The pancreatic islet displays diverse patterns of endocrine cell arrangement. The prototypic islet, with insulin-secreting β-cells forming the core surrounded by other endocrine cells in the periphery, is largely based on studies of normal rodent islets. Recent reports on large animals, including humans, show a difference in islet architecture, in which the endocrine cells are randomly distributed throughout the islet. This particular species difference has raised concerns regarding the interpretation of data based on rodent studies to humans. On the other hand, further variations have been reported in marsupials and some nonhuman primates, which possess an inverted ratio of β-cells to other endocrine cells. This review discusses the striking plasticity of islet architecture and cellular composition among various species including changes in response to metabolic states within a single species. We propose that this plasticity reflects evolutionary acquired adaptation induced by altered physiological conditions, rather than inherent disparities between species.

Francisella tularensis causes lethal pneumonia following infection of the lungs by targeting macrophages for intracellular replication; however, macrophages stimulated with interferon gamma (IFN-␥) can resist infection in vitro. We therefore hypothesized that the protective effect of IFN-␥ against F. tularensis in vivo requires macrophages receptive to stimulation. We found that the lethality of pulmonary F. tularensis LVS infection was exacerbated under conditions of alveolar macrophage depletion and in mice with a macrophage-specific defect in IFN-␥ signaling (termed mice with macrophages insensitive to IFN-␥ [MIIG mice]). We previously found that treatment with exogenous interleukin 12 (IL-12) protects against F. tularensis infection; this protection was lost in MIIG mice. MIIG mice also exhibited reduced neutrophil recruitment to the lungs following infection. Systemic neutrophil depletion was found to render wild-type mice highly sensitive to respiratory F. tularensis infection, and depletion beginning at 3 days postinfection led to more pronounced sensitivity than depletion beginning prior to infection. Furthermore, IL-12-mediated protection required NADPH oxidase activity. These results indicate that lung macrophages serve a critical protective role in respiratory F. tularensis LVS infection. Macrophages require IFN-␥ signaling to mediate protection, which ultimately results in recruitment of neutrophils to further aid in survival from infection.KEYWORDS interferons, lung defense, lung infection, macrophages, neutrophils, tularemia T he tier 1 biothreat Francisella tularensis is a Gram-negative bacterium that is capable of replicating within phagocytes (1-3). In respiratory infection, alveolar macrophages have been reported to be the primary host cell for replication (4-6). It has been suggested that alveolar phagocytes are therefore detrimental to the host during respiratory infection, and a 2005 study reported that depletion of alveolar phagocytes following high challenge doses of LVS resulted in a modestly delayed time to death (7). The course of disease is characterized by a delayed immune response, followed by systemic dissemination and sepsis (8-10). Consequently, the prevailing opinion is that F. tularensis evades destruction by innate immunity and subverts myeloid cells, particularly macrophages, for its own benefit.Despite the proficiency of F. tularensis in subverting and exploiting host immunity, it is possible to stimulate innate immunity to successfully counter F. tularensis infection. Macrophages significantly contribute to in vitro bacterial killing if they are stimulated with interferon gamma (IFN-␥) (2,(11)(12)(13). Correspondingly, IFN-␥ is known to be required for protection in vivo (14). Treatment with exogenous interleukin 12 (IL-12) has been shown to protect mice, and mice lacking either the IL-12p35 or IL-12p40 subunit

Fatal outcomes following influenza infection are often associated with secondary bacterial infections. Allergic airway disease (AAD) is known to influence severe complications from respiratory infections, and yet the mechanistic effect of AAD on influenza virus-Streptococcus pneumoniae coinfection has not been investigated previously. We examined the impact of AAD on host susceptibility to viral-bacterial coinfections. We report that AAD improved survival during coinfection when viral-bacterial challenge occurred 1 week after AAD. Counterintuitively, mice with AAD had significantly deceased proinflammatory responses during infection. Specifically, both CD4+ and CD8+ T cell interferon gamma (IFN-γ) responses were suppressed following AAD. Resistance to coinfection was also associated with strong transforming growth factor β1 (TGF-β1) expression and increased bacterial clearance. Treatment of AAD mice with IFN-γ or genetic deletion of TGF-β receptor II expression reversed the protective effects of AAD. Using a novel triple-challenge model system, we show for the first time that AAD can provide protection against influenza virus-S. pneumoniae coinfection through the production of TGF-β that suppresses the influenza virus-induced IFN-γ response, thereby preserving antibacterial immunity. IMPORTANCE Asthma has become one of the most common chronic diseases and has been identified as a risk factor for developing influenza. However, the impact of asthma on postinfluenza secondary bacterial infection is currently not known. Here, we developed a novel triple-challenge model of allergic airway disease, primary influenza infection, and secondary Streptococcus pneumoniae infection to investigate the impact of asthma on susceptibility to viral-bacterial coinfections. We report for the first time that mice recovering from acute allergic airway disease are highly resistant to influenza-pneumococcal coinfection and that this resistance is due to inhibition of influenza virus-mediated impairment of bacterial clearance. Further characterization of allergic airway disease-associated resistance against postinfluenza secondary bacterial infection may aid in the development of prophylactic and/or therapeutic treatment against coinfection.

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The targeting of immunogens/vaccines to specific immune cells is a promising approach for amplifying immune responses in the absence of exogenous adjuvants. However, the targeting approaches reported thus far require novel, labor-intensive reagents for each vaccine and have primarily been shown as proof-of-concept with isolated proteins and/or inactivated bacteria. We have engineered a plasmid-based, complement receptor-targeting platform that is readily applicable to live forms of multiple gram-negative bacteria, including, but not limited to, Escherichia coli, Klebsiella pneumoniae, and Francisella tularensis. Using F. tularensis as a model, we find that targeted bacteria show increased binding and uptake by macrophages, which coincides with increased p38 and p65 phosphorylation. Mice vaccinated with targeted bacteria produce higher titers of specific antibody that recognizes a greater diversity of bacterial antigens. Following challenge with homologous or heterologous isolates, these mice exhibited less weight loss and/or accelerated weight recovery as compared to counterparts vaccinated with non-targeted immunogens. Collectively, these findings provide proof-of-concept for plasmid-based, complement receptor-targeting of live gram-negative bacteria.

Opsonizing antibody is a critical component of the host protective immune response against many respiratory pathogens. However, the role of antibodies in protection against pulmonary infection with highly virulent strain SchuS4 is unclear, and the mechanism that allows to evade antibody-mediated bacterial clearance is not fully understood. We have now found that depletion of alveolar macrophages reveals an otherwise cryptic protective effect of opsonizing antibody. While antibody opsonization alone failed to confer any survival benefit against SchuS4 lung infection, significant protection was observed when mice were depleted of alveolar macrophages prior to infection. Blood immune signature analyses and bacterial burden measurements indicated that the treatment regimen blocked establishment of productive, systemic infection. In addition, protection was found to be dependent upon neutrophils. The results show for the first time a protective effect of opsonizing antibodies against highly virulent SchuS4 pulmonary infection through depletion of alveolar macrophages, the primary bacterial reservoir, and prevention of systemic dissemination. These findings have important implications for the potential use of therapeutic antibodies against intracellular pathogens that may escape clearance by residing within mucosal macrophages.

Francisella tularensis is the causative agent of tularemia. Due to its ease of dissemination, high lethality, and low infectious dose required for respiratory infection, this bacterium poses a risk as an agent of bioterrorism. Though F. tularensis is believed to be an intracellular pathogen that requires replication within macrophages during respiratory infection, we have found that depletion of alveolar macrophages by intranasal treatment with liposomal Clodronate reduced mean time to death and total survival rates of infected mice. Furthermore, mice with a macrophage-specific insensitivity to interferon-γ (MIIG mice) similarly show heightened sensitivity to pulmonary Francisella infection. Such mice exhibited reduced expression of tumor necrosis factor-α, a potent neutrophil chemoattractant, in lung tissue, and systemic neutrophil depletion resulted in reduced mean time to death and total survival rates. Exogenous interleukin-12 promoted survival and bacterial clearance in mice, but this effect was lost upon neutrophil depletion and in IFN-γ-/- mice. The results suggest that rather than being a required reservoir for bacterial replication, alveolar macrophages are necessary for defense against F. tularensis infection by recruiting neutrophils to the lung, which are the primary cells responsible for clearing the bacterium.

Francisella tularensis causes lethal pneumonia following infection of the lungs. F. tularensis is considered to be an intracellular pathogen that targets macrophages for replication. We therefore hypothesized that depletion of alveolar macrophages would impede bacterial replication and improve host survival. However, it was found that macrophages are essential for survival of F. tularensis infection and depletion of these cells reduced survival rates following infection. Mice with a macrophage-specific defect in interferon gamma (IFN-γ) signaling (MIIG mice) were also highly sensitive to infection, exhibiting reduced survival rates, reduced mean time to death, and increased bacterial burdens in lungs, liver, and spleen relative to wild-type mice. We have previously found that treatment with exogenous interleukin 12 (IL-12) protects against F. tularensis infection; this protection was lost in MIIG mice. MIIG mice also exhibited reduced neutrophil recruitment to the lungs following infection. Systemic neutrophil depletion was found to render mice highly sensitive to pneumonic F. tularensis infection. Furthermore, IL-12-mediated protection required NADPH activity. These results indicate that lung macrophages serve a critical protective role in pneumonic F. tularensis infection. Macrophages require IFN-γ signaling to mediate protection, which ultimately results in recruitment of neutrophils to further aid in survival from infection.