Capacity of Mycobacterium avium isolates to grow well or poorly in murine macrophages resides in their ability to induce secretion of tumor necrosis factor

Subunit vaccine protects against a clinical isolate of Mycobacterium avium in wild type and immunocompromised mouse models

The nontuberculous mycobacteria (NTM) Mycobacterium avium is a clinically significant pathogen that can cause a wide range of maladies, including tuberculosis-like pulmonary disease. An immunocompromised host status, either genetically or acutely acquired, presents a large risk for progressive NTM infections. Due to this quietly emerging health threat, we evaluated the ability of a recombinant fusion protein ID91 combined with GLA-SE [glucopyranosyl lipid adjuvant, a toll like receptor 4 agonist formulated in an oil-in-water stable nano-emulsion] to confer protection in both C57BL/6 (wild type) and Beige (immunocompromised) mouse models. We optimized an aerosol challenge model using a clinical NTM isolate: M. avium 2-151 smt, observed bacterial growth kinetics, colony morphology, drug sensitivity and histopathology, characterized the influx of pulmonary immune cells, and confirmed the immunogenicity of ID91 in both mouse models. To determine prophylactic vaccine efficacy against this M. avium isolate, mice were immunized with either ID91 + GLA-SE or bacillus Calmette-Guérin (BCG). Immunocompromised Beige mice displayed a delayed influx of innate and adaptive immune cells resulting in a sustained and increased bacterial burden in the lungs and spleen compared to C57BL/6 mice. Importantly, both ID91 + GLA-SE and BCG vaccines significantly reduced pulmonary bacterial burden in both mouse strains. This work is a proof-of-concept study of subunit vaccine-induced protection against NTM.

Genetic Variation/Evolution and Differential Host Responses Resulting from In-Patient Adaptation of Mycobacterium avium

Members of the Mycobacterium avium complex (MAC) are characterized as nontuberculosis mycobacteria and are pathogenic mainly in immunocompromised individuals. MAC strains show a wide genetic variability, and there is growing evidence suggesting that genetic differences may contribute to a varied immune response that may impact the infection outcome.

Members of the Mycobacterium avium complex (MAC) are characterized as nontuberculosis mycobacteria and are pathogenic mainly in immunocompromised individuals. MAC strains show a wide genetic variability, and there is growing evidence suggesting that genetic differences may contribute to a varied immune response that may impact the infection outcome. The current study aimed to characterize the genomic changes within M.avium isolates collected from single patients over time and test the host immune responses to these clinical isolates. Pulsed-field gel electrophoresis and whole-genome sequencing were performed on 40 MAC isolates isolated from 15 patients at the Department of Medical Microbiology at St. Olavs Hospital in Trondheim, Norway. Isolates from patients (patients 4, 9, and 13) for whom more than two isolates were available were selected for further analysis. These isolates exhibited extensive sequence variation in the form of single-nucleotide polymorphisms (SNPs), suggesting that M. avium accumulates mutations at higher rates during persistent infections than other mycobacteria. Infection of murine macrophages and mice with sequential isolates from patients showed a tendency toward increased persistence and the downregulation of inflammatory cytokines by host-adapted M. avium strains. The study revealed the rapid genetic evolution of M. avium in chronically infected patients, accompanied by changes in the virulence properties of the sequential mycobacterial isolates.

Host response to nontuberculous mycobacterial infections of current clinical importance

The nontuberculous mycobacteria are a large group of acid-fast bacteria that are very widely distributed in the environment. While Mycobacterium avium was once regarded as innocuous, its high frequency as a cause of disseminated disease in HIV-positive individuals illustrated its potential as a pathogen. Much more recently, there is growing evidence that the incidence of M. avium and related nontuberculous species is increasing in immunocompetent individuals. The same has been observed for M. abscessus infections, which are very difficult to treat; accordingly, this review focuses primarily on these two important pathogens. Like the host response to M. tuberculosis infections, the host response to these infections is of the TH1 type but there are some subtle and as-yet-unexplained differences.

Effects of age and macrophage lineage on intracellular survival and cytokine induction after infection with Rhodococcus equi

Rhodococcus equi, a facultative intracellular pathogen of macrophages, causes life-threatening pneumonia in foals and in people with underlying immune deficiencies. As a basis for this study, we hypothesized that macrophage lineage and age would affect intracellular survival of R. equi and cytokine induction after infection. Monocyte-derived and bronchoalveolar macrophages from 10 adult horses and from 10 foals (sampled at 1-3 days, 2 weeks, 1 month, 3 months, and 5 months of age) were infected ex vivo with virulent R. equi. Intracellular R. equi were quantified and mRNA expression of IL-1β, IL-4, IL-6, IL-8, IL-10, IL-12 p40, IL-18, IFN-γ, and TNF-α was measured. Intracellular replication of R. equi was significantly (P<0.001) greater in bronchoalveolar than in monocyte-derived macrophages, regardless of age. Regardless of the macrophage lineage, replication of R. equi was significantly (P=0.002) higher in 3-month-old foals than in 3-day old foals, 2-week-old foals, 1-month-old foals, and adult horses. Expression of IL-4 mRNA was significantly higher in monocyte-derived macrophages whereas expression of IL-6, IL-18, and TNF-α was significantly higher in bronchoalveolar macrophages. Induction of IL-1β, IL-10, IL-12 p40, and IL-8 mRNA in bronchoalveolar macrophages of 1-3-day old foals was significantly higher than in older foals or adult horses. Preferential intracellular survival of R. equi in bronchoalveolar macrophages of juvenile horses may play a role in the pulmonary tropism of the pathogen and in the window of age susceptibility to infection.

Virulence and immune response induced by Mycobacterium avium complex strains in a model of progressive pulmonary tuberculosis and subcutaneous infection in BALB/c mice

The genus Mycobacterium comprises more than 150 species, including important pathogens for humans which cause major public health problems. The vast majority of efforts to understand the genus have been addressed in studies with Mycobacterium tuberculosis. The biological differentiation between M. tuberculosis and nontuberculous mycobacteria (NTM) is important because there are distinctions in the sources of infection, treatments, and the course of disease. Likewise, the importance of studying NTM is not only due to its clinical significance but also due to the mechanisms by which some species are pathogenic while others are not. Mycobacterium avium complex (MAC) is the most important group of NTM opportunistic pathogens, since it is the second largest medical complex in the genus after the M. tuberculosis complex. Here, we evaluated the virulence and immune response of M. avium subsp. avium and Mycobacterium colombiense, using experimental models of progressive pulmonary tuberculosis and subcutaneous infection in BALB/c mice. Mice infected intratracheally with a high dose of MAC strains showed high expression of tumor necrosis factor alpha (TNF-α) and inducible nitric oxide synthase with rapid bacillus elimination and numerous granulomas, but without lung consolidation during late infection in coexistence with high expression of anti-inflammatory cytokines. In contrast, subcutaneous infection showed high production of the proinflammatory cytokines TNF-α and gamma interferon with relatively low production of anti-inflammatory cytokines such as interleukin-10 (IL-10) or IL-4, which efficiently eliminate the bacilli but maintain extensive inflammation and fibrosis. Thus, MAC infection evokes different immune and inflammatory responses depending on the MAC species and affected tissue.

Capsular Arabinomannans from Mycobacterium avium with Morphotype-specific Structural Differences but Identical Biological Activity

The capsules of two colony morphotypes of Mycobacterium avium strain 2151 were investigated, i.e. the virulent smooth-transparent (SmT1) and the nonvirulent smooth-opaque (SmO) types. From both morphotypes we separated a nonacylated arabinomannan (AM) from an acylated polysaccharide fraction by affinity chromatography, of which the AMs were structurally characterized. The AMs from the virulent morphotype, in contrast to that from the nonvirulent form, possessed a larger mannan chain and a shorter arabinan chain. Incubation of murine bone marrow-derived macrophages and human dendritic cells showed that the acylated polysaccharide fractions were potent inducers of tumor necrosis factor-alpha, interleukin-12, and interleukin-10 compared with nonacylated AMs, which led to only a marginal cytokine release. Further in vitro experiments showed that both the acylated polysaccharide fractions and the nonacylated AMs were able to induce in vitro anti-tumor cytotoxicity of human peripheral blood mononuclear cells. Thus, morphotype-specific structural differences in the capsular AMs of M. avium do not correlate with biological activity; however, their acylation is a prerequisite for effective stimulation of murine macrophages and human dendritic cells.

Mycolactone-mediated inhibition of tumor necrosis factor production by macrophages infected with Mycobacterium ulcerans has implications for the control of infection

The pathogenicity of Mycobacterium ulcerans, the agent of Buruli ulcer, depends on the cytotoxic exotoxin mycolactone. Little is known about the immune response to this pathogen. Following the demonstration of an intracellular growth phase in the life cycle of M. ulcerans, we investigated the production of tumor necrosis factor (TNF) induced by intramacrophage bacilli of diverse toxigenesis/virulence, as well as the biological relevance of TNF during M. ulcerans experimental infections. Our data show that murine bone marrow-derived macrophages infected with mycolactone-negative strains of M. ulcerans (nonvirulent) produce high amounts of TNF, while macrophages infected with mycolactone-positive strains of intermediate or high virulence produce intermediate or low amounts of TNF, respectively. These results are in accordance with the finding that TNF receptor P55-deficient (TNF-P55 KO) mice are not more susceptible than wild-type mice to infection by the highly virulent strains but are more susceptible to nonvirulent and intermediately virulent strains, demonstrating that TNF is required to control the proliferation of these strains in animals experimentally infected by M. ulcerans. We also show that mycolactone produced by intramacrophage M. ulcerans bacilli inhibits, in a dose-dependent manner, but does not abrogate, the production of macrophage inflammatory protein 2, which is consistent with the persistent inflammatory responses observed in experimentally infected mice.

Pathogenesis of Mycobacterium avium infection: typical responses to an atypical mycobacterium?

Studying infections with Mycobacterium avium in mouse models has allowed the dissection of the antimycobacterial pathways of the mammalian host. Whereas the paradigm of cell-mediated immunity to intracellular pathogens has been confirmed, namely with regard to the pivotal roles of CD4+ T cells, macrophages, and the IL12-IFNgamma cytokine axis, atypical features have been uncovered such as the resistance to NO, the involvement of minor players in the induction of type 1 protective immunity (such as TLR2, CD40, and CD30), and the development of immunopathology during the infection with highly virulent strains such as the development of caseous necrosis of granulomas or the progressive emergence of severe lymphopenia.

CD43 is required for optimal growth inhibition of Mycobacterium tuberculosis in macrophages and in mice

We explored the role of macrophage (Mphi) CD43, a transmembrane glycoprotein, in the pathogenesis of Mycobacterium tuberculosis. Using gene-deleted mice (CD43-/-), we assessed the association of the bacterium with distinct populations of Mphi and found that CD43-/- Mphi bound less M. tuberculosis than CD43+/+ Mphi. Increased infective doses did not abrogate this difference. However, reduced association due to the absence of CD43 could be overcome by serum components. Mphi from heterozygote mice, which express 50% of wild-type CD43, bound more bacteria than CD43-/- but less than CD43+/+, proving that the gene dose of CD43 correlates with binding of M. tuberculosis. Furthermore, the reduced ability of CD43-/- Mphi to bind bacteria was restricted to mycobacterial species. We also found that the survival and replication of M. tuberculosis within Mphi was enhanced significantly in the absence of CD43, making this the first demonstration that the mechanism of mycobacterial entry influences its subsequent growth. Most importantly, we show here that the absence of CD43 in mice aerogenically infected with M. tuberculosis results in an increased bacterial load during both the acute and chronic stages of infection and more rapid development of granulomas, with greater lung involvement and distinctive cellularity.

Common and unique gene expression signatures of human macrophages in response to four strains of Mycobacterium avium that differ in their growth and persistence characteristics

Classification of pathogenic species according to the distinct host transcriptional responses that they elicit may become a relevant tool for microarray-based diagnosis of infection. Individual strains of Mycobacterium avium, an opportunistic pathogen in humans, have previously been shown to differ in terms of growth and persistence. In order to cover a wide spectrum of virulence, we selected four M. avium isolates (2151SmO, 2151SmT, SE01, TMC724) that have distinct intramacrophage replication characteristics and cause differential activation in human macrophages. Following infection with each of these strains, the expression of 12,558 genes in human macrophages was systematically analyzed by microarray technology. Fifty genes (including genes encoding proinflammatory cytokines, chemokines, signaling, and adhesion molecules) were differentially expressed more than twofold in response to all of the M. avium isolates investigated and therefore constitute a common macrophage signature in response to M. avium. The magnitude of regulation of most of these genes was directly correlated with the host cell-activating capacity of the particular M. avium strain. The regulation of a number of genes not previously associated with mycobacterial infections was apparent; these genes included genes encoding lymphocyte antigen 64 and myosin X. In addition, individual response patterns typical for some M. avium isolates could be defined by the pronounced upregulation of interleukin-12p40 (IL-12p40) (in the case of 2151SmO) or the specific upregulation of SOCS-1 and IL-10 (in the case of SE01) in macrophages. TMC724, a strain of avian origin, could not be classified by any one of these schemes, possibly indicating the limits of pathogen categorization solely by immune response signatures.

Inhibition of cytokines expression in human microglia infected by virulent and non-virulent mycobacteria

The pathogenesis of tuberculosis (TBC) meningitis is still unknown. As shown by previous studies, human microglia can be the target of mycobacteria, but no data are available about their cellular response to infection. Consequently, we studied the expression of tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1) and IL-10 in human microglia pure cultures infected with the two variants of Mycobacterium avium (domed-opaque (SmD) and transparent (SmT)) and with Mycobacterium tuberculosis. Results showed that microglia was productively infected by mycobacteria which could grow inside the cells. Mycobacteria internalization was more rapid for M. avium, but M. tuberculosis infection turned out to be more efficient due to the incorporation of densely packed bacteria. TNF-alpha expression was not affected by M. avium, whereas an increase followed by a decrease was observed in M. tuberculosis. Both IL-1 and IL-10 cytokine expression was rapidly inhibited by infection with the more virulent bacteria, whereas the non-pathogenic one had almost no effect. Also, the expression of the co-stimulatory molecule CD137, a member of tumor necrosis factor receptor family, was affected by infection with virulent mycobacteria. Our results show that microglia response to mycobacterial infection is modulated in correlation with virulence, mainly toward inhibition of inflammatory response. This observation might be one of the mechanisms by which non-pathogenic mycobacteria are quickly eliminated, explaining one of the bases of virulence.

Experimental murine mycobacteriosis: evaluation of the functional activity of alveolar macrophages in thalidomide- treated mice

Thalidomide is a selective inhibitor of tumor necrosis factor-alpha (TNF-alpha), a cytokine involved in mycobacterial death mechanisms. We investigated the role of this drug in the functional activity of alveolar macrophages in the presence of infection induced by intranasal inoculation of Mycobacterium avium in thalidomide-treated and untreated adult Swiss mice. Sixty animals were inoculated with 5 x 10(6) M. avium by the respiratory route. Thirty animals received daily thalidomide (30 mg/kg mouse) and 30 received water by gavage up to sacrifice. Ten non-inoculated mice were used as a control group. Lots of animals from each group were evaluated until 6 weeks after inoculation. Infection resulted in an increased total number of inflammatory cells as well as increased activity of pulmonary macrophages. Histologically, intranasal inoculation of bacilli resulted in small mononuclear infiltrates located at the periphery of the organ. Culture of lung fragments revealed the presence of bacilli only at the beginning and at the end of the experimental period. Thalidomide administration did not affect the microbiological or histological features of the infection. Thalidomide-treated and untreated animals showed the same amount of M. avium colonies 3 weeks after infection. Although it did not affect bacillary clearance, thalidomide administration resulted in a decreased percent of spread cells and release of hydrogen peroxide, suggesting that factors other than TNF-alpha play a role in the killing of mycobacteria by alveolar macrophages. Thalidomide administration also reduced the number of spread cells among resident macrophages, suggesting a direct effect of the drug on this phenomenon.

Immunopathological studies on feline cutaneous and (muco)cutaneous mycobacteriosis

Eight cases of feline (muco)cutaneous mycobacteriosis were studied to identify the causative agent and examine for phenotype and functional characteristics (expression of interleukin (IL)-1beta, IL-6, IL-12, tumour necrosis factor-alpha and inducible nitric oxide synthase) of the inflammatory cells. Polymerase chain reaction and sequencing identified the causative agents as Mycobacterium tuberculosis or M. avium complex in each four cases. Lesions were characterised by pyogranulomatous infiltration, with variability in the presence and size of necrotic areas, the presence of multinucleated giant cells and the degree of lymphocyte infiltration. Macrophages were positive for myeloid/histiocyte antigen (calprotectin), suggesting they represented freshly recruited monocytes; further differentiation to epithelioid cells and multinucleated giant cells was associated with loss of the myeloid/histiocyte antigen. Lymphocytes were found disseminated in the infiltrate (predominantly T cells) and as B cell-dominated accumulations mainly in the periphery of the lesions. Acid-fast bacilli were numerous. In M. tuberculosis complex infection, extracellular bacilli were most prominent, whereas in M. avium complex infection, bacilli were mainly located intracellularly. All cytokines examined as well as inducible nitric oxide synthase (iNOS) were variably expressed by macrophages, epithelioid cells and multinucleated giant cells. Expression was most intense in degenerating macrophages loaded with intracellular bacilli, but was also seen cell-free within necrotic areas. The intense induction of cytokine and iNOS expression especially in infected macrophages suggests a relatively low virulence for these infectious agents in cats. Furthermore, the confinement of the bacilli to lesions indicates a successful response to infection.

Correlation of serum tumor necrosis factor-alpha, interleukin-4 and soluble interleukin-2 receptor levels with radiologic and clinical manifestations in active pulmonary tuberculosis

The precise clinical manifestations of tuberculosis are likely to result from a complex interaction between the host and the pathogen. We took serum samples from a group of patients with a variety of clinical and radiological stages of pulmonary tuberculosis in order to characterize tumor necrosis factor-alpha (TNF-alpha), interleukin-4 (IL-4) and soluble interleukin-2 receptor (sIL-2R) response. We further evaluated whether the levels of TNF-alpha, IL-4 and soluble IL-2R are related with each other, and also evaluated the levels of TNF-alpha, IL-4 and sIL-2R after anti-tuberculosis therapy and relation with radiologic scores. Forty-three inpatients with active pulmonary tuberculosis and 19 healthy controls participated in the study. Patients were divided into four categories radiologically on chest X-ray (minimal, moderate-advanced, far-advanced and with miliary infiltration). Concentrations of TNF-alpha (20.9+/-10/15.4+/-8 pg/ml) and sIL-2R (2569+/-842/1444+/-514 pg/ml) were statistically different between patients and controls (p=0.02 and p=0.0001, respectively). Before chemotherapy there was a positive correlation between TNF-alpha and sIL-2R (r=0.34), but there was no correlation between IL-4 and TNF-alpha, and between IL-4 and sIL-2R (r=-0.23 and r=-0.22). The TNF-alpha level was not statistically different in four groups before and after chemotherapy. Results of this study provided some evidence confirming the previously reported role of TNF-alpha, IL-4 and sIL 2R in the control of tuberculosis, but these cytokines were not found related with disease severity.

Control of mycobacterial replication in human macrophages: roles of extracellular signal-regulated kinases 1 and 2 and p38 mitogen-activated protein kinase pathways

Intracellular persistence of mycobacteria may result from an intricate balance between bacterial replication and signaling events leading to antimicrobial macrophage activities. Using human monocyte-derived macrophages, we investigated the relevance of mitogen-activated protein kinase activation for the growth control of Mycobacterium avium isolates differing in their abilities to multiply intracellularly. The highly replicative smooth transparent morphotype of M. avium strain 2151 induced significantly less p38 and extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation than the smooth opaque morphotype of the same strain, which was gradually eliminated from macrophage cultures. Inhibition of the p38 pathway by highly specific inhibitors did not significantly affect mycobacterial replication within macrophages, regardless of the in vitro virulence of the M. avium strain. However, repression of the ERK1/2 pathway further enhanced intracellular growth of highly replicative M. avium strains, although it did not increase survival of the poorly replicating M. avium isolate. Inhibition of the ERK1/2 pathway resulted in decreased tumor necrosis alpha (TNF-alpha) secretion irrespective of the virulence of the M. avium isolate used for infection, revealing that TNF-alpha could have been only partially responsible for the control of intracellular M. avium growth. In conclusion, ERK1/2- and TNF-alpha-independent pathways are sufficient to limit intramacrophage growth of less-virulent M. avium strains, but early ERK1/2 activation in infected macrophages is critically involved in controlling the growth of highly replicative M. avium strains.

Activation of the mitogen-activated protein kinase signaling pathway is instrumental in determining the ability of Mycobacterium avium to grow in murine macrophages

Of the two common morphotypes of Mycobacterium avium, designated smooth transparent (SmT) or smooth opaque (SmO), the SmO morphotype is avirulent, whereas the SmT morphotype is virulent. The role of the host macrophage in determining these different virulence phenotypes was analyzed using an in vitro model of macrophage infection. Initial studies confirmed previous reports of the increased ability of the SmT bacteria to grow in macrophages; this increased virulence correlated with reduced induction of inflammatory cytokines. Examination of the response of the mitogen-activated protein kinase (MAPK) pathway following infection with either morphotype revealed that all three members of the MAPK pathway were activated. Pharmacologic inhibition of either the extracellular signal-regulated kinase (ERK) or p38(MAPK) pathways resulted in distinct consequences for the growth of the two morphotypes. In particular, inhibition of the p38(MAPK) resulted in attenuated growth of the SmT morphotype, which correlated with reduced PGE(2) production. Inhibition of cyclooxygenase 2 by indomethacin also inhibited growth of SmT, substantiating the role for PGE(2) in promoting the growth of SmT. In contrast, SmO induction of the ERK pathway was increased compared with the SmT morphotype, and inhibition of ERK resulted in decreased TNF-alpha synthesis and enhanced SmO growth. Pharmacologic inhibitors of the MAPK pathway were present for only the first 4 h of infection and yet had consequences for bacterial growth at 7 days. Therefore, the data suggest that induction of the MAPK pathway during uptake of bacteria is instrumental in determining the eventual fate of the bacteria.

Circulating TNF-alpha, TGF-beta, and IL-10 in tuberculosis patients and healthy contacts

Levels of tumour necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, and interleukin (IL)-10 in plasma of pulmonary tuberculosis (TB) patients and healthy contacts and plasma and pleural fluid of patients with tuberculous pleuritis were examined by enzyme immunoassay. Plasma TNF-alpha and IL-10 were elevated to significant levels in healthy contacts. High levels of TGF-beta and IL-10 were also detected in plasma from TB patients and healthy contacts. Pleural fluid contained all three cytokines with the level of IL-10 being highest followed by TGF-beta and TNF-alpha. Plasma of tuberculous pleuritis patients also had detectable levels of the three cytokines. Increased levels of TNF-alpha in plasma of contacts and to some extent pleural fluid of pleuritis patients, is perhaps to limit the infection, while elevated IL-10 in plasma of TB patients and contacts and pleural fluid would perhaps modulate excess proinflammation. Elevated TGF-beta in TB patients suggests its role in the immunopathogenesis.

Expression of inflammatory cytokine mRNA in lymphoid tissue from swine experimentally infected with Mycobacterium avium serovar 2

OBJECTIVE

To evaluate in situ expression of inflammatory cytokine mRNA in lymphoid tissue of swine experimentally infected with Mycobacterium avium serovar 2.

ANIMALS

7 noninfected pigs and 7 pigs infected with M. avium serovar 2.

PROCEDURE

Expression of mRNA of inflammatory cytokines such as tumor necrosis factor alpha (TNFalpha), interleukin (IL)-1beta IL-6, and IL-8 in formalin-fixed paraffin-embedded blocks of lymphoid tissue (lymph nodes and tonsil) of swine experimentally infected with M. avium serovar 2 was compared with that of noninfected pigs. Tissues were evaluated by use of morphologic localization of cytokine mRNA, using in situ hybridization at 160 days after inoculation.

RESULTS

A noticeable increase in mRNA expression for TNFalpha and mild increases in mRNA expression of IL-8 and IL-1beta were detected in mandibular lymph nodes from infected swine, compared with noninfected swine. Mild increase in mRNA expression for 1L-6 also was observed in tonsils from infected swine. Cytokine mRNA was detected in macrophages and lymphocytes, primarily within cortical follicles and adjacent mantle zones.

CONCLUSIONS AND CLINICAL RELEVANCE

Expression of mRNA for inflammatory cytokines was increased in lymphoid tissue of infected swine, possibly resulting from local factors on, or secreted by, M. avium. These results suggest that alterations in cytokine mRNA expression are important in the pathogenesis and clinical course of mycobacteriosis in swine. Modulation of the immune response by vaccines that selectively target cytokine expression and secretion in response to mycobacterial challenge may be effective in prevention of mycobacteriosis in swine.

Changes in the virulence of Mycobacterium avium after passage through embryonated hens' eggs

Eight-day-old embryonated hen's eggs were used as a model to study Mycobacterium avium virulence. Strains isolated from human patients caused 20-90% mortality when eggs were infected by injection of bacterial suspensions into the amniotic sac. Virulence of examined strains subsequently decreased with passage through eggs to between 0 and 40% mortality in four passages. Virulence of the egg-attenuated strains could be restored by passage through human peripheral blood mononuclear cells. The site of infection in the egg was usually the mesodermal layer of the chorioallantoic membrane. A few small granulomas containing acid-fast bacteria were seen in the liver, but not in other organs. Death of chicken embryos may have resulted from destruction of the mesodermal layer of the chorioallantoic membrane with consequent respiratory failure. PBMCs infected with less virulent egg-passaged strains of M. avium produced higher levels of tumor necrosis factor-alpha than did peripheral blood mononuclear cells infected with more virulent nonpassaged strains.

Minor role played by type I tumour necrosis factor receptor in the control of Mycobacterium avium proliferation in infected mice

Control of mycobacterial growth depends on the concerted activity of different cytokines acting in different stages of the development of innate and adaptive immune responses. Tumour necrosis factor-alpha (TNF-alpha) has been shown to play a protective role in Mycobacterium avium infections. Here we assessed the growth of this mycobacterial species in wild-type mice and in mice with a genetically engineered disruption of the type I receptor for TNF-alpha (p55-KO mice). p55-KO mice infected with a low-virulence strain of M. avium exhibited a slightly delayed capacity to eliminate the micro-organisms from the liver as compared with wild-type animals. However, either the growth of this strain in the other organs studied (spleen and lung) or the growth of two other strains of M. avium with intermediate or high virulence, failed to be affected by mutation of the TNF-alpha receptor. p55-KO mice were also as protected by the administration of recombinant interleukin-12 as the heterozygous p55 +/- mice. We conclude that signalling through the type I TNF receptor plays a small role in vivo in the induction of mycobacteriostasis during M. avium infection but may improve survival during infection with virulent mycobacteria, independently of the extent of their proliferation.

Upregulation of p75 tumor necrosis factor alpha receptor in Mycobacterium avium-infected mice: evidence for a functional role

The bacterial growth and the production of tumor necrosis factor alpha (TNF-alpha) and TNF receptors (TNF-Rs) in the spleen and blood of BALB/c mice challenged with Mycobacterium avium complex (MAC) were monitored. Infection developed in two phases: the first, up to day 21, was associated with rapid MAC multiplication in the spleen and a drop in the mycobacteremia, and the second was associated with control of the infection in both compartments. In the spleen, TNF-alpha and TNF-RII mRNA levels peaked on day 21 and then slowly decreased; however, no increase in the level of TNF-RI mRNA was observed throughout these experiments. The level of circulating soluble TNF-RII (sTNF-RII) was transiently increased after day 21. In a model in which overproduction of bioactive TNF-alpha was triggered in response to a second infection with MAC, an increased production of sTNF-RII by cultured splenocytes was also observed. Administration of an antagonist anti-TNF-RII monoclonal antibody (MAb 6G1) to infected mice inhibited the bacterial growth in the spleen, suggesting that the TNF-RII and/or sTNF-RII was functionally involved in the mechanisms that control the infection. Overall, these observations suggest that upregulation of TNF-RII or sTNF-RII contributes to modulation of the TNF-alpha antibacterial activity in MAC infections.

Therapeutic effects of benzoxazinorifamycin KRM-1648 administered alone or in combination with a half-sized secretory leukocyte protease inhibitor or the nonsteroidal anti-inflammatory drug diclofenac sodium against Mycobacterium avium complex infection in mice

The effects of half-sized secretory leukocyte protease inhibitor or diclofenac sodium administered alone or in combination with the benzoxazinorifamycin KRM-1648 on the therapeutic efficacy of KRM-1648 against Mycobacterium avium complex (MAC) in mice were studied. Neither of the two anti-inflammatory drugs affected the efficacy of KRM-1648, while they exerted significant modulating effects on tumor necrosis factor alpha production by MAC-infected macrophages.

CRL-1072 enhances antimycobacterial activity of human macrophages through interleukin-8

CRL-1072 is a poloxamer surfactant that kills mycobacteria more effectively within macrophages than in broth cultures. Human macrophages treated with CRL-1072 synthesized interleukin-8 (IL-8), tumor necrosis factor-alpha (TNF-alpha), and granulocyte-macrophage colony-stimulating factor (GM-CSF) in a dose-dependent manner. About 3000 pg of IL-8 per million human macrophages accumulated in cultures treated with 100-1500 ng of poloxamer, with mRNA message for IL-8 induced as early as 2 h. As macrophages do not have IL-RA receptors, a transwell culture was used to study the chemotactic and activating effects of IL-8 between CRL-1072-treated human macrophage effectors and polymorphonuclear neutrophil (PMN) targets. PMN were activated by IL-8 and secreted hydrogen peroxide and myeloperoxidase (MPO). MPO derived from PMN, in turn, activated monocytes for an enhanced killing of intracellular Mycobacterium avium. The ability of CRL-1072 to modulate macrophage-mediated activation of neutrophils and receive a feedback activation signal may form one mechanism by which its antimycobacterial activity is achieved in vivo.

Cytokine induction in murine macrophages infected with virulent and avirulent Rhodococcus equi

To look for a possible correlation between the virulence of Rhodococcus equi and its cytokine-inducing capacity, we evaluated intracellular survival and measured cytokine induction by mouse macrophages infected with a virulent strain containing an 85-kb plasmid and expressing VapA (103+), its avirulent plasmid-cured derivative (103-), and heat-killed 103+ (HK). After incubation with similar numbers of bacteria, macrophages infected with 103- contained significantly more organisms than those infected with 103+ or HK. The number of bacteria in the macrophages infected with 103- and HK decreased progressively, whereas the 103+ numbers remained constant over 48 h. Interleukin 1beta (IL-1beta), IL-6, IL-10, IL-12 p40, and tumor necrosis factor alpha (TNF-alpha) mRNA induction peaked at 4 h and returned to baseline between 12 and 48 h postinfection. IL-1beta, IL-6, IL-10, and TNF-alpha concentrations assessed by enzyme-linked immunosorbent assay generally agreed well with mRNA expression; IL-12 could, however, not be detected. For all the cytokines detected, mean concentrations in the supernatants were consistently higher in the 103(-)-infected monolayers than in those infected with 103+, although, with the exception of IL-1beta, the differences were not statistically significant. R. equi HK was a poor inducer of cytokine production. In conclusion, virulent and avirulent R. equi strains induced similar levels of cytokine synthesis. The slightly greater induction of most cytokines observed following infection with 103- is likely secondary to greater uptake by macrophages rather than to a direct role of VapA or another plasmid-encoded product in downregulating cytokine induction.

Expression of virulence of Mycobacterium tuberculosis within human monocytes: virulence correlates with intracellular growth and induction of tumor necrosis factor alpha but not with evasion of lymphocyte-dependent monocyte effector functions

We assessed the applicability of an in vitro model of low-level infection of human monocytes to the characterization of the virulence of strains of the Mycobacterium tuberculosis family. Peripheral blood monocytes were infected at a 1:1 ratio with the virulent M. tuberculosis strain H37Rv, the avirulent M. tuberculosis strain H37Ra, and the attenuated M. bovis strain BCG. Both the percentages of cells infected by the three strains and the initial numbers of intracellular organisms were equivalent, as were levels of monocyte viability up to 7 days following infection. Intracellular growth reflected virulence, as H37Rv replicated in logarithmic fashion throughout the assay, BCG growth reached a plateau at 4 days, and H37Ra did not grow at all. The same patterns of growth were observed following infection of human alveolar macrophages with H37Rv and H37Ra. Monocyte production of tumor necrosis factor alpha was significantly higher following infection with virulent H37Rv than with either BCG or H37Ra. In contrast, there was no clear correlation of interleukin 10 production with virulence. Nonadherent cells of purified-protein-derivative-positive donors mediated equivalent degrees of reduction of the intracellular growth of H37Rv, BCG, and H37Ra. Low-level infection of human monocytes with H37Rv, BCG, and H37Ra thus provides an in vitro model for assessment of the virulence of these M. tuberculosis family strains. Furthermore, it is suggested that the virulence of these strains is expressed primarily by their differing abilities to adapt to the intracellular environment of the mononuclear phagocyte.

The phagosomal environment protects virulent Mycobacterium avium from killing and destruction by clarithromycin

Murine bone marrow-derived macrophages (Mphis) infected with virulent strains of Mycobacterium avium (TMC 724 and a human clinical isolate) or with an avirulent opaque variant that spontaneously dissociates from the virulent human clinical isolate were subjected to a prolonged and continuous treatment with clarithromycin added at the MIC. The efficiency of this antibiotic in terms of inhibition of bacterial growth and bacterial degradation was evaluated during a 21-day treatment period. Growth was assessed by determination of CFU of intracellular bacteria and by a quantitative ultrastructural analysis which allowed us also to determine the extent of bacterial degradation. A similar treatment was applied to the same strains growing in liquid medium. Our data show that in liquid medium, clarithromycin caused a 90% decrease in CFU within 7 days of treatment. When applied to Mphis infected with virulent M. avium, clarithromycin immediately arrested bacterial growth but was unable to fully kill and degrade intracellularly growing virulent bacteria. After 21 days of treatment, 25% of intracellular bacteria were still morphologically intact. These bacteria resumed growth upon removal of the antibiotic, with a normal replication rate. These bacteria had not become more resistant to the drug, since the MIC was unchanged as compared to the one determined for the initial stock used to infect Mphis. Our data therefore suggest that the intraphagosomal environment protects bacteria from degradation. We propose that the inability of the drug to completely destroy bacteria is the result of a limited accessibility of the drug due to prevention of fusions between the immature phagosomes in which virulent bacteria reside and lysosomes in which clarithromycin accumulates. In accord with our proposal, we show that the avirulent opaque variant, which does not prevent phagosome-lysosome fusions (unpublished data), is finally destroyed by clarithromycin even within the phagosomal environment.

Temporal effect of tumor necrosis factor alpha on murine macrophages infected with Mycobacterium avium

Members of the Mycobacterium avium complex are a family of bacteria that persist within macrophages in the face of an immune response. Elimination of these organisms is likely due to cytokine-induced macrophage activation. Because macrophage activation by tumor necrosis factor alpha (TNF-alpha) appears critical for killing of intracellular M. avium, early downregulation of TNF-alpha levels in infected macrophages has been suggested as a survival mechanism for virulent strains of M. avium. We examined the relationship between TNF-alpha and growth of M. avium strains of differing virulence, as measured by their ability to grow in murine bone marrow-derived macrophages. When exogenous TNF-alpha was added immediately following macrophage infection, significant growth inhibition of virulent M. avium strains was observed. If TNF-alpha addition was delayed by 24 h or more, growth inhibition was abrogated. To determine if early downregulation of TNF-alpha levels could explain the differential growth of virulent and avirulent strains, levels of TNF-alpha and prostaglandin E2 (PGE2), which has been shown to suppress TNF-alpha production in uninfected macrophages, were quantified over time. Upregulation of both TNF-alpha and PGE2, as measured by enzyme-linked immunosorbent assay, was evident by 6 h postinfection, indicating that the ability of M. avium to replicate in macrophages was not directly correlated with early downregulation of TNF-alpha production. However, TNF-alpha bioactivity, as measured by cytotoxicity, was significantly decreased in virulent M. avium strains at all time periods examined. Treatment of infected macrophages with gamma interferon immediately after infection resulted in significantly increased levels of nitric oxide but did not affect the growth of virulent M. avium strains. These results suggest that while significant levels of TNF-alpha are present in supernatants from all M. avium strains, levels of biologically active TNF-alpha are significantly reduced in supernatants from virulent M. avium strains. Preliminary results suggest that upregulation of the soluble p75 TNF receptor may be one mechanism by which TNF-alpha bioactivity reduction occurs.

Relationship between virulence of Mycobacterium avium strains and induction of tumor necrosis factor alpha production in infected mice and in in vitro-cultured mouse macrophages

We studied the ability of two Mycobacterium avium strains with different virulences to induce tumor necrosis factor alpha (TNF) synthesis by mouse resident peritoneal macrophages (RPM phi) in vitro in an experiment to look for a possible correlation between virulence and this TNF-inducing capacity. The low-virulence strain, 1983, induced significantly higher production of TNF by RPM phi than did the high-virulence strain, ATCC 25291. TNF neutralization during culture of infected RPM phi resulted in enhancement of growth of strain 1983 and had no effect on growth of strain ATCC 25291; TNF treatment of strain ATCC 25291-infected macrophages had no effect on mycobacterial growth. The extent of M. avium growth and the amount of TNF synthesis were independent of the presence of contaminating T cells or NK cells in the macrophage monolayers. Intraperitoneal administration of anti-TNF monoclonal antibodies to BALB/c mice infected intravenously with M. avium 1983 abrogated the elimination of the bacteria in the liver and caused a slight increase in bacterial growth in the spleen. Neutralization of TNF led to a minor increase in the proliferation of M. avium ATCC 25291 in the liver and spleen of BALB/c mice late in infection. Anti-TNF treatment did not affect the growth of the two M. avium strains in BALB/c.Bcgr (C.D2) mice, suggesting that restriction of M. avium strains to induce TNF production by macrophages may limit their ability to proliferate both in vitro and in vivo.

Tumour necrosis factor-alpha (TNF-alpha) in the host resistance to mycobacteria of distinct virulence

The relative virulence of different isolates of Mycobacterium avium has been linked to their capacity to trigger the secretion of TNF from the macrophages they infect. Smooth opaque (SmOp) variants of Myco. avium have been shown to trigger higher expression of TNF-alpha by macrophages in vitro than the smooth transparent (SmTr) variants. To analyse the role of TNF in resistance to infection by Myco. avium, we studied the infection by two different morphotypes of strain 2.151 of Myco. avium both in vitro and in vivo in the presence or absence of neutralizing antibodies to TNF. No effects were found in vitro regarding the growth of either isolate of Myco. avium. In vivo, only the virulent SmTr morphotype showed enhanced growth in the presence of the neutralizing antibodies. This enhancement occurred relatively late when priming for TNF secretion in vivo was evident. Among four isolates of Myco. avium, three virulent ones induced a marked priming for TNF release and one avirulent strain did not. Mycobacterium tuberculosis H37Ra, which is very active in inducing TNF release due to its lipoarabinomannan moiety, was used to compare with the previous results. The growth of H37Ra in macrophages was increased in vitro by the neutralization of TNF and neutralization of either TNF and/or interferon-gamma (IFN-gamma) enhanced the in vivo proliferation of this microbe in the spleen and liver of infected animals, whereas only the combination of both anti-TNF and anti-IFN-gamma enhanced bacterial proliferation in the lung. We conclude that resistance to the avirulent strains of Myco. avium did not involve TNF, but rather antimicrobial mechanisms expressed constitutively in the mononuclear phagocytes. In contrast, TNF plays an important role in the control of Myco. tuberculosis H37Ra infection.

Opposing effects of interferon-gamma on iNOS and interleukin-10 expression in lipopolysaccharide- and mycobacterial lipoarabinomannan-stimulated macrophages

Previous studies have demonstrated that, like bacterial lipopolysaccharide (LPS), arabinofuranosyl-terminated lipoarabinomannan (AraLAM) from an attenuated strain of Mycobacterium induces potent early gene (c-fos, KC, JE and TNF-alpha) responses in murine macrophages, whereas extensively alpha-Manp capped LAM (ManLAM) from virulent M. tuberculosis do not. In this study we have extended analysis of the influence of mycobacterial LAM on macrophage function by demonstrating that AraLAM (but not ManLAM), like bacterial LPS, is a potent stimulator of inducible nitric oxide synthase (iNOS) expression independent of the autocrine activity of co-stimulated tumour necrosis factor-alpha (TNF-alpha) release. The inability of ManLAM to induce iNOS expression was not due to induction of the 'deactivating' cytokine interleukin-10 (IL-10). Indeed, like LPS, AraLAM was also a potent inducer of IL-10 expression. However, analysis of AraLAM- or LPS-induced responses in the presence of interferon-gamma (IFN-gamma) showed that, whereas IFN-gamma acts as a potent co-stimulus for iNOS, it completely inhibits the IL-10 response. Hence, the presence of IFN-gamma early in infection will have an important immunomodulatory role in determining the macrophage response. These results have important implications for the pathogenesis of virulent and avirulent mycobacteria in vivo.

Production of TNF-alpha, IL-6 and TGF-beta, and expression of receptors for TNF-alpha and IL-6, during murine Mycobacterium avium infection

The Mycobacterium avium complex comprises intracellular bacteria associated with disseminated infection in patients with acquired immune deficiency syndrome (AIDS). Immune defects that lead to infection are unknown but cytokines appear to play an important role in the immunomodulation of host defence mechanisms. We evaluated the cytokine profiles seen temporally after murine M. avium infection. Spleen cells were obtained from M. avium-infected C57BL/6 mice and uninfected mice at weeks 1, 2, 3, 4 and 5. Cells were cultured in vitro and subsequently pulsed with killed M. avium. Supernatants were collected from the cultured splenic cells and the concentrations of interleukin-6 (IL-6), transforming growth factor-beta 1 (TGF-beta 1) and tumour necrosis factor-alpha (TNF-alpha) were measured. TGF-beta 1 was detected at week 1, followed by IL-6 production at week 2. Elevated TNF-alpha levels were observed at week 3. The addition of polyclonal anti-TGF-beta 1 antibody to M. avium-infected peritoneal macrophages in the presence of splenic cell supernatants from weeks 1, 3 and 5 led to decreased bacterial counts compared to controls. Anti-IL-6 antibody did not have any effect on macrophage anti-mycobacterial activity. Concurrently, we observed decreased expression of TNF-alpha receptors on infected macrophages. We propose that the early elevated levels of TGF-beta 1, a known suppressor of macrophage function, in conjunction with down-regulation of TNF-alpha receptors may help explain the suboptimal macrophage response to TNF-alpha, leading to impaired anti-mycobacterial activity.

A bone marrow-derived murine macrophage model for evaluating efficacy of antimycobacterial drugs under relevant physiological conditions

Even though the macrophage is the host cell for the intracellular bacterial parasite Mycobacterium avium, macrophages have undergone only limited evaluation as models for determining the capacities of antimycobacterial drugs to inhibit the growth of M. avium within this relevant intracellular environment. In the present study, we demonstrated that a panel of M. avium isolates could actively infect homogeneous monolayers of murine bone marrow-derived macrophages. A number of established and experimental antimycobacterial drugs were then added to these cultures at a range of concentrations, and their effects on the numbers of surviving bacilli were determined 8 days later. By plotting such numbers versus drug concentrations it was then possible to clearly distinguish between compounds with bactericidal activity (such as rifabutin and PD 125354) and those with bacteriostatic effects (such as clarithromycin), even though several of these compounds had very similar MICs. In addition, an estimate of the potential therapeutic efficiency of each drug could be made by determining the concentration needed to destroy an arbitrary percentage of the inoculum (in this case, the bactericidal concentration destroying 99% of the inoculum). Such values were considerably in excess of the MICs and may more realistically reflect the concentrations in serum required to effectively reduce the bacterial burden in vivo.

Animal and cell-culture models for the study of mycobacterial infections and treatment

Emerging problems with the treatment of infections caused by Mycobacterium avium and Mycobacterium tuberculosis require the development of new models, both in vitro and in vivo, in which new chemotherapeutic and immunotherapeutic approaches can be tested. In this brief review, the use of cell culture models, in which drugs can be tested for their capacity to inhibit mycobacterial growth within the infected host macrophage, and new models in vivo in which drugs and/or cytokines can be tested in infected mice are discussed. In this latter case, new emerging mouse models include animals with engineered gene disruptions, in which severely disseminated infections can be produced, thus mimicking events in severely immunocompromised human patients.

Search for the molecular basis of morphological variation in Mycobacterium avium

Isolates of Mycobacterium avium exhibit three different colonial variations: smooth domed (SmD), smooth transparent (SmT), and rough (Rg). Because the discrimination between morphotypes is founded on morphological rather than molecular principles and because of the absence of consensus over the relevance of morphology to pathogenesis and drug sensitivity, a comparative study at the protein level was undertaken. By direct immunization of BALB/c mice with the soluble sonicate of one of the morphotypes of M. avium serovar 2, eight monoclonal antibodies (MAbs) were identified, of which one was M. avium specific. Cross immunization of syngeneic mice with serum-absorbed antigens allowed the generation of 15 further MAbs; 11 were M. avium or M. avium complex specific, but none of them was morphotype specific. Subcellular fractions analyzed by electrophoresis showed similar profiles, with the exception of a cytosolic protein with a relative molecular mass of ca. 66 kDa (protein SmT 66), which was most highly expressed in SmT variants of M. avium serotypes 2 and 4. Because a well-known, ubiquitous stress-heat shock protein (hsp65) has a similar molecular mass, protein SmT 66 was compared with hsp65. Western blot (immunoblot) analyses using several cross-reacting MAbs and N-terminal amino acid sequencing established that this protein was not the ubiquitous stress protein. Thus, SmT 66 is the first product to be described which might be associated with the SmT morphotype.

Differential release of tumor necrosis factor-alpha from murine peritoneal macrophages stimulated with virulent and avirulent species of mycobacteria

The ability of Mycobacterium tuberculosis H37Rv and H37Ra, M. bovis BCG and M. smegmatis to induce the secretion of tumor necrosis factor-alpha (TNF-alpha) by cultured murine peritoneal macrophages is inversely related to their virulence. The avirulent species of mycobacteria which were unable to persist in macrophages were capable of inducing significant levels of TNF-alpha compared to that formed in cultures infected with the virulent M. tuberculosis H37Rv. This difference was also associated with an inherent toxicity by live H37Rv for macrophage cultures. Heat-killed H37Rv was non-toxic and induced significant levels of TNF-alpha; in contrast, live and heat-killed suspensions of avirulent mycobacteria had an equivalent ability to trigger TNF-alpha secretion. The TNF-alpha response was dose-dependent, related directly to the percentage of infected cells, and peaked 6-12 h post-infection. An early and vigorous TNF-alpha response appears to be a marker of macrophage resistance, while the downregulation of this response seems associated with macrophage toxicity and unrestricted mycobacterial growth.

Cytokines in infectious disease: the battle between host and pathogen

The immune system is poised like a fulcrum to respond quickly to challenge by infectious agents, but can produce excess inflammatory signals or excess suppressive signals when out of balance. During the past year, significant progress has been made in our understanding of how certain pathogens promote immune suppression and shift the balance from the host in their favor. Understanding the mechanisms that underlie excessive inflammatory responses or the suppressive effects of the micro-organism will aid in the development of new therapies.

Regulation of murine macrophage effector functions by lipoarabinomannan from mycobacterial strains with different degrees of virulence

Lipoarabinomannan (LAM) is the major arabinose- and mannose-containing phosphorylated lipopolysaccharide (LPS) in mycobacterial cell walls. LAM preparations from a virulent strain (Erdman) (LAM(Erdman)) and an attenuated strain (H37Ra) (LAMH37Ra) of Mycobacterium tuberculosis, as well as from M. leprae (a virulent mycobacterium), were analyzed for their effects on various macrophage (M phi) effector functions. LAMH37Ra, like gram-negative LPS, exhibited a dose-dependent ability to induce tumor necrosis factor alpha (TNF-alpha) production in normal M phi, and gamma interferon (IFN-gamma) priming of the M phi greatly augmented the levels of TNF-alpha. However, the effects of LAMH37Ra were unaffected by polymyxin B, which totally abrogated the effects of LPS. LAM(Erdman) and LAM from M. leprae, on the other hand, induced virtually no TNF-alpha production. Analysis of M phi mRNA by reverse transcription-polymerase chain reaction revealed that the levels of production. Analysis of M phi mRNA by reverse transcription-polymerase chain reaction revealed that the levels of TNF-alpha mRNA induced by the various preparations correlated with the levels of TNF-alpha protein detected. Interestingly, both LAMH37Ra and LAM(Erdman) could block subsequent IFN-gamma- and LPS-induced M phi activation, a previously reported measure of the potent ability of LAM to down-regulate M phi effector functions. Two lines of evidence suggested, however, that M phi cyclooxygenase products did not play a role in this down-regulation. LAMH37Ra and LPS could induce the production of NO2- in both normal and IFN-gamma-primed M phi, whereas LAM(Erdman) could stimulate NO2- production only in primed M phi. Both LAMH37Ra and LAM(Erdman) could substitute for LPS as a triggering signal for IFN-gamma-primed M phi in a toxoplasma killing assay. The triggering ability of LAM(Erdman), however, was abrogated by an anti-TNF-alpha antibody, suggesting that sufficient TNF-alpha production was stimulated by LAM(Erdman) to drive a M phi function relevant in host resistance. Thus, mycobacterial LAM is a potent regulator of M phi functions, a fact that may have important consequences in mycobacterial disease.

Immunity to mycobacteria

Recent progress in the field of immunity to mycobacteria has centered on T cell subset responses and the cytokines these cells secrete. In addition, there has been steady progress in identifying and characterizing several classes of major mycobacterial proteins; included amongst these are the secreted/export proteins of Mycobacterium tuberculosis, which several laboratories now believe may represent the key protective immunity-inducing antigens of the bacillus.

Infection with Mycobacterium avium induces production of interleukin-10 (IL-10), and administration of anti-IL-10 antibody is associated with enhanced resistance to infection in mice

Organisms of the Mycobacterium avium complex are associated with disseminated infection in patients with AIDS. The mechanisms that account for the survival of the intracellular bacteria are unknown. We document here that infection of C57BL/6 black mice with M. avium 101 triggered interleukin-10 (IL-10) production. The synthesis of IL-10 peaked after 2 weeks of infection and remained elevated throughout the period of infection. Treatment of M. avium-infected peritoneal macrophages with recombinant IL-10 suppressed the stimulatory effect of tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor. To confirm the possible role of IL-10 in the infection in vivo, mice were infected with M. avium 101 and simultaneously received treatment with neutralizing anti-IL-10 antibody. After 4 weeks the animals were harvested and the numbers of viable bacteria were quantitated in the liver, spleen, and blood. The liver and spleen of animals receiving anti-IL-10 antibody had 2 to 3 log units fewer bacteria than did those of control animals. These results suggest a role for IL-10 in the pathogenesis of M. avium infection.

Dissemination of enteric Mycobacterium avium infections in mice rendered immunodeficient by thymectomy and CD4 depletion or by prior infection with murine AIDS retroviruses

This study shows that infection of mice with the murine AIDS virus LP-BM5 or Du5H profoundly depressed the capacity of splenic T cells from these animals to respond to the T-cell mitogen phytohemagglutinin or concanavalin A or to alloantigens. Similar effects were also observed if mice were thymectomized and then infused with monoclonal anti-CD4 antibody (TxCD4- mice). When such mice were infected intravenously with Mycobacterium avium, growth of the infection was markedly exacerbated in the TxCD4- mice or in mice given murine AIDS virus 2 months earlier. In view of these data, we then investigated whether such treatments might cause dissemination of M. avium following enteric implantation of bacteria into the mouse cecum; this route was chosen in an attempt to model events in AIDS patients, in which the gut appears to be one of the major portals of M. avium infection. In this model, the entry and hematogenous dissemination of four clinical isolates of M. avium were monitored against time and found to be accelerated and enhanced in T-cell-deficient mice. In view of this finding, these novel approaches for enteric infection that use immunodeficient mice are presented as potential new models for the evaluation of immunotherapy and chemotherapy in a setting that bears some similarity to events believed to occur in AIDS patients.