Control of natural killer cell-mediated innate resistance against the intracellular pathogen Listeria monocytogenes by gamma/delta T lymphocytes

An alphabeta T-cell-independent immunoprotective response towards gut coccidia is supported by gammadelta cells

Although gammadelta cells are commonly hypothesized to provide a 'first line of defence', gammadelta-cell-deficient mice are generally only marginally more susceptible to pathogens. Because gammadelta cells are enriched within epithelia, it is important to resolve whether immunoprotective capacity towards epithelial-tropic pathogens is absent from the gammadelta-cell compartment, or whether such activity is present but simply redundant with that of alphabeta T cells. In this work, following infection of the intestinal epithelium of alphabeta T-cell-deficient mice with the coccidian parasite, Eimeria vermiformis, gammadelta cells were shown to support the rapid activation of other lymphoid cells and to confer a transferable antipathogen effect that could be eradicated by neutralization of interferon-gamma. However, unlike alphabeta T cells, these effects of gammadelta cells showed no evidence of functional immunological memory. These results are directly relevant to coccidiosis, an economically significant disease of livestock, and should have general relevance to infections involving alphabeta T-cell deficiencies, e.g. cryptosporidiosis in patients with acquired immune deficiency syndrome (AIDS).

Gamma delta-T cells are critical for survival and early proinflammatory cytokine gene expression during murine Klebsiella pneumonia

Although cells of the innate inflammatory response, such as macrophages and neutrophils, have been extensively studied in the arena of Gram-negative bacterial pneumonia, a role for T cells remains unknown. To study the role of specific T cell populations in bacterial pneumonia, mice deleted of their TCR beta- and/or delta-chain were intratracheally inoculated with Klebsiella pneumoniae. Gamma delta T cell knockout mice displayed increased mortality at both early and late time points. In contrast, mice specifically lacking only alpha beta-T cells were no more susceptible than wild-type mice. Pulmonary bacterial clearance in gamma delta-T cell knockout mice was unimpaired. Interestingly, these mice displayed increased peripheral blood dissemination. Rapid up-regulation of IFN-gamma and TNF-alpha gene expression, critical during bacterial infections, was markedly impaired in lung and liver tissue from gamma delta-T cell-deficient mice 24 h postinfection. The increased peripheral blood bacterial dissemination correlated with impaired hepatic bacterial clearance following pulmonary infection and increased hepatic injury as measured by plasma aspartate aminotransferase activity. Combined, these data suggest that mice lacking gamma delta-T cells have an impaired ability to resolve disseminated bacterial infections subsequent to the initial pulmonary infection. These data indicate that gamma delta-T cells comprise a critical component of the acute inflammatory response toward extracellular Gram-negative bacterial infections and are vital for the early production of the proinflammatory cytokines IFN-gamma and TNF-alpha.

Cytokine profile and natural killer cell activity in Listeria monocytogenes infected mice treated orally with Petiveria alliacea extract

In this work, we investigated the effects of Petiveria alliacea extract on the production of Th1-type and Th2-type cytokines and on NK cells activity in normal and Listeria monocytogenes infected mice. Our results demonstrated that in normal/non-infected mice P. alliacea administration led to increased levels of Interleukin-2 (IL-2). The infection alone enhanced INF-gamma levels and NK cell activity at 48 and 72 hours of infection. The treatment with five consecutive doses of 1000 mg/kg/day of P. alliacea extract, given previously to infection, led to further increases in IL-2 levels, in relation to normal/non-infected/P. alliacea treated controls, and in INF-gamma levels at 72 h of infection, compared to infected mice. On the other hand, the production of IL-4 and IL-10 were not altered either by the infection or by the treatment with P. alliacea extract. NK cells activity increased at 48 h and 72 h following the inoculation of the bacteria. When mice were treated with P. alliacea previously to infection, NK activity was higher than that observed at 48 h, 72 h and 120 h of infection in the infected animal. Based on these findings we suggest that P. alliacea up-regulates anti-bacterial immune response by enhancing both Th1 function and the activity of NK cells.

[gamma][delta] cells: a right time and a right place for a conserved third way of protection

The tripartite subdivision of lymphocytes into B cells, alphabeta T cells, and gammadelta cells has been conserved seemingly since the emergence of jawed vertebrates, more than 450 million years ago. Yet, while we understand much about B cells and alphabeta T cells, we lack a compelling explanation for the evolutionary conservation of gammadelta cells. Such an explanation may soon be forthcoming as advances in unraveling the biochemistry of gammadelta cell interactions are reconciled with the abnormal phenotypes of gammadelta-deficient mice and with the striking differences in gammadelta cell activities in different strains and species. In this review, the properties of gammadelta cells form a basis for understanding gammadelta cell interactions with antigens and other cells that in turn form a basis for understanding immunoprotective and regulatory functions of gammadelta cells in vivo. We conclude by considering which gammadelta cell functions may be most critical.

Dominance of IL-12 over IL-4 in gamma delta T cell differentiation leads to default production of IFN-gamma: failure to down-regulate IL-12 receptor beta 2-chain expression

Gamma delta T cells secrete Th1- and Th2-like cytokines that help mediate innate and acquired immunity. We have addressed the mechanism whereby murine gamma delta T cells acquire the capacity to differentially produce such cytokines. Splenic gamma delta T cells could be polarized into IFN-gamma- or IL-4-secreting cells in vitro; however, in contrast to CD4+ alpha beta T cells, gamma delta T cells predominantly produced IFN-gamma, even in the presence of IL-4, a finding independent of genetic background. Like CD4+ Th1 cells, IFN-gamma-producing cells expressed the IL-12 receptor beta 2-chain after activation in the presence of IL-12; however, unlike Th2 cells, IL-4-primed gamma delta T cells also expressed this receptor, even in the absence of IFN-gamma and despite the presence of the transcription factor GATA-3. IL-12 also induced IL-4-primed gamma delta T cells to proliferate and to translocate Stat3/Stat4, indicating signaling through the IL-12 receptor. These molecular events can account for the predominant production of IFN-gamma by gamma delta T cells in the presence of IL-12, despite the availability of IL-4. Early and predominant production of IFN-gamma by gamma delta T cells likely is critical for the roles that these cells play in protection against intracellular pathogens and in tumor immunity.

Role of endogenous interleukin-18 in resolving wild-type and attenuated Salmonella typhimurium infections

The stimulation of gamma interferon (IFN-gamma) has been shown to be essential in resolving infections by intracellular pathogens. As such, several different cytokines including, interleukin-12 (IL-12) and IL-18, can induce IFN-gamma. To resolve Salmonella infections, the stimulation of IL-12 and IFN-gamma are important for mediating its clearance. In this present study, the relevance of IL-18 in protection against oral challenge with Salmonella typhimurium was investigated to determine the role of this IFN-gamma-promoting cytokine. Rabbit anti-murine IL-18 antisera was generated and administered prior to the oral challenge of BALB/c and IL-12p40-deficient knockout (IL-12KO) mice with a wild-type S. typhimurium strain. The median survival time was reduced by 2 days for the anti-IL-18-treated BALB/c mice, while no significant reduction in survival rate for the anti-IL-18-treated IL-12KO mice was observed compared to vehicle-treated mice. To investigate the contribution of IL-18 to resolving Salmonella infections, an attenuated aro-negative mutant (H647) was orally administered to BALB/c mice. This Salmonella infection induced both IL-12 and IFN-gamma in both the Peyer's patches and the spleens. In vehicle-treated mice, Peyer's patch IL-12 peaked by 24 h, while IL-18 levels peaked at 3 days, suggesting sequential support by these cytokines for IFN-gamma. Anti-IL-18 treatment exerted its greatest effect upon the mucosal compartment, limiting early IFN-gamma production. However, anti-IL-18 treatment had little effect upon splenic IFN-gamma levels until late in the response. Infection of IL-12KO mice with H647 strain induced IFN-gamma, but it was not supported by IL-18, although IL-18 levels were reduced by this treatment. These results suggest that IL-18 does contribute to the clearance of S. typhimurium and that endogenously induced IL-18 could not substitute for IL-12.

The effects of Chlorella vulgaris in the protection of mice infected with Listeria monocytogenes. Role of natural killer cells

In this work we have demonstrated the effects of oral administration of Chlorella vulgaris (CV) on Natural Killer cells (NK) activity of mice infected with a sublethal dose of viable Listeria monocytogenes. The treatment with C. vulgaris produced a significant increase on NK cells activity in normal (non-infected) animals compared to the animals that received only vehicle (water) (p < 0.0001). Similarly, the infection alone produced a significant increase on NK cells activity, which was observed at 48 and 72 hours after the inoculation of L. monocytogenes. Moreover, when CV was administered in infected animals, there was an additional increase in NK cells activity which was significantly higher than that found in the infected groups (p < 0.0001). CV treatment (50 and 500mg/Kg) of mice infected with a dose of 3 x 10(5) bacteria/animal, which was lethal for all the non-treated controls, produced a dose-response protection which led to a 20% and 55% survival, respectively (p < 0.0001).

Cytokine/chemokine cascades in immunity to tuberculosis

The relationship between acquired specific resistance and delayed-type hypersensitivity (DTH) in immunity to tuberculosis has long been a topic of debate. Here, Ian Orme and Andrea Cooper propose that the events are separate mechanisms; protection is cytokine driven and initially controls the infection, whereas DTH is primarily chemokine driven and functions to wall off the infection and prevent further dissemination.

Interleukin-15 enhances host protection against acute Toxoplasma gondii infection in T-cell receptor alpha-/-deficient mice

A potential role for T-cell receptor (TCR)-gamma(delta) cells in response to Toxoplasma gondii has been described. This study was undertaken to evaluate whether exogenous rIL-15 can enhance the production of gamma(delta) T cells in response to infection with T. gondii. For this, C57BL/6 and TCR alpha-gene deficient mice (alpha-/-) were vaccinated with Toxoplasma lysate antigen (TLA) in combination with interleukin (IL)-15. The cellular and humoral immune response was assessed following immunization. Administration of IL-15/TLA to alpha-/-mice lengthened survival time post lethal challenge. Several immunological parameters were increased in the alpha-/-mice following immunization with IL-15/TLA including serum immunoglobulin (Ig)G1 and IgG2a antibody titres and splenocyte lymphoproliferation in response to parasite antigen. Further, the CTL response to parasite infected target cells as well as the production of interferon gamma was enhanced by IL-15/TLA administration in the alpha-/-mice. These observations indicate that the gamma(delta) T cells response to this parasite can be enhanced by the administration of exogenous IL-15 in the presence of parasite lysate antigen.

New vaccines against tuberculosis. The status of current research

The increasing realization that the current vaccine for tuberculosis, bacille Calmette-Guérin (BCG), is of varying effectiveness, and is less protective in adults than in children, has prompted new research for a replacement. New research has resulted in innovative approaches, including the use of sub-unit vaccines, auxotropic vaccines, DNA vaccines, and recombinant vaccines, among others. This article reviews these approaches and test results in animal models, and discusses their potential for use in humans.

Early Preferential Stimulation of γδ T Cells by TNF-α

Although recent findings indicate that γδ T cells influence both early innate and Ag-specific adaptive host responses, it has remained unclear what triggers γδ T cell reactivity. Investigating very early T cell activation in mouse and human models of bacterial infection, we measured CD69 expression as an indicator of early cellular activation. Both murine αβ and γδ T cells responded polyclonally to systemic bacterial infections, and to LPS. However, γδ T cells responded more strongly to the bacteria and to LPS. In vitro LPS-stimulated human T cells showed a similar differential response pattern. We identified TNF-α as mediator of the early differential T cell activation, and of differential proliferative responses. The stronger response of γδ T cells to TNF-α was correlated with higher inducible expression levels of TNF-Rp75. Among unstimulated splenocytes, more γδ T cells than αβ T cells expressed CD44 at high levels. The data suggest that TNF-Rp75 determines the differential T cell reactivity, and that most γδ T cells in the normal spleen are present in a presensitized state. As TNF-α stimulates activated T cells, it may early preferentially connect γδ T cell functions with those of cells that produce this cytokine, including activated innate effector cells and Ag-stimulated T lymphocytes.

Disruption of the cellular inflammatory response to Listeria monocytogenes infection in mice with disruptions in targeted genes

The results of this study to dissect the nature of the acquired immune response to infection with Listeria monocytogenes in mice with targetted gene disruptions show that successful resolution of disease requires the essential presence of alphabeta T cells and the capacity to elaborate gamma interferon. In the absence of either of these entities, mice experience increasingly severe hepatitis and tissue necrosis and die within a few days. The data from this study support the hypothesis that the protective process is the efficient replacement of neutrophils in lesions by longer-lived mononuclear phagocytes; alphabeta-T-cell-knockout mice died from progressive infection before neutrophil replacement could occur, whereas in gammadelta-T-cell-knockout mice this replacement process in the liver has previously been shown to be much slower. In the present study we attribute this delay to reduced production of the macrophage-attracting chemokine MCP-1 in the gammadelta-T-cell-knockout animals. These data further support the hypothesis that gammadelta T cells are important in controlling the inflammatory process rather than being essential to the expression of protection.

Primary γδ Cell Clones Can Be Defined Phenotypically and Functionally as Th1/Th2 Cells and Illustrate the Association of CD4 with Th2 Differentiation

The division of CD4+ αβ T cells into Th1 and Th2 subsets has become an established and important paradigm. The respective activities of these subsets appear to have profound effects on the course of infectious and autoimmune diseases. It is believed that specific programs of differentiation induce the commitment of an uncommitted Th0 precursor cell to Th1 or Th2. A component of these programs is hypothesized to be the nature of MHC-peptide antigen presentation to the αβ T cell. It has heretofore remained uncertain whether a Th1/Th2 classification likewise defines, at the clonal level, γδ T cells. Such cells do not, as a general rule, express either CD4 or CD8αβ, and they do not commonly recognize peptide-MHC. In this report, γδ cell clones are described that conform strikingly to the Th1/Th2 classification, both by cytokine expression and by functional activities of the clones in vitro and in vivo. Provocatively, both the γδ cell clones and primary γδ cells in vivo showed a strong association of the Th2 phenotype with CD4 expression. These results are discussed with regard to the immunoregulatory role that is increasingly emerging for γδ cells.

Lethal tuberculosis in interleukin-6-deficient mutant mice

Tuberculosis is a chronic infectious disease which causes major health problems globally. Acquired resistance is mediated by T lymphocytes and executed by activated macrophages. In vitro studies have emphasized the importance of macrophage activation for mycobacterial growth inhibition. In vivo, the protective host response is focused on granulomatous lesions in which Mycobacterium tuberculosis is contained. A cellular immune response of the T helper 1 (Th1) type is considered central for control of tuberculosis. Using interleukin-6 (IL-6)-deficient mice, we here demonstrate a crucial role of this pluripotent cytokine in protection against M. tuberculosis but not against Mycobacterium bovis BCG. Infection with M. tuberculosis was lethal for the IL-6-deficient mice at inocula that were still controlled by IL-6-competent mice. Spleen cells from M. tuberculosis-infected IL-6-/- mouse mutants produced elevated levels of IL-4 and reduced levels of gamma interferon compared to the control levels. Cytofluorometric analyses of spleen cells from M. tuberculosis-infected mice revealed more-profound alterations in T-cell ratios in IL-6-/- mice than in control mice. We assume that IL-6 contributes to host resistance by its proinflammatory activity and by its influence on cytokine secretion.

In vivo and in vitro activation and expansion of gammadelta T cells during Listeria monocytogenes infection in humans

Serial flow cytometry analyses of peripheral blood mononuclear cells obtained from 8 patients infected with Listeria monocytogenes showed a higher percentage (P < 0.01) of gammadelta T cells (median, 11.7; range, 3.7 to 35.3) than did 16 age-matched uninfected controls (1.7, 0.4 to 13). Most in vivo-expanded gammadelta T cells expressed the Vgamma9 and Vdelta2 gene products and displayed a memory phenotype (CD45RO[high]), and patients' gammadelta T cells expressed significantly more (P < 0.01) activation marker HLA-DR than did controls (19.8% [median] and 0.9 to 87.6% [range] versus 2.3% and 0 to 4.7%, respectively). When peripheral blood mononuclear cells from normal donors were cultured in vitro with heat-killed Listeria cells, analysis of CD25 and HLA-DR expression on gammadelta and alphabeta T cells indicated that a high percentage of gammadelta T cells was activated early compared to alphabeta T cells. In addition, depletion of gammadelta T cells before culture abrogated the early lymphocyte proliferative response induced by the pathogen. Taken together, these results argue for the involvement of gammadelta T cells during L. monocytogenes infection in humans.

Interleukin-4 and listeriosis

Experimental infection of mice with Listeria monocytogenes (L. monocytogenes) has served as an appropriate model for analyzing Th1-cell-driven immune responses. Generally, Th2 responses are absent and IL-4 is not detectable. Here, we describe experimental settings under which IL-4 is detectable in listeriosis. Our data suggest that IL-4 is rapidly produced after infection. This prompt IL-4 burst seems to stimulate chemokine responses and, therefore, may participate in the regulation of the early antilisterial host response. Soon thereafter, IL-4 production wanes. At least partially this seems to be caused by downregulation of IL-4-producing CD4+ NK1+ TCR alpha beta int lymphocytes by IL-12. In the absence of IFN-gamma responsiveness, IL-4 production is demonstrable during acquired immunity against L monocytogenes, and this elevated IL-4 production apparently contributes to disease exacerbation. In conclusion, the data are consistent with a detrimental role of IL-4 in listeriosis and active control of IL-4 synthesis by the antilisterial immune response. The rapid, but transient, IL-4 burst in listeriosis probably contributes to host defense without impairing development of the acquired T-cell response because of its shortness.

Interleukin-12 and infectious diseases: a potential novel therapy

Interleukin-12 (IL-12) is emerging as a central component of both innate and acquired immunity. The multiplicity of biological activities associated with this cytokine, particularly the stimulation of cell-mediated immunity, suggests that it may be crucial in the control of extracellular and intracellular infections. In in vitro studies, IL-12 production is initiated rapidly after infection with a variety of viral, parasitic, fungal and bacterial agents. This induction correlates well with the reported resistance or susceptibility of animals to infection with these agents. Other factors may, however, influence responses in vivo, including host genetic make-up, microbial load and the induction of antagonistic cytokine pathways, notably IL-4 and IL-10. In some situations, IL-12 may direct immune responses to inappropriate pathways, and worsen disease, so that careful consideration of the type of required immune response is needed before IL-12 therapy is initiated. IL-12 treatment may also be useful in promoting protective immune responses to vaccines, allowing systemic immunisation with lower doses, or even normally non-immunogenic preparations, of antigen. Finally, IL-12 has been demonstrated to act in concert with standard antimicrobial chemotherapy in viral, parasitic, fungal and bacterial infections, allowing a reduction in the dose of the agent used and providing hope that such combination therapy may more effectively control drug-resistant strains of infectious agents.

Intracellular Salmonella dublin induces substantial secretion of the 40-kilodalton subunit of interleukin-12 (IL-12) but minimal secretion of IL-12 as a 70-kilodalton protein in murine macrophages

The induction by intracellular pathogens of interleukin-12 (IL-12) secretion is of particular importance since this cytokine has been shown to be necessary for optimal cell-mediated immune responses. Several recent investigations have suggested that cultured macrophages are a significant source of IL-12 following intracellular infection with pathogens such as Salmonella spp. In an effort to critically evaluate the magnitude of the IL-12 response in cultured macrophages following interaction with Salmonella dublin, enzyme-linked immunosorbent assays specific for the 40- and 70-kDa subunits of IL-12 (IL-12p40 and IL-12p70) and a sensitive bioassay for IL-12p70 were used. Using BALB/c macrophages, S. dublin at various challenge doses was a potent inducer of IL-12p40 secretion (>6,000 pg/10(7) macrophages). However when secretion of IL-12p70 was evaluated, S. dublin did not induce comparable IL-12p70 production (<80 pg/10(7) macrophages) at any time, despite varying the challenge dose of Salmonella. The limited ability of BALB/c (Ity(s)) macrophages to secrete IL-12p70 in response to Salmonella was not a strain-specific phenomenon since similar results were demonstrated for macrophages isolated from CBA/J (Ity(r)) and C3H/HeJ (lipopolysaccharide [LPS]-hyporesponsive) mice. While intracellular infection with Salmonella was not a potent stimulus for IL-12p70 secretion in these mouse strains, macrophages from these mice responded significantly to a stimulus of gamma interferon plus LPS. Taken together these results demonstrate a limited capacity for intracellular Salmonella to stimulate murine macrophage secretion of IL-12p70, despite being a significant stimulus for IL-12p40 secretion. Furthermore, our results suggest that Salmonella-induced IL-12p40 secretion by macrophages is not solely an LPS-mediated event.

Cytokines in the induction and expression of T-cell-mediated granuloma formation and protection in the murine model of listeriosis

Lymphocyte-mediated inflammation is a hallmark of autoimmune diseases, such as multiple sclerosis. Crohn's disease, rheumatoid arthritis and sarcoidosis. However, this type of inflammation probably developed under evolutionary pressure from pathogenic microorganisms, such as mycobacteria and other intracellular infective agents. One such pathogen, the gram-positive bacterium Listeria monocytogenes (L. monocytogenes), induces a cascade of tissue alterations that ultimately results in the eradication of the bacteria associated with a granulomatous response. Consequently, murine listeriosis has been established as a model to analyze not only T-cell-dependent antibacterial protection but also T-cell-mediated mononuclear inflammation in parenchymal organs. Extensive studies of the molecular basis of the latter phenomenon led to the conclusion that the most decisive step from non-specific microabscess formation to granulomatous inflammation is the activation of non-specifically invading CD4+ T cells, which results in high local concentrations of TNF-alpha and IFN-gamma in the presence of IL-2. This in turn induces CD11b-independent mechanisms of intraparenchymal monocyte accumulation. Because any attempt to neutralize the effects of TNF-alpha and IFN-gamma to modulate T-cell-mediated inflammation will also dramatically decrease host resistance, other anti-inflammatory strategies based on the modulation of TNF-alpha and IFN-gamma-induced mechanisms of monocyte accumulation must be developed. Recalling the classical work by Dienes & Schoenheit on the induction of bacterial allergies (1), the cytokine phenotype of granuloma formation also has implications as regards the most potent adjuvant environment for the development of a T-cell response. The murine listeriosis model is the basis for all conclusions in this article on the role of cytokines in the induction and expression of T-cell-mediated inflammation and, as we will show, promises to yield still more insights into the rational design of vaccines.

Immunity to Rhodococcus equi

Rhodococcal pneumonia is an important, life threatening disease of foals and immunosuppressed humans. Increased knowledge of the mechanisms of protective immunity are required in order to develop an effective immunoprophylaxis strategy for horses and immunotherapeutic regiments for people. Both humoral and cellular components of the immune system may be involved in immune clearance of R. equi. The susceptibility of foals less than 4-6 months of age is postulated to reflect waning maternal antibody, and passive transfer of hyperimmune plasma can provide protection on endemic farms. However, effective clearance is likely to require appropriate cellular responses, including the secretion of cytokines. In murine models, both CD4+ and CD8+ T lymphocytes can reduce bacterial counts in the lung. CD4+ cells appear to be both required and sufficient, and IFN-gamma is a primary mediator. Clearance appears to be a type 1 immune response while type 2 responses may lead to a failure to clear and lesion development. It remains to be determined how the cellular immunity experiments reported in mice relate to horses and humans. Likewise, the role of specific R. equi antigens in protective immunity has not been determined.

Role of alphabeta and gammadelta T cells in the host response to Salmonella infection as demonstrated in T-cell-receptor-deficient mice of defined Ity genotypes

Salmonella spp. are facultative intracellular bacteria which enter the body through the intestinal tract. We studied the roles of T cells expressing either the alpha and beta chains or the gamma and delta chains of the T-cell receptor (alphabeta T cells or gammadelta T cells, respectively) in the host defense against Salmonella using mice genetically deficient in either alphabeta T cells, gammadelta T cells, or both T-cell subsets. These mutant strains of mice were infected orally or intraperitoneally with Salmonella dublin, and the progression of the disease was monitored by determining bacterial numbers in the feces, gut wall, Peyer's patches, mesenteric lymph nodes, spleen, and liver. Since susceptibility to Salmonella infection in mice is strongly affected by the alleles at the Ity locus, T-cell-mutant mice with either the Ity-sensitive or Ity-resistant phenotype were tested for resistance to S. dublin infection. We found that even though large numbers of intraepithelial and mucosal alphabeta and gammadelta T cells populate the normal intestine, they have no role in controlling the invasion of S. dublin into the intestine or the subsequent bacterial replication in the Peyer's patches or gut wall. Furthermore, systemic infections were equally severe for the first 6 days in normal, alphabeta T-cell-deficient, and gammadelta T-cell-deficient mice, and alphabeta but not gammadelta T cells were required for clearance of S. dublin, regardless of the Ity phenotype. However, mice that lacked both T-cell subsets had higher bacterial counts in their livers 15 to 18 days after infection than did alphabeta T-cell-deficient mice, suggesting that gammadelta T cells can contribute to acquired immunity to S. dublin.

Interleukin-12 secretion by Mycobacterium tuberculosis-infected macrophages

Infection with Mycobacterium tuberculosis or phagocytosis of large latex beads induced interleukin-12 (IL-12) production in macrophages. In contrast, tumor necrosis factor (TNF) was produced only in response to M. tuberculosis infection, not after phagocytosis of latex beads. Comparable results were obtained with cells from immunocompetent C57BL/6 and gamma interferon receptor-deficient mutant mice. Thus, phagocytosis by mechanisms not specific for M. tuberculosis was a sufficient trigger for IL-12 secretion, emphasizing the central role of this cytokine in the initiation of anti-infective immunity.

An innate view of gamma delta T cells

Findings made during the past few years demonstrate that gamma delta T cells apparently share with macrophages a propensity to recognize nonpeptidic molecules of the kind most commonly associated with microorganisms and stressed cells. In general, recognition of these antigens by gamma delta T cells involves the antigen receptor but does not require antigen presenting cells to express MHC gene products or to have a functional antigen processing machinery. Other recent advances continue to support the notion that gamma delta T cells can perform specialized functions related to the repair of tissue damage.