Malaria parasite induces tryptophan-related immune suppression in mice.

Plasmodium spp. cause the worst parasitic diseases in humans and evade host immunity in complicated ways. Activated catabolism of tryptophan in dendritic cells is thought to suppress immunity, which is mediated by an inducible rate-limiting enzyme of tryptophan catabolism, indoleamine 2,3 dioxygenase (IDO), via both tryptophan depletion and production of toxic metabolites. In various infections, including malaria, IDO is known to be activated but its biological significance is unclear; therefore, we investigated whether malaria parasites induce IDO to suppress host immune responses. We found that enzymatic activity of IDO was elevated systematically in our mouse malaria model, and was abolished by in vivo IDO inhibition with 1-methyl tryptophan. Experimental infection with Plasmodium yoelii showed that IDO inhibition slightly suppressed parasite density in association with enhanced proliferation and IFN-gamma production by CD4+ T cells in response to malaria parasites. Our observations suggest that induction of IDO is one of the immune mechanisms of malaria parasites.

A novel DNA vaccine based on ubiquitin-proteasome pathway targeting 'self'-antigens expressed in melanoma/melanocyte.

Cancer vaccine that targets 'self'-antigens expressed at high levels in tumor cells is a potentially useful immunotherapy, but immunological tolerance often defeats this strategy. Here, we describe the use of a naked DNA vaccine encoding a self tumor antigen, tyrosinase-related protein 2, to whose N-terminus ubiquitin is fused in a 'nonremovable' fashion. Unlike conventional DNA vaccines, this vaccine broke the tolerance and induced protective immunity to melanoma in C57BL/6 mice, as evaluated by tumor growth, survival rate and lung metastasis. The protective immunity was cancelled in the proteasome activator PA28alpha/beta knockout mice. Moreover, this vaccination exhibited therapeutic effects on melanoma implanted before vaccination. Our findings provide evidence for the first time that naked DNA vaccines encoding a ubiquitin-fused self-antigen preferentially induce the main effector CD8+ T cells through efficient proteolysis mediated by the ubiquitin-proteasome pathway, and lead the way to strategies aimed at targeting tissue differentiation antigens expressed by tumors.

IL-18 gene therapy develops Th1-type immune responses in Leishmania major-infected BALB/c mice: is the effect mediated by the CpG signaling TLR9?

IL-18 regulates either Th1 or Th2 responses depending on the cytokine microenvironment. Administration of recombinant IL-18 (rIL-18) alone does not promote Th1 response, but rather induces Th2 response and exacerbates Leishmania major infection in susceptible BALB/c mice. Here, we treated BALB/c mice with an IL-18-expressing plasmid by using a gene gun weekly after L. major infection. This gene therapy resulted in improved pathogenic process and preferential induction of Th1 responses by inducing the expression of IL-12 p40, but treatment with rIL-18 did not. Notably, simultaneous administration of rIL-18 with an empty plasmid vector rendered BALB/c mice resistant to the infection, despite the fact that treatment with either rIL-18 alone or the plasmid vector alone did not influence the susceptibility. The synergistic role of the vector with rIL-18 was found to depend on CpG motifs, which enhanced expression of proinflammatory cytokines, especially IL-12, from APCs through Toll-like receptor (TLR) 9 ligation. Treatment with methylated plasmid vector in which CpG was disrupted could no longer prevent the disease development in coadministration with rIL-18. Taken together, IL-18 gene therapy was shown to develop Th1-type protective immunity in L. major-infected BALB/c mice without the requirement of exogenous IL-12, probably via CpG-TLR9 signaling pathway.

Antibodies to Plasmodium vivax transmission-blocking vaccine candidate antigens Pvs25 and Pvs28 do not show synergism.

Transmission-blocking vaccines against malaria parasites target molecules expressed by sexual stage parasites to elicit antibodies that prevent the infection of the mosquito vector. Pvs25 and Pvs28, expressed on the surface of ookinetes, are potential candidates for such a vaccine and induce antibodies that block the infectivity of Plasmodium vivax in immunized animals. To improve the ability to induce transmission-blocking antibodies, Pvs25 and Pvs28 were produced as a single fusion protein by the yeast Saccharomyces cerevisiae. Mice immunized with a low dose of the chimeric molecule (Pvs25-28) developed higher antibody responses compared with mice immunized with either Pvs25 or Pvs28. In membrane feeding assays, both anti-Pvs25-28 and anti-Pvs25 antisera had similarly potent transmission-blocking activities (and both were much greater than anti-Pvs28). Furthermore, serum from mice simultaneously immunized with both Pvs25 and Pvs28, or serum mixtures of anti-Pvs25 alone and anti-Pvs28 alone did not enhance the efficacy over anti-Pvs25 serum alone, demonstrating that there is no synergism in the ability to block transmission of P. vivax between anti-Pvs25 and anti-Pvs28 antibodies.

Experimental study of combined treatment with tacrolimus and donor splenocytes via the portal vein in small bowel transplantation.

We previously reported that the combined treatment of perioperative administration of donor splenocytes via the recipient's portal vein (DSPV) and a short-course Tacrolimus significantly prolonged the survival of fully allogenic grafts in rat small bowel transplantation (SBTX). In the present study we examined whether this effect depended on the quantity of the administered alloantigens in DSPV. In addition, we examined the expression of the surface antigen on T cells of the splenocytes and the induced toleragenic factor, according to the tolerant recipients which in our previous report had shown the prolongation of allogenic transplant small bowel graft survival by the combined treatment of DSPV (1 x 10(8) donor splenocytes) and a short-course Tacrolimus. Donor splenocytes were prepared from Brown-Norway (BN (RT1n)) rat spleens for Lewis (LEW (RT1l)) recipients. The recipients (n = 10), treated with a short course of Tacrolimus (0.5 mg/kg, 0 to 3 days postoperatively) only showed graft rejection with an average of 6.3 +/- 1.0 days postoperatively. However, the combined treatment, consisting of DSPV of 1 x 10(8) donor splenocytes and a short course Tacrolimus significantly prolonged graft survival to 12.7 +/- 2.1 days (n = 12, P < 0.01). DSPV of less than 1 x 10(8) donor splenocytes (5 x 10(7) cells and 2.5 x 10(7)) could not prolong the graft or animal survival under a short-course Tacrolimus treatment. In the tolerant recipients, the CD4 and CD8 percentages of splenocytes were not significantly different from those of control rats or recipients that were treated with short-course Tacrolimus alone. Nevertheless, the percentage of Tcr-alpha beta+ cells expressing IL-2 receptor (R) was significantly lower than in either control rats or the recipients with short-course Tacrolimus. In the suppression assay to one-way mixed lymphocyte response, a toleragenic factor was suggested to the present in the serum of the tolerant recipients. In the present study, it was suggested that the effects of the combined treatment of DSPV and short-course Tacrolimus for the prolongation of graft survival in the rat allogenic SBTX should depended on the quantity of the antigens administered into the portal vein. The beneficial effects of this treatment were reflected in the suppression of IL-2R on the recipient's splenocytes, and tolerogenic factor(s) might subsequently be induced in the tolerant recipient's serum.

Granule-dependent killing of Toxoplasma gondii by CD8+ T cells.

Immunization of mice with live bradyzoites of a low-virulent Beverley strain of Toxoplasma gondii has been shown to increase CD8+ T-cell mediated immunity against a highly virulent RH strain. We found that preimmunization with an RH homogenate further enhanced this immunity. Using this model, we investigated the mechanism of CD8+ T-cell mediated protection against T. gondii infection. Splenic cells from mice immunized with RH homogenate and live bradyzoites stimulated apoptosis of RH-infected J774A.1 macrophages in vitro, and at the same time, the immunization significantly suppressed the proliferation of parasites within macrophages, as assessed by measuring 3H-uracil uptake by the parasites. Splenic cells from the immunized mice produced larger amounts of interferon-gamma (IFN-gamma) than did naive splenic cells; however, the production of nitric oxide (NO) by RH-infected macrophages was not enhanced. The elimination of CD8+ T cells from splenic cells significantly reduced their inhibitory action on parasite proliferation as well as their cytotoxic activity against RH-infected macrophages, but it did not affect the production of IFN-gamma. Treatment of CD8+ T-enriched splenic cells from the immunized mice with concanamycin A, but not an anti-Fas ligand monoclonal antibody, significantly reduced their anti-proliferative and killing capabilities, suggesting that the CD8+ T cells induced by immunization with RH antigen and live bradyzoites of the Beverley strain may exert protection against T. gondii infection at least in part through granule-dependent cytotoxic activities.

CD4+ T cells are required for HSP65 expression in host macrophages and for protection of mice infected with Plasmodium yoelii.

We have reported that macrophages expressing heat-shock protein 65 play an essential role in protection of mice infected with Plasmodium yoelii. In this study, we investigated the function and expression mechanism of HSP65 in macrophages of mice infected with P. yoelii. C57BL/6 (B6) mice are susceptible to infection with the lethal (L) strain but resistant to infection with the non-lethal (NL) strain of P. yoelii. The percentage of apoptotic macrophages in mice infected with the L strain was higher than that in mice infected with the NL strain. However, the percentage was low in L strain infected mice if they acquired resistance to the infection by primary infection with the NL strain. That apoptosis was reversely correlated with HSP65 expression in splenic macrophages from mice infected with P. yoelii suggests HSP65 may contribute to protective immunity by preventing apoptosis of macrophages in malarial infection. Cell depletion/transfer experiments showed that CD4+ T cells, but not CD8+ T cells, gammadelta T cells, NK cells or NK T cells, were required for HSP65 expression in macrophages as well as for protection of mice infected with P. yoelii. In conclusion, HSP65 may play a role in preventing apoptosis of macrophages in mice infected with P. yoelii. CD4+ T cells are required for HSP65 expression and for protective immunity against P. yoelii infection.

Treatment with cathepsin L inhibitor potentiates Th2-type immune response in Leishmania major-infected BALB/c mice.

Prior to the activation of CD4 (+) T cells, exogenous proteins must be digested by endo/lysosomal enzymes in antigen-presenting cells (APC) to produce antigenic peptides that are able to be presented on class II molecules of the MHC. Studies described here inspect the functional significance of cathepsin L inhibition for antigen processing and T (h) 1/T (h) 2 differentiation in experimental leishmaniasis. We first demonstrated using in vitro systems that cathepsin L is one of the candidate endo/lysosomal enzymes in processing of soluble Leishmania antigen (SLA) and that its specific inhibitor, CLIK148, modulated the processing of SLA. BALB/c mice are known to be susceptible to infection with Leishmania major. Interestingly, treatment of BALB/c mice with CLIK148 exacerbated the infection by enhancing the development of SLA-specific T (h) 2-type response such as production of IL-4 and generation of T (h) 2-dependent specific IgE/IgG1 antibodies. Moreover, addition of CLIK148 in incubation of a SLA-specific CD4 (+) T cell line with APC up-regulated the production of IL-4. However, CLIK148 did not exert any direct influence on the function of T cells themselves. Taken together, these findings suggest that treatment of host mice with CLIK148 affects the processing of SLA in APC, resulting in the potentiation of T (h) 2-type immune responses and thus leading to exacerbation of the infection. Furthermore, endo/lysosomal cathepsin L was found to be functionally distinct from previously described cathepsins B and D.

Splenic cathepsin L is maturated from the proform by interferon-gamma after immunization with exogenous antigens.

The processing of foreign protein antigens into peptides requires the participation of various endo/lysosomal proteases in antigen-presenting cells (APCs). In this study, a proenzyme of cathepsin L, procathepsin L, was found to be present in the spleens of naive mice, as demonstrated by immunoblotting. Interestingly, the maturation of cathepsin L from procathepsin L was strongly induced when the host BALB/c mice were immunized with ovalbumin or soluble leishmanial antigen, despite the fact that mouse albumin, a kind of self-antigen, did not have such a potential. Furthermore, foreign antigens, but not self-antigens, could increase the activity of cathepsin L, probably being mediated by interferon-gamma, as demonstrated by in vivo and in vitro experiments. As cathepsin L matured, the efficiency of antigen processing was increased in APCs. These results suggest that endo/lysosomal cathepsin L plays an important role in the immune regulation via antigen processing even in peripheral lymphoid tissues as well as in the thymus.

Antibodies specific for heat shock proteins in human and murine malaria.

Heat shock proteins (HSPs) are immunodominant antigens recognized by the host immune system in various infectious diseases. We analyzed HSP-specific antibodies, including immunoglobulin G (IgG), IgM and IgA, in sera from malaria patients in Thailand by using an enzyme-linked immunosorbent assay. All of the antibodies to HSP90 were remarkably increased in the patients compared with those in controls, while only IgM to HSP70 or IgA to HSP65 was significantly elevated. Further experiments showed that anti-HSP IgG was significantly increased in C57BL/6 mice infected with a non-lethal strain of Plasmodium yoelii, with anti-HSP90 IgG being the most elevated. These results suggest that the antigenic potential of HSP90 is higher than those of HSP70 and HSP65 in malaria infection.

Role of innate immune cells in protection against Toxoplasma gondii at inflamed site.

The intraperitoneal infection with Toxoplasma gondii (T. gondii) caused accumulation of gamma delta T, NK, NK1.1+T-like (NKT) cells at inflamed sites. To clarify the roles of these cells in protection against T. gondii at the inflamed sites, BALB/c mice were depleted of gamma delta T, NK, NK and NKT cells by treatment with antibody against TCR-gamma delta, asialoGM1 or Interleukin-2 receptor beta-chain (IL-2 R beta), respectively, prior to infection. Mice treated with anti-TCR-gamma delta monoclonal antibody (mAb) became more susceptible to infection, whereas mice treated with anti-IL-2R beta mAb acquired resistance. Treatment with anti-asialoGM1 Ab showed no effect. We previously reported that heat shock protein 65 (HSP65) in macrophages induced by gamma delta T cells plays an essential role in protective immunity against T. gondii infection, by preventing apoptotic death of infected macrophages. In the present study, we showed that treatment with anti-IL-2R beta mAb, but not with anti-asialoGM1 Ab, enhanced the HSP65 induction in macrophages, and inhibited Interleukin-4 (IL-4) expression in nonadherent peritoneal exudate cells. Furthermore, neutralization of endogenous IL-4 by anti-IL-4 mAb enhanced the HSP65 induction in macrophages. These findings suggest that NKT cells, but not NK cells, negatively regulate the protective immunity against T. gondii infection possibly by producing IL-4 and suppressing HSP65 induction.

Effect of local blood circulation and absolute torque on muscle endurance at two different knee-joint angles in humans.

The effects of the local blood circulation and absolute torque on muscle endurance at different knee-joint angles were determined. The rate of muscle deoxygenation (using near-infrared spectroscopy), and the rate of muscle fatigue (using the slope of integrated electromyography, iEMG) were evaluated concurrently. Nine healthy subjects performed submaximal (50% maximal voluntary contraction. MVC) static knee extension at 50 degrees (extended position, EXT) and 90 degrees (flexed position, FLEX) joint angles until the target torque could no longer be maintained: that time was measured as the endurance time. They exercised with the circulation occluded (OCCL), and without (FREE) to study the possible effects of the local circulation. Although MVC torque was independent of joint angle [mean (SD) FLEX 250.6 (51.7) Nm and EXT 246.5 (46.6) N x m], significantly shorter (P<0.01) endurance time in FLEX [FREE 71.1 (10) s and OCCL 63.1 (8.8) s] than at EXT [FREE 115.3 (30) s and OCCL 106.7 (29.1) s] were obtained in both circulatory conditions. The iEMG-time slope was significantly greater in FLEX at the proximal and distal portion (P<0.05) in both circulatory conditions. Muscle deoxygenation rate in OCCL was significantly greater (P<0.05) at FLEX [20.8 (8.0)%] than EXT [10.9 (4.0)%]. The results would suggest that different knee-joint angle affects muscle endurance even if the local circulation is controlled. Circulatory disturbance would further reduce muscle endurance in EXT, but not in FLEX. Because of the greater muscle internal force in FLEX, local blood flow might be already limited even with a free circulation. The greater muscle deoxygenation and muscle fatigability would be related to the shorter muscle endurance in FLEX.

Different cytokines are required for induction and maintenance of the Th2 type response in DBA/2 mice resistant to infection with Leishmania major.

Experimental cutaneous leishmaniasis is a useful model in studying the mechanism regulating immune responses between T helper type 1 (Th1) and Th2. Mice susceptible to Leishmania major infection such as BALB/c (H-2(d)) are associated with the induction of the disease-promoting Th2 response, while the resistant mice such as DBA/2 (H-2(d)) develop the protective Th1 response. To understand the induction mechanism of Th1 and Th2 responses, it is necessary to establish an immunization scheme by which the induction of each Th response can be easily and experimentally controlled. Adjuvants are known to enhance the immune responses through the combined effect of several factors: prolonged release of antigen, migration of cells, mitogenic effect and so forth. When the genetically resistant DBA/2 mice were immunized twice with soluble leishmanial antigen (SLA), emulsified in incomplete Freund's adjuvant (IFA) before L. major inoculation, these mice mounted a Th2 cell response and suffered from progressive infection. While IL-4 and IL-13 were upregulated early after the infection in both healer and non-healer groups of mice, IL-5 and IL-10 were upregulated only in non-healer mice. From these results, IL-5 and IL-10 appear to have an important role, at least in the early phases of the infection, rather than IL-4 and IL-13 in establishing the disease-promoting Th2 response in leishmaniasis. Further, IL-9 was found to be expressed in both BALB/c and DBA/2 mice immunized with IFA/SLA. This cytokine may support the establishment of a Th2 response in these mice. Therefore it is suggested that Th2 cytokines play different roles between priming and maintaining the Th2 immune response after the infection.

Antibodies to malaria vaccine candidates Pvs25 and Pvs28 completely block the ability of Plasmodium vivax to infect mosquitoes.

Transmission-blocking vaccines are one strategy for controlling malaria, whereby sexual-stage parasites are inhibited from infecting mosquitoes by human antibodies. To evaluate whether the recently cloned Plasmodium vivax proteins Pvs25 and Pvs28 are candidates for a transmission-blocking vaccine, the molecules were expressed in yeast as secreted recombinant proteins. Mice vaccinated with these proteins adsorbed to aluminum hydroxide developed strong antibody responses against the immunogens, although for Pvs28, this response was genetically restricted. Antisera against both recombinant Pvs25 and Pvs28 recognized the corresponding molecules expressed by cultured sexual-stage parasites isolated from patients with P. vivax malaria. The development of malaria parasites in mosquitoes was completely inhibited when these antisera were ingested with the infected blood meal. Pvs25 and Pvs28, expressed in Saccharomyces cerevisiae, are as yet the only fully characterized transmission-blocking vaccine candidates against P. vivax that induce such a potent antiparasite response.

Pepstatin A-sensitive aspartic proteases in lysosome are involved in degradation of the invariant chain and antigen-processing in antigen presenting cells of mice infected with Leishmania major.

We previously reported that CA074, a specific inhibitor of cathepsin B, significantly deviated immune responses from the disease-promoting Th2 type to the protective Th1 type in BALB/c mice infected with Leishmania major. Herein, we found that pepstatin A-sensitive aspartic proteases (PSAP) in lysosomes seem to play a different role from that of cathepsin B in antigen-processing and Ii-degradation. That is, cathepsin B appears to digest 16-, 28-, and 31-kDa peptides of soluble leishmania antigen (SLA), whereas PSAP seems to process mainly 28-kDa peptides. Furthermore, the latter protease contributed to the degradation of Ii but cathepsin B did not. Following treatment with pepstatin A, both Th1 and Th2 responses were profoundly suppressed in resistant DBA/2 mice (H-2(d)) and in susceptible BALB/c mice (H-2(d)), and both strains of mice became markedly susceptible compared with the untreated groups, probably owing to failure in degradation of Ii and partly to failure in digestion of 28-kDa peptide.

CD4(+) v(alpha)14 NKT cells play a crucial role in an early stage of protective immunity against infection with Leishmania major.

The roles of gamma delta T, NK and NKT cells in an early stage of protective immunity against infection with Leishmania major were investigated. Further, the contribution of these innate cells to the expression of 65 kDa heat shock protein (HSP65) in host macrophages was examined, since we found previously that this expression prevents apoptotic death of infected macrophages and is a crucial step in the acquisition of protective immunity against infection with various obligate intracellular protozoa including L. major. C57BL/6 and DBA/2 mice were found to be resistant against the infection on the basis of the parasite burden in their regional lymph nodes, and to strongly express HSP65 in their macrophages, whereas BALB/c mice were susceptible and barely expressed the HSP65. In those resistant mice, CD4(+) NKT cells prominently increased in their regional lymph node and were the main effector cells at least for an early stage of the protective immunity and for the HSP65 expression, whereas this subset did not increase in susceptible BALB/c mice. Further, neither gamma delta T nor NK cells in resistant mice contributed to those protective immune responses. The NKT cell subset bore CD3, CD4, TCR alpha beta, IL-2R beta and NK1.1 but scarcely asialo-GM(1). Moreover, this effector subset was confirmed to be V(alpha)14 NKT cells by using J(alpha)281(-/-) mice.

Gene gun-mediated delivery of an interleukin-12 expression plasmid protects against infections with the intracellular protozoan parasites Leishmania major and Trypanosoma cruzi in mice.

An interleukin-12 (IL-12) expression plasmid was transferred, using a gene gun, to mice infected with Leishmania major or Trypanosoma cruzi. Transfer of the IL-12 gene to susceptible BALB/c mice resulted in regression of lesion size and reduced the number of parasites in draining lymph nodes (LN) at the site of L. major infection. Coincident with these protective effects, the T-helper type (Th) response shifted towards Th1, as evaluated by cytokine production in vitro and L. major-specific antibody responses. Protective effects of the IL-12 gene were also observed in T. cruzi infection. Treatment of BALB/c mice infected with T. cruzi enhanced the production of interferon-gamma (IFN-gamma) by spleen cells, while suppressed production of interleukin-10 (IL-10) compared with control mice. Administration of anti-CD4 or anti-CD8 monoclonal antibody (mAb) abolished the protective immunity against T. cruzi infection, and treatment with the IL-12 gene could not restore the resistance in these mice. Mice depleted of natural killer (NK) cells with anti-asialo GM1 also became susceptible to infection, while the resistance was restored when these mice were treated with the IL-12 gene. Thus, target cells for the treatment appear to be CD4+ and CD8+ T cells, which are ordinarily activated by NK cells. These results suggest that the transfer of cytokine genes using a gene gun is an effective method for investigating the roles of cytokines and gene therapy in infectious diseases.

Lysosomal cathepsin B plays an important role in antigen processing, while cathepsin D is involved in degradation of the invariant chain inovalbumin-immunized mice.

We previously reported that CA074, a specific inhibitor of cathepsin B, modulates specific immune responses from the T helper 2 (Th2) type to Th1 type in BALB/c mice infected with Leishmania major. In the present study, we found that a similar type of immune deviation was also induced in mice immunized with ovalbumin (OVA). However, treatment of mice with pepstatin A, a specific cathepsin D inhibitor, suppressed the OVA-specific proliferation of lymphocytes and blocked the development of both Th1 and Th2 cellular responses. These inhibitors did not appear to have any direct influence in vitro on functions of naive lymphocytes. OVA antigen (47 000 MW) was digested mainly into 40 000 MW protein in vitro by lysosomal proteases from naive BALB/c mice, and its digestion was markedly inhibited by the addition of CA074, but not by addition of pepstatin A, during incubation. However, pepstatin A strongly suppressed the degradation of the major histocompatibility complex class II-associated invariant chain (Ii) molecule in vivo and in vitro. Thus, cathepsin B appears to process antigens directed to preferential activation of Th2 cells, while cathepsin D may be responsible for the degradation of Ii, the processing of which is essential in initiating the antigen-specific activation of Th1 and Th2 CD4+ T cells. These lysosomal proteases may have different functions in regulating immune responses.

Expression and role of heat-shock protein 65 (HSP65) in macrophages during Trypanosoma cruzi infection: involvement of HSP65 in prevention of apoptosis of macrophages.

The 65-kDa heat-shock protein (HSP65) is thought to play a role in host defense against infections with various microbial pathogens and in autoimmune inflammatory disorders. We investigated the biological function and expression mechanism of HSP65 in macrophages of mice infected with Trypanosoma cruzi. BALB/c mice, which are susceptible to T. cruzi, showed high levels of parasitemia, and 80% of these mice died within 42 days after the infection, whereas resistant C57BL/6 or DBA/2 mice showed low levels of transient parasitemia and all survived. HSP65 expression was correlated with resistance to T. cruzi infection; HSP65 was more strongly expressed in macrophages of resistant C57BL/6 and DBA/2 mice than in macrophages of susceptible BALB/c mice. Immunodeficient BALB/c-nu/nu (nude) and C.B-17 scid/scid (SCID) mice were shown to be highly susceptible to this infection, and they did not express detectable levels of HSP65, suggesting that T cells play essential roles in the expression of HSP65 as well as in protective immunity against the infection. CD4(+) T cells, but not CD8(+) T cells or gammadelta T cells, were the cell population responsible for the induction of HSP65 expression in macrophages. Furthermore, depletion of asialo GM-1(+) NK cells made resistant C57BL/6 mice more susceptible to the infection, and HSP65 expression in their macrophages was abolished. Semiquantitative reverse transcription PCR analyses showed that both interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) mRNA levels in CD4(+) T cells became low when resistant C57BL/6 mice were depleted of NK cells, suggesting that NK cells contribute to functional differentiation of CD4(+) T cells and thereby affect the induction of HSP65 expression. To determine the function of HSP65, macrophages were treated in vitro with antisense oligonucleotide for HSP65 prior to inducing HSP65 with IFN-gamma plus TNF-alpha or T. cruzi infection. This treatment did not affect the production of nitric oxide following activation, but the treated macrophages became susceptible to apoptosis. These results indicate that HSP65 plays a role in preventing the apoptosis of macrophages and thereby contributes to host resistance against T. cruzi infection.