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We investigated the mechanisms by which endogenous TNF-α modulates host defenses during experimental primary and secondary pulmonary infection with Histoplasma capsulatum (Hc). Neutralization of TNF-α in vivo resulted in increased CFU and 100% mortality in naive and immune mice challenged with Hc intranasally. Levels of IFN-γ and granulocyte macrophage-CSF were elevated in TNF-α-neutralized naive mice, whereas IL-4, -6, -10 and TGF-β did not differ from controls. In contrast, in secondary histoplasmosis, significant elevations of IL-4 and -10 were observed in TNF-α-depleted mice. Alveolar macrophages (Mφ) did not exert fungistatic activity against Hc after exposure to recombinant murine TNF-α, recombinant murine IFN-γ, or both. The increase in susceptibility to primary Hc infection was associated with diminished production of reactive nitrogen intermediates by alveolar Mφ from TNF-α-depleted mice, whereas production of nitric oxide during secondary histoplasmosis was similar in both groups. Upon secondary challenge, TNF-α-depleted mice were rescued by concomitant neutralization of IL-4 and IL-10, but not either cytokine alone. Thus, TNF-α is critical for controlling primary and secondary infection with Hc, and the mechanisms that lead mice to succumb to primary or secondary infection when endogenous TNF-α is blocked are different.

We examined the immunobiological responses to Histoplasma capsulatum in lungs of gamma interferon (IFN-gamma) knockout mice (GKO mice). Naive GKO mice succumbed by day 9 to intranasal challenge with 2.5 x 10(6) yeasts, whereas all wild-type (WT) mice survived for 45 days. Compared to lungs of WT mice, the lungs of acutely infected GKO mice exhibited dramatically elevated numbers of CFU in lungs and significantly higher levels of tumor necrosis factor alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF) but not interleukin-12 (IL-12) or IL-4. To determine if IFN-gamma is necessary in reexposure histoplasmosis, GKO and WT mice were inoculated with 10(4) yeasts intranasally and given amphotericin B for 3 weeks. Six weeks later, mice were rechallenged with 2.5 x 10(6) yeasts. All GKO mice died by day 6, whereas all WT mice survived for 45 days. Lungs of GKO mice contained substantially elevated numbers of CFU and higher TNF-alpha and GM-CSF levels but not IL-12 or IL-4. Thus, IFN-gamma is requisite for control of pulmonary histoplasmosis in naive and reexposed mice.

The concerted action of several cytokines is necessary for resolution of both primary and secondary infection with Histoplasma capsulatum. Among the soluble factors that contribute to tissue sterilization, TNF-α stands as a central mediator of protective immunity to this fungus. In this study, we explored the regulation of protective immunity by TNFR1 and -2. In primary pulmonary infection, both TNFR1−/− and -2−/− mice manifested a high mortality after infection with H. capsulatum, although TNFR1−/− mice were more susceptible than TNFR2 −/− mice. Overwhelming infection in the former was associated with a pronounced decrement in the number of inflammatory cells in the lungs and elevated IFN-γ and TNF-α levels in the lungs. In contrast, IFN-γ levels were markedly decreased in TNFR2−/− mice, and treatment with this cytokine restored protective immunity. Lung macrophages from both groups of knockout mice released substantial amounts of NO. Upon secondary infection, TNFR2−/− mice survived rechallenge and cleared infection as efficiently as C57BL/6 animals. In contrast, mice given mAb to TNFR1 succumbed to reexposure, and the high mortality was accompanied by a significant increase in fungal burden in the lungs. Both IL-4 and IL-10 were elevated in the lungs of these mice. The results demonstrate the pivotal influence of TNFR1 and -2 in controlling primary infection and highlight the differences between these receptors for regulation reexposure histoplasmosis.

T cells are essential for controlling infection with Histoplasma capsulatum. Because the T cell receptor is vital for transducing the biological activities of these cells, we sought to determine if exposure to this fungus induced an alteration in the Vbeta repertoire in lungs of C57BL/6 mice infected intranasally. Vbeta2(+) cells were elevated on day 3 after infection; Vbeta4(+) cells were higher than controls on days 7, 10, and 14 after infection. Vbeta10(+) cells were increased on days 14 and 21, and Vbeta11(+) exceeded controls only on day 14. We investigated the clonality and function of Vbeta4(+) cells because their expansion transpired during the critical time of infection, that is, when cellular immunity is activated. Sequence analysis demonstrated preferential use of a restricted set of sequences in the complementarity-determining region 3. Elimination of Vbeta4(+) cells from mice impaired their ability to resolve infection. In contrast, depletion of Vbeta7(+) cells, the abundance of which was similar to that of Vbeta4(+), did not alter elimination of the fungus. The identification of clonotypes of Vbeta4(+) cells suggests that a few antigenic determinants may drive proliferation of this subset, which is necessary for optimal clearance.

Heat shock protein (hsp) 70 from several microbes is antigenic in mammals. In this study we sequenced and expressed the gene encoding this protein from Histoplasma capsulatum to study its immunological activity. The deduced amino acid sequence of the gene demonstrated 71 and 76% identity to hsp7O from humans and Saccharomyces cerevisiae, respectively. A cDNA was synthesized by reverse transcription-PCR and was expressed in Escherichia coli. Recombinant protein reacted with a mouse monoclonal antibody raised against human hsp7O. Splenocytes from C57BL/6 mice immunized with recombinant hsp7O emulsified in adjuvant, but not yeast cells, reacted in vitro to the antigen. Recombinant hsp7O elicited a cutaneous delayed-type hypersensitivity response in mice immunized with protein or with viable yeast cells. Mice were injected with recombinant hsp7O and challenged intranasally with a sublethal inoculum of yeast cells. Vaccination did not confer protection in this model. Thus, recombinant hsp7O can induce a cell-mediated immune response but does not induce a protective response.

HIS-62 is a glycoprotein that has been isolated from the cell wall and cell membrane fraction of the pathogenic fungus Histoplasma capsulatum. It is a target of the cellular immune response to this fungus, and it protects mice against a lethal intravenous inoculum of H. capsulatum yeast cells. In this study, we cloned the gene encoding this antigen to reveal its biological nature and studied the immunological activity of recombinant antigen. The amino acid sequences of the NH2 terminus and internal peptides were obtained by Edman degradation. Degenerate oligonucleotides were used to isolate a gene fragment of HIS-62 by PCR. One 680-bp segment that corresponded to the known peptide sequence was amplified from H. capsulatum DNA. This DNA was used to screen a genomic library, and the full-length gene was isolated and sequenced. The deduced amino acid sequence of the gene demonstrated approximately 70 and approximately 50% identity to heat shock protein 60 (hsp 60) from Saccharomyces cerevisiae and hsp 60 from Escherichia coli, respectively. A cDNA was synthesized by reverse transcription PCR and was expressed in E. coli. Recombinant protein reacted with a monospecific polyclonal rabbit antiserum raised against native HIS-62, with monoclonal HIS-62-reactive T cells, and with splenocytes from mice immunized with viable yeast cells. Moreover, vaccination with the recombinant protein conferred protection in mice against a lethal intranasal inoculation with yeast cells. Thus, HIS-62 is a member of the hsp 60 family, and the recombinant hsp 60 is protective against pulmonary histoplasmosis in mice.