Abstract
Understanding mechanisms of cavitation in tuberculosis (TB) is the missing link that could advance the field towards better control of the infection. Descriptions of human TB suggest that postprimary TB begins as lipid pneumonia of foamy macrophages that undergoes caseating necrosis and fragmentation to produce cavities. This study aimed to investigate the various mycobacterial antigens accumulating in foamy macrophages and their relation to tissue destruction and necrosis. Pulmonary tissues from mice with slowly progressive TB were studied for histopathology, acid-fast bacilli (AFB) and presence of mycobacterial antigens. Digital quantification using Aperio ImageScope was done. Until week 12 postinfection, mice were healthy, and lesions were small with scarce AFB and mycobacterial antigens. Colony-forming units (CFUs) increased exponentially. At week 16-33, mice were sick, macrophages attained foamy appearance with an increase in antigens (P<.05), 1.5 log increase in CFUs and an approximately onefold increase in AFB. At week 37-41, mice started dying with a shift in morphology towards necrosis. A >20-fold increase in mycobacterial antigens was observed with only less than one log increase in CFUs and sevenfold increase in AFB. Secreted antigens were significantly (P<.05) higher compared to cell-wall antigens throughout infection. Focal areas of necrosis were associated with an approximately 40-fold increase in antigen MPT46, functionally active thioredoxin, and a significant increase in all secreted antigens. In conclusion, mycobacterial antigens accumulate in the foamy macrophages in TB lesions during slowly progressive murine pulmonary TB. Secreted antigens and MPT46 correlated with necrosis, thereby implying that they might trigger the formation of cavities.
Highlights
Ending tuberculosis (TB) disease remains an unachieved objective for many countries with an estimated 10 million new cases reported in the year 2018
There is a scarcity of information on the immune pathogenesis of postprimary TB which largely accounts for TB cavitation
A large increase in colony-forming unit (CFU) was observed in the early stage with little or no antigen accumulation
Summary
Ending tuberculosis (TB) disease remains an unachieved objective for many countries with an estimated 10 million new cases reported in the year 2018. TB patients with cavities are a major source of disease transmission within a population. Experimental studies using various animal models have given useful insight into the immune pathogenesis of primary TB, characterized by the granulomatous immune response. This immune response is very effective in controlling infection, supported by the epidemiological studies showing that 90%-95% of the infected individuals never develop active TB disease.[3] there is a scarcity of information on the immune pathogenesis of postprimary TB which largely accounts for TB cavitation. Understanding the factors involved in tissue destruction and development of cavitation in postprimary TB may yield novel strategies for reducing disease transmission
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