Integrated platform for linezolid combinations against rifampicin-resistant Mycobacterium tuberculosis: synergy, macrophage apoptosis, and immune modulation

To establish an integrated in vitro evaluation platform for identifying effective antibiotic combinations against rifampicin-resistant Mycobacterium tuberculosis (RR-TB), by simultaneously assessing synergistic drug interactions, intracellular bactericidal activity, macrophage apoptosis, and cytokine responses. Using this platform, we investigated the combinatory potential of linezolid (LZD) with five second-line antitubercular agents. The minimum inhibitory concentrations (MICs) of LZD and five second-line drugs were determined using the Alamar Blue microplate assay. Drug–drug interactions between LZD and cycloserine (CS), clofazimine (CFZ), bedaquiline (BDQ), moxifloxacin (MFX), or levofloxacin (LFX) were evaluated using checkerboard microdilution analysis. Drug pairs demonstrating in vitro synergy were further examined in a macrophage infection model. Intracellular bacterial burden was quantified by colony-forming unit (CFU) enumeration. Macrophage apoptosis was assessed using flow cytometry, and cytokine production (IL-12/23 p40, TNF-α, IL-6, and IL-10) was analyzed to characterize immune modulation. LZD demonstrated synergistic interactions with CS and CFZ, whereas no synergy was observed with BDQ, MFX, or LFX. The synergistic combinations LZD + CS and LZD + CFZ significantly reduced intracellular CFU counts, enhanced macrophage apoptosis, and altered cytokine responses, characterized by increased TNF-α and decreased IL-10 levels in infected macrophages. This study presents a comprehensive and mechanistically informative in vitro methodology for evaluating antibiotic combinations against RR-TB. The platform effectively integrates drug synergy testing with assessments of intracellular killing, apoptosis induction, and immune modulation, offering a promising approach for preclinical screening of novel anti-TB therapeutic strategies.