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Summary CD11b is a cell surface receptor that contributes to many cellular processes which are involved in the generation of a protective immune response against pathogenic organisms. In this work, the natural host-pathogen model of murine Bordetella bronchiseptica infection was used to explore the role of CD11b in respiratory immunity. Following intranasal inoculation, CD11b–/– mice rapidly succumb to B. bronchiseptica respiratory infection, highlighting the prominent role of CD11b in the generation of a protective immune response in this model. CD11b appears to be required for both the control of bacterial numbers and the regulation of cellular responses in the lungs. An increased accumulation of neutrophils in the lungs of CD11b–/– mice as compared with wild-type mice suggests that CD11b contributes to the regulation of cellular responses to respiratory infection. This accumulation may be explained by a decrease in apoptosis that is observed in the absence of CD11b following cellular interactions with B. bronchiseptica. Interestingly, this role for CD11b in the regulation of cellular accumulation appears to be critically important for the resolution of damage associated with the type III secretion system (TTSS) of B. bronchiseptica. These data provide new insight into the key role CD11b plays in the resolution of damage in the lower respiratory tract, as well as the B. bronchiseptica virulence determinant that induces it.

Although the antibacterial effects of Abs are well studied in in vitro systems, the in vivo effects of Abs cannot always be accurately predicted. Complicated cross-talk between different effector functions of Abs and various arms of the immune system can affect their activities in vivo. Using the mouse respiratory pathogen Bordetella bronchiseptica, we examined the mechanisms of Ab-mediated clearance of bacteria from the respiratory tract. Interestingly, although TLR4 was not necessary for protective immunity following infection, it was required for rapid bacterial clearance in mice that were vaccinated or adoptively transferred Abs. TLR4 was important for the rapid recruitment of neutrophils that are necessary for Ab-mediated bacterial clearance via a mechanism that requires both FcγR and CR3. These data are consistent with a model in which TLR4-mediated inflammatory responses aid in the recruitment of neutrophils, which phagocytose Ab- and complement-opsonized bacteria via FcγRs and CR3. Although pattern recognition receptors are known to be involved in innate immunity and the generation of adaptive immunity, their contributions to specific adaptive immune functions should be considered in ongoing efforts to improve vaccine-induced protective immunity.

To efficiently colonize and persist in the lower respiratory tract, bacteria must survive multiple host immune mechanisms.Bordetella bronchisepticais a gram-negative respiratory pathogen that naturally infects mice and persists in the lower respiratory tract for up to 49 days postinoculation. In this work, we examined the effect of mutation of thepagPgene on the persistence ofB. bronchisepticain the lower respiratory tract of mice. ThepagPgene encodes a palmitoyl transferase that is responsible for the addition of a palmitoyl group to the lipid A region ofB. bronchisepticalipopolysaccharide. Data presented here confirm that aB. bronchisepticaΔpagPmutant demonstrates defective persistence in the lower respiratory tract of wild-type mice. We hypothesized that the defective persistence of theB. bronchisepticaΔpagPmutant was due to an increased susceptibility of this mutant to a host immune response. In vivo data indicate that both B cells and the complement component C3 are required for the reduced bacterial numbers of the ΔpagPmutant on day 14 postinoculation. In addition, an in vitro complement killing assay demonstrated thatB. bronchisepticaexhibitspagP-dependent resistance to antibody-mediated complement killing at low concentrations of immune serum. Taken together, these results suggest thatpagPis required forB. bronchisepticato resist antibody-mediated complement lysis during respiratory infection.

To assess the contribution of complement to respiratory immunity in the context of a natural bacterial infection, we used mice genetically deficient in complement components and the murine pathogen Bordetella bronchiseptica. Complement component C3 was not required for the control of bacterial infection or for the generation of infection-induced protective immunity. However, C3-deficient (C3(-/-)) mice were severely defective, compared to wild type, in vaccine-induced protective immunity. Adoptively transferred immune serum from convalescent wild-type or C3(-/-) animals rapidly cleared B. bronchiseptica from the lungs of wild-type mice but did not affect its growth in C3(-/-) mice, indicating that the defect is not in the generation of protective immunity, but in its function. Immune serum was effective in C5-deficient mice but had little effect in the lungs of mice lacking either Fcgamma receptors (FcgammaR) or CR3, suggesting bacterial clearance is not via direct complement-mediated lysis. Together, these data indicate that complement is required for antibody-mediated clearance of Bordetella and suggest the mechanism involves C3 opsonization of bacteria for phagocytosis that is both CR3- and FcgammaR-dependent.

Several species of pathogenic microorganisms have developed strategies to survive and persist in vital organs which are normally maintained as sterile by the generation of strong immune responses. Here, we report an immunomodulation involving theBordetella bronchisepticatype III secretion system (TTSS) which contributes to bacterial survival in the lower respiratory tract of the host. The prolonged persistence ofB. bronchisepticathat was observed in gamma interferon (IFN-γ)−/−mice indicates that the efficient clearance of bacteria from the lower respiratory tract requires not only B cells and antibodies but also IFN-γ production. Our data also suggest that interleukin-10 (IL-10)-producing splenocytes are generated early during infection and that IL-10 inhibits IFN-γ-producing cells and delays the clearance ofB. bronchisepticafrom the lungs. The TTSS ofB. bronchisepticainhibits the generation of IFN-γ-producing splenocytes and is required for long-term bacterial persistence in the lower respiratory tract in wild-type mice. This suggests that a mechanism involving the modulation of IFN-γ production by the TTSS facilitatesB. bronchisepticasurvival in the lower respiratory tract.

Chronic bacterial infection reflects a balance between the host immune response and bacterial factors that promote colonization and immune evasion. Bordetella bronchiseptica uses a type III secretion system (TTSS) to persist in the lower respiratory tract of mice. We hypothesize that colonization is facilitated by bacteria-driven modulation of dendritic cells (DCs), which leads to an immunosuppressive adaptive host response. Migration of DCs to the draining lymph nodes of the respiratory tract was significantly increased in mice infected with wild-type B. bronchiseptica compared with mice infected with TTSS mutant bacteria. Reduced colonization by TTSS-deficient bacteria was evident by 7 days after infection, whereas colonization by wild-type bacteria remained high. This decrease in colonization correlated with peak IFN-γ production by restimulated splenocytes from infected animals. Wild-type bacteria also elicited peak IFN-γ production on day 7, but the quantity was significantly lower than that elicited by TTSS mutant bacteria. Additionally, wild-type bacteria elicited higher levels of the immunosuppressive cytokine IL-10 compared with the TTSS mutant bacteria. B. bronchiseptica colonization in IL-10−/− mice was significantly reduced compared with infections in wild-type mice. These findings suggest that B. bronchiseptica use the TTSS to rapidly drive respiratory DCs to secondary lymphoid tissues where these APCs stimulate an immunosuppressive response characterized by increased IL-10 and decreased IFN-γ production that favors bacterial persistence.

The Friend virus susceptibility gene 2 (Fv2) controls the polyclonal expansion of infected cells that occurs early during Friend erythroleukemia virus infection. Fv2 has recently been shown to encode a truncated form of the Stk receptor tyrosine kinase (Sf-Stk). This observation, coupled with earlier work, suggested that Sf-Stk drives the expansion of infected cells by forming a complex with the Friend virus envelope glycoprotein, gp55, and the erythropoietin receptor. Fv2 has also been implicated in the control of cell cycling in early erythroid progenitors (erythroid blast-forming units [BFU-Es]). Mouse strains that are homozygous for the resistant allele of Fv2 (Fv2rr) have few actively cycling BFU-Es. In this report, we demonstrate that the control of BFU-E cycling is encoded by a gene linked to, but distinct from, Fv2, and suggest that this gene is the dual-specific protein phosphatase Cdc25A, which regulates the G1to S-phase transition of the cell cycle. We show that a naturally occurring allele of Cdc25A, which increases Cdc25A phosphatase activity and promotes cell-cycle progression, segregates in mouse strains that exhibit high levels of BFU-E cell cycling. In wild-type mice, this allele of Cdc25A does not overtly affect erythropoiesis; however, when this allele is combined with a mutation of the Kit receptor (KitWV), the anemia of the mice is enhanced. Furthermore, overexpression of Cdc25A in bone marrow cells causes a defect in the BFU-E colony formation. These results suggest that proper regulation of the cell cycle through Cdc25A is required for normal erythropoiesis. (Blood. 2002;100:3804-3811)

Many pathogens are able to manipulate the signaling pathways responsible for the generation of host immune responses. Here we examine and model a respiratory infection system in which disruption of host immune functions or of bacterial factors changes the dynamics of the infection. We synthesize the network of interactions between host immune components and two closely related bacteria in the genus Bordetellae. We incorporate existing experimental information on the timing of immune regulatory events into a discrete dynamic model, and verify the model by comparing the effects of simulated disruptions to the experimental outcome of knockout mutations. Our model indicates that the infection time course of both Bordetellae can be separated into three distinct phases based on the most active immune processes. We compare and discuss the effect of the species-specific virulence factors on disrupting the immune response during their infection of naive, antibody-treated, diseased, or convalescent hosts. Our model offers predictions regarding cytokine regulation, key immune components, and clearance of secondary infections; we experimentally validate two of these predictions. This type of modeling provides new insights into the virulence, pathogenesis, and host adaptation of disease-causing microorganisms and allows systems-level analysis that is not always possible using traditional methods., Author Summary The immune response is a complex network of processes activated in a host upon infection. Pathogens seek to disrupt or evade these processes to ensure their own survival and proliferation. This article provides a systems-level analysis of the immune response against two related bacterial species in the Bordetella genus, B. bronchiseptica and B. pertussis. B. pertussis, the causative agent of whooping cough, has lost many of the virulence factors of its B. bronchiseptica–like progenitor, and is using different strategies for the modulation of the immune system. We have synthesized two separate network models for the interaction of these pathogens with their hosts. Each network is then translated into a predictive dynamic model and is validated by comparison with available experimental data. The model offers predictions regarding cytokine regulation and the effects of perturbations of the immune system, as well as the time course of infections in hosts that had previously encountered the pathogens. We experimentally validate the prediction that convalescent hosts can rapidly clear both pathogens, while antibody transfer cannot substantially reduce the duration of a B. pertussis infection. This type of modeling provides new insights into the virulence, pathogenesis, and host adaptation of disease-causing microorganisms and can be readily extended to other pathogens.