Experimental cutaneous Bacillus anthracis infections in hairless HRS/J mice

Abstract

Previous studies of experimental Bacillus anthracis cutaneous infections in mice have implicated hair follicles as a likely entry site. Hairless HRS/J mice were used to investigate this possibility because of their non-functional hair follicles that lack penetrating hair shafts. These mice also have diminished macrophage function, increased susceptibility to Listeria, and enhanced neutrophil responses. HRS/J and Balb/c mice were found to be resistant to epicutaneous inoculation with Bacillus anthracis (Sterne) spores onto abraded skin when compared with DBA/2 mice or leucopenic C57BL/6 mice. The HRS/J mice also resisted spore injections that bypassed hair follicles. Haired HRS/J heterozygote mice demonstrated similar reduced susceptibility to B. anthracis spores. Hairless HRS/J mice that were made leucopenic did become susceptible to the epicutaneous spore inoculations. Histologically, the hairless and haired HRS/J mice showed markedly reduced numbers of organisms in hair follicles and the interfollicular dermis when compared even with the resistant Balb/c mice; inflammatory cell infiltrates in the superficial dermis were increased in the HRS/J mice compared with more sensitive strains. Therefore, resistance in the HRS/J mice was apparent at the initial site of epicutaneous inoculation and seemed related to an accumulation of dermal neutrophils rather than to a lack of functional hair follicles.

Figure 1

Bacillus anthracis bacilli infecting hair follicles 24 h after epicutaneous inoculation onto abraded skin in (a) a C57BL/6 mouse, (b) a Balb/c mouse, (c) an HRS/J hr/hr mouse, and (d) a cyclophosphamide-treated HRS/J hr/hr mouse. Slides were stained with a tissue gram stain and photomicrographs taken at an original magnification of 400×; bar represents 100 μm.

Figure 2

Surface touch preparations from inoculated abraded skin 24 h after inoculation with Bacillus anthracis spores in (a) a C57BL/6 mouse, (b) a DBA/2 mouse, (c) an HRS/J hr/hr mouse, and (d) a saline control site in an HRS/J hr/hr mouse. Note that most of the surface exudate cells were neutrophils. Surface touch preparations were stained with the LeukoStat method and photomicrographs taken under oil immersion at an initial magnification of 1000×; bar represents 20 μm.