Mycobacterial antigens accumulation in foamy macrophages of tuberculosis lesions plays a central role in tuberculosis pathogenesis and tissue destruction in chronic murine pulmonary tuberculosis

Field: Medicine (Health Sciences)

Introduction: Tuberculosis (TB) is the ninth leading cause of death worldwide ranking even higher than HIV/AIDS. Multi-drug resistant TB remains a threat. Post-primary cavitary TB, responsible for 80% of disease burden and 100% transmission, occurs in individuals despite strong immunity to TB, with poorly understood immune pathogenesis. Understanding factors involved in pathogenesis and tissue destruction in cavity formation can help advance the field towards developing more effective vaccine and therapies towards elimination of TB disease. Aims: The aim was to study the pulmonary pathology, bacterial growth and expression of various mycobacterial antigens in the pulmonary lesions during the course of slowly progressive murine TB and the factors associated with tissue destruction and disease severity. Methods: B6D2F1 mice were inoculated with Mycobacterium Tuberculosis H37Rv to develop slowly progressive TB. Immunohistochemistry using in-house anti-rabbit polyclonal antibodies was used to investigate the in-situ expression of various mycobacterial antigens. Aperio ScanScope CS® Slide Scanner (Aperio Technologies Inc., Vista, CA, USA) was used to scan slides. For visualization and digital quantification of scanned slides Image Scope software (Aperio Technologies Inc., Vista, CA, USA) and Colour deconvolution algorithm was used. Results: Until week 12 post-infection, mice were healthy, lesions were small, bacterial colony forming units (CFUs) increased exponentially, little mycobacterial antigens were seen. At week 16-33, mice showed disease signs. The macrophages attained foamy appearance. Mycobacterial antigens were significantly higher (p&lt;0.05), and there was 1.5 log increase in CFUs and approximately 1-fold increase in AFB. At week 37-41, mice started dying. There was a shift in morphology towards necrosis and tremendous increase in mycobacterial antigens was observed which was in sharp contrast to slight increase of less than one log in CFUs and approximately 7-fold increase in AFB. Total secreted mycobacterial antigens and individual secreted mycobacterial antigens were significantly (p&lt;0.05) higher along the course of infection compared to cell-wall mycobacterial antigens. Development of focal areas of necrosis was associated with approximately 40-fold increase in antigen MPT46, functionally active thioredoxin of Mycobacterium tuberculosis, and a significant increase in secreted antigens as compared to cell-wall antigens. Conclusion: Mycobacterial antigens accumulate in the foamy macrophages in the TB lesions over the course of infection in the slowly progressive murine pulmonary TB. The accumulation of total secreted mycobacterial antigens, and particularly antigen MPT46 correlated with necrosis, tissue destruction, and mortality rather than an increase in CFUs, AFB or the extent of inflammation. Tissue destruction and necrosis is assumed as the precursor of cavitation, thereby implying the role of MT46 in formation of cavities in TB disease.