Resolution of cutaneous leishmaniasis: interleukin 12 initiates a protective T helper type 1 immune response

Immunoglobulin isotype modulation after administration of IL-12

We have begun a series of experiments assessing the role of IL-12 in the humoral immune response. IL-12 is known to enhance cellular immunity causing a shift toward a Th1, as opposed to a Th2, response. IL-12 is also a potent stimulator of IFN-gamma production which, among other activities, modulates isotype expression particularly with respect to IgG2a. We have performed a series of experiments involving the concurrent dosing of mice with murine IL-12 and TNP-KLH followed by the monitoring of IgG1 and IgG2a anti-TNP responses and total IgG1 and IgG2a levels. Following administration of IL-12, specific anti-TNP titers showed an IgG2a increase while IgG1 responses were markedly lower than those exhibited by animals which did not receive IL-12. Total IgG1 levels in IL-12 treated mice remained at or near baseline while untreated mice demonstrated an increase in total IgG1 levels. In addition, lymph nodes from these mice were removed, stimulated with KLH and assayed for expression of murine IFN-gamma and IL-4. Murine IFN-gamma levels in supernatants obtained from IL-12 treated mice were elevated over those seen in untreated mice while IL-4 levels were suppressed.

Interferon gamma-independent effects of interleukin 12 administered during acute or established infection due to Leishmania major

Interleukin 12 (IL-12) is a powerful stimulus for the growth of activated T and natural killer cells, their generation of interferon gamma (IFN-gamma), and the differentiation of T helper type 1 (Th1) effector cells from naive precursors in vitro. These activities are consistent with the capacity of exogenous IL-12 to heal otherwise susceptible BALB/c mice infected with the intramacrophage parasite Leishmania major. Using this characterized model of CD4 cell subset differentiation, we examined the immunologic effects of IL-12 administered either at the time of infection, when naive T cells are primed, or after 14 days of infection, by which time CD4+ subset differentiation has occurred. Given with the inoculation of parasites, IL-12 induced IFN-gamma and IL-10 and markedly suppressed IL-4. Effects on IL-10 and IL-4 were comparable in mice with homozygous disruption of the IFN-gamma gene (IFN-gamma 0/0), and suppression of IL-4 was unchanged by administration of neutralizing anti-IL-10 antibody. Induction of IFN-gamma and IL-10 mRNA by IL-12 also occurred in infected SCID mice. Given after day 14 of infection, however, IL-12 not only induced IFN-gamma and IL-10 but also induced IL-4 in normal and IFN-gamma 0/0 mice. These data demonstrate direct effects of IL-12 independent of IFN-gamma, IL-10, and IL-4 and demonstrate that the ineffectiveness of IL-12 administered following infection with L. major correlates with resistance of differentiated Th2 cells to the IL-4-suppressing activity of IL-12.

T cell cytokine responses in persons with tuberculosis and human immunodeficiency virus infection

Tuberculosis causes more extensive and life-threatening disease in patients with HIV infection than in immunocompetent persons. To investigate the hypothesis that these severe manifestations of tuberculosis may be due to alterations in cytokine production, we evaluated cytokine patterns in HIV-infected tuberculosis patients. Upon stimulation with Mycobacterium tuberculosis in vitro, PBMC from HIV-infected tuberculosis patients had reduced proliferative and type 1 responses, compared with HIV-seronegative tuberculosis patients. The reduction in proliferative responses was independent of the CD4 cell count, but the reduced type 1 response was a direct result of CD4 cell depletion. There was no enhancement of type 2 cytokine production in HIV-infected patients, although production of IL-10 was prominent in all tuberculosis patients. In HIV-infected tuberculosis patients, M. tuberculosis-induced proliferative responses were significantly enhanced by neutralizing antibodies to IL-10 but not by antibodies to IL-4 or by recombinant IL-12. The M. tuberculosis-induced type 1 response was augmented both by antibodies to IL-10 and by recombinant IL-12. Tuberculosis in the context of HIV infection is characterized by diminished type 1 responses, probably induced by immunosuppressive cytokines produced by macrophages/monocytes, rather than by type 2 cells.

Depletion of interleukin-4 in BALB/c mice with established Leishmania major infections increases the efficacy of antimony therapy and promotes Th1-like responses

Whereas most inbred mouse strains mount a protective Th1 helper T-cell response following infection with Leishmania major, an ineffective Th2 response develops in BALB/c mice, leading to the development of disseminated, ultimately fatal disease. Interleukin-4 (IL-4) production is required for the initiation of the Th2 response, though little is known about the requirements for the long-term maintenance of this response. In order to investigate the role of the expanding parasite population on the Th2 response, mice infected for 2 weeks with L. major, which exhibited a Th2-like cytokine profile, were treated with a leishmanicidal agent (Pentostam) and/or various doses of anti-IL-4 antibody. Untreated mice, mice treated with Pentostam alone, or mice treated with 2.5 mg of anti-IL-4 antibody given at days 13 and 21 of infection developed progressive disease. However, in 8 of 10 mice treated with this dose of anti-IL-4 antibody plus Pentostam lesion development was arrested and lesions were either controlled or eventually healed. Healing was associated with the production of high levels of gamma interferon by spleen cells, and low levels of immunoglobulin E in serum compared with levels for control animals, indicating that a Th1-like response had developed in mice receiving both treatments. Thus, depletion of IL-4 only in combination with a reduction in the parasite burden allowed the expression of a Th1 response. When the dose of anti-IL-4 antibody was increased to 5 mg per injection, all mice treated with this dose of antibody, with or without Pentostam therapy, healed. However, combined therapy with Pentostam in mice treated with this dose of antibody had an additional protective effect. As expected, a Th1 response developed in mice treated with this dose of anti-IL-4 antibody with or without combined therapy with Pentostam, whereas a Th2 response developed in control mice. Thus, a significant effect on the course of disease is noted when mice with established L. major infections are treated with anti-IL-4 antibody in combination with Pentostam, suggesting that the combined effect of inhibiting IL-4 and reducing the parasite burden has a dramatic effect on the development of resistance to L. major.

Interferon-gamma and host antimicrobial defense: current and future clinical applications

In the 1980s, substantive experimental data and emerging clinical results suggested that interferon-gamma (IFN-gamma), a T-cell-derived lymphokine with broad macrophage-activating effects, had considerable potential in the treatment of nonviral infections as a host defense-enhancing antimicrobial agent. During the past 6 years, the breadth of the experimental activity with IFN-gamma against nonviral pathogens has been expanded still further, and pilot studies and formal clinical trials using IFN-gamma have been undertaken in the treatment of patients both at risk for and with active infections. Thus far, IFN-gamma has been approved for use as prophylaxis in patients with chronic granulomatous disease. However, IFN-gamma also appears effective as adjunctive therapy for at least one disseminated intracellular infection (visceral leishmaniasis), and in conjunction with conventional therapy, may benefit patients with certain forms of cutaneous leishmaniasis, disseminated Mycobacterium avium complex infection, and lepromatous leprosy. Despite a rationale for its use, IFN-gamma has not yet been tested in tuberculosis or fungal or common bacterial infections nor sufficiently examined in the prevention and/or treatment of the opportunistic infections related to acquired immunodeficiency syndrome. IFN-gamma remains a promising host defense-enhancing cytokine with still unexplored clinical potential.

Identification and induction of human keratinocyte-derived IL-12

Interleukin 12 is a heterodimeric molecule that serves as a potent co-stimulator enhancing the development of Th1 cells. As one of the classical Th1 cell-mediated responses is contact sensitivity in skin, we wondered whether IL-12 might be produced by epidermal cells and serve as a mediator of this immune response. Using a sensitive, quantitative PCR technique we demonstrate that p35 chain mRNA of IL-12 is produced constitutively by human epidermal cells, whereas p40 chain mRNA can only be detected in epidermis treated with contact allergen, but not epidermis exposed to irritants or tolerogens. Time course studies showed a dramatic induction of IL-12 p40 mRNA 4 h after in vivo allergen treatment reaching peak strength after 6 h. In cell depletion assays we show that epidermal keratinocytes are the major source of this cytokine in the epidermis. This was further supported by analysis of mRNA derived from the human keratinocyte cell line HaCat expressing IL-12 p35 and p40 mRNA upon stimulation. The presence of bioactive IL-12 in supernatants derived from allergen-stimulated epidermal cells was demonstrated by IL-12-specific bioassay. Additional evidence for the functional importance of IL-12 in primary immune reactions in skin was obtained in allogeneic proliferation assays using human haptenated epidermal cells containing Langerhans cells as APC and allogeneic CD4+ T cells as responders. Anti-IL-12 mAb inhibited the proliferation of T cells by approximately 50%. In aggregate our data demonstrate that nonlymphoid keratinocytes are capable of producing functional IL-12 and provide evidence for the functional significance of IL-12 in primary immune responses in skin.

Recombinant soluble interleukin-4 (IL-4) receptor acts as an antagonist of IL-4 in murine cutaneous Leishmaniasis

This study was performed to evaluate the soluble interleukin-4 receptor (sIL-4R) as a potential antagonist of interleukin-4 (IL-4) in an infectious disease. It is shown that antigen-triggered proliferation and cytokine secretion of Leishmania major-specific, cloned Th2 cells in vitro can be inhibited dose dependently by recombinant murine, but not control human, sIL-4R. In vivo, we found that endogenous synthesis of IL-4 mRNA is upregulated during the first week of infection, while an increase of IL-4R mRNA occurred later after infection of BALB/c mice with L. major. To interfere successfully with the IL-4 ligand-receptor interaction, we therefore chose to treat infected BALB/c mice with recombinant sIL-4R during the onset (e.g., days 0 to 7) of the immune response. Treatment with murine, but not with human, sIL-4R during the first week of infection rendered BALB/c mice clinically resistant to L. major, led to a 7- to 12-fold reduction of the parasite load in spleen and lymph nodes at 7 weeks of infection, shifted the pattern of cytokines towards a Th1 type, and provided durable resistance against reinfection. Thus, it could be demonstrated that the balance among sIL-4R, membrane-bound IL-4R, and their ligand IL-4 can be modulated in vivo, thereby modifying the antiparasitic immune response. These results suggest a therapeutic value of sIL-4R in diseases in which neutralization of IL-4 is desirable.

Differential expression of mRNA encoding interleukin-12 p35 and p40 subunits in situ

Interleukin-12 (IL-12) is a heterodimeric cytokine that plays an important role in the regulation of the immune response. For biological activity the expression of both subunits of IL-12, p35 and p40, is required. Moreover, in the mouse the p40 chain of IL-12 specifically inhibits the effects of the IL-12 heterodimer. In the present study we have analyzed by in situ hybridization the expression of the p35 and p40 mRNA in the spleens of BALB/c and mutant (SCID, nude, beige) mice, unstimulated and after in vivo stimulation with lipopolysaccharide (LPS) and with staphylococcal enterotoxin B (SEB). In unstimulated spleens of BALB/c mice p35 and p40 mRNA were only detectable in a few strongly stained single cells, p35 mRNA was expressed in addition weakly in the B cell areas. After injection of LPS or SEB, p40 mRNA was strongly induced in the T cell areas all over the spleen, whereas expression of p35 mRNA and its distribution pattern did not change. Surprisingly, most of the mRNA for p35 and p40 was localized in different areas of the spleen and was apparently produced by different cells. In macrophage-depleted spleens the increased expression of p40 mRNA in response to LPS was reduced but still detectable, demonstrating that other cells besides macrophages can up-regulate IL-12 p40 mRNA. Nude mice showed a stronger expression of p35 mRNA, SCID mice lacked the weak p35 staining of the B cell areas but showed a strong basal expression of both p35 and p40 mRNA and a focal response to LPS. The pattern of IL-12 mRNA expression in beige mice was the same as in normal mice. These data demonstrate a spatial dissociation of expression of the two chains of IL-12 and are compatible with a regulatory role of the isolated IL-12 p40 chain in vivo. In addition, they indicate that the demonstration of mRNA for both chains of IL-12 in whole tissues or cell mixtures is not necessarily indicative of functional IL-12.

Interleukin-12 regulates the proliferation of Th1, but not Th2 or Th0, clones

Our results indicate that interleukin (IL)-12 is an important costimulator of antigen-dependent proliferation of murine Th1 clones. In addition, we demonstrate that IL-10 inhibits splenic antigen-presenting cell (APC)-dependent proliferation of Th1 clones, at least in part, via down-regulation of APC-derived IL-12. Moreover, the failure of activated B cells to provide costimulation via IL-12 accounts for their inability to support optimal proliferative responses of Th1 clones. We also show that IL-12 regulates the ability of Th1 clones to respond to IL-4 and enhances their proliferation in response to IL-2, IL-7, or IL-15. In contrast, Th2 and Th0 clones appear refractory to the effects of IL-12, on antigen-dependent or growth factor-induced proliferation.

Interleukin 12 exerts a differential effect on the maturation of neonatal and adult human CD45R0- CD4 T cells

It is now recognized that IL-12 plays a predominant role in protective immunity against intracellular pathogens by promoting the development of T helper type 1 (Th1) responses. We here report the unexpected observations that IL-12 exerts differential effects on the maturation of "native" human CD4 T cells isolated from umbilical cord blood or from the blood of healthy adults. After priming in the presence of IL-12, naive cells of adult donors, defined as CD45R0- CD4+ T cells, acquire a Th1 phenotype whereas neonatal cells develop into effector cells producing high levels of IL-4 in addition to IFN-gamma. This effect of IL-12 on neonatal T cells is direct inasmuch as it is observed on highly purified CD4 T cells, however, it is not inhibited by CD8 T cells and natural killer cells. Unstimulated neonatal T cells which have been preincubated with IL-12 before the priming behave like adult T cells and acquire a Th1 phenotype after stimulation in the presence of IL-12. Given that IL-4 is a potent antagonist of Th1 responses, the finding that IL-12 promotes the maturation of neonatal T cells into IL-4 producers may explain the increased susceptibility of neonates to intracellular pathogens and should be taken into account for the development of vaccines to be used in the perinatal period.

Cytokines in the treatment of leishmaniasis: from studies of immunopathology to patient therapy

The genus Leishmania, an obligate intramacrophage parasite, causes a wide spectrum of clinical diseases. It is worldwide in distribution and causes 20 million new cases annually with an at risk population of approximately 1.5 billion persons. The most severe forms are associated with high morbidity, mortality and relapses with conventional therapy. The therapeutic issues and responses to standard and alternative therapies are reviewed. Recent developments in molecular biology and immunology methods employed in the study of leishmaniasis have defined an intricate interaction of the parasite with host immune system. Perturbation of the host immune responses may be part of the survival mechanisms of Leishmania. In murine model, the finding of T helper cells that differ by their panel of cytokines has allowed a more precise definition of immunopathogenesis of leishmaniasis. Preliminary data from leishmaniasis patients lend support to this concept of altered immunomodulation. Furthermore, the data from leishmaniasis patients lend support to this concept of altered enhancement of therapeutic response by interferon-gamma has provided a new approach for treatment of patients using recombinant cytokines and for the study of the disease. Current research for early diagnosis, alternative therapies and need for vaccines are reviewed in the context of the immunopathology of leishmaniasis.

Cytokines in the generation of immune responses to, and resolution of, virus infection

A number of immune system components contribute to defense against viral infections. Although some of these overlap in part with those contributing to resistance against non-viral agents, the major anti-viral players comprise a unique subset. In particular, natural killer cells and CD8+ cytotoxic T cells are prominent in defense against viruses. With the exception of interferon-alpha/beta, cytokine responses during viral infections have not been thoroughly characterized and are poorly understood with regard to in vivo expression and function. The availability of recombinant cytokines, assays to measure induced cytokine expression, and cytokine and cytokine receptor negative mice has made it possible to begin to characterize other factors contributing to defense and immune regulation during viral infections. Advances have been made in characterizing the expression and functions of interferon-gamma, IL-2, IL-4, IL-10, transforming growth factor-beta, and IL-12. The results thus far suggest that there are at least three different stages of immune responses to viral infections and that unique cytokine profiles are associated with each of these stages.

B7 and interleukin 12 cooperate for proliferation and interferon gamma production by mouse T helper clones that are unresponsive to B7 costimulation

We have previously shown that dendritic cells isolated after overnight culture, which can express B7 and are potent stimulators of naive T cell proliferation, are relatively poor at inducing the proliferation of a panel of murine T helper 1 (Th1) clones. Maximal stimulation of Th1 clones was achieved using unseparated splenic antigen presenting cells (APC). An explanation for these findings is provided in the present study where we show that FcR+ L cells transfected with B7 stimulate minimal proliferation of Th1 clones in response to anti-CD3 antibodies, in contrast to induction of significant proliferation of naive T cells. However, addition of interleukin 12 (IL-12) to cultures of Th1 cells stimulated with anti-CD3 and FcR+ B7 transfectants resulted in a very pronounced increase in proliferation and interferon gamma (IFN-gamma) production. Exogenous IL-12 did not affect the B7-induced proliferation of naive T cells. This showed that whereas costimulatory signals delivered via B7-CD28 interaction are sufficient to induce significant proliferation of naive T cells activated through occupancy of the T cell receptor, Th1 T cell clones require cooperative costimulation by B7 and IL-12. This costimulation was shown to be specific by inhibition of proliferation and IFN-gamma production using chimeric soluble cytolytic T lymphocyte-associated antigen 4-human IgG1Fc (CTLA4-Ig) and anti-IL-12 antibodies. Furthermore, the significant antigen specific proliferation and IFN-gamma production by Th1 clones observed when splenocytes were used as APC was almost completely abrogated using CTLA4-Ig and anti-IL-12 antibodies. Thus two costimulatory signals, B7 and IL-12, account for the ability of splenic APC to induce maximal stimulation of Th1 clones. IL-10 downregulates the expression of IL-12 by IFN-gamma-stimulated macrophages and this may account largely for t the ability of IL-10 to inhibit APC function of splenic and macrophage APC for the induction of Th1 cell proliferation and IFN-gamma production. Indeed we show that IL-12 can overcome the inhibitory effect of IL-10 for the APC-dependent induction of proliferation and IFN-gamma production by Th1 clones. These results suggest that proliferation by terminally differentiated Th1 clones, in contrast to naive T cells, requires stimulation via membrane-bound B7 and a cytokine, IL-12. It is possible that these signals may result in the activation of unresponsive T cells during an inflammatory response. IL-10, by its role in regulating such innate inflammatory responses, may thus help to maintain these T cells in an unresponsive state.

Th1-Th2 paradigm: insights from leprosy

The mechanism by which T cells and cytokines regulate immune processes in skin can be investigated by studying patients with leprosy. The disease, caused by the obligate intracellular bacterium Mycobacterium leprae, forms a spectrum. At one pole, patients with tuberculoid leprosy are able to restrict the growth of the pathogen and their skin lesions are characterized by a predominance of CD4+ T cells and type 1 cytokines including interleukin 2 and interferon gamma. At the opposite pole, patients with lepromatous leprosy are unable to contain the infection and their skin lesions are characterized by a predominance of CD8+ T cells and type 2 cytokines including interleukins 4 and 10. A key determinant of the T-cell cytokine response may be interleukin 12, which selectively favors expansion of CD4+ T cells producing interferon gamma. By understanding the factors that regulate T-cell and cytokine responses in leprosy, it should be possible to devise specific immunologic interventions in diseases of skin.

Role of cytokines in determining T-lymphocyte function

Early events in an immune response stimulate the production of cytokines that direct the subsequent development of T-helper (Th) subsets with discrete patterns of cytokine production. These events are dictated by the type of antigen/microorganism administered to a host, as well as dose and route of immunization. Bacterial stimuli activate macrophages of the innate immune response to produce IL-12 and drive Th1 development and cell-mediated immunity. Conversely, production of IL-4 early in an immune response favors a Th2 or allergic/humoral immune response. The ability of IL-4 and IL-10 to inhibit Th1 development and effector function, as well as the requirement of committed Th1 cells for co-stimulators to induce maximal IFN-gamma production, suggests that cell-mediated immunity is under strict control, probably to achieve immunity with minimum immunopathology.

Cutaneous leishmaniasis: co-ordinate expression of granzyme A and lymphokines by CD4+ T cells from susceptible mice

We have recently demonstrated that the frequency of T cells expressing granzyme A is significantly higher in skin lesions and spleens of susceptible BALB/c mice compared with resistant C57BL/6 mice infected with Leishmania major, a cause of human cutaneous leishmaniasis. In the present study, we have performed in vitro studies to characterize the subpopulation, the antigen responsiveness and the lymphokine production pattern of granzyme A-expressing T cells in L. major-infected mice. Using a limiting dilution system for functional analysis of selected T cells at the clonal level, we could show that granzyme A activity in infected BALB/c mice can be assigned to L. major-reactive CD4+ T cells secreting interleukin-2 (IL-2) and IL-4. Granzyme A production was most pronounced in the early phase of infection. On the other hand, granzyme A expression could not be detected in C57BL/6-derived T cells responding to L. major. The data support the suggestion that granzyme A is produced by L. major-responsive CD4+ T cells facilitating lesion formation and the dissemination of infection.

Infection with Leishmania major induces interleukin-12 production in vivo

Experimental infections of mice with the protozoan parasite Leishmania major provide an excellent model for defining the conditions required for generation of CD4+ Th1 and Th2 cells in vivo. Since interleukin-12 (IL-12) has been implicated in the development of Th1 cells, we investigated whether L. major stimulates IL-12 production in vitro or in vivo. Surprisingly, macrophages cultured in vitro failed to produce IL-12 following L. major infection. In contrast, lymph node cells from C3H mice infected for 2 days with L. major produced elevated levels of IL-12. In order to determine if the inability to stimulate IL-12 production was limited to in vitro infections, we infected macrophages in vivo by inoculating L. major into the peritoneal cavity. Peritoneal cells isolated 24 h later exhibited a significant increase in the number of cells producing IL-12. In addition, supernatants harvested from these cells following culture contained elevated levels of IL-12. These data indicate that L. major infection induces increased IL-12 production in mice.

Interleukin-12: a bridge between innate resistance and adaptive immunity with a role in infection and acquired immunodeficiency

Interleukin-12 (IL-12) is a disulfide-linked heterodimeric cytokine originally identified as a product of EBV-transformed B cell lines. Monocyte/macrophages are the physiologically most relevant producers of IL-12, in response to both Gram-positive and -negative bacteria, bacterial products, and intracellular parasites. Although IL-12 has an enhancing effect on the survival and growth of early hematopoietic progenitor cells, most of the IL-12 biological activity has been described on T and NK cells, on which it induces production of lymphokines, primarily IFN-gamma, enhances cytotoxic activity, and, in cooperation with other stimuli, increases proliferation. IL-12 is an inducer of development of T helper type 1 (Th-1) cells and the equilibrium between IL-12 and IL-4 is probably important for the balance in vivo between Th-1 and Th-2 responses. IL-12 has an important role in the host resistance to infection, in particular to intracellular pathogens, by activating macrophages through induction of IFN-gamma from NK and T cells and by enhancing cell-mediated immune responses, dependent on Th-1 cell development. Peripheral blood mononuclear cells from HIV-seropositive individuals are impaired in their ability to produce IL-12 in response to bacterial stimulation, and IL-12 restores in vitro some of the depressed immunological functions, suggesting that a defect in IL-12 production may have a pathogenic role in the immunodeficiency of HIV-infected individuals. Natural IL-12 appears to provide a regulatory link between innate resistance and the development of the antigen-specific adaptive immune response and the recombinant protein has therapeutic potential because of its activity against tumors and infections and its effectiveness as an adjuvant enhancing cell-mediated immunity in vaccination.

Effects of interleukin 12 on immune responses and host protection in mice infected with intestinal nematode parasites

The cytokine interleukin (IL) 12 stimulates T cell and natural killer cell production of interferon (IFN) gamma and inhibits T cell production of IL-4. We investigated the effects of IL-12 on cytokine gene expression, immunoglobulin (Ig)E, mucosal mast cell, and eosinophil responses, and the course of infection in mice inoculated with the nematode parasite Nippostrongylus brasiliensis, as well as the IFN-gamma dependence of these effects. IL-12 stimulated IFN-gamma and IL-10 gene expression during primary and secondary N. brasiliensis infections and inhibited IL-3, IL-4, IL-5, and IL-9 gene expression during primary infections but had little inhibitory effect during secondary infections. IL-12 inhibited IgE, mucosal mast cell, and blood and tissue eosinophil responses during primary infections, but only eosinophil responses during secondary infections. IL-12 enhanced adult worm survival and egg production during primary, but not secondary infections. IL-12 needed to be administered by day 4 of a primary infection to inhibit IgE and mucosal mast cell responses, and by day 6 to strongly inhibit eosinophil responses and to enhance worm survival and fecundity. Anti-IFN-gamma mAb inhibited the effects of IL-12 on IgE secretion, intestinal mucosal mastocytosis, and parasite survival and fecundity, but did not affect IL-12 inhibition of eosinophilia. These observations indicate that IL-12, if administered during the initiation of eosinophilia. These observations indicate that IL-12, if administered during the initiation of an immune response, can change the response from one that is characterized by the production of T helper (Th)2-associated cytokines to one characterized by the production of Th-1 associated cytokines. However, IL-12 treatment has less of an effect once the production of Th2-associated cytokines has become established. In addition, our results provide evidence that Th2-associated responses protect against, and/or Th1-associated responses exacerbate, nematode infections.

Interleukin 12 at the site of disease in tuberculosis

Interleukin 12 (IL-12), a heterodimeric cytokine composed of p40 and p35 chains, has potent immunologic effects in vitro. We used tuberculous pleuritis as a model to study the immunoregulatory potential of IL-12 in vivo at the site of human infectious disease. Messenger RNAs for p40 and p35 were detected in pleural fluid from six of six patients by reverse-transcription polymerase chain reaction. By using an ELISA that detected both free p40 and heterodimeric IL-12, we found that mean concentrations were 585 +/- 89 pg/ml in pleural fluid of patients with tuberculous pleuritis, which were significantly higher than those in serum of the same patients (54 +/- 36 pg/ml), or in malignant pleural effusions (123 +/- 35 pg/ml). By using an ELISA specific for heterodimeric IL-12, we found that mean concentrations in pleural fluid of patients with tuberculous pleuritis were 165 +/- 28 pg/ml and undetectable in serum of the same patients, or in malignant pleural effusions. Bioactive IL-12 was detectable in five of five supernatants of pleural fluid cells stimulated with Mycobacterium tuberculosis. Addition of anti-IL-12 antibodies suppressed proliferative responses of pleural fluid cells to M. tuberculosis by 36 +/- 7%. These data indicate that IL-12 may play a role in the human immune response to infectious agents in vivo. We hypothesize that IL-12 contributes to the antimycobacterial immune response by enhancing production of interferon-gamma, facilitating development of Th1 cells and augmenting cytotoxicity of antigen-specific T cells and natural killer cells.

Interleukin 12 induces stable priming for interferon gamma (IFN-gamma) production during differentiation of human T helper (Th) cells and transient IFN-gamma production in established Th2 cell clones

Interleukin 12 (IL-12) facilitates the generation of a T helper type 1 (Th1) response, with high interferon gamma (IFN-gamma) production, while inhibiting the generation of IL-4-producing Th2 cells in polyclonal cultures of both human and murine T cells and in vivo in the mouse. In this study, we analyzed the effect of IL-12, present during cloning of human T cells, on the cytokine profile of the clones. The culture system used allows growth of clones from virtually every T cell, and thus excludes the possibility that selection of precommitted Th cell precursors plays a role in determining characteristics of the clones. IL-12 present during the cloning procedures endowed both CD4+ and CD8+ clones with the ability to produce IFN-gamma at levels severalfold higher than those observed in clones generated in the absence of IL-12. This priming was stable because the high levels of IFN-gamma production were maintained when the clones were cultured in the absence of IL-12 for 11 d. The CD4+ and some of the CD8+ clones produced variable amounts of IL-4. Unlike IFN-gamma, IL-4 production was not significantly different in clones generated in the presence or absence of IL-12. These data suggest that IL-12 primes the clone progenitors, inducing their differentiation to high IFN-gamma-producing clones. The suppression of IL-4-producing cells observed in polyclonally generated T cells in vivo and in vitro in the presence of IL-12 is not observed in this clonal model, suggesting that the suppression depends more on positive selection of non-IL-4-producing cells than on differentiation of individual clones. However, antigen-specific established Th2 clones that were unable to produce IFN-gamma with any other inducer did produce IFN-gamma at low but significant levels when stimulated with IL-12 in combination with specific antigen or insoluble anti-CD3 antibodies. This induction of IFN-gamma gene expression was transient, because culture of the established clones with IL-12 for up to 1 wk did not convert them into IFN-gamma producers when stimulated in the absence of IL-12. These results suggest that Th clones respond to IL-12 treatment either with a stable priming for IFN-gamma production or with only a transient low level expression of the IFN-gamma gene, depending on their stage of differentiation.

T helper type 1 development of naive CD4+ T cells requires the coordinate action of interleukin-12 and interferon-gamma and is inhibited by transforming growth factor-beta

It was observed in vitro and in vivo that both interferon (IFN)-gamma and interleukin (IL)-12 can promote the development of T helper type 1 (TH1) cells. Since IL-12 was shown to be a costimulator for the production of IFN-gamma by T or natural killer (NK) cells, IL-12 might play only an indirect role in TH1 differentiation by providing IFN-gamma which represents the essential differentiation factor. Using anti-CD3 monoclonal antibody (mAb) for activation of naive CD4+ T cells in the absence of accessory cells we could demonstrate that costimulation by IFN-gamma alone results only in marginal TH1 development. Similarly, IL-12 in the absence of IFN-gamma is only a poor costimulator for inducing differentiation towards the TH1 phenotype. Our data indicate that both cytokines are required to allow optimal TH1 development and that IL-12 has a dual role, it promotes differentiation by direct costimulation of the T cells and also enhances the production of IFN-gamma which serves as a second costimulator by an autocrine mechanism. Another cytokine that was reported to favor TH1 differentiation in certain experimental systems is transforming growth factor (TGF)-beta. With naive CD4+ T cells employed in this study TGF-beta strongly inhibited the production of IFN-gamma triggered by IL-12 as well as the IL-12-induced TH1 development. When TGF-beta was combined with anti-IFN-gamma mAb for neutralization of endogenous IFN-gamma the TH1-inducing capacity of IL-12 was completely suppressed.

CD4pos, NK1.1pos T cells promptly produce interleukin 4 in response to in vivo challenge with anti-CD3

Injection of anti-CD3 antibodies causes prompt expression of interleukin (IL)-4, IL-2, and interferon gamma (IFN-gamma) mRNA among spleen cells. The optimal dose of anti-CD3 for such induction was 1.33 microgram/animal; lymphokine mRNA was first observed at 30 min, peaked at 90 min, and was undetectable (for IL-4) or had declined markedly by 4 h. Cells harvested from spleens of mice injected with anti-CD3 90 min earlier secreted IL-4, IL-2, and IFN-gamma without further stimulation. By contrast, in vitro stimulation with anti-CD3 of spleen cell suspensions or splenic fragments from noninjected donors failed to cause prompt production of IL-4 and, even after 24 h of stimulation, the amount of IL-4 produced in such cells was substantially less than that secreted within 1 h by spleen cell suspensions or splenic fragments from mice injected with anti-CD3 90 min earlier. Production of IL-4 by spleen cells from anti-CD3-injected mice was not inhibited by pretreatment with anti-IL-4 antibody or with IFN-gamma or tumor growth factor beta nor enhanced by treatment with IL-4. By contrast, CTLA-4 immunoglobulin (Ig) treatment clearly diminished IL-4 production in response to in vivo anti-CD3, indicating that cellular interactions involving CD28 (or related molecules) were important in stimulation. Cell sorting analysis indicated that the cells that produced IL-4 in response to in vivo injection of anti-CD3 were highly enriched in CD4pos cells with the phenotype leukocyte cell adhesion molecule-1 (LECAM-1)dull, CD44bright, CD45RBdull, NK1.1pos. Indeed, the small population of CD4pos, NK1.1pos cells had the great majority of the IL-4-producing activity of this population. Injection with Staphylococcal enterotoxin B also caused prompt induction of IL-4 mRNA; the cells that were principally responsible for production also had the phenotype of CD4pos, NK1.1pos. These results suggest that possibility that this rare population of T cells may be capable of secreting IL-4 at the outset of immune responses and thus may act to regulate the pattern of priming of naive T cells, by providing a source of IL-4 to favor the development of T cell helper 2-like IL-4-producing cells.

Cytokine treatment of central nervous system infection: efficacy of interleukin-12 alone and synergy with conventional antifungal therapy in experimental cryptococcosis

Cell-mediated immune responses appear to be critical in the outcome of cryptococcosis. Interleukin-12 (IL-12) was studied for its potential use as a therapeutic agent because of its stimulation of natural killer cells and gamma interferon production by stimulated T cells and natural killer cells. Gamma interferon-activated macrophages are important in host resistance against cryptococcosis. In two separate studies, male BALB/c mice were infected intravenously with Cryptococcus neoformans. In the first study, mice received either no treatment, 5.0 mg of fluconazole alone per kg of body weight per day (by gavage twice daily), or IL-12 subcutaneously at 0.01, 0.1, or 1.0 microgram/day once daily (low-dose study) alone or in combination with 5.0 mg of fluconazole per kg/day. In a second study (high dose), the dosages of IL-12 used were 1.0, 2.5, or 5.0 micrograms/day. Therapy was given for 10 consecutive days, and the number of CFU of C. neoformans remaining in various organs was quantitated 1 or 2 days after administration of the last dose. In the low-dose study, IL-12 at 0.1 or 1.0 microgram reduced the level of brain infection by approximately 10-fold (P < 0.05) and IL-12 at 1.0 or 0.1 microgram/day enhanced the efficacy of fluconazole. In liver, both the efficacy of IL-12 alone (0.01 or 0.1 microgram; P < 0.05) and enhancement of the efficacy of fluconazole (P < 0.05) were seen. No efficacy of IL-12 was seen in spleens or lungs, although spleen weights increased fourfold in mice given 1.0 microgram of IL-12 per day. In the high-dose study, all IL-12 doses alone again reduced the levels of brain infection (5- to 8-fold; P < 0.05) when the two were given in combination. No overt toxicities were observed at any dose, and overall, 1.0 microgram of IL-12 per day was found to be the optimal dosage for reducing infection in the brain. To our knowledge, this is the first demonstration of the efficacy of cytokine therapy in systemic and particularly brain infections with C. neoformans. The stimulation of cell-mediated immunity represents a new approach to therapy and can enhance suboptimal antimicrobial chemotherapy. IL-12 should be considered for further study and for clinical trials. These studies suggest that other opportunistic central nervous system pathogens should also be investigated.

Interleukin 12: a key modulator of immune function

Interleukin (IL)-12 was cloned on the basis of its ability to activate natural killer (NK) cells and promote the development of cytolytic T cells. With further understanding of its activities, IL-12 has emerged as an important cytokine, affecting both immune and hematologic functions. It has been shown to be necessary for the T cell independent induction of interferon (IFN)-gamma, critical for the initial suppression of bacterial and parasitic infection; for the development of a Th1 response, critical for effective host defense against intracellular pathogens; and for the activation of differentiated T lymphocytes of both CD4+ and CD8+ phenotype. IL-12 thus functions to activate and to link the innate and acquired immune responses. The therapeutic potential of these activities is suggested by studies in tumor and microbial models. IL-12 has suppressed tumor growth in all murine models examined. Antimicrobial activity has been demonstrated in bacterial, yeast, parasitic, and viral models of infection. In many of these models, activity has been linked to production of IFN-gamma and, in the parasite model, to development of a Th1 response. In addition to the therapeutic potential associated with IL-12 activity in these disease models, the understanding of its role in immune development and interaction with other cytokines, particularly antagonists, such as IL-4 and IL-10, has clarified and extended our understanding of immune regulation and should lead to significant developments in understanding the progression of AIDS and the development of vaccine adjuvants able to direct the immune response.

Leishmania promastigotes evade interleukin 12 (IL-12) induction by macrophages and stimulate a broad range of cytokines from CD4+ T cells during initiation of infection

Leishmania major are intramacrophage parasites whose eradication requires the induction of T helper 1 (Th1) effector cells capable of activating macrophages to a microbicidal state. Interleukin 12 (IL-12) has been recently identified as a macrophage-derived cytokine capable of mediating Th1 effector cell development, and of markedly enhancing interferon gamma (IFN-gamma) production by T cells and natural killer cells. Infection of macrophages in vitro by promastigotes of L. major caused no induction of IL-12 p40 transcripts, whereas stimulation using heat-killed Listeria or bacterial lipopolysaccharide induced readily detectable IL-12 mRNA. Using a competitor construct to quantitate a number of transcripts, a kinetic analysis of cytokine induction during the first few days of infection by L. major was performed. All strains of mice examined, including susceptible BALB/c and resistant C57BL/6, B10.D2, and C3H/HeN, had the appearance of a CD4+ population in the draining lymph nodes that contained transcripts for IL-2, IL-4, and IFN-gamma (and in some cases, IL-10) that peaked 4 d after infection. In resistant mice, the transcripts for IL-2, IL-4, and IL-10 were subsequently downregulated, whereas in susceptible BALB/c mice, these transcripts were only slightly decreased, and IL-4 continued to be reexpressed at high levels. IL-12 transcripts were first detected in vivo by 7 d after infection, consistent with induction by intracellular amastigotes. Challenge of macrophages in vitro confirmed that amastigotes, in contrast to promastigotes, induced IL-12 p40 mRNA. Reexamination of the cytokine mRNA at 4 d revealed expression of IL-13 in all strains analyzed, suggesting that IL-2 and IL-13 may mediate the IL-12-independent production of IFN-gamma during the first days after infection. Leishmania have evolved to avoid inducing IL-12 from host macrophages during transmission from the insect vector, and cause a striking induction of mRNAs for IL-2, IL-4, IL-10, and IL-13 in CD4+ T cells. Each of these activities may favor survival of the organism.

Differential effects of interleukin-12 on the development of naive mouse CD4+ T cells

The influence of interleukin (IL)-12 and IL-4 on the differentiation of naive CD4+ T cells was studied in an accessory cell-free in vitro system. Dense CD4+ T cells were purified from unimmunized mice and activated using immobilized anti-CD3 monoclonal antibodies (mAb) in the presence of IL-4, IL-12, or a combination of both cytokines, and restimulated after 6 days by re-exposure to anti-CD3-coated culture wells. T cells initially activated in the presence of IL-4 produced substantial amounts of IL-4 and trace amounts of interferon (IFN)-gamma after restimulation at day 6 with plate-bound anti-CD3 mAb. By contrast, T cells primed in the presence of IL-12 produced high levels of IFN-gamma and only minimal amounts of IL-4, thus indicating that IL-12 and IL-4 by acting directly on stimulated naive CD4+ T cells support the development of TH1 and TH2 cells, respectively. When naive CD4+ T cells were stimulated in the presence of IL-12 together with IL-4 in comparable concentrations, the effect of IL-12 on TH1 differentiation was largely inhibited by IL-4. On the other hand, IL-12 exerted no inhibitory effect on IL-4-induced TH2 differentiation but rather enhanced the production of IL-4 after restimulation of the respective T cells. Decreasing amounts of IL-4 in combination with a high level of IL-12 led to an increasing production of IFN-gamma by the emerging T cells and, simultaneously, to a relatively high production of IL-4. These data were confirmed by time-course experiments which revealed that the delayed addition of IL-4 to IL-12-primed T cell cultures resulted in a gradual restoration of IFN-gamma production whereas in parallel the secretion of IL-4 was not reduced over a wide period of delay (6-72 h). These results, therefore, demonstrate that (a) IL-4 dominates the effect of IL-12, (b) IL-12 promotes the development of TH1 cells; however, in the presence of IL-12 and relatively high levels of IL-4 also the development of TH2-like cells is slightly but significantly enhanced by IL-12, and (c) high amounts of IL-12 in combination with relatively low levels of IL-4 give rise to a T cell population that upon rechallenge exhibited a cytokine profile resembling that of TH0 cells.

The adjuvant effect of interleukin-12 in a vaccine against Leishmania major

Protection induced by vaccination depends on the capacity of the vaccine to elicit an appropriate immune response. In leishmaniasis, protection requires leishmanial-specific CD4+ T helper (TH) cells. Vaccination of BALB/c mice with leishmanial antigens and interleukin-12 (IL-12) promoted the development of leishmanial-specific CD4+ TH1 cells. These mice were resistant to subsequent infection with Leishmania major. Thus, IL-12 is an effective adjuvant for the initiation of protective cell-mediated immunity against leishmaniasis and may be an important component in other vaccines that need to induce cell-mediated immunity.

Interleukin 12 and the regulation of CD4+ T-cell subset responses during murine Leishmaniasis

Interleukin 12 is unique among cytokines in that it is capable of protecting genetically susceptible mice against progressive infection with Leishmania major. Because of the probable causal relationships between CD4(+) T-cell differentiation, cytokine production and disease outcome, this cytokine may prove useful as a component of cytokine-bosed therapies and Thl-selective vaccines, as discussed here by Frederick Heinzel.

Parasites and T helper cell development: Some insights

Five years after the initial observations implicating the T helper (Th)-cell dichotomy (Th1/Th2) as the focal point in the immunoregulation of murine infection with Leishmania major, investigation has shifted to the factors that govern the differentiation of a specific immune response from its pre-immune of undifferentiated state. In this article, Steven Reiner focuses on the most recent advances concerning the lineage commitment of mature Th-cell populations, showing how new techniques [such as polymerase chain reaction (PCR) and transgenic mice] have allowed for a more-careful dissection of the early evolution of an immune response.

Restoration of HIV-specific cell-mediated immune responses by interleukin-12 in vitro

Peripheral blood mononuclear cells (PBMCs) from many asymptomatic individuals infected with human immunodeficiency virus-type 1 (HIV) are unresponsive as measured by in vitro T cell proliferation and interleukin-2 (IL-2) production to influenza virus and synthetic peptides of HIV envelope (Env). Strong influenza virus- and Env-stimulated IL-2 responses and T cell proliferation were restored when cultures were stimulated in the presence of IL-12. Interferon-gamma production by PBMCs from HIV seropositive (HIV+) patients was also restored with IL-12. Furthermore, in vitro antigen-specific production of IL-2 and proliferation of PBMCs from HIV- donors were suppressed by antibody to IL-12, but were not enhanced by addition of exogenous IL-12. Thus, IL-12 may be limiting in PBMCs from HIV+ but not HIV- individuals. These findings demonstrate that IL-12 can restore HIV-specific cell-mediated immunity in vitro in HIV-infected individuals and suggest a potential use of IL-12 in augmenting the diminished immunologic functions associated with HIV infection.

The relationship of IL-4- and IFN gamma-producing T cells studied by lineage ablation of IL-4-producing cells

Subsets of CD4 T cells are defined by the cytokines that they produce; these cytokines determine the effector function of these cells. Cloned CD4 T cells fall into two subsets, producing either interferon-gamma (IFN gamma) or interleukin-4 (IL-4) in combination with other cytokines, and are called Th1 and Th2 cells, respectively. The lineage relationship between naive T cells and effector Th1- and Th2-type cells is unclear. We generated transgenic mice in which IL-4-producing cells express herpes simplex virus 1 thymidine kinase and are eliminated by ganciclovir (GANC). Activation of transgenic T cells in the presence of GANC eliminates IL-4 and IFN gamma production, showing that IL-4- and IFN gamma-producing cells express or have expressed IL-4. These results show that effector cells producing either IL-4 or IFN gamma have a common precursor, which expresses the IL-4 gene.

Multifactorial nature of human immunodeficiency virus disease: implications for therapy

The immunopathogenic mechanisms underlying human immunodeficiency virus (HIV) disease are extremely complex; the disease process is multifactorial with multiple overlapping phases. Viral burden is substantial and viral replication occurs throughout the entire course of HIV infection. Inappropriate immune activation and elevated secretion of certain cytokines compound the pathogenic process. Profound immunosuppression ultimately occurs together with a disruption of the microenvironment of the immune system, which is probably unable to regenerate spontaneously. Thus, therapeutic strategies in HIV disease must not be unidimensional, but rather must be linked to the complex pathogenic components of the disease and must address where feasible each of the recognized pathogenic processes for the possibility of therapeutic intervention.

Interleukin 12 acts directly on CD4+ T cells to enhance priming for interferon gamma production and diminishes interleukin 4 inhibition of such priming

Naive CD4+ T cells produce interleukin 2 (IL-2) but little IL-4 or interferon gamma (IFN-gamma). In vitro, they develop into IL-4 or IFN-gamma producers depending on the conditions of the priming culture. Using T-cell receptor transgenic CD4+ T cells, the role of IL-12 and IL-4 in antigen-specific priming was examined. IL-12 substantially enhanced the ability of naive CD4+ T cells to develop into cells that produced IFN-gamma upon restimulation. However, it was not essential since anti-IL-12 antibodies failed to block the priming for IFN-gamma observed in the absence of exogenous IL-12. When both IL-12 and IL-4 were present in the priming culture, IL-12 did not inhibit priming for IL-4 production. In contrast, IL-4 diminished but did not abolish priming for IFN-gamma production. In an accessory cell-independent priming system, IL-12 strikingly augmented priming for IFN-gamma production, indicating that it acts directly on T cells. IFN-gamma itself did not enhance priming for IFN-gamma production in either accessory cell-dependent or independent systems. In an accessory cell-dependent system, the IL-12-mediated enhancement was not blocked by adding neutralizing anti-IFN-gamma monoclonal antibody. However, in an accessory cell-independent system, anti-IFN-gamma antibody did inhibit priming for IFN-gamma production leaving open a role for IFN-gamma in the priming process. These data indicate that IL-12 has a major effect on the inductive phase of T-cell priming by enhancing commitment to IFN-gamma production and thus can profoundly influence the state of immunity that develops.

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 and cytokine responses in leishmaniasis

Intracellular pathogens, particularly those that inhabit lymphocytes and macrophages, represent unique challenges to the immune system. Leishmania are protozoan parasites that replicate exclusively in macrophages and are thus in an excellent position to influence lymphocyte responses. T cell responses are critical in determining the outcome of infections with Leishmania. In both murine and human infections, strong T helper type 2 responses have been associated with disease, and a decreased or absent T helper type 2 response has been associated with healing.