Phagocytosis of Mycobacterium fortuitum by Caprine Alveolar Macrophages Is Associated with iNOS and Pro-Inflammatory Markers Expression

Mycobacterial diseases, including tuberculosis (TB), remain the major health and economic challenges in livestock, underscoring the need to characterise the innate immune mechanisms involved in early bacterial containment. Alveolar macrophages (AMs) are the first line of defence against inhaled mycobacteria, yet the functional links between activation, polarisation, and phagocytic capacity in caprine AMs remain poorly defined. In this study, we compared a pH-dependent live-cell fluorescence assay with a culture-based method to evaluate phagocytosis and clearance of <i>Mycobacterium fortuitum</i> under different immunostimulatory conditions. AMs were stimulated in vitro with LPS or heat-inactivated <i>Mycobacterium bovis</i> (HIMB), and phagocytosis was assessed alongside activation and pro-inflammatory markers. Both approaches showed that LPS stimulation significantly enhanced mycobacterial clearance, despite reduced initial bacterial uptake. Moreover, this improved phagocytic capacity was associated with increased expression of the inducible Nitric Oxide Synthase (iNOS), MHCII, CD80, and CD86, as well as an elevated production of some pro-inflammatory cytokines. In contrast, HIMB induced cytokine secretion but failed to enhance activation markers or bacterial clearance. Collectively, these results establish the first association between pro-inflammatory activation and functional mycobacterial phagocytosis in caprine AMs and validate a robust methodological framework for studying innate immune responses relevant to TB and vaccine development in goats.