Antigen processing requirements for T cell activation: differential requirements for presentation of soluble conventional antigen vs cell surface MHC determinants

The present studies were undertaken to characterize the antigen-processing requirements involved in the responses to T cells to soluble antigen (antigen specific), to allogeneic cell surface MHC determinants (alloreactive), and to syngeneic MHC determinants (autoreactive). T cell clones were used that have dual cross-reactive specificities either 1) for self MHC plus soluble antigen and for allogeneic MHC products or 2) for syngeneic MHC and for allogeneic MHC, in order to permit comparison of the processing requirements for responses of the same T cell to distinct antigenic stimuli. The proliferative responses of antigen-specific, Ia-restricted T cell clones to soluble antigens were sensitive to treatment of antigen-presenting cells (APC) with 125 to 250 microM chloroquine, a lysosomotropic agent previously shown to inhibit the processing of soluble antigens. In contrast, the same T cell clones were only minimally affected in their ability to respond to similarly chloroquine-treated APC expressing allogeneic MHC products. The responses of autoreactive T cell clones to syngeneic stimulating cells and their cross-reactive responses to allogeneic cells were both resistant to chloroquine treatment of stimulating cells. The failure of chloroquine to inhibit antigen presentation to autoreactive T cell clones suggests that these clones are specific for self Ia not associated with in vitro processed foreign antigen. Thus, chloroquine sensitivity distinguishes the in vitro antigen-processing requirements for presentation of the soluble antigens tested from the requirements for presentation of syngeneic or allogeneic cell surface MHC determinants to the same T cells.     

Characteristics and functions of Sendai virus-specific T-cell clones

Several murine Sendai virus-specific T-cell clones were characterized in vitro and in vivo. All T-cell clones were phenotypically Thy-1.2+, and most clones were Lyt-1+,2-; one T-cell clone was Lyt-1-,2-. Some of the clones proliferated in response to antigen presented on I region-compatible stimulator cells. Proliferation could be inhibited by monoclonal antibodies directed against class II antigens. Clones which proliferated in response to antigen secreted lymphokines which could be identified as Interleukin 2 and Interleukin 3. All of the clones tested in vivo induced a delayed-type hypersensitivity response in syngeneic mice challenged with antigens. Depending on the experimental conditions chosen, Interleukin 2-producing clones as well as non-Interleukin 2-producing clones mediated help for stimulation of cytolytic T lymphocytes.     

Characterization and function of autoreactive T-lymphocyte clones isolated from normal, unprimed mice

Self-Ia-reactive cloned T-cell lines, designated PK, were established by long-term culture of T cells from normal DBA/2 mice with irradiated syngeneic splenic adherent cells (SAC), rich in macrophages and dendritic cells. The cell lines were Thy 1+, Lyt 1+, Lyt 2-, produced IL-2 following stimulation with syngeneic spleen cells, and did not exhibit alloreactivity when screened against six different H-2 haplotypes. Of the five cloned PK cell lines tested, four were I-Ed restricted while one was I-Ad restricted as determined by genetic mapping and blocking studies carried out with monoclonal anti-Ia sera. Extensive specificity studies suggested that the PK cells reacted to syngeneic Ia molecules alone and not to foreign antigens such as fetal calf serum (FCS) used in the culture medium, in association with self-Ia. SAC pulsed with FCS or other protein antigens such as turkey gamma-globulin (TGG) were tested for their ability to induce proliferation of autoreactive T cells and other antigen-specific T cells using culture conditions consisting of serumless medium and interleukin 2 (IL-2). The data showed that the autoreactive T cells proliferated better in response to antigen-unpulsed SAC, while FCS-specific and TGG-specific cell lines, developed independently, proliferated only in response to FCS- or TGG-pulsed SAC, respectively, but not to antigen-unpulsed SAC. These results clearly distinguished the autoreactive T-cell clones from the antigen-specific T-cell clones. Preliminary studies carried out to investigate the functions of autoreactive T cells suggested that these cells helped in the in vitro differentiation of alloantigen-specific cytotoxic T lymphocytes (CTL) from CTL precursors obtained from the thymus and augmented syngeneic, allogeneic, and antigen-specific immune responses in vitro. The autoreactive T cells were also capable of inducing both proliferation and differentiation of antigen-specific populations of B cells in the absence of antigen. The present investigation suggests that autoreactive, non-antigen-reactive T cells can be cloned from normal, unimmunized mice and that such cell lines may provide a powerful tool for analyzing the role of the syngeneic mixed lymphocyte reaction in induction and maintenance of both T-and B-cell immune responses.     

Cross-reactivity of vector-borne metacyclic forms of Trypanosoma cruzi with mammalian and culture stages

Metacyclic forms of Trypanosoma cruzi isolated from the hindgut of infected insect vectors (Rhodnius prolixus) were found to be immunologically cross-reactive with cultured epimastigote, amastigote, and metacyclic stages of the parasite as well as with bloodstream trypomastigote forms by direct agglutination and indirect immunofluorescence techniques. Sera specific for each of these forms of the parasite systematically yielded maximal antibody titers when measured against the homologous antigen, indicating that antigenic determinants are shared by all of the developmental forms used in this work. Supporting this conclusion were the significant reductions in anti-insect-derived metacyclic antibody titer caused by absorption with any of the other life stages of T. cruzi. These results are relevant to the potential use of laboratory-grown forms of T. cruzi in vaccination against a natural infection with this parasite.     

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)     

Trypanosoma cruzi proteins which are antigenic during human infections are located in defined regions of the parasite

Polyclonal antibodies obtained against antigenic proteins encoded by six recombinant DNA clones of Trypanosoma cruzi were used for the ultrastructural localization of the respective antigens in thin sections of parasites (epimastigote, amastigote and trypomastigote forms of T. cruzi) embedded at low temperature in Lowicryl K4M resin. Antigens of high molecular weight containing tandemly repeated amino acid sequence motifs and recognized by sera from patients with Chagas' disease, were located only in the flagellum, where it contacts the parasite cell body. Other antigens were located on the surface of the parasite while still others were found within the flagellar pocket, as is the case with an antigen released during the acute phase of Chagas' disease. Thus, we conclude that some of the T. cruzi proteins which are antigenic in human infections are located in defined regions of the parasite. Some of the antigens were not expressed to the same extent in the three different developmental stages of the parasite.     

Surface-targeted lysosomal membrane glycoprotein-1 (Lamp-1) enhances lysosome exocytosis and cell invasion by Trypanosoma cruzi

To gain entry into non-phagocytic cells, Trypanosoma cruzi trypomastigotes recruit lysosomes to the host cell surface. Lysosome fusion at the site of parasite entry leads to the formation of a parasitophorous vacuole with lysosomal properties. Here, we show that increased expression of the lysosomal membrane glycoprotein Lamp-1 at the cell surface renders CHO cells more susceptible to trypomastigote invasion in a microtubule-dependent fashion. Mutation of critical residues in the lysosome-targeting motif of Lamp-1 abolished the enhancement of T. cruzi invasion. This suggests that interactions dependent on Lamp-1 cytoplasmic tail motifs, and not the surface-exposed luminal domain, modulate T. cruzi entry. Measurements of Ca2+-triggered exocytosis of lysosomes in these cell lines revealed an enhancement of beta-hexosaminidase release in cells expressing wild-type Lamp-1 on the plasma membrane; this effect was not observed in cell lines transfected with Lamp-1 cytoplasmic tail mutants. These results also implicate Ca2+-regulated lysosome exocytosis in cell invasion by T. cruzi and indicate a role for the Lamp-1 cytosolic domain in promoting more efficient fusion of lysosomes with the plasma membrane.     

Trypanosoma cruzi: maintenance of parasite-specific T cell responses in lymph nodes during the acute phase of the infection

Mice infected with 5 x 10(3) forms of Trypanosoma cruzi showed a transient, but severe impairment of in vitro spleen cell responses to parasite antigens and to Concanavalin A (Con A). In contrast, inguinal and periaortic lymph node (LN) cells displayed high parasite-specific proliferative responses and only a partial reduction of the Con A-induced proliferation during the acute and chronic phases of infection. Lymphocytes that underwent blastic transformation in T. cruzi-stimulated cell cultures were of the L3T4+ phenotype. Suppression of spleen cell responses occurred in the acute phase whether mice were infected with high (3 x 10(5] or low (5 x 10(3] doses of T. cruzi by intraperitoneal or subcutaneous route. Suppression of the T. cruzi-specific proliferative response of LN cells was only observed in mice infected with high subcutaneous inocula. This suppression, however, was restricted to the LNs draining the site of inoculation without affecting distant LNs. Supernatants from parasite-stimulated proliferating LN cells displayed low or undetectable T cell growth factor (TCGF) activity, in contrast with the high TCGF levels found in supernatants of the same cells stimulated with Con A. Low levels of TCGF were also detected in cultures of LN cells from mice immunized with T. cruzi extracts. Neither the T. cruzi antigen used for in vitro stimulation nor the LN cell supernatants from infected mice inhibited TCGF activity. These findings indicate that (1) parasite-specific responses are present in the LN compartment throughout the acute phase of T. cruzi infection in mice and (2) the proliferative response of L3T4+ LN cells from infected mice to T. cruzi antigens is not associated with a high TCGF secretory response.     

Passive transfer of experimental allergic encephalomyelitis by myelin basic protein-specific L3T4+ T cell clones possessing several functions

Mouse myelin basic protein (mBP)-specific T cell clones were generated from lines established from SJL/J mice immunized with mBP in complete Freund's adjuvant. These clones proliferated specifically to mBP and were propagated weekly with the same antigen for up to 8 mo. It is of particular interest that four of these phenotypic T helper clones were able to induce several T cell functions, including that of antibody production. These mBP-reactive T cell clones induced inflammatory infiltrations of the white matter of the central nervous system when transferred i.v. to irradiated (350 R) syngeneic naive recipients in concentrations as low as 0.5 X 10(6) cells/mouse. Lesions characteristic of experimental allergic encephalomyelitis (EAE) were observed as early as 5 days after transfer in the absence of clinical paralysis. Encephalitogenic clones, when added in vitro to a population of mBP-primed B cells in the presence of antigen, induced the production of anti-mBP antibodies determined by ELISA. In addition, the same clones, when transferred i.v., were found to mediate in vivo helper activity by inducing serum anti-mBP antibodies in the recipients. This response was delayed until 20 days after transfer and was abrogated by irradiation of the clones before injection. Finally, these mBP-specific specific clones were capable of mediating a specific delayed-type hypersensitivity (DTH) response. Although all four clones generated displayed the Thy-1.2+, L3T4+, Lyt-2- phenotype and proliferated specifically to mBP, only three were able to induce EAE, transfer DTH, and mediate helper activity.     

Involvement of TSSA (trypomastigote small surface antigen) in Trypanosoma cruzi invasion of mammalian cells

TSSA (trypomastigote small surface antigen) is a polymorphic mucin-like molecule displayed on the surface of Trypanosoma cruzi trypomastigote forms. To evaluate its functional properties, we undertook comparative biochemical and genetic approaches on isoforms present in parasite stocks from extant evolutionary lineages (CL Brener and Sylvio X-10). We show that CL Brener TSSA, but not the Sylvio X-10 counterpart, exhibits dose-dependent and saturable binding towards non-macrophagic cell lines. This binding triggers Ca(2+)-based signalling responses in the target cell while providing an anchor for the invading parasite. Accordingly, exogenous addition of either TSSA-derived peptides or specific antibodies significantly inhibits invasion of CL Brener, but not Sylvio X-10, trypomastigotes. Non-infective epimastigote forms, which do not express detectable levels of TSSA, were stably transfected with TSSA cDNA from either parasite stock. Although both transfectants produced a surface-associated mucin-like TSSA product, epimastigotes expressing CL Brener TSSA showed a ~2-fold increase in their attachment to mammalian cells. Overall, these findings indicate that CL Brener TSSA functions as a parasite adhesin, engaging surface receptor(s) and inducing signalling pathways on the host cell as a prerequisite for parasite internalization. More importantly, the contrasting functional features of TSSA isoforms provide one appealing mechanism underlying the differential infectivity of T. cruzi stocks.     

Distinct proliferative T cell clonotypes are generated in response to a murine retrovirus-induced syngeneic T cell leukemia: viral gp70 antigen-specific MT4+ clones and Lyt-2+ cytolytic clones which recognize a tumor-specific cell surface antigen

After immunization of B6 mice with the syngeneic retrovirus-induced T cell leukemia/lymphoma FBL-3, two major tumor-specific proliferative T cell clonotypes were derived. T cell clones derived from long-term lines propagated by in vitro culture with irradiated tumor cells and syngeneic spleen cells were exclusively of the Lyt-2+ phenotype. Such clones were cytolytic, retained their proliferative phenotype indefinitely when expanded by repeated cycles of reactivation and rest, and recognized a tumor-specific cell surface antigen in association with class I MHC molecules. This tumor cell antigen was not present on nontransformed virus-infected cells. Class II MHC-restricted MT4+ clones specific for the viral antigen gp70 were derived from lymph node T cells of FBL-3 tumor-immune mice only by in vitro culture with purified Friend virus in the presence of syngeneic splenic APC. Once derived, however, such clones could be stimulated in the presence of FBL-3 tumor cells and syngeneic spleen cells, demonstrating the reprocessing of tumor-derived gp70 antigen by APC in the spleen cell population. In contrast, no reprocessing of the tumor cell surface antigen by splenic APC for presentation to the class I MHC-restricted T cell clones could be demonstrated. Evidence is presented that FBL-3 T leukemia/lymphoma cells function as APC for Lyt-2+ class I MHC-restricted clones, and that no concomitant recognition of Ia molecules is required to activate these clones. Both Lyt-2+ and MT4+ clones were induced to proliferate in the presence of exogenous IL2 alone, but this stimulus failed to result in significant release of immune interferon. In contrast, antigen stimulation of both clones resulted in proliferation as well as significant immune interferon release. Immune interferon production is not required for the generation of MHC-restricted cell-mediated cytolytic function.     

Novel strategy in Trypanosoma cruzi cell invasion: implication of cholesterol and host cell microdomains

Trypanosoma cruzi, the etiological agent of Chagas' disease, is an obligatory intracellular parasite in the mammalian host. In order to invade a wide variety of mammalian cells, T. cruzi engages parasite components that are differentially expressed among strains and infective forms. Because the identification of putative protein receptors has been particularly challenging, we investigated whether cholesterol and membrane rafts, sterol- and sphingolipid-enriched membrane domains, could be general host surface components involved in invasion of metacyclic trypomastigotes and extracellular amastigotes of two parasite strains with distinct infectivities. HeLa or Vero cells treated with methyl-beta-cyclodextrin (MbetaCD) are less susceptible to invasion by both infective forms, and the effect was dose-dependent for trypomastigote but not amastigote invasion. Moreover, treatment of parasites with MbetaCD only inhibited trypomastigote invasion. Filipin labeling confirmed that host cell cholesterol concentrated at the invasion sites. Binding of a cholera toxin B subunit (CTX-B) to ganglioside GM1, a marker of membrane rafts, inhibited parasite infection. Cell labeling with CTX-B conjugated to fluorescein isothiocyanate revealed that not only cholesterol but also GM1 is implicated in parasite entry. These findings thus indicate that microdomains present in mammalian cell membranes, that are enriched in cholesterol and GM1, are involved in invasion by T. cruzi infective forms.     

Two types of murine helper T cell clone. II. Delayed-type hypersensitivity is mediated by TH1 clones

We have previously shown that at least two types of Lyt-1+, Lyt-2-, L3T4+ helper T cell clones can be distinguished in vitro by different patterns of lymphokine secretion and by different forms of B cell help. Evidence is presented here to show that one type of helper T cell clone (TH1) causes delayed-type hypersensitivity (DTH) when injected with the appropriate antigen into the footpads of naive mice. The antigen-specific, major histocompatability complex (MHC)-restricted footpad swelling reaction peaked at approximately 24 hr. Footpad swelling was induced by all TH1 clones tested so far, including clones specific for soluble, particulate, or allogeneic antigens. In contrast, local transfer of TH2 cells and antigen did not produce a DTH reaction, even when supplemented with syngeneic spleen accessory cells. Similarly, local transfer of an alloreactive cytotoxic T lymphocyte clone into appropriate recipients did not produce DTH. The requirements for the DTH reaction induced by TH1 cells were investigated further by using TH1 clones with dual specificity for both foreign antigens and M1s antigens. Although these clones responded in vitro to either antigen + syngeneic presenting cells, or M1s disparate spleen cells, they responded in vivo only to antigen + MHC and did not cause footpad swelling in an M1s-disparate mouse in the absence of antigen. Moreover, in vitro preactivation of TH1 or TH2 cells with the lectin concanavalin A was insufficient to induce DTH reactions upon subsequent injection into footpads. From these results, we conclude that the lack of DTH given by TH2 clones in vivo could be due to the inability of the TH2 cells to produce the correct mediators of DTH, or to a lack of stimulation of TH2 clones in the footpad environment.     

Studies of experimental allergic encephalomyelitis by using encephalitogenic T cell lines and clones in euthymic and athymic mice

The role of T-T cell interactions in the clinical course of acute experimental allergic encephalomyelitis (EAE) in mice was investigated. Myelin basic protein (MBP)-reactive and encephalitogenic T cell clones were established from long-term lines derived from susceptible strain SJL/J mice and resistant strain DDD/1 mice. The lines and clones from DDD/1 mice were obtained by immunization of congenitally athymic mice of DDD/1 origin, which had been reconstituted with syngeneic Lyt-2+-depleted splenic T cells. The clones derived from both strains bore surface phenotypes of Lyt-1+, 2- and L3T4+, and proliferated well in response to rat, rabbit, bovine, and guinea pig MBP in the presence of antigen-presenting cells with I-As. Passive EAE could be induced in syngeneic normal recipients by these clones as well as by the lines from which the clones were derived. The clinical features of the clone-induced EAE were essentially the same as those of the line-induced EAE. Furthermore, DDD/1 athymic recipients developed signs of acute EAE by the adoptive transfer of I-A-compatible syngeneic and allogeneic T cell clones, in which there was no significant difference in time of onset, maximum severity, or prognosis. These results indicate that the entire clinical course of acute EAE can be elicited by a single population of MBP-reactive T cells in the absence of the thymus and other populations of primed or unprimed T cells.     

Functional analysis in vitro and in vivo of Mycobacterium bovis strain BCG-specific T cell clones

Several cloned T cell lines specific for PPD and BCG were obtained. All clones were able to secrete lymphokine, i.e., MAF/interferon, upon antigenic stimulation. The surface phenotype of all these different clones was Thy-1.2+, L3T4+, Lyt-2-, suggesting that these lines belonged to the helper/inducer T cell subset. The T cell clones displayed various degrees of helper activity as tested in a secondary antibody response in vitro. The capacity of these clones to elicit DTH reactions in the presence of antigen and their ability to inhibit mycobacterial growth in vivo were tested by transferring locally the different clones to normal mice. The clones which exhibited little or no helper activity were able to elicit DTH responses, whereas the clone with strong helper activity did not. Both types of functionally defined clones had the capacity to inhibit the growth of intracellular mycobacteria in vivo.     

Involvement of L3T4+, LYT2.2+ T cell subsets and non-T cells in the resistance of mice against Trypanosoma cruzi infection

Cellular populations involved in resistance against T. cruzi infection were characterized from mice chronically infected with the parasite. Mice transfused with spleen cells (SC), nylon-wool-non-adherent spleen cells (NWNA) or sera from mice chronically infected with T. cruzi, showed an enhanced resistance against challenge with the parasite. The protective activity of NWNA but not of SC was completely abrogated by treatment with anti-Thy1.2 monoclonal antibodies (mAb) and complement (C). Pretreatment of NWNA cells from chronically infected mice with either anti-L3T4 or anti-Lyt 2.2 mAb partially reduced the transfer of resistance. When both L3T4+ and Lyt2.2+ cells were depleted from NWNA populations, transfer of resistance was abolished. These results appear to indicate that L3T4+, Lyt2.2+ T cell subsets and non-T cells are involved in the immunity to T. cruzi.     

Chicken T lymphocyte clones with specificity for Eimeria tenella. I. Generation and functional characterization

T lymphocyte clones reacting specifically with the antigenic components of Eimeria tenella were generated from splenic lymphocytes of immunized chickens and were maintained for 12 to 14 wk in vitro. These T cell growth factor-dependent T lymphocyte clones from bursectomized and normal chickens proliferated in vitro when stimulated with antigens from different developmental stages of homologous but not heterologous species of the parasite. Specific proliferative responses of the cloned T cells showed an absolute requirement for antigen presentation by histocompatible antigen-presenting cells. Some of the T cell clones exhibited functionally discrete interactions with syngeneic primed B cells; 25% of the T cell clones from immunized normal chickens and 7% of those obtained from immunized bursectomized chickens showed antigen-dependent helper activity and induced specific antibody production by syngeneic primed B cells. Of the T cell clones from immunized normal chickens, 19% showed suppression of in vitro antibody production in comparison to 7% of those isolated from immunized bursectomized chickens. The frequency of cloned T cells with ability to induce cytotoxic activity in macrophages against the sporozoites of E. tenella was much higher in those isolated from bursectomized chickens (80%) than in those isolated from normal chickens. Because both bursectomized and normal chickens can be immunized by repeated infections, differences in the distribution among cloned T cells suggest different effector mechanisms of immunity against coccidiosis in these chickens. Lack of B cells seem to affect the development of T cell immunity as reflected by slower development of immunity and enhanced activation of cytotoxic T cell function.     

In vitro cytocidal effect of novel lytic peptides on Plasmodium falciparum and Trypanosoma cruzi

Plasmodium falciparum and Trypanosoma cruzi were killed by two novel lytic peptides (SB-37 and Shiva-1) in vitro. Human erythrocytes infected with P. falciparum, and Vero cells infected with T. cruzi, were exposed to these peptides. The result, in both cases, was a significant decrease in the level of parasite infection. Furthermore, the peptides had a marked cytocidal effect on trypomastigote stages of T. cruzi in media, whereas host eukaryotic cells were unaffected by the treatments. In view of the worldwide prevalence of these protozoan diseases and the lack of completely suitable treatments, lytic peptides may provide new and unique chemotherapeutic agents for the treatment of these infections.     

Expression and polymorphism of a Trypanosoma cruzi gene encoding a cytoplasmic repetitive antigen.

The study of the expression of a Trypanosoma cruzi gene encoding a cytoplasmic repetitive antigen (CRA) during the metacyclogenesis process shows that this gene is not expressed in metacyclic trypomastigote forms of the parasite. However, a slight increase in CRA expression was observed following the nutritional stress of epimastigotes which precedes T. cruzi metacyclogenesis in vitro. The comparison of the expression of CRA in different T. cruzi strains shows that this gene is highly polymorphic: some strains display one and others display two polypeptides reacting with a CRA antiserum. The comparison of T. cruzi G-49 strain and Dm 28c clone shows that they display rather different Northern and Southern blot profiles when probed with a clone corresponding to the repetitive region of the CRA gene. A similar polymorphism was also observed for the gene encoding a flagellar repetitive antigen, suggesting that gene polymorphism might be a common feature of many T. cruzi genes.     

Molecular and cellular mechanisms involved in the Trypanosoma cruzi/host cell interplay

The protozoan parasite Trypanosoma cruzi has a complex biological cycle that involves vertebrate and invertebrate hosts. In mammals, the infective trypomastigote form of this parasite can invade several cell types by exploiting phagocytic-like or nonphagocytic mechanisms depending on the class of cell involved. Morphological studies showed that when trypomastigotes contact macrophages, they induce the formation of plasma membrane protrusions that differ from the canonical phagocytosis that occurs in the case of noninfective epimastigotes. In contrast, when trypomastigotes infect epithelial or muscle cells, the cell surface is minimally modified, suggesting the induction of a different class of process. Lysosomal-dependent or -independent T. cruzi invasion of host cells are two different models that describe the molecular and cellular events activated during parasite entry into nonphagocytic cells. In this context, we have previously shown that induction of autophagy in host cells before infection favors T. cruzi invasion. Furthermore, we demonstrate that autophagosomes and the autophagosomal protein LC3 are recruited to the T. cruzi entry sites and that the newly formed T. cruzi parasitophorous vacuole has characteristics of an autophagolysosome. This review summarizes the current knowledge of the molecular and cellular mechanisms of T. cruzi invasion in nonphagocytic cells. Based on our findings, we propose a new model in which T. cruzi takes advantage of the upregulation of autophagy during starvation to increase its successful colonization of host cells.