Establishment of stable, cell-mediated immunity that makes "susceptible" mice resistant to Leishmania major

Response of scid mice to establishment of Leishmania major infection

The initiation of Leishmania major infection in susceptible BALB/c mice is regulated by interferon-gamma (IFN-gamma). To examine further the mechanisms of IFN-gamma-dependent regulation of the establishment of L. major, we studied the characteristics of the infection in severe combined immunodeficient (scid) mice. In the first 2 weeks of infection, we observed a delay in the development of the lesions in the footpads and lower numbers of parasites in scid compared with BALB/c mice. By week 5 after infection, the size of the leishmanial lesion was similar in both strains of mice, but the number of parasites in scid mice was 100-fold higher than in BALB/c. Treatment with anti-IFN-gamma during the establishment of L. major did not alter the course of infection in scid mice, while it exacerbated lesion development in BALB/c mice. Macrophages from scid mice were unable to kill L. major when stimulated with IFN-gamma in vitro, and produced lower levels of nitric oxide compared with macrophages from susceptible BALB/c or the resistant C57Bl/6 mice. We examined whether delayed lesion development in scid mice was due to their inability to mount appropriate inflammatory responses. While significantly fewer nucleated cells were present in the footpads of scid mice compared with BALB/c, 2 and 3 weeks after infection, no difference in inflammatory response between scid and BALB/c mice was observed in response to L. major antigen in the footpads. In contrast, there was a dramatic increase in the number of cells in the popliteal lymph nodes of BALB/c mice. Decreased inflammatory responses of scid mice in the footpad (at the site of infection) may contribute to slower development of leishmanial lesions during the first 2 weeks of infection.

Vaccine strategies: targeting helper T cell responses

Vaccine strategies need to take into account the balance of T helper subsets they induce. TH1 cells, which secrete IFN gamma and IL-2, are associated with CMI, rather than humoral responses, and afford protection against intracellular infections including parasites. In contrast, TH2 cells secrete IL-4, IL-5, and IL-10; elicit high-titer antibody responses and poor CMI; and are associated with susceptibility to infection with intracellular pathogens. Depending on the type of TH cell bias required, it is possible to manipulate the immune response to a protein or peptide by employing (1) different adjuvants, (2) conjugating the protein to various carriers, (3) immunizing in the presence of cytokines, (4) using alternative routes of administration, or (5) using different forms or doses of antigen. To apply these approaches to a particular vaccine, it is necessary to identify which component of the infectious agent (e.g., envelope protein or peptide) or allergen to target. Once the type of TH cell response that is protective is identified, it may be possible to combine a protein with an adjuvant or link it to a carrier that will promote responses towards the most advantageous TH subset.

A recombinant Leishmania chagasi antigen that stimulates cellular immune responses in infected mice

Cellular immune mechanisms resulting in gamma interferon production are critical for protection against visceral leishmaniasis. Antigens stimulating T-cell responses are likely present in the intracellular amastigote form of the parasite, since this is the form found in a mammalian host. To identify T-cell antigens of Leishmania chagasi, the parasite causing South American visceral leishmaniasis, we used a double antibody-T-cell technique to screen an amastigote cDNA library. One cDNA selected (Lcr1) encodes an antigen that stimulated proliferation of splenic T lymphocytes from infected mice that were either resistant (C3H.HeJ) or susceptible (BALB/c) to L. chagasi infection. The Lcr1 cDNA contains four highly divergent 201-bp repeats homologous to the 204-bp repeat of a Trypanosoma cruzi flagellar antigen gene. Results are consistent with a single copy of the Lcr1 gene producing an mRNA of > 10 kb and a protein of > 200 kDa. Recombinant Lcr1, cloned adjacent to polyhistidine and purified on a nickel affinity column, stimulated gamma interferon but not interleukin-4 (IL-4), IL-5, or IL-10 secretion by T-cell-enriched splenocytes from either susceptible or resistant mice during L. chagasi infection. Immunization with Lcr1 partially protected BALB/c mice against challenge with L. chagasi, indicating the utility of the double screening approach in selecting relevant T-cell antigens.

Preferential recognition of hepatitis B nucleocapsid antigens by Th1 or Th2 cells is epitope and major histocompatibility complex dependent

Regulatory T-helper (Th) cells have been categorized into two functional subsets, Th1 and Th2 cells, which produce distinct lymphokines. In general, Th1 cells mediate cellular immune responses and Th2 cells mediate humoral immunity. Recent serological studies suggest that the Th1-Th2 balance may be relevant in acute and chronic hepatitis B virus (HBV) infections. The purpose of this study was to determine the potential of the nucleocapsid antigens (Ags) (hepatitis B core and e Ags [HBc/eAg]) of HBV to preferentially elicit either a Th1 or a Th2 dominant response. For this purpose, H-2 congenic B10.S and B10 mice were immunized with HBc/eAg, and Ag-specific T-cell proliferative responses, T-cell helper function, and T-cell cytokine production were analyzed. The results indicated that B10.S mice preferentially develop a Th1-like response whereas B10 mice preferentially develop a Th2-like response after immunization with HBc/eAg. Furthermore, the preferential Th1 and Th2 response patterns were reproduced when 12-residue peptides representing the dominant HBc/eAg-specific T-cell sites for B10.S (peptide 120-131) and B10 (peptide 129-140) mice were used as immunogens. Therefore, the combination of the T-cell site recognized and the major histocompatibility complex restricting element can in large part determine the Th phenotype of the HBc/eAg-specific T-cell response. Other factors that influenced Th phenotype were the presence of exogenous cytokines, Ag structure, and tissue distribution.

FIV vaccine studies. II. Clinical findings, hematological changes and kinetics of blood lymphocyte subsets

Five groups of cats were vaccinated with different recombinant feline immunodeficiency virus (FIV) SU vaccines expressed either in Escherichia coli or in the Baculovirus system. In Part I of this series, we described the humoral immune response and outcome of intraperitoneal FIV challenge exposure. Additionally, all cats were monitored for clinical and hematological changes and the course of blood lymphocyte subsets. These results are described in this present paper. A great increase of antibodies was found after vaccination with different recombinant FIV antigens, which did not protect the cats from intraperitoneal FIV challenge infection. This observation was paralleled by an increase of eosinophils during vaccination which was even more pronounced after challenge infection. After FIV challenge, infection lymphadenopathy, gingivitis, pharyngitis, changes in total leukocytes and neutrophils and a decrease in the CD4+:CD8+ ratio were found in cats of all groups and were considered as a sign of the FIV infection taking place, independent of vaccination. The following observations suggest that in these cats a TH2-like immune response was elicited: the high counts of eosinophils, the nature of antigen and adjuvant (aluminium hydroxide) and the high amounts of antigens used for immunization. Clearly, this type of immune response did not protect the animals from intraperitoneal FIV challenge infection.

Oligomannose-coated liposomes as an adjuvant for the induction of cell-mediated immunity

The effect of the coating of ovalbumin-reconstituted liposomes with various oligosaccharides on their immunogenicity was investigated in mice. The coating of liposomes with oligomannose or yeast mannan drastically enhanced their ability to induce an ovalbumin-specific delayed-type footpad swelling response with a peak at 24 to 48 h post-challenge. Among various oligosaccharides tested, only those with mannose residue at the nonreducing termini manifested the activity when applied to liposomes. Since such oligosaccharides are ubiquitously found in the body, these results suggested the usefulness of oligomannose-coated liposomes as a safe adjuvant for the induction of cell-mediated immunity.

Switch from a type 2 to a type 1 T helper cell response and cure of established Leishmania major infection in mice is induced by combined therapy with interleukin 12 and Pentostam

Successful treatment in allergic, autoimmune, and infectious diseases often requires altering the nature of a detrimental immune response mediated by a particular CD4+ T helper (Th) cell subset. While several factors contribute to the development of CD4+ Th1 and Th2 cells, the requirements for switching an established response are not understood. Here we use infection with Leishmania major as a model to investigate those requirements. We report that treatment with interleukin 12 (IL-12), in combination with the antimony-based leishmanicidal drug Pentostam, induces healing in L. major-infected mice and that healing is associated with a switch from a Th2 to a Th1 response. The data suggest that decreasing antigen levels may be required for IL-12 to inhibit a Th2 response and enhance a Th1 response. These observations are important for treatment of nonhealing forms of human leishmaniasis and also demonstrate that in a chronic infectious disease an inappropriate Th2 response can be switched to an effective Th1 response.

Regulation of the interaction between Th1 and Th2 T cell clones to provide help for antibody production in vivo

On the premise that an individual with an intact immune system has the capability to develop both cellular and antibody immune responses supported by the balance between the lymphokines secreted by T helper (Th) cells, we studied the interaction between different types of Th cell clones in vivo and the parameters that may affect this interaction. We used an adoptive transfer system in which nude or lethally irradiated mice were reconstituted with histocompatible CD4+ keyhole limpet hemocyanin (KLH)-specific T cell clones with defined lymphokine profiles. This approach allowed us to study the effects of the cognate interaction between T and B cells in the presence of a defined set of lymphokines. We demonstrated that the co-transfer of both subsets of Th cells resulted in increased production of IgA, and decreased production of IgE and IgG2a. The concomitant presence of both cell types also increases their functional survival in vivo. We have shown that in the presence of a Th2 clone, higher immunization doses (above 100 micrograms trinitrophenol (TNP)-KLH/mouse) result in increased production of IgE and IgG1. In contrast, when a Th1 clone is present, low immunization doses (less than 50 micrograms TNP-KLH/mouse) resulted in increased production of IgG2a. We were also able to show that the neutralization of interleukin-4(IL-4) and or interferon-gamma (IFN-gamma) was sufficient to abrogate most of the regulatory effects caused by the Th2 or the Th1 clone respectively. Our results indicate that the subset of T cell(s) transferred determines the type of response obtained. In addition, the data presented indicate that the antigen dose used for immunization can modulate the quantitative parameters of the response. Furthermore, we have shown that the interaction between the two subsets of T cells in vivo is characterized by both antagonistic and agonistic effects and that most of the regulatory effects exerted by one subset over the other are mediated by IL-4 or IFN-gamma.

Adjuvants, endocrines and conserved epitopes; factors to consider when designing "therapeutic vaccines"

Research into immunity to complex intracellular parasites has recently placed emphasis on the identification of peptide sequences recognised by T-cells, often with the dual objectives of finding species-specific protective epitopes, and of understanding selection of Th1 versus Th2 response patterns. In this review it is suggested that although such work is interesting, it will not achieve these objectives, which must, however, be addressed before we can design the new generation of therapeutic vaccines which may eventually replace antimicrobial drugs in the treatment of infection. First, we suggest that the balance of Th1 to Th2 lymphocyte activity is not determined by epitopes, but rather by adjuvant effects of microbial components which we have barely begun to define, and local endocrine effects mediated by conversion of prohormones into active metabolites by enzymes in lymph node macrophages. Cytokines play a role as mediators within these pathways. In chronic disease states there is a tendency for T-cell function to shift towards Th2. We describe immunopathological consequences of this tendency, including a putative role for agalactosyl IgG, and review evidence for involvement of changes in the endocrine system, brought about not only by the cytokine-hypothalamus-pituitary-adrenal axis, but also by direct actions on peripheral endocrine organs of excess levels of cytokines such as TNF alpha, TGF beta and IL-6. We summarise evidence that the epitopes that are targets for protective cell-mediated responses to complex organisms are usually not species specific. In tuberculosis, cellular responses to species-specific components appear to be associated with immunopathology rather than protection. Finally, we discuss how application of these principles has led to remarkable results in the immunotherapy of tuberculosis, including multidrug-resistant disease.

T cell genetic background determines default T helper phenotype development in vitro

A host's ability to resist certain pathogens such as Leishmania major can depend upon the phenotype of T helper (Th) subset that develops. Different murine genetic backgrounds are known to significantly alter the direction of Th subset development, although the cellular basis of this influence is poorly understood. To examine the basis of this effect we used an in vitro alpha/beta-T cell receptor (TCR) transgenic system for analysis of Th phenotype development. To control for TCR usage, we derived the DO11.10 alpha/beta-TCR transgene in several genetic backgrounds. Our findings suggest that the effects of genetic background on Th phenotype development reside within the T cell, and not the antigen-presenting cell compartment. Transgenic T cells from both the B10.D2 and BALB/c backgrounds showed development toward either the Th1 or Th2 phenotype under the strong directing influence of interleukin (IL) 12 and IL4, respectively. However, when T cells were activated in vitro under neutral conditions in which exogenous cytokines were not added, B10.D2-derived T cells acquired a significantly stronger Th1 phenotype than T cells from the BALB/c background, correspondent with in vivo Th responses to Leishmania in these strains. Importantly, these cytokine differences resulted in distinct functional properties, because B10.D2- but not BALB/c-derived T cells could induce macrophage production of nitric oxide, an important antimicrobial factor. Thus, the genetically determined default Th phenotype development observed in vitro may correspond to in vivo Th subset responses for pathogens such as Leishmania which do not initiate strong Th phenotype-directing signals.

HIV-specific cytotoxic T-cells in HIV-exposed but uninfected Gambian women

A crucial requirement in the rational design of a prophylactic vaccine against the human immunodeficiency virus (HIV) is to establish whether or not protective immunity can occur following natural infection. The immune response to HIV infection is characterized by very vigorous HIV-specific cytotoxic T-lymphocyte (CTL) activity. We have identified four HIV-1 and HIV-2 cross-reactive peptide epitopes, presented to CTL from HIV-infected Gambians by HLA-B35 (the most common Gambian class I HLA molecule). These peptides were used to elicit HIV-specific CTLs from three out of six repeatedly exposed but HIV-seronegative female prostitutes with HLA-B35. These women remain seronegative with no evidence of HIV infection by polymerase chain reaction or viral culture. Their CTL activity may represent protective immunity against HIV infection.

Low-level infection with Trichuris muris significantly affects the polarization of the CD4 response

Resistance and susceptibility to the intestinal nematode Trichuris muris has been shown to be dependent upon the induction of T helper type 2 (Th2) or Th1 cells, respectively. This study demonstrates that in a normally resistant strain of mouse, i.e. BALB/K which mounts a dominant Th2 response, sub-threshold levels of infection (< 40 eggs) can survive and become sexually mature adult worms (10-20 adults). The immunological basis of this phenomenon was found to be a dramatically altered polarization of the CD4 response. The Th2-response characteristic of this strain of mouse infected with T. muris was shown to be significantly down-regulated as assessed by in vitro cytokine production [interleukin (IL)-4, IL-5 and IL-9]. In contrast, Th1 parameters of infection such as in vitro interferon-gamma production and the presence of parasite-specific IgG2a were greatly up-regulated in these mice.

Depletion of interleukin-4 in BALB/c mice with established Leishmania major infections increases the efficacy of antimony therapy and promotes Th1-like responses

Whereas most inbred mouse strains mount a protective Th1 helper T-cell response following infection with Leishmania major, an ineffective Th2 response develops in BALB/c mice, leading to the development of disseminated, ultimately fatal disease. Interleukin-4 (IL-4) production is required for the initiation of the Th2 response, though little is known about the requirements for the long-term maintenance of this response. In order to investigate the role of the expanding parasite population on the Th2 response, mice infected for 2 weeks with L. major, which exhibited a Th2-like cytokine profile, were treated with a leishmanicidal agent (Pentostam) and/or various doses of anti-IL-4 antibody. Untreated mice, mice treated with Pentostam alone, or mice treated with 2.5 mg of anti-IL-4 antibody given at days 13 and 21 of infection developed progressive disease. However, in 8 of 10 mice treated with this dose of anti-IL-4 antibody plus Pentostam lesion development was arrested and lesions were either controlled or eventually healed. Healing was associated with the production of high levels of gamma interferon by spleen cells, and low levels of immunoglobulin E in serum compared with levels for control animals, indicating that a Th1-like response had developed in mice receiving both treatments. Thus, depletion of IL-4 only in combination with a reduction in the parasite burden allowed the expression of a Th1 response. When the dose of anti-IL-4 antibody was increased to 5 mg per injection, all mice treated with this dose of antibody, with or without Pentostam therapy, healed. However, combined therapy with Pentostam in mice treated with this dose of antibody had an additional protective effect. As expected, a Th1 response developed in mice treated with this dose of anti-IL-4 antibody with or without combined therapy with Pentostam, whereas a Th2 response developed in control mice. Thus, a significant effect on the course of disease is noted when mice with established L. major infections are treated with anti-IL-4 antibody in combination with Pentostam, suggesting that the combined effect of inhibiting IL-4 and reducing the parasite burden has a dramatic effect on the development of resistance to L. major.

A qualitative progression in HIV type 1 glycoprotein 120-specific cytotoxic cellular and humoral immune responses in mice receiving a DNA-based glycoprotein 120 vaccine

The potential for eliciting humoral and cytotoxic T lymphocyte (CTL) responses to HIV-1 gp120 by gene gun-based DNA immunization in mice was examined. We speculated that the induction of de novo antigen production in the epidermis of BALB/c mice following particle bombardment-based gene delivery would result in both MHC class I- and class II-mediated antigen presentation for the elicitation of CTL and antibody responses, respectively. Following epidermal delivery of microgram quantities of an expression plasmid, gp120 production resulted in the appearance of MHC class I-restricted, CD8+ CTL responses. gp120-specific CTL responses peaked following a booster immunization, then declined with the appearance of gp120-specific IgG responses when additional booster immunizations were administered. This qualitative progression in the nature of gp120-specific immune responses with subsequent immunizations was paralleled by a simultaneous shift in the interferon-gamma and interleukin 4 release profiles following antigen stimulation of splenocytes in vitro. The simultaneous shifts in immune responses and cytokine release profiles indicate that the progression of antigen-specific CTL and IgG responses in gp120 DNA-immunized mice may be mediated through changes in the in vivo production of cytokines, such as those associated with the Th1 and Th2 subsets of CD4+ cells.

Cytokine-induced immune deviation as a therapy for inflammatory autoimmune disease

The properties and outcome of an immune response are best predicted by the lymphokine phenotype of the responding T cells. Cytokines produced by CD4+ T helper type 1 (Th1) T cells mediate delayed type hypersensitivity (DTH) and inflammatory responses, whereas cytokines produced by Th2 T cells mediate helper T cell functions for antibody production. To determine whether induction of Th2-like cells would modulate an inflammatory response, interleukin 4 (IL-4) was administered to animals with experimental allergic encephalomyelitis (EAE), a prototypic autoimmune disease produced by Th1-like T cells specific for myelin basic protein (MBP). IL-4 treatment resulted in amelioration of clinical disease, the induction of MBP-specific Th2 cells, diminished demyelination, and inhibition of the synthesis of inflammatory cytokines in the central nervous system (CNS). Modulation of an immune response from one dominated by excessive activity of Th1-like T cells to one dominated by the protective cytokines produced by Th2-like T cells may have applicability to the therapy of certain human autoimmune diseases.

Major histocompatibility complex regulation of T helper functions mapped to a peptide C terminus that controls ligand density

The functional status (Th1- versus Th2-like) of CD4 T cells primed against human collagen type IV (hCol IV) or a single 30mer peptide from the alpha 2 chain of this molecule is predicted by the major histocompatibility complex (MHC) class II (I-A) genotype of the responding mice. H-2s mice elicit Th1-like cell-mediated responses to these antigens, whereas Th2-like humoral responses are primed in H-2b,d,k mice. We now report that the ability of MHC to dictate T helper function in this system depends upon a single amino acid of the minimal alpha 2(IV) peptide. The C terminus of this minimal (12mer) peptide is -G-G-P-K, which is predicted to form a beta-turn. The present data demonstrate that the terminal lysine (K) stabilizes the immunogens full biological effects necessary for exclusive cell-mediated responses in H-2s mice. The lysine-truncated (11mer) peptide with otherwise identical sequence effectively primes T helper function in both H-2b and H-2s genotypes. Most importantly, our direct analysis of these peptides' presentation by live antigen-presenting cells (APC) reveals that the 12mer is bound at a log higher density on H-2s APC than on H-2b APC, and that the 11mer is presented at an equally low relative density on APC from both genotypes. In vitro analyses of 12mer/11mer cross-reactive Th clones demonstrate that I-As restricted clones require about 1-2 log lower doses of 12mer peptide than 11mer peptide to stimulate equivalent thymidine incorporation and cytokine release. By contrast, I-Ab-restricted (12mer/11mer cross-reactive) Th clones show no preference for the 12mer and require relatively high peptide doses similar to those required to stimulate the I-As clones with the 11mer peptide. Thus, the peptide dose requirements of Th clones reflect the high density of presentation associated with the 12mer: I-As ligand. Taken together, the results directly support the role of ligand density as an important control point in the functional decision of CD4 T cells.

Prospects for low dose BCG vaccination against tuberculosis

Efficacious vaccination against fast growing pathogens results in a rapid, secondary immune response on natural infection; this provides protection to the vaccinated individual in the race between developing effective immunity and the rapid multiplication of the pathogen. In certain chronic diseases, due to slow growing pathogens, cell-mediated immunity alone can contain the infection, and yet an antibody response is sometimes induced, at the expense of the cell-mediated response, upon natural infection. Such situations arise in leprosy and the leishmaniases and most probably in tuberculosis. AIDS and syphilis. In these cases, the purpose of vaccination must be to ensure that a stable, protective, cell-mediated immune response is inevitably induced upon natural infection. We believe we have developed a general strategy for causing a pathogen-specific imprint upon the immune system so that a stable, protective, cell-mediated response is inevitably induced in all individuals upon natural infection. BALB/c mice are "susceptible" to Leishmania major in the sense that they mount a non-protective antibody response on substantial infection, and consequently suffer chronic and progressive disease. We have demonstrated that infection with low doses of parasites induces only cellular immunity, and establishes the desired imprint. Mice exposed to low doses and challenged some months later with a substantial, normally pathogenic dose of parasites, mount a stable, protective, cell-mediated response and the vaccinated "susceptible" mice withstand the infection. We have recently managed to achieve a similar lock of the immune response of BALB/c mice to BCG into a cell-mediated mode by low-dose exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of humoral and cellular immunity to simian immunodeficiency virus: what are the requirements for protection?

In an effort to produce a strong humoral and cellular immune response that might protect against simian immunodeficiency virus (SIV) infection, groups of five rhesus macaques each were immunized intramuscularly at 0, 2 and 6 months with 100 micrograms of an inactivated preparation of SIV/Delta B670 in either an oil-in-water emulsion with Ribi Detox, containing mycobacterial cell wall skeleton and monophosphoryl lipid A (CWS/MPL) (group A) or a water-in-oil emulsion with incomplete Freund's adjuvant, containing CWS/MPL for the first two injections (group B). Animals were challenged with 10-100 monkey ID50 of monkey-cell-grown SIVmac251 3 months after the last injection, along with a group of four unvaccinated controls. Group B animals demonstrated the strongest immune responses following immunization, including neutralizing antibody titres against the challenge virus ranging from 160 to 320 and SIV-specific ELISA titres ranging from 10(5)-10(6) on the day of challenge, as well as strong in vitro lymphoproliferative and interleukin-2 (IL-2) production responses to the immunogen. Neutralizing antibody was not detectable in group A animals, ELISA titres were lower (10(2)-10(4)), no in vitro lymphoproliferative responses were observed, and in vitro IL-2 production was less pronounced. No protection against challenge was observed in either group. Moreover, group B animals exhibited a more pronounced clinical response following challenge than either group A animals or controls, consisting of hyperthermia and a greater degree of lymphadenopathy on day 7, followed by hypothermia and generally higher levels of serum viraemia on day 14.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokines in the treatment of leishmaniasis: from studies of immunopathology to patient therapy

The genus Leishmania, an obligate intramacrophage parasite, causes a wide spectrum of clinical diseases. It is worldwide in distribution and causes 20 million new cases annually with an at risk population of approximately 1.5 billion persons. The most severe forms are associated with high morbidity, mortality and relapses with conventional therapy. The therapeutic issues and responses to standard and alternative therapies are reviewed. Recent developments in molecular biology and immunology methods employed in the study of leishmaniasis have defined an intricate interaction of the parasite with host immune system. Perturbation of the host immune responses may be part of the survival mechanisms of Leishmania. In murine model, the finding of T helper cells that differ by their panel of cytokines has allowed a more precise definition of immunopathogenesis of leishmaniasis. Preliminary data from leishmaniasis patients lend support to this concept of altered immunomodulation. Furthermore, the data from leishmaniasis patients lend support to this concept of altered enhancement of therapeutic response by interferon-gamma has provided a new approach for treatment of patients using recombinant cytokines and for the study of the disease. Current research for early diagnosis, alternative therapies and need for vaccines are reviewed in the context of the immunopathology of leishmaniasis.

Advances in immunology

Much was accomplished in the last decade in understanding how the adaptive immune system evolved to combat pathogens. Essential features of antigen presentation and T lymphocyte recognition were deciphered, setting the stage for further studies that elucidated basic elements of lymphocyte differentiation (including positive and negative selection during lymphocyte ontogeny) and the major interactions that occur among cells in secondary lymphoid organs in an ongoing immune response. The major challenges of today are found in the burgeoning fields of programmed cells death, enzymology of recombination and somatic mutation, development of memory, and the recognition of pathogens by unconventional lymphocytes.

Site-specific immunity to Leishmania major in SWR mice: the site of infection influences susceptibility and expression of the antileishmanial immune response

Inbred strains of mice usually develop either of two divergent patterns of infection in response to Leishmania major. Resistant mice, which develop self-limiting infections, respond immunologically with the activation of gamma interferon-secreting Th1 helper T cells, while nonhealing infections in susceptible mice are characterized by the proliferation of interleukin-4-secreting Th2 cells. Development of these divergent responses is dependent primarily on the strain of mouse infected, although factors such as the infective dose, species, and strain of parasite can also influence the degree of resistance. In this study, we show that a single mouse strain, SWR, can develop totally divergent patterns of infection depending on the site of parasite inoculation. Both SWR mice and highly susceptible BALB/c mice developed progressive, ultimately fatal disease when inoculated in the dorsal skin over the base of the tail. However, SWR mice infected in the hind footpad developed far less severe infections, which were for the most part controlled, whereas BALB/c mice infected in this site developed severe, nonhealing lesions. Production of gamma interferon and interleukin-4 and measurement of immunoglobulin E levels in serum were used to assess the degree of Th1 and Th2 cell activation in infected mice. Cytokine profiles early in infection had characteristics of a mixed Th1-Th2 response and were similar in SWR mice infected at either site. These early cytokine responses were not predictive of the ultimate disease outcome, since lymph node cells from healing mice eventually produced higher levels of gamma interferon than did those from nonhealing mice, and healing mice had lower levels of immunoglobulin E in serum, suggesting a functional bias toward Th1 cell activity in these animals. The differential ability of SWR mice to heal infections at different cutaneous sites provides a new model for the study of resistance to cutaneous leishmaniasis. Unlike traditional models of infection in which resistant and susceptible strains of mice are compared, this model allows for the study of factors that contribute to healing and nonhealing infections in a genetically identical strain of mouse.

Immune responses to Yersinia enterocolitica in susceptible BALB/c and resistant C57BL/6 mice: an essential role for gamma interferon

Susceptibility of mice to infection with Yersinia enterocolitica has been shown to be related to neither the Ity locus encoding for resistance to Salmonella typhimurium and other pathogens nor the H-2 locus. Recent studies in our laboratory have demonstrated that T-cell-mediated immune responses are required for overcoming primary Yersinia infection. In the present study, we investigated the course of infection with Y. enterocolitica and the resulting immune responses in Yersinia-susceptible BALB/c and Yersinia-resistant C57BL/6 mice. In the early phase of infection, the clearance of the pathogen was comparable in both strains of mice, suggesting similar mechanisms of innate resistance. Splenic T cells from Yersinia-infected C57BL/6 mice exhibited marked proliferative responses and produced gamma interferon (IFN-gamma) upon exposure to heat-killed yersiniae. By contrast, the Yersinia-specific T-cell response in BALB/c mice was weak, and IFN-gamma production could not be detected before day 21 postinfection. T cells isolated from C57BL/6 mice 7 days after infection mediated immunity to Y. enterocolitica but those from BALB/c mice did not, while at 21 days postinfection T cells from both strains mediated protection. Neutralization of IFN-gamma abrogated resistance to yersiniae in C57BL/6 mice but to a far smaller extent in BALB/c mice. Administration of recombinant IFN-gamma or anti-interleukin-4 antibodies rendered BALB/c mice resistant to yersiniae, whereas this treatment did not significantly affect the course of the infection in C57BL/6 mice. These results indicate that the cellular immune response, in particular the production of IFN-gamma by Yersinia-specific T cells, is associated with resistance of mice to Y. enterocolitica.

Effective vaccination of mice against Mycobacterium tuberculosis infection with a soluble mixture of secreted mycobacterial proteins

An experimental vaccine that was based on secreted proteins of Mycobacterium tuberculosis was investigated in a mouse model of tuberculosis. I used a short-term culture filtrate (ST-CF) containing proteins secreted from actively replicating bacteria grown under defined culture conditions. The immunogenicity of the ST-CF was investigated in combination with different adjuvants, and peak proliferative responses were observed when ST-CF was administered with the surface-active agent dimethyldioctadecylammonium chloride. The immunity induced by this vaccine was dose dependent, and, in the optimal concentration, the vaccine induced a potent T-helper 1 response which efficiently protected the animals against a subsequent challenge with virulent M. tuberculosis. Antigenic targets for the T cells generated were mapped by employing narrow-molecular-weight fractions of ST-CF. The experimental vaccine primed a broadly defined T-cell repertoire directed to multiple secreted antigens present in ST-CF. A vaccination with viable Mycobacterium bovis bacillus Calmette-Guérin (BCG), in contrast, induced a restricted T-cell reactivity directed to two secreted protein fractions with molecular masses of 5 to 12 and 25 to 35 kDa. The protective efficacy of the ST-CF vaccine was compared with that of a BCG standard vaccine, and both induced a highly significant protection of equal magnitude. The vaccination with ST-CF gave rise to a population of long-lived CD4 cells which could be isolated 22 weeks after the vaccination and could adoptively transfer acquired resistance to T-cell-deficient recipients. My results confirm the hypothesis that M. tuberculosis cells release protective antigens during growth. The high efficacy of a subunit vaccine observed in the present study is discussed as a possible alternative to a live recombinant vaccine carrier.

Role of cytokines in determining T-lymphocyte function

Early events in an immune response stimulate the production of cytokines that direct the subsequent development of T-helper (Th) subsets with discrete patterns of cytokine production. These events are dictated by the type of antigen/microorganism administered to a host, as well as dose and route of immunization. Bacterial stimuli activate macrophages of the innate immune response to produce IL-12 and drive Th1 development and cell-mediated immunity. Conversely, production of IL-4 early in an immune response favors a Th2 or allergic/humoral immune response. The ability of IL-4 and IL-10 to inhibit Th1 development and effector function, as well as the requirement of committed Th1 cells for co-stimulators to induce maximal IFN-gamma production, suggests that cell-mediated immunity is under strict control, probably to achieve immunity with minimum immunopathology.

B cells are required for the switch from Th1- to Th2-regulated immune responses to Plasmodium chabaudi chabaudi infection

The induction of T-helper cell subsets during the course of blood stage Plasmodium chabaudi chabaudi infection was compared in immunologically intact NIH mice and mice that were depleted of B cells from birth by treatment with anti-mu antibodies. For intact mice, in which the acute primary parasitemia peaked 10 days following infection, purified splenic CD4+ T cells recovered during the ascending parasitemia produced high levels in vitro of interleukin 2 (IL-2) (peak levels on day 10) and gamma interferon (IFN-gamma) (peak levels on day 7). Sera collected from these mice at around this time contained relatively high levels of P. c. chabaudi-specific immunoglobulin 2a (peak levels on day 12), and serum nitric oxide activity was significantly elevated at peak parasitemia. During the descending primary parasitemia, production of IFN-gamma and IL-2 decreased, while levels of IL-4 and IL-10 produced by splenic CD4+ T cells were significantly raised from the time at which subpatency was recorded (day 17) and persisted for at least 50 days. This was concomitant with a significant increase in levels of parasite-specific immunoglobulin G1, which peaked at around the time of recrudescence. Thus, in normal mice, sequential appearance of Th1 and Th2 responses was observed. In contrast, in B-cell-depleted mice, recovery from acute primary parasitemia was followed by a persistent patent infection which did not drop below 0.1% for at least 75 days after initiation of infection. These mice were unable to mount a significant Th2 response, manifest as an enduring inability of splenic CD4+ T cells to produce significant levels of IL-4 and IL-10. IL-2 and IFN-gamma levels remained significantly elevated throughout the 50-day observation period, and there was sustained production of nitric oxide. These data show that immune responses mediated by CD4+ T cells of the Th1 subset are capable of limiting infection beyond the initial acute phase, but that they do not eliminate parasitemia. Furthermore, as the progression from a Th1-regulated to a Th2-regulated immune response fails to occur in B-cell-depleted mice, the data suggest that B cells are required for the downregulation of Th1-mediated and/or the generation of Th2-mediated protective immunity to P. c. chabaudi.

Antigen-specific activation, tolerization, and reactivation of the interleukin 4 pathway in vivo

The outcome of immune responses critically depends on the pattern of lymphokines secreted by CD4+ T cells. CD4+ T cells may differentiate into interleukin 2 (IL-2) and interferon gamma secreting T helper 1 (Th1)-like cells or IL-4/IL-5/IL-10 secreting Th2-like cells. However, the mechanisms that regulate production of IL-4 or other T cell lymphokines in vivo remain unknown. We use the superantigen, Staphylococcus enterotoxin A (SEA), as a model antigen to characterize the signals that regulate the production of IL-4 in vivo. Induction of IL-4 in normal CD4+ T cells required stimulation with both antigen and IL-4. SEA-specific CD4+ T cells produced large amounts of IL-4 when restimulated within 10 d after in vivo priming. Repetitive application of both signals was required to prevent downregulation of IL-4 production. Although controversy exists regarding the susceptibility of Th2-like cells to tolerogenic signals, high doses of superantigen readily abolished the capacity to produce IL-4 in both naive T cells and in T cells already primed for IL-4 production. Infection with the nematode, Nippostrongylus brasiliensis, reversed the established T cell tolerance, whereas the signals which induced IL-4 production in normal T cells, antigen and IL-4, were not capable of reversing superantigen-specific tolerance in vivo. The major parameter that correlated with the capacity of parasitic infection to break tolerance was magnitude of the lymphoproliferation seen during the course of the infection. The capacity to activate or tolerize the IL-4 pathway in an antigen-specific fashion should prove useful in the design of antigen-specific therapies for autoimmune and allergic diseases.

Leishmania-reactive CD4+ and CD8+ T cells associated with cure of human cutaneous leishmaniasis

Fourteen patients suffering from American cutaneous leishmaniasis were studied. Assays of the lymphocyte proliferative response induced in vitro by Leishmania braziliensis antigens were performed. After 5 days in culture, L. braziliensis-stimulated blast T cells were harvested for CD4+ and CD8+ phenotype analysis. When results before and at the end of therapy were compared, leishmaniasis patients showed an increase in the percentage of CD8+ blast T cells and a decline in the proportion of CD4+ blast T cells in cultures. The levels of gamma interferon in T-cell culture supernatants showed a tendency to increase when the patients were cured. These results show a pattern of higher proportions of Leishmania-reactive CD8+ T cells and lower proportions of Leishmania-reactive CD4+ T cells after cure.

Is the incidence of AIDS-associated Mycobacterium avium-intracellulare disease affected by previous exposure to BCG, M. tuberculosis or environmental mycobacteria?

The incidence of disease due to Mycobacterium avium-intracellulare (MAI) in AIDS patients varies from region to region. It has been previously postulated that this difference is the result of a protective effect of neonatal BCG vaccination. The basis of this hypothesis is examined and it is further postulated that a similar protective effect may be afforded by self-limiting tuberculosis in childhood and by exposure to certain environmental mycobacteria. The possibility of developing vaccination strategies to limit the incidence of AIDS-related MAI disease thus requires consideration.

Th1 and Th2 cell-associated cytokines in experimental visceral leishmaniasis

In experimental Leishmania donovani infection in BALB/c mice, initial susceptibility gives way to T-cell-dependent acquired resistance and eventual control over visceral infection. Since various cytokines appear to underlie the host response to Leishmania infection, we examined infected liver tissue for gene expression of cytokines associated with Th1 (gamma interferon [IFN-gamma] and interleukin-2 [IL-2]) and Th2 cells (IL-4 and IL-10). By Northern (RNA) blot analysis, only IFN-gamma mRNA expression was detected in livers of infected euthymic mice. To determine whether activation of Th1 cells develops selectively in this model, qualitative PCR analysis was used. These results indicated that mRNAs for IFN-gamma, IL-2, IL-4, and IL-10 were all induced by L. donovani infection. The potentially negative Th2 cell-associated response did not appear to play a functional role, however, since resistance was acquired, anti-IL-4 monoclonal antibody treatment did not accelerate control over visceral infection, and serum immunoglobulin E levels remained low. As judged by PCR analysis, IL-4 and IL-10 mRNAs were also expressed under three other conditions without apparent effect: in naive euthymic mice treated with IL-2, which induces leishmanicidal activity; in rechallenged immune mice, which resist reinfection; and in nude mice, which fail to control L. donovani. These results suggest that, like other Leishmania species, L. donovani infection may trigger a potentially suppressive Th2 cell-associated cytokine response. However, in T-cell-intact mice able to control L. donovani, this response either is insufficient to influence outcome or more likely is overshadowed by the Th1 cell response.

Signals and signs for lymphocyte responses

The adaptive immune response protects us from infection in a world of pathogens that is forever evolving new variants. As the system is built on the generation of an open repertoire of receptors, the recognition of self is unavoidable, and is guarded against by deletion during lymphocyte development of those cells that are specific for ubiquitous self antigens, and the silencing of those that are specific for self antigens only encountered after cells achieve functional maturity in the periphery. This silencing occurs when lymphocytes recognize antigens in the absence of suitable costimulatory molecules. By contrast, when the same cell encounters the same ligand on a cell that expresses costimulatory molecules, it will proliferate and differentiate into an effector cell. These effector cells mediate protective immunity when the antigen is carried by a pathogen, but they can mount autoimmune responses if the antigen is derived from self. The major costimulatory molecules for CD4 T cells appear to be B7 and B7.2 that bind to the CD28 and CTLA-4 receptors on the T cell. The signals from the TCR appear to be integrated with those from the costimulator receptor, and the T cell response depends on the precise nature of these signals, further conditioned by cytokines present in the environment of the responding cell. B cells can be viewed in a similar way, with the costimulatory molecule CD40 ligand and cytokines coming mainly from CD4 helper T cells determining the fate of the responding B cell. The TCR is not simply an on and off switch, since the precise way in which the TCR is ligated determines the differentiation of the T cell and can alter the effector responses of established T cell lines. Thus, the response capabilities of T cells are more flexible than originally believed, and much of this flexibility comes from the interplay of TCR signals and signs from the environment. If the biochemical nature of these differential signaling pathways were known, it might be possible to develop simple pharmacological agents capable of diverting T cell responses from harmful to innocuous by getting the T cell to reinterpret the signals it is receiving via its receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Prior infection by respiratory syncytial virus or parainfluenza viruses augments virus-specific IgG responses induced by the measles/mumps/rubella vaccine

We found previously that immunizing cyclophosphamide-treated mice with one Paramyxoviridae virus mixed with dimethyl dioctadecyl ammonium bromide induces T cells which apparently also recognize other Paramyxoviridae viruses. This finding and the fact that respiratory syncytial virus (RSV) and parainfluenza viruses (PIVs) infect children early in life led us to ask if prior RSV or PIV infections influence the antibody response to measles and mumps vaccine viruses. Detection of virus-specific IgG in serum specimens collected randomly or at defined times after measles/mumps/rubella (MMR) vaccination was done with solid-phase enzyme immunoassays. The antibody-binding data obtained were converted to serum antibody titers by an immunoassay curve-fitting computer program. Prior infection by RSV and PIVs correlated with an augmented IgG response not only to measles and mumps virus, but also to rubella virus. Furthermore, the augmentation was greater for responders below the median response. These data show that common early childhood viral infections can influence immunity induced by the MMR vaccine.

Cytokines in infectious disease: the battle between host and pathogen

The immune system is poised like a fulcrum to respond quickly to challenge by infectious agents, but can produce excess inflammatory signals or excess suppressive signals when out of balance. During the past year, significant progress has been made in our understanding of how certain pathogens promote immune suppression and shift the balance from the host in their favor. Understanding the mechanisms that underlie excessive inflammatory responses or the suppressive effects of the micro-organism will aid in the development of new therapies.

Cytokines in leishmaniasis: a complex network of stimulatory and inhibitory interactions

The work of immunologists, cell biologists and parasitologists in the field of leishmaniasis has not only provided important insights into the immunopathogenesis of this disease, but also yielded fundamental contributions to our understanding of basic immunological phenomena and of host-parasite interactions. The ability of recombinant interferon-gamma to induce the microbicidal activity of phagocytes and the opposite effect of inhibitory cytokines was first demonstrated with Leishmania-infected macrophages. The selective development of protective and disease-mediating CD4+ T lymphocytes as well as their differential influence on the course of the disease has been long investigated in the murine Leishmania major model and now represents one of the best examples for the in vivo induction of type 1 versus type 2 T helper lymphocytes. At the same time, this model has also been extensively used for immunization studies and cytokine therapy, which shed light on the functions of cytokines in vivo as well as on the mechanism(s) of disease resistance and susceptibility. In this review we will discuss the present picture of the cytokine network in murine L. major infections.

HIV and human complement: mechanisms of interaction and biological implication

Human complement, although not lytic for HIV-1, interacts with the virus and is closely involved in the infectious process. It enhances infection in the absence of antibody, and turns neutralizing antibodies into agents which increase viral infectivity. In this review M.P. Dierich et al. summarize available information and discuss possible biological implications.

T-cell and cytokine responses in leishmaniasis

Intracellular pathogens, particularly those that inhabit lymphocytes and macrophages, represent unique challenges to the immune system. Leishmania are protozoan parasites that replicate exclusively in macrophages and are thus in an excellent position to influence lymphocyte responses. T cell responses are critical in determining the outcome of infections with Leishmania. In both murine and human infections, strong T helper type 2 responses have been associated with disease, and a decreased or absent T helper type 2 response has been associated with healing.

Chemically modified antigen preferentially elicits induction of Th1-like cytokine synthesis patterns in vivo

Differential activation of CD4+ T cell subsets in vivo leads to the development of qualitatively different effector responses. We identify an approach that allows selective activation of strongly Th1-dominated immune responses to protein antigens. Whereas in vivo administration of ovalbumin (OVA) induces cytokine synthesis that is neither Th1 nor Th2 dominated, administration of glutaraldehyde polymerized, high relative molecular weight OVA (OA-POL) leads to 20-fold increase in the ratio of interferon gamma (IFN-gamma)/IL-4 and IFN-gamma/IL-10 synthesis observed after short-term, antigen-mediated restimulation directly ex vivo. In contrast, concurrent in vivo administration of anti-IFN-gamma mAb and OVA or OA-POL results in marked increases in IL-4 and IL-10, and decreased IFN-gamma production, reflecting a polarization of the response towards a Th2-like pattern of cytokine synthesis. These observations may be useful in clinical settings including hypersensitivity, autoimmune diseases, and vaccine development where the ability to actively select specific patterns of cytokine gene expression would be advantageous.

Immune responses associated with susceptibility of C57BL/10 mice to Leishmania amazonensis

Leishmaniae are protozoans which, depending upon both the host and parasite species, can cause either a healing or nonhealing infection. While C57BL/10 mice are able to heal following infection with Leishmania major, they fail to heal following infection with Leishmania amazonensis. In order to address the role of Th1 and Th2 cell responses in the outcome of these infections in C57BL/10 mice, gamma interferon (IFN-gamma) and interleukin-4 (IL-4) production was assessed. While cells from L. major-infected C57BL/10 mice produced high levels of IFN-gamma, cells from L. amazonensis-infected animals produced little or no IFN-gamma. On the other hand, IL-4 was produced only by cells from L. amazonensis-infected C57BL/10 mice, but this production was restricted to the first few weeks of infection. Later in infection, when lesions were evident, no IL-4 was detected. Treatment of BALB/c mice with a monoclonal antibody (11B11) directed against IL-4 induced a dramatic reduction in L. amazonensis lesions. This reduction was associated with a decrease in IL-4 levels and an increase in IFN-gamma production. However, only a slight reduction in lesion sizes and parasite numbers was observed when anti-IL-4-treated C57BL/10 mice were infected with L. amazonensis. These results suggest that IL-4 may have an important role in mediating susceptibility to L. amazonensis in BALB/c mice, as previously demonstrated for L. major. More importantly, however, the data suggest that susceptibility to L. amazonensis in C57BL/10 mice is due to the absence of a Th1 cell response, rather than to the presence of a Th2 cell response.

Selective differentiation of CD4+ T helper cell subsets

During the past year significant advances have been made in our understanding of the factors contributing to the differentiation of CD4 T helper cell subsets. These have been driven, in part, by the realization that signals (cytokines) from the innate immune response are critical in T cell subset differentiation. In addition, several studies have also elucidated how the antigen-presenting cell, the antigenic epitope and the antigen concentration may contribute to the selective development of a particular T helper cell subset.

Identifying strategies for immune intervention

In recent years the molecular basis of antigen recognition by T cells has been unraveled and the various pathways that control T cell activation and functional specialization have been defined. Consequently, it is now possible to delineate various strategies for intervention with the immune system to design protective vaccines, to induce an effective response to tumor antigens, and to control graft rejection and autoimmune diseases.

A TH1-->TH2 switch is a critical step in the etiology of HIV infection

This viewpoint proposes that an imbalance in the TH1-type and TH2-type responses contributes to the immune dysregulation associated with HIV infection, and that resistance to HIV infection and/or progression to AIDS is dependent on a TH1-->TH2 dominance. This hypothesis is based on the authors' findings that: (1) progression to AIDS is characterized by loss of IL-2- and IFN-gamma production concomitant with increases in IL-4 and IL-10; and (2) many seronegative, HIV-exposed individuals generate strong TH1-type responses to HIV antigens.

Dynamic interaction between Leishmania infection in mice and Th1-type CD4+ T-cells: complexity in outcome without a requirement for Th2-type responses

The interaction between Leishmania parasites and Th1 cells is investigated using a simple mathematical model of immunological responses and parasite population growth within the host. The model generates patterns of resistance and susceptibility to infection that mirror observed trends in experimental infections of mice and of humans exposed to infection in areas of endemic transmission. The heterogeneity in outcome predicted by the model can arise either through differences in the values of the parameters that characterize the genetic background of the host or as a consequence of differences in the size of the infecting inoculum of the parasite. Detailed analyses of equilibrium states and of the time course of infection within a host suggest that a limitation in the availability of precursor T-cells, as a consequence of high levels of recruitment into the activated pool, may play a significant role in the progression of infection in susceptible hosts. A brief discussion is presented of the implications of model prediction for therapeutic intervention.

An hypothesis to explain why cell-mediated immunity alone can contain infections by certain intracellular parasites and how immune class regulation of the response against such parasites can be subverted

Cells with a low density of parasite-specific antigens on their surface are postulated to be susceptible to a cell-mediated attack but not to effector mechanisms normally activated following the binding of specific antibody to the infected cell. It is further postulated that such infected cells normally induce a cell-mediated response, and that cells infected with slow-growing intracellular parasites have a low density of parasite-specific antigens on their surface. Despite these general postulates, cell-mediated immunity is not invariably induced following natural infection by certain slow-growing parasites, such as those responsible for leprosy, tuberculosis, and the leishmaniases, and antibody can be induced that is exclusive of a strong, cell-mediated response. It is proposed that certain events in such cases subvert the normal regulatory processes that control the class of immunity induced. In these cases, the parasite-infected cells, bearing a low representation of parasite antigens, induce antibody even though they are not susceptible to antibody-dependent effector mechanisms, and so they are not eliminated. In this case, chronic infection and uncontrolled growth of the parasite occurs, often with fatal consequences.