Resistance to malarial infection is achieved by the cooperation of NK1.1(+) and NK1.1(-) subsets of intermediate TCR cells which are constituents of innate immunity

NK1.1 Expression Defines a Population of CD4+ Effector T Cells Displaying Th1 and Tfh Cell Properties That Support Early Antibody Production During Plasmodium yoelii Infection

Early plasmablast induction is a hallmark of Plasmodium infection and is thought to contribute to the control of acute parasite burden. Although long understood to be a T-cell dependent phenomenon, regulation of early plasmablast differentiation, however, is poorly understood. Here, we identify a population of CD4⁺ T cells that express the innate NK cell marker NK1.1 as an important source of T cell help for early plasmablast and parasite-specific Ab production. Interestingly, NK1.1⁺ CD4⁺ T cells arise from conventional, naive NK1.1⁻ CD4⁺ T cells, and their generation is independent of CD1d but critically reliant on MHC-II. CD4⁺ T cells that express NK1.1 early after activation produce IFN-γ and IL-21, and express the follicular helper T (Tfh) cell markers ICOS, PD-1 and CXCR5 more frequently than NK1.1⁻ CD4⁺ T cells. Further analysis of this population revealed that NK1.1⁺ Tfh-like cells were more regularly complexed with plasmablasts than NK1.1⁻ Tfh-like cells. Ultimately, depletion of NK1.1⁺ cells impaired class-switched parasite-specific antibody production during early Plasmodium yoelii infection. Together, these data suggest that expression of NK1.1 defines a population of rapidly expanding effector CD4⁺ T cells that specifically promote plasmablast induction during Plasmodium infection and represent a subset of T cells whose modulation could promote effective vaccine design.

Reactivity of autoantibodies against not only erythrocytes but also hepatocytes in sera of mice with malaria

In order to further examine the reactivity of autoantibodies, mice were infected with a non-lethal strain of Plasmodium yoelii. Parasitemia appeared between days 10 and 21. During this period, hyperglycemia and hypothermia were serially obeserved and this phenomenon resembled stress-associated responses. In parallel with parasitemia, autoantibodies appeared against nucleus and double-stranded DNA in the sera. To examine further the reactivity of autoantibodies against tissues, immunohistochemical staining using sera from mice with or without malaria was conducted. Autoantibodies contained reactivity to erythrocytes in the spleen, bone marrow and peripheral blood, especially against tissues obtained from mice with malaria. In the liver and intestine, autoantibodies reacted with hepatocytes and intestinal epithelial cells, respectively. These results suggested that the reactivity of autoantibodies against erythrocytes and hepatocytes might be associated with the modulation of the disease course in malaria.

The importance of evolution in the development and course of rheumatoid arthritis

Rheumatoid arthritis (RA) is a complex autoimmune disease of recent evolutionary origin. Genetic drift determines diverse polymorphisms implicated in the susceptibility to RA including the major histocompatibility complex (MHC) class II genes in the so-called shared epitope. These genes originated after the divergence between Homo and Pan from their common ancestry Ardipithecus ramidus about 5 million years ago. Natural selection determined the particular changes in the legs (bipedal position), hands, neck, brain and eusociality in humans which influence the clinical presentation of RA. In this article, we hypothesized that the origin and course of RA may be explainable in the light of evolution.

Human natural killer cells control Plasmodium falciparum infection by eliminating infected red blood cells

Immunodeficient mouse-human chimeras provide a powerful approach to study host-specific pathogens, such as Plasmodium falciparum that causes human malaria. Supplementation of immunodeficient mice with human RBCs supports infection by human Plasmodium parasites, but these mice lack the human immune system. By combining human RBC supplementation and humanized mice that are optimized for human immune cell reconstitution, we have developed RBC-supplemented, immune cell-optimized humanized (RICH) mice that support multiple cycles of P. falciparum infection. Depletion of human natural killer (NK) cells, but not macrophages, in RICH mice results in a significant increase in parasitemia. Further studies in vitro show that NK cells preferentially interact with infected RBCs (iRBCs), resulting in the activation of NK cells and the elimination of iRBCs in a contact-dependent manner. We show that the adhesion molecule lymphocyte-associated antigen 1 is required for NK cell interaction with and elimination of iRBCs. Development of RICH mice and validation of P. falciparum infection should facilitate the dissection of human immune responses to malaria parasite infection and the evaluation of therapeutics and vaccines.

The role of invariant natural killer T cells in microbial immunity

Invariant natural killer T cells (iNKT cells) are unique lymphocytes with characteristic features, such as expression of an invariant T-cell antigen receptor (TCR) α-chain, recognition of glycolipid antigens presented by CD1d molecules, and ability to rapidly produce large amounts of cytokines, including interferon-γ (IFN-γ) and interleukin 4 (IL-4) upon TCR stimulation. Many studies have demonstrated that iNKT cells participate in immune response against diverse microbes, including bacteria, fungi, protozoan parasites, and viruses. Generally, these cells play protective roles in host defense against infections. However, in some contexts they play pathogenic roles, by inducing or augmenting inflammation. Recent reports show that iNKT cells recognize glycolipid antigens from pathogenic bacteria including Streptococcus pneumoniae, and they contribute to host defense against infection. iNKT cell responses to these microbial glycolipid antigens are highly conserved between rodents and humans, suggesting that iNKT cells are evolutionally conserved because their invariant TCR is useful in detecting certain pathogens. Furthermore, glycolipid-mediated iNKT cell activation during immunization has adjuvant activity, enhancing humoral and cell-mediated responses. Therefore, iNKT cell activation is an attractive target for developing new vaccines for infectious diseases.

Testosterone-induced persistent susceptibility to Plasmodium chabaudi malaria: long-term changes of lincRNA and mRNA expression in the spleen

Testosterone (T) is known to induce persistent susceptibility to blood-stage malaria of Plasmodium chabaudi in otherwise resistant female C57BL/6 mice, which is associated with permanent changes in mRNA expression of the liver. Here, we investigate the spleen as the major effector against blood-stage malaria for any possible T-induced long-term effects on lincRNA and mRNA expression. Female C57BL/6 mice were treated with T for 3 weeks, then T was withdrawn for 12 weeks before challenging with P. chabaudi. LincRNA and mRNA expression was examined after 12 weeks of T-withdrawal and after subsequent infections using Agilent whole mouse genome oligo microarrays. Our data show for the first time long-term effects of T on lincRNA expression evidenced directly as persistent changes after T-withdrawal for 12 weeks and indirectly as altered responsiveness of expression to P. chabaudi infections. There are 3 lincRNA-species upregulated and 10 lincRNAs downregulated by more than 2-fold (p<0.01). In addition, 11 and 10 mRNAs are persistently up- and downregulated by T, respectively. These changes remain not sustained during infections at peak parasitemia, when 15 other lincRNAs and 9 other mRNAs exhibit an altered expression. The only exception is the Tnk1-mRNA encoding the non-receptor tyrosine kinase 1 that is persistently downregulated by 0.34-fold after T-withdrawal and that becomes upregulated by 5.9-fold upon infection at peak parasitemia, suggesting an involvement of tyrosine phosphorylation by Tnk1 in mediating long-term effects of T in the spleen. The T-induced changes in splenic mRNA expression are totally different to those previously observed in the liver. Collectively, our data support the view that T induces long-term organ-specific changes in both lincRNA and mRNA expression, that presumably contribute to organ-specific dysfunctions upon infection with blood-stage malaria of P. chabaudi.

Possible influence of resistance to malaria in clinical presentation of rheumatoid arthritis: biological significance of natural selection

Rheumatoid arthritis (RA) is a common autoimmune disease that affects all ethnic groups. Genetic factors, mainly HLA alleles, are highly associated with increased risk to develop RA. However, there are few available data about the role of these genetic polymorphisms in the prevalence or severity of RA in the Afrodescendant population, who have evolutionarily and by natural selection developed mutations that allowed them to acquire resistance to infectious diseases like malaria. Some of the mechanisms, by which this resistance was developed as a product of natural selection, are involved in different forms of immunological response, many of them of a well-known importance in the pathophysiology of RA. This paper focuses on presenting the known mechanisms of resistance to malaria and their possible contribution to the pathophysiology of RA, including "loss-of-function" mutations, lack of expression of chemokine receptors, decrease of immune complexes clearance by asplenia, or increase of immune reactivity mediated by B cells, among other mechanisms in this special group of patients.

Biology of autoreactive extrathymic T cells and B-1 cells of the innate immune system

Cumulative evidence has shown that extrathymic T cells can be autoreactive and that B-1 cells may produce autoantibodies. These T and B-1 cells, which form part of the innate immune system, tend to be activated simultaneously when conventional T and B cells are in a suppressive state, for example, when thymic atrophy occurs by stress or involution with aging. In other words, autoreactive T cells and autoantibody-producing B cells are different from thymus-derived T cells and bone marrow-derived B cells. Activated extrathymic T cells and B-1 cells are often observed in numerous autoimmune diseases, aging, malarial infection and chronic graft-versus-host disease. It is thought that the autoreactivity of extrathymic T cells and B-1 cells may be important for the elimination of "abnormal self" tissues or cells. However, over-activation of innate lymphocytes may be related to the onset of disease or self-tissue destruction. However, it must be emphasized that the autoreactivity of innate lymphocytes is not generated by failure of the thymic pathway of T-cell differentiation or the conventional pathway of B-2 cells.

Organ-specific testosterone-insensitive response of miRNA expression of C57BL/6 mice to Plasmodium chabaudi malaria

Increasing evidence critically implicates miRNAs in the pathogenesis of diseases, but little is known in context with infectious diseases. This study investigates as to whether the testosterone-induced persistent susceptibility to blood-stage malaria of Plasmodium chabaudi coincides with changes in miRNA expression of the anti-malaria effectors sites spleen and liver. Female C57BL/6 mice were treated with vehicle or testosterone (T) for 3 weeks. Then, T treatment was discontinued for 12 weeks before challenge with 10(6) P. chabaudi-parasitized erythrocytes. The miRNA expression was examined after 12 weeks of T withdrawal and during infections at peak parasitemia on day 8 p.i. using miRXplore™ microarray technology. P. chabaudi infections induce an organ-specific response of miRNA expression. We can identify 25 miRNA species to be downregulated by more than 2-fold in the spleen and 169 miRNA species in the liver. Among these 194 miRNA species, there are 12 common miRNA species that are downregulated by 0.48-0.14-fold in both spleen and liver, which are miR-194, miR-192, miR-193A-3P, miR-145, miR-16, miR-99A, miR-99B, miR-15A, miR-152, let-7G, let-7B, and miR-455-3P. Only in the liver, there is an upregulation of the miR-142-5p by 2.5-fold and miR-342-3p by 5.1-fold. After 12 weeks of T withdrawal, the spleen exhibits only the miR-200A that is upregulated by 2.7-fold. In the liver, miR-376B, miR-493*, and miR-188-3P are upregulated by 2.4-fold, 2.2-fold, and 2.1-fold, respectively, and miR-347, miR-200A, and miR-200B are downregulated by approximately 0.4-fold. Upon infection, however, these changes are not sustained, i.e., the miRNA expressions of both spleen and liver of T-pretreated mice exhibit the same response to P. chabaudi malaria as that of vehicle-treated control mice. Our data suggest (1) that the P. chabaudi-induced downregulation of miRNA expression in spleen and liver is required to allow the upregulation of their numerous target genes in response to infection, and (2) that the T-induced persistent susceptibility to P. chabaudi does not affect the responsiveness of miRNA expression in spleen and liver to blood-stage malaria.

Monoclonal auto-antibodies and sera of autoimmune patients react with Plasmodium falciparum and inhibit its in vitro growth

The relationship between autoimmunity and malaria is not well understood. To determine whether autoimmune responses have a protective role during malaria, we studied the pattern of reactivity to plasmodial antigens of sera from 93 patients with 14 different autoimmune diseases (AID) who were not previously exposed to malaria. Sera from patients with 13 different AID reacted against Plasmodium falciparum by indirect fluorescent antibody test with frequencies varying from 33-100%. In addition, sera from 37 AID patients were tested for reactivity against Plasmodium yoelii 17XNL and the asexual blood stage forms of three different P. falciparum strains. In general, the frequency of reactive sera was higher against young trophozoites than schizonts (p < 0.05 for 2 strains), indicating that the antigenic determinants targeted by the tested AID sera might be more highly expressed by the former stage. The ability of monoclonal auto-antibodies (auto-Ab) to inhibit P. falciparum growth in vitro was also tested. Thirteen of the 18 monoclonal auto-Ab tested (72%), but none of the control monoclonal antibodies, inhibited parasite growth, in some cases by greater than 40%. We conclude that autoimmune responses mediated by auto-Ab may present anti-plasmodial activity.

Induction of ssDNA-binding autoantibody secreting B cell immunity during murine malaria infection is a critical part of the protective immune responses

Although it has been hypothesized that autoimmune-like phenomena may play a critical role in the protective immune responses to both human and animal malaria, there are still no evidence-based data to support this view. In this study we demonstrate that the majority of anti-single stranded (ss) DNA autoantibody secreting B cells were confined to B220(+)CD21(+)CD23(-) cells and that these cells expanded significantly in the spleen of C57BL/6 mice infected with Plasmodium yoelii 17X non-lethal (PyNL). To determine the role of ssDNA-binding autoantibody secreting B cell responses in murine malaria, we conjugated generation 6 (poly) amidoamine dendrimer nanoparticles with ssDNA to deplete ssDNA-binding autoreactive B cells in vivo. Our data revealed that 55.5% of mice died after DNA-coated nanoparticle-mediated in vivo depletion of ssDNA-specific autoreactive B cells and subsequent challenge using PyNL. Adoptive transfer of B cells with ssDNA specificity to mice, followed by PyNL infection, caused a later appearance and inhibition of parasitemia. The possible mechanism by which the ssDNA-binding autoantibody secreting B cells is involved in the protection against murine malaria has also been demonstrated.

The spleen CD4+ T cell response to blood-stage Plasmodium chabaudi malaria develops in two phases characterized by different properties

The pivotal role of spleen CD4⁺ T cells in the development of both malaria pathogenesis and protective immunity makes necessary a profound comprehension of the mechanisms involved in their activation and regulation during Plasmodium infection. Herein, we examined in detail the behaviour of non-conventional and conventional splenic CD4⁺ T cells during P. chabaudi malaria. We took advantage of the fact that a great proportion of CD4⁺ T cells generated in CD1d^-/- mice are I-A^b-restricted (conventional cells), while their counterparts in I-A^b-/- mice are restricted by CD1d and other class IB major histocompatibility complex (MHC) molecules (non-conventional cells). We found that conventional CD4⁺ T cells are the main protagonists of the immune response to infection, which develops in two consecutive phases concomitant with acute and chronic parasitaemias. The early phase of the conventional CD4⁺ T cell response is intense and short lasting, rapidly providing large amounts of proinflammatory cytokines and helping follicular and marginal zone B cells to secrete polyclonal immunoglobulin. Both TNF-α and IFN-γ production depend mostly on conventional CD4⁺ T cells. IFN-γ is produced simultaneously by non-conventional and conventional CD4⁺ T cells. The early phase of the response finishes after a week of infection, with the elimination of a large proportion of CD4⁺ T cells, which then gives opportunity to the development of acquired immunity. Unexpectedly, the major contribution of CD1d-restricted CD4⁺ T cells occurs at the beginning of the second phase of the response, but not earlier, helping both IFN-γ and parasite-specific antibody production. We concluded that conventional CD4⁺ T cells have a central role from the onset of P. chabaudi malaria, acting in parallel with non-conventional CD4⁺ T cells as a link between innate and acquired immunity. This study contributes to the understanding of malaria immunology and opens a perspective for future studies designed to decipher the molecular mechanisms behind immune responses to Plasmodium infection.

T-cell Subsets and Antifungal Host Defenses

It has been long appreciated that protective immunity against fungal pathogens is dependent on activation of cellular adaptive immune responses represented by T lymphocytes. The T-helper (Th)1/Th2 paradigm has proven to be essential for the understanding of protective adaptive host responses. Studies that have examined the significance of regulatory T cells in fungal infection, and the recent discovery of a new T-helper subset called Th17 have provided crucial information for understanding the complementary roles played by the various T-helper lymphocytes in systemic versus mucosal antifungal host defense. This review provides an overview of the role of the various T-cell subsets during fungal infections and the reciprocal regulation between the T-cell subsets contributing to the tailored host response against fungal pathogens.

Comparative analysis of activation phenotype, proliferation, and IFN-gamma production by spleen NK1.1(+) and NK1.1(-) T cells during Plasmodium chabaudi AS malaria

The NK1.1 molecule participates in NK, NKT, and T-cell activation, contributing to IFN-gamma production and cytotoxicity. To characterize the early immune response to Plasmodium chabaudi AS, spleen NK1.1(+) and NK1.1(-) T cells were compared in acutely infected C57BL/6 mice. The first parasitemia peak in C57BL/6 mice correlated with increase in CD4(+)NK1.1(+)TCR-alphabeta(+), CD8(+)NK1.1(+)TCR-alphabeta(+), and CD4(+)NK1.1(-)TCR-alphabeta(+) cell numbers per spleen, where a higher increment was observed for NK1.1(+) T cells compared to NK1.1(-) T cells. According to the ability to recognize the CD1d-alpha-GalCer tetramer, CD4(+)NK1.1(+) cells in 7-day infected mice were not predominantly invariant NKT cells. At that time, nearly all NK1.1(+) T cells and around 30% of NK1.1(-) T cells showed an experienced/activated (CD44(HI)CD69(HI)CD122(HI)) cell phenotype, with high expression of Fas and PD-L1 correlating with their low proliferative capacity. Moreover, whereas IFN-gamma production by CD4(+)NK1.1(+) cells peaked at day 4 p.i., the IFN-gamma response of CD4(+)NK1.1(-) cells continued to increase at day 5 of infection. We also observed, at day 7 p.i., 2-fold higher percentages of perforin(+) cells in CD8(+)NK1.1(+) cells compared to CD8(+)NK1.1(-) cells. These results indicate that spleen NK1.1(+) and NK1.1(-) T cells respond to acute P. chabaudi malaria with different kinetics in terms of activation, proliferation, and IFN-gamma production.

A double-edged sword: the role of NKT cells in malaria and HIV infection and immunity

NKT cells are known to play a role against certain microbial infections, including malaria and HIV, two major global infectious diseases. NKT cells exhibit either protective or pathogenic role against malaria. They are depleted by HIV infection and have a direct pathogenic role against many opportunistic infections common in end-stage AIDS. This review discusses the various features of the interaction between NKT cells and malaria parasites and HIV, and the potential to harness this interaction for therapeutic and vaccine strategies.

Appearance of B220low autoantibody-producing B-1 cells at neonatal and older stages in mice

In this study, normal adult mice carried B220(high) conventional B cells in the spleen and liver, but carried both B220(high) and B220(low) in the bone marrow. However, at the neonatal stage, only B220(low) unconventional B cells were found in all these organs. This pattern continued up to 2 weeks after birth, and at this stage autoantibodies were detected in the sera. This phenomenon was seen in all tested young mice (1-2 weeks), irrespective of their gender. Furthermore, at older stages (more than 20 weeks), B220(low) cells reappeared in the spleen and liver, and these B220(low) cells became dominant in the bone marrow. Autoantibodies also reappeared in the sera of these older mice. Cell-sorting experiments revealed that B220(low) cells were able to produce autoantibodies upon lipopolysaccharide stimuli in vitro. These results suggest that B220(low) cells appear at both neonatal and older stages as physiological responses and eventually produce autoantibodies.

Immunologic states of autoimmune diseases

The etiology and immunologic states of autoimmune diseases have mainly been discussed without consideration of extrathymic T cells, which exist in the liver, intestine, and excretion glands. Because extrathymic T cells are autoreactive and are often simultaneously activated along with autoantibody-producing B-1 cells, these extrathymic T cells and B-1 cells should be introduced when considering the immunologic states of autoimmune diseases. The immunologic states of autoimmune diseases resemble those of aging, chronic GVH disease, and malarial infection. Namely, under all these conditions, conventional T and B cells are rather suppressed concomitant with thymic atrophy or involution. In contrast, extrathymic T cells and B-1 cells are inversely activated at this time. These facts suggest that the immunologic states of autoimmune diseases should be reevaluated by introducing the concept of extrathymic T cells and autoantibody-producing B-1 cells, which might be primordial lymphocytes in phylogeny.

Multipotential acceptance of Peyer's patches in the intestine for both thymus-derived T cells and extrathymic T cells in mice

Peyer's patches (PP) are important inductive sites for the mucosal immune response. It is well known that lymphocytes that migrate into PP are mainly of T-cell lineage from thymus-derived cells (i.e. alphabetaTCR(high) cells). In this study, we further characterized the properties of PP lymphocytes in mice using a mouse model of colitis induced by dextran sulphate sodium (DSS). Although the major site of the inflammation induced by DSS is known to be the large intestine, the small intestine was also damaged. When mice developed DSS-induced colitis, CD3+CD8+B220+ gammadelta T cells increased in PP in the small intestine. These gammadelta T cells, which are not seen in the PP of normal mice, resembled intraepithelial lymphocytes (IEL) in the small intestine in terms of their expression of CD5, CD103 and Thy1.2. In addition, the Vgamma/delta repertoire of these gammadelta T cells was similar to that of gammadelta IEL. When DSS-treated mice were injected with IEL isolated from normal mice, IEL including gammadelta T cells preferentially migrated to PP, raising the possibility that B220+ T cells seen in PP of diseased mice may derive from IEL in the small intestine. Our present study suggests that PP might be able to accept T-cell lineages from intestinal IEL as well as from thymus-derived T cells.

Malaria protection in beta 2-microglobulin-deficient mice lacking major histocompatibility complex class I antigens: essential role of innate immunity, including gammadelta T cells

It is still controversial whether malaria protection is mediated by conventional immunity associated with T and B cells or by innate immunity associated with extrathymic T cells and autoantibody-producing B cells. Given this situation, it is important to examine the mechanism of malaria protection in beta(2)-microglobulin-deficient (beta(2)m(-/-)) mice. These mice lack major histocompatibility complex class I and CD1d antigens, which results in the absence of CD8(+) T cells and natural killer T (NKT) cells. When C57BL/6 and beta(2)m(-/-) mice were injected with parasitized (Plasmodium yoelii 17XNL) erythrocytes, both survived from the infection and showed a similar level of parasitaemia. The major expanding T cells were NK1.1(-) alphabeta T-cell receptor(int) cells in both mice. The difference was a compensatory expansion of NK and gammadelta T cells in beta(2)m(-/-) mice, and an elimination experiment showed that these lymphocytes were critical for protection in these mice. These results suggest that malaria protection might be events of the innate immunity associated with multiple subsets with autoreactivity. CD8(+) T and NKT cells may be partially related to this protection.

CD1-restricted T cells in host defense to infectious diseases

CD1 has been clearly shown to function as a microbial recognition system for activation of T cell responses, but its importance for mammalian protective responses against infections is still uncertain. The function of the group 1 CD1 isoforms, including human CD1a, CDlb, and CDLc, seems closely linked to adaptive immunity. These CD1 molecules control the responses of T cells that are highly specific for particular lipid antigens, the best known of which are abundantly expressed by pathogenic mycobacteria such as Mycobacterium tuberculosis and Mycobacterium leprae. Studies done mainly on human circulating T cells ex vivo support a significant role for group I CD1-restricted T cells in protective immunity to mycobacteria and potentially other pathogens, although supportive data from animal models is currently limited. In contrast, group 2 CD1 molecules, which include human CD1d and its orthologs, have been predominantly associated with the activation of CD1d-restricted NKT cells, which appear to be more appropriately viewed as a facet of the innate immune system. Whereas the recognition of certain self-lipid ligands by CD d-restricted NKT cells is well accepted, the importance of these T cells in mediating adaptive immune recognition of specific microbial lipid antigens remains controversial. Despite continuing uncertainty about the role of CD 1d-restricted NKT cells in natural infections, studies in mouse models demonstrate the potential of these T cells to exert various effects on a wide spectrum of infectious diseases, most likely by serving as a bridge between innate and adaptive immune responses.

The unique role of natural killer T cells in the response to microorganisms

Natural killer T (NKT) cells combine features of the innate and adaptive immune systems. Recently, it has become evident that these T cells have crucial roles in the response to infectious agents. The antigen receptor expressed by NKT cells directly recognizes unusual glycolipids that are part of the membrane of certain Gram-negative bacteria and spirochetes. Moreover, even in the absence of microbial glycolipid antigens, these T cells respond to innate cytokines produced by dendritic cells that have been activated by microbes. This indirect sensing of infection, by responding to cytokines from activated dendritic cells, allows NKT cells to react to a broad range of infectious agents.

Contribution of T Cells and Neutrophils in Protection of Young Susceptible Rats from Fatal Experimental Malaria

In human malaria, children suffer very high rates of morbidity and mortality. To analyze the mechanisms involved in age-dependent protection against malaria, we developed an experimental model of infection in rats, where young rats are susceptible to Plasmodium berghei and adult rats control blood parasites and survive thereafter. In this study, we showed that protection of young rats could be achievable by adoptive transfer of spleen cells from adult protected rats, among which T cells could transfer partial protection. Transcriptome analysis of spleen cells transferring immunity revealed the overexpression of genes mainly expressed by eosinophils and neutrophils. Evaluation of the role of neutrophils showed that these cells were able to transfer partial protection to young rats. This antiparasitic effect was shown to be mediated, at least in part, through the neutrophil protein-1 defensin. Further adoptive transfer experiments indicated an efficient cooperation between neutrophils and T cells in protecting all young recipients. These observations, together with those from in vitro studies in human malaria, suggest that the failure of children to control infection could be related not only to an immaturity of their adaptive immunity but also to a lack in an adequate innate immune response.

Protection against malaria due to innate immunity enhanced by low-protein diet

Mice were fed ad libitum with a normal diet (25% protein) or low-protein diets (0-12.5% protein) for a wk and then infected with a nonlethal or lethal strain of Plasmodium yoelii, that is, blood stage infection. The same diet was continued until recovery. Mice fed with a normal diet showed severe parasitemia during nonlethal infection, but survived the infection. They died within 2 wk in the case of lethal infection. However, all mice fed with low-protein diets survived without apparent parasitemia (there were small peaks of parasitemia) in cases of both nonlethal and lethal strains. These surviving mice were found to have acquired potent innate immunity, showing the expansion of NK1.1 -TCRint cells and the production of autoantibodies during malarial infection. Severe combined immunodeficiency (scid) mice, which lack TCRint cells as well as TCRhigh cells, did not survive after malarial infection of lethal strain of P. yoelii, even when low-protein diets were given. These results suggest that low-protein diets enhanced innate immunity and inversely decreased conventional immunity, and that these immunological deviations rendered mice resistant against malaria. The present outcome also reminds us of our experience in the field study of malaria, in which some inhabitants eventually avoided contracting malaria even after apparent malarial infection.

Contribution of NK, NK T, gamma delta T, and alpha beta T cells to the gamma interferon response required for liver protection against Trypanosoma cruzi

In the present work, we show that intracellular Trypanosoma cruzi is rarely found in the livers of acutely infected mice, but inflammation is commonly observed. The presence of numerous intrahepatic amastigotes in infected gamma interferon (IFN-gamma)-deficient mice corroborates the notion that the liver is protected by an efficient local immunity. The contribution of different cell populations was suggested by data showing that CD4- and CD8-deficient mice were able to restrain liver parasite growth. Therefore, we have characterized the liver-infiltrating lymphocytes and determined the sources of IFN-gamma during acute T. cruzi infection. We observed that natural killer (NK) cells increased by day 7, while T and B cells increased by day 14. Among CD3+ cells, CD4+, CD8+, and CD4- CD8- cell populations were greatly expanded. A large fraction of CD3+ cells were positive for PanNK, a beta1 integrin expressed by NK and NK T cells. However, these lymphocytes were not classic NK T cells because they did not express NK1.1 and showed no preferential usage of Vbeta8. Otherwise, liver NK T (CD3+ NK1.1+) cells were not increased in acutely infected mice. The majority of PanNK+ CD4+ and PanNK+ CD8+ cells expressed T-cell receptor alphabeta (TCRalphabeta), whereas PanNK+ CD4- CD8- cells were positive for TCRgammadelta. In fact, gammadelta T cells showed the most remarkable increase (40- to 100-fold) among liver lymphocytes. Most importantly, intracellular analysis revealed high levels of IFN-gamma production at day 7 by NK cells and at day 14 by CD4+, CD8+, and CD4- CD8- TCRgammadelta+ cells. We concluded that NK cells are a precocious source of IFN-gamma in the livers of acutely infected mice, and, as the disease progresses, conventional CD4+ and CD8+ T cells and gammadelta T cells, but not classic NK-T cells, may provide the IFN-gamma required for liver protection against T. cruzi.

Reasons why DBA/2 mice are resistant to malarial infection: expansion of CD3int B220+ gammadelta T cells with double-negative CD4- CD8- phenotype in the liver

DBA/2 (H-2(d)) mice are known to be more resistant than C57BL/6 (B6, H-2(b)) mice to the non-lethal 17XNL strain of Plasmodium yoelii. This is a very strange phenomenon because the functions of conventional T cells, especially CD8(+) T cells, are known to be somewhat lower in DBA/2 mice than in other strains of mice. We examined herein how immune responses differed between DBA/2 mice and B6 mice during malarial infection. DBA/2 mice and (DBA/2 x B6)F(1) (BDF(1), H-2(b/d)) mice were found to have milder parasitaemia and to recover more quickly from malarial infection than B6 mice. These DBA/2 and BDF(1) mice were also found to experience a marked expansion of interleukin (IL)-2Rbeta(+) CD3(int) cells and gammadelta T cells in the liver, especially in the recovery phase. The expansion of unconventional T cells (i.e. B220(+) T cells) was also marked in DBA/2 and BDF(1) mice. The majority of B220(+) T cells were gammadelta T cells and these T cells were double-negative CD4(-) CD8(-). More importantly, the production of immunoglobulin M (IgM)-type anti-DNA autoantibody was also higher in DBA/2 and BDF(1) mice than in B6 mice. In conjunction with data on cytokine production, these results indicate that primitive T and B cells, namely autoreactive extrathymic T cells and autoantibody-producing B cells, may be much more activated in DBA/2 mice and therefore resistant to the non-lethal 17XNL strain of P. yoelii.

Protection against malaria by anti-erythropoietin antibody due to suppression of erythropoiesis in the liver and at other sites

We have previously reported that erythropoiesis commences in the liver and spleen after malarial infection, and that newly generated erythrocytes in the liver are essential for infection of malarial parasites as well as continuation of infection. At this time, erythropoietin (EPO) is elevated in the serum. In the present study, we administered EPO or anti-EPO antibody into C57BL/6 (B6) mice to modulate the serum level of EPO. When mice were infected with a non-lethal strain (17NXL) of Plasmodium yoelii (blood-stage infection of 10(4) parasitized erythrocytes per mouse), parasitemia continued for 1 month, showing a peak at day 17. Daily injection of EPO (200 IU/day per mouse) from day five to day 14 prolonged parasitemia, whereas injection of anti-EPO antibody (1.5 mg/day per mouse) every second day from day five to day 28 decreased it. Erythropoiesis was confirmed in the liver, spleen and bone marrow by the appearance of nucleated erythrocytes (TER119+). When anti-EPO antibody was injected by the same protocol into mice infected with a lethal strain (17XL) of P. yoelii, all mice showed decreased parasitemia and recovered from the infection. These results suggest that the use of anti-EPO antibody after malarial infection may be of therapeutic value in severe cases of malaria.

Plasmodium chabaudi chabaudi (AS): Differential cellular responses to infection in resistant and susceptible mice

The infection with blood stages of Plasmodium chabaudi chabaudi (AS) was followed in BALB/c and DBA/2 mice. Both strains show a peak parasitemia by 7-9 days after infection, display splenic hypercellularity of T and B cells, thymic atrophy, nearly complete depletion of B cells in the bone marrow, and mount comparable polyclonal IgM and IgG responses in the serum. In contrast, these strains diverge in some aspects of the immune response and susceptibility to infection: while BALB/c survive, 70-80% of DBA/2 die within 2 weeks; BALB/c but not DBA/2 show marked increases in the levels of splenic gamma/delta and regulatory T cells, dendritic cells and macrophages and parasite-specific IgM and IgG levels; however, lower levels of TNF-alpha and IL-12 were observed. These results suggest the relevance of different cell populations that are known to participate/regulate specific antibody responses and cytokine production in the susceptibility to infection.

Generation of B220low B cells and production of autoantibodies in mice with experimental amyloidosis: association of primordial T cells with this phenomenon

To investigate the immunological state in amyloidosis, mice were twice intraperitoneally injected (2-week interval) with casein emulsified in complete Freund's adjuvant. Two weeks after the treatment, amyloid deposits were detected in the spleen and other organs of these mice. The number of lymphocytes yielded by the liver and spleen increased significantly. The most affected lymphocyte subset was found to be B cells, namely, the total number of B cells increased and unusual B220low B cells were newly generated in the liver and spleen. In other words, not only normal B220high B cells but also unusual B220low B cells were detected in these organs of mice with amyloidosis. In parallel with this phenomenon, autoantibodies against denatured DNA were detected in sera. Since such autoantibodies are known to accompany the functional activation of NKT cells, NKT cell-deficient mice were used for the induction of amyloidosis. Such mice showed less formation of amyloidosis and lower levels of autoantibodies in sera. Athymic nude mice were NKT cell-deficient but NK1.1- TCRint cells were present. These athymic mice showed an intermediate induction of amyloidosis. The cytokine profile seen in mice with amyloidosis was the Th0 type, showing simultaneous production of IL-4 and IFNgamma. These results suggest that the generation of B220low B cells and the production of autoantibodies in aid of primordial T cells may be major immunological mechanisms in amyloidosis mice.

The age-related resistance of rats to Plasmodium berghei infection is associated with differential cellular and humoral immune responses

In this study, we investigated how the age of rats would affect the course of infection of and the immune response to Plasmodium berghei. Both young (4-week-old) and adult rats (8-week-old) can be infected with P. berghei ANKA strain, with significantly higher levels of infected red blood cells in young rats. While 100% of young rats succumbed to infection, adult rats were able to clear blood parasites and no mortality was observed. Analysis of cellular distribution and circulating cytokines demonstrated the persistence of CD4+/CD25+ T cells and high expression of circulating interleukin-10 (IL-10) during the progression of infection in young-susceptible rats, whereas high levels of CD8+ T cells and natural killer T cells are detected in adult-resistant rats. Analysis of antibody isotypes showed that adult rats produced significantly higher levels of interferon-gamma (IFN-gamma)-dependent IgG2c antibodies than young rats during infection. Further evaluation of the role of IL-10, IFN-gamma and of immune cells showed that only the adoptive transfer of spleen cells from adult-resistant rats was able to convert susceptibility of young-susceptible rats to a resistant phenotype. These observations suggest that cell-mediated mechanisms are crucial for the control of a primary infection with P. berghei in young rats.

16alpha-Bromo-epiandrosterone therapy modulates experimental feline immunodeficiency virus viremia: initial enhancement leading to long-term suppression

16alpha-Bromo-epiandrosterone (epiBr), a synthetic derivative of the natural hormone dehyroepiandrosterone (DHEA), was evaluated for its effects on feline immunodeficiency virus (FIV) infection in experimental cats. The rationale for this study was based on the ability of DHEA to significantly reduce the mortality to viral infections in mice. DHEA and epiBr also have demonstrable in vitro anti-viral activity for both HIV-1 and FIV. Preliminary pharmacokinetic studies in cats demonstrated that subcutaneously injected epiBr was rapidly absorbed, completely metabolized, and nontoxic. Metabolites were excreted in both urine and feces, with the latter having the most complex pattern of breakdown products. Cats were then divided into four groups; two groups were infected with FIV and two uninfected. Two groups, one infected and one uninfected were treated on 5 consecutive days of weeks 0, 4, 8, 12 and 16 with epiBr. The remaining two groups were mock treated with the drug vehicle alone. Treatment started 1 week prior to infection and extended for 4 weeks after infection. Cats were observed for 20 weeks post-FIV infection. Infected cats had identical decreases in blood neutrophil and lymphocyte counts following, regardless of whether they were treated with epiBr or vehicle alone. The CD4/CD8 T-cell ratio was decreased following FIV exposure, but was significantly more decreased for the epiBr treated animals from week 2 post-infection onward. CD4+ T cells were decreased in FIV-infected cats treated with epiBr compared to their untreated cohort, while CD8+ T cells tended to be higher in treated animals. FIV infected cats that were treated with epiBr had over one-log higher virus loads at week 2 post-infection than non-epiBr treated cohorts. In spite of this enhanced initial viremia, the subsequent levels of virus in the blood were significantly lower in epiBr treated versus untreated animals. EpiBr treated cats had significantly higher FIV-p24 antibody responses than control cats receiving vehicle alone, although primary and secondary antibody responses to a T-cell dependent non-FIV antigen, keyhole limpet hemocyanin (KLH), were unaffected. EpiBr treatment significantly decreased the expected FIV-induced suppression of IL-12 p40 mRNA levels in peripheral blood mononuclear cells (PBMCs) observed at weeks 4, 5, 8, 9 and 16 post-infection, but had no influence on FIV-induced changes in IL-4, IL-6, IL-10, IFN-gamma, MIP-1alpha and RANTES.

AIM inhibits apoptosis of T cells and NKT cells in Corynebacterium-induced granuloma formation in mice

Apoptosis inhibitor expressed by macrophages (AIM) inhibits apoptosis of CD4(+)CD8(+) (CD4/CD8) double-positive thymocytes, and supports the viability of these cells on the thymic selection. However, pleiotropic functions of AIM have been suggested. In this study, heat-killed Corynebacterium parvum (C. parvum) was injected into mice carrying the homozygous mutation (AIM(-/-)) and wild-type (AIM(+/+)) mice, to investigate the role of AIM in the formation of hepatic granulomas. In AIM(-/-) mice, the size and the number of hepatic granulomas were larger, and the resorption of granulomas was more delayed than in AIM(+/+) mice. The production of interleukin-12 was more prominent in AIM(-/-) mice than in AIM(+/+) mice. In the liver of AIM(+/+) mice, expression of AIM messenger ribonucleic acid (mRNA) increased after C. parvum injection. In situ hybridization demonstrated that AIM mRNA was expressed in Kupffer cells and exudate macrophages in the liver, especially in granulomas. Larger numbers of T cells and natural killer T (NKT) cells underwent apoptosis in the granulomas of AIM(-/-) mice, suggesting that AIM prevents apoptosis of NKT cells and T cells in C. parvum-induced inflammation. Recombinant AIM (rAIM) protein significantly inhibited apoptosis of NKT cells and T cells obtained from C. parvum-stimulated livers in vitro. These results indicate that AIM functions to induce resistance to apoptosis within NKT cells and T cells, and supports the host defense in granulomatous inflammation.

Essential role of extrathymic T cells in protection against malaria

Athymic nude mice carry neither conventional T cells nor NKT cells of thymic origin. However, NK1.1(-)TCR(int) cells are present in the liver and other immune organs of athymic mice, because these lymphocyte subsets are truly of extrathymic origin. In this study, we examined whether extrathymic T cells had the capability to protect mice from malarial infection. Although B6-nu/nu mice were more sensitive to malaria than control B6 mice, these athymic mice were able to survive malaria when a reduced number of parasitized erythrocytes (5 x 10(3) per mouse) were injected. At the fulminant stage, lymphocytosis occurred in the liver and the major expanding lymphocytes were NK1.1(-)TCR(int) cells (IL-2Rbeta(+)TCRalphabeta(+)). Unconventional CD8(+) NKT cells (V(alpha)14(-)) also appeared. Similar to the case of B6 mice, autoantibodies (IgM type) against denatured DNA appeared during malarial infection. Immune lymphocytes isolated from the liver of athymic mice which had recovered from malaria were capable of protecting irradiated euthymic and athymic mice from malaria when cell transfer experiments were conducted. In conjunction with the previous results in euthymic mice, the present results in athymic mice suggest that the major lymphocyte subsets associated with protection against malaria might be extrathymic T cells.

Extrathymic T cells in malaria protection, including evidence for the onset of erythropoiesis in the liver during infection

This review proposes the possibility that malarial protection might not be achieved through the process of acquired immunity in which the constituents are conventional T and B (B-2) cells. On the other hand, malarial protection might be achieved by the process of innate immunity in which the constituents are extrathymic T cells and autoantibody-producing B-1 cells. Accordingly, mice infected with malaria exhibited severe thymic atrophy, and the expansion of IL-2Rbeta+ CD3int cells and its subset of NK1.1-CD3int cells were simultaneously induced. In parallel with the expansion of extrathymic T cells in the liver, extramedullary erythropoiesis was found to begin in the liver of these mice. Interestingly, malarial protozoa were primarily seen in only these nucleated erythrocytes in the liver at the early stage of infection. These results suggest that malaria immunology falls into a new field of immunology, namely, innate immunity. The similarity of the immune states among malaria, aging, and autoimmune diseases also suggest that the immunosuppression of a conventional, acquired immune system is more likely the common mechanism underlying these diseases or physiological responses.