Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Prolonged antigen presentation is required for optimal CD8+ T cell responses against malaria liver stage parasites

Immunization with irradiated sporozoites is currently the most effective vaccination strategy against liver stages of malaria parasites, yet the mechanisms underpinning the success of this approach are unknown. Here we show that the complete development of protective CD8+ T cell responses requires prolonged antigen presentation. Using TCR transgenic cells specific for the malaria circumsporozoite protein, a leading vaccine candidate, we found that sporozoite antigen persists for over 8 weeks after immunization--a remarkable finding since irradiated sporozoites are incapable of replication and do not differentiate beyond early liver stages. Persisting antigen was detected in lymphoid organs and depends on the presence of CD11c+ cells. Prolonged antigen presentation enhanced the magnitude of the CD8+ T cell response in a number of ways. Firstly, reducing the time primed CD8+ T cells were exposed to antigen in vivo severely reduced the final size of the developing memory population. Secondly, fully developed memory cells expanded in previously immunized mice but not when transferred to naïve animals. Finally, persisting antigen was able to prime naïve cells, including recent thymic emigrants, to become functional effector cells capable of eliminating parasites in the liver. Together these data show that the optimal development of protective CD8+ T cell immunity against malaria liver stages is dependent upon the prolonged presentation of sporozoite-derived antigen.

F4/80+Alternatively Activated Macrophages Control CD4+T Cell Hyporesponsiveness at Sites Peripheral to Filarial Infection

Both T cells and APC have been strongly implicated in the immune suppression observed during filarial nematode infections, but their relative roles are poorly understood, particularly in regard to timing and locality of action. Using Litomosoides sigmodontis infection of susceptible BALB/c mice, we have studied the progression of filarial immunosuppression leading to patent infection with blood microfilaremia. Patent infection is associated with decreased immune responsiveness in the draining thoracic lymph nodes (tLN) and intrinsically hyporesponsive CD4+ T cells at the infection site. We now show that we are able to separate, both in time and space, different suppressive mechanisms and cell populations that contribute to filarial hyporesponsiveness. L. sigmodontis infection recruited a F4/80+ population of alternatively activated macrophages that potently inhibited Ag-specific CD4+ T cell proliferative responses even in the presence of competent naive APC. T cell responsiveness was partially restored by neutralizing TGF-beta, but not by blocking IL-10 or CTLA-4 signaling. During prepatent infection, the macrophage population was restricted to the infection site. However, once infection became patent with systemic release of microfilariae, the suppressive macrophage activity extended peripherally into the tLN. In contrast, the hyporesponsive CD4+ T cell phenotype remained localized at the infection site, and the tLN CD4+ T cell population recovered full Ag responsiveness in the absence of suppressive macrophages. Filarial immunosuppression, therefore, evolves over time at sites increasingly distal to infection, and the mechanisms of filarial down-regulation are dependent on proximity to the infection site.

DM peptide-editing function leads to immunodominance in CD4 T cell responses in vivo

DM functions as a peptide editor for MHC class II-bound peptides. We examined the hypothesis that DM peptide editing plays a key role in focusing the in vivo CD4 T cell responses against complex pathogens and protein Ags to only one, or at most a few, immunodominant peptides. Most CD4 T cells elicited in the wild-type BALB/c (H-2d) mice infected with Leishmania major predominantly recognize a single epitope 158-173 within Leishmania homologue of activated receptor for c-kinase (LACK), as is the case when these mice are immunized with rLACK. Using DM-deficient (DM-/-) H-2d mice, we now show that in the absence of DM, the in vivo CD4 T cell responses to rLACK are skewed away from the immunodominant epitopes and are diversified to include two novel epitopes (LACK 33-48 and 261-276). DM-/- B10.BR (H-2k) mice showed similar results. These results constitute the first demonstration of the role of DM peptide editing in sculpting the specificity and immunodominance in in vivo CD4 T cell responses.

A helminth glycan induces APC maturation via alternative NF-kappa B activation independent of I kappa B alpha degradation

Activation of APCs via TLRs leads to activation of NF-kappaB, a key transcription factor in cells of the immune system most often associated with induction of Th1-type and proinflammatory responses. The neoglycoconjugate lacto-N-fucopentaose III (12-25 molecules)-dextran (LNFPIII-Dex) activates dendritic cells (DCs) via TLR4, as does LPS. However, unlike LPS, LNFPIII-Dex-activated cells induce Th2-type CD4+ T cell responses. This observation led us to ask whether LNFPIII-activated APCs were differentially activating NF-kappaB, and if so, could this partly account for how DCs mature in response to these two different pathogen-associated molecular patterns (PAMPs). In this study, we show that LNFPIII-Dex stimulation of APCs induces rapid, but transient NF-kappaB translocation and activity in the nucleus, in comparison with the persistent activation induced by LPS. We then demonstrate that transient vs persistent NF-kappaB activation has important implications in the development of the APC phenotype, showing that the second wave of NF-kappaB translocation in response to LPS is required for production of the proinflammatory mediator NO. In contrast to LPS, LNFPIII-stimulated APCs that only transiently activate NF-kappaB do not induce degradation of the known IkappaB family members or production of NO. However, cells stimulated with LNFPIII rapidly accumulate p50, suggesting that an alternative p105 degradation-dependent mechanism is primarily responsible for NF-kappaB activation downstream of LNFPIII. Finally, we show that while NF-kappaB translocation in LNFPIII-stimulated APCs is transient, it is required for the development of the DC 2 phenotype, confirming a crucial and multifaceted role for NF-kappaB in innate immune responses.

Leishmania spp.: on the interactions they establish with antigen-presenting cells of their mammalian hosts

Identification of macrophages as host cells for the mammalian stage of Leishmania spp. traces back to about 40 years ago, but many questions concerning the ways these parasites establish themselves in these cells, which are endowed with potent innate microbicidal mechanisms, are still unanswered. It is known that microbicidal activities of macrophages can be enhanced or induced by effector T lymphocytes following the presentation of antigens via MHC class I or class II molecules expressed at the macrophage plasma membrane. However, Leishmania spp. have evolved mechanisms to evade or to interfere with antigen presentation processes, allowing parasites to partially resist these T cell-mediated immune responses. Recently, the presence of Leishmania amastigotes within dendritic cells has been reported suggesting that they could also be host cells for these parasites. Dendritic cells have been described as the only cells able to induce the activation of naive T lymphocytes. However, certain Leishmania species infect dendritic cells without inducing their maturation and impair the migration of these cells, which could delay the onset of the adaptive immune responses as both processes are required for naive T cell activation. This review examines how Leishmania spp. interact with these two cell types, macrophages and dendritic cells, and describes some of the strategies used by Leishmania spp. to survive in these inducible or constitutive antigen-presenting cells.

Dendritic cells: the host Achille's heel for mucosal pathogens?

Mucosal surfaces represent the main sites of interaction with environmental microorganisms and antigens. Sentinel cells, including epithelial cells and dendritic cells (DCs), continuously sense the environment and coordinate defenses for the protection of mucosal tissues. DCs play a central role in the control of adaptive immune responses owing to their capacity to internalize foreign materials, to migrate into lymph nodes and to present antigens to naive lymphocytes. Some pathogenic microorganisms trigger epithelial responses that result in the recruitment of DCs. These pathogens hijack the recruited DCs to enable them to infect the host, escape the host's defense mechanisms and establish niches at remote sites.

Proinflammatory Cytokines Dominate the Early Immune Response to Filarial Parasites

Although the early human immune response to the infective-stage larvae (L3) of Brugia malayi has not been well-characterized in vivo (because of the inability to determine the precise time of infection), the consensus has been that it must involve a predominant Th2 environment. We have set up an in vitro system to study this early immune response by culturing PBMC from unexposed individuals with live L3 of B. malayi. After 24 h of culture, T cell responses were examined by flow cytometry and by quantitative real-time RT-PCR for multiple cytokines. T cells were activated early following exposure to L3 as indicated by up-regulation of surface markers CD69 and CD71. The frequency of T cells expressing proinflammatory Th1 cytokines (IFN-gamma, TNF-alpha, GM-CSF, IL-1alpha, and IL-8) but not Th2 cytokines (IL-4, IL-5, IL-6, IL-10, and IL-13) was significantly increased in response to L3. This T cell response occurred in both the CD4 and CD8 T cell compartment and was restricted to the effector/memory pool (CD45RO(+)). This T cell response was not due to LPS activity from the parasite or from its endosymbiont, Wolbachia; moreover, it required the presence of APC as well as direct contact with live L3. Real-time RT-PCR analysis of multiple cytokines in the T cells confirmed the increased expression of proinflammatory Th1 cytokines. Up-regulation of these cytokines suggests that the primary immune response to the live infective stage of the parasite is not predominantly Th2 in nature but rather dominated by a proinflammatory response.

Dendritic cell-based immunotherapy combined with antimony-based chemotherapy cures established murine visceral leishmaniasis

Dendritic cells (DCs) have been proposed to play a critical role as adjuvants in vaccination and immunotherapy. In this study we evaluated the combined effect of soluble Leishmania donovani Ag (SLDA)-pulsed syngeneic bone marrow-derived DC-based immunotherapy and antimony-based chemotherapy for the treatment of established murine visceral leishmaniasis. Three weekly injections of SLDA-pulsed DCs into L. donovani-infected mice reduced liver and splenic parasite burden significantly, but could not clear parasite load from these organs completely. Strikingly, the conventional antileishmanial chemotherapy (sodium antimony gluconate) along with injections of SLDA-pulsed DCs resulted in complete clearance of parasites from both these organs. Repetitive in vitro stimulation of splenocytes from uninfected or L. donovani-infected mice with SLDA-pulsed DCs led to the emergence of CD4(+) T cells with characteristics of Th1 cells. Our data indicate that DC-based immunotherapy enhances the in vivo antileishmanial potential of antimony or vice versa.

Toxoplasma gondii inhibits MHC class II expression in neural antigen-presenting cells by down-regulating the class II transactivator CIITA

Major histocompatibility complex (MHC) class II expression by microglia and astrocytes is critical for CD4+-mediated immune responses within the central nervous system. Here, we demonstrate that the obligate intracellular parasite, Toxoplasma gondii, down-regulates activation-induced MHC class II expression in human-derived glioblastoma cells as well as in primary astrocytes and microglia from cortices of rat fetuses. Down-regulation of MHC class II proteins was predominantly observed in parasite-positive, but not parasite-negative, host cells of T. gondii-infected cell cultures. MHC class II transcript levels induced by IFN-gamma alone or in combination with TNF-alpha were also clearly diminished after parasitic infection. Furthermore, T. gondii dose-dependently down-regulated the transcript levels of the class II transactivator CIITA. These results suggest that T. gondii partially evade CD4+-mediated intracerebral immune responses, a mechanism which may contribute to long-term persistence of the parasite within the CNS.

Nematode infection enhances survival of activated T cells by modulating accessory cell function

The type of immune response generated following exposure to Ag depends on a variety of factors, including the nature of the Ag, the type of adjuvant used, the site of antigenic entry, and the immune status of the host. We have previously shown that infection of rodents with Nippostrongylus brasiliensis (Nb) shifts the development of type 1 allo-specific responses toward type 2 immunity, suggesting nematode modulation of T cell activation. In this report we explore the immunomodulatory effects of Nb on T cell activation. We found that spleen cells from Nb-infected mice exhibited dramatically increased proliferation in response to Con A and anti-CD3. This hyperproliferation could be transferred in vitro to naive splenocytes by coculture with mitomycin C-treated cells from Nb-infected animals. The transfer was mediated by non-T accessory cells and supernatants derived from Con A-activated non-T cells, suggesting the involvement of a soluble factor secreted by accessory cells. The accessory cells secreted high levels of IL-6, and anti-IL-6 treatment abrogated the supernatant-induced hyperproliferation, thus confirming that IL-6 was mediating the effect. Further, spleen cells from Nb-infected mice were more resistant to activation-induced cell death (AICD) following mitogenic stimulation. Reduced AICD was also transferable and IL-6 dependent. Thus, the hyperproliferation was in part due to enhanced activated T cell survival. These phenomena mediated by accessory cells may contribute to the powerful polyclonal activation of type 2 immunity caused by nematode infection.

Delayed clearance of filarial infection and enhanced Th1 immunity due to modulation of macrophage APC functions in xid mice

Bruton's tyrosine kinase (Btk) mutant CBA/N mice show delayed clearance of injected microfilaria (mf) compared with wild-type CBA/J mice. Anti-mf T cells from CBA/N mice make relatively more IFN-gamma than those from CBA/J mice. The anti-mf T cell proliferative responses are also greater in CBA/N mice. This CBA/N immune phenotype is not restricted to filarial Ags, because immunization with pure proteins also yields T cell responses of greater proliferative magnitude skewed away from Th2 cytokines in CBA/N compared with CBA/J mice. The increased magnitude of CBA/N T cell proliferative responses is reflected in increases in both precursor frequencies and clonal burst sizes of responding Ag-specific T cells, and is independent of the source of re-stimulating APCs. Transfer of CBA/J peritoneal resident cells (PRCs) into CBA/N mice before pure protein immunization leads to a wild-type immune phenotype in the recipient CBA/N mice, with a reduction in the proliferative response and a relative decrease in the IFN-gamma produced. When wild-type PRC subpopulations are similarly transferred, the wild-type immune phenotype is transferred by macrophages rather than by B cells. Transfer of wild-type PRCs into CBA/N mice before injection of mf also causes similar changes in the anti-mf T cell responses and enhances the clearance of mf. Thus, Btk is involved in critical macrophage APC functions regulating priming of T cells, and can modulate these responses in pathophysiologically relevant fashion in vivo.

Induction of immunity to Schistosoma mansoni: interaction of schistosomula with accessory leucocytes in murine skin and draining lymph nodes

A single exposure to radiation-attenuated cercariae of Schistosoma mansoni induces a high level of protective immunity in C57BL/6 mice, which is mediated by Th1 responses. Events in the skin and/or draining lymph nodes early after exposure are crucial for the induction of protection, and we have investigated the interactions of vaccinating parasites with host leucocytes in these 2 locations. We observed extensive lateral spreading of cercarial secretions along layers of the stratum corneum but not between keratinocytes. There was little direct contact with host leucocytes during the first 1-2 days when the parasites lay at the base of the epidermis, but cells accumulated in the underlying dermis. In contrast to normal parasites, attenuated larvae persisted in the dermis for > 10 days, often surrounded by aggregates of macrophages/dendritic cells. Whilst cells bearing MHC II, CD11b or CD11c markers were present in the lymph nodes, particularly in the periphery and paracortical areas, no obvious redistribution was seen as a result of parasite residence there for 5-15 days. However, ultrastructural observations revealed numerous cells with macrophage/dendritic morphology in the vicinity of parasites, in some instances closely adherent to the tegument. The observations strongly suggest that the tegument is a potent source of the antigens which prime the immune system in the lymph nodes of vaccinated mice for a protective response.

Activation of CD4+ and CD8+ parasite -specific T-cells by macrophages infected with live T. cruzi amastigotes

T. cruzi-infected macrophages are potential candidates for the presentation of parasite antigens to T. cruzi-specific T lymphocytes. To assess this question, we examine the ability of peritoneal exudate macrophages to process exogenous live or dead parasites and to activate defined populations of T. cruzi-specific immune T-cells. Macrophages infected with live amastigotes activated both lymph node CD4+ and spleen CD8 + T-primed cells that proliferated and secreted cytokines. Lymph node CD4+ T-cells produced IFN-gamma and IL-10 while CD8 + T-cells produced IFN-gamma. In contrast, macrophages pulsed with dead parasites activated only lymph node CD4+ T-cells, which proliferated and secreted IFN-gamma. Interestingly, the immunization with heat-killed parasites primed mice for CD8+ T-cells which were expanded in vitro by recognition of infected macrophages. Taken together, these results demonstrated that amastigote infected macrophages present parasite peptides associated with MHC I and II molecules, activating both CD4 + and CD8+ T-cells. Furthermore, the development of T. cruzi-specific CD8+ T-cells in vivo using the immunization protocol with non-living parasites as described in this report could be explored for further studies on the role of CTL in the outcome of infection.

Role of CD80 (B7.1) and CD86 (B7.2) in the immune response to an intracellular pathogen

The costimulatory ligands CD80 and CD86 play a crucial role in the initiation and maintenance of an immune response. We demonstrate that whereas infection of human monocytes with viable tachyzoites of Toxoplasma gondii resulted in rapid induction of expression of CD80 and up-regulation of expression of CD86, incubation with killed organisms failed to alter the levels of expression of these costimulatory ligands. The T. gondii-mediated changes in levels of expression of these molecules are critical to the T cell response to the parasite. Proliferation of resting T cells in response to parasite-infected cells was dependent on both CD80 and CD86. More importantly, early production of IFN-gamma in response to T. gondii by T cells from T. gondii-seronegative individuals occurred only after stimulation with monocytes that exhibited increased expression of CD80 and CD86 (monocytes infected with viable parasites) and was almost completely ablated by the combination of anti-CD80 plus anti-CD86 mAb. Moreover, proliferation and IFN-gamma production by CD4+ CD45RA+ T cells from unexposed individuals were dependent on both CD80 and CD86. These data indicate that pathogen-monocyte interaction influences the ensuing T cell response.

CD4-mediated and CD8-mediated cytotoxic and proliferative immune responses to Toxoplasma gondii in seropositive humans

Both CD4+ and CD8+ cytotoxic T lymphocytes (CTL) are part of the human immune response to Toxoplasma gondii infection. To further our understanding of Toxoplasma immunity, we investigated factors influencing stimulation of CD4+ or CD8+ human T. gondii-specific immune cells. Both antigen-pulsed and Toxoplasma-infected antigen-presenting cells (APC) induced cell proliferation. Toxoplasma-infected APC elicited strong proliferation of CD4+ cells, but little or no proliferation of CD8+ cells, unless high antigen loads were used. Toxoplasma-infected APC stimulated specific cytotoxicity poorly or not at all, owing to death of stimulated cultures, whereas antigen-pulsed APC strongly elicited specific cytotoxicity. Cytotoxicity elicited by either type of APC resided exclusively in CD4+ T cells in polyclonal cultures. Thus, Toxoplasma-infected APC elicited stronger CD4-mediated than CD8-mediated cell proliferation and generated CD4+ CTL more readily than CD8+ CTL. Nonetheless, specific CD8+ memory cells were demonstrated, and rare CD8+ Toxoplasma-specific CTL were subcloned. Fixed Toxoplasma-infected APC (which induce CD8+ CTL) also elicited cell proliferation, but polyclonal cultures stimulated with these infected APC did not die. Unfixed Toxoplasma-infected APC strongly inhibited phytohemagglutinin-induced cell proliferation, whereas fixed APC did not. These data suggested that infected APC were inhibitory or lethal to some immune cells. Further investigations into interactions between immune cells and Toxoplasma-infected cells likely will help elucidate factors involved in the immunopathogenesis of Toxoplasma infection. As other intracellular parasites, including Plasmodium spp. and Leishmania spp., also elicit CD4+ CTL, such work may help establish paradigms governing immunity to intracellular parasites.

Parasite accessory cell interactions in murine leishmaniasis. I. Evasion and stimulus-dependent suppression of the macrophage interleukin 1 response by Leishmania donovani

Interleukin 1 (IL 1) is a principal mediator of the host immune response to microbial challenge. Accessory cells of the monocyte-macrophage series are a major source of this cytokine and are also chronically parasitized by protozoa of the genus Leishmania. This suggests that characterization of the macrophage IL 1 response to Leishmania would increase our understanding of the regulation of host immunity to these organisms. In the present study, the macrophage IL 1 response to Leishmania donovani was examined because infections with this organism have findings consistent with parasite-specific T cell unresponsiveness. Cytokine activity was measured either by direct stimulation or by co-stimulation of thymocytes. Conditioned media from BALB/c resident peritoneal macrophages infected with amastigotes of L. donovani contained no more IL 1 than did supernatant fluids of control cells. In contrast, supernatants from cells stimulated with lipopolysaccharide or heat-killed Listeria monocytogenes had significantly increased cytokine content. Resident cells infected with L. donovani for 4 hr before being stimulated with Listeria demonstrated a suppressed IL 1 response (approximately 40% of Listeria alone) to this secondary particulate stimulus. In contrast, the secondary response of leishmania-preinfected cells to lipopolysaccharide was not affected. To examine whether accessory cell nonresponsiveness to L. donovani (with respect to IL 1) was related to the state of macrophage activation, elicited peritoneal macrophages obtained by injection of proteose peptone were also studied. These cells responded to stimulation with lipopolysaccharide and fixed Staphylococcus aureus with increases in intracellular, membrane, and secreted cytokine activities. In contrast, L. donovani failed to induce any of these activities. This was found to be the case irrespective of whether amastigotes were alive or killed or opsonized with specific antibodies. Elicited cells preinfected with Leishmania responded normally to secondary stimulation with lipopolysaccharide, but not S. aureus (64% of Staphylococcus alone). In addition, attachment and penetration of L. donovani promastigotes and their subsequent conversion to amastigotes within macrophages failed to induce IL 1 synthesis. The findings of this study indicate that L. donovani has the ability to both evade and suppress the macrophage IL 1 response. Because this monokine provides an obligatory signal during macrophage-dependent T cell activation, evasion of signal transduction for IL 1 synthesis may be related to defects in cell-mediated immunity which occur during infections with this organism.

Parasite-accessory cell interactions in murine leishmaniasis. II. Leishmania donovani suppresses macrophage expression of class I and class II major histocompatibility complex gene products

In the present study, we examined the modulation of MHC class II and class I gene products on BALB/c macrophages infected with the obligate intracellular protozoan Leishmania donovani. Our findings indicated that this organism suppressed macrophage expression of both classes of MHC antigens. These effects varied somewhat, depending on whether cells were in the basal state or were stimulated with interferon-gamma. Thus, class II density on interferon-gamma-treated infected macrophages was suppressed by as much as 90%, relative to lymphokine-stimulated control cells. Induction of H-2K and H-2D by lymphokine treatment of infected macrophages was also markedly reduced. In the basal (non-lymphokine-treated) state, infected cells also showed reduced expression of H-2K and H-2D, but not I-A or I-E. The latter result was related to minimal levels of class II molecules on normal, in vitro cultured macrophages. Suppression of MHC gene products correlated with both the duration and intensity of leishmania infection and could not be overcome by increasing doses of interferon-gamma. Culture of cells under conditions of cyclooxygenase inhibition completely abolished elevated synthesis of prostaglandin E2 by infected macrophages and augmented their responsiveness to lymphokine induction of class II antigens by 60 to 80%. These results indicate that L. donovani is capable of subverting a critical macrophage accessory function required for the induction of T lymphocyte immunity. This mechanism could account, at least in part, for defective parasite-specific cell-mediated immunity seen during infections with this protozoan.