Genome wide transcriptome profiling of a murine acute melioidosis model reveals new insights into how Burkholderia pseudomallei overcomes host innate immunity

Abstract

Background: At present, very little is known about how Burkholderia pseudomallei (B. pseudomallei) interacts with its host to elicit melioidosis symptoms. We established a murine acute-phase melioidosis model and used DNA microarray technology to investigate the global host/pathogen interaction. We compared the transcriptome of infected liver and spleen with uninfected tissues over an infection period of 42 hr to identify genes whose expression is altered in response to an acute infection.

Results: Viable B. pseudomallei cells were consistently detected in the blood, liver and spleen during the 42 hr course of infection. Microarray analysis of the liver and spleen over this time course demonstrated that genes involved in immune response, stress response, cell cycle regulation, proteasomal degradation, cellular metabolism and signal transduction pathways were differentially regulated. Up regulation of toll-like receptor 2 (TLR2) gene expression suggested that a TLR2-mediated signalling pathway is responsible for recognition and initiation of an inflammatory response to the acute B. pseudomallei infection. Most of the highly elevated inflammatory genes are a cohort of "core host immune response" genes commonly seen in general inflammation infections. Concomitant to this initial inflammatory response, we observed an increase in transcripts associated with cell-death, caspase activation and peptidoglysis that ultimately promote tissue injury in the host. The complement system responsible for restoring host cellular homeostasis and eliminating intracellular bacteria was activated only after 24 hr post-infection. However, at this time point, diverse host nutrient metabolic and cellular pathways including glycolysis, fatty acid metabolism and tricarboxylic acid (TCA) cycle were repressed.

Conclusions: This detailed picture of the host transcriptional response during acute melioidosis highlights a broad range of innate immune mechanisms that are activated in the host within 24 hrs, including the core immune response commonly seen in general inflammatory infections. Nevertheless, this activation is suppressed at 42 hr post-infection and in addition, suboptimal activation and function of the downstream complement system promotes uncontrolled spread of the bacteria.

Figure 1

Bacterial loads in BALB/c mice. The bacterial loads in the (A) liver, (B) spleen and (C) blood of BALB/c mice at 16 hr, 24 hr and 42 hr time points after intravenous infection with 1.1 × 10³ CFU of B. pseudomallei D286 are shown. Each symbol represents one mouse. Horizontal line indicates the geometric mean for each group. The control mice are not represented as no colonies grew from their organ homogenates or blood samples.

Figure 2

Leukocyte differential counts during acute B. pseudomallei infection. Changes in differential leukocyte counts after intravenous infection with 1.1 × 10³ CFU B. pseudomallei D286. Values are given as that of pooled-blood from infected mice for each group at the particular time point.

Figure 3

Differential gene expression in acute B. pseudomallei infection over 42 hrs. Hierarchical clustering of the expression profile of (A) liver (L) and (D) spleen (S) infected with B. pseudomallei for 16, 24 and 42 hpi; Number of genes modulated during acute B. pseudomallei D286 infection in BALB/c mice at 16 hpi, 24 hpi and 42 hpi in both (B) liver and (E) spleen; Major biological processes consistently modulated throughout the acute phase of infection in (C) liver and (F) spleen as determined by GOTerm Finder analysis.

Figure 4

Transcriptional responses to acute B. pseudomallei infection. Hierarchical clustering of the expression profile of liver and spleen infected with B. pseudomallei at 16, 24 and 42 hpi according to functional categories. The heat maps indicate the fold change in liver or spleen gene expression greater than (red) or less than (green) 2-fold at least once during the time course. Genes whose expression did not change are coloured in black. *, immune-related genes known to be associated with the general bacterial infection.

Figure 5

Expression profiling of some immune-related genes over 42 hrs. (A) Decrease in transcriptional expression of potent cytokines and chemokines in the spleen at 42 hpi, (B) qPCR analysis of host-cell genes found to be differentially expressed by microarray.