Non-specific immunosuppression by Cryptococcus neoformans infection

Basic principles of the virulence of Cryptococcus

Among fungal pathogens, Cryptococcus neoformans has gained great importance among the scientific community of several reasons. This fungus is the causative agent of cryptococcosis, a disease mainly associated to HIV immunosuppression and characterized by the appearance of meningoencephalitis. Cryptococcal meningitis is responsible for hundreds of thousands of deaths every year. Research of the pathogenesis and virulence mechanisms of this pathogen has focused on three main different areas: Adaptation to the host environment (nutrients, pH, and free radicals), mechanism of immune evasion (which include phenotypic variations and the ability to behave as a facultative intracellular pathogen), and production of virulence factors. Cryptococcus neoformans has two phenotypic characteristics, the capsule and synthesis of melanin that have a profound effect in the virulence of the yeast because they both have protective effects and induce host damage as virulence factors. Finally, the mechanisms that result in dissemination and brain invasion are also of key importance to understand cryptococcal disease. In this review, I will provide a brief overview of the main mechanisms that makes C. neoformans a pathogen in susceptible patients.

Abbreviations: RNS: reactive nitrogen species; BBB: brain blood barrier; GXM: glucuronoxylomannan; GXMGal: glucuronoxylomannogalactan

The Role of Cryptococcus in the Immune System of Pulmonary Cryptococcosis Patients

Objectives

To investigate the role of Cryptococcus in the immune system of immunocompetent patients with pulmonary cryptococcosis (PC) by analysing the dynamic changes of patients’ immune status before and after antifungal therapy.

Methods

The level of the serum interferon-γ (IFN-γ) and interleukin (IL)-2, -4, -10 and -12 was measured before and after 6-months of treatment. Peripheral blood samples were obtained from 30 immunocompetent PC patients and 30 age- and gender-matched healthy controls. Peripheral blood mononuclear cells (PBMCs) were isolated and incubated with recombinant human IL-12 (rhIL-12) for 48 h. Then the concentrations of IFN-γ and IL-4 in the supernatant were analysed.

Results

Baseline serum IFN-γ level was significantly lower in the PC patients as compared with the control group (P < 0.001). The serum IL-2 and IFN-γ of PC patients were significantly increased after appropriate treatments (P < 0.05 and P < 0.001 when compared to their baseline levels). The productions of IFN-γ in the culture supernatant of PBMCs showed no significant difference between the control and PC patients both before and after antifungal treatments. RhIL-12 is a potent stimulus for IFN-γ production. Culture PBMCs collected from PC patients before treatments had a smaller increase of IFN-γ production in the present of rhIL-12 than the control (P < 0.01); PBMCs from PC patients completing 6-months of treatment showed a comparable increase of IFN-γ production by rhIL-12 stimulation to the control group.

Conclusions

In apparently immunocompetent patients with PC, a normalization of serum IFN-γ was achieved after recovery from infection. This suggests that Cryptococcus infection per se can suppress the immune system and its elimination contributes to the reestablishment of an immune equilibrium.

Cryptococcus neoformans induces antimicrobial responses and behaves as a facultative intracellular pathogen in the non mammalian model Galleria mellonella

Cryptococcus neoformans is an encapsulated opportunistic fungal pathogen that is found in multiple niches in the environment and that can cause fatal meningoencephalitis in susceptible patients, mainly HIV+ individuals. Cryptococcus also infects environmental hosts such as nematodes, insects and plants. In particular, C. neoformans can kill the lepidopteran Galleria mellonella, which offers a useful tool to study microbial virulence and drug efficacy. Galleria mellonella immunity relies on innate responses based on melanization, accumulation of antimicrobial peptides, and cellular responses as phagocytosis or multicellular encapsulation. In this work we have investigated the immune response of G. mellonella during cryptococcal infection. We found that G. mellonella infected with C. neoformans had a high lytic activity in their hemolymph. This response was temperature- and capsule-dependent. During interaction with phagocytic cells, C. neoformans behaved as an intracellular pathogen since it could replicate within hemocytes. Non-lytic events were also observed. In contrast to Candida species, C. neoformans did not induce melanization of G. mellonella after infection. Finally, passage of C. neoformans through G. mellonella resulted in changes in capsule structure as it has been also reported during infection in mammals. Our results highlight that G. mellonella is an optimal model to investigate innate immune responses against C. neoformans.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Regulation of cytokine expression in mice immunized with cryptococcal polysaccharide, a glucuronoxylomannan (GXM), associated with peritoneal antigen-presenting cells (APC): requirements for GXM, APC activation, and interleukin-12

Mice immunized with peritoneal exudate cells (PEC; used as antigen-presenting cells [APC]) that are pulsed ex vivo with cryptococcal capsular polysaccharide, a glucuronoxylomannan (GXM), exhibit increased survival times and delayed-type hypersensitivity reactions when they are infected with Cryptococcus neoformans. These responses are GXM specific. The present study revealed that GXM-APC immunization enhanced development of anticryptococcal type-1 cytokine responses (interleukin-2 [IL-2] and gamma interferon) in mice infected with C. neoformans. The enhancement was not GXM specific, because immunization with GXM-APC and immunization with APC alone had similar effects. GXM-APC (or APC) immunization caused small increases in the expression of type-2 cytokines (IL-4 and IL-5), but the increases were not always statistically significant. IL-10 levels were not regulated by immunization with GXM-APC or APC. GXM-APC prepared with PEC harvested from mice injected with complete Freund's adjuvant (CFA) enhanced type-1 cytokine responses, while GXM-APC prepared with PEC induced with incomplete Freund's adjuvant were ineffective. The CFA-induced PEC had an activated phenotype characterized by increased numbers of F4/80(+) cells that expressed CD40, B7-1, and B7-2 on their membranes. The immunomodulatory activity of the CFA-induced APC population was not attributed to their production of IL-12 because GXM-APC prepared with peritoneal cells harvested from IL-12 knockout mice or their wild-type counterparts were equally effective in augmenting the type-1 response. Blocking of IL-12 in the recipients of GXM-APC early after APC infusion revealed that early induction of IL-12 secretion was not responsible for the immunomodulatory response elicited by GXM-APC. These data, considered together with previously reported data, reveal that the protective activity of GXM-APC immunization involves both antigen-specific and nonspecific activities of GXM-APC.

Immunoadjuvant effect of inactivated Cryptococcus neoformans

Cryptococcus neoformans disseminated into the central nervous system (CNS) of intraperitoneally inoculated adult ICR mice, but did not potentiate penetration of concurrently given Bhanja virus (Bunyaviridae) into the CNS. Likewise, Bhanja virus infection did not affect significantly the course of murine cryptococcosis. However, formalin-killed C. neoformans cells non-specifically increased production of antibodies against the virus infection in the animals.

What makes Cryptococcus neoformans a pathogen?

Life-threatening infections caused by the encapsulated fungal pathogen Cryptococcus neoformans have been increasing steadily over the past 10 years because of the onset of AIDS and the expanded use of immunosuppressive drugs. Intricate host-organism interactions make the full understanding of pathogenicity and virulence of C. neoformans difficult. We discuss the current knowledge of the characteristics C. neoformans must possess to enter the host and establish progressive disease: basic growth requirements and virulence factors, such as the polysaccharide capsule; shed products of the organism; melanin production; mannitol secretion; superoxide dismutase; proteases; and phospholipases.

Presentation of cryptococcal capsular polysaccharide (GXM) on activated antigen-presenting cells inhibits the T-suppressor response and enhances delayed-type hypersensitivity and survival

A hallmark of infection with Cryptococcus neoformans is depression of the immune system characterized by poor inflammatory responses and loss of delayed-type hypersensitivity (DTH) and antibody responses. T-suppressor cell (Ts) responses, elicited by the capsular polysaccharide (GXM) of the organism, are known to develop during infection. This study was undertaken to develop a method to inhibit the anti-GXM Ts response and thereby study the influence of the Ts response on immune responsiveness and survival in cryptococcosis. Antigen-presenting cells (APC), elicited with complete Freund's adjuvant (CFA), were treated in vitro with GXM (GXM-APC). The GXM-APC were injected intravenously into normal mice. These mice were resistant to induction of anti-GXM Ts cells when soluble GXM was administered in tolerogenic doses or when animals were infected with C. neoformans. Inhibition of the anti-GXM Ts response was specific to GXM as levan-APC did not inhibit induction of anti-GXM Ts cells. Inhibition of the anti-GXM Ts response could not be attributed to increased clearance of GXM due to induction of anti-GXM antibodies or other mechanisms. Anti-cryptococcal DTH responses were lost in mice by the second week of infection. However, treatment with GXM-APC, but not levan-APC, allowed mice to maintain their DTH response. GXM-APC pretreatment enhanced survival of infected mice compared with mice pretreated with levan-APC. These results show that GXM-APC induces immune responses that inhibit the induction of Ts responses and enhances DTH responses in infected mice. These responses correlate with enhanced survival after cryptococcal infection.

Cryptococcal capsular polysaccharide utilizes an antigen-presenting cell to induce a T-suppressor cell to secrete TsF

A T-T hybridoma (F6.6.2) which secretes a T-suppressor factor (TsF) specific for cryptococcal capsular polysaccharide (glucuronoxylomannan, GXM) was tested to determine if antigen-presenting cells (APC) were necessary for activation of the hybridoma to secrete TsF. Normal, syngeneic spleen cells were required along with GXM before TsF could be detected in culture supernatants. Ts cells did not secrete TsF unless the APC were obtained from mice which were identical at the "so-called' I-J sublocus as defined by the difference between B10.A(3R) and B10.A(5R) mice. The APC was adherent and could be depleted from spleen cell suspensions by treatment with anti-I-J and complement but not anti-I-A and complement. Additionally, treatment with anti-T cell serum or anti-immunoglobulin and complement did not remove the APC function of the spleen cell population. A role for I-E antigens in the function of the APC was determined by blocking antigen presentation to the suppressor cell with anti-I-E antibodies. The polysaccharide was associated with splenic adherent cells as extensive washing of the APC after incubation with GXM did not eliminate the antigen presenting function of the population.

Functional equivalence of cryptococcal and haptene-specific T suppressor factor (TsF). II. Monoclonal anti-cryptococcal TsF inhibits both phagocytosis by a subset of macrophages and transfer of contact sensitivity

Monoclonal anti-cryptococcal TsF (which inhibits phagocytosis by macrophages) and anti-picryl TsF use the same two circuits to block the transfer of contact sensitivity (CS). Both arm macrophages which then release a macrophage suppressor factor (MSF) when exposed to antigen. This MSF depresses the transfer of CS. The evidence suggests that a single molecular species of TsF (MW ca. 70 kDa), which bears an antigen-binding site and I-J determinant, is responsible for MSF production and inhibition of phagocytosis. Anti-cryptococcal TsF also arms the T acceptor cell which then releases nsTsF-1 after triggering with a specific antigen (SCPA). This nsTsF-1, which depresses the transfer of contact sensitivity, was authentic, as shown by its I-J positivity (in contrast to MSF) and its role in the production of nsTsF-2. As anti-picryl TsF also inhibits phagocytosis, it was concluded that anti-cryptococcal TsF, originally detected by the inhibition of phagocytosis, and anti-picryl TsF, originally detected by inhibition of CS, are functionally equivalent.

T cell-mediated immunity in the lung: a Cryptococcus neoformans pulmonary infection model using SCID and athymic nude mice

T cells are important in systemic anticryptococcal defenses, but a role in controlling an initial pulmonary infection has not been demonstrated. A murine model with intratracheal inoculation was developed to study the acquisition and expression of pulmonary T cell-mediated immunity against Cryptococcus neoformans. Infections with four strains of C. neoformans (305, 68A, 613D, and 52D) in two strains of mice (BALB/c and C57BL/6) were examined. Unencapsulated strain 305 and slowly growing strain 68A were readily controlled apparently by nonimmune pulmonary defenses, and no extrapulmonary dissemination was detected. Strain 613D grew progressively in the lungs and disseminated to the brain and spleen. Strain 52D initially grew rapidly in the lungs and disseminated to the spleen, but a clearance mechanism developed in the lungs after day 7 postinfection and in the spleen after day 28. SCID and athymic nude mice were unable to clear a strain 52D pulmonary infection, and a lethal disseminated infection occurred. Pulmonary clearance could be adoptively transferred into SCID mice infected with strain 52D by use of immune T cells from the spleen and lungs and hilar lymph nodes of infected immunocompetent donors. Furthermore, pulmonary clearance was almost 100-fold better in SCID mice that received immune T cells from the lungs and hilar lymph nodes than in those that received immune T cells from the spleen, even though equivalent levels of delayed-type hypersensitivity were transferred by both cell populations. These adoptive transfer studies suggested that the lung and hilar lymph node T cells from immune animals either are enriched in such a way as to mediate protective immunity or home to the lungs better than do splenic T cells.

Pathogenesis of Cryptococcus neoformans in congenitally immunodeficient beige athymic mice

Mortality after intravenous challenge with 10(4) Cryptococcus neoformans demonstrated that doubly immunodeficient beige athymic (bg/bg nu/nu) mice were more susceptible to systemic cryptococcosis than either bg/bg or nu/nu mice. Infected bg/bg nu/nu mice also had a shortened lifespan compared with their bg/bg nu/+ littermates. Beige athymic (bg/bg nu/nu) but not bg/bg nu/+mice developed cryptococcal lesions in the skin, demonstrating that C. neoformans is dermatotropic in a T-cell-deficient host. Higher numbers of C. neoformans were isolated from the lungs and spleen of infected bg/bg nu/nu than bg/bg nu/+ mice as early as day 3 after challenge, indicating that in lymphoid-rich organs, T cells can alter the course of systemic cryptococcosis early in the infection. Despite extensive abscess formation in the brains of bg/bg nu/+ mice, dissemination and growth rate of C. neoformans in the brain was similar in both genotypes. The primary histopathological feature in tissues from bg/bg nu/nu mice infected with C. neoformans consisted of foci of encapsulated yeast cells with minimal to no inflammatory response. In contrast to bg/bg nu/nu mice, bg/bg nu/+ mice mounted a vigorous inflammatory response to C. neoformans that progressed from acute to chronic inflammation. Beige athymic mice are a new animal model that will be useful in clarifying the innate and acquired immune factors important in resistance to cryptococcosis.

Inhibition of macrophage phagocytosis in cryptococcosis: phenotypic analysis of the suppressor cell

Our laboratory has previously reported a suppressor cell mechanism to occur late in the course of a lethal infection with Cryptococcus neoformans. A soluble factor was found to be responsible for inhibition of the phagocytic activity of a subpopulation of peritoneal macrophages. The suppressor cell was identified as a T cell which required in vitro stimulation with specific antigen before the phagocytosis-inhibiting lymphokine (PIL) was produced. PIL action was allospecific and occurred in animals given tolerogenic doses of cryptococcal and noncryptococcal antigens. The current investigation has further characterized the T lymphocyte responsible for PIL activity. The suppressor cell was found to be in a cyclophosphamide-sensitive pathway. PIL activity was not detected when spleen cell populations were treated with anti-I-J and complement or anti-Lyt-2 and complement. Likewise, a mixture of anti-I-J-treated and anti-Lyt-2-treated cells was incapable of synthesizing the lymphokine. Treatment of spleen cells with anti-Lyt-1.2 or anti-L3T4 and complement did not eliminate PIL synthesis. Further analysis of the genetic restrictions associated with the PIL-macrophage interaction revealed regulation by the I-J subregion of the major histocompatibility complex.

Induction and expression of cell-mediated immune responses in inbred mice infected with Coccidioides immitis

Comparisons of the course of coccidioidomycosis in two strains of inbred mice established that BALB/c mice are significantly more susceptible to pulmonary infection with Coccidioides immitis than are DBA/2 mice. The susceptibility of BALB/c mice does not reside in their inability to mount a delayed-type hypersensitivity response to C. immitis antigen. That is, BALB/c mice manifested footpad hypersensitivity to coccidioidin early during the course of disease, to a level comparable to that of DBA/2 mice. In contrast to the more resistant DBA/2 mouse strain, however, BALB/c mice developed anergy by day 15 postinfection. Suppression of the delayed-type hypersensitivity response was not specific for C. immitis antigen, as evidenced by the finding that BALB/c mice immunized with mycobacterial purified protein derivative prior to infection with C. immitis were suppressed in their footpad response to mycobacterial antigen at day 15 postinfection. Taken together, these results establish that genetically determined susceptibility to this fungus is associated with an acquired suppression of cell-mediated immune reactivity.

Induction of a macrophage-suppressive lymphokine by soluble cryptococcal antigens and its association with models of immunologic tolerance

Soluble extracts of Cryptococcus neoformans were examined for their ability to induce a macrophage-regulatory T-suppressor cell known to appear in the spleens of mice infected with cryptococci. Suppressor cells were induced by injection of extracts of encapsulated or thinly encapsulated strains of cryptococci. Dose-response analysis showed that as little as 25 micrograms of soluble capsular polysaccharide antigen could induce significant suppressor cell activity, with maximum suppression occurring at a dose of 100 micrograms. The suppressor cells appeared within 1 week of injection of antigen and persisted for at least 2 months. Suppressor cells were induced in animals given tolerogenic doses of levan, human gamma globulin, and soluble capsular polysaccharide antigen. When these same antigens were administered in immunogenic form, no suppressor cell activity was detected. Therefore, the suppressive mechanism was common to models of immunologic tolerance and was not unique to cryptococcal disease or cryptococcal capsular polysaccharide antigen. The phagocytosis-inhibiting lymphokine produced by the suppressor cell population completely inhibited the phagocytic activity of only a portion of peritoneal exudate cells. Other macrophages in the population were not totally inhibited but exhibited a reduction in the number of yeast cells engulfed.

Modulation of Bhanja virus infection in mice

Randombred (ICR) and inbred (C57B1/6) 4-wk-old SPF male mice were infected extraneurally with Bhanja virus (Bunyaviridae) and subjected to various treatments. Immunosuppression with cyclophosphamide (CPA) affected the course of the infection when a higher dose (10(6) suckling mouse intracerebral LD50) of the virus and 2 or 3 injections of CPA (150 mg/kg each) were given: then a part of the animals died due to viral encephalitis, whereas all the CPA-untreated infected mice survived. A dual peripheral infection with Bhanja and Lipovnik (Reoviridae) viruses did not cause any symptomatic response, and the host's humoral antibody was slightly stimulated. When Bhanja virus was given prior to, or simultaneously with, tick-borne encephalitis virus (Flaviviridae), a moderate decrease of the mortality (due to tick-borne encephalitis) occurred. A mixed peripheral infection of mouse with Bhanja virus and Cryptococcus neoformans, did not result in a fatal virus encephalitis of the host, nor was cryptococcosis affected substantially. However, formalin-killed cells of the fungus ("cryptococcin") administered before the extraneural inoculation of Bhanja virus caused an 8-fold increase of antibodies neutralizing the virus; a mild therapeutic or protective effect of cryptococcin on encephalitis after an intracerebral application of Bhanja virus was also observed.