the most common clinical form representing 90% of cases . This rare disease has a prevalence of 6C7 individuals per million in the general population, occurs in all ethnic groups and is approximately twice as common in males, presumably due to Cichoric Acid the association with smoking. Individuals usually present in the third to fourth decades of life with progressive dyspnea of insidious onset, diffuse bilateral lung infiltrates on radiological evaluation, and pulmonary pathology comprised of well-preserved alveoli filled with a granular, eosinophilic, periodic acid-Schiff (PAS)-positive material. Secondary infections can occur and account for 18% of the reported mortality in PAP , presumably due to myeloid cell defects (see below). The GM-CSF autoantibodies are polyclonal, comprised primarily of IgG1 and IgG2, with only small amounts of IgG3 and IgG4 . They recognize human GM-CSF with high avidity (207.5 pM) and high specificity  and are capable of neutralizing far more GM-CSF than is physiologically present . Interestingly, GM-CSF autoantibodies have been consistently detected and comprise the major anti-cytokine activity in pharmaceutical immunoglobulin prepared from pooled serum from healthy individuals . Further, GM-CSF autoantibodies were also present in healthy donors, albeit at levels (median [interquartile range] = 1.04 [0.63C1.72] g/ml)  far lower than those present in PAP patients (59.8 [27.4C116.5] g/ml). These results suggested the hypothesis that Cichoric Acid the risk of PAP is increased when GM-CSF autoantibody levels are increased above a critical threshold, which has been reported to be ~5 g/ml  (Figure 3). The function of GM-CSF autoantibodies in healthy individuals is uncertain. However, the finding that GM-CSF levels in serum are higher than earlier reports Rabbit Polyclonal to SFRS17A suggested and that more than 99% of serum GM-CSF in healthy individuals and PAP patients exists in the form of immune complexes suggesting that GM-CSF autoantibodies may have a scavenging role . Open in a separate window Figure 3 Relationship between GM-CSF autoantibody concentration, GM-CSF bioactivity, and GM-CSF dependent myeloid functionsAt GM-CSF autoantibody concentrations below the critical threshold, GM-CSF bioactivity and GM-CSF dependent functions (i.e., CD11b stimulation, neutrophil phagocytosis) vary inversely with autoantibody Cichoric Acid concentration. At and above the critical threshold, GM-CSF bioactivity and GM-CSF-dependent functions are reduced minimum values (i.e., alveolar macrophage catabolism of surfactant). The hatched region represents the inverse relationship; the black bar represents priming caused by supranormal GM-CSF levels present during infection or exogenous administration; the white bar represents residual, non-zero minimal values of some function (e.g., phagocytosis). The critical threshold concept provides Cichoric Acid an explanation of why a high level of GM-CSF autoantibody is virtually diagnostic of autoimmune PAP and yet autoantibody levels (which are generally well above the critical threshold value) do not reflect lung disease severity. Adapted from reference . Reproduction of Autoimmune PAP in Healthy, Non-Human Primates In the context of data from GM-CSF deficient mice, the strong association of neutralizing GM-CSF autoantibodies in patients with PAP is compelling evidence.