Prior exposure to live Mycobacterium bovis BCG decreases Cryptococcus neoformans-induced lung eosinophilia in a gamma interferon-dependent manner

Prospects on Repurposing a Live Attenuated Vaccine for the Control of Unrelated Infections

Live vaccines use attenuated microbes to acquire immunity against pathogens in a safe way. As live attenuated vaccines (LAVs) still maintain infectivity, the vaccination stimulates diverse immune responses by mimicking natural infection. Induction of pathogen-specific antibodies or cell-mediated cytotoxicity provides means of specific protection, but LAV can also elicit unintended off-target effects, termed non-specific effects. Such mechanisms as short-lived genetic interference and non-specific innate immune response or long-lasting trained immunity and heterologous immunity allow LAVs to develop resistance to subsequent microbial infections. Based on their safety and potential for interference, LAVs may be considered as an alternative for immediate mitigation and control of unexpected pandemic outbreaks before pathogen-specific therapeutic and prophylactic measures are deployed.

The Regulation of Inflammation by Innate and Adaptive Lymphocytes

Inflammation plays an essential role in the control of pathogens and in shaping the ensuing adaptive immune responses. Traditionally, innate immunity has been described as a rapid response triggered through generic and nonspecific means that by definition lacks the ability to remember. Recently, it has become clear that some innate immune cells are epigenetically reprogrammed or “imprinted” by past experiences. These “trained” innate immune cells display altered inflammatory responses upon subsequent pathogen encounter. Remembrance of past pathogen encounters has classically been attributed to cohorts of antigen-specific memory T and B cells following the resolution of infection. During recall responses, memory T and B cells quickly respond by proliferating, producing effector cytokines, and performing various effector functions. An often-overlooked effector function of memory CD4 and CD8 T cells is the promotion of an inflammatory milieu at the initial site of infection that mirrors the primary encounter. This memory-conditioned inflammatory response, in conjunction with other secondary effector T cell functions, results in better control and more rapid resolution of both infection and the associated tissue pathology. Recent advancements in our understanding of inflammatory triggers, imprinting of the innate immune responses, and the role of T cell memory in regulating inflammation are discussed.

Severity of allergic airway disease due to house dust mite allergen is not increased after clinical recovery of lung infection with Chlamydia pneumoniae in mice

Chlamydia pneumoniae is associated with chronic inflammatory lung diseases like bronchial asthma and chronic obstructive pulmonary disease. The existence of a causal link between allergic airway disease and C. pneumoniae is controversial. A mouse model was used to address the question of whether preceding C. pneumoniae lung infection and recovery modifies the outcome of experimental allergic asthma after subsequent sensitization with house dust mite (HDM) allergen. After intranasal infection, BALB/c mice suffered from pneumonia characterized by an increased clinical score, reduction of body weight, histopathology, and a bacterial load in the lungs. After 4 weeks, when infection had almost resolved clinically, HDM allergen sensitization was performed for another 4 weeks. Subsequently, mice were subjected to a methacholine hyperresponsiveness test and sacrificed for further analyses. As expected, after 8 weeks, C. pneumoniae-specific antibodies were detectable only in infected mice and the titer was significantly higher in the C. pneumoniae/HDM allergen-treated group than in the C. pneumoniae/NaCl group. Intriguingly, airway hyperresponsiveness and eosinophilia in bronchoalveolar lavage fluid were significantly lower in the C. pneumoniae/HDM allergen-treated group than in the mock/HDM allergen-treated group. We did observe a relationship between experimental asthma and chlamydial infection. Our results demonstrate an influence of sensitization to HDM allergen on the development of a humoral antibacterial response. However, our model demonstrates no increase in the severity of experimental asthma to HDM allergen as a physiological allergen after clinically resolved severe chlamydial lung infection. Our results rather suggest that allergic airway disease and concomitant cellular changes in mice are decreased following C. pneumoniae lung infection in this setting.

Virus-like particle-induced protection against MRSA pneumonia is dependent on IL-13 and enhancement of phagocyte function

The importance of the priming of the lung environment by past infections is being increasingly recognized. Exposure to any given antigen can either improve or worsen the outcome of subsequent lung infections, depending on the immunological history of the host. Thus, an ability to impart transient alterations in the lung environment in anticipation of future insult could provide an important novel therapy for emerging infectious diseases. In this study, we show that nasal administration of virus-like particles (VLPs) before, or immediately after, lethal challenge with methicillin-resistant Staphylococcus aureus (MRSA) of mice i) ensures complete recovery from lung infection and near absolute clearance of bacteria within 12 hours of challenge, ii) reduces host response-induced lung tissue damage, iii) promotes recruitment and efficient bacterial clearance by neutrophils and CD11c⁺ cells, and iv) protects macrophages from MRSA-induced necrosis. VLP-mediated protection against MRSA relied on innate immunity. Complete recovery occurred in VLP-dosed mice with severe combined immunodeficiency, but not in wild-type mice depleted of either Ly6G⁺ or CD11c⁺ cells. Early IL-13 production associated with VLP-induced CD11c⁺ cells was essential for VLP-induced protection. These results indicate that VLP-induced alteration of the lung environment protects the host from lethal MRSA pneumonia by enhancing phagocyte recruitment and killing and by reducing inflammation-induced tissue damage via IL-13–dependent mechanisms.

The antiasthma effect of neonatal BCG vaccination does not depend on the Th17/Th1 but IL-17/IFN-γ balance in a BALB/c mouse asthma model

OBJECTIVE

This study aimed to determine whether the protective effects of the Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccination on allergic asthma are associated with the T helper (Th) 17/Th1 balance in a murine asthma model.

METHODS

BALB/c neonates were vaccinated with BCG on the first day after birth, sensitized with ovalbumin, and then challenged with allergen. The resulting airway inflammation and responsiveness were measured. The levels of IL-17 and interferon (IFN)-γ in BALF and ratio of Th17/Th1 were investigated.

RESULTS

We found that although BCG neonatal vaccination inhibited airway hyperresponsiveness and inflammation following allergen challenge in a BALB/c mouse asthma model, reduced levels of Th2 cytokines were not observed. However, BCG neonatal vaccination reduced IL-17 production and increased IFN-γ production in both the bronchoalveolar lavage fluid and the lung lymphocytes in asthmatic mice.

CONCLUSION

The antiasthma effects of neonatal BCG vaccination reversed the IL-17/IFN-γ imbalance in a murine asthma model but did not depend on modifying the Th17/Th1 balance.

Immune homeostasis in the respiratory tract and its impact on heterologous infection

Innate immunity at mucosal surfaces requires additional restraint to prevent inflammation to innocuous antigens or commensal microorganisms. The threshold above which airway macrophages become activated is raised by site-specific factors including the receptors for transforming growth factor beta, interleukin 10 and CD200; the ligands for which are produced by, or expressed on, respiratory epithelium. We discuss such site-specific regulation and how this is continually altered by prior infections. Resetting of innate reactivity represents a strategy for limiting excessive inflammation, but in some may pre-dispose to secondary bacterial pneumonia.

Protective T cell immunity against respiratory syncytial virus is efficiently induced by recombinant BCG

Respiratory syncytial virus (RSV) is one of the leading causes of childhood hospitalization and a major health burden worldwide. Unfortunately, because of an inefficient immunological memory, RSV infection provides limited immune protection against reinfection. Furthermore, RSV can induce an inadequate Th2-type immune response that causes severe respiratory tract inflammation and obstruction. It is thought that effective RSV clearance requires the induction of balanced Th1-type immunity, involving the activation of IFN-gamma-secreting cytotoxic T cells. A recognized inducer of Th1 immunity is Mycobacterium bovis bacillus Calmette-Guérin (BCG), which has been used in newborns for decades in several countries as a tuberculosis vaccine. Here, we show that immunization with recombinant BCG strains expressing RSV antigens promotes protective Th1-type immunity against RSV in mice. Activation of RSV-specific T cells producing IFN-gamma and IL-2 was efficiently obtained after immunization with recombinant BCG. This type of T cell immunity was protective against RSV challenge and caused a significant reduction of inflammatory cell infiltration in the airways. Furthermore, mice immunized with recombinant BCG showed no weight loss and reduced lung viral loads. These data strongly support recombinant BCG as an efficient vaccine against RSV because of its capacity to promote protective Th1 immunity.

Manipulation of acute inflammatory lung disease

Inflammatory lung disease to innocuous antigens or infectious pathogens is a common occurrence and in some cases, life threatening. Often, the inflammatory infiltrate that accompanies these events contributes to pathology by deleterious effects on otherwise healthy tissue and by compromising lung function by consolidating (blocking) the airspaces. A fine balance, therefore, exists between a lung immune response and immune-mediated damage, and in some the "threshold of ignorance" may be set too low. In most cases, the contributing, potentially offending, cell population or immune pathway is known, as are factors that regulate them. Why then are targeted therapeutic strategies to manipulate them not more commonplace in clinical medicine? This review highlights immune homeostasis in the lung, how and why this is lost during acute lung infection, and strategies showing promise as future immune therapeutics.

Neonatal vaccination with Bacillus Calmette-Guérin elicits long-term protection in mouse-allergic responses

BACKGROUND

Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccination has been shown to inhibit allergic airway inflammation in animal models, associated with the regulation of allergen-specific T-cell immunity. However, little is known about whether neonatal BCG treatment could inhibit allergic inflammation by regulating allergen-specific T-cell response in aged mice. This study was aimed to investigate the impact of neonatal BCG treatment on allergic asthma and possible mechanism(s) underlying the action of BCG in different ages of mice.

METHODS

C57BL/6 neonates were vaccinated with BCG on days 1, 7 and 14, sensitized with ovalbumin (OVA) at 5 and 7 weeks of age, and then challenged with allergen at 9 or 45 weeks of age for early- or late-challenged asthma. Their airway inflammation and allergen-specific T-cell responses were characterized.

RESULTS

Following early-challenge, BCG vaccination inhibited airway hyper-responsiveness (AHR), infiltration of eosinophils and mucous overproduction (P < 0.05), and shifted OVA-specific predominant Th2- to Th1-type cytokine responses in both the bronchoalveolar lavage fluid and the splenocyte supernatants (P < 0.05). In late-challenged mice, neonatal BCG treatment attenuated AHR and eosinophilia (P < 0.05), but failed to modulate allergen-specific cytokine responses.

CONCLUSIONS

Our data suggest that neonatal BCG vaccination has a long-term effect on inhibiting AHR and eosinophilia, which is associated with the modulation of Th1/Th2 cytokine production in early-, but not in late-challenged mice. Thus, different mechanisms may mediate the long-term protective effect of BCG neonatal vaccination differently in younger adult and aged mice.

Respiratory infections: do we ever recover?

Although the outcome of respiratory infection alters with age, nutritional status, and immunologic competence, there is a growing body of evidence that we all develop a unique but subtle inflammatory profile. This uniqueness is determined by the sequence of infections or antigenic insults encountered that permanently mold our lungs through experience. This experience and learning process forms the basis of immunologic memory that is attributed to the acquired immune system. But what happens if the pathogen is not homologous to any preceding it? In the absence of cross-specific acquired immunity, one would expect a response similar to that of a subject who had never been infected with anything before. It is now clear that this is not the case. Prior inflammation in the respiratory tract alters immunity and pathology to subsequent infections even when they are antigenically distinct. Furthermore, the influence of the first infection is long lasting, not dependent on the presence of T and B cells, and effective against disparate pathogen combinations. We have used the term "innate imprinting" to explain this phenomenon, although innate education may be a closer description. This educational process, by sequential waves of infection, may be beneficial, as shown for successive viral infections, or significantly worse, as illustrated by the increased susceptibly to life-threatening bacterial pneumonia in patients infected with seasonal and pandemic influenza. We now examine what these long-term changes involve, the likely cell populations affected, and what this means to those studying inflammatory disorders in the lung.

The impact of successive infections on the lung microenvironment

The effect of infection history on the immune response is ignored in most models of infectious disease and in preclinical vaccination studies. No one, however, is naive and repeated microbial exposure, in particular during childhood, shapes the immune system to respond more efficiently later in life. Concurrent or sequential infections influence the immune response to secondary unrelated pathogens. The involvement of cross-reactive acquired immunity, in particular T-cell responses, is extensively documented. In this review, we discuss the impact of successive infections on the infected tissue itself, with a particular focus on the innate response of the respiratory tract, including a persistent alteration of (1) epithelial or macrophage expression of Toll-like receptors or adherence molecules used by subsequent bacteria to invade the host, (2) the responsiveness of macrophages and neutrophils and (3) the local cytokine milieu that affects the activation of local antigen-presenting cells and hence adaptive immunity to the next infection. We emphasize that such alterations not only occur during coinfection, but are maintained long after the initial pathogen is cleared. As innate responses are crucial to the fight against local pathogens but are also involved in the maintenance of the homeostasis of mucosal tissues, dysregulation of these responses by repeated infections is likely to have a major impact on the outcome of infectious or allergic disease.

Role of granulocyte macrophage colony-stimulating factor in host defense against pulmonary Cryptococcus neoformans infection during murine allergic bronchopulmonary mycosis

We investigated the role of granulocyte macrophage colony-stimulating factor (GM-CSF) in host defense in a murine model of pulmonary cryptococcosis induced by intratracheal inoculation of Cryptococcus neoformans. Pulmonary C. neoformans infection of C57BL/6 mice is an established model of an allergic bronchopulmonary mycosis. Our objective was to determine whether GM-CSF regulates the pulmonary Th2 immune response in C. neoformans-infected C57BL/6 mice. Long-term pulmonary fungistasis was lost in GM-CSF knockout (GM(-/-)) mice, resulting in increased pulmonary burden of fungi between weeks 3 and 5. GM-CSF was required for the early influx of macrophages and CD4 and CD8 T cells into the lungs but was not required later in the infection. Lack of GM-CSF also resulted in reduced eosinophil recruitment and delayed recruitment of mononuclear cells into the airspace. Macrophages from GM(+/+) mice showed numerous hallmarks of alternatively activated macrophages: higher numbers of intracellular cryptococci, YM1 crystals, and induction of CCL17. These hallmarks are absent in macrophages from GM(-/-) mice. Mucus-producing goblet cells were abundantly present within the bronchial epithelial layer in GM(+/+) mice but not in GM(-/-) mice at week 5 after infection. Production of both Th1 and Th2 cytokines was impaired in the absence of GM-CSF, consistent with both reduced C. neoformans clearance and absence of allergic lung pathology.

Modulation of immunity to respiratory viral infection

Acute respiratory tract infections cause approximately 4 million deaths globally each year. Current strategies to combat these infections include vaccines and antiviral drugs. However, a significant proportion of the pathology and illness induced by respiratory viral infection is attributed to an excessive host immune response, rather than the cytopathic nature of the pathogen. Distinct respiratory viruses elicit common immune mediators and cells, which are subsequently the cause of pathology. Therefore, strategies that seek to attenuate the potency of the host’s immune response may provide generic relief from multiple respiratory infections.

Role of local pulmonary IFN-gamma expression in murine allergic airway inflammation

Generalized underrepresentation of IFN-gamma has been implicated in the development of allergic asthma. However, the role of local IFN-gamma in the lung during the development of this disease has not been completely elucidated. We studied the influence of local pulmonary IFN-gamma expression on the development of allergen-induced lung inflammation. To restrict our analysis to IFN-gamma expression in the lung and to exclude influences of systemic IFN-gamma production, we generated a transgenic mouse line with a targeted deletion of the IFN-gamma gene and constitutive, lung-specific IFN-gamma expression (Clara cell 10 [CC10]-IFN-gamma-tg-IFN-gamma-KO mice), and compared allergen-induced airway inflammation in these mice with that of wild-type and IFN-gamma- KO mice on the C57BL/6 background. Cytokine quantification in lungs of mice with allergic airway inflammation revealed that pulmonary IFN-gamma expression increased expression of IL-5 and IL-13. Consistent with this observation, eosinophilia in bronchoalveolar lavage of CC10-IFN-gamma-tg-IFN-gamma-KO mice was profoundly increased, indicating that this critical component of asthma is enhanced by local IFN-gamma expression. In contrast, airway hyperresponsiveness and anti-ovalbumin-IgE serum levels were reduced by local IFN-gamma expression. Together, our results demonstrate pleiotropic action of constitutive IFN-gamma expression in the lung, and question the therapeutic value of IFN-gamma in allergic asthma. Local expression of IFN-gamma in the lung increases markers of allergic airway inflammation, but decreases airway hyperresponsiveness in a murine model of allergic-asthma.

Feasibility and prospects for a vaccine to prevent cryptococcosis

Cryptococcosis is a relatively common fungal disease caused by Cryptococcus neoformans that has high morbidity and mortality. Numerous studies have established the feasibility of enhancing host immunity to C neoformans in naive immunocompetent animal models by vaccination. Several antigens have been identified that appear to be suitable vaccine candidates. Induced immune responses can mediate protection through both humoral and cellular immunity. Hence, a vaccine against cryptococcosis in humans is probably feasible but there are significant obstacles to vaccine development that range from uncertainties about the pathogenesis of infection to economic considerations.

The expanding realm of heterologous immunity: friend or foe?

Antecedent or current infections can alter the immunopathologic outcome of a subsequent unrelated infection. Immunomodulation by co-infecting pathogens has been referred to as 'heterologous immunity' and has been postulated to play a role in host susceptibility to disease, tolerance to organ transplant, and autoimmune disease. The effect of various infections on heterologous immune responses has been well studied in the context of shared epitopes and cross-reactive T cells. It has been shown that prior infections can modulate protective immunity and immunopathology by forming a pool of memory T cells that can cross-react with antigens from heterologous organisms or through the generation of a network of regulatory cells and cytokines. While it is not feasible to alter a host's history of prior infection, understanding heterologous immune responses in the context of simultaneous unrelated infections could have important therapeutic implications. Here, we outline key evidence from animal and human studies demonstrating the effect of heterologous immunity on the outcome of disease. We briefly review the role of T cells, but expand our discussion to explore other immune mechanisms that may modulate the response to concurrent active infections. In particular, we underscore the role of the innate immune system, polarized responses and regulatory mechanisms on heterologous immune responses.

Role of IFN-gamma in regulating T2 immunity and the development of alternatively activated macrophages during allergic bronchopulmonary mycosis

Pulmonary Cryptococcus neoformans infection of C57BL/6 mice is an established model of a chronic pulmonary fungal infection accompanied by an "allergic" response (T2) to the infection, i.e., a model of an allergic bronchopulmonary mycosis. Our objective was to determine whether IFN-gamma plays a role in regulating the pulmonary T2 immune response in C. neoformans-infected C57BL/6 mice. Long-term pulmonary fungistasis was lost in IFN-gamma knockout (KO) mice, resulting in an increased pulmonary burden of fungi at wk 3. IFN-gamma was required for the early influx of leukocytes into the lungs but was not required later in the infection. By wk 3, eosinophil and macrophage numbers were elevated in the absence of IFN-gamma. The inducible NO synthase to arginase ratio was lower in the lungs of IFN-gamma KO mice and the macrophages had increased numbers of intracellular cryptococci and YM1 crystals, indicative of alternatively activated macrophages in these mice. There was evidence of pulmonary fibrosis in both wild-type and IFN-gamma KO mice by 5 wk postinfection. IFN-gamma production was not required for the development of T2 cytokine (IL-4, IL-5, IL-13) producing cells in the lungs and lung-associated lymph nodes or induction of an IgE response. At a number of time points, T2 cytokine production was enhanced in IFN-gamma KO mice. Thus, in the absence of IFN-gamma, C57BL/6 mice develop an augmented allergic response to C. neoformans, including enhanced generation of alternatively activated macrophages, which is accompanied by a switch from a chronic to a progressive pulmonary cryptococcal infection.

Distinct roles for IL-4 and IL-10 in regulating T2 immunity during allergic bronchopulmonary mycosis

Pulmonary Cryptococcus neoformans infection of C57BL/6 mice is an established model of an allergic bronchopulmonary mycosis that has also been used to test a number of immunomodulatory agents. Our objective was to determine the role of IL-4 and IL-10 in the development/manifestation of the T2 response to C. neoformans in the lungs and lung-associated lymph nodes. In contrast to wild-type (WT) mice, which develop a chronic infection, pulmonary clearance was significantly greater in IL-4 knockout (KO) and IL-10 KO mice but was not due to an up-regulation of a non-T cell effector mechanism. Pulmonary eosinophilia was absent in both IL-4 KO and IL-10 KO mice compared with WT mice. The production of IL-4, IL-5, and IL-13 by lung leukocytes from IL-4 KO and IL-10 KO mice was lower but IFN-gamma levels remained the same. TNF-alpha and IL-12 production by lung leukocytes was up-regulated in IL-10 KO but not IL-4 KO mice. Overall, IL-4 KO mice did not develop the systemic (lung-associated lymph nodes and serum) or local (lungs) T2 responses characteristic of the allergic bronchopulmonary C. neoformans infection. In contrast, the systemic T2 elements of the response remained unaltered in IL-10 KO mice whereas the T2 response in the lungs failed to develop indicating that the action of IL-10 in T cell regulation was distinct from that of IL-4. Thus, although IL-10 has been reported to down-regulate pulmonary T2 responses to isolated fungal Ags, IL-10 can augment pulmonary T2 responses if they occur in the context of fungal infection.

Role of atypical bacterial infection of the lung in predisposition/protection of asthma

Asthma is a common inflammatory disease of the airways that results in airway narrowing and wheezing. Allergic asthma is characterised by a T-helper cell-type (Th) 2 response, immunoglobulin (Ig) E production, and eosinophilic influx into the airways. Recently, many clinical studies have implicated Mycoplasma pneumoniae and Chlamydia pneumoniae in the development and exacerbation of both chronic and acute asthma. It is widely accepted that M. pneumoniae and C. pneumoniae infections require Th1 immunity for clearance; therefore, according to the hygiene hypothesis, these infections should be protective against asthma. Here, we review the clinical evidence for the association and mechanisms of predisposition to and protection against asthma by these infections. We will examine the following question: Is it the absence of infection or the age of the individual on infection that confers susceptibility or resistance to asthma and does this vary between normal and predisposed individuals? We put forward a hypothesis of the effects of these infections on the development and prevention of asthma and how novel preventative and treatment strategies involving these microbes may be targeted against asthma.