Profiling the immune response to Mycobacterium tuberculosis Beijing family infection: A perspective from the transcriptome

Tuberculosis continues as an important public health problem. Particularly considering Beijing-family strains of Mycobacterium tuberculosis, which have been associated with drug-resistance and hypervirulence. The Beijing-like SIT190 (BL) is the most prevalent Beijing strain in Colombia. The pathogenic mechanism and immune response against this pathogen is unknown. Thus, we compared the course of pulmonary TB in BALB/c mice infected with Classical-Beijing strain 391 and BL strain 323. The disease course was different among infected animals with Classical-Beijing and BL strain. Mice infected with BL had a 100% mortality at 45 days post-infection (dpi), with high bacillary loads and massive pneumonia, whereas infected animals with Classical-Beijing survived until 60 dpi and showed extensive pneumonia and necrosis. Lung RNA extraction was carried out at early (day 3 dpi), intermediate (day14 dpi), and late (days 28 and 60 dpi) time points of infection. Transcriptional analysis of infected mice with Classical-Beijing showed several over-expressed genes, associated with a pro-inflammatory profile, including those for coding for CCL3 and CCL4 chemokines, both biomarkers of disease severity. Contrary, mice infected with BL displayed a profile which included the over-expression of several genes associated with immune- suppression, including Nkiras, Dleu2 and Sphk2, highlighting an anti-inflammatory milieu which would allow high bacterial replication followed by an intense inflammatory response. In summary, both Beijing strains induced a non-protective immune response which induced extensive tissue damage, BL strain induced rapidly extensive pneumonia and death, whereas Classical-Beijing strain produced slower extensive pneumonia later associated with extensive necrosis. In this study, the progressive pulmonary TB model in BALB/c mice were used; groups of 50 male animals per group were anaesthetized with sevoflurane (100 uL per mice) and infected by intratracheal inoculation with 250.000 bacteria/100uL of each strain. At day 1, 3, 7, 14, 21, 28 and 60 (with surviving mice) after infection groups of six mice were euthanized by exsanguination under anesthesia with pentobarbital. Left lungs lobes and spleens were removed for CFU processing and histopathology. Here, higher virulence of Beijing like strain was characterized by earlier and higher mortality, higher pulmonary bacillary loads with extensive tissue damage (pneumonia). In contrast, classical Beijing 391 strain induced slower disease progression, allowing animals live up to day 60, however at this day the pulmonary histology showed necrotic areas, but with lower bacillary burdens than the animals infected with Beijing Like