Significance of thymus-derived lymphocytes in immunity elicited by immunization with ribosomes or live yeast cells of Histoplasma capsulatum

Fungal cell wall vaccines: an update

This discussion is intended to be an overview of current advances in the development of fungal cell wall vaccines with an emphasis on Candida; it is not a comprehensive historical review of all fungal cell wall vaccines. Selected, more recent, innovative strategies for developing fungal vaccines will be highlighted. Both scientific and logistical obstacles related to the development of, and clinical use of, fungal vaccines will be discussed.

Horizontal transmission of Candida albicans and evidence of a vaccine response in mice colonized with the fungus

Disseminated candidiasis is the third leading nosocomial blood stream infection in the United States and is often fatal. We previously showed that disseminated candidiasis was preventable in normal mice by immunization with either a glycopeptide or a peptide synthetic vaccine, both of which were Candida albicans cell wall derived. A weakness of these studies is that, unlike humans, mice do not have a C. albicans GI flora and they lack Candida serum antibodies. We examined the influence of C. albicans GI tract colonization and serum antibodies on mouse vaccination responses to the peptide, Fba, derived from fructose bisphosphate aldolase which has cytosolic and cell wall distributions in the fungus. We evaluated the effect of live C. albicans in drinking water and antimicrobial agents on establishment of Candida colonization of the mouse GI tract. Body mass, C. albicans in feces, and fungal-specific serum antibodies were monitored longitudinally. Unexpectedly, C. albicans colonization occurred in mice that received only antibiotics in their drinking water, provided that the mice were housed in the same room as intentionally colonized mice. The fungal strain in unintentionally colonized mice appeared identical to the strain used for intentional GI-tract colonization. This is the first report of horizontal transmission and spontaneous C. albicans colonization in mice. Importantly, many Candida-colonized mice developed serum fungal-specific antibodies. Despite the GI-tract colonization and presence of serum antibodies, the animals made antibodies in response to the Fba immunogen. This mouse model has potential for elucidating C. albicans horizontal transmission and for exploring factors that induce host defense against disseminated candidiasis. Furthermore, a combined protracted GI-tract colonization with Candida and the possibility of serum antibody responses to the presence of the fungus makes this an attractive mouse model for testing the efficacy of vaccines designed to prevent human disseminated candidiasis.

Production and evaluation of reagents for detection of Histoplasma capsulatum antigenuria by enzyme immunoassay

The detection of urinary Histoplasma capsulatum polysaccharide antigen (HPA) by enzyme immunoassay (EIA) has proven useful for the presumptive diagnosis of histoplasmosis in AIDS patients. Assay limitations include (i) detection of a largely uncharacterized antigen and (ii) difficulty in reproducibly generating antibodies for use in the EIA. To improve antibody production for use in this test and to better understand the antigen being detected, we compared rabbit antibodies elicited using various immunization schedules, routes, and H. capsulatum-derived antigens. Antibodies were evaluated by EIA for their ability to detect purified H. capsulatum C antigen (C-Ag) and antigenuria. Reported as enzyme immunoassay (EI) units (the A(450) with antigen divided by the A(450) without antigen), results demonstrated that intravenous immunization of rabbits with whole, killed yeast-phase cells (yeast-i.v. regimen) produced antibodies giving the highest EI values in the C-Ag EIA (mean EI units +/- standard deviation, 14.9 +/- 0.6 versus 6.4 +/- 0.4 for rabbits immunized with C-Ag versus 2.4 +/- 0.3 for all other regimens combined). Yeast-i.v. antibodies were highly sensitive for the detection of antigenuria in patients with histoplasmosis, as shown by the following results: 12/12 patients compared to 10/12, 6/12, 3/12, and 3/12, respectively, for antibodies from rabbits immunized with (i) C-Ag; (ii) whole, killed yeast-phase cells administered subcutaneously and intramuscularly; (iii) yeast-phase culture filtrates; and (iv) HPA-positive urine. Rabbits immunized using the yeast-i.v. regimen also gave higher peak antibody titers than rabbits immunized by any other regimen (P < 0.03), and their antibodies were most comparable in reactivity to antibodies produced for use in the standard HPA-EIA test (P < 0.001). Therefore, rabbits immunized using the yeast-i.v. regimen produced the most sensitive antibodies with the highest titers for detection of C-Ag and antigenuria in histoplasmosis patients.

Molecular cell biology and molecular genetics of Histoplasma capsulatum

Histoplasma capsulatum is a dimorphic ascomycete which is capable of producing a broad spectrum of disease ranging from mild asymptomatic, pulmonary illness to severe, life-threatening systemic mycosis. Regulatory mechanisms that use temperature and other environmental cues are paramount to the successful adaptation of the organism as an effective intracellular pathogenic yeast. Although the biochemistry and phenomenology of reversible morphogenesis have been well examined in Histoplasma, the identification and functional characterization of genes and their products that are required for early establishment or maintenance of the parasitic yeast phase in intracellular host compartments have only recently been fruitful. Advances in the molecular biology of Histoplasma, including approaches to introduce telomeric plasmids, reporter fusion constructs, and gene disruption cassettes into the fungus are poised to solidify the pre-eminence of this fungus as a model system which can be applied to other dimorphic fungal pathogens that exhibit similar cellular and immunological complexities. This review centers on recent developments in the molecular cell biology and molecular genetics of Histoplasma capsulatum that provide important new avenues for examining the mold-to-yeast phase transition beyond the historical, binary view of dimorphism and the implications that these successful approaches may have on seminal issues in fungal pathogenesis.

Prospects for the development of fungal vaccines

In an era that emphasizes the term "cost-effective," vaccines are the ideal solution to preventing disease at a relatively low cost to society. Much of the previous emphasis has been on childhood scourges such as measles, mumps, rubella, poliomyelitis, and Haemophilus influenzae type b. The concept of vaccines for fungal diseases has had less impact because of the perceived limited problem. However, fungal diseases have become increasingly appreciated as serious medical problems that require recognition and aggressive management. The escalation in the incidence and prevalence of infection has prompted a renewed interest in vaccine development. Herein, I discuss the most recent developments in the search for vaccines to combat fungal infections. Investigators have discovered several inert substances from various fungi that can mediate protection in animal models. The next challenge will be to find the suitable mode of delivery for these immunogens.

Immunity in histoplasmosis: identification of protective immunogens from Histoplasma capsulatum

The complexities of the protective immune response to Histoplasma capsulatum remain poorly understood. It is clear that an interaction between antigen-reactive T cells and macrophages is essential for successful resolution of infection. The primary focus in our laboratory is to identify the antigens from this fungus that stimulate T cells to mediate growth inhibition of H. capsulatum. We describe herein the results of studies that centered on the identification of two antigens that trigger T-cell proliferation and can confer protective immunity. These antigens potentially can be used to generate a vaccine against this pathogenic fungus. Key words: histoplasmosis, T cells, antigens, protective immunity, HIS-62.

Protective efficacy of a 62-kilodalton antigen, HIS-62, from the cell wall and cell membrane of Histoplasma capsulatum yeast cells

We reported previously that a detergent extract of the cell wall and cell membrane of Histoplasma capsulatum yeast cells contains antigens recognized by T cells. In T-cell immunoblot analysis, a region encompassing 62 kDa was stimulatory for an H. capsulatum-reactive T-cell line and T-cell clones derived from C57BL/6 mice. In this study, we isolated a 62-kDa band, termed HIS-62, from electrophoresed cell wall and cell membrane of H. capsulatum yeast cells and examined its antigenicity and immunogenicity. C57BL/6, BALB/c, and CBA/J mice that were immunized with viable H. capsulatum yeast cells mounted a delayed-type hypersensitivity response to HIS-62 that was stronger than that of normal controls. Spleen cells from each strain of mouse immunized with viable yeast cells proliferated vigorously in response to HIS-62; conversely, splenocytes from control animals did not recognize this antigen. A T-cell line and 5 of 5 T-cell clones from C57BL/6 mice, 10 of 15 BALB/c T-cell hybridomas, and 8 of 12 CBA/J T-cell hybridomas recognized HIS-62. A cutaneous delayed-type hypersensitivity response to the antigen was apparent in each strain of mouse that was injected with 80 micrograms of HIS-62 mixed with Freund adjuvant. In addition, spleen cells from HIS-62-immunized mice proliferated in vitro in response to this antigen. Vaccination of each strain of mouse with 80 micrograms of HIS-62 conferred protection against a lethal intravenous challenge with H. capsulatum yeast cells. Thus, HIS-62 appears to be an important target of the cellular immune response to H. capsulatum and induces a protective immune response in mice.

Antigenicity and immunogenicity of an extract from the cell wall and cell membrane of Histoplasma capsulatum yeast cells

In order to identify T-cell antigens from Histoplasma capsulatum yeast cells, we prepared a detergent extract of the cell wall and cell membrane of yeast-phase H. capsulatum G217B and analyzed its antigenicity and immunogenicity. Mice injected with viable H. capsulatum yeast cells or with 500 or 1,000 micrograms of the extract mounted a delayed-type hypersensitivity response to solubilized cell wall and cell membrane. Vaccination with this antigenic preparation conferred a protective immune response in mice that were challenged intravenously with H. capsulatum yeast cells. The extract induced in vitro proliferation by splenocytes from mice injected with either viable yeast cells or the soluble cell wall and cell membrane preparation. We also examined the profile of in vitro responses by a murine T-cell line and by cloned T cells to soluble cell wall and cell membrane by employing the technique of T-cell immunoblotting. Two prominent regions that stimulated the T-cell line and cloned T cells were identified. Fractions encompassing an area between 53 and 64 kDa caused proliferation by a T-cell line and five of six clones. Antigens recognized by the T-cell line and by three of six clones were contained in another area that extended from 69 to 82 kDa. The data demonstrate that this soluble extract from cell wall and cell membrane contains antigens recognized by T cells and mediates protective immunity. Moreover, T-cell immunoblotting provides a useful technique for mapping immunoreactive molecules from H. capsulatum yeast cells.

Evaluation of an ELISA for the detection of anti-Histoplasma ribosomal and antihistoplasmin antibodies in histoplasmosis

We have developed an indirect sandwich enzyme-lined immunosorbent assay (ELISA) for the detection of antibodies to Histoplasma ribosomes and histoplasmin; we used this test for demonstration of these antibodies in sera from proven cases of histoplasmosis and other infections. Serum dilutions from five negative controls used in each experiment were normalized against 50 normal sera, and a factor of the mean absorbance was used to establish a positive reaction. Antiribosomal antibodies were detected in 97% of the known histoplasmosis patients with ELISA titers ranging from 1:100 to over 1:12,800. In contrast, antibodies to histoplasmin were detected in only 75% of these sera; titers ranged from 1:100 to 1:12,800. Cross-reactions with sera from other fungal infections (blastomycosis, coccidioidomycosis, paracoccidioidomycosis, cryptococcosis, candidiasis, and aspergillosis) were seen in 46% of the cases with ribosomes and 37% with histoplasmin. Fifty percent of the sera from tuberculosis patients gave positive reactions with ribosomes and 29% with histoplasmin. These results warrant further studies on the significance of antibodies to ribosomes and histoplasmin in immunity to histoplasmosis.

Role of L3T4+ T cells in host defense against Histoplasma capsulatum

Cell-mediated immunity is critical in host resistance against the pathogenic fungus Histoplasma capsulatum. To explore the role of L3T4+ T cells in protection of mice against H. capsulatum infection, we examined the effect of in vivo treatment with anti-L3T4 monoclonal antibody (MAb) GK1.5 on the course of murine disseminated histoplasmosis. Treatment with anti-L3T4 antibody caused a profound and selective depletion of L3T4+ T cells that was associated with a significant increase in the number of H. capsulatum CFU recovered from the spleens of mice infected for 1 week. In addition, none of the infected mice treated with MAb GK1.5 survived a sublethal challenge with H. capsulatum yeasts. Histopathological examination of spleens from mice infected for 1 week revealed the presence of granulomatous inflammation in mice depleted of L3T4+ T cells and in infected controls. However, silver stains demonstrated that spleens of infected mice given MAb GK1.5 contained a greater number of yeasts than did spleens from infected controls. MAb GK1.5 did not cause reactivation of infection when administered for 2 weeks beginning 4 weeks after inoculation of Histoplasma yeasts. MAb GK1.5 did not alter the functional properties of murine macrophages as measured by antigen presentation, production of interleukin-1 in response to lipopolysaccharide, and phagocytosis of H. capsulatum yeasts. These results suggest that the L3T4+ T-cell subset is an essential constituent of the cell-mediated immune defense against H. capsulatum infection.

Chronic pulmonary and disseminated paracoccidioidomycosis in mice: quantitation of progression and chronicity

An animal model of chronic paracoccidioidomycosis was established in male adult BALB/cByJIMR mice by intranasal instillation of different doses of yeast form Paracoccidioides brasiliensis. The inoculum was standardized in terms of virulence, age of the culture, percentage of multicellular fungal units containing 1-3 cells, and viability. Progression and chronicity of the infection was measured by quantitative counts of colony forming units (CFU) of P. brasiliensis from infected lungs, spleens, and livers in a newly developed culture medium. The body weight of the mice and the organ weights were also used to monitor the disease process. Infection with several challenge sizes progressed in the lungs until a maximum of 10(7)-10(8) CFU per lung was reached; in general, the higher the inoculum, the sooner this maximum was reached. In mice infected with 2.5 X 10(6) CFU the maximum was reached at 8 weeks, whereas in mice infected with 3 X 10(4) CFU the maximum was reached 14 weeks after infection. Dissemination of the disease progressed until there were 10(6)-10(7) CFU per spleen or liver. The higher the infective dose, the shorter the time required to reach a maximum stable population of yeasts in spleen and liver (12 weeks with inoculum of 2.5 X 10(6) CFU, 18-26 weeks with inoculum of 7.0 X 10(3) CFU). The body weight of mice tended to diminish with time after infection compared to uninfected controls. In contrast, the weight of lung and spleen increased with time after infection. This model of chronic paracoccidioidomycosis permits evaluation of progression of the disease process and of the multiplication of the yeast in organs.

Vaccines against fungal infections

The state-of-the-art reached in developing protective immunity against fungal infections through vaccination makes a survey of methodologies and results timely. This review describes experimental vaccinations against dermatophytes, pathogenic yeasts, and dimorphic fungi with special attention to the anti-Coccidioides immitis vaccine, which has reached clinical trials, and to the anti-Candida albicans and anti-Histoplasma capsulatum ribosomal vaccines. Also covered are vaccination experiments in compromised hosts aimed at eliciting acquired resistance to opportunistic fungal infections which constitute risk factors for these hosts. Immunization procedures include live, killed, and attenuated organisms, as well as different subcellular fractions such as cytoplasmic extracts, fungal culture filtrates, cell walls, or ribosomal fractions. A variety of experimental animal models and isolated human trials constitute the subjects in these studies. Acquired immunity has been evaluated through assessment of resistance to infection and determination of specific immune responses. It has been demonstrated that fungal vaccines do elicit both humoral and cell-mediated immunity in the immunized host. For some vaccines (e.g., H. capsulatum), a correlation between the induced immunity and protection was observed and the immunity could be adoptively transferred. In view of the potential of vaccines against fungal infections, a perspective on their applicability, significance, and value for human use is discussed.

An overview of macrophage-fungal interactions

A review of the literature (148 references) on the interactions of fungi with polymorphonuclear cells, monocytes and macrophages is presented. The interactions of Aspergillus species, Coccidioides immitis, Blastomyces dermatitidis, Histoplasma capsulatum, Cryptococcus neoformans, Candida albicans, and Candida species with human and experimental animal derived immune cells are examined in this overview. An effort has been made to present the reader with a comprehensive list of references with the intent of encouraging additional reading and research in this important area.

Diagnosis of disseminated histoplasmosis by detection of Histoplasma capsulatum antigen in serum and urine specimens

The diagnosis of Histoplasma capsulatum infection by serologic testing for the presence of antibodies is limited by a high rate of false positive and false negative results and by the requirement that the patient have a normal immune response. We have developed a radioimmunoassay for the detection of H. capsulatum antigen in urine and serum specimens. Antigenuria was noted in 20 of 22 episodes of disseminated histoplasmosis that occurred in 16 patients, in 6 of 32 patients with self-limited infection, in 2 of 32 patients with cavitary histoplasmosis, and in 4 of 8 patients with a sarcoid-like illness caused by H. capsulatum. The detection of antigen in urine was reproducible in 38 of 41 (93 percent) retests of specimens. H. capsulatum antigen was also detected in the serum during 11 of the 22 episodes of disseminated histoplasmosis, in none of the 12 episodes of other types of histoplasmosis in patients with antigenuria, in 1 of the 33 patients with histoplasmosis who lacked the urinary antigen, and in none of the 50 controls. Antigenemia and antigenuria decreased after initiation of antifungal therapy and recurred in patients who had a relapse. We conclude that this radioimmunoassay for H. capsulatum antigen represents a useful new method for the rapid diagnosis of disseminated histoplasmosis.

Immunology of fungal infections in animals

The nature of immunity to fungal infection is discussed predominantly for mammals and birds. T-cell-mediated immunity seems essential for recovery both from cutaneous and mucosal infections (Candida, Malassezia and dermatophytes) and from infections of systemic fungal pathogens (Cryptococcus, Blastomyces, Histoplasma, and Coccidioides). Often chronic progressive disease caused by these fungi is associated with a depression or absence of T-cell-mediated immunity to antigens of the infecting fungus. In contrast recovery from disease, or absence of clinical disease after exposure to these fungi, is associated with the presence of strong T-cell-mediated immune responses to the fungus. The activation of macrophages and the stimulation of epidermal growth and keratinization are the processes induced by T-cell-mediated immunity which result in the resolution of systemic or cutaneous and mucosal disease. Other cell types, for example NK cells and PMNs (polymorphonuclear leucocytes), may be important in these diseases in reducing the effective amount of inoculum to which an animal is exposed and thereby reducing the likelihood of disseminated disease. Invasive opportunistic fungi (Candida, Aspergillus, Mucorales) are resisted by PMNs which attach to the hyphae or pseudohyphae and damage them via an extracellular mechanism. Other host cell types may be important in natural resistance, fungal spores being handled by the macrophages which, under conditions when animals are not immunosuppressed, are likely to be an effective first line of defense. Subcutaneous pathogens and miscellaneous other fungal diseases are discussed from a point of view of host immunity and immunodiagnosis. Vaccine development for ringworm and for other mycoses is discussed.

T lymphocyte abnormalities in disseminated histoplasmosis

Disseminated histoplasmosis is associated with depression of T cell-mediated immunity and in some cases anergy. In this report, two patients with disseminated disease are described. Both had a depression of T cell-mediated immunity as well as other abnormalities of immune response. In one, a patient with relapse, a marked depression in the ratio of T helper to T suppressor cells was noted. Neither patient had any predisposing condition known to be associated with disseminated disease.

Blastogenic responses of lymphocytes from mice immunized by sublethal infection with yeast cells of Histoplasma capsulatum

Blastogenic responses of spleen cells to histoplasmin and ribosomal antigens and to the mitogens concanavalin A. phytohemagglutinin, and lipopolysaccharide were studied in normal and immunized mice (10(5) live yeast cells of Histoplasma capsulatum given by the subcutaneous route). Cells (10(6) per well) were cultured with and without antigens and mitogens in microtiter plates with RPMI 1640-5% heat-inactivated normal mouse serum for 72 h at 37 degrees C. Cells were harvested after a 16- to 18-h pulse with 1 microCi of [3H]thymidine (6.7 Ci/mol), and thymidine incorporation was measured by scintillation counting. The initial blastogenic response to concanavalin A (54 X 10(3) cpm) was suppressed (P less than 0.001) from 4 to 14 days post-immunization and returned to control levels on day 21. The response to phytohemagglutinin was suppressed up to 21 days. Lipopolysaccharide responses, however, were affected to a lesser degree. Blastogenic responses to histoplasmin and H. capsulatum ribosomes were similar on day 0 in normal and immune lymphocytes, but by day 4 cells from immunized mice were more responsive (P less than 0.01). The maximum response to H. capsulatum antigens was detected on day 42 and was 9- to 16-fold higher than in controls. These results demonstrate in vitro responses of primed lymphocytes on exposure to H. capsulatum antigens and suppressed responses to mitogens during early stages of the immune response.

A microsomal fraction of Cryptococcus neoformans induces lymphocyte blastogenesis in infected guinea pigs

Differential centrifugation of a homogenate from a mechanically disrupted, acapsular isolate of Cryptococcus neoformans resulted in a 105,000 x g supernatant (105 K) and a microsomal fraction (MS), both of which were capable of eliciting specific delayed cutaneous hypersensitivity and in vitro blastogenesis in infected guinea pigs. Polyacrylamide gel electrophoresis revealed two major proteins in the MS and seven proteins in the 105 K fractions. Electron microscopy of the MS showed both membranes and ribosomes. In vitro lymphocyte blastogenesis elicited by 1 to 10 micrograms/ml of antigens was maximal after 4 days of incubation; the reacting populations were peripheral blood leukocytes (PBL) and peritoneal exudate cells (PEC). Spleen cells of infected animals were unresponsive to in vitro antigenic stimulation. A simplified schedule of priming animals was infection with a single dose of virulent cryptococci. Under these conditions 3 of 6 animals' PBL responded with stimulation ratios of 6.55, 21.1, 35.42 to the MS and 1.41, 9.33, 17.39 to the 105 K antigens at 1 microgram/ml. Four of six animals' PEC response were positive with stimulation ratios of 2.62, 2.72, 4.02 and 7.20 towards MS, and 2.62, 5.13, 5.71, 10.01 to the 105 K antigens at 1 microgram/ml. When small capsule and large capsule isolates were used for infection, the small capsule form was not isolated from the brain, in contrast to its isolation from 2 of 3 animals receiving large capsule forms. Two of three animals in each group responded with blastogenic indices more vigorous in the PBL, and the most potent antigen was MS. There was no obvious difference in lymphocyte reactivity between the two groups.