Help for cytotoxic-T-cell responses is mediated by CD40 signalling

Cytotoxic T lymphocytes (CTLs) which carry the CD8 antigen recognize antigens that are presented on target cells by the class I major histocompatibility complex. CTLs are responsible for the killing of antigen-bearing target cells, such as virus-infected cells. Although CTL effectors can act alone when killing target cells, their differentiation from naive CD8-positive T cells is often dependent on 'help' from CD4-positive helper T (TH) cells. Furthermore, for effective CTL priming, this help must be provided in a cognate manner, such that both the TH cell and the CTL recognize antigen on the same antigen-presenting cell. One explanation for this requirement is that TH cells are needed to convert the antigen-presenting cell into a cell that is fully competent to prime CTL. Here we show that signalling through CD40 on the antigen-presenting cells can replace the requirement for TH cells, indicating that T-cell 'help', at least for generation of CTLs by cross-priming, is mediated by signalling through CD40 on the antigen-presenting cell.

Neither the Bvg- phase nor the vrg6 locus of Bordetella pertussis is required for respiratory infection in mice

In Bordetella species, the BvgAS sensory transduction system mediates an alteration between the Bvg+ phase, characterized by expression of adhesins and toxins, and the Bvg- phase, characterized by the expression of motility and coregulated phenotypes in Bordetella bronchiseptica and by the expression of vrg loci in Bordetella pertussis. Since there is no known environmental or animal reservoir for B. pertussis, the causative agent of whooping cough, it has been assumed that this phenotypic alteration must occur within the human host during infection. Consistent with this hypothesis was the observation that a B. pertussis mutant, SK6, containing a TnphoA insertion mutation in a Bvg-repressed gene (vrg6) was defective for tracheal and lung colonization in a mouse model of respiratory infection (D. T. Beattie, R. Shahin, and J. Mekalanos, Infect. Immun. 60:571-577, 1992). This result was inconsistent, however, with the observation that a Bvg+ phase-locked B. bronchiseptica mutant was indistinguishable from the wild type in its ability to establish a persistent respiratory infection in rabbits and rats (P. A. Cotter and J. F. Miller, Infect. Immun. 62:3381-3390, 1994; B. J. Akerley, P. A. Cotter, and J. F. Miller, Cell 80:611-620, 1995). To directly address the role of Bvg-mediated signal transduction in B. pertussis pathogenesis, we constructed Bvg+ and Bvg- phase-locked mutants and compared them with the wild type for their ability to colonize the respiratory tracts of mice. Our results show that the Bvg+ phase of B. pertussis is necessary and sufficient for respiratory infection. By constructing a strain with a deletion in the bvgR regulatory locus, we also show that ectopic expression of Bvg- phase phenotypes decreases the efficiency of colonization, underscoring the importance of Bvg-mediated repression of gene expression in vivo. Finally, we show that the virulence defect present in strain SK6 cannot be attributed to the vrg6 mutation. These data contradict an in vivo role for the Bvg- phase of B. pertussis.

The role of the thymus and recent thymic migrants in the maintenance of the adult peripheral lymphocyte pool

The thymus is essential for the initial seeding of T cells to the periphery, but its role in maintaining the adult T cell pool remains poorly defined. We investigated whether changes to the rate of T cell export could form part of the mechanism(s) controlling the homeostatic regulation of the size and composition of the peripheral T cell pool. Using neonatal thymi grafted under the kidney capsule, we found that irrespective of whether the pool was oversupplied (by thymic grafts) or undersupplied (due to neonatal thymectomy), the thymic export rate was constant from both the host and graft thymus, and the periphery remained constant in size. Recent thymic emigrants (RTE) were also tracked to determine the extent of their acceptance into the T cell pool of a normal mouse. As a population, RTE are phenotypically mature, but were distinct from resident T cells in the periphery, being released in a CD4/CD8 ratio approximately twice that of established peripheral T cells. This export ratio is similar to that of T cells in the mature thymic compartment, but soon after entry into the periphery, the ratio falls, indicating separate thymic and peripheral regulation of the CD4/CD8 ratio. RTE may also be preferentially incorporated into the periphery, causing displacement of resident T cells, thus maintaining the size of the peripheral pool. Although not vital for the maintenance of a functional T cell pool, the acceptance of RTE in a "full" peripheral pool would ensure that the T cell receptor repertoire is kept diverse and that the T cell population encompasses a broad range of naive as well as memory T cells.

The BvgAS virulence control system regulates type III secretion in Bordetella bronchiseptica

The BvgAS signal transduction system in Bordetella spp. mediates a transition between infectious (Bvg+) and non-infectious (Bvg-) phases by sensing environmental conditions and regulating gene expression. Using differential display, arbitrary-primed polymerase chain reaction (PCR), we identified a gene expressed in the Bvg+ phase of Bordetella bronchiseptica that shows a high degree of sequence similarity to a locus involved in providing energy for type III secretion in pathogenic gram-negative bacteria (yscN in Yersinia spp.). We determined that the expression of this homologue in B. bronchiseptica (designated bscN) is regulated by bvg. Several open reading frames surrounding the bscN locus also show sequence similarity to loci encoding type III secretion apparatus components in other bacteria. An in-frame deletion of bscN in B. bronchiseptica leads to decreased secretion of several proteins, decreased cytotoxicity towards cultured cell lines and a defect in causing tyrosine dephosphorylation of specific proteins in infected cells in vitro. The deletion strain also revealed that bscN-mediated secretion is required for persistent colonization of the trachea in a rat infection model. Loci encoding type III secretion homologues were identified in four strains of B. pertussis and two strains of B. parapertussis. B. pertussis strain 18323 and an ovine isolate of B. parapertussis show significant transcription of the genes in vitro.

The threshold for autoimmune T cell killing is influenced by B7-1

The concept that naive CD4+ and CD8+ T cells require co-stimulatory signals for activation and proliferation is well documented. Less clear is the need for co-stimulation during the effector phase of the T cell response. Here we examined the influence of B7-1 (CD80) during the effector phase of an autoimmune response to pancreatic islets using transgenic mouse lines which expressed B7-1 in either all or only some of their beta cells ("confluent" or "patchy" RIP-B7-1 mice). Transgenic expression of B7-1 in normal mouse islets that co-expressed the pro-inflammatory cytokine, IL-2, resulted in early spontaneous autoimmunity. Islets with IL-2 and "confluent" B7-1 expression were destroyed whereas islets with IL-2 and "patchy" B7-1 expression showed selective killing of the B7-1+ beta cells. Islet-reactive T cells, circulating in the RIP-B7-1/IL-2 mice, rejected syngeneic islet grafts, but only if these expressed B7-1. Introduction of the B7-1 transgene into the nonobese diabetic (NOD) genetic background likewise resulted in early spontaneous autoimmunity, but splenocytes from the diabetic animals could only transfer diabetes to NOD scid recipients that expressed B7-1 on their beta cells. In both these transgenic models, therefore, islet destruction required continuous B7-1 expression by target beta cells. Thus, although the normal repertoire contains T cells with potential islet reactivity, these T cells remain harmless because parenchymal cells like the beta cell cannot normally express B7-1. Our results also have implications for tumor immunotherapy in that the ability of T cells to kill poorly immunogenic targets may be dependent upon B7-1 expression by the target cell itself.

Compartmentalization of bacterial antigens: differential effects on priming of CD8 T cells and protective immunity

Bacterial pathogens synthesize numerous proteins that are either secreted or localized within bacterial cells. To address the impact of antigen compartmentalization on T cell immunity, we constructed recombinant Listeria monocytogenes that express a model CD8T cell epitope as a secreted or nonsecreted fusion protein. Both forms of the antigen, either secreted into the host cell cytoplasm or retained within bacterial cells, efficiently prime CD8 T cell responses. However, epitope-specific CD8 T cells confer protection only against bacteria secreting the antigen but not against the bacteria expressing the nonsecreted form of the same antigen. This dichotomy as a result of antigen compartmentalization suggests that bacterial antigens are presented by multiple MHC class I pathways to prime CD8 T cells, but only the endogenous pathway provides target antigens for CD8 T cell-mediated protective immunity.

In vivo and ex vivo regulation of bacterial virulence gene expression

Bacteria are remarkably adaptable organisms that are able to survive and multiply in diverse and sometimes hostile environments. Adaptability is determined by the complement of genetic information available to an organism and by the mechanisms that control gene expression. In general, gene products conferring a growth or survival advantage in a particular situation are expressed, while unnecessary or deleterious functions are not. Expression of virulence gene products that allow pathogenic bacteria to multiply on and within host cells and tissues are no exception to this rule. Being of little or no use to the bacterium except during specific stages of the infectious cycle, these accessory factors are nearly always subject to tight and coordinate regulation. As a result of recent advances, we are beginning to appreciate the complexities of the interactions between bacteria and their hosts. The ability to probe virulence gene regulation in vivo has broadened our perspectives on pathogenesis.

The structure of mother-child play: young children with Down syndrome and typically developing children

This study explored developmental changes and effects of Down syndrome on mothers' structuring of their children's play. Mothers and their young children with Down syndrome (n = 28) were compared with socioeconomically matched samples of mothers and their mental age-matched (n = 28) and chronological age-matched (n = 28) typically developing children. Mothers of typically developing children exhibited more object demonstrations with their developmentally younger children, who exhibited less object play. Mothers of children with Down syndrome were more directive and supportive than were mothers of younger and older typically developing children, who did not differ in their frequencies of these behaviors. More maternal supportive object behavior was associated with more object play and vocalization by children with Down syndrome. Mothers and children in all 3 groups were contingently responsive to their partners' behavioral signals.

CD4+ T cell help impairs CD8+ T cell deletion induced by cross-presentation of self-antigens and favors autoimmunity

Self-antigens expressed in extrathymic tissues such as the pancreas can be transported to draining lymph nodes and presented in a class I-restricted manner by bone marrow-derived antigen-presenting cells. Such cross-presentation of self-antigens leads to CD8+ T cell tolerance induction via deletion. In this report, we investigate the influence of CD4+ T cell help on this process. Small numbers of autoreactive OVA-specific CD8+ T cells were unable to cause diabetes when adoptively transferred into mice expressing ovalbumin in the pancreatic beta cells. Coinjection of OVA-specific CD4+ helper T cells, however, led to diabetes in a large proportion of mice (68%), suggesting that provision of help favored induction of autoimmunity. Analysis of the fate of CD8+ T cells indicated that CD4(+) T cell help impaired their deletion. These data indicate that control of such help is critical for the maintenance of CD8+ T cell tolerance induced by cross-presentation.

An experimental investigation of the spray issued from a pMDI using laser diagnostic techniques

This research was concerned with the experimental investigation of the spray issued from a pressurised metered-dose inhaler (pMDI) using laser diagnostic techniques and has been motivated by the urgent need to find suitable replacements to the environmentally destructive CFC propellants currently used in the device. The experimental work was conducted using phase-Doppler particle analysis (PDPA), a single particle light scattering technique that provides the simultaneous measurement of drop size, velocity, and concentration, yielding the most detailed temporal and spatial analysis of the pMDI spray to date. Three formulations were studied to compare the performance of an "ozone-friendly" hydrofluoroalkane propellant against that of a traditional CFC propellant mixture and a commercially available CFC formulation containing drug and surfactant. The PDPA analysis was complemented by a visual investigation of the near-orifice flow field using copper laserstrobe microcinematography to obtain information on the primary atomization process of the pMDI. This work was conducted in parallel with the theoretical investigation of the spray issued from a pMDI.

Recombinant Listeria monocytogenes vaccination eliminates papillomavirus-induced tumors and prevents papilloma formation from viral DNA

Listeria monocytogenes is a gram-positive, facultative intracellular bacterium that enters the cytoplasm of infected cells and spreads directly into neighboring cells without encountering the extracellular environment. Cytoplasmic L. monocytogenes efficiently presents secreted proteins to the major histocompatibility complex class I pathway which can stimulate protective T-cell-mediated immune responses. We have used a cottontail rabbit papillomavirus (CRPV) rabbit model to test the ability of recombinant L. monocytogenes strains secreting the viral E1 protein (E1-rLm) to protect outbred rabbits against CRPV- and CRPV DNA-induced tumors. CRPV infection of outbred rabbits serves as a model for oncogenic papillomaviruses since CRPV-induced papillomas progress with high frequency to malignant carcinoma. Rabbits were vaccinated with wild-type L. monocytogenes or E1-rLm and then challenged with CRPV or viral DNA. In contrast to 0% papilloma regression in control animals, 77% of E1-rLm-vaccinated rabbits generated protective immunity that controlled and induced complete regression of tumors induced by CRPV. Latent viral DNA was not detected at 71% of the papilloma regression sites examined 4.5 months postregression. E1-rLm responder rabbits were completely resistant to papilloma formation from viral DNA. In contrast to controls, peripheral blood mononuclear cells from E1-rLm responder rabbits were able to proliferate in response to in vitro E1 stimulation. These results indicate that E1-rLm immunization generated a systemic anti-CRPV E1 cell-mediated immune response which protected outbred rabbits from tumors induced by CRPV or CRPV DNA challenge.

Dilute topical proparacaine for pain relief after photorefractive keratectomy

PURPOSE

The purpose of the study is to determine whether there is a nonanesthetic and nontoxic concentration of topical proparacaine that can be applied repeatedly to the cornea to reduce pain after photorefractive keratectomy (PRK).

METHODS

Part I: To determine a nonanesthetic concentration, the corneal sensitivity of 50 healthy volunteers was assessed using aesthesiometry before and after a drop of either 0.01%, 0.025%, 0.05%, 0.1%, or 0.2% topical proparacaine. Ten volunteers similarly were tested with multiple doses of 0.05% proparacaine. To evaluate toxicity, ten healthy volunteers self-administered 0.05% proparacaine to one eye and placebo to the other eye every 15 minutes for 12 hours on day 1 and every hour for 12 hours on days 2 through 7. Subjects were assessed throughout the week using visual acuity, slitlamp examination, aesthesiometry, and ultrasonic pachometry. Part II: In a prospective, double-masked study, 34 patients who underwent PRK (48 eyes) self-administered either topical 0.05% proparacaine or placebo for 1 week after PRK as needed to reduce pain. Patients recorded their pain score before and after drop use and answered a pain-relief questionnaire.

RESULTS

Part I: Proparacaine concentrations greater than or equal to 0.1% eliminated sensation from some corneas; concentrations of less than or equal to 0.05% were never fully anesthetic. No corneal toxicity was observed except for some minimal punctate staining in both treatment and placebo eyes. Part II: Patients in the treatment group had significantly more pain relief (P < 0.001) for a longer period (P < 0.001) than did patients in the control group. Average change in pain score was significantly greater in the treatment group (P < 0.002). No significant difference in the number of days needed to reach complete epithelial healing was found between the two groups (P < 0.18).

CONCLUSIONS

Dilute (0.05%) topical proparacaine is nonanesthetic and nontoxic, and can be used safely for at least 1 week to reduce pain after PRK.

Recombinant Listeria monocytogenes as a live vaccine vehicle and a probe for studying cell-mediated immunity

The ability of Listeria monocytogenes (L. monocytogenes) to enter the cytosol of host cells allows secreted proteins to efficiently enter the endogenous antigen-processing pathway leading to presentation by MHC class I molecules. L. monocytogenes has recently been exploited as a live vaccine vehicle for the induction of immunological memory against heterologous antigens. We have established a genetic system for site-specific integration of antigen expression cassettes into the Listeria genome which allows regulated expression and secretion of heterologous proteins. The ability of recombinant strains to stimulate long-term immunological memory and CD8+ T-cell-mediated protective immunity was investigated using the lymphocytic choriomeningitis virus (LCMV) murine infection model. Vaccination of mice with recombinant Listeria strains expressing LCMV antigens induced LCMV-specific CD8+ T cells which protected mice against LCMV challenge. We have also used a cottontail rabbit papillomavirus model to test the ability of recombinant Listeria strains to stimulate protective antitumor immunity in domestic rabbits. These studies have demonstrated the protective efficacy of recombinant L. monocytogenes vaccines and have established an experimental system for systematic analysis of cytotoxic T-cell induction by an intracellular bacterium.

Class I-restricted cross-presentation of exogenous self-antigens leads to deletion of autoreactive CD8(+) T cells

In this report, we show that cross-presentation of self-antigens can lead to the peripheral deletion of autoreactive CD8(+) T cells. We had previously shown that transfer of ovalbumin (OVA)-specific CD8(+) T cells (OT-I cells) into rat insulin promoter-membrane-bound form of OVA transgenic mice, which express the model autoantigen OVA in the proximal tubular cells of the kidneys, the beta cells of the pancreas, the thymus, and the testis of male mice, led to the activation of OT-I cells in the draining lymph nodes. This was due to class I-restricted cross-presentation of exogenous OVA on a bone marrow-derived antigen presenting cell (APC) population. Here, we show that adoptively transferred or thymically derived OT-I cells activated by cross-presentation are deleted from the peripheral pool of recirculating lymphocytes. Such deletion only required antigen recognition on a bone marrow-derived population, suggesting that cells of the professional APC class may be tolerogenic under these circumstances. Our results provide a mechanism by which the immune system can induce CD8(+) T cell tolerance to autoantigens that are expressed outside the recirculation pathway of naive T cells.

Induction of a CD8+ cytotoxic T lymphocyte response by cross-priming requires cognate CD4+ T cell help

Class I-restricted presentation is usually associated with cytoplasmic degradation of cellular proteins and is often considered inaccessible to exogenous antigens. Nonetheless, certain exogenous elements can gain entry into this so-called endogenous pathway by a mechanism termed cross-presentation. This is known to be effective for class I-restricted cytotoxic T lymphocyte (CTL) cross-priming directed against a variety of exogenous tumor, viral, and minor transplantation antigens. The related effect of cross-tolerance can also effectively eliminate responses to selected self components. In both cases, this presentation appears to require the active involvement of a bone marrow-derived antigen presenting cell (APC). Here, we show that CTL induction by cross-priming with cell-associated ovalbumin requires the active involvement of CD4+ helper T cells. Importantly, this CD4+ population is only effective when both the helper and CTL determinants are recognized on the same APC. Moreover, we would argue that the cognitive nature of this event suggests that the CD4+ T cell actively modifies the APC, converting it into an effective stimulator for the successful priming of the CTL precursor.

Roles for motility in bacterial-host interactions

The ability to move in a directed manner may confer distinct advantages upon host-adapted prokaryotes. Potential benefits of motility include increased efficiency of nutrient acquisition, avoidance of toxic substances, the ability to translocate to preferred hosts and access optimal colonization sites within them, and dispersal in the environment during the course of transmission. The costs of motility also may be significant. These include the metabolic burden of synthesizing flagellar components, the energetic expense of fuelling flagellar motors and the presentation of polymeric and highly antigenic targets to the immune system. It is therefore not surprising that synthesis of the motility apparatus is usually subject to strict control. Studies of a variety of bacterial-host interactions demonstrate roles for motility, and its regulation, at points throughout the infectious cycle.

Spiritual well-being, religiosity, hope, depression, and other mood states in elderly people coping with cancer

PURPOSE/OBJECTIVES

To determine the relationships among spiritual well-being, religiosity, hope, depression, and other mood states in elderly people coping with cancer and if differences in hope, depression, and other mood states exist between those elderly with high and low intrinsic religiosity and spiritual well-being.

DESIGN

Descriptive correlational and descriptive comparison.

SETTING

Acute care units of two hospitals located in the midwestern United States.

SAMPLE

100 elderly people with diagnosis of cancer and a mean age of 73 years. Thirty-three of the subjects were male, and 67 were female. Sixty-two percent had either lung, breast, or colon cancer.

METHODS

Each subject was administered an intrinsic and extrinsic religiosity index, a spiritual well-being scale, a geriatric depression scale, the Miller hope scale, and the Profile of Mood States scale.

MAIN RESEARCH VARIABLES

Spiritual well-being, religiosity, hope, depression, and mood.

FINDINGS

A consistent positive correlation was found among intrinsic religiosity, spiritual well-being, hope, and other positive mood states. A consistent negative correlation among intrinsic religiosity, depression, and other negative mood states existed. Analysis of variance indicated that significantly higher levels of hope and positive moods existed in elderly patients with high levels of intrinsic religiosity and spiritual well-being.

CONCLUSION

Intrinsic religiosity and spiritual well-being are associated with hope and positive mood states in elderly people coping with cancer.

IMPLICATIONS FOR NURSING PRACTICE

Nurses must assess and support intrinsic religiosity and promote spiritual well-being in elderly people coping with cancer.

A mutation in the Bordetella bronchiseptica bvgS gene results in reduced virulence and increased resistance to starvation, and identifies a new class of Bvg-regulated antigens

The Bordetella BvgAS signal-transduction system has traditionally been viewed as mediating a transition between two distinct phenotypic phases: the Bvg+ phase, characterized by the expression of adhesins and toxins, and the Bvg-phase, characterized by motility in Bordetella bronchiseptica and by the expression of vrg loci in Bordetella pertussis. In B. bronchiseptica, the Bvg+ phase is necessary and sufficient for respiratory tract colonization whereas the Bvg phase is required for growth under nutrient-limiting conditions. This report describes the characterization of a mutant that is locked in a Bvg-intermediate (Bvg[i]) phase. The mutation conferring this phenotype, designated bvgS-I1, results in a threonine-to-methionine substitution near the primary site of phosphorylation in BvgS. Compared to its Bvg+-phase-locked parent, the Bvg(i) mutant displays increased resistance to nutrient limitation and reduced virulence. Molecular analyses indicate that the mutant has lost the ability to express a subset of Bvg+-phase factors and has gained the ability to express factors unique to the Bvg(i) phase. Although identified by mutation, this work indicates that the Bvg(i) phase is expressed by wild-type B. bronchiseptica in response to certain (semi-modulating) environmental conditions. The identification of Bvg(i)-specific antigens suggests the existence of a new class of Bvg-regulated genes. We hypothesize that BvgAS is capable of mediating the expression of a spectrum of phenotypic phases in response to the various environments encountered as Bordetella travels within and between mammalian hosts.

T cell tolerance and autoimmunity

Many T cells with auto-aggressive potential are deleted in the thymus. Although some of these escape to the general circulation, they do not usually damage organs such as the pancreas. To investigate the mechanisms preventing autoimmunity, we generated transgenic mice expressing known genes under the control of various promoters. We found that the occurrence of autoaggression depended on factors such as the precursor frequency of responding T cells, their state of activation, their accessibility to the autoantigen, the physicochemical properties of the autoantigen, the possibility of priming by environmental antigens which mimic the target antigen, and some inflammatory reaction in the target site.

Central role of the BvgS receiver as a phosphorylated intermediate in a complex two-component phosphorelay

Two-component systems use phosphorylation reactions to regulate stimulus/response pathways. In Bordetella pertussis, a human respiratory pathogen, the infectious cycle of the organism is controlled by the BvgAS two-component system. BvgS has similarities to sensor and response regulator components and is an autophosphorylating kinase that phosphorylates BvgA. BvgA, a response regulator, is a DNA-binding protein that activates virulence gene transcription. Three phosphorylated BvgS domains, the transmitter, receiver, and C terminus, are essential for signal transduction. We now demonstrate that the BvgS transmitter is sufficient for autophosphorylation but is unable to phosphorylate the C terminus or BvgA. The BvgS receiver regulates several phenotypes: dephosphorylation of both the BvgS transmitter and C terminus as well as transfer of a phosphoryl group from the transmitter to the C terminus. Our results indicate that BvgAS signal transduction initiates with autophosphorylation of the transmitter followed by phosphotransfer to the receiver. The phosphorylated receiver can donate to the C terminus or to water. The phosphorylated C terminus is then able to transfer the phosphoryl group to BvgA.

Induction of autoimmune diabetes by oral administration of autoantigen

An antigen administered orally can induce immunological tolerance to a subsequent challenge with the same antigen. Evidence has been provided for the efficacy of this approach in the treatment of human autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. However, oral administration of autoantigen in mice was found to induce a cytotoxic T lymphocyte response that could lead to the onset of autoimmune diabetes. Thus, feeding autoantigen can cause autoimmunity, which suggests that caution should be used when applying this approach to the treatment of human autoimmune diseases.

Comparative analysis of the virulence control systems of Bordetella pertussis and Bordetella bronchiseptica

Bordetella pertussis and Bordetella bronchiseptica contain nearly identical BvgAS signal-transduction systems that mediate a biphasic transition between virulent (Bvg+) and avirulent (Bvg-) phases. In the Bvg+ phase, the two species express a similar set of adhesins and toxins, and in both organisms the transition to the Bvg- phase occurs in response to the same environmental signals (low temperature or the presence of nicotinic acid or sulphate anion). These two species differ, however, with regard to Bvg(-)-phase phenotypes, host specificity, the severity and course of the diseases they cause, and also potentially in their routes of transmission. To investigate the contribution of the virulence-control system to these phenotypic differences, we constructed a chimeric B. bronchiseptica strain containing bvgAS from B. pertussis and compared it with wild-type B. bronchiseptica in vitro and in vivo. The chimeric strain was indistinguishable from the wild type in its ability to express Bvg(+)- and Bvg(-)- phase-specific factors. However, although the chimeric strain responded to the same signals as the wild type, it differed dramatically in sensitivity to these signals; significantly more nicotinic acid or MgSO4 was required to modulate the chimeric strain compared with the wild-type strain. Despite this difference in signal sensitivity, the chimeric strain was indistinguishable from the wild type in its ability to cause respiratory-tract infections in rats, indicating that the bvgAS loci of B. pertussis and B. bronchiseptica are functionally interchangeable in vivo. By exchanging discrete fragments of bvgAS, we found that the periplasmic region of BvgS determines signal sensitivity.

Mechanisms of tolerance to self

Several mechanisms exist to prevent lymphocytes from reacting against self-antigens. As T cells develop in the thymus and express antigen-specific receptors, those with high-affinity to self-antigens existing within the thymus are deleted. Low-affinity self-reactive T cells and T cells with receptors against antigens not represented intrathymically will mature and join the peripheral T cell pool. They may either ignore self-antigens expressed by tissues unable to activate T cells through a lack of the appropriate costimulator signals, or they may, under certain conditions, be deleted or rendered anergic and unable to respond. Likewise, B cells that express surface Ig receptors with high binding affinity to membrane-bound self-antigens present in the bone marrow will be rescued by receptor editing or will be deleted, whereas those of lower affinity will migrate to the periphery in either an anergic or indifferent state depending on the degree of receptor engagement by antigen. Once there, their ultimate fate is determined by the availability of T cell help.