Microorganism-Dependent Phagocytic Responses in Murine Alveolar Macrophages

Background. Macrophages are a highly heterogeneous immune cell subset that play essential roles in innate and adaptive immunity. Alveolar macrophages (AMs) represent the major resident immune cell population in the distal airways and constitute the first line of cellular defense against inhaled microorganisms. Phagocytosis is a fundamental function of AMs, supporting both pathogen clearance and lung homeostasis. However, the efficiency with which AMs internalize different classes of microorganisms remains incompletely explored, despite its relevance for early host–pathogen interactions in the lung. This study aimed to quantitatively compare the phagocytic activity of murine alveolar macrophages toward Gram-negative bacteria (Escherichia coli), Gram-positive bacteria (Staphylococcus aureus), and the yeast Candida albicans under non-opsonized, non-inflammatory in vitro conditions using flow cytometry. Methods. Murine AMs were isolated from BALB/c mice by bronchoalveolar lavage. Phagocytic activity was assessed using FITC-labeled, inactivated microorganisms, including heat-killed Staphylococcus aureus (Gram-positive), heat-killed Escherichia coli (Gram-negative), and ethanol-fixed Candida albicans. AMs were incubated with microbial targets for 30 minutes at 37 °C. Phagocytosis was quantified by flow cytometry as the percentage of FITC-positive cells and the phagocytic index, expressed as mean fluorescence intensity. Results. Analysis of AM phagocytic activity revealed pronounced target-dependent differences. Uptake was lowest for Escherichia coli, intermediate for Staphylococcus aureus, and highest for Candida albicans. This pattern was consistent for both the percentage of phagocytosing cells and the phagocytic index. Conclusions. Taken together, murine alveolar macrophages exhibit higher phagocytic activity toward Candida albicans than toward Gram-positive or Gram-negative bacteria under non-opsonized, non-inflammatory conditions. These results highlight pathogen-dependent differences in alveolar macrophage phagocytosis relevant to early innate immune defense in the lung.