Heterosubtypic immunity to influenza A virus infection requires a properly diversified antibody repertoire

Germline-enforced enrichment for charged amino acids in TCR beta chain (TCRβ) complementarity determining region 3 (CDR-B3) alters T cell development, repertoire content, and antigen recognition

T cell receptor beta chain (TCRβ) diversity (Dβ) gene segments are highly conserved across evolution, with trout Dβ1 sequence identical to human and mouse Dβ1. A key conserved feature is enrichment for glycine in all three Dβ reading frames (RFs). Previously, we found that replacement of mouse Dβ1 with a typical immunoglobulin D_H sequence, which unlike Dβ is enriched for tyrosine, leads to an increase in the use of tyrosine in TCRβ complementarity determining region 3 (CDR-B3) after thymic selection, altering T cell numbers, CDR-B3 diversity, and T cell function. To test whether the incorporation of charged amino acids into the Dβ sequence in place of glycine would also influence T cell biology, we targeted the TCRβ locus with a novel glycine-deficient DβDKRQ allele that replaces Dβ1 coding sequence with charged amino acids in all three reading frames. Developing T cells using DβDKRQ expressed TCR CDR-B3s depleted of tyrosine and glycine and enriched for germline-encoded lysine, arginine, and glutamine. Total thymocytes declined in number during the process of β selection that occurs during the transition from the DN3bc to DN4 stage. Conventional thymocyte and T cell numbers remained reduced at all subsequent thymic stages and in the spleen. By contrast, regulatory T cell numbers were increased in Peyer's patches and the large intestine. In terms of functional consequences, T cell reactivity to an ovalbumin immunodominant epitope was reduced. These findings buttress the view that natural selection of Dβ sequence is used to shape the pre-immune TCRβ repertoire, affecting both conventional and regulatory T cell development and influencing epitope recognition.

Sex Influences Age-Related Changes in Natural Antibodies and CD5+ B-1 Cells

Natural Abs are primarily produced by B-1 cells and are essential for protection against Streptococcus pneumoniae The incidence and mortality rate for pneumococcal infection increases dramatically after age 65, disproportionately affecting males in both human and murine systems. To date, there is a significant gap in our understanding of the relationship among sex, aging, natural IgM efficacy, and the natural IgM repertoire. Our investigation demonstrates that the protective capacity of serum IgM against pneumococcal infection is maintained in IgM obtained from aged female mice but absent in IgM from aged male mice. To understand this difference in protective capacity, we examined serum Ig, discovering that the protective change was not associated with shifts in levels of phosphorylcholine (PC)- or pneumococcal capsular polysaccharide serotype 3-specific IgM. Interestingly, we observed that aged females have an increase in the total number of CD5⁺ B-1 cells, higher serum IL-5 levels, and a larger percentage of aged female CD5⁺ B-1 cells that express CD86 as compared with aged males. Furthermore, single-cell IgM repertoire analysis from peritoneal PC⁺, splenic PC⁺, and bone marrow CD5⁺ B-1 cell subsets demonstrated greater diversity with age and a higher level of germline status in female mice than previously observed in studies of aged male mice. Aged female CD5⁺ B-1 cells also expressed higher levels of transcripts associated with cell activity and self-renewal, such as Nanog and Hmga2 Taken together, these data indicate that females maintain a more diverse and active CD5⁺ B-1 cell pool and natural IgM repertoire, which has implications for sex-related susceptibility to infection and disease.

ICAMs are dispensable for influenza clearance and anti-viral humoral and cellular immunity

αLβ2 (LFA-1) mediated interactions with ICAM-1 and ICAM-2 predominate leukocyte-vascular interactions, but their functions in extravascular cell-cell communications is still debated. The roles of these two ligands in leukocyte trafficking, lymphocyte differentiation, and immunity to influenza infections were dissected in the present study. Surprisingly, double ICAM-1 and ICAM-2 knock out mice (herein ICAM-1/2^-/- mice) infected with a lab adapted H1N1 influenza A virus fully recovered from infection, elicited potent humoral immunity, and generated normal long lasting anti-viral CD8⁺ T cell memory. Furthermore, lung capillary ICAMs were dispensable for both NK and neutrophil entry to virus infected lungs. Mediastinal lymph nodes (MedLNs) of ICAM-1/2^-/- mice poorly recruited naïve T cells and B lymphocytes but elicited normal humoral immunity critical for viral clearance and effective CD8⁺ differentiation into IFN-γ producing T cells. Furthermore, whereas reduced numbers of virus specific effector CD8⁺ T cells accumulated inside infected ICAM-1/2^-/- lungs, normal virus-specific T_RM CD8⁺ cells were generated inside these lungs and fully protected ICAM-1/2^-/- mice from secondary heterosubtypic infections. B lymphocyte entry to the MedLNs and differentiation into extrafollicular plasmablasts, producing high affinity anti-influenza IgG2a antibodies, were also ICAM-1 and ICAM-2 independent. A potent antiviral humoral response was associated with accumulation of hyper-stimulated cDC2s in ICAM null MedLNs and higher numbers of virus-specific T follicular helper (Tfh) cells generated following lung infection. Mice selectively depleted of cDC ICAM-1 expression supported, however, normal CTL and Tfh differentiation following influenza infection, ruling out essential co-stimulatory functions of DC ICAM-1 in CD8⁺ and CD4⁺ T cell differentiation. Collectively our findings suggest that lung ICAMs are dispensable for innate leukocyte trafficking to influenza infected lungs, for the generation of peri-epithelial T_RM CD8⁺ cells, and long term anti-viral cellular immunity. In lung draining LNs, although ICAMs promote lymphocyte homing, these key integrin ligands are not required for influenza-specific humoral immunity or generation of IFN-γ effector CD8⁺ T cells. In conclusion, our findings suggest unexpected compensatory mechanisms that orchestrate protective anti-influenza immunity in the absence of vascular and extravascular ICAMs.

Improved immunologic responses to heterologous influenza strains in children with low preexisting antibody response vaccinated with MF59-adjuvanted influenza vaccine

Vaccination is the most effective approach to reduce the substantial morbidity and mortality caused by influenza infection. Vaccine efficacy is highly sensitive to antigenic changes causing differences between circulating and vaccine viruses. Adjuvants such as MF59 increase antibody-mediated cross-reactive immunity and therefore may provide broader seasonal protection. A recent clinical trial showed that an MF59-adjuvanted vaccine was more efficacious than a nonadjuvanted comparator in subjects < 2 years of age, although not in those ≥ 2 years, during influenza seasons in which the predominant circulating virus was an A/H3N2 strain that was antigenically different from the vaccine virus. This finding suggested that the increased efficacy of the adjuvanted vaccine in younger subjects may be mediated by strain cross-reactive antibodies. A subset of the trial population, representing subjects with distinct age and/or immunological history, was tested for antibody responses to the vaccine A/H3N2 strain as well as A/H3N2 drifted strains antigenically matching the viruses circulating during the trial seasons. The neutralizing tests showed that, compared with nonadjuvanted vaccine, the adjuvanted vaccine improved not only the neutralizing antibody response to the vaccine strain but also the cross-reactive antibody response to the drifted strains in subjects with lower preexisting antibody titers, regardless of their age or vaccine history. The results demonstrated an immunological benefit and suggested a potential efficacy benefit by adjuvanted vaccine in subjects with lower preexisting antibody responses.

Attenuated asthma phenotype in mice with a fetal-like antigen receptor repertoire

We hypothesized that the scarcity of N-nucleotides might contribute to the inability of the neonate to mount a robust allergic immune response. To test this, we used terminal deoxyribunucleotidyl Transferase deficient (TdT^-/-) mice, which express "fetal-like" T cell receptor and immunoglobulin repertoires with largely germline-encoded CDR3 regions. Intraperitoneal sensitization was followed by aerosol provocation with either PBS or the allergen OVA in both TdT^-/- mice and wild-type mice to develop allergic respiratory inflammation. The effects of this procedure were investigated by lung function test, immunological analysis of serum and brochoalveolar lavage. The local T_H2 cytokine milieu was significantly attenuated in TdT^-/- mice. Within this group, the induction of total IgE levels was also significantly reduced after sensitization. TdT^-/- mice showed a tendency toward reduced eosinophilic inflow into the bronchial tubes, which was associated with the elimination of respiratory hyperreactivity. In conclusion, in a murine model of allergic airway inflammation, the expression of fetal-like antigen receptors was associated with potent indications of a reduced ability to mount an asthma phenotype. This underlines the importance of somatically-generated antigen-receptor repertoire diversity in type one allergic immune responses and suggests that the fetus may be protected from allergic responses, at least in part, by controlling N addition.

Accounting for cross-immunity can improve forecast accuracy during influenza epidemics

Previous exposure to influenza viruses confers cross-immunity against future infections with related strains. However, this is not always accounted for explicitly in mathematical models used for forecasting during influenza outbreaks. We show that, if an influenza outbreak is due to a strain that is similar to one that has emerged previously, then accounting for cross-immunity explicitly can improve the accuracy of real-time forecasts. To do this, we consider two infectious disease outbreak forecasting models. In the first (the "1-group model"), all individuals are assumed to be identical and cross-immunity is not accounted for. In the second (the "2-group model"), individuals who have previously been infected by a related strain are assumed to be less likely to experience severe disease, and therefore recover more quickly, than immunologically naive individuals. We fit both models to estimated case notification data (including symptomatic individuals as well as laboratory-confirmed cases) from Japan from the 2009 H1N1 influenza pandemic, and then generate synthetic data for a future outbreak by assuming that the 2-group model represents the epidemiology of influenza infections more accurately. We use the 1-group model (as well as the 2-group model for comparison) to generate forecasts that would be obtained in real-time as the future outbreak is ongoing, using parameter values estimated from the 2009 epidemic as informative priors, motivated by the fact that without using prior information from 2009, the forecasts are highly uncertain. In the scenario that we consider, the 1-group model only produces accurate outbreak forecasts once the peak of the epidemic has passed, even when the values of important epidemiological parameters such as the lengths of the mean incubation and infectious periods are known exactly. As a result, it is necessary to use the more epidemiologically realistic 2-group model to generate accurate forecasts. Accounting for cross-immunity driven by exposures in previous outbreaks explicitly is expected to improve the accuracy of epidemiological modelling forecasts during influenza outbreaks.

Replacement of TCR Dβ With Immunoglobulin DH DSP2.3 Imposes a Tyrosine-Enriched TCR Repertoire and Adversely Affects T Cell Development

Enrichment for tyrosine in immunoglobulin CDR-H3 is due in large part to natural selection of germline immunoglobulin DH sequence. We have previously shown that when DH sequence is modified to reduce the contribution of tyrosine codons, epitope recognition is altered and B cell development, antibody production, autoantibody production, and morbidity and mortality following pathogen challenge are adversely affected. TCRβ diversity (Dβ) gene segment sequences are even more highly conserved than DH, with trout Dβ1 identical to human and mouse Dβ1. We hypothesized that natural selection of Dβ sequence also shapes CDR-B3 diversity and influences T cell development and T cell function. To test this, we used a mouse strain that lacked Dβ2 and contained a novel Dβ1 allele (DβYTL) that replaces Dβ1 with an immunoglobulin DH, DSP2.3. Unlike Dβ1, wherein glycine predominates in all three reading frames (RFs), in DSP2.3 there is enrichment for tyrosine in RF1, threonine in RF2, and leucine in RF3. Mature T cells using DβYTL expressed TCRs enriched at particular CDR-B3 positions for tyrosine but depleted of leucine. Changing Dβ sequence altered thymocyte and peripheral T cell numbers and the T cell response to an ovalbumin immunodominant epitope. The differences in tyrosine content might explain, at least in part, why TCRs are more polyspecific and of lower affinity for their cognate antigens than their immunoglobulin counterparts.

Antigen Receptor Specificity and Cell Location Influence the Diversification and Selection of the B-1a Cell Pool with Age

B-1a cells provide immediate and essential protection from infection through production of natural Ig, which is germline-like due to minimal insertion of N region additions. We have previously demonstrated peritoneal B-1a cell-derived phosphorylcholine-specific and total IgM moves away from germline (as evidenced by an increase in N-additions) with age as a result of selection. In young mice, anti-phosphatidylcholine Abs, like anti-phosphorylcholine Abs, contain few N-additions, and have been shown to be essential in protection from bacterial sepsis. In this study, we demonstrate the germline-like status of phosphatidylcholine (PtC)-specific (PtC⁺) peritoneal B-1a cell IgM does not change with age. In direct contrast, the splenic PtC⁺ B-1a cell population does not preserve its IgM germline status in the aged mice. Furthermore, splenic PtC⁺ B-1a cells displayed more diverse variable gene segments of the H chain (V_H) use in both the young and aged mice as compared with peritoneal PtC⁺ B-1a cells. Whereas the peritoneal PtC⁺ population increased V_H12 use with age, we observed differential use of V_H11, V_H12, and V_H2 between the peritoneal and splenic PtC⁺ populations with age. These results suggest disparate selection pressures occur with age upon B-1a cells expressing different specificities in distinct locations. Overall, these results illuminate the need to further elucidate how B-1a cells are influenced over time in terms of production and selection, both of which contribute to the actual and available natural IgM repertoire with increasing age. Such studies would aid in the development of more effective vaccination and therapeutic strategies in the aged population.

A Complex Dance: Measuring the Multidimensional Worlds of Influenza Virus Evolution and Anti-Influenza Immune Responses

The human antibody response to influenza virus infection or vaccination is as complicated as it is essential for protection against flu. The constant antigenic changes of the virus to escape human herd immunity hinder the yearly selection of vaccine strains since it is hard to predict which virus strains will circulate for the coming flu season. A "universal" influenza vaccine that could induce broad cross-influenza subtype protection would help to address this issue. However, the human antibody response is intricate and often obscure, with factors such as antigenic seniority or original antigenic sin (OAS), and back-boosting ensuring that each person mounts a unique immune response to infection or vaccination with any new influenza virus strain. Notably, the effects of existing antibodies on cross-protective immunity after repeated vaccinations are unclear. More research is needed to characterize the mechanisms at play, but traditional assays such as hemagglutinin inhibition (HAI) and microneutralization (MN) are excessively limited in scope and too resource-intensive to effectively meet this challenge. In the past ten years, new multiple dimensional assays (MDAs) have been developed to help overcome these problems by simultaneously measuring antibodies against a large panel of influenza hemagglutinin (HA) proteins with a minimal amount of sample in a high throughput way. MDAs will likely be a powerful tool for accelerating the study of the humoral immune response to influenza vaccination and the development of a universal influenza vaccine.

The sequences encoded by immunoglobulin diversity (DH ) gene segments play key roles in controlling B-cell development, antigen-binding site diversity, and antibody production

Although at first glance the diversity of the immunoglobulin repertoire appears random, there are a number of mechanisms that act to constrain diversity. For example, key mechanisms controlling the diversity of the third complementarity determining region of the immunoglobulin heavy chain (CDR-H3) include natural selection of germline diversity (D_H ) gene segment sequence and somatic selection upon passage through successive B-cell developmental checkpoints. To test the role of D_H gene segment sequence, we generated a panel of mice limited to the use of a single germline or frameshifted D_H gene segment. Specific individual amino acids within core D_H gene segment sequence heavily influenced the absolute numbers of developing and mature B-cell subsets, antibody production, epitope recognition, protection against pathogen challenge, and susceptibility to the production of autoreactive antibodies. At the tip of the antigen-binding loop (PDB position 101) in CDR-H3, both natural (germline) and somatic selection favored tyrosine while disfavoring the presence of hydrophobic amino acids. Enrichment for arginine in CDR-H3 appeared to broaden recognition of epitopes of varying hydrophobicity, but led to diminished binding intensity and an increased likelihood of generating potentially pathogenic dsDNA-binding autoreactive antibodies. The phenotype of altering the sequence of the D_H was recessive for T-independent antibody production, but dominant for T-cell-dependent responses. Our work suggests that the antibody repertoire is structured, with the sequence of individual D_H selected by evolution to preferentially generate an apparently preferred category of antigen-binding sites. The result of this structured approach appears to be a repertoire that has been adapted, or optimized, to produce protective antibodies for a wide range of pathogen epitopes while reducing the likelihood of generating autoreactive specificities.

Blood hematological values of Barb horses in Algeria

The Barb horse this famous Maghreb horse, symbol of the pride of the Berber peoples It is particularly significant in the Algerian history, tradition and culture. This study consists in establishing the hematologic values specific to the Barb horse in the national stud farm "Chaou-Chaoua" in Tiaret. Algeria. Age of the study animals varied between 1 and 23 years. The effects of age and sex were analyzed by dividing animals into four groups, A (1 - 2 years, n=5), B (3-5 years, n=5), C (5-10 years, n=5) and D (11-23 years, n=5); for females and E (2 - 3 years, n=5), F (5-7, n=5), G (10, n=5) and H (11-25, n=5) for males. Animals studied were clinically healthy. A blood sampling was carried out from all study animals, the following hematologic variables have been determined: erytrocytes (RBC), hematocrit (Ht), hemoglobin (Hb), mean cell volume (MCV), mean corpuscular hemoglobin concentration (MCHC), mean hemoglobin content (MHC), white blood cells (WBC), neutrophils, eosinophils, basophils, lymphocytes, monocytes, platelets, total number of platelets (PLT) and mean platelet volume (MPV). The obtained results showed that the hematologic values are within established standards for the other breeds of hot-blooded horses; these values approached the lower limits of the standards for the RBCs, HCT, Hb, WBCs and the lymphocytes. Statically significant (P<0.05) differences were observed between females in different age groups for the MCV, MCHC, Platelets and the MPV. Groups of males showed significant differences for lymphocytes and platelets and between males and females, significant differences related to the MCV, Neutrophils, Basophils Monocytes, Platelet and the MPV. This research showed that these hematologic parameters are weak, they approach the lower limits of the standards and that neither the age, nor the sex seem to have an influence on the hematologic values of the Barb horse.

Antiviral Activity of Fermented Ginseng Extracts against a Broad Range of Influenza Viruses

Ginseng products used as herb nutritional supplements are orally consumed and fermented to ginsenoside compounds by the intestinal microbes. In this study, we investigated antiviral protective effects of fermented ginseng extracts against different strains of influenza viruses in genetically diverse mouse models. Intranasal coinoculation of mice with fermented ginseng extract and influenza virus improved survival rates and conferred protection against H1N1, H3N2, H5N1, and H7N9 strains, with the efficacy dependent on the dose of ginseng samples. Antiviral protection by fermented ginseng extract was observed in different genetic backgrounds of mice and in the deficient conditions of key adaptive immune components (CD4, CD8, B cell, MHCII). The mice that survived primary virus inoculation with fermented ginseng extract developed immunity against the secondary infection with homologous and heterosubtypic viruses. In vitro cell culture experiments showed moderate virus neutralizing activity by fermented ginseng extract, probably by inhibiting hemagglutination and neuraminidase activity. This study suggests that fermented ginseng extracts might provide a means to treat influenza disease regardless of virus strains.

Nucleotide Composition of Human Ig Nontemplated Regions Depends on Trimming of the Flanking Gene Segments, and Terminal Deoxynucleotidyl Transferase Favors Adding Cytosine, Not Guanosine, in Most VDJ Rearrangements

The formation of nontemplated (N) regions during Ig gene rearrangement is a major contributor to Ab diversity. To gain insights into the mechanisms behind this, we studied the nucleotide composition of N regions within 29,962 unique human V_HDJ_H rearrangements and 8728 unique human DJ_H rearrangements containing exactly one identifiable D gene segment and thus two N regions, N1 and N2. We found a distinct decreasing content of cytosine (C) and increasing content of guanine (G) across each N region, suggesting that N regions are typically generated by concatenation of two 3' overhangs synthesized by addition of nucleoside triphosphates with a preference for dCTP. This challenges the general assumption that the terminal deoxynucleotidyl transferase favors dGTP in vivo. Furthermore, we found that the G and C gradients depended strongly on whether the germline gene segments were trimmed or not. Our data show that C-enriched N addition preferentially happens at trimmed 3' ends of V_H, D, and J_H gene segments, indicating a dependency of the transferase mechanism upon the nuclease mechanism.

Immunogenicity and efficacy of replication-competent recombinant influenza virus carrying multimeric M2 extracellular domains in a chimeric hemagglutinin conjugate

Current influenza vaccines provide hemagglutinin (HA) strain-specific protection. To improve cross protection, we engineered replication-competent influenza A virus to express tandem repeats of heterologous M2 extracellular (M2e) domains in a chimeric HA. M2e epitopes conjugated to HA glycoproteins (M2e4x-HA) were found to be expressed on the surfaces of a replicable influenza virus as examined by electron microscopy. The recombinant influenza virus containing M2e4x-HA was moderately attenuated but superior to the parental virus in inducing M2e specific antibodies without compromising HA immunogenicity. Recombinant influenza virus immune mice showed better cross protection than parental virus immune mice. Immune sera from the mice with inoculation of live recombinant influenza virus expressing M2e4x-HA were effective in conferring protection against H1, H3, and H5 subtype influenza viruses. This study indicates that recombinant influenza virus expressing conserved protective epitopes in an HA chimeric form can provide a new approach for improving the efficacy of influenza vaccines.

Seasonality in risk of pandemic influenza emergence

Influenza pandemics can emerge unexpectedly and wreak global devastation. However, each of the six pandemics since 1889 emerged in the Northern Hemisphere just after the flu season, suggesting that pandemic timing may be predictable. Using a stochastic model fit to seasonal flu surveillance data from the United States, we find that seasonal flu leaves a transient wake of heterosubtypic immunity that impedes the emergence of novel flu viruses. This refractory period provides a simple explanation for not only the spring-summer timing of historical pandemics, but also early increases in pandemic severity and multiple waves of transmission. Thus, pandemic risk may be seasonal and predictable, with the accuracy of pre-pandemic and real-time risk assessments hinging on reliable seasonal influenza surveillance and precise estimates of the breadth and duration of heterosubtypic immunity.

Non-neutralizing antibodies induced by seasonal influenza vaccine prevent, not exacerbate A(H1N1)pdm09 disease

The association of seasonal trivalent influenza vaccine (TIV) with increased infection by 2009 pandemic H1N1 (A(H1N1)pdm09) virus, initially observed in Canada, has elicited numerous investigations on the possibility of vaccine-associated enhanced disease, but the potential mechanisms remain largely unresolved. Here, we investigated if prior immunization with TIV enhanced disease upon A(H1N1)pdm09 infection in mice. We found that A(H1N1)pdm09 infection in TIV-immunized mice did not enhance the disease, as measured by morbidity and mortality. Instead, TIV-immunized mice cleared A(H1N1)pdm09 virus and recovered at an accelerated rate compared to control mice. Prior TIV immunization was associated with potent inflammatory mediators and virus-specific CD8 T cell activation, but efficient immune regulation, partially mediated by IL-10R-signaling, prevented enhanced disease. Furthermore, in contrast to suggested pathological roles, pre-existing non-neutralizing antibodies (NNAbs) were not associated with enhanced virus replication, but rather with promoted antigen presentation through FcR-bearing cells that led to potent activation of virus-specific CD8 T cells. These findings provide new insights into interactions between pre-existing immunity and pandemic viruses.

Towards the knowledge-based design of universal influenza epitope ensemble vaccines

MOTIVATION

Influenza A viral heterogeneity remains a significant threat due to unpredictable antigenic drift in seasonal influenza and antigenic shifts caused by the emergence of novel subtypes. Annual review of multivalent influenza vaccines targets strains of influenza A and B likely to be predominant in future influenza seasons. This does not induce broad, cross protective immunity against emergent subtypes. Better strategies are needed to prevent future pandemics. Cross-protection can be achieved by activating CD8+ and CD4+ T cells against highly conserved regions of the influenza genome. We combine available experimental data with informatics-based immunological predictions to help design vaccines potentially able to induce cross-protective T-cells against multiple influenza subtypes.

RESULTS

To exemplify our approach we designed two epitope ensemble vaccines comprising highly conserved and experimentally verified immunogenic influenza A epitopes as putative non-seasonal influenza vaccines; one specifically targets the US population and the other is a universal vaccine. The USA-specific vaccine comprised 6 CD8+ T cell epitopes (GILGFVFTL, FMYSDFHFI, GMDPRMCSL, SVKEKDMTK, FYIQMCTEL, DTVNRTHQY) and 3 CD4+ epitopes (KGILGFVFTLTVPSE, EYIMKGVYINTALLN, ILGFVFTLTVPSERG). The universal vaccine comprised 8 CD8+ epitopes: (FMYSDFHFI, GILGFVFTL, ILRGSVAHK, FYIQMCTEL, ILKGKFQTA, YYLEKANKI, VSDGGPNLY, YSHGTGTGY) and the same 3 CD4+ epitopes. Our USA-specific vaccine has a population protection coverage (portion of the population potentially responsive to one or more component epitopes of the vaccine, PPC) of over 96 and 95% coverage of observed influenza subtypes. The universal vaccine has a PPC value of over 97 and 88% coverage of observed subtypes.

AVAILABILITY AND IMPLEMENTATION

http://imed.med.ucm.es/Tools/episopt.html CONTACT: d.r.flower@aston.ac.uk.

HIV-1 gp140 epitope recognition is influenced by immunoglobulin DH gene segment sequence

Complementarity Determining Region 3 of the immunoglobulin (Ig) H chain (CDR-H3) lies at the center of the antigen-binding site where it often plays a decisive role in antigen recognition and binding. Amino acids encoded by the diversity (DH) gene segment are the main component of CDR-H3. Each DH has the potential to rearrange into one of six DH reading frames (RFs), each of which exhibits a characteristic amino acid hydrophobicity signature that has been conserved among jawed vertebrates by natural selection. A preference for use of RF1 promotes the incorporation of tyrosine into CDR-H3 while suppressing the inclusion of hydrophobic or charged amino acids. To test the hypothesis that these evolutionary constraints on DH sequence influence epitope recognition, we used mice with a single DH that has been altered to preferentially use RF2 or inverted RF1. B cells in these mice produce a CDR-H3 repertoire that is enriched for valine or arginine in place of tyrosine. We serially immunized this panel of mice with gp140 from HIV-1 JR-FL isolate and then used enzyme-linked immunosorbent assay (ELISA) or peptide microarray to assess antibody binding to key or overlapping HIV-1 envelope epitopes. By ELISA, serum reactivity to key epitopes varied by DH sequence. By microarray, sera with Ig CDR-H3s enriched for arginine bound to linear peptides with a greater range of hydrophobicity but had a lower intensity of binding than sera containing Ig CDR-H3s enriched for tyrosine or valine. We conclude that patterns of epitope recognition and binding can be heavily influenced by DH germ line sequence. This may help explain why antibodies in HIV-infected patients must undergo extensive somatic mutation in order to bind to specific viral epitopes and achieve neutralization.

Prior infection with influenza virus but not vaccination leaves a long-term immunological imprint that intensifies the protective efficacy of antigenically drifted vaccine strains

The role of pre-existing immunity for influenza vaccine responses is of great importance for public health, and thus has been studied in various contexts, yet the impact of differential priming on vaccine responses in the midst of antigenic drift remains to be elucidated. To address this with antigenically related viruses, mice were first primed by either infection or immunization with A/Puerto Rico/8/34 (PR8) virus, then immunized with whole-inactivated A/Fort Monmouth/1/47 (FM1) virus. The ensuing vaccine responses and the protective efficacy of FM1 were superior in PR8 infection-primed mice compared to PR8 immunization-primed or unprimed mice. Increased FM1-specific Ab responses of PR8 infection-primed mice also broadened cross-reactivity against contemporary as well as antigenically more drifted strains. Further, prior infection heightened the protective efficacy of antigenically distant strains, such as A/Brisbane/59/2006 infection followed by immunization with split pandemic H1N1 vaccine (A/California/07/2009). Therefore, influenza infection is a significant priming event that intensifies future vaccine responses against drift strains.

Increased exposure to Plasmodium chabaudi antigens sustains cross-reactivity and avidity of antibodies binding Nippostrongylus brasiliensis: dissecting cross-phylum cross-reactivity in a rodent model

Mounting an antibody response capable of discriminating amongst and appropriately targeting different parasites is crucial in host defence. However, cross-reactive antibodies that recognize (bind to) multiple parasite species are well documented. We aimed to determine if a higher inoculating dose of one species, and thus exposure to larger amounts of antigen over a longer period of time, would fine-tune responses to that species and reduce cross-reactivity. Using the Plasmodium chabaudi chabaudi (Pcc)–Nippostrongylus brasiliensis (Nb) co-infection model in BALB/c mice, in which we previously documented cross-reactive antibodies, we manipulated the inoculating dose of Pcc across 4 orders of magnitude. We investigated antigen-specific and cross-reactive antibody responses against crude and defined recombinant antigens by enzyme linked immunosorbent assay, Western blot and antibody depletion assays. Contrary to our hypothesis that increasing exposure to Pcc would reduce cross-reactivity to Nb, we found evidence for increased avidity of a subpopulation of antibodies that recognized shared antigens. Western blot indicated proteins of apparent monomer molecular mass 28 and 98 kDa in both Nb and Pcc antigen preparations and also an Nb protein of similar size to recombinant Pcc antigen, merozoite surface protein-119. The implications of antibodies binding antigen from such phylogenetically distinct parasites are discussed.

Delta Inulin Adjuvant Enhances Plasmablast Generation, Expression of Activation-Induced Cytidine Deaminase and B-Cell Affinity Maturation in Human Subjects Receiving Seasonal Influenza Vaccine

There is a major need for new adjuvants to improve the efficacy of seasonal and pandemic influenza vaccines. Advax is a novel polysaccharide adjuvant based on delta inulin that has been shown to enhance the immunogenicity of influenza vaccine in animal models and human clinical trials. To better understand the mechanism for this enhancement, we sought to assess its effect on the plasmablast response in human subjects. This pilot study utilised cryopreserved 7 day post-vaccination (7dpv) peripheral blood mononuclear cell samples obtained from a subset of 25 adult subjects from the FLU006-12 trial who had been immunized intramuscularly with a standard dose of 2012 trivalent inactivated influenza vaccine (TIV) alone (n=9 subjects) or combined with 5mg (n=8) or 10mg (n=8) of Advax adjuvant. Subjects receiving Advax adjuvant had increased 7dpv plasmablasts, which in turn exhibited a 2-3 fold higher rate of non-silent mutations in the B-cell receptor CDR3 region associated with higher expression of activation-induced cytidine deaminase (AID), the major enzyme controlling BCR affinity maturation. Together, these data suggest that Advax adjuvant enhances influenza immunity in immunized subjects via multiple mechanisms including increased plasmablast generation, AID expression and CDR3 mutagenesis resulting in enhanced BCR affinity maturation and increased production of high avidity antibody. How Advax adjuvant achieves these beneficial effects on plasmablasts remains the subject of ongoing investigation.

Maximal immune response and cross protection by influenza virus nucleoprotein derived from E. coli using an optimized formulation

The highly conserved internal nucleoprotein (NP) is a promising antigen to develop a universal influenza A virus vaccine. In this study, mice were injected intramuscularly with Escherichia coli-derived NP protein alone or in combination with adjuvant alum (Al(OH)3), CpG or both. The results showed that the NP protein formulated with adjuvant was effective in inducing a protective immune response. Additionally, the adjuvant efficacy of Al(OH)3 was stronger than that of CpG. Optimal immune responses were observed in BALB/c mice immunized with a combination of NP protein plus Al(OH)3 and CpG. These mice also showed maximal resistance following challenge with influenza A virus PR8 strain. Most importantly, 10 µg NP formulated with Al(OH)3 and CpG induced higher protection than did 90 µg NP. These findings indicated that a combination of Al(OH)3 and CpG may be an efficient adjuvant in the NP formulation.

Comparison of the effectiveness of antibody and cell-mediated immunity against inhaled and instilled influenza virus challenge

BACKGROUND

To evaluate immunity against influenza, mouse challenge studies are typically performed by intranasal instillation of a virus suspension to anesthetized animals. This results in an unnatural environment in the lower respiratory tract during infection, and therefore there is some concern that immune mechanisms identified in this model may not reflect those that protect against infectious virus particles delivered directly to the lower respiratory tract as an aerosol.

METHOD

To evaluate differences in protection against instilled and inhaled virus, mice were immunized with influenza antigens known to induce antibody or cell-mediated responses and then challenged with 100 LD50 A/PR/8/34 (PR8) in the form of aerosol (inhaled) or liquid suspension (instilled).

RESULTS

Mice immunized with recombinant adenovirus (Ad) expressing hemagglutinin were protected against weight loss and death in both challenge models, however immunization with Ad expressing nucleoprotein of influenza A (NPA) or M2 resulted in greater protection against inhaled aerosolized virus than virus instilled in liquid suspension. Ad-M2, but not Ad-NPA-immunized mice were protected against a lower instillation challenge dose.

CONCLUSIONS

These results demonstrate differences in protection that are dependent on challenge method, and suggest that cell-mediated immunity may be more accurately demonstrated in mouse inhalation studies. Furthermore, the data suggest immune mechanisms generally characterized as incomplete or weak in mouse models using liquid intranasal challenge may offer greater immunity against influenza infection than previously thought.

Evaluation of mismatched immunity against influenza viruses

Prior immunity against influenza A viruses generates sterilizing immunity against matched (homologous) viruses and varying levels of protection against mismatched (heterologous) viruses of the same or different subtypes. Natural immunity carries the risk of high morbidity and mortality, therefore immunization offers the best preventative measure. Antibody responses against the viral hemagglutinin protein correlate with protection in humans and evidence increasingly supports a role for robust cellular immune responses. By exploiting mismatched immunity, current conventional and experimental vaccine candidates can improve the generation of cross-protective immune responses against heterologous viruses. Experimental vaccines such as virus-like particles, DNA vectors, viral vectors and broadly neutralizing antibodies are able to expand cross-protection through mismatched B- and T-cell responses. However, the generation of mismatched immune responses can also have the opposite effect and impair protective immunity. This review discusses mismatched immunity in the context of natural infection and immunization. Additionally, we discuss strategies to exploit mismatched immunity in order to improve current conventional and experimental influenza A virus vaccines.

Profiling of humoral immune responses to influenza viruses by using protein microarray

The emergence of pandemic A(H1N1) 2009 influenza showed the importance of rapid assessment of the degree of immunity in the population, the rate of asymptomatic infection, the spread of infection in households, effects of control measures, and ability of candidate vaccines to produce a response in different age groups. A limitation lies in the available assay repertoire: reference standard methods for measuring antibodies to influenza virus are haemagglutination inhibition (HI) assays and virus neutralization tests. Both assays are difficult to standardize and may be too specific to assess possible partial humoral immunity from previous exposures. Here, we describe the use of antigen-microarrays to measure antibodies to HA1 antigens from seven recent and historical seasonal H1, H2 and H3 influenza viruses, the A(H1N1) 2009 pandemic influenza virus, and three avian influenza viruses. We assessed antibody profiles in 18 adult patients infected with A(H1N1) 2009 influenza virus during the recent pandemic, and 21 children sampled before and after the pandemic, against background reactivity observed in 122 persons sampled in 2008, a season dominated by seasonal A(H1N1) influenza virus. We show that subtype-specific and variant-specific antibody responses can be measured, confirming serological responses measured by HI. Comparison of profiles from persons with similar HI response showed that the magnitude and broadness of response to individual influenza subtype antigens differs greatly between individuals. Clinical and vaccination studies, but also exposure studies, should take these findings into consideration, as they may indicate some level of humoral immunity not measured by HI assays.

Antibodies against conserved antigens provide opportunities for reform in influenza vaccine design

High-performance neutralizing antibody against influenza virus typically recognizes the globular head region of its hemagglutinin (HA) envelope glycoprotein. To-date, approved human vaccination strategies have been designed to induce such antibodies as a sole means of preventing the consequences of this infection. However, frequent amino-acid changes in the HA globular head allow for efficient immune evasion. Consequently, vaccines inducing such neutralizing antibodies need to be annually re-designed and re-administered at a great expense. These vaccines furthermore provide little-to-no immunity against antigenic-shift strains, which arise from complete replacement of HA or of neuraminidase genes, and pose pandemic risks. To address these issues, laboratory research has focused on inducing immunity effective against all strains, regardless of changes in the HA globular head. Despite prior dogma that such cross-protection needs to be induced by cellular immunity alone, several advances in recent years demonstrate that antibodies of other specificities are capable of cross-strain protection in mice. This review discusses the reactivity, induction, efficacy, and mechanisms of antibodies that react with poorly accessible epitopes in the HA stalk, with the matrix 2 membrane ion channel, and even with the internal nucleoprotein. These advances warrant further investigation of the inducibility and efficacy of such revolutionary antibody strategies in humans.

Age-related changes in durability and function of vaccine-elicited influenza-specific CD4(+) T-cell responses

The major antigenic component of licensed influenza vaccines, hemagglutinin (HA), elicits predominantly type-specific antibody responses, thus necessitating frequent antigenic updates to the annual vaccine. However, accumulating evidence suggests that influenza vaccines can also induce significant cross-reactive T-cell responses to highly divergent, heterosubtypic HA antigens not included in the vaccine. Influenza vaccines are less effective among the elderly and studies that characterize cross-reactive T-cell immunity in this vulnerable population are much needed. Here, we systematically compare the ex vivo frequency, cytokine profile and phenotype of vaccine-elicited HA-specific T-cell responses among a cohort of young (18-49 years old) and elderly (≥70 years old) vaccinees, as well as the maturation and activation phenotype of total CD4(+) and CD8(+) T-cells. IFN-γ production after in vitro expansion and HA-specific Ab titers were also determined. We find that vaccine-elicited ex vivo frequencies of CD4(+) T-cells elicited by vaccination reactive to any given homo- or heterosubtypic Ag were comparable across the two age groups. While, no differences were observed between age groups in the phenotype of Ag-specific or total CD4(+) T-cells, PBMC from young adults were superior at producing IFN-γ after short-term Ag-specific culture. Significantly, while vaccine-elicited T-cell responses were durable among the younger vaccinees, they were short-lived among the elderly. These results have important ramifications for our understanding of vaccine-induced changes in the magnitude and functionality of HA-specific CD4(+) T-cells, as well as age-related alterations in response kinetics.

Polymerases in nonhomologous end joining: building a bridge over broken chromosomes

Repair of double-strand breaks in chromosomal DNA is essential. Unfortunately, a paradigm central to most DNA repair pathways--damaged DNA is replaced by polymerases, by using an intact, undamaged complementary strand as a template--no longer works. The nonhomologous end joining (NHEJ) pathway nevertheless still uses DNA polymerases to help repair double-strand breaks. Bacteria use a member of the archaeo-eukaryal primase superfamily, whereas eukaryotes use multiple members of the polymerase X family. These polymerases can, depending on the biologic context, accurately replace break-associated damage, mitigate loss of flanking DNA, or diversify products of repair. Polymerases specifically implicated in NHEJ are uniquely effective in these roles: relative to canonic polymerases, NHEJ polymerases have been engineered to do more with less.

Regulation of antinucleoprotein IgG by systemic vaccination and its effect on influenza virus clearance

Seasonal influenza epidemics recur due to antigenic drift of envelope glycoprotein antigens and immune evasion of circulating viruses. Additionally, antigenic shift can lead to influenza pandemics. Thus, a universal vaccine that protects against multiple influenza virus strains could alleviate the continuing impact of this virus on human health. In mice, accelerated clearance of a new viral strain (cross-protection) can be elicited by prior infection (heterosubtypic immunity) or by immunization with the highly conserved internal nucleoprotein (NP). Both heterosubtypic immunity and NP-immune protection require antibody production. Here, we show that systemic immunization with NP readily accelerated clearance of a 2009 pandemic H1N1 influenza virus isolate in an antibody-dependent manner. However, human immunization with trivalent inactivated influenza virus vaccine (TIV) only rarely and modestly boosted existing levels of anti-NP IgG. Similar results were observed in mice, although the reaction could be enhanced with adjuvants, by adjusting the stoichiometry among NP and other vaccine components, and by increasing the interval between TIV prime and boost. Importantly, mouse heterosubtypic immunity that had waned over several months could be enhanced by injecting purified anti-NP IgG or by boosting with NP protein, correlating with a long-lived increase in anti-NP antibody titers. Thus, current immunization strategies poorly induce NP-immune antibody that is nonetheless capable of contributing to long-lived cross-protection. The high conservation of NP antigen and the known longevity of antibody responses suggest that the antiviral activity of anti-NP IgG may provide a critically needed component of a universal influenza vaccine.

Hallmarks of CD4 T cell immunity against influenza

The mechanisms responsible for heterosubtypic immunity to influenza virus are not well understood but might hold the key for new vaccine strategies capable of providing lasting protection against both seasonal and pandemic strains. Memory CD4 T cells are capable of providing substantial protection against influenza both through direct effector mechanisms and indirectly through regulatory and helper functions. Here, we discuss the broad impact of memory CD4 T cells on heterosubtypic immunity against influenza and the prospects of translating findings from animal models into improved human influenza vaccines.

Contributions of antinucleoprotein IgG to heterosubtypic immunity against influenza virus

Influenza A virus causes recurring seasonal epidemics and occasional influenza pandemics. Because of changes in envelope glycoprotein Ags, neutralizing Abs induced by inactivated vaccines provide limited cross-protection against new viral serotypes. However, prior influenza infection induces heterosubtypic immunity that accelerates viral clearance of a second strain, even if the external proteins are distinct. In mice, cross-protection can also be elicited by systemic immunization with the highly conserved internal nucleoprotein (NP). Both T lymphocytes and Ab contribute to such cross-protection. In this paper, we demonstrate that anti-NP IgG specifically promoted influenza virus clearance in mice by using a mechanism involving both FcRs and CD8(+) cells. Furthermore, anti-NP IgG rescued poor heterosubtypic immunity in B cell-deficient mice, correlating with enhanced NP-specific CD8 T cell responses. Thus, Ab against this conserved Ag has potent antiviral activity both in naive and in influenza-immune subjects. Such antiviral activity was not seen when mice were vaccinated with another internal influenza protein, nonstructural 1. The high conservation of NP Ag and the known longevity of Ab responses suggest that anti-NP IgG may provide a critically needed component of a universal influenza vaccine.

The Role of Animal Models In Influenza Vaccine Research

A major challenge for research on influenza vaccines is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans. Vaccines for seasonal influenza have been available for decades and there is a wealth of data available on the immune response to these vaccines in humans, with well-established correlates of protection for inactivated influenza virus vaccines. Many of the seminal studies on vaccines for epidemic influenza have been conducted in human subjects. Studies in humans are performed less frequently now than they were in the past. Therefore, as the quest for improved influenza vaccines continues, it is important to consider the use of animal models for the evaluation of influenza vaccines, and a major challenge is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans.

The emergence of highly pathogenic H5N1 avian influenza (AI) viruses and the threat of a pandemic caused by AI viruses of this or another subtype has resulted in a resurgence of interest in influenza vaccine research. The development of vaccines for pandemic influenza presents a unique set of obstacles, not the least of which is that the demonstration of efficacy in humans is not possible. As the correlates of protection from pandemic influenza are not known, we rely on extrapolation of the lessons from seasonal influenza vaccines and on data from the evaluation of pandemic influenza vaccines in animal models to guide our decisions on vaccines for use in humans. The features and contributions of commonly used animal models for influenza vaccine research are discussed. The recent emergence of the pandemic 2009 H1N1 influenza virus underscores the unpredictable nature of influenza viruses and the importance of pandemic preparedness.

The CDR-H3 repertoire from TdT-deficient adult bone marrow is a close, but not exact, homologue of the CDR-H3 repertoire from perinatal liver

Compared with adult bone marrow (BM), the composition of the perinatal liver CDR-3 of the Ig H chain (CDR-H3) repertoire is marked by a paucity of N nucleotides and by enrichment for use of J(H) proximal DQ52 and D(H) proximal V(H) and J(H) gene segments. To test the extent to which these differences reflect limited perinatal TdT activity versus differences in the fetal/adult environment, we used the Hardy scheme to sort fractions B-F B lineage cells from TdT-deficient BALB/c adult BM. V(H)7183-containing VDJCμ transcripts from these cells were amplified, cloned, sequenced, and compared with transcripts from wild-type perinatal liver and adult BM. The pattern of V(H)DJ(H) usage in TdT-deficient BM largely matched that of TdT-sufficient adult cells. What minor differences were detected in the pro-B cell stage tended to diminish with B cell maturation, suggesting strong environmental or Ag-driven pressure to achieve a specific range of V(H)DJ(H) usage regardless of the extent of N nucleotide addition. However, although the patterns of V(H)DJ(H) usage in the TdT-deficient B lineage cells paralleled that of wild-type adult cells, the length distribution, global amino acid composition, and charge distribution of the CDR-H3 repertoire proved to be a close, although not exact, homologue of the CDR-H3 repertoire first expressed by late pre-B cells in the TdT-insufficient perinatal liver. Thus, although differing in V(H) content, TdT-deficient mice appear to represent a good, although not perfect, model for testing the role of perinatal CDR-H3 limitations on late B cell development and Ab responses.

Genetic control of DH reading frame and its effect on B-cell development and antigen-specifc antibody production

The power of the adaptive immune system to identify novel antigens depends on the ability of lymphocytes to create antigen receptors with diverse antigen-binding sites. For immunoglobulins, CDR (complementarity-determining region)-H3 lies at the center of the antigen-binding site, where it often plays a key role in antigen binding. It is created de novo by VDJ rearrangement and is thus the focus for rearrangement-dependent diversity. CDR-H3 is biased for the inclusion of tyrosine. In seeking to identify the mechanisms controlling CDR-H3 amino acid content, we observed that the coding sequence of DH gene segments demonstrate conservation of reading frame (RF)-specific sequence motifs, with RF1 enriched for tyrosine and depleted of hydrophobic and charged amino acids. Use of DH RF1 in functional VDJ transcripts is preferred from the earliest stages of B-cell development, "pushing" CDR-H3 to include specific categories of tyrosine-enriched antigen-binding sites. With development and maturation, the composition of the CDR-H3 repertoire appears to be pulled into a more refined specific range. Forcing the use of alternative DH RFs by means of gene targeting alters the expressed repertoire, enriching alternative sequence categories. This change in the repertoire variably affects antibody production and the development of specific B-cell subsets.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.

Vaccination with whole inactivated virus vaccine affects the induction of heterosubtypic immunity against influenza virus A/H5N1 and immunodominance of virus-specific CD8+ T-cell responses in mice

It was recently shown that the use of an experimental subunit vaccine protected mice against infection with a human A/H3N2 influenza virus, but consequently affected the induction of heterosubtypic immunity to a highly pathogenic A/H5N1 influenza virus, which was otherwise induced by the A/H3N2 infection. As whole inactivated virus (WIV) vaccines are widely used to protect against seasonal influenza and also contain inner viral proteins such as the nucleoprotein (NP), the potential of a WIV vaccine to induce protective immunity against infection was tested with a homologous A/H3N2 (A/Hong Kong/2/68) and a heterosubtypic A/H5N1 influenza virus (A/Indonesia/5/05). As expected, the vaccine afforded protection against infection with the A/H3N2 virus only. In addition, it was demonstrated that the use of WIV vaccine for protection against A/H3N2 infection affected the induction of heterosubtypic immunity that was otherwise afforded by A/H3N2 influenza virus infection. The reduction in protective immunity correlated with changes in the immunodominance patterns of the CD8(+) T-cell responses directed to the epitopes located in the acid polymerase subunit of the viral RNA polymerase (PA(224-233)) and the NP (NP(366-374)). In unvaccinated mice that experienced infection with the A/H3N2 influenza virus, the magnitude of the CD8(+) T-cell response to both peptides was similar on secondary infection with A/H5N1 influenza virus. In contrast, prior vaccination with WIV affected the immunodominance pattern and skewed the response after infection with influenza virus A/Indonesia/5/05 towards a dominant NP(366-374)-specific response. These findings may have implications for vaccination strategies aimed at the induction of protective immunity to seasonal and/or pandemic influenza.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.

Terminal deoxynucleotidyl transferase is required for an optimal response to the polysaccharide α-1,3 dextran

An understanding of Ab responses to polysaccharides associated with pathogenic microorganisms is of importance for improving vaccine design, especially in neonates that respond poorly to these types of Ags. In this study, we have investigated the role of the lymphoid-specific enzyme TdT in generating B cell clones responsive to alpha-1,3 dextran (DEX). TdT is a DNA polymerase that plays a major role in generating diversity of lymphocyte AgRs during V(D)J recombination. In this study, we show that the DEX-specific Ab response is lower, and the dominant DEX-specific J558 idiotype (Id) is not detected in TdT(-/-) mice when compared with wild-type (WT) BALB/c mice. Nucleotide sequencing of H chain CDR3s of DEX-specific plasmablasts, sorted postimmunization, showed that TdT(-/-) mice generate a lower frequency of the predominant adult molecularly determined clone J558. Complementation of TdT expression in TdT(-/-) mice by early forced expression of the short splice variant of TdT-restored WT proportions of J558 Id+ clones and also abrogated the development of the minor M104E Id+ clones. J558 Id V(D)J rearrangements are detected as early as 7 d after birth in IgM-negative B cell precursors in the liver and spleen of WT and TdT-transgenic mice but not in TdT(-/-) mice. These data show that TdT is essential for the generation of the predominant higher-affinity DEX-responsive J558 clone.

Pre-existing immunity against swine-origin H1N1 influenza viruses in the general human population

A major concern about the ongoing swine-origin H1N1 influenza virus (S-OIV) outbreak is that the virus may be so different from seasonal H1N1 that little immune protection exists in the human population. In this study, we examined the molecular basis for pre-existing immunity against S-OIV, namely the recognition of viral immune epitopes by T cells or B cells/antibodies that have been previously primed by circulating influenza strains. Using data from the Immune Epitope Database, we found that only 31% (8/26) of B-cell epitopes present in recently circulating H1N1 strains are conserved in the S-OIV, with only 17% (1/6) conserved in the hemagglutinin (HA) and neuraminidase (NA) surface proteins. In contrast, 69% (54/78) of the epitopes recognized by CD8(+) T cells are completely invariant. We further demonstrate experimentally that some memory T-cell immunity against S-OIV is present in the adult population and that such memory is of similar magnitude as the pre-existing memory against seasonal H1N1 influenza. Because protection from infection is antibody mediated, a new vaccine based on the specific S-OIV HA and NA proteins is likely to be required to prevent infection. However, T cells are known to blunt disease severity. Therefore, the conservation of a large fraction of T-cell epitopes suggests that the severity of an S-OIV infection, as far as it is determined by susceptibility of the virus to immune attack, would not differ much from that of seasonal flu. These results are consistent with reports about disease incidence, severity, and mortality rates associated with human S-OIV.

Yearly influenza vaccinations: a double-edged sword?

Yearly vaccination against seasonal influenza viruses is recommended for certain individuals at high risk of complications associated with influenza. It has been recommended in some countries, including the USA, that all children aged 6-59 months are vaccinated against seasonal influenza. However, it has been shown-mainly in animals-that infection with influenza A viruses can induce protective immunity to influenza A viruses of other unrelated subtypes. This so-called heterosubtypic immunity does not provide full protection, but can limit virus replication and reduce morbidity and mortality of the host. This type of immunity might be relevant to human beings when a new subtype of influenza A virus is introduced into the population, such as the new influenza A H1N1 virus responsible for the present influenza pandemic and the highly pathogenic avian influenza H5N1 viruses that are causing an ever increasing number of human infections with high mortality rates. Preventing infection with seasonal influenza viruses by vaccination might prevent the induction of heterosubtypic immunity to pandemic strains, which might be a disadvantage to immunologically naive people-eg, infants.

Case-based surveillance of influenza hospitalizations during 2004-2008, Colorado, USA

Case-based surveillance provides more information than any other influenza surveillance component.

Colorado became the first state to make laboratory-confirmed influenza-associated hospitalizations a case-based reportable condition in 2004. We summarized surveillance for influenza hospitalizations in Colorado during the first 4 recorded influenza seasons (2004–2008). We highlight the similarities and differences among influenza seasons; no 2 seasons were entirely the same. The 2005–06 influenza season had 2 distinct waves of activity (types A and B), the 2006–07 season was substantially later and milder, and 2007–08 had substantially greater influenza B activity. The case-based surveillance for influenza hospitalizations provides information regarding the time course of seasonal influenza activity, reported case numbers and population-based rates by age group and influenza virus type, and a measure of relative severity. Influenza hospitalization surveillance provides more information about seasonal influenza activity than any other surveillance measure (e.g., surveillance for influenza-like illness) currently in widespread use among states. More states should consider implementing case-based surveillance for influenza hospitalizations.

Regulation of repertoire development through genetic control of DH reading frame preference

In jawed vertebrates most expressed Ig H chains use only one of six possible D(H) reading frames. Reading frame (RF)1, the preferred reading frame, tends to encode tyrosine and glycine, whereas the other five RFs tend to be enriched for either hydrophobic or charged amino acids. Mechanisms proposed to favor use of RF1 include a preference for deletion over inversion that discourages use of inverted RF1, RF2, and RF3; sequence homology between the 5' terminus of the J(H) and the 3' terminus of the D(H) that promotes rearrangement into RF1; an ATG start site upstream of RF2 that permits production of a truncated Dmicro protein; stop codons in RF3; and, following surface expression of IgM, somatic, presumably Ag receptor-based selection favoring B cells expressing Igs with tyrosine- and glycine-enriched CDR-H3s. By creating an IgH allele limited to the use of a single, frameshifted DFL16.1 D(H) gene segment, we tested the relative contribution of these mechanisms in determining reading frame preference. Dmicro-mediated suppression via an allelic exclusion-like mechanism dominated over somatic selection in determining the composition of the CDR-H3 repertoire. Evidence of somatic selection for RF1-encoded tyrosine in CDR-H3 was observed, but only among the minority of recirculating, mature B cells that use D(H) in RF1. These observations underscore the extent to which the sequence of the D(H) acts to delimit the diversity of the Ab repertoire.

Preferential use of DH reading frame 2 alters B cell development and antigen-specific antibody production

All jawed vertebrates limit use of D(H) reading frames (RFs) that are enriched for hydrophobic amino acids. In BALB/c mice, DFL16.1 RF2 encodes valine and isoleucine. To test whether increased use of RF2 affects B cell function, we examined B cell development and Ab production in mice with an IgH allele (DeltaD-DmicroFS) limited to use of a single, frameshifted DFL61.1 gene segment. We compared the results of these studies to wild-type mice, as well as those previously obtained in mice limited to use of either a single normal D(H) or a single inverted D(H) that forces use of arginine in CDR-H3. All three of the mouse strains limited to a single D(H) produced fewer immature B cells than wild type. However, whereas mice limited to a single normal D(H) achieved normal B cell numbers in the periphery, mice forced to preferentially use RF2 had reduced numbers of mature B cells in the spleen and bone marrow, mirroring the pattern previously observed in mice enriched for charged CDR-H3s. There were two exceptions. B cells in the mice using RF2 normally populated the marginal zone and peritoneal cavity, whereas mice using inverted RF1 had increased numbers of marginal zone B cells and decreased numbers of B1a cells. When challenged with several T-dependent or T-independent Ags, Ag-specific Ab titers in the mice forced to use RF2 were altered. These findings indicate that B cell development and Ag-specific Ab production can be heavily influenced by the global amino acid content of the CDR-H3 repertoire.

Altered immunodominance hierarchies of influenza A virus-specific H-2(b)-restricted CD8+ T cells in the absence of terminal deoxynucleotidyl transferase

Immunodominance is considered an obstacle to successful T cell-based vaccination, and constant efforts are made to uncover the underlying mechanisms for this phenomenon. We have examined the contribution of terminal deoxynucleotidyl transferase (TdT), whose function accounts for approximately 90% of T cell receptor diversity, to dominance hierarchies of H-2(b)-restricted flu-specific T(CD8+). Using intracellular cytokine staining to quantitatively detect epitope-specific T(CD8+), we demonstrate that TdT-deficient mice exhibit a distinct hierarchical pattern in their primary and recall T(CD8+) responses to influenza A viruses, which results from skewed responsiveness towards select influenza epitopes. Our data establish a link between TdT and immunodominance in H-2(b)-restricted antiviral T(CD8+) responses.

Heterosubtypic immunity to influenza A virus: where do we stand?

Influenza A virus (IAV) strains are denoted by the subtype of their hemagglutinin (HA) and neuraminidase (NA) virion surface proteins. Major changes in HA subtype among strains circulating in humans are referred to as "antigenic shift". Antigenic shift can occur by two means: direct transmission of a zoonotic strain to humans or through reshuffling of the segmented genome in cells co-infected with animal and human strains. The lack of circulating anti-HA antibodies in human populations to a novel IAV results in extremely high frequency of illness and the potential for severe morbidity and mortality on a world-wide basis; the dreaded pandemic. Such pandemics could be partially controlled by developing a vaccine that generates effective heterosubtypic immunity (HSI) based on immune recognition of IAV antigens conserved across all viral strains. While it has long been known that T cells exhibit such broad cross-reactive specificity that could provide effective HSI, recent animal studies suggest a potential role for antibodies as well. Here we review current knowledge of the mechanisms contributing to HSI to influenza and speculate on the potential for this approach to contribute to public health.