Resistance of Athymic Nude Mice to Experimental Cutaneous Bacillus anthracis Infection

Anthrax in humans is ~95% cutaneous and ~5% respiratory in form [1, 2]. Because animals rarely develop cutaneous anthrax, there have been few experimental studies of this form of the disease. Therefore, the pathogenesis of cutaneous anthrax and the local host defenses against it have received relatively little study. To learn more about this disease, we recently developed a murine model of cutaneous anthrax, using the Sterne strain of Bacillus anthracis [3]. This unencapsulated strain is essentially noninfectious for humans but retains significant pathogenicity for mice (which are more sensitive to anthrax than are humans). Many previous studies have used experimental infections with the Sterne strain in mice as a model for human anthrax, with the caveat that lack of a capsule could cause differences in the way the organism is handled by the host. Initial experiments in our cutaneous infection system demonstrated that hair follicles were a major site of organism proliferation in the inoculated skin. Further experiments were done with hairless HRS/J mice to determine whether the lack of hair follicles with properly formed hair shafts would decrease susceptibility to B. anthracis after epicutaneous inoculation. Indeed, these animals were found to be extremely resistant to this type of infection. However, this resistance was found to be unrelated to the abnormal hair follicles of these animals by a number of lines of evidence, including (1) resistance to intradermal and subcutaneous spore injections (which bypass the hair follicles as portals of entry), (2) resistance of the haired heterozygotes of this strain, and (3) development of susceptibility in hairless HRS/J mice after they were rendered leukopenic [4]. Further examination of infections in HRS/J mice revealed that these animals were able to mount a superficial neutrophilic response in the cutaneous exudate fluids of the abraded inoculated skin and that this response appeared to clear the organisms before they could infect the underlying skin [5]. Previous studies have shown that various inbred mice strains vary markedly in their susceptibility to anthrax in a number of experimental infection models, including the cutaneous system described above [3, 4, 6]. Therefore, HRS/J mice may be more resistant because of some as yet undefined characteristic in their genetic background. On the other hand, these mice have some other traits in addition to their lack of hair. They do have a mutation in the hr gene of chromosome 14 that causes them to lose their hair and to have shallow, nonfunctioning hair follicles, but they have also been found to undergo late thymic cortical atrophy, resulting in significant T cell abnormalities that predispose them to Listeria infections [7–9]. Haired HRS/J mice that are heterozygous for the hr gene mutation have also been found to be susceptible to Listeria infections and to have an increased rate of lymphomagenesis late in life [7–10]. Therefore, it would appear that HRS/J mice (either hairless hr/hr mutants or haired hr/+ heterozygotes) have some relative defects in T cell function and cell-mediated immunity, although these are perhaps not as severe as those seen in athymic nude mice. The comparison is interesting, however, because athymic nude mice have been shown to be very resistant to certain infections, including those caused by Pseudomonas aeruginosa and Candida albicans [11–13], and T cell deficiency could be protective against specific infections. On the other hand, the background of HRS/J mice is complex and based on several strains. Their immunity to anthrax could be due in some way to their T cell deficiency or, perhaps, to some other aspect of their genetic background. Because athymic nude mice of the C57BL/6 strain are available, the present study was undertaken to compare athymic with euthymic C57BL/6 mice to determine whether the former might be more resistant to experimental cutaneous anthrax. We wanted the experiments to answer 2 fundamental questions: (1) Do athymic nude mice have increased resistance to experimental cutaneous infections with B. anthracis? (2) If so, is this resistance due to an enhanced superficial neutrophilic exudation, as in the HRS/J animals?