The production of disseminated histoplasmosis in the mouse: the eff4ects of changes in reticulo-endothelial function

Distinct CD8 T cell functions mediate susceptibility to histoplasmosis during chronic viral infection

It has long been recognized that some viral infections result in generalized immune suppression. In acute infections, this period of suppressed immunity is relatively short. However, chronic infections associated with a prolonged period of immune suppression present far greater risks. Here, we examined the role of CD8 T cell responses following viral infection in immunity to systemic histoplasmosis. Although wild-type mice with systemic histoplasmosis were able to control the infection, those simultaneously infected with lymphocytic choriomeningitis virus clone 13 showed reduced immunity with greater fungal burden and high mortality. The immune suppression was associated with loss of CD4 T cells and B cells, generalized splenic atrophy, and inability to mount a granulomatous response. Removing the anti-viral CD8 T cells in the coinfected mice enabled them to reduce the fungal burden and survive the infection. Their lymphoid organs were replenished with CD4 T and B cells. In contrast to wild-type mice, perforin-deficient mice infected with lymphocytic choriomeningitis virus clone 13 and Histoplasma showed an absence of immunopathology, but the animals still died. These results show that CD8 T cells can suppress immunity through different mechanisms; although immunopathology is perforin-dependent, lethality is perforin-independent.

Effect of macrophage blockade on the resistance of inbred mice to Paracoccidioides brasiliensis infection

The effect of macrophage blockade on the natural resistance and on the adaptative immune response of susceptible (B10.D2/oSn) and resistant (A/Sn) mice to Paracoccidioides brasiliensis infection was investigated. B10.D2/oSn and A/Sn mice previously injected with colloidal carbon were infected ip with yeast cells to determine the 50% lethal dose, and to evaluate the anatomy and histopathology, macrophage activation, antibody production and DTH reactions. Macrophage blockade rendered both resistant and susceptible mice considerably more susceptible to infection, as evidenced by increased mortality and many disseminated lesions. P. brasiliensis infection and/or carbon treatment increased the ability of macrophages from resistant mice to spread up to 25 days after treatment. In susceptible mice the enhanced spreading capacity induced by carbon treatment was impaired at all assayed periods except at 1 week after infection. Macrophage blockade enhanced DTH reactions in resistant mice, but did not alter these reactions in susceptible mice, which remained anergic. To the contrary, macrophage blockade enhanced specific antibody production by susceptible mice, but did not affect the low levels produced by resistant mice. The effect of macrophage blockade confirms the natural tendency of resistant animals to mount DTH reactions in the course of the disease and the preferential antibody response developed by susceptible mice after P. brasiliensis infection. On the whole, macrophage functions appear to play a fundamental role in the natural and acquired resistance mechanisms to P. brasiliensis infection.

Phagocytosis of Histoplasma capsulatum yeasts and microconidia by human cultured macrophages and alveolar macrophages. Cellular cytoskeleton requirement for attachment and ingestion

Phagocytosis of Histoplasma capsulatum (Hc) yeasts and microconidia by human macrophages (M phi) was quantified by a fluorescence quenching technique. Phagocytosis of unopsonized Hc yeasts by monocyte-derived M phi and human alveolar M phi (AM) was rapid. After 60 min, 79% of cultured M phi and 59% of AM had ingested an average of 9.8 and 11 yeasts/M phi, respectively. In contrast, only 26% of monocytes ingested 4.5 yeasts/cell after 60 min. Phagocytosis of unopsonized microconidia by cultured M phi and by AM was equivalent. Monoclonal antibodies specific for the alpha-chains and beta-chain of the CD18 family of adhesion receptors inhibited the binding of Hc yeasts and microconidia to cultured M phi and AM. Thus, the M phi CD18 complex mediates recognition of both phases of this dimorphic fungus. Disruption of actin microfilaments with cytochalasin D inhibited both attachment and ingestion of yeasts by M phi. In contrast, nocodazole, which prevents polymerization of microtubules, did not inhibit binding or ingestion. Both drugs inhibited ingestion, but neither drug inhibited binding of C3b- and C3bi-coated sheep erythrocytes to complement receptors type one (CR1) or type three (CR3), respectively. Therefore, different signal transducing mechanisms for phagocytosis appear to be triggered by the binding of Hc yeasts to CD18, and by the binding of EC3bi to CD11b/CD18, respectively.

Relative susceptibilities of inbred mouse strains C57BL/6 and A/J to infection with Histoplasma capsulatum

Differences in the 30-day survival of Histoplasma capsulatum after intravenous injection indicated that the A/J strain of inbred mouse was more resistant to experimental infection than was the C57BL/6 strain. CFU from the spleens of infected animals increased during the first week after injection but gradually declined over the next 3 weeks. The CFU per gram of tissue in the C57BL/6 animals were 10- to 100-fold higher than were those in the A/J mice during the time between 7 and 28 days after infection. The units of gamma interferon (IFN-gamma) in supernatants of spleen cells stimulated with heat-killed yeast cells of H. capsulatum reached a peak at the time of the largest number of CFU per gram of tissue. The titers of IFN-gamma at days 3 to 5 were higher in the A/J mice than they were in the C57BL/6 mice, but from days 7 to 28, the titers of IFN-gamma were not correlated with the more efficient clearance of the fungus from the spleens of A/J mice. The L3T4+ spleen cells were shown to be active IFN-gamma producers. Treatment of Histoplasma-infected mice with anti-IFN-gamma antibody resulted in much larger tissue burdens of the fungus in the lungs and spleens of treated animals than in untreated animals. There was no marked difference in the result of treatment with anti-IFN-gamma antibody between A/J and C57BL/6 mice. Treatment of Histoplasma-infected mice with recombinant murine IFN-gamma did not alter the course of infection in either inbred strain of mouse.

Interactions between Histoplasma capsulatum and macrophages from normal and treated mice: comparison of the mycelial and yeast phases in alveolar and peritoneal macrophages

Interactions between macrophages (alveolar and peritoneal) from normal, vaccinated (with heat-killed yeast cells), and Mycobacterium bovis BCG-treated mice and the mycelial and yeast phases of Histoplasma capsulatum were observed. Phagocytosis of microconidia, small hyphal fragments, and yeast cells occurred 4 to 6 h after the infection of macrophage cultures. Conversion to the yeast phase began at 6 to 7 h and was complete after a 72-h incubation at 37 degrees C. Macrophages surrounded and adhered to macroconidia and large hyphal elements. More macrophages (65 to 68%) from BCG-treated mice contained fungi at 24 h than did macrophages from normal or vaccinated mice. Although there was no increase in the number of fungi in macrophages from vaccinated mice, only the macrophages from BCG-treated mice contained fewer fungi after 48 h of infection with the mycelial phase of H. capsulatum. Fungal growth was not inhibited in any of the macrophage cultures when infected with the yeast phase. The macrophages infected with yeast cells were destroyed after 48 to 72 h in the culture. Only BCG-treated macrophages survived infection with the mycelial phase, whereas macrophages from normal and vaccinated mice were destroyed by the infection.