Biomedical subjects

F Kierszenbaum

Publications and source records attributed to F Kierszenbaum.

At least 37 records · Page 2Linked to original sources

PubMed1993

Alterations induced by Trypanosoma cruzi in activated mouse lymphocytes.

Although a number of immunological anomalies have been shown to occur during the acute period of Trypanosoma cruzi infection, the contribution of the parasite has not been clarified. In this work, we co-cultured activated splenic mononuclear cells (SMC) from normal outbred (CD1) or inbred (CBA/J) mice with purified T. cruzi trypomastigotes and studied ensuing T- and B-lymphocyte alterations. In the presence of parasites, phytohaemagglutinin-stimulated SMC from either mouse background manifested a marked reduction in both lymphoproliferative capacity (i.e., 3H-thymidine incorporation) and cell membrane level of interleukin-2 receptors (IL-2R; determined by flow cytometry) relative to SMC from parasite-free cultures. Thus, substantial proportions of activated SMC either became unable to express detectable levels of IL-2R or expressed this receptor in significantly lower numbers than control SMC. Supernatants from T. cruzi suspensions reproduced these suppressive effects on phytohaemagglutinin-stimulated SMC from normal or chronically infected CD1 or CBA/J mice. Similar results were obtained with SMC activated with a bacterial lipopolysaccharide. Since IL-2R expression is required for activated lymphocytes to progress through the cell cycle and multiply to mount effective immune responses, impaired IL-2R expression by T. cruzi provides a plausible hypothesis for the wide-ranged immunosuppression that occurs in the infected host.

Animals↗

PubMed1993

Inhibition of host cell invasion and intracellular replication of Trypanosoma cruzi by N,N'-bis(benzyl)-substituted polyamine analogs.

We studied the effects of two N,N'-bis(benzyl)-substituted polyamine analogs on the capacities of Trypanosoma cruzi to invade and multiply within a mammalian host cell. At concentrations as low as 1 microM, these compounds reduced significantly the infectivity of the parasite for rat heart myoblasts in a time-dependent manner. Pretreatment of virulent T. cruzi trypomastigotes, but not myoblast pretreatment, reduced the level of infectivity. The inhibitory effects started to subside 3 h after removal of the drugs and were no longer detectable after 4 h. A significant decrease in the rate of intracellular amastigote multiplication was also seen when the drugs were added to myoblast cultures which had been previously infected with untreated T. cruzi. These results show that N,N'-bis(benzyl)-substituted polyamine analogs meet the two most important criteria for potential chemotherapeutic agents against T. cruzi infection, namely, inhibition of both host cell invasion and intracellular replication by this parasite.

Animals↗

PubMed1993

Molecular basis of Trypanosoma cruzi-induced immunosuppression. Altered expression by activated human lymphocytes of molecules which regulate antigen recognition and progression through the cell cycle.

The mechanisms by which Trypanosoma cruzi causes dysfunction in normal human lymphocytes was studied by using an in vitro system in which purified parasites and normal peripheral blood mononuclear cells are co-cultured in the presence or absence of mitogens. Our results have shown that T. cruzi impairs the expression of receptors for interleukin-2 (IL-2R) and transferrin, activated lymphocyte membrane molecules which play key roles in controlling progression through the cell cycle. T. cruzi also downregulates the expression of constitutive lymphocyte molecules (e.g., CD4, and CD8) involved in the interactions between antigen-presenting cells and T lymphocytes as well as the expression of T cell receptor (TCR) and CD3 molecules. The latter molecular structures are physically associated and are responsible for signaling and transducing activation events resulting from antigen binding. Stimulated B lymphocytes also display reduced IL-2R expression in the presence of T. cruzi. In contrast, neither the expression of EA-1 molecules by T lymphocytes nor that of CD19 and CD20 molecules by B lymphocytes is affected by this parasite. Thus, the T. cruzi effects are selective, not indiscriminate. The activated T cell populations affected by T. cruzi show concomitant reductions in the levels of expression of IL-2R and CD4, IL-2R and CD8, IL-2R and CD3 or IL-2R and TCR as well as in their capacity to proliferate; 3H-thymidine uptake decreases and there is a massive arrest of cells at the G0/G1a phase of the cell cycle. The immunosuppressive effects of T. cruzi are reproduced by a protein molecule(s) released spontaneously by the parasite termed TIF (for trypanosomal immunosuppressive factor). We report herein that TIF does not compete with IL-2 for binding to IL-2R and that shedding of IL-2R is decreased in the presence of T. cruzi. Moreover, the intracellular level of IL-2R was found to be lower than that found in control cells cultured in the absence of parasites. These results suggest that suppressed IL-2R reflects a modification induced by T. cruzi at a time coinciding with or preceding IL-2R mRNA translation. Studies are underway to identify the earliest process targeted by T. cruzi.

Animals↗

PubMed1993

Does interleukin-2 restore lymphocyte responses suppressed by Trypanosoma cruzi?

There has been disagreement about the ability of exogenous interleukin-2 (IL-2) to restore responsiveness to lymphocytes from either Trypanosoma cruzi-infected animals or normal individuals co-cultured with this parasite. The discrepancy has been attributed to the use of different strains of mice or T. cruzi isolates, or to the use of lymphoid cells from different organs. As T. cruzi inhibits the expression of IL-2 receptors by activated lymphocytes in vitro, we were able to test whether restoration of responsiveness by exogenous IL-2 might depend on the level of suppression present in the system. Human or mouse lymphocytes stimulated with phytohaemagglutinin (PHA) exhibited gradual decreases in IL-2 receptor expression, [3H]thymidine incorporation and IL-2 secretion as the concentration of T. cruzi in the culture increased. Exogenous IL-2 afforded a degree of restoration of both IL-2 receptor expression and [3H]thymidine uptake which was substantial at the lower, but very small--if any--at the higher, parasite concentrations tested. Trypanosoma cruzi could not have competed with the lymphocytes for IL-2 because it did not bind significant amounts of this cytokine. These results suggested that the controversy about the corrective effects of IL-2 may be more apparent than real, reflecting variations in the extent of immunosuppression present in different model systems of T. cruzi-associated immunosuppression.

Animals↗

PubMed1993

Inhibition of S-adenosyl-L-methionine (AdoMet) decarboxylase by the decarboxylated AdoMet analog 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811) decreases the capacities of Trypanosoma cruzi to infect and multiply within a mammalian host cell.

A decarboxylated S-adenosyl-L-methionine (AdoMet) analog, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), that specifically and irreversibly inhibits AdoMet decarboxylase (DC) was used to investigate the role of AdoMetDC in the regulation of host cell invasion and intracellular replication by Trypanosoma cruzi. The presence of MDL 73811 in cocultures of T. cruzi and rat heart myoblasts (RHM) significantly inhibited host cell infection in a dose-dependent manner. This effect, evidenced by reductions in the proportion of infected RHM and the number of parasites per 100 RHM, was due to MDL 73811 action on T. cruzi as it was reproduced when the parasites, but not the RHM, were pretreated with the inhibitor. Significant inhibition of infectivity required a 10-min treatment with MDL 73811 although greater effects ensued with additional incubation time. We could not detect signs of recovered infectivity up to 6 hr after the removal of nonincorporated MDL 73811, suggesting that either AdoMetDC turnover in T. cruzi is a relatively slow process or the amounts of MDL 73811 taken up could not be blocked or eliminated during this time period. MDL 73811-mediated inhibition of infectivity was not bypassed by the addition of exogenous spermidine or spermine, suggesting that the mechanism of action does not involve reduced production of these polyamines due to AdoMetDC inhibition. Intracellular accumulation of AdoMet appeared to be a more plausible explanation in view of the fact that exogenous AdoMet significantly inhibited infectivity. When added to infected RHM cultures, MDL 73811 or AdoMet inhibited also intracellular T. cruzi growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosylmethionine Decarboxylase↗

PubMed1992

Suppression by Trypanosoma cruzi of T-cell receptor expression by activated human lymphocytes.

The immunosuppression that develops during Chagas' disease and African sleeping sickness is thought to facilitate survival of the causative agents in their mammalian hosts. Whereas a number of manifestations of immunosuppression manifested during the course of these diseases has been reported in patients and animals, the mechanisms by which they are induced remain obscure. An in vitro system in which phytohaemagglutinin (PHA)-stimulated human peripheral blood mononuclear (PBMC) were co-cultured with purified Trypanosoma cruzi or T. brucei rhodesiense was used in the present work to establish whether these organisms were able to alter the capacity of activated helper/inducer (CD4+) or cytotoxic/suppressor (CD8+) cells to express T-cell receptor (TcR). Suppressed interleukin-2 receptor (IL-2R), known to be caused by both the trypanosomes and supernatants containing their secretion products, was the independent parameter used to demonstrate the occurrence of immunosuppression in all experiments. We found marked reductions in the percentage of TcR+ cells in T. cruzi-containing cultures as early as 18 hr after PHA stimulation. This alteration was still readily demonstrable after 72 hr of culture, i.e. when last tested for. Suppressed TcR expression occurred concomitantly with reduced levels of CD4 or CD8 molecules on the surface of helper/inducer and cytotoxic/suppressor T lymphocytes, respectively, indicating that the parasite had induced more than one alteration in the same cells. These effects were reproduced when the trypanosomes were separated from the PBMC by a 0.45 micron pore size filter or when filtrates from T. cruzi suspensions substituted for the parasite in the cultures, indicating that TcR suppression was mediated by a parasite secretion product(s). Interestingly, neither T. b. rhodesiense nor filtrates of suspensions of this organism altered significantly the level of TcR expression in cultures in which suppressed IL-2R expression by activated human T cells took place. Thus despite sharing the ability to impair IL-2R expression, T. cruzi and T.b. rhodesiense appear to differ in other mechanisms by which they affect human T-cell function. If occurring in infected hosts, the alterations that T. cruzi causes in the expression of TcR, CD4, CD8 and IL-2R--all molecules playing important roles in lymphocyte activation--could contribute to the development of the immunosuppression observed during the acute phase of Chagas' disease.

Animals↗

PubMed1992

Trypanosoma cruzi suppresses the expression of the p75 chain of interleukin-2 receptors on the surface of activated helper and cytotoxic human lymphocytes.

When Trypanosoma cruzi is co-cultured with activated human lymphocytes the latter manifest a marked level of suppression in their capacity to express high-affinity interleukin-2 receptors (IL-2R). In this study we asked whether this effect could result from reduced expression of the p75 component of the IL-2R, a condition which would limit the number of heterodimeric receptors that could be assembled by activated T cells. The results of two-colour flow cytometric analyses revealed that both the percentage and surface density of p75 molecules on phytohaemagglutinin-stimulated CD3+, CD4+ or CD8+ lymphocytes were significantly reduced in the presence of T. cruzi, whether or not the parasites were separated from the lymphocytes by a cell-impermeable filter. These results suggested that reduced expression of p75 chains would limit the number of IL-2R assembled by activated T lymphocytes, contributing in this manner to the immunosuppression that occurs during acute T. cruzi infection.

Antigens, CD↗

PubMed1992

Biochemical evidence for the presence of arginine decarboxylase activity in Trypanosoma cruzi.

Trypanosoma cruzi was found to release 14CO2 from radiolabeled arginine, and this effect was inhibited by either DL-alpha-difluoromethylarginine or monofluoromethylagmatine, both specific inhibitors of arginine decarboxylase (ADC). Furthermore, agmatine, which can be derived metabolically only by ADC-mediated arginine decarboxylation, was produced when T. cruzi was incubated with radiolabeled arginine, and agmatine production was inhibited in the presence of DL-alpha-difluoromethylarginine. These results constitute direct biochemical evidence for the presence in T. cruzi of ADC, an enzyme that does not occur in mammalian cells.

Agmatine↗

PubMed1992

DL-alpha-difluoromethylarginine inhibits intracellular Trypanosoma cruzi multiplication by affecting cell division but not trypomastigote-amastigote transformation.

DL-alpha-difluoromethylarginine (DFMA), a specific, irreversible inhibitor of arginine decarboxylase (ADC), decreases the capacity of Trypanosoma cruzi to invade and multiply within different types of mammalian host cells in vitro. In this work we found that inhibition of intracellular growth results from selective impairment of amastigote division without appreciable alteration of the capacity of the invading trypomastigotes to transform into the replicative amastigote form. Addition of agmatine, the product of arginine decarboxylation, reversed the inhibitory effect of DFMA. Inhibition of ornithine decarboxylase activity by DL-alpha-difluoromethylornithine present in the medium prior to and during infection did not affect trypomastigote transformation or amastigote replication and did not change the magnitude of the inhibitory effect of DFMA on parasite multiplication. Hence, neither polyamine synthesis via the ornithine decarboxylase pathway nor salvage of host cell polyamines by T. cruzi appeared to be a likely explanation for the normal rate of parasite transformation that was seen in the presence of DFMA. Two clones of T. cruzi, TMSU-1 and TMSU-2, were tested for their degrees of sensitivity to the inhibitory effects of DFMA. Both trypomastigote association with (i.e., binding to and penetration of) myoblasts, and intracellular amastigote multiplication by either clone were found to be significantly (P less than 0.05) but not completely inhibited by DFMA. Therefore, the partial inhibition of T. cruzi infectivity and replication caused by DFMA is unlikely to represent a composite of effects of the drug on DFMA-sensitive and insensitive clones.(ABSTRACT TRUNCATED AT 250 WORDS)

Agmatine↗

PubMed1991

Suppression by Trypanosoma brucei rhodesiense of the capacities of human T lymphocytes to express interleukin-2 receptors and proliferate after mitogenic stimulation.

We studied the suppressive effects induced in phytohemagglutinin (PHA)-stimulated human peripheral blood mononuclear cells (PBMC) by purified blood forms of Trypanosoma brucei rhodesiense. The parasite was found to markedly impair lymphocyte proliferation (measured in terms of [3H]thymidine incorporation). The extent of this effect increased with parasite concentration and was not due to mitogen absorption, depletion of medium nutrients, or PBMC killing by the parasite. Significant reductions in interleukin-2 receptor (IL-2R) expression, determined by flow cytometric analysis, were also observed in PHA-stimulated PBMC cultured in the presence of T. b. rhodesiense as evidenced by marked decreases in the surface density of the receptor. Concomitant decreases in the percentage of IL-2R+ cells were recorded in approximately half of the experiments. A discrete, dimly stained subpopulation of IL-2R+ cells were consistently demonstrable whether or not a reduction in the percentage of IL-2R+ cells occurred. Living, but not glutaraldehyde-fixed, parasites suppressed IL-2R expression. In kinetic studies, a low but reproducible level of suppression of IL-2R was demonstrable as early as 6 h after PHA stimulation; the extent of this effect became considerably more pronounced as additional culture time elapsed. Levels of IL-2 biological activity in cocultures of T. b. rhodesiense with PHA-stimulated PBMC were comparable with or higher than those present in control cultures lacking the parasite. Therefore, insufficient levels of this cytokine would be an unlikely explanation for the noted suppression of IL-2R expression and lymphoproliferation. These effects of T. b. rhodesiense could represent an important component of the mechanism by which immunosuppression develops in African sleeping sickness.

Animals↗

PubMed1991

Trypanosoma cruzi induces suppression of DNA synthesis and inhibits expression of interleukin-2 receptors by stimulated human B lymphocytes.

Trypanosoma cruzi, the causative agent of Chagas' disease, suppresses immune responses during the acute phase and has been shown to induce multiple cellular alterations in activated human T lymphocytes. However, no information is available regarding the effects of this parasite on human B cells. Using an in vitro culture system, in which purified T. cruzi are co-cultured with either peripheral blood mononuclear cells (PBMC) or B-cell-enriched preparations (BCE), we studied whether the organism can induce alterations in DNA synthesis after stimulation with Pansorbin (PS). This response was markedly reduced by the parasite at both suboptimal and optimal PS concentrations, and the extent of the inhibition was augmented as the parasite concentration was increased. Maximal reduction in DNA synthesis was observed when the trypanosomes were incorporated into the cultures at 0 time (i.e. together with PS); the effect was of a much lesser magnitude and undetectable when the parasites were added at 24 and 48 hr, respectively. These results imply that T. cruzi affects a relatively early event during B-cell stimulation. This inference was confirmed by the finding that the proportion of PS-stimulated B cells expressing interleukin-2 (IL-2) receptors was significantly reduced when the parasite was present in the culture. Addition of recombinant human IL-2 did not restore B-cell responsiveness to normal levels. Suppressed B-cell responses were also observed when T. cruzi was separated from the PBMC or the BCE by a cell-impermeable filter, indicating that a soluble factor(s) released by the organism mediated the effect. Accordingly, supernatants of T. cruzi suspensions were found to be suppressive. These results demonstrate for the first time that T. cruzi can affect human B-cell responses and that the mechanism involves inhibition of IL-2 receptor expression.

Animals↗

PubMed1991

Binding of the specific ligand to Fc receptors on Trypanosoma cruzi increases the infective capacity of the parasite.

The infective capacity of Trypanosoma cruzi was significantly increased after treatment with monoclonal IgG1 antibodies, whether or not specific for the parasite; minimal or no change in infectivity was seen after treatment with IgG2a, IgG2b or IgG3 monoclonal antibodies. The stimulatory effect was evidenced by elevated numbers of trypanosomes invading mammalian host cells in vitro compared to parasites treated with medium alone. Greater infectivity was also induced by pure human Fc, suggesting a role for Fc receptors on the organism. This inference received support in the fact that protein A inhibited the stimulatory effect of Fc. In addition, Fc-treated parasites incubated with fluorescein-labelled F(ab')2 from goat anti-human IgG exhibited fluorescence detectable by both ultraviolet microscopy and flow cytometry. 125I-Fc binding to T. cruzi was found to be saturable at 0 degrees and was inhibited by cold Fc but not by bovine serum albumin (BSA) or orosomucoid. Interestingly, 125I-Fc binding was greater at 37 degrees and it was not saturable with the concentrations that did saturate at 0 degrees. Possibly, Fc might up-regulate expression of its own receptor and greater endocytosis could take place at 37 degrees. Significant increases in infectivity were detectable after a 40 min pretreatment with Fc--hinting that Fc could trigger a chain of biochemical events underlying the phenomenon--and were reversible, becoming undetectable 2 hr after Fc removal. The average number of Fc receptors per parasite, determined at 0 degrees (at which binding saturation was possible), was estimated as 5 x 10(5), the dissociation constant was of the order of 10(-6)-10(7)M. The present results define an important biological role for an Fc-binding T. cruzi surface component and expose the capacity of this organism to exploit even elements of the immune system in its quest to attain intracellular localization, required for multiplication.

Animals↗

PubMed1991

Trypanosoma cruzi suppresses the ability of activated human lymphocytes to enter the cell cycle.

Using an in vitro system in which human peripheral blood mononuclear cells stimulated with the T-lymphocyte mitogen phytohemagglutinin were cocultured with Trypanosoma cruzi trypomastigotes, we demonstrated a marked accumulation of cells in G0/G1a during the 72-hr observation period whereas mitogen-stimulated cells in parallel cultures lacking the parasite readily entered the G1b, S, and G2/M phases. These results suggest that the immunological alterations that occur in acute Chagas' disease may result from an early cell cycle blockade induced by the parasite.

Animals↗

PubMed1991

A soluble factor from Trypanosoma brucei rhodesiense that prevents progression of activated human T lymphocytes through the cell cycle.

African sleeping sickness is accompanied by a severe immunosuppression. As part of our efforts to examine the mechanisms by which this suppressive state is induced, we studied alterations in human T-lymphocyte function caused by Trypanosoma brucei rhodesiense. To this end, we used an in vitro system in which phytohaemagglutinin (PHA)-stimulated human peripheral blood mononuclear cells (PBMC) were cultured in a medium containing soluble, non-dialysable parasite products. We were able to demonstrate significant suppression of both lympho-proliferation and interleukin-2 receptor (IL-2R) expression. These effects were found to be dose-dependent and reversible after 48 hr of culture. The suppressive effects of living trypanosomes and the soluble parasite products on lympho-proliferation and interleukin-2 receptor expression were similar in that both precluded the entry of PHA-activated PBMC into the cell cycle. Eighty to ninety-eight per cent of the activated cells remained arrested in the G0/G1a (early G1) phase even 48 hr after stimulation, i.e. when last tested. Parasite-induced expression could not be overcome by the addition of recombinant human IL-2. These results suggest that immunosuppression associated with African trypanosomiasis may result from parasite-induced alteration of very early events during lymphocyte activation, leading to a virtually complete block in cell cycle progression and inhibition of IL-2R expression.

Animals↗

PubMed1990

Trypanosomal immunosuppressive factor: a secretion product(s) of Trypanosoma cruzi that inhibits proliferation and IL-2 receptor expression by activated human peripheral blood mononuclear cells.

Coculture of blood forms of Trypanosoma cruzi with human PMBC suppresses the expression of several molecules involved in lymphocyte activation, including receptors for IL-2. Our work was initially undertaken to establish whether this effect required physical parasite-PBMC contact or was mediated by a T. cruzi secretion product. Using culture inserts with cell-impermeable membranes, we were able to demonstrate significant suppression of PHA-induced lymphoproliferation whether the trypanosomes were placed in the same compartment as, or separated from, the PBMC. Similar effects were observed by using supernatants from T. cruzi suspensions. These supernatants, which we refer to as trypanosomal immunosuppressive factor, also inhibited IL-2R expression in response to PHA stimulation. The suppressive effect of the secretion product(s) of T. cruzi was reversible, as evidenced by significant recovery of the proliferative capacity of PBMC after removal of the parasite-containing inserts. Moreover, the extent of the suppression produced by trypanosomal immunosuppressive factor subsided as culture time increased. Treatment of trypanosomal immunosuppressive factor with proteases abrogated its suppressive activity, suggesting that the relevant principle(s) was of protein nature. From ultrafiltration experiments, the molecular mass of the suppressive molecule(s) was estimated to be between 30,000 and 100,000 Da. These results demonstrate for the first time the capacity of T. cruzi to spontaneously secrete a factor that suppresses human lymphocyte responses in vitro. This factor, which may play a role in the down-regulation of host immune function observed in acute chagasic patients, might be a useful tool in exploring the mechanisms that regulate the expression of IL-2R and other surface molecules playing key roles in lymphocyte activation.

Animals↗

PubMed1990

Trypanosoma cruzi inhibits the expression of CD3, CD4, CD8, and IL-2R by mitogen-activated helper and cytotoxic human lymphocytes.

We studied the effects of Trypanosoma cruzi secretion products on the capacity of helper (Th) and cytotoxic (Tc) cells to express IL-2R, CD3, CD4, and CD8 in response to PHA or anti-CD3 stimulation. To this end, we used a culture system in which blood forms of the parasite were cocultured with human PBMC. Two-color flow cytometry studies revealed that, under these conditions, there was a significant decrease in the percentage of both Th and Tc cells expressing IL-2R (inhibition range = 30 to 65%). These effects were already demonstrable 18 h after mitogenic activation, and maximal reductions occurred after 48-72 h. T cruzi-induced suppression of IL-2R expression was accompanied by a marked decrease in surface CD3, CD4, and CD8. The surface densities of these markers on the CD3+, CD4+, and CD8+ cells was markedly inhibited by the parasite. This effect was sometimes accompanied by a significant drop in the percentage of positive cells. Diminished expression of all of these surface molecules was observed on activated, but not on resting cells, i.e., no effects occurred in the absence of mitogen. The suppressed expression of CD3, CD4, and CD8 was also seen at 18 h, and occurred concomitantly with a marked decrease in cell proliferation. The inhibited expression of IL-2R, CD3, CD4, and CD8 molecules by Th and Tc might underlie the immunosuppression that occurs during the acute phase of Chagas disease (caused by T. cruzi). Our recent observation that the parasite suppresses responses of human lymphocytes separated by a cell-impermeable membrane, raises hopes that parasite product(s) may be useful reagents for studying the regulation of the expression of IL-2R, CD3, CD4, and CD8, which are known to play key roles during lymphocyte activation.

Animals↗

PubMed1990

Mechanisms underlying immunosuppression induced by Trypanosoma cruzi.

Trypansoma cruzi affects immune responsiveness in mammalian hosts. Studies with patients and infected animals have defined some of the immunological dysfunctions but not the underlying mechanisms. Recent work using an in vitro model system of T. cruzi-human lymphocyte interactions has made it possible to uncover specific alterations in human lymphocyte activation induced by this parasite. Felipe Kierzenbaum and Marcelo Sztein discuss recent advances in our understanding of the processes that lead to impaired human lymphocyte function and that might be involved in the immunosuppression seen in the acute phase of Chagas disease.

Journal Article↗

PubMed1990

Regulation of Trypanosoma cruzi infectivity by alpha- and beta-adrenergic agonists: desensitization produced by prolonged treatments or increasing agonist concentrations.

We previously reported that blood forms of Trypanosoma cruzi express alpha- and beta-adrenergic receptors and that binding of specific agonists to these receptors modifies the infective capacity of the parasite in vitro. The present study has revealed that the inhibitory effect of the beta-adrenergic agonist L-isoproterenol and the stimulatory effect of the alpha-adrenergic agonist L-phenylephrine are not produced when the parasite is subjected to prolonged exposure to otherwise effective doses of these agonists or when supraoptimal doses of these agonists are used. We refer to these phenomena as 'desensitization' because of their analogy with vertebrate cells becoming desensitized by prolonged exposure to, or relatively high concentrations of, adrenergic agonists. At a constant agonist concentration, T. cruzi desensitization was time-dependent and, when the time of parasite treatment with the agonists was not changed, the higher concentrations of the agonist tested were the most effective in producing desensitization. The reduced infectivity resulting from treatment with optimal doses of L-isoproterenol was accompanied by elevated levels of cyclic adenosine monophosphate (cAMP) which were not detectable when L-isoproterenol concentrations producing desensitization were used. This finding implicated cAMP as a likely second signal in the inhibitory mechanisms of this agonist. No significant change in cAMP was detectable in parasites treated with L-phenylephrine, leaving open the question about how optimal doses of this alpha-adrenergic agonist enhance T. cruzi infectivity. Parasite responsiveness to alpha- and beta-adrenergic agonists as well as the desensitization effects define a system which regulates infectivity and could be modified at the host tissue level by naturally occurring agonists.

Animals↗