Lipoprotein-Dependent and -Independent Immune Responses to Spirochetal Infection

In this study, we used the epidermal suction blister technique, in conjunction with multiparameter flow cytometry, to analyze the cellular and cytokine responses elicited by intradermal injection of human volunteers with synthetic analogs for spirochetal lipoproteins and compared the responses to findings previously reported from patients with erythema migrans (EM). Compared with peripheral blood (PB), lipopeptides derived from the N termini of the Borrelia burgdorferi outer surface protein C and the 17-kDa lipoprotein of Treponema pallidum (OspC-L and 17-L, respectively) elicited infiltrates enriched in monocytes/macrophages and dendritic cells (DCs) but also containing substantial percentages of neutrophils and T cells. Monocytoid (CD11c⁺) and plasmacytoid (CD11c⁻) DCs were selectively recruited to the skin in ratios similar to those in PB, but only the former expressed the activation/maturation surface markers CD80, CD83, and DC-SIGN. Monocytes/macrophages and monocytoid DCs, but not plasmacytoid DCs, displayed significant increases in surface expression of Toll-like receptor 1 (TLR1), TLR2, and TLR4. Staining for CD45RO and CD27 revealed that lipopeptides preferentially recruited antigen-experienced T-cell subsets; despite their lack of antigenicity, these agonists induced marked T-cell activation, as evidenced by surface expression of CD69, CD25, and CD71. Lipopeptides also induced significant increases in interleukin 12 (IL-12), IL-10, gamma interferon, and most notably IL-6 without corresponding increases in serum levels of these cytokines. Although lipopeptides and EM lesional infiltrates shared many similarities, differences were noted in a number of immunologic parameters. These studies have provided in situ evidence for a prominent “lipoprotein effect” during human infection while at the same time helping to pinpoint aspects of the cutaneous response that are uniquely driven by spirochetal pathogens.     

Immune Responses of the Protein-Deficient Guinea Pig to BCG Vaccination

Two groups of guinea pigs were maintained on high-protein and low-protein diets and immunized with intradermal BCG. Protein deficiency was accompanied by marked inhibition of local and systemic immune responses: a) The BCG nodule was poorly formed. There was marked delay and deficiency in the mobilization of macrophages. b) The draining lymph node was atrophic and showed little or no proliferation of lymphoid cells in the paracortical area. Macrophage accumulation occurred late but became diffuse and marked, in contrast to its consistent scarcity in the BCG nodule. c) In either location epithelioid cell transformation was retarded. Well-formed mature epithelioid cell granulomas were not seen. d) Bacilli persisted for a long time in the skin and lymph node lesions, e) Tuberculin sensitivity was greatly impaired in one-fifth of animals and absent in others. These findings were suggestive of macrophage dysfunction and depression of cell-mediated immunity to BCG in the protein-deficient guinea pig.     

Differential Immune Responses to Primary Measles-Mumps-Rubella Vaccination in Israeli Children

Measles remains an important cause of morbidity and mortality worldwide, primarily due to problems associated with delivery of the live attenuated vaccine to susceptible populations. In some developed countries, there is concern about the effects of immunization on the immune system. In this study, we analyzed the responses of 12-month-old Bedouin and Jewish children living in Israel to routine measles-mumps-rubella (MMR) vaccination. Seroconversion to measles was 99% in Bedouin and 79% in Jewish children (P < 0.01), and that to mumps and rubella was 92 to 100% in both groups. Measles neutralizing antibody titers were higher in Bedouin (333 ± 39 mIU/ml) than Jewish (122 ± 60 mIU/ml) children (P < 0.002). Immunoglobulin G levels were higher in Bedouin than Jewish children (P = 0.007) and increased after vaccination (P = 0.0009). Leukocyte (P < 0.02) and lymphocyte (P = 0.04) counts were higher and CD4 lymphocyte percentages were lower (P < 0.001) in Bedouin than Jewish children before and after vaccination. Leukocyte counts and natural killer cell numbers did not change after vaccination, but lytic activity increased in Bedouin children (P < 0.005). Spontaneous proliferation of cultured peripheral blood mononuclear cells increased with vaccination, but there were no changes in the proliferative responses to phytohemagglutinin or tetanus toxoid. In summary, no adverse effects of MMR vaccination on immune function were detected. However, there were differences in underlying immunologic parameters and in response to the measles component of the vaccine between Bedouin and Jewish children. It is not known whether genetic differences or environmental exposure accounts for these differences.     

HBV和HIV的复合型DNA免疫可诱发BALB/c小鼠的细胞免疫应答

本研究是通过细胞免疫检测来研究含有两种不同病毒基因的复合型DNA免疫的效果 ,同时和单一病毒基因的DNA免疫作比较。在本实验中 ,将HBV的S基因和 (或 )HIV 1的gp12 0基因插入到真核表达载体pcDNA3中 ,得到能表达HBsAg和gp12 0融合蛋白的重组体pcDNA S 12 0 ,和能表达HBsAg的重组体pcDNA S。在实验中 ,将重组质粒DNA直接注射到BALB/c小鼠股四头肌内 ,按每只小鼠 10 0 μg/ 10 0 μl的剂量经 3次免疫后 ,在T细胞增殖实验中 ,用HBsAg蛋白刺激体外培养的被免疫小鼠T细胞后 ,出现了明显的T细胞增殖。采用51Cr释放法检测特异性CTL杀伤作用时 ,发现致敏的CTL对HIVgp12 0多肽孵育后的靶细胞P815有明显的杀伤作用。     

Serological Responses to a Norovirus Nonstructural Fusion Protein after Vaccination and Infection

The performance of an assay to detect antibodies to a norovirus nonstructural fusion protein, designated VPR and consisting of three proteins (GI.1 virus protein genome-linked [VPg], a virus protease, and an RNA-dependent RNA polymerase), was evaluated. The assay sensitivity and specificity were 74.5% and >95%, respectively, for identifying GI.1 norovirus infection among persons who received either a monovalent GI.1 norovirus virus-like particle (VLP) vaccine or placebo by the intranasal route followed by an oral live GI.1 norovirus challenge.     

Revisiting Immune Exhaustion During HIV Infection

Chronic immune activation is a hallmark of HIV infection, yet the underlying triggers of immune activation remain unclear. Persistent antigenic stimulation during HIV infection may also lead to immune exhaustion, a phenomenon in which effector T cells become dysfunctional and lose effector functions and proliferative capacity. Several markers of immune exhaustion, such as PD-1, LAG-3, Tim-3, and CTLA-4, which are also negative regulators of immune activation, are preferentially upregulated on T cells during HIV infection. It is not yet clear whether accumulation of T cells expressing activation inhibitory molecules is a consequence of general immune or chronic HIV-specific immune activation. Importantly, however, in vitro blockade of PD-1 and Tim-3 restores HIV-specific T-cell responses, indicating potential for immunotherapies. In this review we discuss the evolution of our understanding of immune exhaustion during HIV infection, highlighting novel markers and potential therapeutic targets.     

Divergent Immune Responses to Mycobacterium avium subsp. paratuberculosis Infection Correlate with Kinome Responses at the Site of Intestinal Infection

Mycobacterium avium subsp. paratuberculosis is the causative agent of Johne''s disease (JD) in cattle. M. avium subsp. paratuberculosis infects the gastrointestinal tract of calves, localizing and persisting primarily in the distal ileum. A high percentage of cattle exposed to M. avium subsp. paratuberculosis do not develop JD, but the mechanisms by which they resist infection are not understood. Here, we merge an established in vivo bovine intestinal segment model for M. avium subsp. paratuberculosis infection with bovine-specific peptide kinome arrays as a first step to understanding how infection influences host kinomic responses at the site of infection. Application of peptide arrays to in vivo tissue samples represents a critical and ambitious step in using this technology to understand host-pathogen interactions. Kinome analysis was performed on intestinal samples from 4 ileal segments subdivided into 10 separate compartments (6 M. avium subsp. paratuberculosis-infected compartments and 4 intra-animal controls) using bovine-specific peptide arrays. Kinome data sets clustered into two groups, suggesting unique binary responses to M. avium subsp. paratuberculosis. Similarly, two M. avium subsp. paratuberculosis-specific immune responses, characterized by different antibody, T cell proliferation, and gamma interferon (IFN-γ) responses, were also observed. Interestingly, the kinomic groupings segregated with the immune response groupings. Pathway and gene ontology analyses revealed that differences in innate immune and interleukin signaling and particular differences in the Wnt/β-catenin pathway distinguished the kinomic groupings. Collectively, kinome analysis of tissue samples offers insight into the complex cellular responses induced by M. avium subsp. paratuberculosis in the ileum and provides a novel method to understand mechanisms that alter the balance between cell-mediated and antibody responses to M. avium subsp. paratuberculosis infection.     

Induction of HIV-1 Specific Mucosal Immune Responses by DNA Vaccination

DNA vaccination has been shown to induce immunity against several different pathogens including HIV-1. The authors demonstrate here that administration of DNA vaccines via the intranasal route is sufficient to induce immune responses both at distal mucosal sites and systemically. Since transmission of HIV-1 occurs largely across mucosal surfaces, the intranasal route provides a further means of application for DNA immunization.     

Analysis of the Immune Responses of Mice to Infection with Leishmania braziliensis

Leishmania major and Leishmania braziliensis both cause cutaneous leishmaniasis, but the former kills BALB/c mice while the latter is killed by the mice. This killing of L. braziliensis occurred by a gamma interferon-dependent mechanism, potentially made possible by the observed lack of high interleukin-4 production.     

Changes in Intestinal Fluid Transport and Immune Responses to Enterotoxins Due to Concomitant Parasitic Infection

The effect of a parasitic infection on enterotoxic diarrhea and on local and systemic formation of antibody to the toxin after immunization was studied in mice. Trichinella spiralis infection was chosen as the model, since the effects of the parasite when residing in both intestinal and extraintestinal sites can be studied. It was found that during the intestinal stage of the infection, the fluid response to cholera toxin as well as dibutyryl-cyclic adenosine 3′,5′ -monophosphate was greatly enhanced and that this was associated with a marked reduction in the absorption of fluid from the intestine. Later in the infection (migration stage), fluid accumulation in response to cholera toxin was significantly reduced, whereas absorption was normal and secretion in response to dibutyryl-cyclic adenosine 3′,5′-monophosphate was somewhat increased. Still later in the infection (muscular stage), the fluid-secretory response to cholera toxin was normal. There was a drastic depression of local formation of antitoxin of both immunoglobulin and immunoglobulin classes in mice given the first two of four oral immunizations with cholera toxin during the intestinal stage of T. spiralis infection. When the priming was given before or after the intestinal stage, the local antitoxin response was not affected. The titers of circulating antibodies were also depressed in mice given the first immunizations during the intestinal stage. In addition, significant though less pronounced depression of the serum antibody response was observed in mice primed during the extraintestinal stage.     

Helicobacter pylori-Specific Immune Responses of Children: Implications for Future Vaccination Strategy

We analyzed the specific anti-Helicobacter pylori immunoglobulin G (IgG) antibody profile for a sample of 824 asymptomatic schoolchildren in southern Germany (mean age, 10.7 ± 0.65 years) with an H. pylori-specific IgG enzyme-linked immunosorbent assay and Western blot analysis. The prevalence of infection was 19.8% (95% confidence interval, 17.1 to 22.7%). The immunoresponses were characterized predominantly by antibodies against low-molecular-mass antigens of 14 and 29 kDa, with a significant difference between children of German and Turkish nationalities (P = 0.0012 and P < 0.0001, respectively).     

Humoral and Cellular Immune Responses to Yersinia pestis Infection in Long-Term Recovered Plague Patients

Plague is one of the most dangerous diseases and is caused by Yersinia pestis. Effective vaccine development requires understanding of immune protective mechanisms against the bacterium in humans. In this study, the humoral and memory cellular immune responses in plague patients (n = 65) recovered from Y. pestis infection during the past 16 years were investigated using a protein microarray and an enzyme-linked immunosorbent spot assay (ELISpot). The seroprevalence to the F1 antigen in all recovered patients is 78.5%. In patients infected more than a decade ago, the antibody-positive rate still remains 69.5%. There is no difference in the antibody presence between gender, age, and infected years, but it seems to be associated with the F1 antibody titers during infection (r = 0.821; P < 0.05). Except F1 antibody, the antibodies against LcrV and YopD were detected in most of the patients, suggesting they could be the potential diagnostic markers for detecting the infection of F1-negative strains. Regarding cellular immunity, the cell number producing gamma interferon (IFN-γ), stimulated by F1 and LcrV, respectively, in vitro to the peripheral blood mononuclear cells of 7 plague patients and 4 negative controls, showed no significant difference, indicating F1 and LcrV are not dominant T cell antigens against plague for a longer time in humans. Our findings have direct implications for the future design and development of effective vaccines against Y. pestis infection and the development of new target-based diagnostics.     

Mouse Strain-Dependent Differences in Susceptibility to Neisseria gonorrhoeae Infection and Induction of Innate Immune Responses

Acute gonorrhea in women is characterized by a mucopurulent exudate that contains polymorphonuclear leukocytes (PMNs) with intracellular gonococci. Asymptomatic infections are also common. Information on the innate response to Neisseria gonorrhoeae in women is limited to studies with cultured cells, isolated immune cells, and analyses of cervicovaginal fluids. 17β-Estradiol-treated BALB/c mice can be experimentally infected with N. gonorrhoeae, and a vaginal PMN influx occurs in 50 to 80% of mice. Here, we compared the colonization loads and proinflammatory responses of BALB/c, C57BL/6 and C3H/HeN mice to N. gonorrhoeae. BALB/c and C57BL/6 mice were colonized at similar levels following inoculation with 10⁶ CFU of N. gonorrhoeae. BALB/c, but not C57BL/6, mice exhibited a marked vaginal PMN influx. Tumor necrosis factor alpha, interleukin-6, macrophage inflammatory protein 2 (MIP-2), and keratinocyte-derived chemokine were elevated in vaginal secretions from infected BALB/c mice, but not in those from C57BL/6 mice. MIP-2 levels positively correlated with a vaginal PMN influx. In contrast to BALB/c and C57BL/6 mice, C3H/HeN mice were resistant to infection, and there was no evidence of an inflammatory response. We conclude that N. gonorrhoeae causes a productive infection in BALB/c mice that is characterized by the induction of proinflammatory cytokines and chemokines and the recruitment of PMNs. Infection of C57BL/6 mice, in contrast, is more similar to asymptomatic infection. C3H/HeN mice are inherently resistant to N. gonorrhoeae infection, and this resistance is not due to an overwhelming inflammatory response to infection. Host genetic factors can therefore impact susceptibility and the immune response to N. gonorrhoeae.Uncomplicated gonorrhea is most commonly an infection of the urethra in men and the cervix. The female urethra may also be infected, and rectal and pharyngeal infection can occur in either sex. The hallmark of symptomatic gonococcal infection is the presence of a purulent exudate containing numerous polymorphonuclear leukocytes (PMNs), many of which contain intracellular gonococci. Asymptomatic infections are also common, particularly in females. Epithelial cells that line the genital mucosal surface are the first line of defense against this human-specific pathogen, and Neisseria gonorrhoeae produces a robust proinflammatory cytokine and chemokine response when incubated with cultured human vaginal, endocervical, ectocervical (12, 33), urethral (17), endometrial (3), and fallopian tube (31) tissue culture cells. Similarly, studies using the complex fallopian tube organ culture model suggest that N. gonorrhoeae induces the proinflammatory cytokines interleukin-1α (IL-1α) and tumor necrosis factor alpha (TNF-α) (29). Signaling through cellular receptors on epithelial cells results in the activation and recruitment of phagocytic cells, including PMNs and macrophages. Primary macrophages and peripheral blood mononuclear cells also elicit a proinflammatory response when incubated with N. gonorrhoeae (30, 34, 39). These innate immune cells further contribute to the array of proinflammatory cytokines and antimicrobial factors.Due to the multiple cell types that contribute to the host innate response to infection, it is important that whole model systems be utilized to measure the impact of N. gonorrhoeae infection on the host immune response. Experimental urethral colonization in male volunteers with N. gonorrhoeae evokes a strong innate response that is characterized by the production of proinflammatory cytokines (37, 38). Similar studies with female subjects are not feasible due to the risk of complications of gonococcal infection in women. Therefore, features of the innate response to N. gonorrhoeae in the female genital tract are predicted solely from tissue culture systems and the analysis of clinical samples. It is unclear whether women elicit a cytokine response to gonococcal infection. The reason why some infections are asymptomatic is also not known. Hedges et al. (18, 19) were unable to detect local proinflammatory cytokines in cervicovaginal secretions from infected women and detected a low anti-gonococcal antibody response. Based on these observations, it was proposed that N. gonorrhoeae fails to induce host inflammatory responses or is actively immunosuppressive. This finding is in marked contrast with the robust induction of proinflammatory cytokines observed from in vitro cell lines that constitute the female genital tract. The absence of various other cell types in tissue culture cell models could influence the cytokine response to infection; alternatively, the timing of sample collection from infected subjects may also influence the data. Therefore, a systematic analysis of cytokine induction over the course of infection in a female animal model is needed.The 17β-estradiol-treated mouse model is the only small-animal model available for studying the immune response to N. gonorrhoeae genital tract infection. While the mechanism by which estradiol promotes long-term colonization in female mice is not known, it is likely that promotion of an estrus-like state is beneficial for the gonococcus based on the fact that untreated mice can be transiently colonized with N. gonorrhoeae provided they are inoculated in the proestrus stage of the reproductive cycle (7, 46). The 17β-estradiol-treated mouse model has been a useful system for studying many aspects of gonococcal infection, including gonococcal evasion of PMN killing (43, 49) and antimicrobial peptides (23, 48), antigenic variation in vivo (41), and interactions between N. gonorrhoeae and commensal flora (32). This model is based on the use of BALB/c mice. Approximately 50 to 80% of infected BALB/c mice that are treated with a slow-release estradiol pellet exhibit a significant vaginal PMN response following inoculation with N. gonorrhoeae strain FA1090 based on examination of stained vaginal smears (21, 22, 43), and PMNs and macrophages are also found in vaginal and cervical tissue samples from infected mice (44). Gonococci are localized within vaginal and cervical tissue, and similar to that which occurs in humans, an insignificant and transient humoral response to N. gonorrhoeae occurs which was not protective against reinfection with the same strain (44). A recent modification of the model utilizes water-soluble estradiol to reduce the length of time that mice are exposed to nonphysiological concentrations of estradiol. A vaginal PMN influx also occurs during infection of mice treated with water-soluble estradiol, and as with pelleted mice, infection persists despite the presence of PMNs (44).One advantage of using inbred mouse strains for studies of infectious diseases is that environmental and genetic components can be controlled. Interestingly, the susceptibility to infectious agents can often vary with the genetic background of the mouse. One example in the area of sexually transmitted infections is that genetically controlled differences in the development of infertility in inbred mouse strains following inoculation with chlamydia have been reported, with pregnancy rates following infection of C3H/HeN mice being significantly lower than those of C57BL/6 mice (10). Darville and colleagues (8) found similar results, and their data suggested that an earlier and more severe acute inflammatory response in the C57BL/6 strain may lead to earlier eradication of the infection, thus protecting the upper tract from disease. Numerous examples of vulnerability have been found for other infectious agents, including Leishmania major, Listeria monocytogenes, Salmonella enterica serovar Typhimurium, Plasmodium chabaudi, Legionella pneumophila, and Mycobacterium tuberculosis (reviewed by Kramnik and Boyartchuk [27]).In this study, we sought to characterize in greater detail the cytokine and inflammatory response to genital tract infection with N. gonorrhoeae in 17β-estradiol-treated BALB/c mice and to determine if susceptibility to colonization and the host inflammatory response to infection vary between inbred mouse strains. Our data demonstrate that BALB/c, C57BL/6, and C3H/HeN mouse strains differ widely in their response to infection. While both BALB/c and C57BL/6 strains support colonization with the gonococcus, only the BALB/c strain appears to mount an inflammatory response. In contrast, the C3H/HeN strain appears to be resistant to colonization with the gonococcus. These data demonstrate significant divergence among inbred mouse strains in terms of susceptibility and inflammatory response to gonococcal infection, and they suggest that future studies can be designed to correlate genetic markers with the host response.     

Ageing Down‐Modulates Liver Inflammatory Immune Responses to Schistosome Infection in Mice

Ageing is associated with several alterations in the immune system. Our aim in this study was to compare the development of immunity to Schistosoma mansoni infection in young versus aged C57Bl/6 mice using the liver as the main organ to evaluate pathological alterations and immune responses. In the acute phase, young mice had large liver granulomas with fibrosis and inflammatory cells. Chronic phase in young animals was associated with immunomodulation of granulomas that became reduced in size and cellular infiltrate. On the other hand, aged animals presented granulomas of smaller sizes already in the acute phase. Chronic infection in these mice was followed by no alteration in any of the inflammatory parameters in the liver. In concert with this finding, there was an increase in activated CD4+ T, CD19+ B and NK liver cells in young mice after infection whereas old mice had already higher frequencies of activated B, NK and CD4+ T liver cells and infection does not change these frequencies. After infection, liver production of inflammatory and regulatory cytokines such as IFN‐γ, IL‐4 and IL‐10 increased in young but not in old mice that had high levels of IL‐4 and IL‐10 regardless of their infection status. Our data suggest that the unspecific activation status of the immune system in aged mice impairs inflammatory as well as regulatory immune responses to S. mansoni infection in the liver, where major pathological alterations and immunity are at stage. This poor immune reactivity may have a beneficial impact on disease development.     

Immune Responses of Bison and Efficacy after Booster Vaccination with Brucella abortus Strain RB51

Thirty-one bison heifers were randomly assigned to receive saline or a single vaccination with 10¹⁰ CFU of Brucella abortus strain RB51. Some vaccinated bison were randomly selected for booster vaccination with RB51 at 11 months after the initial vaccination. Mean antibody responses to RB51 were greater (P < 0.05) in vaccinated bison after initial and booster vaccination than in nonvaccinated bison. The proliferative responses by peripheral blood mononuclear cells (PBMC) from the vaccinated bison were greater (P < 0.05) than those in the nonvaccinated bison at 16 and 24 weeks after the initial vaccination but not after the booster vaccination. The relative gene expression of gamma interferon (IFN-γ) was increased (P < 0.05) in the RB51-vaccinated bison at 8, 16, and 24 weeks after the initial vaccination and at 8 weeks after the booster vaccination. The vaccinated bison had greater (P < 0.05) in vitro production of IFN-γ at all sampling times, greater interleukin-1β (IL-1β) production in various samplings after the initial and booster vaccinations, and greater IL-6 production at one sampling time after the booster vaccination. Between 170 and 180 days of gestation, the bison were intraconjunctivally challenged with approximately 1 × 10⁷ CFU of B. abortus strain 2308. The incidences of abortion and infection were greater (P < 0.05) in the nonvaccinated bison after experimental challenge than in the bison receiving either vaccination treatment. Booster-vaccinated, but not single-vaccinated bison, had a reduced (P < 0.05) incidence of infection in fetal tissues and maternal tissues compared to that in the controls. Compared to the nonvaccinated bison, both vaccination treatments lowered the colonization (measured as the CFU/g of tissue) of Brucella organisms in all tissues, except in retropharyngeal and supramammary lymph nodes. Our study suggests that RB51 booster vaccination is an effective vaccination strategy for enhancing herd immunity against brucellosis in bison.     

Compartmentalization of Immune Responses during Staphylococcus aureus Cranial Bone Flap Infection

Decompressive craniectomy is often required after head trauma, stroke, or cranial bleeding to control subsequent brain swelling and prevent death. The infection rate after cranial bone flap replacement ranges from 0.8% to 15%, with an alarming frequency caused by methicillin-resistant Staphylococcus aureus, which is problematic because of recalcitrance to antibiotic therapy. Herein we report the establishment of a novel mouse model of S. aureus cranial bone flap infection that mimics several aspects of human disease. Bacteria colonized bone flaps for up to 4 months after infection, as revealed by scanning electron microscopy and quantitative culture, demonstrating the chronicity of the model. Analysis of a human cranial bone flap with confirmed S. aureus infection by scanning electron microscopy revealed similar structural attributes as the mouse model, demonstrating that it closely parallels structural facets of human disease. Inflammatory indices were most pronounced within the subcutaneous galeal compartment compared with the underlying brain parenchyma. Specifically, neutrophil influx and chemokine expression (CXCL2 and CCL5) were markedly elevated in the galea, which demonstrated substantial edema on magnetic resonance images, whereas the underlying brain parenchyma exhibited minimal involvement. Evaluation of immune mechanisms required for bacterial containment and inflammation revealed critical roles for MyD88-dependent signaling and neutrophils. This novel mouse model of cranial bone flap infection can be used to identify key immunologic and therapeutic mechanisms relevant to persistent bone flap infection in humans.Decompressive craniectomy is performed after head trauma, stroke, or cranial bleeding, where a portion of the skull is removed to control subsequent brain swelling and prevent death. After removal, the bone flap is often cryopreserved until replacement; however, this increases the likelihood of destroying its blood supply, which substantially augments risk of infection.¹ The prevalence of infection after craniotomy ranges from 0.8% to 15%, with an alarming frequency caused by methicillin-resistant Staphylococcus aureus (MRSA), a major community and nosocomial gram-positive pathogen.² This high infection rate subjects patients to at least two additional surgical procedures since it is not possible to clear the infected bone in situ because of its recalcitrance to antibiotic therapy.³ In the first procedure, the infected skull flap is removed, and after a variable period of antibiotic therapy ranging from 6 weeks to >12 months, a second procedure is performed to place an expensive custom alloplastic flap composed of either acrylic resins, titanium mesh, or hydroxyapatite.^3,4 In approximately 13% of patients, prolonged absence of the skull flap can lead to syndrome of the trephined, a series of adverse effects that can include headache, seizures, mood swings, and behavioral disturbances.^5–7 Treatment of trephine syndrome consists of replacement of the original bone flap or synthetic device^8,9; however, this cannot be performed until there is convincing evidence that any residual infection associated with the original bone/artificial flap has been eliminated.Currently, cranial bone flap infections cannot be prevented or effectively treated without removal of the infected flap, and little information is available about the immune or microbial attributes that contribute to disease chronicity. These are important issues because a better appreciation of key pathogenic factors may reveal new targets to prevent and/or treat bone flap infections. Here we report a novel mouse model of S. aureus cranial bone flap infection that accurately mimics several facets of human disease in response to a relatively low infectious inoculum. This model has revealed that distinct immune responses are elicited within the subcutaneous space and brain parenchyma even though both bone flap surfaces communicate with these compartments and harbor similar numbers of bacteria. This information will help to facilitate the future design of novel therapeutic targets to prevent and/or treat bacterial cranial bone flap infections.