This murine model of intratracheally induced histoplasmosis resembles natural infection following inhalation exposure. Animals develop a diffuse pulmonary infection followed by hematogenous dissemination to the liver and spleen (3
). The severity of the infection is determined by the size of the inoculum and the immune status of the host.
Immunosuppression by a variety of means has been shown to worsen the outcome of infection with H. capsulatum
both in humans (19
) and in animals. Immunosuppressed animal models have been established by administration of corticosteroids (15
), cyclophosphamide (5
), or antibodies to lymphocytes (1
), CD4 cells (8
), interleukin 12 (27
), or gamma interferon (2
). In addition, homozygous nude mice (10
), SCID mice (28
), or gamma interferon knockout mice (2
) have been used to create immunosuppressed animal models.
CD4 and CD8 depletion was chosen for this study to create an immunosuppressed state resembling that seen in persons with advanced AIDS. The level of CD4 and CD8 depletion used in this model may have been greater than that experienced in some AIDS patients with disseminated histoplasmosis, however, since CD4 counts may be above 200/μl in up to 15% of patients and CD8 counts are not uniformly or greatly suppressed (13
). In our model, CD4 depletion alone increased mortality and fungal burden but not to the extent observed with both CD4 and CD8 depletion (Schnizlein-Bick et al., Abstr. 34th Annu. Meet. Infect. Dis. Soc. Am.). The CD4- and CD8-depleted model was chosen to provide the most extreme challenge by which to evaluate antifungal therapy.
Posaconazole previously showed activity comparable to that of amphotericin B for the treatment of histoplasmosis in immunocompetent animals (3
). In the current study, it was shown to be highly effective for the treatment of CD4- and CD8-depleted animals with disseminated histoplasmosis. Itraconazole at 10 mg/kg/day prolonged survival but failed to prevent deaths, which occurred in 40% of animals during week 4 of infection. Deaths in that group likely would have continued had the animals not been sacrificed at day 29. Posaconazole at 1.0 and 0.1 mg/kg/day was as effective as itraconazole at 75 mg/kg/day and amphotericin B at 2 mg/kg qod in preventing death. Amphotericin B was not effective when used at 0.2 mg/kg qod.
Posaconazole was also highly effective in reducing the fungal burden in tissues. Posaconazole at as low as 1 mg/kg/day markedly reduced fungal burden and sterilized lung and spleen tissues in 70% of mice sacrificed on day 14 of infection. Posaconazole at 0.1 mg/kg/day reduced fungal burden by 1 log unit in the spleen, while the results for the lungs were inconsistent. Itraconazole at 75 mg/kg/day also sterilized 70% of lung and spleen tissues but, at 10 mg/kg/day, failed to sterilize lung or spleen tissues from any animals. Amphotericin B at 2 mg/kg qod reduced fungal burden in comparison to that in untreated control animals but failed to sterilize lung tissues in any animals.
An interesting discrepancy was noted between the survival and fungal burden experiments. While posaconazole at 1 or 0.1 mg/kg/day, amphotericin B at 2 mg/kg qod, and itraconazole at 75 mg/kg/day all prevented death over the 29 days of observation and, except for posaconazole at 0.1 mg/kg/day, greatly reduced fungal burden at day 14, fungal burden at day 29 was high. At day 14, sterile cultures were observed in at least 80% of mice treated with the higher doses of amphotericin B, itraconazole, or posaconazole. No cultures in any group were sterile at day 29. Similar findings were reported for SCID mice treated with amphotericin B (28
). Statistical comparisons of day-14 and day-29 data involved comparisons of groups that were infected at different times. Nevertheless, the differences observed between days 14 and 29 were sufficiently great to suggest that growth was suppressed only at day 14, despite negative culture results for the majority of animals, and that the infection recrudesced after discontinuation of therapy. These data suggest that a 2-week course of treatment is not curative for immunosuppressed animals. Our earlier report with immunocompetent animals did not show fungal burden rebound after discontinuation of therapy (3
). These findings are consistent with the experience that treatment is usually curative in nonimmunocompromised human patients (19
) but not in those with AIDS (23
In conclusion, posaconazole was at least as effective as amphotericin B and more effective than itraconazole in this model of intratracheally induced histoplasmosis in CD4- and CD8-depleted animals.