Role of T-cell derived cytokines in the downregulation of immune responses in parasitic and retroviral infection

Cytokines and T-cell response in malaria

The intracellular protozoan Plasmodium sp induces a complex immune response which sometimes implies serious pathological effects for the host. According to in vitro studies and epidemiological surveys, several effector mechanisms are displayed against plasmodial blood stages and a large interaction between humoral and cell-mediated immunity is presumed to occur among protected individuals. The key role of T cells in the antiplasmodial immune response is now well established, but all the regulatory heterogenous mechanisms are not yet fully known. An increasing body of data shows a dual role during malaria attack for some cytokines released by monocytes and macrophages (TNF, IL-1, IL-6) or by T cells (IFN-gamma, lymphotoxin (LT), IL-4). The importance of some plasmodial proteins in the cytokine-induced pathology and the stimulation of a preferential TH1 or TH2 mediated immune response to achieve protective immunity against Plasmodium sp are discussed.

Endogenous IL-10 and IFN-gamma production controls thymic cell proliferation in mice acutely infected by Trypanosoma cruzi

Thymocytes from mice with experimental Trypanosoma cruzi infection respond poorly to Con-A stimulation. However, the proliferative capacity of these cells is not impaired, as demonstrated by the fact that at high doses, exogenous rIL-2 restores thymidine uptake. This finding could be explained either by insufficient IL-2 production or by the appearance of inhibitory factors during T. cruzi infection. This paper shows that in response to Con A, IL-2 production is decreased in the model. Furthermore, the whole profile of cytokine production is modified, with a striking increase in IL-10, IFN-gamma, IL-4, IL-5 and IL-6 production. The results indicate that IL-10 plus IFN-gamma are responsible for the decrease in the Con A-induced proliferation since a normal proliferative response as well as normal IL-2 production can be restored if both cytokines are neutralized by adding their monoclonal antibodies (MoAbs). Evidence is provided also for an enhanced non-specific cytotoxicity of thymic cells from infected mice that might involve IL-4, IL-5 and IL-6. This is the first study demonstrating an alteration of thymic cell function by T. cruzi infection which results from overstimulation of IL-10 and IFN-gamma production.

Quantitative differences in interleukin-2 and interleukin-4 production by antigen-stimulated splenocytes from individually- and group-housed mice

The effects of differential housing on T-helper (TH) cell activation were investigated. BALB/c and C57Bl/6 male mice housed 1 or 4 per cage were administered three i.p. injections of keyhole limpet hemocyanin (KLH) over several weeks. Effects of differential housing on in vitro antigen-specific interleukin (IL)-2 (a TH1 cell derived cytokine) and IL-4 (a TH2 cell derived cytokine) production were observed. BALB/c mice housed alone produced significantly more IL-4 than BALB/c mice housed in groups. C57Bl/6 mice housed alone produced significantly more IL-2 than C57Bl/6 mice housed in groups. Differential housing did not influence either IL-2 production among BALB/c mice or IL-4 production among C57Bl/6 mice. These data demonstrate that environmental conditions can influence cytokine production by both TH-1 dominant (C57Bl/6) and TH-2 dominant (BALB/c) mice.

Interleukin 13, an interleukin 4-like cytokine that acts on monocytes and B cells, but not on T cells

Interleukin 13 (IL-13) is a recently described protein secreted by activated T cells which is a potent in vitro modulator of human monocyte and B-cell functions. The data, reviewed here by Gerard Zurawski and Jan de Vries, shows that IL-13 shares biological activities with IL-4, their genes are closely linked in both the human and mouse genomes, and there is sequence homology between IL-13 and IL-4 proteins. Although the cloned IL-4 receptor protein (IL-4R) does not bind IL-13, it appears that the functional IL-4R and IL-13R share a common subunit that is important for signal transduction.

Breaking the asymptomatic phase of HIV-1 infection

AIDS typically consists of three phases: (1) an acute, infectious mononucleosis-like syndrome followed by (2) a prolonged asymptomatic stage ending in (3) the appearance of frank AIDS. The asymptomatic phase may last for years and its presence suggests a persistent conflagration between the virus and the host's immune response. There is considerable evidence that an immune response develops but the response is ultimately inadequate. From the work of others as well as our own, we have constructed a hypothesis which attempts to explain some aspects of the immune response. We propose that HIV-1 preferentially infects a subset of CD4+ lymphocytes which are then either destroyed or altered in their biological functions. Further, we suggest that this subset represents the CD4+ TH1 lymphocyte population. By decreasing the quantity of IL-2 and interferon-gamma produced by TH1 lymphocytes, the production of cytokines by TH2 cells is increased. One of the cytokines produced by TH2 lymphocytes is IL-10, a polypeptide with significant inhibitory properties towards lymphocytes. Sera from patients with frank AIDS have significant lymphocyte inhibitory activities some of which operate through IL-10. Thus, a gradual shift to a TH2-type response and release of increasing amounts of inhibitors eventually prevents the host from replacing destroyed cells or mounting new and appropriate immune responses.

Cytokine dysregulation in HIV-associated neurological disease

AIDS is associated with three major neurological syndromes: dementia (HIVD), vacuolar myelopathy (VM) and plainful sensory neuropathy (PSN). The pathogenesis of these conditions remains unclear although they all demonstrate a marked increase in macrophage number and activation despite systemic immunosuppression. It was therefore of interest to determine the profile of cytokine and HIV expression in brain, spinal cord and peripheral nerves of AIDS patients with AD, VM and PSN, as compared to AIDS patients without neurological disease and seronegative controls. RNA was extracted from brain, spinal cord and peripheral nerve and RT/PCR for cytokine and HIV mRNA was performed. In situ RT/PCR was performed to determine the number and type of cells expressing cytokine message and this was compared to the number of cells containing HIV DNA detected with in situ PCR. We found a consistent profile of increased TNF alpha and decreased IFN gamma and IL4 in all three syndromes compared to AIDS patients without neurological disease. IL1 did not increase in parallel with TNF alpha IL10 was decreased in the VM tissue. HIV transcripts were increased in the AD brains compared to non-demented controls but were detected only occasionally in spinal cord and not at all in peripheral nerve. Preliminary data from in situ RT/PCR suggests that a large number of cells are expressing. TNF alpha but only a small number are infected with HIV.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential susceptibilities of mice genomically deleted of CD4 and CD8 to infections with Trypanosoma cruzi or Trypanosoma brucei

The role of CD4+ and CD8+ T cells in the surveillance of Trypanosoma cruzi or Trypanosoma brucei brucei was studied in mice which lacked CD4 or CD8 molecules and which were generated by embryonic stem cell technology. Whereas wild-type mice infected with T. cruzi (Tulahuén strain) displayed low levels of parasitemia and no mortality, striking increases in parasite growth and mortality occurred in both CD8- and CD4- mice. On the contrary, CD8- and, to a lesser degree, CD4- mice showed enhanced resistance to T. b. brucei. T-cell-dependent immunoglobulin G-specific responses were produced in CD8- but not CD4- mice. Normal T-cell proliferative responses were measured in both CD4- and CD8- mice. Interleukin-4 production after concanavalin A or anti-CD3 monoclonal antibody stimulation was strikingly enhanced in CD8- but not CD4- spleen cells, whereas gamma interferon production was normal in both CD4- and CD8- spleen cells. Spleen and lymph node cells from CD8- (but not CD4-) mice at 20 days postinfection with T. cruzi had higher levels of interleukin-4 mRNA than the wild-type controls, as shown in a competitive polymerase chain reaction assay. On the other hand, CD4- (but not CD8-) mice at 20 days postinfection with T. cruzi had lower levels of gamma interferon mRNA than the wild-type mice.

T-cell subsets in autoimmunity

Although differential cytokine production has been best characterized in CD4+ T cells, it is becoming clear that CD8+ T cells may also be heterogeneous at the level of cytokine production, and that this determines whether they exhibit inflammatory- or suppressor-type properties. Compelling evidence has accumulated in the past few years that cytokines such as interleukin-4, interleukin-10 and transforming growth factor-beta may serve as regulators of cell-mediated immunopathologies by inhibiting the development or effector function of inflammatory T cells that produce cytokines such as interferon-gamma or lymphotoxin.

Interleukin-4 and interleukin-10 synergize to inhibit cell-mediated immunity in vivo

The lack of cell-mediated (Th1-like) immunity that is often associated with strong humoral immune responses is thought to be due in part to the inhibition of Th1 effector function by the Th2-derived cytokine interleukin-10 (IL-10). This hypothesis, however, is based entirely on results from in vitro studies, wherein IL-10 has been shown to inhibit Th1 cytokine synthesis. In this study we have compared the regulatory effects of both IL-4 and IL-10 on the development of a more complex Th1 effector function in vivo, the development of delayed-type hypersensitivity (DTH) to Leishmania major in mice immune to Leishmania. The results revealed two findings unexpected from in vitro studies with Th1 clones. First, optimal inhibition of the DTH response (up to 70%), assessed by footpad swelling and leukocytic infiltration, required the combination of IL-4 and IL-10, indicating that these two activities synergized to inhibit DTH reactivity. Second, IL-4 inhibited interferon-gamma (IFN-gamma) production by lymph node cells draining the site of antigen challenge as well as did IL-10. The combination of both cytokines was no more effective than either alone. The mechanism by which IL-4 and IL-10 acted to inhibit DTH responses did not appear to be through inhibition of IFN-gamma or tumor necrosis factor production as treatment with antibodies which neutralized these activities failed to inhibit DTH responses. Inhibition of the DTH with IL-4 and IL-10 is the most effective specific regulator of DTH responses reported and the only one capable of modulating tuberculin DTH. These data establish IL-4 and IL-10 as potent inhibitors of Th1 effector function in vivo and suggest their utility in controlling deleterious Th1-mediated inflammatory responses such as occur in some infectious and autoimmune diseases.

Distinct H-2 complex control of mortality, and immune responses to tuberculosis infection in virgin and BCG-vaccinated mice

We have studied the impact of distinct haplotypes and of different alleles at specific H-2 loci on: (i) the susceptibility to lethal form of experimental tuberculosis; (ii) the level of DTH to mycobacterial antigens; (iii) the efficacy of vaccination with bacille Calmette-Guérin (BCG); and (iv) the IgG production and T cell proliferative response to H37Rv antigens. On the basis of median survival time (MST) following primary inoculation with lethal dose of Mycobacterium tuberculosis, susceptibility to infection associated with I-Ab and Db alleles, host resistance associated with I-Ak and Dd alleles. Mice bearing a disease-resistant phenotype also developed a vigorous DTH response. Vaccination with BCG before H37Rv infection significantly prolonged the survival time of both resistant and susceptible animals, except in B10.M (H-2f) mice. The latter exhibited intermediate resistance to infection before but slight decrease in the MST following a high-dose BCG vaccination. Distinct H-2 regulation of susceptibility to lethal infection and of BCG vaccination efficacy was confirmed in another relatively resistant H-2f-bearing strain A.CA, in which mortality occurred more rapidly in vaccinated compared with primarily infected animals. The expression of the H-2f haplotype was associated with a low DTH response to tuberculin following vaccination and subsequent lethal infection. The lack of BCG protection against Myco, tuberculosis challenge in B10.M mice associated with the high titre of specific IgG. In addition, these mice exhibited a unique ability to respond to 65-kD antigen by both IgG synthesis and T cell proliferation.

Hepatic fibrosis and gene expression changes induced by praziquantel treatment during immune modulation of Schistosoma japonicum infection

In the present study fibrogenic gene expression was determined in murine Schistosoma japonicum infection during the progression of immune modulation of infection and following chemotherapy during the course of immune modulation. Histomorphometric analysis of granuloma size and collagen deposition revealed peak granuloma size in acute infection (5 weeks) and peak hepatic collagen content at 16 weeks of infection. Peak Type I collagen gene expression was concomitant with TGF-beta 1 gene expression at 8-11 weeks. Chemotherapy during either acute (9 weeks) or chronic (24, 28 weeks) infection resulted in increased collagen deposition and increased gene expression of Type I collagen and TGF-beta 1. However, chemotherapy at 14-16 weeks resulted in decreased levels of TGF-beta 1 gene expression and essentially minimal change in Type I collagen deposition and gene expression. These data indicate that chemotherapy of schistosomiasis japonica does not reverse hepatic fibrogenesis when administered in acute infection-when granuloma size is maximal-or in chronic infection. However, a beneficial effect on hepatic fibrogenesis is seen when chemotherapy is administered at 14-16 weeks post-infection, a time of decreasing granuloma size and maximal hepatic collagen content. Thus the ability to reverse schistosomal-induced hepatic fibrogenesis by chemotherapy may depend on disease stage.

Cytokines in leishmaniasis: a complex network of stimulatory and inhibitory interactions

The work of immunologists, cell biologists and parasitologists in the field of leishmaniasis has not only provided important insights into the immunopathogenesis of this disease, but also yielded fundamental contributions to our understanding of basic immunological phenomena and of host-parasite interactions. The ability of recombinant interferon-gamma to induce the microbicidal activity of phagocytes and the opposite effect of inhibitory cytokines was first demonstrated with Leishmania-infected macrophages. The selective development of protective and disease-mediating CD4+ T lymphocytes as well as their differential influence on the course of the disease has been long investigated in the murine Leishmania major model and now represents one of the best examples for the in vivo induction of type 1 versus type 2 T helper lymphocytes. At the same time, this model has also been extensively used for immunization studies and cytokine therapy, which shed light on the functions of cytokines in vivo as well as on the mechanism(s) of disease resistance and susceptibility. In this review we will discuss the present picture of the cytokine network in murine L. major infections.

T cell-mediated cognate signaling of nitric oxide production by macrophages. Requirements for macrophage activation by plasma membranes isolated from T cells

Macrophage generation of reactive nitrogen intermediates (RNI) represents a major effector mechanism in anti-microbial immunity and non-septic inflammatory reactions. The induction of macrophage RNI production has been demonstrated to require at least two signals which in microbial infections can be provided by interferon (IFN)-gamma and lipopolysaccharide (LPS). The current study demonstrates that, in the absence of LPS, T lymphocytes can provide cognate signal(s) which synergize with IFN-gamma in stimulating macrophage RNI production, as evidenced by the ability of plasma membranes from T cell clones to activate IFN-gamma-primed macrophages. Although viable resting T cells can activate IFN-gamma-primed macrophages by an interaction that is antigen specific, plasma membranes from resting T cells do not active macrophages. Plasma membranes from T cells activated by immobilized anti-CD3 were able to effectively induce RNI production in IFN-gamma-primed macrophages. However, in contrast to the antigen-specific interaction of macrophages with viable resting T cells, the activation of IFN-gamma-primed macrophages by membranes from activated T cells does not display antigen specificity. Plasma membranes from activated T helper TH2 and from activated TH1 cells were equally effective in activating IFN-gamma-primed macrophages, suggesting that the dominance of TH1 over TH2 cells in cell-mediated responses involving macrophage effectors is not a reflection of differences in their ability to interact with macrophages but rather is a reflection of their different pattern of cytokine production. These results suggest that the T cell-macrophage interaction involves reciprocal activation of both cells--an antigen-specific activation of the T cells which results in the acquisition of T cell membrane components involved in antigen-nonspecific stimulation of the macrophages.

Interleukin-10-deficient mice develop chronic enterocolitis

Interleukin-10 (IL-10) affects the growth and differentiation of many hemopoietic cells in vitro; in particular, it is a potent suppressor of macrophage and T cell functions. In IL-10-deficient mice, generated by gene targeting, lymphocyte development and antibody responses are normal, but most animals are growth retarded and anemic and suffer from chronic enterocolitis. Alterations in intestine include extensive mucosal hyperplasia, inflammatory reactions, and aberrant expression of major histocompatibility complex class II molecules on epithelia. In contrast, mutants kept under specific pathogen-free conditions develop only a local inflammation limited to the proximal colon. These results indicate that the bowel inflammation in the mutants originates from uncontrolled immune responses stimulated by enteric antigens and that IL-10 is an essential immunoregulator in the intestinal tract.

An Abd transgene prevents diabetes in nonobese diabetic mice by inducing regulatory T cells

Susceptibility to the human autoimmune disease insulin-dependent diabetes mellitus is strongly associated with particular haplotypes of the major histocompatibility complex (MHC). Similarly, in a spontaneous animal model of this disease, the nonobese diabetic (NOD) mouse, the genes of the MHC play an important role in the development of diabetes. We have produced transgenic NOD mice that express the class II MHC molecule I-Ad in addition to the endogenous I-Ag7 molecules in order to study the role of these molecules in the disease process. Although the inflammatory lesions within the islets of Langerhans in the pancreas appear similar in transgenic and nontransgenic animals, transgenic mice develop diabetes with greatly diminished frequency compared to their nontransgenic littermates (10% of transgenic females by 30 weeks of age compared to 45% of nontransgenic females). Furthermore, adoptive transfer experiments show that T cells present in the transgenic mice are able to interfere with the diabetogenic process caused by T cells from nontransgenic mice. Thus, the mechanism by which I-Ad molecules protect mice from diabetes includes selecting in the thymus and/or inducing in the periphery T cells capable of inhibiting diabetes development.

Molecular dissection of the mouse interleukin-4 promoter

Understanding the molecular mechanisms regulating the expression of interleukin 4 (IL-4) may shed light on the differentiation of lymphokine-producing phenotypes of CD4+ T cells. We have identified two DNA segments that are necessary for full phorbol 12-myristate 13-acetate (PMA)-induced activity of the IL-4 promoter region in the thymoma cell line EL4. Through deletion and mutation analyses, one of these segments (-57 through -47) was shown to be indispensable for promoter function. We designated this sequence consensus sequence 1 (CS1), as it shares homology with a sequence (ATTTTCCNNTG) that appears five times in the proximal 302-base-pair (bp) region 5' of the gene. We examined CS1 in further detail, as well as a second consensus sequence, CS2, located at nucleotides -75 through -65; both are within a minimal 83-bp construct that expresses full promoter activity. CS1- and CS2-spanning oligonucleotides bound apparently distinct PMA-inducible, sequence-specific factors in mobility-shift assays. Multimer constructs linking CS1- or CS2-spanning oligonucleotides to a heterologous promotor revealed that the CS1 construct had the greater enhancer activity in EL4 cells. Mutating the CS1 sequence within the context of the 302-bp promoter abolished all activity of the promoter, while mutating the CS2 sequence alone had little effect. Furthermore, a CS1 multimer could drive a heterologous promoter in an IL-4-producing [helper T-cell type 2 (TH2-type)] T-cell clone but not in a non-IL-4-producing (TH1-type) clone, suggesting a mechanism by which IL-4 production could be differentially regulated in TH subsets.

Lymphokine mRNA expression in the spinal cords of Lewis rats with experimental autoimmune encephalomyelitis is associated with a host recruited CD45R hi/CD4+ population during recovery

To evaluate CD4+ T cell subpopulations involved in the induction and recovery from experimental autoimmune encephalomyelitis (EAE), the CD45R phenotype and lymphokine mRNA profile was evaluated for encephalitogenic CD4+ T cell lines in vitro and compared to CD4+ T cells isolated from the spinal cord of Lewis rats with EAE. All of the myelin basic protein (MBP)-specific T cell lines and clones that adoptively transferred EAE were > 90% CD4+ and > 90% CD45R lo. A time course of EAE disease progression was monitored as a function of the percentage of CD45R hi/CD4+ T cells isolated from the spinal cords of diseased animals. The majority of CD4+ T cells found in the central nervous system during the early phase of passive EAE were CD45R lo (the same as the encephalitogenic lines/clones). A large increase of the CD45R hi/CD4+ T cells (up to 45%) was observed during the peak and recovery phases of EAE. Lymphokine mRNA production was analyzed from antigen-stimulated MBP-specific lines, and from spinal cord lymphocytes isolated from rats with EAE. The BP-specific lines produced Th1 lymphokines (IL-2, IFN-gamma, and TNF-alpha), while the spinal cord lymphocytes produced the same Th1 lymphokines as well as IL-4 and IL-10. The CD45R hi/CD4+ T cells isolated from the spinal cords were larger and expressed more lymphokine RNA per cell than the CD45R lo/CD4+ T cells. The encephalitogenic cells (CD45R lo) were detected in the spinal cords of rats with a fluorescent dye and by allelic transfers and all of the CD45R hi/CD4+ T cells were found to be host recruited. Thus, it appears that the CD45R hi/CD4+ lymphocytes found in the spinal cord represent a host-recruited, activated cellular infiltrate that increased in number in the recovery phase of EAE and synthesized both Th1 and Th2 lymphokines.

A Sporozoite-based vaccine for Theileria parva

East Coast fever, which is caused by Theileria parva infection in cattle, is of major economic importance in eastern and central Africa. Until recently, the only available method of immunization against East Coast fever was the infection with live sporozoites and simultaneous treatment with a long-acting oxytetracycline. This method has two major disadvantages: (I) it uses live organisms; and (2) the immunity engendered is parasite strain specific. In this article, Antony Musoke, Vishvonath Nene and Subhosh Morzoria review the progress made in developing an alternative method o f immunization based on a defined sporozoite antigen.

I-E+ nonobese diabetic mice develop insulitis and diabetes

The development of type I diabetes in the nonobese diabetic (NOD) mouse is under the control of multiple genes, one or more of which is linked to the major histocompatibility complex (MHC). The MHC class II region has been implicated in disease development, with expression of an I-E transgene in NOD mice shown to provide protection from insulitis and diabetes. To examine the effect of expressing an I-E+ or I-E- non-NOD MHC on the NOD background, three I-E+ and three I-E- NOD MHC congenic strains (NOD.H-2i5, NOD.H-2k, and NOD.H-2h2, and NOD.H-2h4, NOD.H-2i7, and NOD.H-2b, respectively) were developed. Of these strains, both I-E+ NOD.H-2h2 and I-E- NOD.H-2h4 mice developed insulitis, but not diabetes. The remaining four congenic strains were free of insulitis and diabetes. These results indicate that in the absence of the NOD MHC, diabetes fails to develop. Each NOD MHC congenic strain was crossed with the NOD strain to produce I-E+ and I-E- F1 mice; these mice thus expressed one dose of the NOD MHC and one dose of a non-NOD MHC on the NOD background. While a single dose of a non-NOD MHC provided a large degree of disease protection to all of the F1 strains, a proportion of I-E+ and I-E- F1 mice aged 5-12 mo developed insulitis and cyclophosphamide-induced diabetes. When I-E+ F1 mice were aged 9-17 mo, spontaneous diabetes developed as well. These data are the first to demonstrate that I-E+ NOD mice develop diabetes, indicating that expression of I-E in NOD mice is not in itself sufficient to prevent insulitis or diabetes. In fact, I-E- F1 strains were no more protected from diabetes than I-E+ F1 strains, suggesting that other non-NOD MHC-linked genes are important in protection from disease. Finally, transfer of NOD bone marrow into irradiated I-E+ F1 recipients resulted in high incidences of diabetes, indicating that expression of non-NOD MHC products in the thymus, in the absence of expression in bone marrow-derived cells, is not sufficient to provide protection from diabetes.

Interleukin-4 and interleukin-10 synergize to inhibit cell-mediated immunity in vivo

The lack of cell-mediated (TH1-like) immunity that is often associated with strong humoral immune responses is thought to be due in part to the inhibition of Th1 effector function by the Th2-derived cytokine interleukin-10 (IL-10). This hypothesis, however, is based entirely on results from in vitro studies, wherein IL-10 has been shown to inhibit Th1 cytokine synthesis. In this study we have compared the regulatory effects of both IL-4 and IL-10 on the development of a more complex Th1 effector function in vivo, the development of delayed-type hypersensitivity (DTH) to Leishmania major in mice immune to Leishmania. The results revealed two findings unexpected from in vitro studies with Th1 clones. First, optimal inhibition of the DTH response (up to 70%), assessed by footpad swelling and leukocytic infiltration, required the combination of IL-4 and IL-10, indicating that these two activities synergized to inhibit DTH reactivity. Second, IL-4 inhibited interferon-gamma (IFN-gamma) production by lymph node cells draining the site of antigen challenge as well as did IL-10. The combination of both cytokines was no more effective than either alone. The mechanism by which IL-4 and IL-10 acted to inhibit DTH responses did not appear to be through inhibition of IFN-gamma or tumor necrosis factor production as treatment with antibodies which neutralized these activities failed to inhibit DTH responses. Inhibition of the DTH with IL-4 and IL-10 is the most effective specific regulator of DTH responses reported and the only one capable of modulating tuberculin DTH. These data establish IL-4 and IL-10 as potent inhibitors of Th1 effector function in vivo and suggest their utility in controlling deleterious Th1-mediated inflammatory responses such as occur in some infectious and autoimmune diseases.

Immunity and immunoregulation in helminth infections

Parasitic helminths continue to be a major cause of morbidity in human populations, particularly in the tropics and subtropics. The need for effective vaccines that minimize worm burdens, thus reducing associated pathology, is evident. With this goal in mind, an intense research effort is in progress to characterize immune responses to helminths, especially in the context of recent developments in our understanding of the cytokine network. The growing realization that the parasites can themselves subvert host immune responses to their own advantage makes the task of vaccine development that much harder.

Immunodeficiency-inducing retroviruses

The plethora of disease syndromes (dystrophy of various organ systems, malignancies and opportunistic infections) caused by HIV are all potentiated by the profound virus-induced immunosuppression that accompanies this infection. The mechanism of this severe immunosuppression is poorly understood and the subject is currently being pursued in studies of HIV-infected patients and in animals infected with other immunodeficiency-inducing retroviruses.

Interleukin 10 production correlates with pathology in human Leishmania donovani infections

We have found that an important Th2 cytokine, IL-10, is produced by tissues from patients acutely infected with Leishmania donovani. In all individuals tested, IL-10 mRNA production was increased in lymph nodes taken during acute disease over that observed in postacute samples. In contrast, both pre- and posttreatment lymph nodes had readily detected mRNA for IFN-gamma and IL-2. A down-regulating effect of IL-10 on leishmania-induced proliferative responses was demonstrated when Hu rIL-10 was added to cultures of PBMC from clinically cured individuals. PBMC from individuals with acute visceral leishmaniasis responded to stimulation with leishmania lysate by producing IL-10 mRNA. Simultaneously cultured PBMC collected from the same patients after successful chemotherapy produced no detectable IL-10 mRNA after leishmania antigen stimulation. Neutralizing anti-IL-10 mAb added to PBMC from patients with acute visceral leishmaniasis markedly increased the proliferative response to leishmania lysate. Finally, we observed mRNA for IL-10 and IFN-gamma concurrently in a lesion from a patient with post-kala-azar dermal leishmaniasis (PKDL). These results indicate the production of IL-10 during L. donovani infection, and suggest a role for this cytokine in the regulation of immune responsiveness during visceral leishmaniasis.

Chemically modified antigen preferentially elicits induction of Th1-like cytokine synthesis patterns in vivo

Differential activation of CD4+ T cell subsets in vivo leads to the development of qualitatively different effector responses. We identify an approach that allows selective activation of strongly Th1-dominated immune responses to protein antigens. Whereas in vivo administration of ovalbumin (OVA) induces cytokine synthesis that is neither Th1 nor Th2 dominated, administration of glutaraldehyde polymerized, high relative molecular weight OVA (OA-POL) leads to 20-fold increase in the ratio of interferon gamma (IFN-gamma)/IL-4 and IFN-gamma/IL-10 synthesis observed after short-term, antigen-mediated restimulation directly ex vivo. In contrast, concurrent in vivo administration of anti-IFN-gamma mAb and OVA or OA-POL results in marked increases in IL-4 and IL-10, and decreased IFN-gamma production, reflecting a polarization of the response towards a Th2-like pattern of cytokine synthesis. These observations may be useful in clinical settings including hypersensitivity, autoimmune diseases, and vaccine development where the ability to actively select specific patterns of cytokine gene expression would be advantageous.

Signal transduction events leading to T-cell lymphokine gene expression

The expression of T-cell derived lymphokines is regulated by signal transduction events initiated by the T-cell antigen receptor and other T-cell surface molecules. Substantial progress has been made in characterizing the signal transduction events initiated at the plasma membrane of the T cell and their targets which control lymphokine gene expression in the nucleus. This review will summarize recent progress in this area of investigation.

Switch of CD8 T cells to noncytolytic CD8-CD4- cells that make TH2 cytokines and help B cells

CD8+ T cells are a major defense against viral infections and intracellular parasites. Their production of interferon-gamma (IFN-gamma) and their cytolytic activity are key elements in the immune response to these pathogens. Mature mouse CD8+ T cells that were activated in the presence of interleukin-4 (IL-4) developed into a CD8-CD4- population that was not cytolytic and did not produce IFN-gamma. However, these CD8- cells produced large amounts of IL-4, IL-5, and IL-10 and helped activate resting B cells. Thus, CD8 effector functions are potentially diverse and could be exploited by infectious agents that switch off host protective cytolytic responses.

Macrophage activation by T cells: cognate and non-cognate signals

Both tumor necrosis factor-alpha and interferon-gamma are involved in the activation of macrophage cytocidal/cytostatic effector function. Recent studies provide evidence that, in non-septic inflammatory disease, T cells may activate macrophages primed by interferon-gamma either by providing tumor necrosis factor-alpha (in soluble or membrane-anchored form) or by inducing macrophage tumor necrosis factor-alpha production by antigen-non-specific cognate interactions. Conversely, T cells may inhibit macrophage activation by producing cytokines that inhibit either tumor necrosis factor-alpha production or interferon-gamma receptor signaling.

Cytokine regulation of T-cell function: potential for therapeutic intervention

CD4+ T cells, via the cytokines that they produce, play a pivotal role in the induction and regulation of cell-mediated and humoral immunity. Recently it has become clear that the CD4+ T-cell population is heterogeneous and that distinct CD4+ T-cell subsets, defined by their cytokine repertoire, regulate cell-mediated and humoral immune responses. Protective responses to pathogens are dependent on activation of the appropriate TH subset accompanied by its characteristic set of immune effector functions. Evidence to date suggests that the cytokines produced by the TH cells themselves are important regulators of TH subset activation and differentiation. Here, Fiona Powrie and Robert Coffman discuss how manipulation of the levels of these cytokines can be used to alter the balance of TH cell subsets and illustrate some clinical situations where this may be beneficial.

Cytokine regulation of T-cell function: potential for therapeutic intervention

CD4+ T cells, via the cytokines that they produce, play a pivotal role in the induction and regulation of cell-mediated and humoral immunity. Recently it has become clear that the CD4+ T-cell population is heterogeneous and that distinct CD4+ T-cell subsets, defined by their cytokine repertoire, regulate cell-mediated and humoral immune responses. Protective responses to pathogens are dependent on activation of the appropriate TH subset accompanied by its characteristic set of immune effector functions. Evidence to date suggests that the cytokines produced by the TH cells themselves are important regulators of TH subset activation and differentiation. Fiona Powrie and Robert Coffman discuss how manipulation of the levels of these cytokines can be used to alter the balance of TH-cell subsets and illustrate some clinical situations where this may be beneficial.

Development of TH1 CD4+ T cells through IL-12 produced by Listeria-induced macrophages

Development of the appropriate CD4+ T helper (TH) subset during an immune response is important for disease resolution. With the use of naïve, ovalbumin-specific alpha beta T cell receptor transgenic T cell, it was found that heat-killed Listeria monocytogenes induced TH1 development in vitro through macrophage production of interleukin-12 (IL-12). Moreover, inhibition of macrophage production of IL-12 may explain the ability of IL-10 to suppress TH1 development. Murine immune responses to L. monocytogenes in vivo are of the appropriate TH1 phenotype. Therefore, this regulatory pathway may have evolved to enable innate immune cells, through interactions with microbial pathogens, to direct development of specific immunity toward the appropriate TH phenotype.

T-cell dysfunction in HIV infection: anergy due to defective antigen-presenting cell function?

Before CD4+ T cells are depleted, T cells in asymptomatic HIV-infected individuals are functionally abnormal. These T cells are programmed for death, are non-responsive and fail to produce interleukin-2 after antigenic stimulation. Our view is that these different T-cell abnormalities are explained by the effects of HIV on antigen-presenting cells. Alteration of the functions of the antigen-presenting cell may program T cells for activation-induced death, and may induce anergy in interleukin-2 and interferon-gamma secreting TH1 cells. This results in predominance of TH2 allergic responses instead of cellular immunity dependent on TH1 cells.

A TH1-->TH2 switch is a critical step in the etiology of HIV infection

This viewpoint proposes that an imbalance in the TH1-type and TH2-type responses contributes to the immune dysregulation associated with HIV infection, and that resistance to HIV infection and/or progression to AIDS is dependent on a TH1-->TH2 dominance. This hypothesis is based on the authors' findings that: (1) progression to AIDS is characterized by loss of IL-2- and IFN-gamma production concomitant with increases in IL-4 and IL-10; and (2) many seronegative, HIV-exposed individuals generate strong TH1-type responses to HIV antigens.

Activation of cytokine genes in T cells during primary and secondary murine influenza pneumonia

The patterns of cytokine mRNA expression in mice with primary or secondary influenza pneumonia have been assessed by in situ hybridization analysis of cells from both the mediastinal lymph node (MLN) and the virus-infected lung. Evidence of substantial transcriptional activity was found in all lymphocyte subsets recovered from both anatomical sites. The kinetics of cytokine mRNA expression after primary infection with an H3N2 virus were in accord with the idea that the initial response occurs in regional lymphoid tissue, with the effector T cells later moving to the lung. This temporal separation was much less apparent for the more rapid secondary response resulting from challenge of H3N2-primed mice with an H1N1 virus. Among the T cell receptor alpha/beta+ subsets, transcripts for interferon (IFN) gamma and tumor necrosis factor beta were most commonly found in the CD8+ population whereas mRNA for interleukin (IL) 4 and IL-10 was much more prevalent in CD4+ T cells. The gamma/delta T cells expressed mRNA for all cytokines tested, with IL-2, IL-4, and IFN-gamma predominating among those recovered from the inflammatory exudate. At particular time points, especially early in the MLN and late in the infected lung, the frequency of mRNA+ lymphocytes was much higher than would be expected from current understanding of the prevalence of virus-specific precursors and effectors. If this response is typical, induction of cytokine gene expression for T cells that are not responding directly to the invading pathogen may be a prominent feature of acute virus infections.

Effect of anti-gamma-interferon and anti-interleukin-4 administration on the resistance of mice against infection with reticulotropic and myotropic strains of Trypanosoma cruzi

We studied the effect of in vivo administration of anti-gamma-IFN and anti-IL-4 monoclonal antibodies on the resistance of mice against myotropic and reticulotropic strains of Trypanosoma cruzi. Anti-gamma-IFN treatment augmented the susceptibility of mice when infected with the reticulotropic RA and Tulahuén strains of T. cruzi but did not alter the course of infection with the myotropic CA-I strain of the parasite. In vivo administration of anti-IL-4 enhanced the resistance of mice when infected with either Tulahuén or RA strains but did not affect the course of parasitemia when infected with CA-I. The possible biological relevance of these observations is discussed.

Interleukin 10 inhibits macrophage microbicidal activity by blocking the endogenous production of tumor necrosis factor alpha required as a costimulatory factor for interferon gamma-induced activation

Interleukin 10 (IL-10) inhibits interferon gamma-induced macrophage activation for cytotoxicity against larvae of the human parasite Schistosoma mansoni by suppressing production of the toxic effector molecule nitric oxide (NO). In this study, the mechanism of IL-10 action was identified as inhibition of endogenous tumor necrosis factor alpha (TNF-alpha) production by interferon gamma-activated macrophages. TNF-alpha appears to serve as a cofactor for interferon gamma-mediated activation, since both schistosomulum killing and NO production were inhibited by anti-TNF-alpha antibody, whereas TNF-alpha alone was unable to stimulate these macrophage functions. IL-10 blocked TNF-alpha production by interferon gamma-treated macrophages at the levels of both protein and mRNA synthesis. Addition of exogenous TNF-alpha reversed IL-10-mediated suppression of macrophage cytotoxic activity as well as NO production. Likewise, addition of a macrophage-triggering agent (bacterial lipopolysaccharide or muramyl dipeptide), which induced the production of TNF-alpha, also reversed the suppressive effect of IL-10 on cytotoxic function. In contrast to IL-10, two other cytokines, IL-4 and transforming growth factor beta, which also inhibit macrophage activation for schistosomulum killing and NO production, did not substantially suppress endogenous TNF-alpha production. These results, therefore, describe a separate pathway by which macrophage microbicidal function is inhibited by the down-regulatory cytokine IL-10.

Interleukin 10: an overview

Since the original description of interleukin-10, a wealth of information concerning its biological properties has been gathered. Studies in vitro have rapidly identified both immunostimulatory and immunosuppressive activities for IL-10. Based on these findings, in vivo studies were initiated in a variety of animal disease models to assess the importance of these activities. This review will summarize the pleiotropic properties of IL-10 and will survey current research regarding the potential of IL-10 to regulate acute and chronic inflammatory reactions.