Allelic Variation on Murine Chromosome 11 Modifies Host Inflammatory Responses and Resistance to Bacillus anthracis

Abstract

Anthrax is a potentially fatal disease resulting from infection with Bacillus anthracis. The outcome of infection is influenced by pathogen-encoded virulence factors such as lethal toxin (LT), as well as by genetic variation within the host. To identify host genes controlling susceptibility to anthrax, a library of congenic mice consisting of strains with homozygous chromosomal segments from the LT-responsive CAST/Ei strain introgressed on a LT-resistant C57BL/6 (B6) background was screened for response to LT. Three congenic strains containing CAST/Ei regions of chromosome 11 were identified that displayed a rapid inflammatory response to LT similar to, but more severe than that driven by a LT-responsive allele of the inflammasome constituent NRLP1B. Importantly, increased response to LT in congenic mice correlated with greater resistance to infection by the Sterne strain of B. anthracis. The genomic region controlling the inflammatory response to LT was mapped to 66.36–74.67 Mb on chromosome 11, a region that encodes the LT-responsive CAST/Ei allele of Nlrp1b. However, known downstream effects of NLRP1B activation, including macrophage pyroptosis, cytokine release, and leukocyte infiltration could not fully explain the response to LT or the resistance to B. anthracis Sterne in congenic mice. Further, the exacerbated response in congenic mice is inherited in a recessive manner while the Nlrp1b-mediated response to LT is dominant. Finally, congenic mice displayed increased responsiveness in a model of sepsis compared with B6 mice. In total, these data suggest that allelic variation of one or more chromosome 11 genes in addition to Nlrp1b controls the severity of host response to multiple inflammatory stimuli and contributes to resistance to B. anthracis Sterne. Expression quantitative trait locus analysis revealed 25 genes within this region as high priority candidates for contributing to the host response to LT.

Rapid Vascular Responses to Anthrax Lethal Toxin in Mice Containing a Segment of Chromosome 11 from the CAST/Ei Strain on a C57BL/6 Genetic Background

“…administration of LT, consistent with previous findings [37]. Following the ∼1 h observation period, animals were prepared for analysis by intravital microscopy.…”

“…An increased production of IL-1β in response to LT was also observed in congenic mice that have a segment of chromosome 11 from the CAST/Ei strain introgressed onto a C57BL/6J background (B6.CAST.11M) [37]. Both B6 Nlrp1b (129S1) and B6.CAST.11M strains exhibit an early response phenotype (ERP) that typically has an initial presentation 0.5–1 h following LT exposure and includes hypothermia, ataxia, bloat, loose feces, abdominal breathing and/or dilated vessels on pinnae [7], [37]. The ERP and increased responsivity of macrophages to LT are associated with increased resistance to challenge with B. anthracis , and signaling through the IL-1 receptor is required for the increased resistance in B6 Nlrp1b (129S1) mice [37].…”

“…Other published findings also support a role for an additional genetic determinant(s), other than Nlrp1b , on chromosome 11 contributing to inflammatory responses [37]. For example, ataxia and hypothermia are enhanced in B6.CAST.11M mice compared to C57BL/6J mice in response to MDP + LPS challenge [37], yet the response to this challenge is not thought to be controlled by the Nlrp1b locus.…”

“…Indeed serum levels of 23 cytokines increased in association with the ERP in B6.CAST.11M animals [37]. The ERP has been linked to a genetic locus within a critical region in chromosome 11 that accounts for a high level of responsivity to LT and resistance to B. anthracis [37]. This region encodes numerous genes associated with immune function including the inflammasome component Nlrp1b .…”

“…Transgenic mice with a sensitive 129S1 Nlrp1b allele on resistant C57BL/6J background (B6 Nlrp1b (129S1) ) exhibit heightened serum levels of IL-1β compared to C57BL/6J mice in response to LT [7]. An increased production of IL-1β in response to LT was also observed in congenic mice that have a segment of chromosome 11 from the CAST/Ei strain introgressed onto a C57BL/6J background (B6.CAST.11M) [37]. Both B6 Nlrp1b (129S1) and B6.CAST.11M strains exhibit an early response phenotype (ERP) that typically has an initial presentation 0.5–1 h following LT exposure and includes hypothermia, ataxia, bloat, loose feces, abdominal breathing and/or dilated vessels on pinnae [7], [37].…”