Experimental onchocerciasis in chimpanzees. Antibody response and antigen recognition after primary infection with Onchocerca volvulus.

Nine of 18 chimpanzees inoculated with 250 infective third-stage larvae (L3) each developed patent (i.e., positive for microfilariae) Onchocerca volvulus infection. Four of 6 infected chimpanzees that received 200 micrograms/kg ivermectin at 28 days postinfection (pi) became patent, whereas, when ivermectin was given concurrently with L3 challenge only 1 of 6 infected animals developed patent infection. The antibody response to O. volvulus adult worm-derived antigens (OvAg) showed clear differences between patent and nonpatent chimpanzees. Three months pi, all sera detected several OvAg in the range of M(r) 35-120 k. Sera collected 6 mo pi from later patent animals recognized increasing numbers of OvAg, especially in the lower MW range of M(r) 13 to 33 k. Beginning 10 months pi Onchocerca-antigens of M(r) 21, 24, 26, and 28 k were detected only by patent chimpanzee's sera. The antibody response in nonpatent chimpanzees consistently recognized fewer OvAg, most of which were limited to the higher M(r) range (35-120 k). The reactivity of sera from infected chimpanzees to a low molecular weight fraction (LMW) of total OvAg doubled within 6 months pi, and increased continuously in patent animals from 13 until 30 months pi. Serological reactivity of nonpatent animals to LMW-OvAg remained low. The titers of circulating IgG directed against total OvAg increased in all infected chimpanzees, and continued to rise with patency. In nonpatent chimpanzees the antibody production gradually returned to preinfection values. Total and OvAg-specific IgE increased in patent and nonpatent chimpanzees. Also, during prepatency the granulocyte and antibody-mediated in vitro killing of microfilariae of O. volvulus increased in subsequently patent chimpanzees. The in vitro immobilization of L3 remained low.     

Transmission intensity determines lymphocyte responsiveness and cytokine bias in human lymphatic filariasis

Humans living in areas where filariasis is endemic vary greatly in their exposure to mosquito-borne infective third-stage larvae (L3) of these parasitic helminths. Because the intensity of exposure to Ags affects T cell differentiation and susceptibility to parasitic infections in murine models, we compared T cell and cytokine responses in 97 residents of two villages in Papua New Guinea, where transmission intensity of Wuchereria bancrofti differed by 63-fold (37 vs 2355 L3 per person per year). Residents of the high transmission village had 4- to 11-fold lower proliferation and IFN-gamma responses to filarial Ags, nonparasite Ag, and PHA by PBMC compared with the low transmission village (p < 0.01) even when subjects were matched for intensity of infection. In contrast, filarial Ag-driven IL-5 production was 5.5-fold greater (p < 0.001), and plasma IL-4 and TGF-beta levels were 4-fold and 34% higher, respectively, in residents of the high transmission village. IL-4 and IL-10 responses by PBMC differed little according to village, and increased production of the counterregulatory cytokines IL-10 or TGF-beta by PBMC did not correlate with weak proliferation and IFN-gamma responses. Plasma IL-5, IFN-gamma, and IL-10 levels were similar in the two villages. These data demonstrate that the intensity of exposure to L3 affects lymphocyte responsiveness and cytokine bias possibly by a mechanism that alters APC function.     

Early stage-specific immune responses in primary experimental human hookworm infection

To determine the cellular immune response during different stages of hookworm infection, we infected two human volunteers with Necator americanus and followed their immune responses against stage-specific, crude antigen extracts through larval migration, pre-patency, and early patency. After chemotherapy, the volunteers were followed for an additional 6 months. Low-dose N. americanus infection resulted in mild clinical signs and peripheral blood eosinophilia occurred during the estimated time of arrival of juvenile worms in the intestine. After an initial rise in proliferative responses against larval and adult worm antigens, the cellular reactivity decreased until the end of pre-patency, rose again during patency, and dropped after chemotherapy. Before infection and during the course of infection, elevated concentrations of TNF-alpha were detected in supernatants of peripheral blood mononuclear cells (PBMC) stimulated in vitro with third-stage (L3) or adult worm excretory-secretory (ES) antigens, which dropped off after chemotherapy. After stimulation with L3 antigen, elevated concentrations of CCL17 were detected in supernatants during pre-patency and patency. Interestingly, a prominent rise in IL-10 secretion was detected in ES antigen-stimulated PBMC cultures during late pre-patency. During reinfection studies in endemic areas, such distinct cytokine and chemokine profiles might be additional markers to better classify egg-negative patients.     

Onchocerca volvulus: comparative analysis of antibody responses to recombinant antigens in two animal models of onchocerciasis

Experimental infections of chimpanzees with Onchocerca volvulus and cattle with Onchocerca ochengi provide model systems for research in human onchocerciasis. These infections share many similarities from the standpoint of parasite biology, but little is known about the comparability of immune responses in the two systems. To make a direct comparison between the models in terms of immune responsiveness to defined parasite products, three recombinant antigens of O. volvulus (Ov7, Ov103, and B20) were used to analyze the kinetics of antibody production following experimental infection. Each of the antigens was derived from adult cDNA libraries following immunoscreening with sera from chimpanzees (Ov7, Ov103) or cattle (B20). All chimpanzees (n = 12) and cattle (n = 8) displayed responses to Ov7 and Ov103, and all cattle, but only 33% of chimpanzees, showed responses to B20. The dynamics of the response to individual antigens showed further similarities between the chimpanzees and the cattle, with responses to Ov7 and Ov103 peaking after, and B20 before, the onset of patent infections. We conclude that there is good preliminary evidence of concordance in the kinetics of serological responses in the two models. However, individual antigens many be more or less immunogenic in the two systems, making it inadvisable to extrapolate between models concerning the relative immunodominance of specific parasite products.     

Onchocerca volvulus: immunization of chimpanzees with X-irradiated third-stage (L3) larvae.

To provide a theoretical basis for the potential development of vaccines against Onchocerca volvulus (Ov) a trial has been conducted to assess the protective efficacy of immunization of chimpanzees with X-irradiated L3 larvae. Approximately 1000 larvae were injected at 0, 1, and 7 months. The immunized animals, and unimmunized controls, were then challenged with 250 live L3. In order to provide possibly protective exposure to the immunologically distinct L4 epicuticle, a radiation dose (45 krad) was chosen which preserved about 50% of the molting ability of unirradiated larvae. Despite the presence of a strong immune response to crude adult worm extracts, and to cloned Ov antigens, at the time of challenge little or no significant protection against patent infection was observed: three of four immunized animals developed patent infection as compared to four of four controls. One immunized animal failed to become patent or to manifest the late antibody response to adult worm antigens seen in both subpatent and patent infections in this model, and may have been protected from infection. The implications of these studies for future attempts to immunize against O. volvulus are discussed.     

Cell-mediated immune responses in human infection with Onchocerca volvulus

Mechanisms involved in modulation of the immune response in persons with chronic Onchocerca volvulus infection are poorly understood. In this study in vitro reactivity of PBMC to O. volvulus antigen (Ovag), streptolysin O (SL-O) and the mitogen PHA was tested in 62 infected individuals (INF), 17 persons living in the endemic area with exposure to the infection, but with no detectable infection (END), and 7 healthy controls (CTRL) in Liberia, West Africa. Mean blastogenic responses to Ovag were minimal and did not differ between the groups. There was, however, heterogenous reactivity to Ovag in the INF and END. For example, individuals with a history of therapy, and half of those less than 17 yr old who were tested, showed high responses. No significant differences in the response to SL-O or PHA were detected between the groups. IL-2 production in response to Ovag was minimal in the majority of infected subjects. Exogenous IL-2 was found to cause a significant increase in mean responses to Ovag and SL-O in INF and END only. Similarly, Ovag did not stimulate IL-1 production in most INF, whereas stimulation with LPS led to significantly greater production of IL-1. Depletion of plastic and nylon wool adherent cells did not increase responses to parasite-related antigen in INF, END or CTRL; however, responses to SL-O were augmented in INF, an effect that was also observed in CTRL. Finally, depletion of CD8 or CD16 cells in INF by C lysis did not increase blastogenic responses. These results indicate that cell-mediated immunity to parasite-related Ag as reflected in lymphocyte responses in vitro is diminished in infected individuals, and that this may be caused by defects in T cell activation.     

Reduction in the number of UCHL-1+ cells and IL-2 production in the peripheral blood of patients with visceral leishmaniasis.

PBMC from patients with visceral leishmaniasis (VL), before and after successful antimony therapy, were analyzed for their phenotypes and for their ability to produce IL-2 and IFN-gamma and to proliferate against PHA and leishmanial Ag. In agreement with results of earlier studies, PBMC from active VL patients showed a markedly reduced proliferative response and IL-2 and IFN-gamma production, compared with those of healthy controls. The levels of CD4+ and CD8+ T cells were within the normal range, but there was a significant decrease in UCHL-1+ cells (helper-inducer), compared with healthy individuals. The inhibited cellular responses, and lymphokine secretion and decreased level of UCHL-1+ cells in the PBMC of the VL patients returned to the normal range after successful chemotherapy. PBMC from active VL patients were fractionated into adherent cells and nonadherent cells, and the non-adherent were further fractionated into UCHL-1+ and UCHL-1- subpopulations. Results from cell depletion and reconstitution experiments suggest that the IL-2 production by nonadherent cells stimulated with PHA was inhibited by adherent cells, but the IL-2 production by nonadherent cells in response to specific Ag was not. In contrast, UCHL-1- cells seem to mediate the inhibition of Ag-driven IL-2 production by nonadherent cells but not mitogen-stimulated IL-2 secretion by nonadherent cells. Ag-specific IL-2 production principally involves UCHL-1+ cells.     

Brugia malayi: intravenous injection of microfilariae in ferrets as an experimental method for occult filariasis

Microfilaremia, immune responses, and pathology were compared in ferrets infected with 100 third-stage larvae of Brugia malayi (subperiodic strain) or injected intravenously with 10(6) microfilariae. Ferrets (Mustela putorius furo) inoculated with third-stage larvae typically became patent during the third month after infection, with a mean patency of 123 +/- 25 (SE) days. Ferrets injected intravenously with microfilariae exhibited a relatively constant microfilaremia for 3-4 weeks and usually cleared microfilariae before the fourth month. Ferrets that cleared microfilariae after intravenous injection of microfilariae or after infection with third-stage larvae failed to become patent or became amicrofilaremic within 3 weeks after a challenge intravenous injection of 10(6) microfilariae. Clearance of circulating microfilariae was associated with eosinophilia and serum antibody specific for the microfilarial sheath in ferrets injected with microfilariae and in most ferrets infected with third-stage larvae. Ferrets infected with third-stage larvae and necropsied after clearance of microfilariae had tissue inflammatory reactions to microfilariae characteristic of occult filariasis (tropical eosinophilia) in man; these ferrets exhibited immediate cutaneous hypersensitivity and circulating reaginic antibody to antigens of microfilariae. In ferrets necropsied following two intravenous injections of microfilariae, the majority of ferrets examined within 10 days after clearance of microfilariae had visible liver lesions to microfilariae identical to those of the ferrets infected with third-stage larvae; immediate cutaneous hypersensitivity and reaginic antibody were not consistently detected in ferrets injected with microfilariae. Sera from ferrets that had cleared circulating microfilariae were transferred passively into ferrets made microfilaremic by intravenous injection of microfilariae. Sera with microfilarial sheath-reactive IgG antibody titers (greater than or equal to 1:200) and microfilarial agglutination titers (greater than or equal to 1:40) rapidly cleared injected microfilariae (less than 24 hr); this serum also cleared or greatly reduced circulating microfilariae established by an infection with third-stage larvae; only the IgG-containing fraction of the sera was active in immune clearance. Sera that cleared microfilariae of B. malayi did not clear circulating microfilariae of Dirofilaria immitis or prevent recurrence of circulating microfilariae of B. malayi in ferrets infected with adult filariae.(ABSTRACT TRUNCATED AT 400 WORDS)     

IL-12, IFN-gamma, and T cell proliferation to measles in immunized infants

Measles infection in infants is associated with severe complications, and secondary infections are attributed to generalized immunosuppression. Measles binding to its monocyte receptor down-regulates IL-12 which is expected to diminish Th1-like cytokine responses, including IFN-gamma. Whether young infants can be immunized effectively against measles is an important public health issue. We evaluated Ag-specific IL-12, IFN-gamma, and T cell responses of infants at 6 (n = 60), 9 (n = 46), or 12 mo (n = 56) of age and 29 vaccinated adults. IL-12 and IFN-gamma release by PBMC stimulated with measles Ag increased significantly after measles immunization in infants. IL-12 and IFN-gamma concentrations were equivalent in younger and older infants, but IL-12 concentrations were significantly lower in infants than in adults (p = 0.04). IL-12 production by monocytes was down-regulated by measles; the addition of recombinant human IL-12 enhanced IFN-gamma production by PBMC stimulated with measles Ag, but infant T cells released significantly less IFN-gamma than adult T cells under this condition. Of particular interest, the presence of passive Abs to measles had no effect on the specific T cell proliferation or IFN-gamma production after measles stimulation. Cellular immunity to measles infection and vaccination may be limited in infants compared with adults as a result of less effective IFN-gamma and IL-12 production in response to measles Ags. These effects were not exaggerated in younger infants compared with effects in infants who were immunized at 12 mo. In summary, infant T cells were primed with measles Ag despite the presence of passive Abs, but their adaptive immune responses were limited compared with those of adults.     

Defective production of and response to IL-2 in acute human falciparum malaria

Patients with acute Plasmodium falciparum malaria have defective cell-mediated immune responses to malaria-specific Ag (MA). This immunologic defect may partially explain the difficulty with which natural immunity to falciparum malaria develops and may have important implications for the efficacy of potential malaria vaccines in endemic areas. To investigate the basis of this immune defect, we have examined the capacity of PBMC from patients with acute falciparum malaria to produce IL-2 and to express I1-2R in response to Ag stimulation. The effect of exogenous IL-1 and IL-2 on lymphocyte proliferation was studied. Soluble IL-2R levels were measured in acute and convalescent sera. Our results showed that no detectable IL-2 was produced and no IL-2R were expressed by PBMC in response to MA during the acute infection. IL-2 production and IL-2R expression were also depressed when PBMC were exposed to streptococcal Ag. The specific immune defect was not reconstituted by the addition of graded doses of purified human IL-1 or IL-2 and could not be attributed to suppressor adherent cells. In contrast to the absence of IL-2 and cell-bound IL-2R, circulating soluble IL-2R was elevated in acute sera. These findings suggest that the lack of IL-2, through either a defect in its production or inhibition of its activity, may be the basis of the Ag-specific immune unresponsiveness in acute P. falciparum malaria.     

CD40 ligand trimer and IL-12 enhance peripheral blood mononuclear cells and CD4+ T cell proliferation and production of IFN-gamma in response to p24 antigen in HIV-infected individuals: potential contribution of anergy to HIV-specific unresponsiveness

It has been suggested that CD4+ T cell proliferative responses to HIV p24 Ag may be important in the control of HIV infection. However, these responses are minimal or absent in many HIV-infected individuals. Furthermore, while in vitro and in vivo responses to non-HIV recall Ags improve upon administration of highly active antiretroviral therapy, there does not appear to be a commensurate enhancement of HIV-specific immune responses. It is possible that CD4+ p24-specific T cells are deleted early in the course of infection. However, it is also possible that a discrete unresponsiveness, or anergy, contributes to the lack of proliferation to p24. To evaluate the possible contribution of unresponsiveness to the lack of CD4+ T cell proliferation to p24 in HIV-infected individuals, we attempted to overcome unresponsiveness. CD40 ligand trimer (CD40LT) and IL-12 significantly increased PBMC and CD4+ T cell proliferative responses to p24 Ag in HIV-infected, but not uninfected, individuals. No increase in proliferative response to CMV Ag was observed. CD40LT exerted its effect through B7-CD28-dependent and IL-12- and IL-15-independent mechanisms. Finally, the increase in proliferation with CD40LT and IL-12 was associated with an augmented production of IFN-gamma in most, but not all, individuals. These data suggest the possible contribution of HIV-specific unresponsiveness to the lack of CD4+ T cell proliferation to p24 Ag in HIV-infected individuals and that clonal deletion alone does not explain this phenomenon. They also indicate the potential for CD40LT and IL-12 as immune-based therapies for HIV infection.     

Proinflammatory cytokines in bovine colostrum potentiate the mitogenic response of peripheral blood mononuclear cells from newborn calves through IL-2 and CD25 expression

Abstract: Bovine colostrum contains high concentrations of cytokines, and colostral cytokines are considered to be an important factor in stimulation of maturation of the immune system in newborns. In this study, 5 proinflammatory cytokines (IL-1beta, IL-6, TNF-alpha, IFN-gamma and IL-1 receptor antagonist, IL-1ra) present in colostrum were tested for their potential to enhance mitogenic response and to elicit expression of IL-2 mRNA and CD25 in peripheral blood mononuclear cells (PBMC) from newborn calves before being fed colostrum. PBMC were pretreated with each recombinant bovine cytokine for 2 hr before stimulation with concanavalin A (ConA). Pretreatment of PBMC from newborn calves with IL-1beta, TNF-alpha or IFN-gamma significantly enhanced the ConA response, whereas IL-1ra inhibited the response. The degree of enhancement or inhibition of mitogenic response by these cytokines was more pronounced in PBMC from newborn calves than in those from adult cows. Although IL-2 mRNA expression in ConA-stimulated PBMC from newborn calves was weaker than that in those from adult cows of ConA-stimulated controls, the expression levels became comparable after pretreatment with IL-1beta, TNF-alpha or IFN-gamma. The CD25 expression in PBMC from newborn calves was also enhanced by pretreatment with IL-1beta, TNF-beta and IFN-gamma. These results suggest that pretreatment of neonatal PBMC with IL-1beta, TNF-alpha or IFN-gamma promotes mitogenic response to ConA through up-regulating the production of IL-2 and the expression of the mature IL-2 receptor.     

Role of IL-15 on monocytic resistance to human herpesvirus 6 infection

Interleukin-15 (IL-15) is a cytokine that possesses a variety of biological functions, including stimulation and maintenance of cellular immune responses. Recently, it has been demonstrated that Human Herpes virus type 6 (HHV-6) enhances NK activity of human PBMC by inducing IL-15. HHV-6 is a typical immunosuppressive agent, as suggested by its tropism for both CD4+ and CD8+ T cells, B cells, monocytes/macrophages, megakaryocytes and NK cells. Moreover, several studies have indicated that mononuclear phagocyte resistance to virus infection is influenced by the cellular differentiation state. This paper describes the effect of pretreatment "in vitro" with IL-15 on the resistance of human monocytes (HM) to HHV-6 infection. Our results demonstrate that undifferentiated HM were highly resistant to HHV-6 infection, whereas HM pretreated with human recombinant IL-15 showed an increased permissiveness for HHV-6 infection. This permissiveness was characterised by higher release of extracellular virus as well as an increased percentage of antigen positive cells. Moreover, we evaluated IL-15 production after the addition of HHV-6 to monocytes precultured in different experimental conditions. Our data indicate that HHV-6-induced IL-15 production by human monocytes is not affected by the condition of "in vitro" precultivation/differentiation. Furthermore, the neutralization of IL-15 induced by HHV-6 in differentiated monocytes did not affect viral replication. These findings suggest that IL-15 acts only on the mechanisms of cellular differentiation, rendering HM more susceptible to HHV-6 infection, without interfering with virus replication.     

Comparison of Th1- and Th2-associated immune reactivities stimulated by single versus multiple vaccination of mice with irradiated Schistosoma mansoni cercariae.

Mice immunized against Schistosoma mansoni by a single percutaneous exposure to radiation-attenuated parasite larvae demonstrate partial resistance to challenge infection that has been shown to correlate with development of cell-mediated immunity, whereas mice hyperimmunized by multiple exposure to attenuated larvae produce antibodies capable of transferring partial protection to naive recipients. Measurement of Ag-specific lymphokine responses in these animals suggested that the difference in resistance mechanisms may be due to the differential induction of Th subset response by the two immunization protocols. Thus, upon Ag stimulation, singly immunized mice predominantly demonstrated responses associated with Th1 reactivity, including IL-2 and IFN-gamma production, whereas multiply immunized animals showed increased IL-5, IL-4, and IgG1 antibody production associated with enhanced Th2 response. These responses demonstrated some degree of organ compartmentalization, with splenocytes demonstrating higher Th1-related lymphokine production and cells from draining lymph nodes showing stronger proliferation and Th2 type reactivity. However, hyperimmunized mice also continued to demonstrate substantial Th1-associated immune reactivity. Moreover, in vivo Ag challenge elicited activated larvacidal macrophages in hyperimmunized animals. These observations indicate that protective cell-mediated mechanisms associated with induction of CD4+ Th1 cell reactivity predominate in singly vaccinated mice. Further vaccination stimulates Th2 responses, such as enhanced IgG1 production, that may also contribute to protective immunity.     

Differential control of IFN-gamma and IL-2 production during Trypanosoma cruzi infection

In murine infection with Trypanosoma cruzi, immune responsiveness to parasite and non-parasite Ag becomes suppressed during the acute phase of infection, and this suppression is known to extend to the production of IL-2. To determine whether suppression of lymphokine production was specific for IL-2, or was a generalized phenomenon involving suppressed production of other lymphokines, we have begun an investigation of the ability of mice to produce of a number of lymphokines during infection, initially addressing this question by studying IFN-gamma production. Supernatants from Con A-stimulated spleen cells from infected resistant (C57B1/6) and susceptible (C3H) mice were assayed for IFN-gamma. Supernatants known to be suppressed with respect to IL-2 production from both mouse strains contained IFN-gamma at or above that of supernatants from normal spleen cells. Samples were assayed in an IFN bioassay to ensure that the IFN-gamma detected by ELISA was biologically active. Thus, suppression during T. cruzi infection does not extend to the production of all lymphokines. The stimulation of IFN-gamma production was confirmed by detection of IFN-gamma mRNA in unstimulated spleen cells from infected animals, and in Con A, Con A + PMA, and in some cases, parasite Ag-stimulated spleen cells from infected animals. IFN-gamma mRNA levels in mitogen-stimulated spleen cells equalled or exceeded those found in similarly stimulated normal cells. In contrast, stimulated spleen cells from infected animals had reduced levels of IL-2 mRNA relative to normal spleen cells. Thus at both the protein and mRNA level, IFN-gamma production is stimulated by T. cruzi infection, whereas IL-2 production is suppressed. Serum IFN-gamma in infected C57B1/6 and C3H mice was detected 8 days after infection, peaked on day 20 of infection, and subsequently fell, but remained detectable at low levels throughout the life of infected mice. Infected animals were depleted of cell populations known to be capable of producing IFN-gamma, and Thy-1+, CD4-, CD8-, NK- cells, and to a lesser degree, CD4+ and CD8+ cells were found to be responsible for the production of IFN-gamma during infection. We also report that IL-2 can induce IFN-gamma production in vitro and in vivo by spleen cells from infected animals, and that IL-2 can synergize with epimastigote or trypomastigote antigen to produce high levels of IFN-gamma comparable to those found in supernatants from mitogen-stimulated cells.     

Prostaglandin E precursor fatty acids inhibit human IL-2 production by a prostaglandin E-independent mechanism

Essential fatty acids, from which PG derive, can participate in development and regulation of immune responses and have been shown to suppress inflammation and tissue injury in animal models. In this report, we investigate the effects of the immediate (DGLA, precursor to PGE1), arachidonic acid (AA, PGE precursors, dihomogamma linolenic acid (DGLA, precursor to PGE1), arachidonic acid (AA, precursor to PGE2), and eicosapentaenoic acid (EPA, precursor to PGE3) on IL-2 production by PHA-stimulated human PBMC. DGLA and AA inhibited IL-2 production in a dose-dependent manner: half-maximal inhibition was obtained by using the fatty acids at the dose of 10 micrograms/ml without significant effects on cell viability. EPA inhibited IL-2 production by PBMC of only some donors. Incubation of cells in the presence of oleic, stearic, and palmitic acids, which are not PG precursors, did not affect mitogen-induced IL-2 production. A progressive increase in incorporation of DGLA into cellular lipids was observed over a 48-h incubation period. IL-2 production was reduced also when PBMC were pretreated overnight with DGLA or AA and washed before exposure to PHA. Whereas addition of the cyclo-oxygenase inhibitor, indomethacin, at the time of mitogenic stimulation led to increased IL-2 production and prevented mitogen- and fatty acid-induced increases in PGE release, it had no significant effect on the capacity of the fatty acids to suppress IL-2 production. Time course experiments showed that DGLA and AA inhibited IL-2 production even at times of minimal or no PGE release by the treated cultures. Moreover, DGLA and AA inhibited IL-2 production by the human leukemia T cell line Jurkat which, when appropriately induced, is able to release high levels of IL-2 in the absence of accessory cells and measurable PGE production. Taken together, these data indicate that essential fatty acids inhibit IL-2 production directly without conversion into their cyclo-oxygenase pathway products, and suggest that human lymphocyte function may be altered profoundly by small changes in their fatty acid profile.     

Repeated exposure induces periportal fibrosis in Schistosoma mansoni-infected baboons: role of TGF-beta and IL-4

Recently, we observed that repeated Schistosoma mansoni infection and treatment boost Th2-associated cytokines and TGF-beta production in baboons. Other studies have shown that some chronically infected baboons develop hepatic fibrosis. Because TGF-beta, IL-2, and IL-4 have been shown to participate in development of fibrosis in murine schistosomiasis, the present study examined whether repeated exposure stimulates hepatic fibrosis in olive baboons. To test this hypothesis, animals were exposed to similar numbers of S. mansoni cercariae given once or repeatedly. After 19 wk of infection, animals were cured with praziquantel and reinfected once or multiple times. Hepatic granulomatous inflammation and fibrosis were assessed from serial liver biopsies taken at weeks 6, 9, and 16 after reinfection and egg Ag (schistosome egg Ag)-specific cytokine production by PBMC were measured simultaneously. Periportal fibroblast infiltration and extracellular matrix deposition (fibrosis), angiogenesis, and biliary duct hyperplasia developed in some animals. The presence and amount of fibrosis directly correlated with the frequency of exposure. Fibrosis was not associated with adult worm or tissue egg burden. The amount of fibrosis correlated with increased schistosome egg Ag-driven TGF-beta at 6, 9, and 16 wk postinfection (rs = 0.9, 0.8, and 0.54, respectively, all p < 0.01) and IL-4 production (p = 0.02) at 16 wk postinfection and not IFN-gamma, IL-2, IL-5, or IL-10. These data suggest that repeated exposure is a risk factor for periportal fibrosis by a mechanism that primes lymphocytes to produce increased levels of profibrotic molecules that include TGF-beta and IL-4.     

Novel mechanism for Trypanosoma cruzi-induced suppression of human lymphocytes. Inhibition of IL-2 receptor expression

Co-culture of blood forms of Trypanosoma cruzi, the causative agent of Chagas' disease, with human PBMC impaired the capacity of T lymphocytes to express surface receptors for IL-2. This effect was evidenced by marked reductions in both the proportion of Tac+ cells and the density of Tac Ag on the surface of the positive cells, determined by flow cytometry. The extent of the inhibition increased with parasite concentration. Under optimal or suboptimal conditions of stimulation with either PHA or monoclonal anti-CD3, specific for an epitope of the T3-Ti human T cell Ag receptor complex, the presence of T. cruzi curtailed the capacity of T lymphocytes to proliferate and express Il-2R but did not affect IL-2 production. Furthermore, the addition of exogenous IL-2 did not restore the responsiveness of suppressed human lymphocytes but did when mouse lymphocytes were used instead. Therefore, unlike mouse lymphocytes, human lymphocyte suppression by T. cruzi did not involve deficient IL-2 production and was accompanied by impaired IL-2 utilization. Co-culture of human monocytes/macrophages with suppressive concentrations of T. cruzi increased IL-1 production, and the parasite did not decrease IL-1 secretion stimulated by a bacterial LPS. Therefore, the suppression of IL-2R expression and lymphoproliferation is not likely to have been an indirect consequence of insufficient IL-1 production due to infection of monocytes or macrophages. We have shown that suppression of human lymphocyte proliferation by T. cruzi is not caused by nutrient consumption, absorption of IL-2, lymphocyte killing, or mitogen removal by the parasite. Therefore, these results uncover a novel suppressive mechanism induced by T. cruzi, involving inhibited expression of IL-2R after lymphocyte activation and rendering T cells unable to receive the IL-2 signal required for continuation of their cell cycle and mounting effective immune responses.     

Regulation of murine lymphokine production in vivo. Ultraviolet radiation exposure depresses IL-2 and enhances IL-4 production by T cells through an IL-1-dependent mechanism

The exposure of experimental animals to the inflammatory effects of ultraviolet radiation (UVR) is known to cause depressions in their ability to initiate and effectuate various types of cellular immune responses. Contact-type and delayed-type hypersensitivity, plus the ability to generate protective forms of anti-viral and anti-tumor immunity, are all affected by the prior exposure of normal animals to the effects of this physical agent. Presently, the cellular and molecular mechanism(s) responsible for mediating the changes in immune function observed in UVR-exposed animals is not fully understood. Herein we report that one reproducible consequence of exposing normal mice to low doses of UVR is a dramatic change in the pattern of lymphokines secreted by their activated T cells. Lymphocytes isolated from UVR-exposed donors produce/secrete greatly reduced levels of the T cell lymphokines IL-2 and IFN-gamma activation in vitro with protein Ag of the polyclonal T cell stimulant anti-CD3. The secretion of IL-4 by these lymphocyte cultures, however, is consistently elevated in comparison to normal controls. Further studies determined that a similar change in lymphokine production was induced when mice were treated with either bacterial LPS or rIL-1 beta, a cytokine known to be elevated in vivo after UVR or LPS exposure. The ability of IL-1 to facilitate a change in the capacity of T lymphocytes to produce/secrete lymphokines after in vitro activation does not appear to represent a direct effect of this cytokine on lymphocyte or accessory cell targets because addition of IL-1 beta to cultures of Ag-primed lymphocytes obtained from normal donors was incapable of altering the pattern of lymphokine production. Collectively, our present results add further support to the hypothesis that UVR-induced elevations in endogenous IL-1 are, in part, responsible for the immunomodulatory effects of UVR. These findings provide compelling evidence that UVR, plus other agents capable of endogenously stimulating the production of IL-1, may function to alter the expression of different effector mechanisms in vivo. This could be facilitated through selective reductions in lymphokines produced by Th-1-type cells (IL-2 and IFN-gamma) and a simultaneous augmentation in a lymphokine produced by Th-2-type cells (IL-4).