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

Intranasal vaccination induces protective immunity against intranasal infection with virulent Francisella tularensis biovar A

The inhalation of Francisella tularensis biovar A causes pneumonic tularemia associated with high morbidity and mortality rates in humans. Exposure to F. tularensis usually occurs by accident, but there is increasing awareness that F. tularensis may be deliberately released in an act of bioterrorism or war. The development of a vaccine against pneumonic tularemia has been limited by a lack of information regarding the mechanisms required to protect against this disease. Vaccine models for F. tularensis in inbred mice would facilitate investigations of the protective mechanisms and significantly enhance vaccine development. Intranasal vaccination with the attenuated live vaccine strain (LVS) of F. tularensis reproducibly protected BALB/c mice, but not C57BL/6 mice, against intranasal and subcutaneous challenges with a virulent clinical isolate of F. tularensis biovar A (NMFTA1). The resistance of LVS-vaccinated BALB/c mice to intranasal NMFTA1 challenge was increased 100-fold by boosting with live NMFTA1 but not with LVS. The protective response was specific for F. tularensis and required both CD4 and CD8 T cells. The vaccinated mice appeared outwardly healthy for more than 2 months after NMFTA1 challenge, even though NMFTA1 was recovered from more than half of the vaccinated mice. These results show that intranasal vaccination induces immunity that protects BALB/c mice from intranasal infection by F. tularensis biovar A.

B cell response during infection with the MAT a and MAT alpha mating types of Cryptococcus neoformans

In the present study, we compared the B cell response of BALB/c and C57Bl/6 mice during Cryptococcus neoformans infection. This response was investigated using virulent serotype D forms of mating types alpha and a (MAT alpha and MAT a). C57Bl/6 mice showed massive (mainly cerebral) infection by both types, while BALB/c were resistant to infection. Some resistance of C57Bl/6 mice was induced by previous immunization with the capsular polysaccharide from MAT alpha. Passive immunization of C57Bl/6 mice with purified antibody (Ab) obtained from capsular polysaccharide-immunized mice also increased resistance to infection. Both mouse strains showed comparable low IgM response to the capsular polysaccharide from MAT alpha, and only C57Bl/6 mice produced IgM to the polysaccharide of MAT a. Comparable levels of different immunoglobulin (Ig) isotypes against capsular components of MAT alpha and MAT a were detected, and the response of C57Bl/6 mice was higher when compared to that of BALB/c mice. FACS analysis indicated an increase in the percentage of a high-granulosity (side-scatter) splenic subpopulation and in the percentage of splenic Gr-1+ cells in infected C57Bl/6 mice. In addition, the percentage of follicular splenic B cells was decreased after C. neoformans infection of C57Bl/6 mice. This response was more pronounced when we investigated infection induced by the MAT a mating type. Taken together, our results indicate that capsular polysaccharide derived from MAT alpha and MAT a types of C. neoformans have a stimulatory effect upon B cells but that there is no correlation between resistance of BALB/c mice and Ab production. However, the increase in resistance of C57Bl/6 mice parallels the production of Abs and a major change in splenic cell populations.

Involvement of CD14, toll-like receptors 2 and 4, and MyD88 in the host response to the fungal pathogen Cryptococcus neoformans in vivo

The major capsular polysaccharide of Cryptococcus neoformans, glucuronoxylomannan (GXM), is recognized by Toll-like receptor 2 (TLR2), TLR4, and CD14. In these studies, mice deficient in CD14, TLR2, TLR4, and the TLR-associated adaptor protein, MyD88, were utilized to investigate the contribution of TLRs and CD14 to in vivo host defenses against C. neoformans. MyD88(-/-) mice had significantly reduced survival compared with wild-type C57BL/6 mice after intranasal (i.n.) and intravenous (i.v.) infection with live C. neoformans. CD14(-/-) mice had reduced survival when infected i.v., while TLR2(-/-) mice died significantly earlier after i.n. infection. Mortality was similar comparing TLR4 mutant C3H/HeJ mice and control C3H/HeOuJ mice following i.v. or i.n. challenge with C. neoformans. The course of pulmonary cryptococcosis was studied in more detail in the CD14(-/-), TLR2(-/-), and MyD88(-/-) mice. MyD88(-/-) mice infected i.n. had higher numbers of CFU in the lungs as well as higher GXM levels in the sera and lungs 7 days after infection than wild-type mice did. Surprisingly, there were no major differences in the levels of tumor necrosis factor alpha, interleukin-4 (IL-4), IL-10, IL-12p70, or gamma interferon in the lungs of C. neoformans-infected knockout mice compared with wild-type mice. Histopathologic analysis of the lungs on day 7 postinfection revealed minimal inflammation in all mouse groups. These studies demonstrate a major role for MyD88 and relatively minor roles for CD14 and TLR2 in the response to cryptococcal infection, with the decreased survival of MyD88(-/-) mice correlating with increased numbers of lung CFU and serum and lung GXM levels.

Age-related resistance of C57BL/6 mice to Cryptococcus neoformans is dependent on maturation of NKT cells

Conflicting results have been reported regarding the ability of C57BL/6 mice to clear infections due to Cryptococcus neoformans. Examination of the various experimental protocols used suggested that C57BL/6 mice might develop the ability to resist infection as they mature. We analyzed the ability of C57BL/6 mice of different ages to respond to immunization with cryptococcal antigen or to clear a cryptococcal infection. Mice were immunized with a soluble cryptococcal culture filtrate antigen (CneF) emulsified in complete Freund's adjuvant (CneF-CFA). Delayed-type hypersensitivity (DTH) reactions elicited by the immunization were significantly stronger in 15-week-old C57BL/6 mice than in 7-week-old mice. Analysis of cryptococcal CFU 8 weeks following intratracheal infection of 7-week-old mice or 15-week-old mice revealed a relative inability of the younger animals to control the infection. Six-week-old immunized and infected mice cleared cryptococci from brain, spleen, and liver in a manner similar to that of immunized and infected 15-week-old mice. However, the older mice cleared cryptococci much more efficiently from the lungs. The possible role for NKT cells was determined by passive transfer of thymocytes from 10-week-old mice (containing mature NKT cells) or 2-week-old mice (containing immature NKT cells) to 6-week-old mice. The 10-week-old thymocytes significantly enhanced the ability of the mice to develop a DTH response after immunization with CneF-CFA, while animals treated with 2-week-old thymocytes did not improve their DTH response after immunization. The cells in the 10-week-old thymocyte population responsible for improvement of DTH responses were identified as being NK1.1 positive.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

OX40 ligation on activated T cells enhances the control of Cryptococcus neoformans and reduces pulmonary eosinophilia

Pulmonary eosinophilia induced in C57BL/6 mice after Cryptococcus neoformans infection is driven by CD4(+) Th2 cells. The immunological mechanisms that protect against eosinophilia are not fully understood. Interaction of OX40 (CD134) and its ligand, OX40L, has been implicated in T cell activation and cell migration. Unlike CD28, OX40 is only expressed on T cells 1-2 days after Ag activation. Manipulation of this pathway would therefore target recently activated T cells, leaving the naive repertoire unaffected. In this study, we show that engagement of OX40 by an OX40L:Ig fusion protein drives IFN-gamma production by CD4(+) T cells and reduces eosinophilia and C. neoformans burden in the lung. Using gene-depleted mice, we show that reduction of eosinophilia and pathogen burden requires IL-12 and/or IFN-gamma. C. neoformans infection itself only partially induces OX40L expression by APCs. Provision of exogenous OX40L reveals a critical role of this pathway in the prevention of C. neoformans-induced eosinophilia.

Prior exposure to live Mycobacterium bovis BCG decreases Cryptococcus neoformans-induced lung eosinophilia in a gamma interferon-dependent manner

Some common childhood infections appear to prevent the development of atopy and asthma. In some Mycobacterium bovis BCG-vaccinated populations, strong delayed-type hypersensitivity responses to mycobacterial antigens are associated with a reduced risk of atopy. Although BCG exposure decreases allergen-induced lung eosinophilia in animal models, little attention has been given to the effect of immunity to BCG on responses against live pathogens. We used the murine Cryptococcus neoformans infection model to investigate whether prior BCG infection can alter such responses. The present study shows that persistent pulmonary BCG infection of C57BL/6 mice induced an increase in gamma interferon, a reduction in interleukin-5, and a decrease in lung eosinophilia during subsequent Cryptococcus infection. This effect was long lasting, depended on the presence of live bacteria, and required persistence of mycobacterial infection in the lung. Reduction of eosinophilia was less prominent after infection with a mutant BCG strain (DeltahspR), which was rapidly cleared from the lungs. These observations have important implications for the development of vaccines designed to prevent Th2-mediated disease and indicate that prior lung BCG vaccination can alter the pattern of subsequent host inflammation.

Primary dendritic cells phagocytose Cryptococcus neoformans via mannose receptors and Fcgamma receptor II for presentation to T lymphocytes

Different "professional" antigen-presenting cells (APC) have unique characteristics that favor or restrict presentation of microbial antigens to T cells, depending on the organism. Cryptococcus neoformans is a pathogenic yeast that presents unique challenges to APC, including its large size, its rigid cell wall, and its ability to stimulate T cells as a mitogen. T-cell proliferation in response to the C. neoformans mitogen (CnM) requires phagocytosis and processing of the organisms by accessory cells prior to presentation of CnM to T cells. Because of the requirement for uptake of the organism and more limited costimulatory requirements of mitogens, macrophages might be the most likely cellular source for the accessory cell. However, the present study demonstrates that a transiently adherent cell that was CD3(-), CD14(-), CD19(-), CD56(-), HLA-DR(+), and CD83(+) with a dendritic morphology, rather than monocyte-derived or tissue (alveolar) macrophages, was the most efficient APC for presentation of CnM. A large number of these cells bound and internalized the organism, and only a small number of dendritic cells were required for presentation of the mitogen to T cells. Further, the mannose receptor and Fcgamma receptor II were required for presentation of C. neoformans, as blocking either of these receptors abrogated both uptake of C. neoformans and lymphocyte proliferation in response to CnM. These studies demonstrate the surprising fact that dendritic cells are the most efficient accessory cells for CnM.

Cryptococcus neoformans pulmonary granuloma formation is associated with matrix metalloproteinase-2 expression

The aim of this study was to investigate matrix metalloproteinase (MMP) expression during the immune response to pulmonary Cryptococcus neoformans (Cne) infection. The immune response generated in C.B-17 and C57BL/6 mice to pulmonary Cne infection has previously been characterized as type 1 and type 2, respectively, differing in the cytokines produced and leukocytes recruited during infection, influencing the extent of Cne clearance from the lung. The focus of this study was to examine changes in expression of MMP-2 and tissue inhibitor of metalloproteinase (TIMP)-2 in the lungs of Cne-infected mice during the two types (type 1 vs. type 2) of responses. C.B-17 mice that formed well-defined granulomas had elevated levels of pulmonary MMP-2 mRNA early during infection. C57BL/6 mice that had poorly defined cellular aggregates did not express detectable levels of pulmonary MMP-2 mRNA until later in the infection. Specific expression of MMP/TIMP was correlated with the type of immune response present, resolution of Cne infection and the resulting lung pathology.

Complementation of a capsule deficient Cryptococcus neoformans with CAP64 restores virulence in a murine lung infection

Cryptococcosis is a systemic infection in humans caused by the opportunistic fungal pathogen, Cryptococcus neoformans. The infection usually presents as chronic meningoencephalitis, but infects via the respiratory tract. A polysaccharide capsule is a major virulence factor, which allows the yeast to resist host defenses. However, the essential role of the capsule in allowing it to resist host defenses during the initial lung infection has not been clearly shown. A mutant acapsular C. neoformans strain 602 was complemented with the CAP64 gene to obtain an encapsulated strain, TYCC38-602. TYCC38-602 persisted in the lungs of C.B-17 mice after intratracheal inoculation and disseminated to the brain, whereas the mutant acapsular 602 and the plasmid control transformant CIP3-602 strains grew less readily in the lung and were infrequently detected in the brain. T cell-mediated immunity, developed to the encapsulated organism, was required to control growth within the lungs and had a significant impact on numbers of yeasts detected in the brain. The parent acapsular strain, but not the transformant control, also required T cells for optimal inhibition of growth within the lung, but not for maintaining control of the colony-forming units (cfu) in the brain. In summary, the cryptococcal capsule plays an important role in lung virulence and dissemination to the brain, and intact immunity is required to control lung growth of the encapsulated yeast.

Urokinase-type plasminogen activator is required for the generation of a type 1 immune response to pulmonary Cryptococcus neoformans infection

Urokinase-type plasminogen activator (uPA)(-/-) mice cannot mount protective host defenses during infection with the opportunistic yeast Cryptococcus neoformans (52D). Because effective host defense against C. neoformans requires specific immune responses and the generation of type 1 (T1) cytokines, we determined how the absence of uPA impacts these processes. Wild-type (WT) and uPA(-/-) mice were inoculated with C. neoformans. Macrophage antifungal activity was assessed histologically, T lymphocyte responses in vivo and proliferation in vitro were quantified, and cytokine concentrations were determined by ELISA. uPA(-/-) macrophages have impaired antimicrobial activity. Regional lymph nodes of infected uPA(-/-) mice contained fewer cells than WT, suggesting impaired T cell proliferation in response to the pathogen in vivo. In vitro, uPA(-/-) T lymphocytes had impaired proliferative responses to C. neoformans rechallenge compared with WT. Infected WT mice generated T1 cytokines in the lung, characterized by high levels of IFN-gamma and IL-12. uPA(-/-) mice had decreased levels of IFN-gamma and IL-12, and increased IL-5, a type 2 cytokine. In the absence of uPA, the cytokine profile of regional lymph nodes shifted from a T1 pattern characterized by IFN-gamma and IL-2 to a weak, nonpolarized response. We conclude that in the absence of uPA, lymphocyte proliferative responses are diminished, and mice fail to generate protective T1 cytokines, resulting in impaired antimicrobial activity. This study provides novel evidence that uPA is a critical modulator of immune responses and of immune cell effector functions in vivo.

Phenotypic switching of Cryptococcus neoformans occurs in vivo and influences the outcome of infection

Phenotypic switching has been linked to the virulence of many pathogens, including fungi. However, it has not been conclusively shown to occur in vivo or to influence the outcome of infection. Cryptococcus neoformans undergoes phenotypic switching in vitro to colony types that differ in their virulence in mice. In this study, we asked whether C. neoformans undergoes phenotypic switching in vivo and whether this phenomenon contributes to virulence. By using a small inoculum to preclude the introduction of variants that had already switched during in vitro propagation, we demonstrated that in vivo switching to a mucoid phenotype occurred in two mice strains and was associated with a lethal outcome. Phenotypic switching resulted in changes of the capsular polysaccharide that inhibited phagocytosis by alveolar macrophages. This promoted a more vigorous inflammatory response and rapid demise. These data document in vivo switching in a fungus and associate this phenomenon with enhanced virulence and a lethal outcome. The importance of this finding is underscored by the increased likelihood of phenotypic switching in chronic cryptococcosis; thus this mechanism may account for the inability to eradicate the organism in immunocompromised hosts.

The capsular polysaccharides of Cryptococcus neoformans activate normal CD4(+) T cells in a dominant Th2 pattern

Capsular components of Cryptococcus neoformans induce several deleterious effects on T cells. However, it is unknown how the capsular components act on these lymphocytes. The present study characterized cellular and molecular events involved in immunoregulation of splenic CD4(+) T cells by C. neoformans capsular polysaccharides (CPSs). The results showed that CPSs induce proliferation of normal splenic CD4(+) T cells, but not of normal CD8(+) T or B lymphocytes. Such proliferation depended on physical contact between CPSs and viable splenic adherent cells (SAC) and CD40 ligand-induced intracellular signal transduction. The absence of lymphoproliferation after fixation of SAC with paraformaldehyde has discarded the hypothesis of a superantigen-like activation. The evaluation of a cytokine pattern produced by the responding CD4(+) T lymphocytes revealed that CPSs induce a dominant Th2 pattern, with high levels of IL-4 and IL-10 production and undetectable inflammatory cytokines, such as TNF-alpha and IFN-gamma. Blockade of CD40 ligand by relevant mAb down-regulated the CPS-induced anti-inflammatory cytokine production and abolished the enhancement of fungus growth in cocultures of SAC and CD4(+) T lymphocytes. Our findings suggest that CPSs induce proliferation and differentiation of normal CD4(+) T cells into a Th2 phenotype, which could favor parasite growth and thus important deleterious effects to the host.

Regulatory effects of macrophage inflammatory protein 1alpha/CCL3 on the development of immunity to Cryptococcus neoformans depend on expression of early inflammatory cytokines

Macrophage inflammatory protein 1alpha (MIP-1alpha)/CCL3 prevents the development of eosinophilic pneumonia (EP) driven by a nonprotective T2-type immunity during infection with a highly virulent strain of Cryptococcus neoformans. The present study evaluated the interaction of MIP-1alpha with other innate immune system cytokines by comparing the immune responses that followed pulmonary infections with high- (C. neoformans 145A) and low (C. neoformans 52D)-virulence strains. In contrast to what was found for C. neoformans 145A infection, lack of MIP-1alpha in C. neoformans 52D infection did not cause the development of EP. C. neoformans 52D induced tumor necrosis factor alpha (TNF-alpha), gamma interferon (IFN-gamma), and MCP-1 in the lungs of infected wild-type (WT) and MIP-1alpha knockout (KO) mice by day 7 postinfection. Both WT and MIP-1alpha KO mice subsequently cleared this infection. Thus, the robust expression of early inflammatory cytokines in C. neoformans 52D-infected mice promoted the development of protective immunity even in the absence of MIP-1alpha. Alternatively, C. neoformans 145A-infected WT and MIP-1alpha KO mice had diminished TNF-alpha, IFN-gamma, and macrophage chemoattractant protein 1 (MCP-1) responses, indicating that virulent C. neoformans 145A evaded early innate host defenses. However C. neoformans 145A-infected WT mice had an early induction of MIP-1alpha and subsequently did not develop EP. In contrast, C. neoformans 145A-infected MIP-1alpha KO mice developed EP and had increased C. neoformans dissemination into the brain by day 35. We conclude that, in the absence of other innate immune response effector molecules, MIP-1alpha is crucial to prevent the development of EP and to control C. neoformans dissemination to the brain.

Both Th1 and Th2 cytokines affect the ability of monoclonal antibodies to protect mice against Cryptococcus neoformans

Variable-region-identical mouse immunoglobulin G1 (IgG1), IgG2b, and IgG2a monoclonal antibodies to the capsular polysaccharide of Cryptococcus neoformans prolong the lives of mice infected with this fungus, while IgG3 is either not protective or enhances infection. CD4+ T cells are required for IgG1-mediated protection, and CD8+ T cells are required for IgG3-mediated enhancement. Gamma interferon is required for both effects. These findings revealed that T cells and cytokines play a role in the modulation of cryptococcal infection by antibodies and suggested that it was important to more fully define the cytokine requirements of each of the antibody isotypes. We therefore investigated the efficacy of passively administered variable-region-identical IgG1, IgG2a, IgG2b, and IgG3 monoclonal antibodies against intravenous infection with C. neoformans in mice genetically deficient in interleukin-12 (IL-12), IL-6, IL-4, or IL-10, as well as in the parental C57BL/6J strain. The relative inherent susceptibilities of these mouse strains to C. neoformans were as follows: IL-12(-/-) > IL-6(-/-) > C57BL/6J approximately IL-4(-/-) >> IL-10(-/-). This is consistent with the notion that a Th1 response is necessary for natural immunity against cryptococcal infection. However, none of the IgG isotypes prolonged survival in IL-12(-/-), IL-6(-/-), or IL-4(-/-) mice, and all isotypes significantly enhanced infection in IL-10(-/-) mice. These results indicate that passive antibody-mediated protection against C. neoformans requires both Th1- and Th2-associated cytokines and reveal the complexity of the mechanisms through which antibodies modulate infection with this organism.

The role of macrophage inflammatory protein-1 alpha/CCL3 in regulation of T cell-mediated immunity to Cryptococcus neoformans infection

Macrophage inflammatory protein-1alpha (MIP-1alpha/CCL3) is a CC chemokine required for optimal recruitment of leukocytes in response to cryptococcal Ags. MIP-1alpha is expressed in the lungs by day 6 post Cryptococcus neoformans infection and could play a role in the development of cell-mediated immunity. To address this possibility, wild-type (MIP-1alpha(+/+)) mice and MIP-1alpha knockout (MIP-1alpha(-/-)) mice were infected intratracheally with a highly virulent strain of C. neoformans (145A). MIP-1alpha message was detected in the lungs on days 3, 7, and 14 in MIP-1alpha(+/+) mice, but it was undetectable in MIP-1alpha(-/-) mice. On day 16, MIP-1alpha(-/-) mice had a 7-fold increase in C. neoformans burden in the lungs, but no decrease in pulmonary leukocyte recruitment. MIP-1alpha(+/+) and MIP-1alpha(-/-) mice had similar numbers of recruited lymphocytes and monocytes/macrophages. Notably, MIP-1alpha(-/-) mice had a significantly greater number of eosinophils. MIP-1alpha(-/-) mice had extremely high levels of serum IgE. This switch of immune response to a T(2) phenotype was associated with enhanced IL-4 and IL-13 expression in the lungs of MIP-1alpha(-/-) mice compared with MIP-1alpha (+/+) mice. Progression of pulmonary cryptococcosis in the presence of nonprotective T(2) immunity resulted in profound lung damage in MIP-1alpha(-/-) mice (eosinophilic crystal deposition, destruction of lung parenchyma, and pulmonary hemorrhage). Twelve-week survival was dramatically decreased in MIP-1alpha(-/-) mice. These studies, together with our previous studies, demonstrate that MIP-1alpha plays a role in both the afferent (T(1)/T(2) development) and efferent (T(1)-mediated leukocyte recruitment) phases of cell-mediated immunity to C. neoformans.

Phagocytosis and protein processing are required for presentation of Cryptococcus neoformans mitogen to T lymphocytes

In addition to eliciting antigen specific T-cell-mediated immunity, Cryptococcus neoformans possesses a mitogen (CnM) that activates naive T cells to proliferate. This mechanism of T-cell activation is accessory cell dependent and major histocompatibility complex unrestricted. CnM-induced T-cell proliferation correlates with internalization of the organism, suggesting that intracellular processing is required to liberate CnM prior to presentation to T cells. To determine whether phagocytosis and processing are required, various inhibitors of accessory cell uptake and processing were used. C. neoformans was observed within the accessory cells. Paraformaldehyde fixation of the accessory cell abrogated presentation of CnM to T cells, indicating that a dynamic accessory cell surface was required. A lysosomotropic agent abrogated the response to CnM but had no effect on a control stimulus that did not require processing. Both aspartic acid and cysteine protease inhibitors blocked effective processing of CnM, so that it was unable to stimulate T cells. Finally, an inhibitor of microfilament polymerization abrogated proliferation to CnM. These results indicate that the mitogenic activity of C. neoformans requires phagocytosis of the organism, lysosomal or endosomal processing, proteolytic activity, and microfilament polymerization and intracellular transport as a prerequisite for T-cell proliferation.

Animal models of pulmonary infection in the compromised host: potential usefulness for studying health effects of inhaled particles

Pulmonary infection leading to pneumonia is a significant cause of morbidity and mortality worldwide. Airborne particles have been associated with pneumonia through epidemiological research, but the mechanisms by which particles affect the incidence of pneumonia are not well established. The purpose of this review is to examine the potential of animal models to improve our understanding of the mechanisms by which inhaled particles might affect the incidence and resolution of pulmonary infection. The pathogenesis of pneumonia in most animal models differs from that in humans because humans frequently have underlying diseases that predispose them to infection with relatively low doses of pathogens. Normal, healthy animals lack the underlying pathology often found in humans and clear bacteria and viruses rapidly from their lungs. To overcome this, animals are administered large inocula of pathogens, are treated with agents that cause mucosal lesions, or are treated with immunosuppressive drugs. Alternatively, pathogenic bacteria are protected from phagocytosis by encasing them in agar. No one animal model will replicate a human disease in its entirety, and the choice of model depends upon how well the animal infection mimics the particular human response being examined. The advantages and disadvantages of animal models in current use for bacterial and viral infections important in the etiology of human pneumonia are reviewed in detail. Considerable data indicate that prior exposure to particles compromises the ability of experimental animals to resolve a subsequent infection. In addition, information is available on the effects of particle exposure on various portions of respiratory defense including phagocytic function, ciliary movement, inflammation, and antibody response in the absence of infection. In contrast, little research to date has examined the consequences of particle exposure on the host defense mechanisms of animals already infected or on their ability to resolve their infection.

Pulmonary immune responses during primary mycobacterium bovis- Calmette-Guerin bacillus infection in C57Bl/6 mice

Mechanisms of protective immunity to mycobacterial infection in the lung remain poorly defined. In this study, T-cell subset expansion and cytokine expression in bronchoalveolar spaces, lung parenchyma, and mediastinal lymph nodes of mice infected intratracheally with Mycobacterium bovis-Calmette-Guerin bacillus (BCG) were analyzed in parallel with histopathology and bacterial burden. M. bovis-BCG was cleared rapidly from bronchoalveolar spaces without evidence for persistence. In lung parenchyma bacteria grew during the first 4 wk followed by gradual clearance with less than 0.1% of the original inoculum persisting for more than 8 mo. Clearance of M. bovis-BCG from bronchoalveolar lavage was associated with recruitment of both neutrophils and lymphocytes. Lung CD4(+), CD8(+), and gammadelta T-cell receptor-positive T cells expanded maximally by Week 4, and declined by Week 8 to control values despite bacterial persistence. Both CD4(+) and CD8(+) lung T cells produced interferon (IFN)-gamma in response to M. bovis-BCG. Four distinct pathologic states of lung parenchymal infection were noted. Early focal sub-bronchial inflammation with transmigration of cells into airways was followed by diffuse peribronchitis, perivasculitis, and alveolitis with activated macrophages, lymphoblasts, and occasional giant cells. The latter stage corresponded to maximal M. bovis-BCG growth. Resolving infection consisted of small lymphocytes and foamy macrophages, which coincided with decreasing M. bovis-BCG colony-forming units, T-cell infiltration, and IFN-gamma expression. A final quiescent phase consisted of residual lymphoid aggregates and perivasculitis associated with persistent spontaneous IFN-gamma production. Bacterial dissemination to lymph node and spleen occurred by Week 4 and declined in parallel to lung. In contrast to lung, IFN-gamma secretion was detected only late despite early expansion of CD4(+) and CD8(+) T cells. By reverse transcriptase/polymerase chain reaction, IFN-gamma and interleukin (IL)-12 p40 messenger RNA (mRNA) in lung paralleled IFN-gamma protein production. Tumor necrosis factor-alpha, IL-4 and IL-10 mRNA expression was not increased during M. bovis-BCG lung infection. Thus, protective immunity to M. bovis-BCG in the lung evolved differently in air space, lung, and lymph node.

Requirement for CD4(+) T lymphocytes in host resistance against Cryptococcus neoformans in the central nervous system of immunized mice

The importance of cell-mediated immunity (CMI) and CD4(+) T lymphocytes in host resistance against Cryptococcus neoformans is well documented and is exemplified by the high susceptibility to progressive infection with this pathogen of AIDS patients with reduced CD4(+) T-cell numbers. Although much has been learned about the role of CMI in the clearance of C. neoformans from the lungs and other internal organs, less is known about the protective mechanisms in the brain, the organ most frequently involved with a fatal outcome of cryptococcosis. We hypothesized that host resistance mechanisms against C. neoformans in the central nervous system (CNS) were similar to those outside the CNS (i.e., gamma interferon [IFN-gamma], CD4(+) T cells, and others). To test this hypothesis, we used a murine model of cryptococcal meningitis whereby cryptococci are introduced directly into the CNS. In experiments where mice were immunized to mount an anticryptococcal CMI response, our results indicate that immunization induced protective mechanisms that could be detected in the CNS by inhibition of the growth of viable yeast cells. Flow cytometric analyses of leukocytes in brain and spinal cord homogenates revealed that T lymphocytes, macrophages, and neutrophils accumulated in C. neoformans-infected brains of immune mice. In vivo depletion of CD4(+) T cells, but not CD8(+) T cells, resulted in significantly reduced leukocyte accumulation in the brains of immune mice. Furthermore, depletion of CD4(+) T cells or neutralization of IFN-gamma exacerbated CNS infection in immune mice, suggesting a critical role for CMI mechanisms in acquired protection in the CNS.

Ig Heavy Chain Complex-Linked Genes Influence the Immune Response in a Murine Cryptococcal Infection

A murine pulmonary infection with Cryptococcus neoformans (Cne) has been used to determine mechanisms regulating effective T cell-mediated immunity in the lungs. In BALB/c and C.B-17 mice, following intratracheal deposition of Cne, the fungus initially grows rapidly and is then progressively cleared from the lungs. Cne clearance in C.B-17 mice requires CD4 and CD8 T cells, IFN-γ, and NO. Clearance in congenic BALB/c mice proceeds more slowly than in C.B-17 mice, even though the only genetic difference between these strains is at the Ig H chain-containing region of chromosome 12. Examination of the pulmonary immune response in the two strains revealed that both cleared lung Cne by T cell-dependent mechanisms and generated equivalent levels of NO. Furthermore, both strains recruited equal numbers of macrophages, lymphocytes, and neutrophils to the lungs, although BALB/c mice recruited higher numbers of eosinophils. Notably, leukocytes isolated from BALB/c lungs during infection secreted lower levels of IFN-γ and higher levels of the Th2 cytokines IL-4 and IL-5 as compared with lung leukocytes from C.B-17 mice. Furthermore, serum levels of IgM, IgG1, IgG2a, and IgG3 anti-Cne Abs generated during infection were significantly greater in BALB/c mice than C.B-17 mice. These data suggest that although both BALB/c and C.B-17 mice clear pulmonary cryptococcosis through T cell-mediated mechanisms, Ig H chain-linked genes in BALB/c mice are associated with a decreased effectiveness of the host response, which we suggest might influence the balance in Th1/Th2 T cell subset development or increase anti-Cne Abs, or both.

Human defenses against Cryptococcus neoformans: an update

Cryptococcus neoformans var. neoformans is an opportunistic fungal pathogen, especially in AIDS patients, and is found world-wide. On the other hand, Cryptococcus neoformans var. gatti (CN-g) is restricted to an association with two species of Eucalyptus trees. Alveolar macrophages (AM) constitute the first line of defense to Cryptococcus neoformans and offers some resistance. The inflammatory response to Cryptococcus neoformans with an influx of neutrophils and monocytes affords a second line of defense. Secretion of proinflammatory monokines by human AM is now being defined. The inflammatory phagocytes are efficient in killing Cryptococcus neoformans and offer strong resistance. T and B cell responses to infection, a third line of defense, results in production of lymphokines (IFNg, etc.) and specific antibodies. Enhancement of lymphocyte responses by IL-12 and IL-18 to Cryptococcus neoformans infection appears to be critical. Susceptibility of AIDS patients to Cryptococcus neoformans is associated with low CD4+ T cell counts and likely reduced efficacy of the second line of defense.

Role of the C-C Chemokine, TCA3, in the Protective Anticryptococcal Cell-Mediated Immune Response

Activated T lymphocytes play a crucial role in orchestrating cellular infiltration during a cell-mediated immune (CMI) reaction. TCA3, a C-C chemokine, is produced by Ag-activated T cells and is chemotactic for neutrophils and macrophages, two cell types in a murine CMI reaction. Using a gelatin sponge model for delayed-type hypersensitivity (DTH), we show that TCA3 is a component of the expression phase of an anticryptococcal CMI response in mice. TCA3 mRNA levels are augmented in anticryptococcal DTH reactions at the same time peak influxes of neutrophils and lymphocytes are observed. Neutralization of TCA3 in immunized mice results in reduced numbers of neutrophils and lymphocytes at DTH reaction sites. However, when rTCA3 is injected into sponges in naive mice, only neutrophils are attracted into the sponges, indicating TCA3 is chemotactic for neutrophils, but not lymphocytes. We show that TCA3 is indirectly attracting lymphocytes into DTH-reactive sponges by affecting at least one other chemokine that is chemotactic for lymphocytes. Of the two lymphocyte-attracting chemokines assessed, monocyte-chemotactic protein-1 and macrophage-inflammatory protein-1α (MIP-1α), only MIP-1α was reduced when TCA3 was neutralized, indicating that TCA3 affects the levels of MIP-1α, which attracts lymphocytes into the sponges. TCA3 also plays a role in protection against Cryptococcus neoformans in the lungs and brains of infected mice, as evidenced by the fact that neutralization of TCA3 results in increased C. neoformans CFU in those two organs.

Urokinase is required for T lymphocyte proliferation and activation in vitro

We have previously demonstrated that urokinase-deficient (uPA-/-) mice do not increase lung T lymphocyte number and fail to mount protective immune responses during pulmonary Cryptococcus neoformans infection. These observations suggest a previously unconsidered role for urokinase-type plasminogen activator (uPA) in T lymphocyte-mediated immune responses. Accordingly, we sought to determine whether uPA is required for T cell receptor-mediated (TCR-mediated) lymphocyte proliferation and activation. Splenocytes from uPA-/- and uPA+/+ mice were stimulated with concanavalin A (Con A). The uPA-/- mice had diminished T cell proliferation as compared with uPA+/+ mice. Coculturing uPA-/- T cells with uPA+/+ accessory cells led to the restoration of proliferation. Similarly, T cell proliferation induced by CD3 cross-linking was diminished in uPA-/- mice as compared with uPA+/+ mice. T lymphocyte activation, defined as the induced expression of antigens and the elaboration of cytokines, was determined. The expression of CD69 and that of CD49d were diminished in response to Con A stimulation in uPA-/- mice as compared with uPA+/+ mice. The elaboration of cytokines in response to Con A was also altered in the uPA-/- mice. The production of the Th1 cytokines interferon-gamma and interleukin-12 was diminished in uPA-/- mice as compared with uPA+/+ mice. The uPA-/- mice produced increased amounts of interleukin-10, a Th2 cytokine. We conclude that the lack of uPA results in impaired T cell activation and proliferation in response to TCR-mediated signaling and the expression of a less Th1-polarized profile of cytokines. These findings suggest that the inability of uPA-/- mice to combat Cryptococcus neoformans infection may be caused by the impairment of T lymphocyte immune responses in the absence of uPA.

Urokinase-type plasminogen activator in inflammatory cell recruitment and host defense against Pneumocystis carinii in mice

Effective host defense against Pneumocystis carinii depends upon the integrated actions of inflammatory cells and mediators in the lungs. Using immunocompetent and immunosuppressed mice, our laboratory has defined inflammatory changes in the lungs in response to P. carinii. However, the essential molecules and mechanisms required for cellular recruitment and for host defense against P. carinii are undefined. We hypothesized that urokinase-type plasminogen activator (uPA), a protease intimately involved in inflammatory cell migration and activation, is required for clearance of P. carinii. To test this hypothesis in vivo, we compared the intensity of P. carinii infection and inflammation in the lungs of mice lacking the uPA gene (uPA knockout mice) and in the lungs of wild-type mice. After intratracheal inoculation with P. carinii organisms, uPA knockout mice developed uniformly heavy P. carinii pneumonia while wild-type mice cleared the P. carinii inoculum. Bronchoalveolar lavage fluid from uPA knockout mice contained significantly smaller numbers of cells than did lavage fluid from wild-type mice. We conclude that deletion of the uPA gene prevents the clearance of P. carinii and reduces inflammatory cell recruitment. Therefore, uPA is an important participant in the network of inflammatory events required for the clearance of P. carinii, confirming an important role for this molecule in pulmonary host defense against opportunistic pathogens.

Decreased resistance to primary intravenous Cryptococcus neoformans infection in aged mice despite adequate resistance to intravenous rechallenge

It is often stated that impaired immune functions in the aged underlie their greater susceptibility to infections. Indeed, in many experimental settings, T-cell responses in aged mice have been shown to be deficient compared with those from young adults. Nonetheless, there are very few examples where a greater susceptibility to infection in aged mice has been demonstrated to result from impaired T-cell function. The clinical importance of understanding the basis for increased susceptibility to infection that accompanies advanced age dictates a need for experimental models with which to study the effect that aging has on immunological resistance to infection. This study was undertaken to investigate whether aged mice were less resistant than young adult control mice to infection with the fungus Cryptococcus neoformans. After a primary intravenous challenge with yeast, aged mice died sooner and developed higher organ burdens of yeast than did young adults. Deficient in vitro responses were observed in T cells from aged mice; however, greater susceptibility to intravenous infection appeared not to result from less effective T-cell-dependent resistance in vivo. In fact, T-cell-replete aged mice were more susceptible to intravenous cryptococcal infection than were T-cell-depleted young adults. Furthermore, aged mice were as resistant to primary pulmonary challenge with Cryptococcus as were young adults. Similarly, vaccinated aged mice were as resistant to rechallenge as were young adult counterparts. Therefore, despite demonstrably deficient in vitro responses of T cells from aged mice, their T-cell-dependent resistance to C. neoformans is as effective as that of young adults.

IL-5 Is Required for Eosinophil Recruitment, Crystal Deposition, and Mononuclear Cell Recruitment During a Pulmonary Cryptococcus neoformans Infection in Genetically Susceptible Mice (C57BL/6)

CBA/J (highly resistant), BALB/c (moderately resistant), and C57BL/6 (susceptible) mice displayed three resistance patterns following intratracheal inoculation of Cryptococcus neoformans 52. The inability to clear the infection correlated with the duration of the eosinophil infiltrate in the lungs. The role of IL-5 in promoting the pulmonary eosinophilia and subsequent inflammatory damage in susceptible C57BL/6 mice was investigated. C57BL/6 mice developed a chronic alveolar, peribronchiolar, and perivascular eosinophilia following C. neoformans infection. This resulted in the accumulation of intracellular Charcot-Leyden-like crystals in alveolar macrophages by wk 4 and the extracellular deposition of these crystals in the bronchioles with associated destruction of airway epithelium by wk 6. IL-5 mRNA was expressed in the lungs, and injections of anti-IL-5 mAb prevented eosinophil recruitment and crystal deposition but did not alter cryptococcal clearance. Depletion of CD4+ T cells (but not CD8+) ablated IL-5 production by lung leukocytes in vitro and eosinophil recruitment in vivo. Neutralization of IL-5 also inhibited the recruitment of macrophages, CD8+ T lymphocytes, and B lymphocytes by 47 to 57%. Anti-IL-5 mAb inhibited CD4+ T lymphocyte recruitment by 30% but did not affect neutrophil recruitment. Thus, the development of a chronic eosinophil infiltrate in the lungs of C. neoformans-infected C57BL/6 mice is a nonprotective immune response that causes significant lung pathology. Furthermore, IL-5 promotes the recruitment and activation of eosinophils, resulting in the recruitment of additional macrophages and lymphocytes into the lungs.

Microevolution of a standard strain of Cryptococcus neoformans resulting in differences in virulence and other phenotypes

Cryptococcus neoformans is a major fungal pathogen for patients with debilitated immune systems. However, no information is available on the stability of virulence or of phenotypes associated with virulence for C. neoformans laboratory strains. A serendipitous observation in our laboratory that one isolate of C. neoformans ATCC 24067 (strain 52D) became attenuated after continuous in vitro culture prompted us to perform a comparative study of nine strain 24067 isolates obtained from six different research laboratories. Each isolate was characterized by DNA typing, virulence for mice, proteinase production, extracellular protein synthesis, melanin synthesis, carbon assimilation pattern, antifungal drug susceptibility, colony morphology, growth rate, agglutination titers, phagocytosis by murine macrophages, capsule size, and capsular polysaccharide structure. All isolates had similar DNA typing patterns consistent with their assignment to the same strain, although minor chromosome size polymorphisms were observed in the electrophoretic karyotypes of two isolates. Several isolates had major differences in phenotypes that may be associated with virulence, including growth rate, capsule size, proteinase production, and melanization. These findings imply that C. neoformans is able to undergo rapid changes in vitro, probably as a result of adaptation to laboratory conditions, and suggest the need for careful attention to storage and maintenance conditions. In summary, our results indicate that C. neoformans (i) can become attenuated by in vitro culture and (ii) is capable of microevolution in vitro with the emergence of variants exhibiting new genotypic and phenotypic characteristics.

IL-12 and IFN-gamma are required for initiating the protective Th1 response to pulmonary cryptococcosis in resistant C.B-17 mice

A murine model was used to assess the role of cytokines in initiating protective T-cell-mediated immunity in the lung. A pulmonary infection was initiated by intratracheal inoculation of Cryptococcus neoformans (Cne). Previously, we had established that Cne lung clearance was mouse-strain-specific: C.B-17 mice were resistant and developed a Th1-like response, whereas C57BL/6 mice were susceptible and did not develop a Th1 response. In the present study we showed that monoclonal anti-interferon-gamma (IFN-gamma) and anti-interleukin-12 (IL-12) antibody administration prior to infection of resistant C.B-17 mice inhibited lung clearance of Cne. Cytokine profiles of lung and lung-associated lymph nodes (LALN) from monoclonal antibody (mAb)-treated C.B-17 mice were switched from Th1-like to Th2-like, and mAb-treated C.B-17 mice exhibited lung eosinophilia, which was absent in control C.B-17 mice. Additionally, C.B-17 mice treated with anti-IFN-gamma and anti-IL-12 mAb demonstrated a significantly lower percentage of lung macrophages expressing inducible nitric oxide synthase (iNOS) than did control mice. These studies clearly demonstrate that both IFN-gamma and IL-12 are required for initiation of a Th1 response in resistant C.B-17 mice.

Cryptococcus neoformans infection in mice previously infected with LP-BM5 MuLV, the agent of murine AIDS (MAIDS)

We studied susceptibility to experimental systemic cryptococcosis in mice previously infected with the retroviral complex LP-BM5 (responsible for murine AIDS). LP-BM5 was inoculated to C57B1/6 mice by intravenous (i.v.) injection 8 weeks before an i.v. challenge with 4 x 10(3) CFU of Cryptococcus neoformans. Uninfected and singly infected mice were used as controls. LP-BM5 infection did not result in a significant increase in fungal burdens in the lungs or brains of co-infected animals compared to mice infected with C. neoformans alone. However, mortality was enhanced in the co-infected animals. The kinetics of splenocyte subsets differed in co-infected mice and LP-BM5-infected mice; the increase in CD4+, CD8+ and Ly5+ cells was only moderate in the former. Cytokine production by concanavalin A (Con A)-stimulated splenocytes from co-infected mice showed a marked decrease in the Th1 response (IFN-gamma, IL-2) and an increase in the Th2 response (IL-4, IL-10). Furthermore, cryptococcosis altered the course of MAIDS, inhibiting splenomegaly. This effect was not related to a decrease in ecotropic virus titres in the spleen or to improved in vitro responsiveness of spleen cells to Con A. The marked decrease in IFN-gamma production in co-infected animals could partly explain the inhibition of LP-BM5-induced splenomegaly. This model of murine retroviral infection does not seem to be suitable for studying cryptococcosis in immunosuppressed animals, but remains valuable for investigating in vivo interactions between two pathogens.

Different susceptibility of three clinically isolated strains of Cryptococcus neoformans to the fungicidal effects of reactive nitrogen and oxygen intermediates: possible relationships with virulence

We investigated the susceptibility of three clinically isolated strains of Cryptococcus neoformans with different virulences to reactive nitrogen and oxygen intermediates (RNI and ROI, respectively), representing two important mediators of macrophage microbicidal activity. All mice infected with the highly virulent strain of C. neoformans, YC-11, died within 3 to 6 weeks because of rapid multiplication of the organism in the lungs and dissemination to the brain. In contrast, a weakly virulent strain, YC-13, was almost completely eradicated from the lungs and did not disseminate to the brain, leading to survival of all infected animals during the period of observation (15 weeks). The virulence of the third strain, YC-5, was intermediate between the other two strains. To examine the susceptibility of C. neoformans to the fungicidal effect of nitric oxide (NO) and superoxide anions (O2-), the organisms were exposed to these oxidants, which were chemically generated in a cell-free system. Interestingly, the number of live YC-13 yeast cells was markedly reduced after exposure to NO and O2-. In contrast, YC-11 was almost completely resistant to the killing effect of these oxidants. YC-5 showed an intermediate susceptibility. Our results demonstrate that the resistance of C. neoformans to the fungicidal effects of RNI and ROI is related to virulence, and suggest that the resistance to nitrogen- and oxygen-derived oxidants may be one of the factors to determine the outcome of infection with C. neoformans.

Macrophages in resistance to candidiasis

Candida albicans, an increasingly common opportunistic pathogenic fungus, frequently causes disease in immunodeficient but not immunocompetent hosts. Clarifying the role of the phagocytic cells that participate in resistance to candidiasis not only is basic to understanding how the host copes with this dimorphic pathogen but also will expedite the development of innovative prophylactic and therapeutic approaches for treating the multiple clinical presentations that candidiasis encompasses. In this review, we present evidence that a diverse population of mononuclear phagocytes, in different states of activation and differentiation and from a variety of host species, can phagocytize C. albicans blastoconidia via an array of opsonic and nonopsonic mechanisms and can kill C. albicans blastoconidia and hyphae by means of oxygen-dependent and -independent mechanisms. Reactive nitrogen intermediates should now be added to the well-established candidacidal reactive oxygen intermediates of macrophages. Furthermore, what were thought to be two independent pathways, i.e., nitric oxide and superoxide anion, have now been shown to combine to form a potent macrophage candidacidal molecule, peroxynitrite. In contrast to monocytes and neutrophils, which are important in resistance to early stages of C. albicans infections, more differentiated macrophages activated by cytokines such as gamma interferon participate in the acquired resistance of hosts with C. albicans-specific, cell-mediated immunity. Evidence presented in this review demonstrates that mononuclear phagocytes, in some instances in the absence of other professional phagocytes such as neutrophils, play an import role in resistance to systemic and mucosal candidiasis.

A 77-kilodalton protein of Cryptococcus neoformans, a member of the heat shock protein 70 family, is a major antigen detected in the sera of mice with pulmonary cryptococcosis

Heat shock proteins (HSPs) from several pathogenic microbes have been shown to be target molecules of humoral responses as well as cellular immune responses. However, little is known about target molecules in pulmonary cryptococcosis. Western blotting analysis revealed that experimentally induced pulmonary cryptococcosis in (BALB/c x DBA/2)F1 mice was associated with the appearance of serum antibodies to a 77-kDa protein derived from Cryptococcus neoformans as well as to 18-, 22-, 25-, 36-, and 94-kDa proteins. Since the 77-kDa band also reacted with rabbit polyclonal antibodies against 70-kDa HSP (HSP70) family members, the protein was predicted to be a member of the HSP70 family. We also purified HSP70 directly from a C. neoformans cell extract by Mono Q fast protein liquid chromatography and ATP-agarose affinity column chromatography and showed that it was positive in immunoblot analysis using either serum from C. neoformans-infected mice or rabbit anti-HSP70 antibodies. N-terminal amino acid sequencing of this purified protein confirmed that the 77-kDa protein was a member of the HSP70 protein family. A 66-kDa protein, which coincidentally purified with the HSP70 protein and was identified as a member of the HSP60 family by N-terminal amino acid sequencing, was not reactive with sera from C. neoformans-infected mice. Thus, a protein associated with the HSP70 family and derived from C. neoformans was a major target molecule of the humoral response in murine pulmonary cryptococcosis.

T cells cooperate with passive antibody to modify Cryptococcus neoformans infection in mice

Cryptococcus neoformans is an encapsulated fungus that is a major cause of meningitis in patients with AIDS. In immunocompetent mice, administration of IgG1 mAb protects against cryptococcal infection, whereas administration of IgG3 is not protective and can accelerate the infection. In beige mice with impaired natural killer cell function, the effects of IgG1 and IgG3 are similar to those observed in immunocompetent mice, suggesting that natural killer cells are not crucial for antibody-mediated modulation of cryptococcal infection. In mice lacking CD4+ T cells, IgG1 is not protective and IgG3 accelerates infection, indicating that CD4+ T cells are required for antibody-mediated protection. In mice lacking CD8+ T cells, both IgG1 and IgG3 antibodies prolong survival, indicating that acceleration of the disease process by IgG3 involves CD8+ T cells. Both IgG1-mediated protection and IgG3-mediated acceleration of infection require interferon gamma. These results reveal a functional dependence of passively administered antibody on cellular immunity in cryptococcal infection in mice and have implications for antibody-based therapies in humans in the setting of CD4+ lymphopenia.

Cell-mediated immunity in host resistance against infection caused by Penicillium marneffei

Penicillium marneffei is one of the most important opportunistic infectious pathogens in AIDS patients in Thailand and Southeast Asia. However, very little is known about the host defence mechanisms against P. marneffei infection. In the present study, we established the first experimental murine model of chronic pulmonary and disseminated infection using P. marneffei, and examined the immunological response to such infection in euthymic and athymic mice. In this model, micro-organisms inoculated intratracheally multiplied progressively in the lungs and disseminated to the liver and spleen. However, the number of organisms decreased gradually in these organs. In contrast, congenitally athymic mice developed severe pulmonary and disseminated systemic mycosis. Pulmonary penicilliosis marneffei was associated with a marked cellular inflammatory response as evident by histological abnormalities and increased intraparenchymal leucocyte count. To confirm the importance of cell-mediated immunity in host resistance to P. marneffei infection, we transferred nylon wool non-adherent spleen cells into the athymic mice. Such treatment significantly reduced the number of yeasts in the organs of athymic mice. Taken together, our results demonstrate that the cell-mediated immunity play a central role in a host defence mechanism against infection with P. marneffei, and suggest that our new model may be a useful approach for studying the pathogenesis of this fungal disease.

Experimental systemic cryptococcosis in SCID mice

The susceptibility of severe combined immune deficiency (SCID) mice to experimental systemic cryptococcosis was studied. SCID mice were at least 16-fold more susceptible to lethal infection than were immunocompetent control mice (P < 0.001). Histological assessment showed that control mice responded with a granulomatous inflammatory response, whereas SCID mice showed no evidence of a cellular host response. In a therapeutic study, 5 mg kg-1 day-1 of fluconazole caused a significant reduction in the infectious burden in the spleen, liver, kidney and lungs of treated mice compared with no treatment. These results indicate that a model of systemic cryptococcosis with meningitis in SCID mice may have utility in the study of immunology and therapy of this disease in the severely immunocompromised host.

Urokinase is required for the pulmonary inflammatory response to Cryptococcus neoformans. A murine transgenic model

Urokinase (uPA) is hypothesized to provide proteolytic activity enabling inflammatory cells to traverse tissues during recruitment, and it is implicated as a cytokine modulator. Definitive evaluation of these hypotheses in vivo has previously been impossible because uPA could not completely and irreversibly be eliminated. This limitation has been overcome through the development of uPA-deficient transgenic mice (uPA-/-). Using these mice, we evaluated the importance of uPA in the pulmonary inflammatory response to Cryptococcus neoformans (strain 52D). C. neoformans was inoculated into uPA-/- and control mice (uPA+/+), and cell recruitment to the lungs was quantitated. The number of CFU in lung, spleen and brain was determined to assess clearance, and survival curves were generated. By day 21 after inoculation, uPA-/- mice had markedly fewer pulmonary inflammatory (CD45+), CD4+, and CD11b/CD18+ cells compared with uPA+/+ controls (P<0.0007); pulmonary CFUs in the uPA-/- mice continued to increase, whereas CFUs diminished in uPA+/+ mice(P<0.005). In survival studies, only 3/19 uPA+/+ mice died, whereas 15/19 uPA-/- mice died (p<0.001). We have demonstrated that uPA is required for a pulmonary inflammatory response to C. neoformans. Lack of uPA results in inadequate cellular recruitment, uncontrolled infection, and death.

Role of tumor necrosis factor and gamma interferon in acquired resistance to Cryptococcus neoformans in the central nervous system of mice

Although naive C.B-17 and BALB/cBy mice die of meningoencephalitis within 5 weeks of intravenous infection with an opportunistic strain of Cryptococcus neoformans, immunized mice express an acquired, CD4+ T-cell-dependent immunity and survive an intravenous infection. Infusion of lymphocytes from immune mice into severe combined immunodeficiency (SCID) mice renders these mice more resistant to cryptococcal brain infection than uninfused controls. We have investigated the role of gamma interferon (IFN-gamma) and tumor necrosis factor (TNF) in acquired resistance to C. neoformans. Neutralization of either IFN-gamma or TNF impaired resistance of immune BALB/cBy or C.B-17 mice to cryptococci. At 10 days postinfection, there were approximately 10 times as many yeast cells in the brains of mice treated with either anticytokine antibody as in the brains of mice treated with control antibody. Simultaneous neutralization of IFN-gamma and TNF further exacerbated infection. Neutralization of IFN-gamma or TNF also impaired resistance in immune lymphocyte-infused SCID mice, resulting in significantly higher yeast burdens in brains of cytokine-neutralized mice than in brains of controls. Concurrent neutralization of IFN-gamma and TNF rendered SCID recipients of immune cells equivalent to uninfused SCID mice with respect both to brain yeast burdens at 10 days and to survival. Anti-TNF treatment alone also curtailed survival. Histological examination of the brains of cytokine-neutralized mice revealed deficiencies in ability to focus inflammatory cells at brain lesions. These data demonstrate that both IFN-gamma and TNF are important mediators of acquired resistance to cryptococcal meningoencephalitis.

Effects of immunization with Cryptococcus neoformans cells or cryptococcal culture filtrate antigen on direct anticryptococcal activities of murine T lymphocytes

Immunizing CBA/J mice with intact Cryptococcus neoformans cells or with a cryptococcal culture filtrate antigen (CneF) induces an anticryptococcal delayed-type hypersensitivity response. Recently, it has been shown that two phenotypically different T-cell populations are responsible for delayed-type hypersensitivity reactivity in mice immunized with intact cryptococcal cells, whereas only one of those populations is present in mice immunized with soluble cryptococcal antigens in complete Freund's adjuvant (CFA). The purpose of this study was to determine if differences occur with regard to direct anticryptococcal activity between T-lymphocyte-enriched populations from mice immunized with intact viable or dead cryptococcal cells and similar cell populations from mice immunized with the soluble cryptococcal culture filtrate antigen, CneF, emulsified in CFA. The percentage of lymphocytes which form conjugates with C. neoformans and the percentage of cryptococcal growth inhibition in vitro are greater with T-lymphocyte-enriched populations from mice sublethally infected with C. neoformans or from mice immunized with intact heat-killed cryptococcal cells in the presence or absence of CFA than with lymphocyte populations from mice immunized with CneF-CFA. Enhanced anticryptococcal activity of T lymphocytes could be induced by immunizing mice with heat-killed C. neoformans cells of serotype A, B, C, or D as well as by immunizing with a similar preparation of an acapsular C. neoformans mutant but not by immunizing with CFA emulsified with CneF prepared from any one of the C. neoformans isolates. These data indicate that the soluble cryptococcal culture filtrate antigens do not induce the same array of functional T lymphocytes as whole cryptococcal cells.

T cell-dependent activation of macrophages and enhancement of their phagocytic activity in the lungs of mice inoculated with heat-killed Cryptococcus neoformans: involvement of IFN-gamma and its protective effect against cryptococcal infection

Previous investigations have demonstrated that macrophages play a critical role in the first-line cellular defense mechanism against infection with Cryptococcus neoformans. In the present study, to elucidate the way in which anticryptococcal activity of macrophages is regulated at the site of infection, pulmonary intraparenchymal macrophages were directly analyzed for expression of their surface molecules and their phagocytic activities against the organism, and the effects of depletion of T cells and endogenous IFN-gamma in vivo on these parameters were examined. In the lungs of mice intratracheally inoculated with heat-killed C. neoformans, macrophages were activated, as indicated by augmented expression of MHC class II, intercellular adhesion molecule-1 (ICAM-1) and Fc receptor (FcR), and about two-thirds of macrophages were found to have ingested an average of 3.77 +/- 0.12 yeast cells per macrophage. In mice depleted of both CD4+ and CD8+ T cells by injecting the specific monoclonal antibodies (mAbs) or anti-IFN-gamma mAb, not only augmentation of the expression of macrophage activation markers but also phagocytosis of C. neoformans was significantly reduced. These results suggest that anticryptococcal activity of macrophages is regulated by IFN-gamma endogenously produced by T cells. Additionally, treatment with IFN-gamma were shown to significantly prolong the survival time of mice infected with viable C. neoformans. Additionally, preimmunization with heat-killed C. neoformans significantly prolonged the survival time of mice which received the following infection.

The regulation of pulmonary immunity

No evidence has emerged which suggests that the principles of immunity derived from studies on cells from other body sites are contradicted in the lung and its associated lymphoid tissue. What is clear, however, is that the environment dictates the types of cells, their relationship to one another, and what perturbing events will set in motion either the development of an "active" immune response or tolerance. Investigating mechanisms for the development of lung immunity has increased our understanding of how human diseases develop and is continuing to suggest new ways to manipulate pulmonary immune responses. Demonstration that lung cells regulate both nonspecific inflammation and immunity through the expression of adhesion molecules and the secretion of cytokines offers hope for ways to design more effective vaccines, enhance microbial clearance in immunosuppressed hosts, and to suppress manifestations of immunologically mediated lung disease. Important lung diseases targeted for intensive research efforts in the immediate future are tuberculosis, asthma, and fibrotic lung disease. Perhaps even the common cold might be conquered. Considering the pace of current research on lung immunity, it may not be too ambitious to predict that these diseases may be conquered in the next decade.

Pathogenesis of pulmonary Cryptococcus neoformans infection in the rat

The pathogenesis of Cryptococcus neoformans pulmonary infection in the rat was studied after intratracheal inoculation. Lungs were examined at various times following infection for histopathology in conjunction with macrophage markers, proliferating cell nuclear antigen (PCNA), and capsular glucuronoxylomannan (GXM) antigen. Serum GXM, immunoglobulin M (IgM) and IgG titers and organ fungal burden were compared with pathological findings. C. neoformans organisms were in the lung parenchyma 2 h postinoculation, and GXM antigen was present in surrounding tissues shortly thereafter. Extrapulmonary dissemination occurred early in infection. Two phases of host cellular inflammatory response were discernible: early local macrophage recruitment at 2 to 4 days followed by granulomatous inflammation, which reached maximum intensity 14 days after infection. The granulomatous phase was preceded by lymphocyte influx with macrophage proliferation and maturation into epithelioid histiocytes; this was paralleled by a shift of yeasts from extracellular to intracellular spaces. Tissue IgG deposits, serum IgG to GXM, and localization of tissue GXM immunoreactivity to epithelioid cells were noted at 2 to 4 weeks. A 10-fold decrease in lung fungal burden occurred 25 days postinfection and was associated with resolving granulomas, fewer proliferating cells, and decreased tissue GXM. The present study demonstrates that (i) C. neoformans penetrates the lung parenchyma shortly after infection; (ii) immunocompetent rats control pulmonary cryptococcosis efficiently, with minimal extrapulmonary dissemination and low levels of serum GXM; and (iii) macrophage activation is likely to play a crucial role in limiting C. neoformans infection in the rat lung.

Anticryptococcal resistance in the mouse brain: beneficial effects of local administration of heat-inactivated yeast cells

Using a murine model, we have previously shown that brain resistance to local infection with opportunistic fungi is affected by manipulation of the host myelomonocytic compartment. Here, we demonstrate that intracerebral administration of heat-inactivated Cryptococcus neoformans (H-CN) yeast cells results in a consistent enhancement of mouse survival to subsequent local challenge with lethal doses of C. neoformans. The phenomenon, more pronounced upon double H-CN treatment, is associated with (i) massive local inflammatory response, (ii) reduced growth of the fungus within the brain, and (iii) induction of delayed-type hypersensitivity. Moreover, H-CN treatment confers protection against local heterologous challenges. Our data provide initial evidence that intracerebral administration of H-CN results in the establishment of aspecific and specific immune responses; the mechanisms of elicitation and relative contributions to host antimicrobial resistance remains to be elucidated.

Regulation of cytokine production during the expression phase of the anticryptococcal delayed-type hypersensitivity response

Effects of both positive and negative regulatory T cells on cellular infiltration and cytokine production during the expression phase of the anticryptococcal immune response were examined. Tamp cells, which are induced by cryptococcal antigen, significantly amplify the anticryptococcal delayed-type hypersensitivity response, whereas a cascade of T suppressor (Ts) cells inhibits the response and decreases the clearance of Cryptococcus neoformans during an infection. By using the gelatin sponge implantation model, we found that Tamp cells do not stimulate a significant increase in cellular infiltration into the sponges in response to cryptococcal antigen compared with that into delayed-type hypersensitivity-reactive sponges in immune control mice. However, Tamp cells do stimulate significant increases in the production of gamma interferon and interleukin-2 (IL-2) in the antigen-injected sponges over the level of the representative cytokine in antigen-injected sponges from the immune control mice. Likewise, Ts1 cells, induced with cryptococcal antigen, do not significantly affect antigen-induced cellular infiltration into sponges in immune mice. In contrast, decreased levels of gamma interferon and IL-2 are observed in antigen-injected sponges from Ts1-cell-recipient, immunized mice compared with those of the positive immune controls. The presence of either Tamp or Ts1 cells in immunized mice stimulates increased production of IL-5 but not IL-4 over that of the positive immune controls.

Polysaccharide antigens of the capsule of Cryptococcus neoformans

The major significance of the capsular polysaccharide of C. neoformans is its role in potentiating opportunistic infections by the yeast. It has the ability to exert a broad spectrum of influences on the immune response, from activation of phagocytic cells and complement components of the alternative pathway, to the induction of specific antibody, T-suppressor cells, DTH responses, and cytokines (51). These biological properties along with the serotype specificities are all determined by the physical properties and chemical structures of the polysaccharide antigens that compose the capsule. There is evidence not only for an association of lethal infections with serotype A in patients with advanced AIDS (34, 56), but also for a role for the capsule in directly influencing the infection of CD4+ cells by HIV (57). Together, these phenomena raise intriguing questions about the possible connection between the chemistry of these capsular antigens and cryptococcal infections in AIDS patients. One speculation is that AIDS creates the optimal physiological conditions for the establishment and spread of cryptococcosis. It has been observed that during the progression of AIDS there is a shift towards a T-2 response (14). This could lead to conditions that would inhibit the cellular immune responses that block dissemination of cryptococcal infections. Thus, an important consideration in the application of vaccine or immune modulation therapies in the treatment of cryptococcosis in AIDS victims would be the design of vaccines that could boost the T-1 immune response. It has been shown that the form and dose of an antigenic challenge can influence the induction of a T-1 or T-2 immune response (61). Recently, Murphy has reported that gamma interferon and interleukin 2 are up-regulated in the spleens of mice that produce anticryptococcal TDH and TAMP cells in response to immunogenic doses of cryptococcal culture filtrate antigen given with Freund's complete adjuvant (49). Perhaps purified cryptococcal antigens (e.g., MP) conjugated to an appropriate carrier or adjuvant could be used in therapeutic strategies to limit cryptococcosis in immunocompromised individuals. Future investigations of virulence and pathogenicity in the context of defined polysaccharide antigens from encapsulated strains of C. neoformans will contribute to a better understanding of the regulation of cryptococcal infection and immunity at the cellular and molecular levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of macrophages and expansion of specific T lymphocytes in the lungs of mice intratracheally inoculated with Cryptococcus neoformans

A quantitative and qualitative change in inflammatory cells in the lungs of mice after intratracheal inoculation of heat-killed Cryptococcus neoformans was examined by direct analysis of the pulmonary intraparenchymal leucocytes. Macrophages and T and B lymphocytes increased, peaked at day 7, and then gradually decreased to the basal level. Macrophages were activated 7 days after the inoculation, as indicated by the enhanced expression of MHC class II, intercellular adhesion molecule-1 (ICAM-1) and Fc receptor (FcR), which have been known as their activation markers. T cells were also activated, as indicated by the induction of IL-2 receptor (IL-2R) and the enhanced expression of leucocyte function-associated molecule-1 (LFA-1) and ICAM-1, a pair of adhesion molecules which have also been regarded as T cell activation markers. CD4+ T cells preferentially accumulated in lungs, and proliferated in vitro by stimulation with heat-killed whole yeast cells, suggesting that at least some of the infiltrated T cells expand locally in response to the organisms. These results demonstrate that the activation of macrophages and T cells reactive to C. neoformans is induced in lungs after intratracheal inoculation of heat-killed organisms, and suggest that these cells interact to eliminate organisms more efficiently from the host.

Role for C5 and neutrophils in the pulmonary intravascular clearance of circulating Cryptococcus neoformans

Although C5a-induced intravascular pulmonary sequestration of neutrophils has been investigated with regards to lung injury, relatively few studies have addressed the possible role for this mechanism in the intravascular clearance of circulating microorganisms. A murine model was used in which the complement-fixing, encapsulated yeast Cryptococcus neoformans (Cne) was inoculated intravenously (IV), and lung clearance of the organism was measured 24 h later. In normal mice, clearance was remarkably effective, but in leukocyte-depleted or C5-deficient (C5-) animals, clearance was significantly decreased. In vitro assays indicated that C5 was necessary for neutrophils to kill encapsulated Cne and evidence was obtained that C5a was involved. In vivo studies using light and electron microscopy demonstrated that 30 min after an IV inoculation of encapsulated yeast into C5-sufficient (C5+) mice, neutrophils accumulated in pulmonary vessels and engulfed Cne. However, in C5- mice, neutrophils failed to accumulate in pulmonary vessels and there was no endocytosis of encapsulated yeasts. These studies suggested that following Cne interaction with complement in the blood, release of C5a activated circulating neutrophils to adhere to Cne, and perhaps to adjacent endothelium, which facilitated rapid phagocytosis and killing of the organism. In contrast to the IV infection model, when Cne was inoculated into the tracheas of C5+ and C5- mice, no evidence was obtained for an early PMN-C5-dependent clearance mechanism. C5a-dependent neutrophil killing in the lung vasculature may provide important host protection against Cne during vascular dissemination and perhaps against other disseminating microorganisms that activate complement.