Immunization with Babesia bigemina rhoptry-associated protein 1 induces a type 1 cytokine response

Anaplasma marginale field challenge: Protection by an inactivated immunogen that shares partial sequence of msp1α variable region with the challenge strain

Twenty four Hereford heifers free of anaplasmosis were allotted into three groups of eight animals each and inoculated three times with adjuvant in Puck saline as control or 50 microg and 100 microg of total protein of Anaplasma marginale initial bodies from three Mexican strains which share the same variable region of msp1alpha and msp4. Inoculation with the adjuvant or the immunogen at either of the two protein doses did not induce any undesirable changes attributable to inoculation in vaccinates or controls. On day 78 post vaccination animals were released in a ranch where bovine Anaplasmosis is endemic. The A. marginale strain prevalent in this ranch shares some of the msp1alpha tandem repeats with and the strains used in the vaccine. After release, all animals became infested with Boophilus microplus ticks and flies. During the challenge period, between days 279 and 300, loss of PCV due to clinical anaplasmosis in control animals was statistically higher from vaccinated animals. Likewise, controls mean peak rickettsemia was also significantly higher (p< or =0.01) than vaccinates' rickettsemias. The antibody responses of all vaccinates after the third vaccination reached OD values above 2.0 on day 49 and were different from controls (p<0.01). IgG(2) responses from both groups of vaccinates were different from controls (p<0.01). Vaccinates which required treatment, also showed the lowest IgG(2) and substantial IgG(1) responses. After contact with the rickettsia, controls developed clinical disease and 7 out of 8 required treatment, while vaccinates in general showed no substantial changes in hematocrit or rickettsemia and only one animal in each group required treatment. Our present results show that vaccination with either 50 microg or 100 microg of protein from purified IB derived from three strains induced protection to resist the challenge with the a field strain that shares some of the tandem repeats of MSP1a.

Prospects for recombinant vaccines against Babesia bovis and related parasites

Babesial parasites infect cattle in tropical and temperate regions of the world and cause significant morbidity and mortality. Discovery of protective antigens that could be used in a killed vaccine has been slow and to date there are few promising vaccine candidates for cattle Babesia. This review describes mechanisms of protective innate and adaptive immune responses to babesial parasites and different strategies to identify potentially protective protein antigens of B. bovis, B. bigemina, and B. divergens. Successful parasites often cause persistent infection, and this paper also discusses how B. bovis evades and regulates the immune response to promote survival of parasite and host. Development of successful non-living recombinant vaccines will depend on increased understanding of protective immune mechanisms and availability of parasite genomes.

Protective efficacy of antigenic fractions in mouse models of cryptococcosis

Infections due to the encapsulated fungus Cryptococcus neoformans are a significant cause of morbidity and mortality in patients with impaired T-cell function, particularly those with AIDS. Presumably then, T-cell responses to cryptococcal antigens are critical for protection against this ubiquitous fungus. To test the protective efficacy of these antigens as vaccine candidates, secreted cryptococcal antigens were separated by concanavalin A affinity chromatography into adherent (mannoprotein [MP]) and nonadherent (flowthrough [FT]) fractions, and the fractions were tested in murine models of disseminated cryptococcosis. Compared with adjuvant alone, C57BL/6 mice that received two inoculations of MP and FT exhibited prolonged survival and reduced brain and kidney fungal loads following intravenous challenge with C. neoformans strain B3501. MP-immunized animals had increased brain levels of tumor necrosis factor alpha, gamma interferon, and interleukin-2. Histopathologic examination revealed that compared with organs from mice that received only adjuvant, MP-immunized mice were able to recruit a stronger cellular infiltrate in brain, kidney, and liver in response to cryptococcal infection. Conjugated O-linked glycans were necessary for optimal MP-mediated protection, because chemical O deglycosylation reduced the protective efficacy of MP immunization. FT and MP immunization protected B-cell-deficient, but not T-cell-deficient mice, suggesting that protection was T-cell mediated. CBA/J mice also benefited from immunization with FT and MP, although the benefits were more modest than those seen with C57BL/6 mice. Thus, both MP and FT fractions of C. neoformans contain components that protect mice from disseminated cryptococcosis, and this protection appears to be T-cell mediated.

Bovine T lymphocyte responses to Brucella abortus

The long-held paradigm of T lymphocyte-mediated activation of mononuclear phagocytes (Mø) as the major mechanism of protection against facultative intracellular pathogens such as Brucella has been modified to include killing of infected Mø by various subsets of T lymphocytes. Remnants of killed infected cells are phagocytosed by immunologically-activated Mø, which are much more efficient at killing such pathogens. Most of the detailed information regarding immunity in general and that of brucellosis specifically has been obtained using murine infection models rather than in cattle. However, there has been considerable definition of cellular phenotypes, cytokines and functional characteristics of T lymphocytes in cattle over the last decade. This was mainly due to development of monoclonal antibodies against cell surface markers and application of molecular cloning and polymerase chain reaction (PCR) for isolation, characterization and detection of genes encoding bovine cytokines. This review discusses cellular and molecular immunity in bovine brucellosis as pertains to T lymphocyte interactions with the Mø. Although current knowledge directly obtained from brucellosis immunity studies in the bovine host is limited and incomplete, the many parallels between the bovine and murine immune systems allow for some extrapolation in the description of bovine host defense mechanisms. Direct information from studies with immunized cattle supports the concepts of coordinate activation of uninfected Mø and killing of Brucella-infected Mø by antigen-specific T lymphocytes as major mechanisms of host defense in bovine brucellosis. There also appears to be a bias in the T lymphocyte compartment towards recognition of particular bacterial stress proteins following immunization with live Brucella vaccines.

Regulation of bovine IL-12R beta 2 subunit mRNA expression in bovine lymph node cells

The beta 2 subunit of the interleukin (IL)-12 receptor (IL-12R beta 2) has been shown to play an essential role in differentiation of T helper 1 (Th1) cells in the murine and human system, and antibodies raised against IL-12R beta 2 recognized this molecule on human Th1 but not Th2 cells. However, while the cytokines secreted by clones of murine cells allowed the definition of distinct T helper cell subsets, bovine clones with polarized Th1 and Th2 cytokine profiles were rarely found. This raised important questions about the regulation of immune responses in cattle. We therefore cloned bovine IL-12R beta2 (boIL-12R beta 2) DNA complementary to RNA (cDNA) from the start codon to the 3' end of the mRNA. Comparison of boIL-12R beta 2 cDNA with human and murine IL-12R beta 2 cDNA sequences revealed homologies of 85 and 78%, respectively. The deduced protein sequence showed the hallmark motifs of the cytokine receptor superfamily including the four conserved cysteine residues, the WSXWS motif and fibronectin domains in the extracellular part as well as a STAT4 binding site in the intracellular part of the molecule. Using real-time reverse transcription-polymerase chain reaction, upregulation of mRNA expression of this molecule could be demonstrated in cultured bovine lymph node cells stimulated with phytohemagglutinin. Furthermore, cells with upregulated boIL-12R beta 2 mRNA responded with enhanced expression of interferon gamma to treatment with interleukin 12.

Interferon-gamma and interleukin-4 mRNA expression by peripheral blood mononuclear cells from pregnant and non-pregnant cattle seropositive for bovine viral diarrhea virus

The acceptance of the fetal allograft by pregnant women and mice seems to be associated with a shift from a Th 1 dominated to a Th 2 dominated immune response to certain infectious agents. The goal of this study was to examine cytokine expression in peripheral blood mononuclear cells (PBMCs) from cattle immune to bovine viral diarrhea virus (BVDV) to determine whether pregnancy also has an influence on the type of immune response in this species. Forty-six heifers and cows between 14 months and 13 years of age were included in this study. Twenty-four were seropositive and 22 seronegative for BVDV. Eleven of the seropositive animals and 11 of the seronegative animals were in the eighth month of gestation, the remaining animals were virgin heifers. PBMC from these animals were analyzed for Interferon (IFN)-gamma and Interleukin (IL)-4 mRNA expression by real-time RT-PCR after stimulation with a non-cytopathic strain of BVDV. Additionally, an ELISA was performed to measure IFN-gamma in the supernatants of stimulated cell cultures. In BVDV seropositive animals, IFN-gamma mRNA levels were significantly higher than in BVDV seronegative animals and there was a significant positive correlation between the changes in IFN-gamma and IL-4 mRNA expression. There was, however, no significant difference in IFN-gamma and IL-4 mRNA levels between pregnant and non-pregnant animals. These results are inconsistent with BVDV inducing a Th1 or Th2 biased immune response. Furthermore, a shift in the cytokine pattern during bovine pregnancy was not evident.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Interleukin-12 as an adjuvant promotes immunoglobulin G and type 1 cytokine recall responses to major surface protein 2 of the ehrlichial pathogen Anaplasma marginale

Anaplasma marginale is a tick-transmitted pathogen of cattle closely related to the human ehrlichiae, Ehrlichia chaffeensis and the agent of human granulocytic ehrlichiosis (HGE). These pathogens have in common a structurally conserved outer membrane protein (OMP) designated the major surface protein 2 (MSP-2) in A. marginale and HGE and OMP-1 in E. chaffeensis. Protective immunity against ehrlichial pathogens is believed to require induction of gamma interferon (IFN-gamma) and opsonizing immunoglobulin (Ig) subclasses directed against OMP epitopes that, in concert, activate macrophages for phagocytosis and killing. Because interleukin-12 (IL-12) acts as an adjuvant for protein immunization to induce IFN-gamma and protective immunity against intracellular pathogens, we hypothesized that as an adjuvant with MSP-2, IL-12 would augment type 1 recall responses to A. marginale. IL-12 was coadsorbed with MSP-2 to alum and shown to significantly enhance IFN-gamma production by lymph node cells (LNC) and LNC-derived CD4(+) T-cell lines from immunized calves following recall stimulation with A. marginale. LNC proliferation and IL-2 production were also enhanced in IL-12-treated calves. Elevated recall proliferative responses by peripheral blood mononuclear cells were still evident 9 months after immunization. Serum IgG levels were consistently increased in IL-12 immunized calves, predominantly due to higher IgG1 responses. The results support the use of IL-12 coadsorbed with OMP of ehrlichial pathogens in alum to amplify both antibody and type-1 cytokine responses important for protective immunity.

Designing blood-stage vaccines against Babesia bovis and B. bigemina

The tick-transmitted apicomplexan parasites Babesia bovis and B. bigemina cause significant disease in cattle in many tropical and temperate areas of the world. These parasites present a challenge for vaccine development, and yet provide a system for studying the pathogenesis, mechanisms of protective immunity and regulation of host immune responses associated with intraerythrocytic protozoan parasites in a non-rodent species. In this article, Wendy Brown and Guy Palmer review strategies for identifying candidate vaccine antigens of B. bovis and B. bigemina and for priming immune responses to evoke strain crossprotective immunity.

Upregulation of interleukin-4 and IFN-gamma expression by IFN-tau, a member of the type I IFN family

Trophoblast interferon-tau (IFN-tau) is a new member of the type I IFN family that is produced in large quantities by the ruminant conceptus. Like other type I IFN, IFN-tau inhibits viral replication and activates natural killer (NK)-mediated cytotoxicity. In mice and humans, type I IFN enhances type 1 T helper (Th) cell responses, but the effects of type I IFN, including IFN-tau, on cytokine expression by bovine Th cells have not been described. The present study determined the effects of IFN-tau on interleukin-4 (IL-4), IFN-gamma, and IL-10 expression by antigen-specific, CD4+ T cell lines derived from cattle immune to either Babesia bovis, Babesia bigemina rhoptry-associated protein-1, or Anaplasma marginale. IFN-tau upregulated IFN-gamma secretion and steady-state levels of IFN-gamma and IL-4 mRNA by cell lines cultured for 3-6 weeks. In contrast, the steady-state levels of IL-10 mRNA were either not changed or inhibited at these times. Similar effects were obtained with human IFN-alpha. Comparison of the quantities of IFN-gamma, IL-4, and IL-10 transcripts in IFN-tau-treated or IFN-alpha-treated cultures revealed that even though IFN-gamma was the predominant cytokine expressed by all T cell lines, both IFN-gamma and IL-4 steady-state transcript levels were upregulated by a comparable degree. Thus, these studies demonstrate that IFN-tau is an immunomodulatory cytokine that promotes enhanced IL-4 and IFN-gamma responses by effector T cells but not, strictly speaking, Thl-biased responses in cattle. These results indicate the potential use of this cytokine as an adjuvant in ruminants to boost cell-mediated immune responses.

Development and in vitro characterization of recombinant vaccinia viruses expressing bovine leukemia virus gp51 in combination with bovine IL4 or IL12

Type 1 and type 2 immune responses are modulated by IL12 or IL4, respectively, at the time of lymphocyte priming. Importantly, type 1 responses have been associated with resistance to retroviral infection in mice, humans, and ruminants. Specifically, vaccination of sheep with vaccinia virus expressing bovine leukemia virus (BLV) gp51 resulted in protective immunity with the characteristics of a type 1 response, whereas vaccination of cattle resulted in a non-protective type 2 response. In order to test the hypothesis that cattle inoculated with BLV gp51 and IL12 will respond with a type 1 response, a recombinant vaccinia virus expressing BLV gp51 together with bovine IL12 was developed and characterized in vitro. For induction of type 2 responses a recombinant vaccinia virus expressing gp51 with bovine IL4 was similarly constructed and characterized. In this study recombinant cassettes were developed containing either the BLVenv gene alone or in combination with bovine IL4 or the two genes, p35 and p40, encoding bovine IL12. Correct alignment with p7.5 or p11 vaccinia promoters and orientation was confirmed by complete sequencing. Recombinant vaccinia viruses were generated by homologous recombination, selected based on large plaque formation due to reconstitution of the vp37 gene, and structurally confirmed by Southern blotting. Transcription of recombinant BLVenv, bovine IL4, p35 and p40 was demonstrated by RT-PCR. Expression of BLVenv gp51 protein and bovine IL4 was shown by immunofluorescence and immunoblotting. Biologically active bovine IL4 expressed by vaccinia virus stimulated lymphoblast proliferation, B lymphocyte proliferation in the presence of CD40L, and inhibited IFN gamma secretion from PHA activated PBMC in a dose dependent fashion. Finally, bovine IL12 expression and biological function was confirmed by dose dependent induction of IFN gamma secretion by PHA activated PBMC and the moderate enhancement of lymphoblast proliferation. In conclusion, bovine IL12 and IL4 expressed by recombinant vaccinia virus in vitro clearly exhibited type 1-type 2 modulating properties.

Bovine CD4(+) T-lymphocyte clones specific for rhoptry-associated protein 1 of Babesia bigemina stimulate enhanced immunoglobulin G1 (IgG1) and IgG2 synthesis

Optimal protective immunity against babesial infection is postulated to require both complement-fixing and opsonizing antibodies in addition to gamma interferon (IFN-gamma)-mediated macrophage activation. The rhoptry-associated protein 1 (RAP-1) of Babesia bigemina induces partial protective immunity and is a candidate vaccine antigen. Previous studies demonstrated that cattle immunized with native protein that were subsequently protected against challenge had a strong IFN-gamma and weaker interleukin-4 (IL-4) response in immune lymph node lymphocytes that reflected the cytokine profile of the majority of CD4(+) T-cell clones obtained from peripheral blood. RAP-1-specific T helper (Th) cell clones that coexpress IFN-gamma and IL-4 are typical of numerous parasite-specific clones examined. However, the function of such cells as helper cells to enhance immunoglobulin secretion by bovine B cells has not been reported. In cattle, both immunoglobulin G1 (IgG1) and IgG2 can fix complement, but IgG2 is the superior opsonizing subclass. Therefore, studies were undertaken to ascertain the functional relevance of RAP-1-specific, CD4(+) Th0 cells as helper cells to enhance IgG1 and/or IgG2 production by autologous B lymphocytes. For comparison, Th0 clones specific for the metazoan parasite Fasciola hepatica that expressed relatively more IL-4 than the B. bigemina-specific Th cells were similarly assayed. B. bigemina RAP-1-specific clones could enhance production of both IgG1 and IgG2 by autologous B cells, whereas Th cell clones specific for F. hepatica enhanced predominantly IgG1 production. The capacity to enhance IgG2 production was associated with production of IFN-gamma by Th cells cocultured with B cells, antigen, and IL-2. The in vitro helper T-cell activity of these T-cell clones was representative of the in vivo serologic responses, which were composed of a mixed IgG1-IgG2 response in B. bigemina RAP-1 immune cattle and a biased IgG1 response in F. hepatica-immune cattle.

Bovine type 1 and type 2 responses

The Th1/Th2 paradigm has provided a useful framework for understanding the observed bias in immune responses that are often dominated by either cell-mediated or humoral responses, and for devising therapeutic strategies to stimulate T cell- or antibody-mediated immunity. However this paradigm is an oversimplification of a much more complex immunoregulatory network. Studies with bovine Th cell clones and immunoregulatory cytokines support this viewpoint. This paper highlights the progress that has been made in defining type 1 and type 2 responses in cattle. Evidence is presented for the presence of different subtypes of antigen-specific Th cell clones of cattle which constitute a spectrum of cell phenotypes, and for cytokine-mediated regulation of Th cell responses that differs from that observed in mice. The majority of over 60 parasite antigen-specific Th cell clones coexpress IL-4 and IFN-gamma, and polarized cytokine profiles were rarely observed. Furthermore, IL-2 and IL-10 expression was not restricted to IFN-gamma or IL-4-producing cells, respectively. This lack of coordinate regulation of "Th1" and "Th2" cytokines strengthens the emerging viewpoint that Th1 and Th2 responses, per se, do not typify the immune response to most pathogens. In addition, we provide evidence that major regulatory cytokines, IL-4, IL-10, and IL-12, do not selectively exert their negative (IL-4 and IL-10) or positive (IL-12) effects on Th1-like cells.

Helper T-cell epitopes encoded by the Babesia bigemina rap-1 gene family in the constant and variant domains are conserved among parasite strains

Among important candidates for babesial vaccines are apical complex proteins, including rhoptry-associated protein 1 (RAP-1) from Babesia bovis and B. bigemina, which have been shown to induce partial immunity. Four variant B. bigemina rap-1 transcripts identified in a clone of the Mexico strain have highly conserved sequence in the central region but vary in sequence at the amino and carboxy termini (NT and CT) of the predicted proteins, resulting in different combinations of NT and CT domains in the individual gene products. Cattle were immunized with native protein consisting of the RAP-alpha1 variant, which contains NT-1 and CT-1 domains, and T-cell responses were characterized. We previously reported the identification of two T helper (Th) cell epitopes in B. bigemina RAP-1alpha1 protein (I. Hötzel, W. C. Brown, T. F. McElwain, S. D. Rodriguez, and G. H. Palmer, Mol. Biochem. Parasitol. 81:89-99, 1996). One epitope mapped to the constant domain of RAP-1 (amino acids [aa] 144 to 187), and one mapped to the CT-1 variable domain (aa 386 to 480). Th1-like clones responding to these epitopes proliferated differentially to different strains of B. bigemina, raising the possibilities that the T-cell epitopes may vary antigenically and that CT-1 may be differentially expressed with respect to the other RAP-1 CT domains in the different strains. In this report, we definitively map the T-cell epitope identified in the constant domain of RAP-1 to aa 159 to 187 (FVVSLLKKNVVRDPESNDVENFASQYFYM) and show that the predicted amino acid sequence is completely conserved among seven strains. The T-cell epitope in the CT-1 domain was mapped to aa 436 to 465 (VNSEKVDADDAGNAETQQLPDAENEVRADD), which is also completely conserved among eight strains of B. bigemina. We further show that the RAP-1alpha1-immunized cattle were protected against homologous B. bigemina challenge, thus suggesting an association between protective immunity and the helper T-cell response against the two epitopes. The immunogenic and highly conserved nature of these T-cell epitopes and their ability to stimulate functionally relevant Th cells that express gamma interferon support their inclusion in a vaccine.