Polarized helper-T-cell responses against Leishmania major in the absence of B cells

The involvement of host circadian clocks in the regulation of the immune response to parasitic infections in mammals

Circadian rhythms are recurring variations of physiology with a period of ~24 hours, generated by circadian clocks located throughout the body. Studies have shown a circadian regulation of many aspects of immunity. Immune cells have intrinsic clock mechanisms, and innate and adaptive immune responses - such as leukocyte migration, magnitude of inflammation, cytokine production and cell differentiation - are under circadian control. This circadian regulation has consequences for infections including parasitic infections. In the context of Leishmania infection, the circadian clock within host immune cells modulates the magnitude of the infection and the inflammatory response triggered by the parasite. As for malaria, rhythms within the immune system were shown to impact the developmental cycles of Plasmodium parasites within red blood cells. Further, host circadian rhythms impact infections by multicellular parasites; for example, infection with helminth Trichuris muris shows different kinetics of worm expulsion depending on time of day of infection, a variation that depends on the dendritic cell clock. Although the research on the circadian control of immunity in the context of parasitic infections is in its infancy, the research reviewed here suggests a crucial involvement of host circadian rhythms in immunity on the development and progression of parasitic infections.

Dectin-1 Positive Dendritic Cells Expand after Infection with Leishmania major Parasites and Represent Promising Targets for Vaccine Development

Resistant mouse strains mount a protective T cell-mediated immune response upon infection with Leishmania (L.) parasites. Healing correlates with a T helper (Th) cell-type 1 response characterized by a pronounced IFN-γ production, while susceptibility is associated with an IL-4-dependent Th2-type response. It has been shown that dermal dendritic cells are crucial for inducing protective Th1-mediated immunity. Additionally, there is growing evidence that C-type lectin receptor (CLR)-mediated signaling is involved in directing adaptive immunity against pathogens. However, little is known about the function of the CLR Dectin-1 in modulating Th1- or Th2-type immune responses by DC subsets in leishmaniasis. We characterized the expression of Dectin-1 on CD11c+ DCs in peripheral blood, at the site of infection, and skin-draining lymph nodes of L. major-infected C57BL/6 and BALB/c mice and in peripheral blood of patients suffering from cutaneous leishmaniasis (CL). Both mouse strains responded with an expansion of Dectin-1+ DCs within the analyzed tissues. In accordance with the experimental model, Dectin-1+ DCs expanded as well in the peripheral blood of CL patients. To study the role of Dectin-1+ DCs in adaptive immunity against L. major, we analyzed the T cell stimulating potential of bone marrow-derived dendritic cells (BMDCs) in the presence of the Dectin-1 agonist Curdlan. These experiments revealed that Curdlan induces the maturation of BMDCs and the expansion of Leishmania-specific CD4+ T cells. Based on these findings, we evaluated the impact of Curdlan/Dectin-1 interactions in experimental leishmaniasis and were able to demonstrate that the presence of Curdlan at the site of infection modulates the course of disease in BALB/c mice: wild-type BALB/c mice treated intradermally with Curdlan developed a protective immune response against L. major whereas Dectin-1-/- BALB/c mice still developed the fatal course of disease after Curdlan treatment. Furthermore, the vaccination of BALB/c mice with a combination of soluble L. major antigens and Curdlan was able to provide a partial protection from severe leishmaniasis. These findings indicate that the ligation of Dectin-1 on DCs acts as an important checkpoint in adaptive immunity against L. major and should therefore be considered in future whole-organism vaccination strategies.

IL-4-producing B cells regulate T helper cell dichotomy in type 1- and type 2-controlled diseases

Interleukin-4 (IL-4)-induced T helper (Th) 2 cells promote susceptibility to the protozoan parasite Leishmania major, while conferring immunity to the intestinal trematode Schistosoma mansoni Here, we report that abrogation of IL-4 receptor alpha (IL-4Rα) signaling on B cells in BALB/c mice (mb1^creIL-4Rα^-/lox) transformed nonhealer BALB/c to a healer phenotype with an early type 1 and dramatically reduced type 2 immune response and an absence of ulceration and necrosis during cutaneous leishmaniasis. From adoptive reconstitution and mixed bone-marrow chimera studies in B cell-deficient (µMT) mice, we reveal a central role for B cell-derived IL-4 and IL-4Rα in the optimal induction of the susceptible type 2 phenotype to L. major infection. We further demonstrate that the absence of IL-4Rα signaling on B cells exacerbated S. mansoni-induced mortality and pathology in BALB/c mice, due to a diminished type 2 immune response. In both disease models, IL-4Rα-responsive B cells displayed increased IL-4 production as early as day 1 after infection. Together, these results demonstrate that IL-4-producing and IL-4Rα-responsive B cells are critical in regulating and assisting early T helper dichotomy toward Th2 responses, which are detrimental in cutaneous leishmaniasis but beneficial in acute schistosomiasis.

Immune responses during cutaneous and visceral leishmaniasis

Leishmania are protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world, resulting in an estimated 1·3 million new cases and 30 000 deaths annually. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective in several endemic regions. To date there is no vaccine against leishmaniasis, although extensive evidence from studies in animal models indicates that solid protection can be achieved upon immunization. This review focuses on immune responses to Leishmania in both cutaneous and visceral forms of the disease, pointing to the complexity of the immune response and to a range of evasive mechanisms utilized by the parasite to bypass those responses. The amalgam of innate and acquired immunity combined with the paucity of data on the human immune response is one of the major problems currently hampering vaccine development and implementation.

T helper1/t helper2 cells and resistance/susceptibility to leishmania infection: is this paradigm still relevant?

Work in large part on Leishmania major in the 1980s identified two distinct apparently counter-regulatory CD4⁺ T cell populations, T helper (h)1 and Th2, that controlled resistance/susceptibility to infection respectively. However, the generation of IL-4^−/− mice in the 1990s questioned the paramount role of this Th2 archetypal cytokine in the non-healing response to Leishmania infection. The more recent characterization of CD4⁺ T cell regulatory populations and further effector CD4⁺ T helper populations, Th17, Th9, and T follicular (f)h cells as well as the acknowledged plasticity in T helper cell function has further added to the complexity of host pathogen interactions. These interactions are complicated by the multiplicity of cells that respond to CD4⁺ T cell subset signatory cytokines, as well as the diversity of Leishmania species that are often subject to significantly different immune-regulatory controls. In this article we review current knowledge with regard to the role of CD4⁺ T cells and their products during Leishmania infection. In particular we update on our studies using conditional IL-4Rα gene-deficient mice that have allowed dissection of the cell interplay dictating the disease outcomes of the major Leishmania species infecting humans.

B cell: T cell interactions occur within hepatic granulomas during experimental visceral leishmaniasis

Hepatic resistance to Leishmania donovani infection in mice is associated with the development of granulomas, in which a variety of lymphoid and non-lymphoid populations accumulate. Although previous studies have identified B cells in hepatic granulomas and functional studies in B cell-deficient mice have suggested a role for B cells in the control of experimental visceral leishmaniasis, little is known about the behaviour of B cells in the granuloma microenvironment. Here, we first compared the hepatic B cell population in infected mice, where ≈60% of B cells are located within granulomas, with that of naïve mice. In infected mice, there was a small increase in mIgM(lo)mIgD(+) mature B2 cells, but no enrichment of B cells with regulatory phenotype or function compared to the naïve hepatic B cell population, as assessed by CD1d and CD5 expression and by IL-10 production. Using 2-photon microscopy to quantify the entire intra-granuloma B cell population, in conjunction with the adoptive transfer of polyclonal and HEL-specific BCR-transgenic B cells isolated from L. donovani-infected mice, we demonstrated that B cells accumulate in granulomas over time in an antigen-independent manner. Intra-vital dynamic imaging was used to demonstrate that within the polyclonal B cell population obtained from L. donovani-infected mice, the frequency of B cells that made multiple long contacts with endogenous T cells was greater than that observed using HEL-specific B cells obtained from the same inflammatory environment. These data indicate, therefore, that a subset of this polyclonal B cell population is capable of making cognate interactions with T cells within this unique environment, and provide the first insights into the dynamics of B cells within an inflammatory site.

A MATHEMATICAL MODEL OF IMMUNE RESPONSE IN CUTANEOUS LEISHMANIASIS

The TH1/TH2 paradigm has been largely used in the interpretation of several diseases, particularly in leishmaniasis. As far as we know there is no mathematical description of this model related to leishmaniasis. We have extended and modified a previous published set of equations1in order to adapt it to leishmanial disease particularities. The main modifications were: (1) the analysis of logistic and exponential parasite growth curves, (2) the assumption of the TH2 arm of the immune response having a positive action on parasite growth. The set of three simultaneous differential equations describing the TH1 arm, TH2 arm and parasite growth were analyzed for conditions of existence and stability of the solutions.

Stable solutions valid for the logistic and exponential parasite growth models, with its possible clinical correlations, were obtained in the following situations: (1) parasite and TH2 extinction [TH1 cure], (2) parasite extinction and TH1/TH2 co-existence [TH1/TH2 cure], (3) TH1 and parasite co-existence, TH2 extinction [stable TH1 infection], and (4) TH1, TH2 and parasite co-existence [stable TH1/TH2 infection]. TH2 and parasite co-existence associated to TH1 extinction [stable TH2 infection] was obtained only with the logistic growth model. The model also provides an alternative hypothesis for TH1 bias in resistant mice and emphazises the importance of natural immunity for the existence of chronic states.

Cell biology and immunology of Leishmania

More than 20 years ago, immunologists discovered that resistance and susceptibility to experimental infection with the intracellular protozoan Leishmania major was associated with the development of T-helper 1 (Th1)- and Th2-dominated immune responses, respectively. This infectious disease model was later used to identify and assess the role of key factors, such as interleukin-12 (IL-12) and IL-4, in Th1 and Th2 maturation. While infection by Leishmania remains a popular model for immunologists who wish to assess the role of their favorite molecule in T-cell differentiation, other investigators have tried to better understand how Leishmania interact with its insect and mammalian hosts. In this review, we discuss some of these new data with an emphasis on the early events that shape the immune response to Leishmania and on the immune evasion mechanisms that allow this parasite to avoid the development of sterilizing immunity and to secure its transmission to a new host.

The contribution of Toll-like receptor 2 to the innate recognition of a Leishmania infantum silent information regulator 2 protein

We have characterized a Leishmania protein belonging to the silent information regulator 2 (SIR2) family [SIR2 related protein 1 (SIR2RP1)] that might play an immunoregulatory role during infection through its capacity to trigger B-cell effector functions. We report here that SIR2RP1 leads to the proliferation of activated B cells, causing increased expression of major histocompatibility complex (MHC) II and the costimulatory molecules CD40 and CD86, which are critical ligands for T-cell cross-talk during the development of adaptive immune responses. In contrast, B cells isolated from Toll-like receptor 2 (TLR2) knockout mice were unable to respond to the SIR2RP1 stimulus. Similarly, SIR2RP1 induced the maturation of dendritic cells (DCs) in a TLR2-dependent manner with the secretion of pro-inflammatory cytokines [interleukin (IL)-12 and tumour necrosis factor (TNF)-alpha] and enhanced the costimulatory properties of DCs. Nevertheless, immunization assays demonstrated that TLR2-deficient mice were able to mount a specific humoral response to SIR2RP1. Interestingly, further investigations showed that macrophages were activated by SIR2RP1 even in the absence of TLR2. Therefore, a different type of interplay between SIR2RP1 and the major antigen-presenting cells in vivo could explain the immune response observed in TLR2-deficient mice. Together, these results demonstrate that TLR2 signalling contributes to SIR2RP1 recognition by innate immune host cells.

Leishmania major-specific B cells are necessary for Th2 cell development and susceptibility to L. major LV39 in BALB/c mice

B lymphocytes are considered to play a minimal role in host defense against Leishmania major. In this study, the contribution of B cells to susceptibility to infection with different strains of L. major was investigated in BALB/c mice lacking mature B cells due to the disruption of the IgM transmembrane domain (microMT). Whereas BALB/c microMT remained susceptible to infection with L. major IR173 and IR75, they were partially resistant to infection with L. major LV39. Adoptive transfer of naive B cells into BALB/c microMT mice before infection restored susceptibility to infection with L. major LV39, demonstrating a role for B cells in susceptibility to infection with this parasite. In contrast, adoptive transfer of B cells that express an IgM/IgD specific for hen egg lysozyme (HEL), an irrelevant Ag, did not restore disease progression in BALB/c microMT mice infected with L. major LV39. This finding was likely due to the inability of HEL Tg B cells to internalize and present Leishmania Ags to specific T cells. Furthermore, specific Ig did not contribute to disease progression as assessed by transfer of immune serum in BALB/c microMT mice. These data suggest that direct Ag presentation by specific B cells and not Ig effector functions is involved in susceptibility of BALB/c mice to infection with L. major LV39.

Pathogenic role of B cells and antibodies in murine Leishmania amazonensis infection

Leishmania amazonensis infection, occurring predominantly in Central and South America, can manifest itself in several forms, including those of cutaneous and diffuse cutaneous leishmaniasis. The outcome of L. amazonensis infection depends largely on host immune responses to the parasites. While CD4+ T cell activation is a prerequisite for pathogenesis in L. amazonensis-infected mice, the roles of B cells and their antibody production are unclear. In this study, we provide evidence suggesting that B cells and antibodies are involved in disease pathogenesis. We documented a correlation between B cell activation and lesion progress in immunocompetent mice. In the absence of functional B cells and antibodies, JhD mice showed a delayed onset of disease and developed small lesions. Histological examination of these mice revealed a significant reduction in CD4+ and CD8+ T cells, but not in MAC1+ macrophages, at the infection site. In contrast to the wild-type mice that showed typical tissue necrosis, L. amazonensis-infected JhD mice showed no or minimal signs of necrotic foci. A marked reduction in CD4+ T cell proliferation and cytokine (IFN-gamma and IL-10) production in infected JhD mice suggested an involvement of B cells and antibodies in the priming of parasite-specific T cells. This notion was further supported by the observations that adoptive transfer of B cells or antibodies could restore CD4+ T cell activation and migration in infected JhD mice. Moreover, antibody coating of parasites could stimulate dendritic cells to produce high levels of cytokines and increase their ability to prime nai ve CD4+ T cells. Since CD4+ T cells are crucial to disease pathogenesis, this study suggests that B cells and their antibody production enhanced L. amazonensis infection, partially by promoting T cell priming and cellular migration to the infection site.

BOB.1/OBF.1 controls the balance of TH1 and TH2 immune responses

BOB.1/OBF.1 is a transcriptional coactivator essential at several stages of B-cell development. In T cells, BOB.1/OBF.1 expression is inducible by co-stimulation. However, a defined role of BOB.1/OBF.1 for T-cell function had not been discovered so far. Here, we show that BOB.1/OBF.1 is critical for T helper cell function. BOB.1/OBF.1(-/-) mice showed imbalanced immune responses, resulting in increased susceptibility to Leishmania major infection. Functional analyses revealed specific defects in TH1 and TH2 cells. Whereas expression levels of TH1 cytokines were reduced, the secretion of TH2 cytokines was increased. BOB.1/OBF.1 directly contributes to the IFNgamma and IL2 promoter activities. In contrast, increased TH2 cytokine production is controlled indirectly, probably via the transcription factor PU.1, the expression of which is regulated by BOB.1/OBF.1. Thus, BOB.1/OBF.1 regulates the balance of TH1 versus TH2 mediated immunity.

CD83 is a regulator of murine B cell function in vivo

The transmembrane glycoprotein CD83 has been described as a specific maturation marker for dendritic cells and several lines of evidence suggest that CD83 regulates thymic T cell maturation as well as peripheral T cell activation. Here we show for the first time that CD83 is involved also in the regulation of B cell function. CD83 is up-regulated on activated B cells in vivo, specifically in the draining lymph nodes of Leishmania major-infected mice. The ubiquitous transgenic (Tg) expression of CD83 interferes with Leishmania-specific T cell-dependent and with T cell-independent antibody production. This defect is restricted to the B cell population since the antigen-specific T cell response of CD83Tg mice to L. major infection is unchanged. The defective immunoglobulin (Ig) response is due to Tg expression of CD83 on the B cells because wild-type B cells display normal antigen-specific responses in CD83Tg hosts and CD83Tg B cells do not respond to immunization in a mixed wild-type/CD83Tg bone marrow chimera. Finally, the treatment of non-Tg C57BL/6 mice with anti-CD83 mAb induces a dramatic increase in the antigen-specific IgG response to immunization, thus demonstrating a regulatory role for naturally induced CD83 on wild-type B cells.

Epidermal inoculation of Leishmania-antigen by gold bombardment results in a chronic form of leishmaniasis

Experimental leishmaniasis represents a suitable model to analyze Th1-type associated immunity. In C57BL/6 mice healing of leishmaniasis correlates with activation of Th1 cells. Recently, it could be demonstrated that dermal dendritic cells rather than epidermal Langerhans cells are responsible for the activation of Th1 cells after infection, indicating a necessary reconsideration of the role of Langerhans cells. In our current work, epidermal application of Leishmania-antigen prior to infection resulted in an atypical course of disease that is characterized by an impaired Leishmania-specific Th1 response. Consequently, these mice cannot manage an efficient elimination of the parasites at the site of infection. These data point to the activation of immunomodulatory effects by epidermal incorporation of antigen.

Mixed inflammatory/regulatory cytokine profile marked by simultaneous raise of interferon-gamma and interleukin-10 and low frequency of tumour necrosis factor-alpha(+) monocytes are hallmarks of active human visceral Leishmaniasis due to Leishmania chagasi infection

Considering the complexity of the immunological events triggered during active visceral Leishmaniasis (VL), the relevance of the segregation of the immune response during human VL into type 1 and type 2 still remains unclear. For this purpose, in individuals living in risk areas for VL, we have evaluated especially asymptomatic individuals and patients with active VL, the plasmatic levels of cytokines and reactive nitrogen species under ex vivo conditions. In addition, we have also performed an analysis of intracellular cytokine patterns of circulating leucocytes after short-term culture, particularly in the absence of antigenic-specific stimulation, in order to reflect dynamic events of immune response in vivo during Leishmania chagasi infection. Although asymptomatic individuals and non-infected subjects presented a similar immunological profile, an outstanding inflammatory/regulatory profile, based on higher plasmatic levels of cytokines such as interleukin (IL)-8, interferon (IFN)-gamma, tumour necrosis factor (TNF)-alpha, IL-6 and IL-10, was associated with clinical status observed in active VL. In this context, we hypothesize that IL-10, through its ability to inhibit anti-leishmanial macrophage activation, associated with the lower frequency of TNF-alpha(+) monocytes and ordinary levels of nitrite and nitrate are the major mechanisms associated with disease onset.

Uptake of Leishmania major by dendritic cells is mediated by Fcgamma receptors and facilitates acquisition of protective immunity

Uptake of Leishmania major by dendritic cells (DCs) results in activation and interleukin (IL)-12 release. Infected DCs efficiently stimulate CD4- and CD8- T cells and vaccinate against leishmaniasis. In contrast, complement receptor 3-dependent phagocytosis of L. major by macrophages (MPhi) leads exclusively to MHC class II-restricted antigen presentation to primed, but not naive, T cells, and no IL-12 production. Herein, we demonstrate that uptake of L. major by DCs required parasite-reactive immunoglobulin (Ig)G and involved FcgammaRI and FcgammaRIII. In vivo, DC infiltration of L. major-infected skin lesions coincided with the appearance of antibodies in sera. Skin of infected B cell-deficient mice and Fcgamma-/- mice contained fewer parasite-infected DCs in vivo. Infected B cell-deficient mice as well as Fcgamma-/- mice (all on the C57BL/6 background) showed similarly increased disease susceptibility as assessed by lesion volumes and parasite burdens. The B cell-deficient mice displayed impaired T cell priming and dramatically reduced IFN-gamma production, and these deficits were normalized by infection with IgG-opsonized parasites. These data demonstrate that DC and MPhi use different receptors to recognize and ingest L. major with different outcomes, and indicate that B cell-derived, parasite-reactive IgG and DC FcgammaRI and FcgammaRIII are essential for optimal development of protective immunity.

Control of infection with Leishmania major in susceptible BALB/c mice lacking the common gamma-chain for FcR is associated with reduced production of IL-10 and TGF-beta by parasitized cells

Previous studies have shown that the in vitro ligation of FcgammaRs with IgG-opsonized Leishmania amastigotes promotes IL-10 production by macrophages. In addition, infection of either BALB/c mice lacking the common gamma-chain of Fc receptors (FcgammaR(-/-)) or mice genetically altered to lack circulating Ab (J(H)D) with Leishmania pifanoi results in reduced and delayed lesion development and a deficit in the recruitment of inflammatory cells into infected lesions. We show in this study that FcgammaR(-/-) mice can control infection with Leishmania major and totally resolve cutaneous lesions. The ability to eventually control infection is not associated with a reduction in lesion inflammation or a reduction in the ability of Leishmania to parasitize cells through week 6 of infection. The immune response in healing FcgammaR(-/-) mice is associated with a reduction in numbers of cells producing Th2-type cytokines, including IL-4 and IL-10, but not an increase in numbers of IFN-gamma-producing cells characteristic of a dominant Th1-type response. Instead, we observe a reduction in levels of IL-10 and TGF-beta within infected lesions, including reduced levels of these cytokines within parasitized macrophages. Together, these results suggest that uptake of opsonized parasites via FcgammaRs may be a strong in vivo stimulus for the production of anti-inflammatory cytokines that play a role in susceptibility to infection.

Does the Leishmania major paradigm of pathogenesis and protection hold for New World cutaneous leishmaniases or the visceral disease?

Parasitic protozoa of the genus Leishmania have provided a useful perspective for immunologists in terms of host defense mechanisms critical for the resolution of infection caused by intracellular pathogens. These organisms, which normally reside in a late endosomal, major histocompatibility complex (MHC) class II(+) compartment within host macrophages cells, require CD4(+) T-cell responses for the control of disease. The paradigm for the CD4(+) T-helper 1 (Th1)/Th2 dichotomy is largely based on the curing/non-curing responses, respectively, to Leishmania major infection. However, this genus of parasitic protozoa is evolutionarily diverse, with the cutaneous disease-causing organisms of the Old World (L. major) and New World (Leishmania mexicana/ Leishmania amazonensis) having diverged 40-80 million years ago. Further adaptations to survive within the visceral organs (for Leishmania donovani, Leishmania chagasi, and Leishmania infantum) must have been required. Consequently, significant differences in host-parasite interactions have evolved. Different virulence factors have been identified for distinct Leishmania species, and there are profound differences in the immune mechanisms that mediate susceptibility/resistance to infection and in the pathology associated with disease. These variations not only point to interesting features of the host-pathogen interaction and immunobiology of this genus of parasitic protozoa, but also have important implications for immunotherapy and vaccine development.

Contrasting roles of dendritic cells and B cells in the immune control of Epstein-Barr virus

The human gamma-herpesvirus, Epstein-Barr virus (EBV), has growth-transforming potential in vivo and in vitro. Despite this, most healthy carriers remain free of EBV-associated malignancies because of effective T cell-mediated immune control of the virus. A better understanding of these highly efficient control mechanisms is important in the development of new treatment strategies for EBV-associated malignancies. A rational approach to EBV immunotherapy requires answering two questions about the initiation of the protective EBV-specific immune response. The first question is, what is the antigen-presenting cell responsible for priming EBV specific immunity? Second, which viral antigen is central to protective EBV adaptive immunity seen in healthy carriers of the virus? We provide evidence in this review that dendritic cells rather than EBV-transformed B cells are responsible for orchestrating protective EBV immunity and that the EBV nuclear antigen 1 (EBNA1)-specific CD4+ T cell response probably plays a role in resistance against all types of EBV-associated malignancies in healthy carriers. This implies that EBNA1 targeting to dendritic cells should be a component of vaccine and immunotherapy development against EBV-associated malignancies.

Leishmania pifanoi pathogenesis: selective lack of a local cutaneous response in the absence of circulating antibody

Recently, a role for B cells in the pathogenesis associated with infection by Leishmania (Leishmania mexicana complex and L. donovani) has been established. In the case of L. mexicana complex parasites (L. mexicana, L. pifanoi, and L. amazonensis), a critical role for immunoglobulin G-mediated mechanisms for the amastigote stage in the host is evident; however, the immunological mechanisms involved remain to be established. In vitro analysis of the kinetics of parasite uptake by macrophages failed to indicate a major effect of antibody opsonization. Given the importance of CD4(+) T cells in the development of disease caused by these parasites, the possibility that the lack of pathogenesis was due to the lack of development of an immune response at the local site (draining lymph node and/or cutaneous site) was explored. Interestingly, the level of CD4(+)-T-cell activation (proliferation and cytokine) in draining lymph nodes from mice lacking circulating antibody (resistant) was found to be comparable to that in nodes from wild-type mice (susceptible) at 2, 5, and 10 weeks postinfection. However, antibody-deficient animals had markedly reduced numbers of monocytes and lymphocytes recruited or retained at the site of cutaneous infection in comparison to wild-type mice, indicating a selective impairment in the local cutaneous immune response. In vitro antigen presentation studies employing tissue-derived (opsonized) amastigotes demonstrated that L. pifanoi-infected FcR(-/-) macrophages, in contrast to comparably infected wild-type cells, failed to activate Leishmania antigen-specific T lymphocytes. These data, taken together, suggest that one possible mechanism for the role of antibody in pathogenesis may be to mediate parasite uptake and regulate the immune response at the local cutaneous site of infection.

B-cell infiltration and frequency of cytokine producing cells differ between localized and disseminated human cutaneous leishmaniases

Biopsies from human localized cutaneous lesions (LCL n = 7) or disseminated lesions (DL n = 8) cases were characterized according to cellular infiltration,frequency of cytokine (IFN-gamma, TNF-alpha) or iNOS enzyme producing cells. LCL, the most usual form of the disease with usually one or two lesions, exhibits extensive tissue damage. DL is a rare form with widespread lesions throughout the body; exhibiting poor parasite containment but less tissue damage. We demonstrated that LCL lesions exhibit higher frequency of B lymphocytes and a higher intensity of IFN-gamma expression. In both forms of the disease CD8+ were found in higher frequency than CD4+ T cells. Frequency of TNF-alpha and iNOS producing cells, as well as the frequency of CD68+ macrophages, did not differ between LCL and DL. Our findings reinforce the link between an efficient control of parasite and tissue damage, implicating higher frequency of IFN-gamma producing cells, as well as its possible counteraction by infiltrated B cells and hence possible humoral immune response in situ.

B cells and antibodies are required for resistance to the parasitic gastrointestinal nematode Trichuris muris

Previous studies using cell transfers and antibody receptor knockout mice have shown that B cells and antibodies are not essential components of the expulsion mechanism in Trichuris muris infections. Serum transfer experiments have given mixed results regarding the importance of antibodies in this infection model, and the role of B cells in initiating or maintaining T-cell responses has not been addressed. We used B-cell-deficient muMT mice to determine if B cells play a role in anti-T. muris immune responses. In contrast to wild-type C57BL/6 mice, muMT mice were susceptible to infection. Antigen-restimulated mesenteric lymph node cells from infected muMT mice produced only naive levels of Th2-associated cytokines but had increased levels of gamma interferon. However, these mice appeared capable of mounting a Th2-dependent mucosal mastocytosis, though this was significantly delayed compared to that seen in wild-type mice. Resistance to T. muris was restored following reconstitution with naive C57BL/6 splenic B cells, as was in vitro Th2 cytokine production in response to parasite antigen. Treatment of muMT mice with anti-interleukin-12 monoclonal antibody during the first 2 weeks of infection also restored immunity, suggesting that muMT mice can be manipulated to expel worms at the time of T-cell priming. Additionally, treatment of muMT mice with parasite-specific immunoglobulin G1 purified from the serum of resistant NIH mice prevented worm establishment, suggesting an important role for antibodies. Our results as a whole describe the first detailed report of a critical role for B cells in resistance to an intestinal nematode.

Selective activation and expansion of high-affinity CD4+ T cells in resistant mice upon infection with Leishmania major

Using multimers of MHC class II molecules linked to a peptide derived from the Leishmania LACK antigen, we have compared the fate of parasite-specific CD4+ T cells in resistant and susceptible mice transgenic for the beta chain of a LACK-specific TCR. Activated T cells were readily detected in the draining lymph nodes of infected animals. Although the kinetics of activation and expansion were similar in both strains, T cells from susceptible and resistant mice expressed low- and high-affinity TCR, respectively. As T cells from resistant mice produced more IFN-gamma and less IL-4 than those from susceptible animals, our results suggest that differences in TCR usage between MHC-matched animals may influence the development of the antiparasite immune response.

Pathogenesis of herpes simplex virus-induced ocular immunoinflammatory lesions in B-cell-deficient mice

The role of B cells and humoral immunity in herpes simplex virus (HSV) ocular infections was studied in immunoglobulin mu chain gene-targeted B-cell-deficient mice (muK/O). At doses of virus well tolerated by immunocompetent mice, heightened susceptibility of muK/O mice to herpetic encephalitis as well as to herpetic stromal keratitis (HSK) was observed. An explanation was sought for the increased severity of HSK in the muK/O mice. First, the lack of antibody responses in muK/O mice resulted in longer viral persistence and dissemination to the corneal stroma, the site of inflammation. Prolonged virus expression in the corneal stroma was suggested to cause bystander activation of Th1-type CD4(+) T cells, further contributing to the severity of HSK lesion expression in muK/O mice. Second, muK/O mice generated minimal Th2 cytokine responses compared to wild-type mice. Such responses might serve to downregulate the severity of Th1-mediated HSK lesions.

B cell-deficient mice are highly resistant to Leishmania donovani infection, but develop neutrophil-mediated tissue pathology

Resolution of Leishmania infection is T cell-dependent, and B lymphocytes have been considered to play a minimal role in host defense. In this study, the contribution of B lymphocytes to the response against Leishmania donovani was investigated using genetically modified IgM transmembrane domain (muMT) mutant mice, which lack mature B lymphocytes. When compared with wild-type mice, muMT mice cleared parasites more rapidly from the liver, and infection failed to establish in the spleen. The rapid clearance of parasites in muMT mice was associated with accelerated and more extensive hepatic granuloma formation compared with wild-type mice. However, the liver of infected muMT mice also showed signs of destructive pathology, associated with the presence of increased numbers of neutrophils. The role of neutrophils in controlling parasite growth in the viscera was determined by depletion with the mAb RB6-8C5. This treatment led to a dramatic enhancement of parasite growth in both the liver and spleen of muMT and wild-type mice. As assessed by transfer of both normal and chronic-infection serum, Ig protects microMT mice from destructive hepatic pathology, but minimally alters their resistance compared with wild-type mice. However, adoptive transfer of CD4+ and CD8+ T cells into recombinase activating gene 1 (RAG1-/-) recipients, suggested that T cell function was not altered by maturation in a B cell-deficient environment. Taken together, these data suggest an inhibitory role for B lymphocytes in resistance to L. donovani unrelated to the presence or absence of Ig. However, Ig protects muMT mice from the exaggerated pathology that occurs during infection.

Interleukin 1alpha activity of peritoneal and bone marrow macrophages infected with Leishmania major and Leishmania donovani in vitro

In this study, the pattern of interleukin-1alpha (IL-1alpha) production by both peritoneal (PM) and bone marrow macrophages (BMM) from resistant (C3H/HeJ) and susceptible (BALB/c) mice was investigated, using a bioassay and an IL-1alpha-specific ELISA kit. PM from normal uninfected mice showed either an initial high (C3H/HeJ) or a neglected (BALB/c) level of IL-1alpha activity, respectively, probably due to thioglycollate stimulation. Infection with Leishmania major induced only a marginal effect on IL-1 production by both cells. Normal, uninfected and unstimulated BMM from both mice did not produce IL-1alpha over a 7-day period of cultivation in vitro. Upon stimulation with either lipopolysaccharide (LPS) (BALB/c) or concanavalin A (Con A) (C3H/HeJ), both cell types produced IL-1alpha that peaked within the first 12-24 h following stimulation. BMM from C3H/HeJ and BALB/c mice failed to produce IL-1alpha when infected in vitro with L. major or L. donovani promastigotes. However, infection with these two parasites did not interfere with the capability of the host cell to produce IL-1alpha when stimulated with LPS or Con A. The level of IL-1alpha production was independent of the degree of parasitization of the macrophages. Similar results were observed with IL-1beta and IL-6 production by BMM, even though their levels were generally slightly higher than those obtained with IL-1alpha.

An Essential Role for Antibody in Neutrophil and Eosinophil Recruitment to the Cornea: B Cell-Deficient (μMT) Mice Fail to Develop Th2-Dependent, Helminth-Mediated Keratitis

Invasion of the corneal stroma by neutrophils and eosinophils and subsequent degranulation disrupts corneal clarity and can result in permanent loss of vision. In the current study, we used a model of helminth-induced inflammation to demonstrate a novel role for Ab in mediating recruitment of these inflammatory cells to the central cornea. C57BL/6 and B cell-deficient (μMT) mice were immunized s.c. and injected intrastromally with Ags from the parasitic helminth Onchocerca volvulus (which causes river blindness). C57BL/6 mice developed pronounced corneal opacification, which was associated with an Ag-specific IL-5 response and peripheral eosinophilia, temporal recruitment of neutrophils and eosinophils from the limbal vessels to the peripheral cornea and subsequent migration to the central cornea. In contrast, the corneas of μMT mice failed to develop keratitis after intrastromal injection of parasite Ags unless Ags were injected with immune sera. Eosinophils were recruited from the limbal vessels to the peripheral cornea in μMT mice, but failed to migrate to the central cornea, whereas neutrophil recruitment was impaired at both stages. With the exception of IL-5, T cell responses and peripheral eosinophils were not significantly different between C57BL/6 and μMT mice. Taken together, these findings not only demonstrate that Ab is required for the development of keratitis, but also show that recruitment of neutrophils to the cornea is Ab-dependent, whereas eosinophil migration is only partially dependent upon Ab interactions.

BCL-6-Deficient Mice Reveal an IL-4-Independent, STAT6-Dependent Pathway That Controls Susceptibility to Infection by Leishmania major

The BCL-6 gene negatively regulates Th2 responses as shown by the finding that BCL-6-deficient (BCL-6−/−) mice develop a lethal Th2-type inflammatory disease. The response of inbred mouse strains to infection with Leishmania major is under genetic control; BALB/c mice are susceptible and develop a progressive parasite burden, whereas most other common laboratory strains of mice are resistant to infection. We found that BCL-6−/− mice on a resistant genetic background (C57BL/6 × 129 intercrossed mice) were highly susceptible to L. major infection; they resembled BALB/c mice in terms of lesion size, parasite load, and the production of Th2 cytokines. BCL-6−/−IL-4−/− double-mutant mice were also susceptible to L. major infection and produced 10-fold higher levels of the Th2 cytokine IL-13 than IL-4−/− littermate controls. By contrast, BCL-6−/−STAT6−/− double-mutant mice were resistant to L. major infection despite also producing elevated levels of IL-13. These results show that STAT6 is required for susceptibility to L. major infection and suggest that IL-13 signaling through STAT6 may contribute to a nonhealing, exacerbated L. major infection.