The chosen few: Mycobacterium tuberculosis isolates for IMPAc-TB

Abstract

The three programs that make up the Immune Mechanisms of Protection Against Mycobacterium tuberculosis Centers (IMPAc-TB) had to prioritize and select strains to be leveraged for this work. The CASCADE team based at Seattle Children's Research Institute are leveraging M.tb H37Rv, M.tb CDC1551, and M.tb SA161. The HI-IMPACT team based at Harvard T.H. Chan School of Public Health, Boston, have selected M.tb Erdman as well as a novel clinical isolate recently characterized during a longitudinal study in Peru. The PHOENIX team also based at Seattle Children's Research Institute have selected M.tb HN878 and M.tb Erdman as their isolates of choice. Here, we describe original source isolation, genomic references, key virulence characteristics, and relevant tools that make these isolates attractive for use. The global context for M.tb lineage 2 and 4 selection is reviewed including what is known about their relative abundance and acquisition of drug resistance. Host-pathogen interactions seem driven by genomic differences on each side, and these play an important role in pathogenesis and immunity. The few M.tb strains chosen for this work do not reflect the vast genomic diversity within this species. They do, however, provide specific virulence, pathology, and growth kinetics of interest to the consortium. The strains selected should not be considered as "representative" of the growing available array of M.tb isolates, but rather tools that are being used to address key outstanding questions in the field.

Keywords: IMPAc-TB; Mycobacterium tuberculosis; TB; diversity; isolates; lineages; tuberculosis.

Figure 1

Survival of mice challenged with Beijing isolates of M.tb. Male Beige mice were challenged with a low-dose aerosol of either M.tb HN878 (open black circles) or M.tb SA161 (open red squares). At 4 weeks post-challenge, (A) lung and (B) spleen bacterial burden were evaluated in n=6-7. Two-tailed unpaired Student’s t test found no significant differences between lung (p = 0.4997) or spleen (p = 0.3365) groups. Mean Log₁₀ CFU for lung = 7.1 HN878 and 6.9 for SA161. Mean Log₁₀ CFU for spleen = 4.2 HN878 and 4.0 for SA161 (C) Cohorts of = 10 mice per challenge (M.tb HN878 = black line or M.tb SA161 = red line) isolate were followed over time with weekly or more frequent weighing as a measure of morbidity due to infection. When mice reached 20% weight loss, they were considered moribund and humanely euthanized. Asterisks represent significance (p < 0.0001) between the groups using a Log-rank Mantel–Cox test comparing survival curves. Data representative of one experiment.

Figure 2

Comparable pulmonary bacterial burden between M.tb H37Rv and Erdman after ultra-low-dose challenge. Female C57BL/6 mice were challenged between 8 and 27 weeks of age with an ultra-low-dose challenge of either M.tb Erdman or M.tb H37Rv. At 35–42 days post-infection, lung bacterial burden was evaluated. Data depict six different challenge experiments, denoted in the figure as different-colored shapes, where each shape represents an individual mouse whose CFU was over 1. Two-tailed unpaired Student’s t test found no significant differences between the challenged groups (p = 0.5196) where the mean CFU for M.tb Erdman was 3.4 × 10⁴ and for M.tb H37Rv was 4.2 × 10⁴.