Trypanosoma cruzi: effect on B-cell-responsive and -responding clones

Trypanosoma cruzi infection does not impair major histocompatibility complex class I presentation of antigen to cytotoxic T lymphocytes

Trypanosoma cruzi (T. cruzi), the etiological agent of Chagas' disease, lives free within the cytoplasm of infected host cells. This intracellular niche suggests that parasite antigens may be processed and presented on major histocompatibility complex (MHC) class I molecules for recognition by CD8+ T cells. However, the parasite persists indefinitely in the mammalian host, indicating its success at evading immune clearance. It has been shown that T. cruzi interferes with processing and presentation of antigenic peptides in the MHC class II pathway. This investigation sought to determine whether interference in MHC class I processing and presentation occurs with T. cruzi infection. Surface expression of MHC class I molecules was found to be unaffected or up-regulated by T. cruzi infection in vitro. A model system employing a beta-galactosidase (beta-gal)-specific murine cytotoxic T lymphocyte (CTL) line (0805B) showed: (i) in vitro infection of mouse peritoneal macrophages or J774 cells with T. cruzi did not inhibit MHC class I presentation of exogenous peptide (a nine-amino acid epitope of beta-gal) to the CTL line, (ii) in vitro infection of a beta-gal-expressing 3T3 cell line (LZEJ) with T. cruzi did not inhibit MHC class I presentation of the endogenous protein to the CTL line and (iii) mouse renal adenocarcinoma cells infected with T. cruzi and subsequently infected with adenovirus expressing beta-gal were able to present antigen to the beta-gal-specific CTL line. These findings indicate that the failure of the immune response to clear T. cruzi does not result from global interference by the parasite with MHC class I processing and presentation. Parasites engineered to express beta-gal were unable to sensitize infected antigen-presenting cells in vitro to lysis by the CTL 0805B line. This was probably due to the intracellular localization of the beta-gal within the parasite and its inaccessibility to the host cell cytoplasm.

Mechanisms of development of immunosuppression during Trypanosoma infections

Acute infection with Trypanosoma cruzi or its African relatives, including T. brucei rhodesiense, T. b. gambiense, T. b. brucei and T. congolense, is frequently accompanied by manifestations of immunological dysfunction. Initially investigators catalogued the ensuing immunologic alterations and identified a number of modifications in lymphoid or accessory cell properties. More recently, the emphasis has switched towards the molecular underpinnings of immunosuppression in these infections. In this article, Marcelo Sztein and Felipe Kierszenboum focus on recent progress made in the quest to delineate the mechanisms behind altered lymphocyte functions in tryponosomal infections, point out particular and common features of immunosuppression induced by T. cruzi and African trypanosomes, and outline possible directions for future research.

Effect of antigen-specific T helper cells or interleukin-2 on suppressive ability of macrophage subsets detected in spleens of Trypanosoma cruzi-infected mice as determined by limiting dilution-partition analysis

Trypanosoma cruzi, a protozoan parasite and the causative agent of Chagas' disease, induces a state of lymphocyte hyporesponsiveness to both mitogenic and antigenic stimuli in mice during the acute phase of infection. Addition of spleen cells from T. cruzi-infected mice (SCinf) to microcultures of spleen cells from noninfected mice (SCn) suppresses the responsiveness of such cultures to antigenic challenge and to mitogenic stimulation. We analyzed the regulatory cell populations in SCinf by limiting dilution-partition analysis and found a complex regulatory circuit in T. cruzi-infected mice consisting of two suppressive macrophage subsets and an enhancing T-cell population. This T-cell population was able to abrogate or escape the suppressive ability of one suppressor macrophage subset, yet was suppressed by the other macrophage subset. To further study the cellular interactions of this regulatory circuit and analyze the suppressive abilities of the two suppressor macrophage subsets, we examined the effect of adding either primed T helper cells of known specificity or interleukin-2 to the limiting dilution-partition analysis microcultures. The results of these experiments suggest that one suppressor macrophage subset, which is abundant and, therefore, detected with low doses of SCinf, is able to suppress both mitogen- and primary antigen-specific responses but is unable to inhibit cells once they are already activated or primed. The other macrophage subset, which is presumably a less abundant or less active population (since high doses of SCinf are required to detect it), is able to suppress the response of activated or primed T cells by the inhibition of interleukin-2 production.

Polyclonal activation of B lymphocytes during experimental infection with Schistosoma mansoni

A significant polyclonal activation of B lymphocytes was observed during experimental infection of C57BL/10J mice with Schistosoma mansoni. The isotypic pattern of this expansion, assessed by the Protein-A plaque-forming cell method, was compared with and found to differ from those occurring after infection by Trypanosoma cruzi or injection of bacterial LPS. In the infection of S. mansoni an early expansion of most immunoglobulin isotypes occurs together with a late, sustained expansion of IgG1-secreting cells. High levels of polyclonal B cell activation were observed after adoptive transfer of spleen cells from infected mice to isogenic recipients pre-treated with hydroxyurea.

Pregnancy and humoral immune response in mice chronically infected by Trypanosoma cruzi

The effect of pregnancy on the humoral immune response induced by Trypanosoma cruzi was studied in groups of chronically infected and pregnant mice (IP) or chronically infected and nonpregnant mice (INP) of strain BALB/c. Groups of noninfected and nonpregnant mice (NINP) or noninfected and pregnant mice (NIP) served as controls. The pregnant mice were killed on day 17 of pregnancy. Anti-T. cruzi immunoglobulin G (IgG) and IgM antibodies, detected by immunofluorescence or enzyme-linked immunosorbent assay or both, underwent a pregnancy-associated decrease of 20 to 40%, whereas complement-mediated lytic antibodies were unaffected by pregnancy. Immunoblotting analysis indicated identical specificities of the anti-T. cruzi antibodies in IP and INP groups. The levels of all the immunoglobulin isotypes (particularly IgG2a and IgG3), circulating immune complexes, rheumatoid-like factor, and anti-DNA antibodies were considerably increased during chronic infection (NINP versus INP), which could be related to the high degree of polyclonal B-cell activation occurring in T. cruzi infection. However, pregnancy significantly decreased (by 20 to 60%) such parameters. IgG levels were particularly affected (by 40 to 60%), and the decreases could be ordered as follows: IgG3 greater than IgG2a greater than IgG1 greater than IgG2b for IP versus INP. Comparisons between the noninfected groups indicated differences only in IgG levels. These results indicate the following. (i) The specific humoral anti-T. cruzi immune response is weakly affected by pregnancy, which is not sufficient to modify the course of the mother's infection. (ii) Pregnancy does not modify the expression of the anti-T. cruzi antibody repertory. (iii) Pregnancy reduces the polyclonal B-cell activation, particularly the levels of the IgG isotypes undergoing the greatest activation.

Nematospiroides dubius and Nippostrongylus brasiliensis: delayed type hypersensitivity responses to ovalbumin in the infected mouse

Mice (C57BL) infected with the intestinal nematode Nematospiroides dubius showed depressed delayed type hypersensitivity responses to ovalbumin administered subcutaneously in Freund's complete adjuvant. IgG and IgM responses to this inoculum were unaffected. It is unlikely that the depression arose from impairment of the ear test response because responses to an extract of the adult parasite were measurable and ear testing with lipopolysaccharide yielded normal responses in infected mice. Furthermore, mice immunized on the day of infection responded normally, whilst long term infected mice ear challenged with antigen pulsed macrophages gave depressed responses. The in vitro proliferative responses of cells from the spleens and from the lymph nodes draining the site of immunization were enhanced marginally by N. dubius infection. Furthermore, these cells induced normal or elevated adoptive delayed-type hypersensitivity and IgG responses in irradiated recipients. These findings suggest that N. dubius does not compromise the development of ovalbumin specific T cells involved in a delayed type hypersensitivity response. Evidence for the induction of suppressor cells by N. dubius is discussed, and the findings are compared with results obtained with Nippostrongylus brasiliensis, a parasite which is rejected rapidly from the mouse.

Polyclonal lymphocyte responses to murine Trypanosoma cruzi infection. I. Quantitation of both T- and B-cell responses

Lymphoid activity was studied in spleen and lymph node cells from Trypanosoma cruzi-infected mice. Blast transformation in each lymphocyte class was assessed by dual parameter analysis for size and surface markers by both FACS and conventional immunofluorescence, while proliferative activity was measured by tritiated thymidine uptake, autoradiography, and analysis of DNA content in single cells. Acute infection results in rapid blast transformation and proliferative activity of all three lymphocyte classes (Ig+, L3T4+, and Lyt 2+). At 2 weeks of infection most cells in these organs are enlarged and more than half are dividing. By 2 and 6 months after infection (chronic phase of resistant strains), large numbers of activated B lymphocytes and, to a lesser extent, of Lyt 2+ T cells are still detected. Similar results were obtained in C57BL/6 (resistant) and C3H/HeJ (susceptible) mouse strains. The implications of this massive polyclonal lymphocyte response to the parasite for the physiopathology of acute and chronic infection are discussed.

Interleukin 2 enhances specific and nonspecific immune responses in experimental Chagas' disease

Mice infected with Trypanosoma cruzi develop an early and profound immunosuppression of responses to heterologous antigens. Recently it has been demonstrated that this immunosuppression is linked, in part, to deficiency in the production of interleukin 2 (IL-2), and that the addition of IL-2 to cultures of normally unresponsive spleen cells from infected mice will restore responsiveness to sheep erythrocytes (SRBC) and enhance parasite-specific immune responses. In the present study, the effect of administration of ultrapure or recombinant IL-2 on immune responses to SRBC and parasite-specific responses in vivo was examined. It was found that a single injection of 1,500 U of IL-2 provided at the same time as SRBC more than doubled the number of direct plaque-forming cells to SRBC and that multiple injections of 1,500 U of IL-2 were no more restorative than a single injection. Anti-SRBC responses of normal mice were unaffected by injection of IL-2. Single or multiple injections of recombinant human IL-2, with and without gelatin, into highly susceptible C3H(He) mice induced greater parasite-specific immunity as reflected by significantly reduced levels of parasitemia and increased longevity. Three injections of 1,500 U each of recombinant human IL-2 on days 10, 14, and 18 was found to be the most efficacious in reducing parasitemia and increasing longevity. Injection of IL-2 with gelatin did not enhance the effect of IL-2 alone.

Immunopathological aspects of experimental Trypanosoma cruzi infection: correlation of immune complexes and other serological features with muscle lesions during the infection

The pathogenesis of the tissue lesions observed in American trypanosomiasis (Chagas' disease) seems to depend on a variety of mechanisms. The present study was dedicated to the investigation of the evolution of muscle lesions and its relationship with immunological and parasitological parameters. Mice were infected i.p. with 10(5) blood forms of Trypanosoma cruzi (Columbian strain). A progressive increase in parasitaemia was observed which correlated with an increase in serum levels of IgG and IgM Trypanosoma-specific antibodies. Immune complex like material was detected in the serum (125I-C1q binding assay) on day 8 and reached maximum level between day 21 and 28. A concomitant fall in C3 levels was also observed. Inflammatory lesions and parasites were first observed in the striated muscle 2 weeks after infection. The cellular infiltrates involved macrophages and lymphoid cells; later on (week 3 onwards), intense necrosis appears and at the same time immunoglobulin and complement deposition were observed. These observations may indicate that, in addition to the cellular immune response, humoral mechanisms could contribute to the worsening of tissue lesions.

Measurement of parasite-specific immune responses in vitro: evidence for suppression of the antibody response to Trypanosoma cruzi

Using the Mishell and Dutton culture system, we have developed an assay for eliciting and quantifying parasite-specific immune responses in vitro. The ability of spleen cells from noninfected and Trypanosoma cruzi-infected mice to respond to parasite-associated antigens was assessed by examining the primary plaque-forming cell response to trinitrophenylated T. cruzi (TNP-TC). The response to TNP-TC of both normal, noninfected, unprimed mice and mice infected with T. cruzi is T cell dependent and appeared to involve recognition of parasite antigens by T. cruzi-specific T cells. In most experiments, spleen cells from infected mice respond to TNP-TC at levels equal to, or below, that of spleen cells from normal mice. This near "normal" response is in apparent contrast to the suppressed response of spleen cells from infected mice to another antigen (sheep red blood cells) or TNP on a different carrier (TNP-chicken erythrocytes). Demonstration that the response of spleen cells from infected mice to TNP-TC can be potentiated by addition of interleukin 2-containing supernatants or by depletion of plastic and Sephadex G-10-adherent cells suggests that the mechanisms which control the response of infected mice to nonparasite antigens may also limit parasite-specific immune responses.

Very large and isotypically atypical polyclonal plaque-forming cell responses in mice infected with Trypanosoma cruzi

Normal C3H/HeJ mice, acutely infected with T. cruzi, develop large numbers of splenic Ig-secreting plaque-forming cells (PFC). IgG2a, IgG2b and IgG1 PFC account for over 90% of all PFC, while the numbers of IgG3- and IgA-secreting PFC are lower than in normal animals. These effects appear to be due to both T helper-dependent regulation and to a mitogenic activity associated with the parasites themselves.

Effects of extracts of Trypanosoma cruzi on immune responses: induction of a nonspecific suppressor factor

Extracts of epimastigotes of Trypanosoma cruzi were examined for the ability to augment or suppress in vivo and in vitro responses of mouse spleen cells. When administered to mice 4 days before immunization with sheep erythrocytes, the 12,000 X g pellet and the 104,000 X g pellet and supernatant fractions of extracts of T. cruzi had no effect on the subsequent plaque-forming cell responses. Testing of the two 104,000 X g fraction in vitro resulted in a slight dose-dependent modification of anti-sheep erythrocyte responses. Also, these two fractions had a moderate dose-dependent blastogenic effect on normal spleen cells. Most significantly, the 104,000 X g supernatant fraction elicited the production of a suppressor factor when incubated with spleen cells from infected mice. This suppressor factor was also apparent when spleen cells from infected mice were incubated with intact epimastigotes or trypomastigotes. Production of the suppressor factor was insensitive to indomethacin and, therefore, presumably was not a prostaglandin.

Changes in cell populations and immunoglobulin-producing cells in the spleens of mice infected with Trypanosoma cruzi: correlations with parasite-specific antibody response

Infection of mice with Trypanosoma cruzi elicits the production of parasite-specific antibodies which reach high levels and remain elevated for at least 105 days of infection. The more susceptible C3H(He) mouse actually has a higher level of "natural" antibodies for T. cruzi but may show a greater lag time in the production of antibodies in response to infection than the more resistant C57BL/6 mouse. Comparison of the kinetics of antibody production against T. cruzi and the numbers of immunoglobulin-producing cells in the spleen during the course of infection suggests that a large number of the immunoglobulin-producing cells are probably producing antibodies directed against the parasite and are not the result of an exhaustive polyclonal B-cell activation. Cell numbers in the spleen change dramatically both in total numbers and in the percentage of different cell types during infection with T. cruzi. The percentage of T cells in the spleen remains relatively unchanged throughout infection in both mouse strains tested but numbers of Ig-positive cells decrease markedly during the acute phase of infection while macrophage numbers increase up to sixfold. Cell numbers and proportions of B cells, T cells, and macrophages return to near normal values by 105 days of infection in the C57BL/6 mouse.

Cellular immunity in Chagas' disease patients. Lymphoproliferative response to subcellular fractions of Trypanosoma cruzi

The ability of peripheral mononuclear cells from Chagas' disease patients (CDP) to generate specific proliferative responses in vitro to whole homogenate (WH) and subcellular fractions of T. cruzi was investigated. Flagellar (F), microsomal (Mc) and cell sap (CS) fractions were isolated from a WH obtained by disruption by pressure-depressure. Parasite-free human peripheral mononuclear cells were isolated by centrifugation of leukocyte-rich plasma over discontinuous gradients of Ficoll-Hypaque and cultured for 6 days in the presence of the different fractions of T. cruzi. Eighty-six percent (54/63) of the CDP showed good capacity to respond to at least one of the fractions assayed while only 18% (4/22) of the controls reacted. The Mc fraction showed the best specificity as well as the maximal stimulating effect in CDP (37/55 vs 0/18 in controls) while WH (62% vs 10%), F (54% vs 16%) and CS (52% vs 6%) showed a lesser antigenic capacity. These results suggest that recognition mechanisms and proliferative responses are functional in CDP. No correlation was found between the stimulating capacity of fractions in vitro and their immunogenic capacity previously observed in vivo.