The Blockade of Interleukin-2 During the Acute Phase of Trypanosoma cruzi Infection Reveals Its Dominant Regulatory Role

Trypanosoma cruzi infection causes Chagas’ disease in humans. The infection activates the innate and adaptative immunity in an orchestrated immune response to control parasite growth, guaranteeing host survival. Despite an effective immune response to the parasite in the acute phase, the infection progresses to a chronic stage. The parasite infects different tissues such as peripheral neurons, the brain, skeletal muscle, and heart muscle, among many others. It is evident now that tissue-specific immune responses may develop along with anti-parasite immunity. Therefore, mechanisms to regulate immunity and to ensure tissue-specific tolerance are operating during the infection. Studying those immunoregulatory mechanisms is fundamental to improve host protection or control inflammatory reactions that may lead to pathology. The role of IL-2 during T. cruzi infection is not established. IL-2 production by T cells is strongly down-modulated early in the disease by unknown mechanisms and remains low during the chronic phase of the disease. IL-2 activates NK cells, CD4, and CD8 T cells and may be necessary to immunity development. Also, the expansion and maintenance of regulatory T cells require IL-2. Thus, IL-2 may be a key cytokine involved in promoting or down-regulating immune responses, probably in a dose-dependent manner. This study blocked IL-2 during the acute T. cruzi infection by using a neutralizing monoclonal antibody. The results show that parasitemia and mortality rate was lower in animals treated with anti-IL-2. The percentages and total numbers of CD4⁺CD25⁺Foxp3⁺ T cells diminished within three weeks of infection. The numbers of splenic activated/memory CD4 and CD8 splenic T cells increased during the acute infection. T cells producing IFN-γ, TNF-α and IL-10 also augmented in anti-IL-2-treated infected mice. The IL-2 blockade also increased the numbers of inflammatory cells in the heart and skeletal muscles and the amount of IL-17 produced by heart T cells. These results suggest that IL-2 might be involved in the immune regulatory response during the acute T. cruzi infection, dampening T cell activation through the expansion/maintenance of regulatory T cells and regulating IL-17 production. Therefore, the IL-2 pathway is an attractive target for therapeutic purposes in acute and chronic phases of Chagas’ disease.

Drug Discovery for Kinetoplastid Diseases: Future Directions

Kinetoplastid parasites have caused human disease for millennia. Significant achievements have been made toward developing new treatments for leishmaniasis (particularly on the Indian subcontinent) and for human African trypanosomiasis (HAT). Moreover, the sustained decrease in the incidence of HAT has made the prospect of elimination a tantalizing reality. Despite the gains, no new chemical or biological entities to treat kinetoplastid diseases have been registered in more than three decades, and more work is needed to discover safe and effective therapies for patients with Chagas disease and leishmaniasis. Advances in tools for drug discovery and novel insights into the biology of the host-parasite interaction may provide opportunities for accelerated progress. Here, we summarize the output from a gathering of scientists and physicians who met to discuss the current status and future directions in drug discovery for kinetoplastid diseases.

Administration of a nondepleting anti-CD25 monoclonal antibody reduces disease severity in mice infected with Trypanosoma cruzi

The role of CD25+ regulatory T cells during the course of Trypanosoma cruzi infection has been previously analyzed, and the bulk of results have shown a limited role for this T cell subpopulation. In this study, we have used an IgM, nondepleting monoclonal antibody (mAb) aiming at blocking interleukin (IL)-2 activity on CD25+ T cells. The administration of this antibody 10 days before infection increased the resistance of outbred Swiss mice to the Colombian strain of T. cruzi. Anti-CD25-treated mice had lower parasitemia and augmented numbers of effector memory T cells. In addition, these animals showed higher numbers of splenic T cells secreting IFN-γ and TNF-α, both cytokines described to be involved in the resistance to T. cruzi infection. The same treatment also increased the numbers of splenic T cells that produced homeostatic and regulatory cytokines, such as IL-2 and IL-10, and CD4+CD25+ T cells. The administration of nondepleting anti-CD25 mAb at the beginning of the chronic phase, when parasites were cleared from the blood, halted the inflammatory process in the heart, without any signs of infection reactivation. These results indicate that nondepleting anti-CD25 monoclonal antibodies may be useful to treat chronic Chagas' disease.

NK1.1+ cells and T-cell activation in euthymic and thymectomized C57Bl/6 mice during acute Trypanosoma cruzi infection

Natural killer (NK) cells may provide the basis for resistance to Trypanosoma cruzi infection, because the depletion of NK1.1 cells causes high levels of parasitemia in young C57Bl/6 mice infected with T. cruzi. Indeed, NK1.1 cells have been implicated in the early production of large amounts of interferon (IFN)-gamma, an important cytokine in host resistance. The NK1.1 marker is also expressed on special subpopulations of T cells. Most NK1.1+ T cells are of thymic origin, and their constant generation may be prevented by thymectomy. This procedure, by itself, decreased parasitemia and increased resistance in young mice. However, the depletion of NK1.1+ cells by the chronic administration of a monoclonal antibody (MoAb) (PK-136) did not increase the parasitemia or mortality in thymectomized C57Bl/6 mice infected with T. cruzi (Tulahuen strain). To study the cross-talk between NK1.1+ cells and conventional T cells in this model, we examined the expression of activation/memory markers (CD45RB) on splenic CD4+ and CD8+ T cells from young euthymic or thymectomized mice with or without depletion of NK1.1+ cells and also in aged mice during acute infection. Resistance to infection correlated with the amount of CD4+ T cells that are already activated at the moment of infection, as judged by the number of splenic CD4+ T cells expressing CD45RB(-). In addition, the specific antibody response to T. cruzi antigens was precocious and an accumulation of immunoglobulin (Ig)M with little isotype switch occurred in euthymic mice depleted of NK1.1+ cells. The data presented here suggest that NK1.1+ cells have important regulatory functions in euthymic, but not in thymectomized mice infected with T. cruzi. These regulatory functions include a helper activity in the generation of effector or activated/memory T cells.

Modulation of the severity of experimental autoimmune encephalomyelitis by gammadelta T lymphocytes activated by mycobacterial antigens

Immunity to mycobacterial antigens may contribute to the maintenance of self-tolerance. Exposure of the immune system to mycobacterial antigen might well stimulate the immune system to exert control over unwanted self-reactive clones. We demonstrated that in vivo administration of Mycobacterium tuberculosis, PPD, and PPD peptide (180-196) prior to immunization with Myelin Basic Protein (MBP) led to a moderate increase of gammadelta T cells, suppression of the immune response, and reduction in the severity of Experimental Autoimmune Encephalomyelitis. The immunosuppression observed is due, at least in part, to the production of Transforming growth factor-beta (TGFbeta) by the gammadelta T lymphocytes.

Prevention of lung eosinophilic inflammation by oral tolerance

Airway inflammation plays a major role in human asthma. Increasing evidence points to a close correlation between eosinophil infiltration and allergic lung disease. A new murine model of eosinophilic lung inflammation has recently been developed; it consists of immunizing mice with small fragments of solidified hen egg white implanted (EWI) into the subcutaneous tissue. In this model, which is further characterized here, mice challenged with ovalbumin (OVA) present an intense and persistent lung eosinophilia, as well as histopathological findings that resemble human asthma. In the present work, the effect of oral tolerance on the development of allergic lung inflammation in B6 mice immunized with antigen plus adjuvant or with EWI is investigated. It was found that in mice rendered orally tolerant by previous exposure to antigen in the drinking water, the T-helper type 2 cell (Th2)-associated allergic responses in both protocols of immunization were almost completely abolished. The allergic responses were assessed by pulmonary and bone marrow eosinophilia, lung histopathology and antigen-specific IgE and IgG1 production. These findings provide the first indication that Th2-associated lung pathology can be prevented by oral tolerance.

The role of the thymus in modulating gammadelta T cell suppressor activity during experimental Trypanosoma cruzi infection

We have previously shown that splenic gammadelta T cells from young but not aged BALB/c mice possess suppressor activity in vivo and in vitro during the acute phase of Trypanosoma cruzi infection. The present work was undertaken to investigate the suppressor activity of gammadelta T cells from T. cruzi-infected euthymic or athymic mice and the role of the thymus in modulating non-adherent spleen cell suppressor activity during the acute phase of infection. Splenic gammadelta T cells from aged or athymic BALB/c mice reconstituted with total spleen cells or non-reconstituted did not exhibit suppressor activity when added to full allogeneic, mixed lymphocyte cultures. In contrast, splenic gammadelta T cells from young euthymic BALB/c mice showed suppressor activity in vitro. Thymectomy reduced the splenic gammadelta T cell suppressor activity of young BALB/c mice in a time-dependent manner, following a T. cruzi challenge. The continuous provision of thymocytes to aged mice, young thymectomized mice or total spleen cell-reconstituted athymic mice could re-establish the gammadelta T cell suppressor activity. Of particular significance was the observation that the depletion of gammadelta T cells during the acute phase of T. cruzi infection restored the capacity of these mice to mount a humoral immune response to a non-related antigen such as ovalbumin. These results indicate that gammadelta T cells of extrathymic origin cannot mediate suppression and that the thymus has a role in the regulation of suppression during the acute phase of T. cruzi infection.

Terminal maturation of resting B cells induced by macrophage factors

Mouse splenic macrophages from BALB/c nude mice (purified by plastic adherence) or cloned macrophage hybridomas stimulated with jacalin (12.5 micrograms/ml), a D-Gal binding lectin, produce one or more B-cell stimulatory factors which cause splenic B cells from BALB/c or C3H/HeJ mice to secrete immunoglobulin in a polyclonal manner as detected by reverse protein A plaque assays. Jacalin-stimulated macrophage supernatants (JacSup) activate both normal and Percoll gradient-purified small high-density (resting) B cells. Supernatants from total or resting BALB/c spleen cells cultured for 7 days in the presence of JacSup (derived from splenic BALB/c nude mice macrophages) were assayed for immunoglobulin isotypes by ELISA. Resting B cells produce only IgG3 and IgM, whereas total B cells secrete IgG3 and IgM as well as IgG1, IgG2a, IgG2b and IgA. Resting and total B cells from BALB/c nude mice are also stimulated by macrophage supernatants to secrete immunoglobulin, thus indicating that this activity is likely to be T cell independent. Moreover, jacalin-stimulated macrophage supernatants did not induce spleen cells or purified B cells to proliferate. Fractionation of factor-rich supernatants on a Sephacryl S-200 column revealed that the factor activity is located in fractions corresponding to a molecular mass of 25-27 kDa. Taken together, these results suggest that upon the action of a macrophage factor(s) resting B cells undergo terminal differentiation without proliferation in the absence of T cells.

Mouse ear spleen grafts: a model for intrasplenic immunization with minute amounts of antigen

The production of monoclonal antibodies to protein antigens which can only be obtained in tiny amounts has been a major task, since classical in vivo immunization procedures are not always efficient. In order to circumvent this problem, two methods have been developed: (1) in vitro immunization, in which the immunogen is presented directly to spleen cell cultures; (2) intrasplenic immunization, a technique in which the immunogen is deposited in the spleen tissue. The latter approach requires less laboratory work and the risk of contamination, often a problem with in vitro cultures (Nilsson and Larsson, Immunol. Today (1990) 11, 10), is greatly reduced. Here, we describe a novel method of grafting neonatal spleens in the pinna of the mouse ear. Histological and functional studies show that these spleen grafts have white and red pulp and contain normal percentages of functional T and B cells. The results indicate that this procedure is extremely efficient in priming mice for a secondary humoral immune response, since very small amounts of soluble antigen (ovalbumin) were required. The data are discussed in terms of the advantages of this new technique over current procedures for intrasplenic immunization.

Anti-gamma delta T cell antibody blocks the induction and maintenance of oral tolerance to ovalbumin in mice

The oral administration of antigens is one of the means of inducing tolerance in adult mammals. In this report, the role of gamma delta T cells in the induction and maintenance of orally-induced tolerance to ovalbumin was investigated. The injection of a monoclonal anti-gamma delta T cell monoclonal antibody blocked the induction of oral tolerance, because the secondary immune responses to ovalbumin in these animals were comparable to the corresponding responses in ovalbumin-immunized control mice. Furthermore, depletion of gamma delta T cells either in vivo or in vitro abolished already established oral-tolerance. The fact that the state of tolerance could be adoptively transferred to naive recipients by CD3+ alpha beta- gamma delta + spleen cells from tolerant mice. These results suggest that systemic oral tolerance is induced and actively maintained by mechanisms involving gamma delta T cells.

T cell-dependent antibody response to staphylococcal enterotoxin B

Treatment of mice with staphylococcal enterotoxin B (SEB) induces specific T-cell tolerance to this superantigen, characterized by partial deletion of V beta 8+ T cells in vivo and T cell anergy in vitro. In this study we examined the humoral response to SEB in BALB/c mice. Immunization of mice with SEB results in a detectable anti-SEB antibody response. Upon further treatment of mice with SEB, specific antibody levels increase significantly and the response is accelerated--characteristics of a secondary humoral response. The secondary antibody response is T cell dependent, can be transferred to T cell deficient mice with splenocytes and is composed mainly of IgM, IgG1 and IgG2b isotypes, suggesting that Th2 cells provide B cell help in this response. These data demonstrate that at the same time as inducing in vitro unresponsiveness, SEB primes SEB-specific T helper cells to provide help for B cells in a secondary antibody response.

Anti-IL-10 treatment does not block either the induction or the maintenance of orally induced tolerance to OVA

Herein, the role of IL-10 in the induction and maintenance of oral tolerance was evaluated. The results show that: (1) mice treated with MoAb anti-IL-10 are permissive to the induction of oral tolerance to OVA; (2) anti-IL-10 treatment did not reverse the in vitro blocking of T cell proliferative response found in orally-tolerized mice; and (3) orally-induced tolerance could not be broken by anti-IL-10 treatment. Taken together, these results suggest that IL-10 is not a fundamental cytokine for the establishment and maintenance of oral tolerance.

The development of humoral immunological memory to a T-cell-dependent antigen requires thymic emigrant cells

Immunological memory is embodied in the rapid and enhanced immune responsiveness to previously encountered antigens. Classically, memory would depend on the presence of small resting long-lived specific lymphocytes which, through clonal expansion after priming with antigen, would be present at higher frequencies than in naive animals. Here we report that T-cell-reconstituted athymic mice, which lack recent thymic emigrants, mount a primary response to a T-cell-dependent antigen, but do not develop memory or the capacity to produce specific anti-TNP IgG1 antibodies during the secondary immune response. On the other hand, if thymocytes are continuously provided during the secondary response, a typical secondary immune response is achieved with high levels of specific IgG1. These results lead us to propose that the development of humoral immunological memory cannot be explained solely by the long life span of primed T lymphocytes, but is rather a dynamic state dependent on the continuous presence of recent thymic emigrants and qualitative functional differences in responder T cells.

Hydroxyurea before oral antigen blocks the induction of oral tolerance

1. Treatment with hydroxyurea (HU, 1 mg/g ip, 2 doses applied 7 h apart) eliminates the majority of cells undergoing mitosis (cycling cells) without affecting non-cycling cells. Oral tolerance, induced by a single gavage with 20 mg of ovalbumin, results in a drastic inhibition of anti-Ova antibody responses in young adult mice. Oral tolerance is actively maintained by the presence of specific suppressor T cells which may adoptively transfer the tolerance to naive syngeneic recipients. Under the clonal selection hypothesis, the induction of oral tolerance should be blocked by HU treatment applied soon after oral exposure to the antigen by the elimination of specific clones of lymphocytes activated by tolerogenic presentation of the antigen. 2. However, treatment with HU initiated 3, 6 or 24 h after oral exposure to ovalbumin had no effect on the induction of oral tolerance in B6D2F1 mice. However, treatment with HU 24 h before antigen exposure, totally blocked the induction of tolerance. Treatment with HU 72 h before ovalbumin had no effect. 3. In animals treated with HU 24 h before, the adoptive transfer of normal thymus, bone marrow or spleen cells partially restored the susceptibility to the induction of oral tolerance. 4. The results suggest that cycling cells, which may be totally regenerated within 72 h after treatment with HU, and are present in normal thymus, bone marrow and spleen, are crucially important for the induction of oral tolerance.

An age-related gamma delta T cell suppressor activity correlates with the outcome of autoimmunity in experimental Trypanosoma cruzi infection

In this work the suppressive activity of splenic T cells from young and aged BALB/c mice infected with Trypanosoma cruzi were compared and correlated with the development of autoimmune myocarditis. The T cells from young adult BALB/c mice with acute T. cruzi infection exhibit suppressor activity when added to full allogeneic or Mls-disparate mixed lymphocyte cultures. This suppression could not be reverted by exogenous interleukin (IL)-2 and was not directly dependent on the presence of IL-4, IL-10 or transforming growth factor-beta. Further characterization of the T cell lineage responsible for the suppressor activity by in vitro and/or in vivo depletion with monoclonal antibody to alpha beta or gamma delta T cell receptor revealed that splenic gamma delta T cells function as suppressor lymphocytes in young T. cruzi-infected mice. In addition, these young adult BALB/c mice do not develop autoimmune myocarditis and showed a low incidence of syngeneic heart graft rejection in the early chronic phase of the infection. In contrast, T cells from acutely infected aged BALB/c mice lacked demonstrable T suppressor activity. Furthermore, these mice developed a severe autoimmune myocarditis as early as 2 months after the onset of the infection, when the majority of them reject syngeneic heart grafts. These findings suggest that a gamma delta T cell-mediated suppressor mechanism may operate in the avoidance of the breaking of tissue-specific tolerance during the acute infection. Moreover, such a mechanism is likely related to the immune system chronobiology.

An activated murine B cell lymphoma line (A-20) produces a factor-like activity which is functionally related to human natural killer cell stimulatory factor

In the present article we show that supernatants derived from lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA)-stimulated A-20 B cell lymphoma are able to induce polyclonal immunoglobulin (Ig) secretion by normal B cells in a T-cell-dependent manner. This activity could be blocked by neutralizing monoclonal antibodies against interferon-gamma, but not by monoclonal antibodies against interleukin (IL)-2, IL-4, IL-5, IL-6, IL-10 and granulocyte macrophage colony-stimulating factor (GM-CSF) or even a polyclonal antibody against tumor necrosis factor (TNF)-alpha. Furthermore, A-20 supernatants induced the production of measurable amounts of interferon-gamma by normal murine spleen cells and activates natural killer (NK) cells. Fractionation of factor-rich supernatants on a Sephacryl S-200 column revealed that the factor activity is located in the fractions corresponding to a molecular mass of 160-150 kDa and 80-70 kDa. The biological activities found in the A-20 supernatant are very similar to the ones described for the recently cloned human IL-12/NK cell stimulatory factor. These results suggest the existence of a murine analogous factor for the human IL-12 produced by A-20 B cell lymphoma.

Anti-CD4 abrogates rejection and reestablishes long-term tolerance to syngeneic newborn hearts grafted in mice chronically infected with Trypanosoma cruzi

The contribution of autoimmunity in the genesis of chronic Chagas' heart pathology is not clear. In the present study, we show that: (a) BALB/c mice chronically infected with Trypanosoma cruzi reject syngeneic newborn hearts; (b) in vivo treatment with anti-CD4 but not anti-CD8 monoclonal antibodies (mAbs) abrogates rejection; (c) CD4+ T cells from chronically infected mice proliferate in vitro to syngeneic myocardium antigens and induce heart graft destruction when injected in situ; (d) anti-CD4 treatment of chronically infected mice establishes long-term tolerance to syngeneic heart grafts; and (e) the state of tolerance is related to in vitro and in vivo unresponsiveness of the CD4+ T cells. These findings allow us to suggest that autoimmunity is the major mechanism implicated in the rejection of syngeneic heart tissues grafted into the pinna of the ear of mice chronically infected with T. cruzi. The similarity of the lesions to those found in humans suggests that autoimmunity is involved in the pathogenesis of chagasic cardiomyopathy in humans. Moreover, this could imply therapeutic strategies by reestablishing long-term tissue-specific tolerance with anti-CD4 mAb treatment, mediating anergy, or deleting the responder CD4+ T cells to heart tissue antigens.

Proliferative T cell anergy to MIs-1a does not correlate with in vivo tolerance

Intravenous or intraperitoneal priming of MIs-1b mice with cells from MIs-1a donors drastically reduces secondary in vitro proliferative responses to specific stimulation. We show here that: (i) priming leads to blast transformation of essentially all CD4+ T cells bearing V beta 6 receptors in spleen and lymph nodes, and to their marked clonal expansion; (ii) secondary in vivo (or in vitro) challenges have no effect on the state of activation and numbers of V beta 6 CD4 T cells, which, however, migrate to the site of antigenic exposure; (iii) priming results in the differentiation of specific V beta 6 CD4 T cells to effector helper activities, manifested in vivo by marked increases in the numbers of splenic plasma cells, which include terminally differentiated donor MIs-1a B cells; (iv) primed mice show accelerated 'second set' rejection of antigenic cells; and (v) MIs-1b mice, thymectomized as adults before exposure to MIs-1a cells, show immune responses that are equivalent to those of control animals. We conclude that, in this experimental system, proliferative 'anergy' does not correlate with tolerance but with memory, and relates to the determination of class in immune responses.