Immunology and Pathogenesis of Pulmonary Aspergillosis

Aspergillus is a ubiquitous saprophytic fungus and has adapted to the environment and developed mechanisms to colonize, invade, and proliferate within the human host, leading to a spectrum of pulmonary disorders, including allergic bronchopulmonary aspergillosis (ABPA), chronic pulmonary aspergillosis (CPA), and invasive pulmonary aspergillosis (IPA). ABPA primarily affects individuals with cystic fibrosis (CF) or asthma and is characterized by a hypersensitivity reaction to persistent Aspergillus fumigatus colonization. Genetic predispositions, such as polymorphisms in surfactant protein A2 (SP-A2) and HLA-DR, and cystic fibrosis transmembrane conductor (CFTR) genes play a critical role. The pathogenesis involves a skewed Th2 response, leading to elevated IgE levels, eosinophilic infiltration, and chronic inflammation confined within bronchial mucus plugs. CPA, on the other hand, predominantly occurs in immunocompetent individuals with underlying pulmonary diseases like tuberculosis or chronic obstructive pulmonary disease (COPD). The condition is marked by Aspergillus colonization, chronic inflammation, cavity formation, and progressive fibrosis. Dysregulated cytokine production, alongside genetic factors, exacerbates the disease, with polymorphisms in mannose-binding lectin (MBL) and TLR4 contributing to host susceptibility. IPA, the most severe form, occurs in immunocompromised patients and is distinguished by rapid pulmonary epithelial destruction, vascular invasion, and thrombosis, often leading to lung infarction. The immune response is dominated by neutrophils and Th1 cells, while Th2 responses may worsen outcomes. Despite extensive research into the immune deficiencies in IPA, the precise roles of innate and adaptive immunity in CPA and ABPA require further elucidation. A deeper understanding of the immunopathogenesis across this disease spectrum is imperative for developing targeted therapeutic interventions and improving patient outcomes.