CD11b Regulates Fungal Outgrowth but Not Neutrophil Recruitment in a Mouse Model of Invasive Pulmonary Aspergillosis

Background and Aims: In immunosuppressed individuals Aspergillus (A.) fumigatus is a frequent cause of invasive pulmonary aspergillosis (IPA) which is highly associated with relevant morbidity and mortality. Moreover, it often occurs in patients suffering from leukocyte-adhesion deficiency type 1 (LAD1) which is triggered by a functional loss of CD18 in ß2 integrin receptors as these receptors consist of an alpha subunit (CD11a-CD11d) and CD18 as the common beta subunit. ß2 integrin receptors are differentially expressed by leukocytes, and are required for cell-cell interaction, transendothelial migration, uptake of opsonized pathogens, and cell signaling processes. Here, we asked for the importance of CD11b/CD18 also termed MAC-1 which is required for phagocytosis of opsonized A. fumigatus conidia by polymorphonuclear neutrophils (PMN) for control of pulmonary A. fumigatus infection. Methods: We used a murine IPA model (C57BL/6) and challenged CD11b deficient (CD11b-/-) or wild type (WT) mice with A. fumigatus conidia intratracheally. Afterwards, some mice were sacrificed 24h after infection. In these mice PMN recruitment and cytokine patterns were examined by analyzing bronchoalveolar lavage fluid (BALF) and peripheral blood (PB) using flow cytometry, cytospin analysis, and cytometric bead array. Additionally, pulmonary fungal clearance and inflammation in murine lungs were analyzed by fungal culture assays and histopathologic examination using a scoring system. Furthermore, survival was studied with neutropenic animals serving as positive controls. To determine PMNs phagocytic activity and fungal killing capacity ex vivo, PMN were purified from bone marrow of CD11b-/- or WT mice by magnetic cell sorting using Ly6G specific antibodies. Afterwards, isolated PMN were stimulated with pacific blue-labeled tomato red-fluorescent modified A. fumigatus conidia and analyzed by flow cytometry. Results: We found that lung homogenates from CD11b-/- mice obtained 24h after infection showed an enhanced fungal burden as compared to lungs from WT mice. In contrast, lung tissue from infected CD11b-/- mice displayed impaired pulmonary inflammation as assessed by Hematoxilin-Eosin (HE) staining. Furthermore, the number of mucus-producing cells in bronchi of A. fumigatus infected CD11b-/- mice was decreased compared to cells in bronchi of WT mice. However, we observed markedly higher numbers of PMN in BALF of infected CD11b-/- mice compared to corresponding WT mice samples