Single-Cell Chromatin Modification Profiling Reveals Increased Epigenetic Variations with Aging.

Post-translational modifications of histone proteins and exchanges of histone variants of chromatin are central to the regulation of nearly all DNA-templated biological processes. However, the degree and variability of chromatin modifications in specific human immune cells remain largely unknown. Here, we employ a highly multiplexed mass cytometry analysis to profile the global levels of a broad array of chromatin modifications in primary human immune cells at the single-cell level. Our data reveal markedly different cell-type- and hematopoietic-lineage-specific chromatin modification patterns. Differential analysis between younger and older adults shows that aging is associated with increased heterogeneity between individuals and elevated cell-to-cell variability in chromatin modifications. Analysis of a twin cohort unveils heritability of chromatin modifications and demonstrates that aging-related chromatin alterations are predominantly driven by non-heritable influences. Together, we present a powerful platform for chromatin and immunology research. Our discoveries highlight the profound impacts of aging on chromatin modifications.

Variation in the human immune system is largely driven by non-heritable influences.

There is considerable heterogeneity in immunological parameters between individuals, but its sources are largely unknown. To assess the relative contribution of heritable versus non-heritable factors, we have performed a systems-level analysis of 210 healthy twins between 8 and 82 years of age. We measured 204 different parameters, including cell population frequencies, cytokine responses, and serum proteins, and found that 77% of these are dominated (>50% of variance) and 58% almost completely determined (>80% of variance) by non-heritable influences. In addition, some of these parameters become more variable with age, suggesting the cumulative influence of environmental exposure. Similarly, the serological responses to seasonal influenza vaccination are also determined largely by non-heritable factors, likely due to repeated exposure to different strains. Lastly, in MZ twins discordant for cytomegalovirus infection, more than half of all parameters are affected. These results highlight the largely reactive and adaptive nature of the immune system in healthy individuals.

Signatures of immune dysfunction in HIV and HCV infection share features with chronic inflammation in aging and persist after viral reduction or elimination.

Chronic inflammation is thought to be a major cause of morbidity and mortality in aging, but whether similar mechanisms underlie dysfunction in infection-associated chronic inflammation is unclear. Here, we profiled the immune proteome, and cellular composition and signaling states in a cohort of aging individuals versus a set of HIV patients on long-term antiretroviral therapy therapy or hepatitis C virus (HCV) patients before and after sofosbuvir treatment. We found shared alterations in aging-associated and infection-associated chronic inflammation including T cell memory inflation, up-regulation of intracellular signaling pathways of inflammation, and diminished sensitivity to cytokines in lymphocytes and myeloid cells. In the HIV cohort, these dysregulations were evident despite viral suppression for over 10 y. Viral clearance in the HCV cohort partially restored cellular sensitivity to interferon-α, but many immune system alterations persisted for at least 1 y posttreatment. Our findings indicate that in the HIV and HCV cohorts, a broad remodeling and degradation of the immune system can persist for a year or more, even after the removal or drastic reduction of the pathogen load and that this shares some features of chronic inflammation in aging.

Determinants governing T cell receptor α/ß-chain pairing in repertoire formation of identical twins.

The T cell repertoire in each individual includes T cell receptors (TCRs) of enormous sequence diversity through the pairing of diverse TCR α- and ß-chains, each generated by somatic recombination of paralogous gene segments. Whether the TCR repertoire contributes to susceptibility to infectious or autoimmune diseases in concert with disease-associated major histocompatibility complex (MHC) polymorphisms is unknown. Due to a lack in high-throughput technologies to sequence TCR α-ß pairs, current studies on whether the TCR repertoire is shaped by host genetics have so far relied only on single-chain analysis. Using a high-throughput single T cell sequencing technology, we obtained the largest paired TCRαß dataset so far, comprising 965,523 clonotypes from 15 healthy individuals including 6 monozygotic twin pairs. Public TCR α- and, to a lesser extent, TCR ß-chain sequences were common in all individuals. In contrast, sharing of entirely identical TCRαß amino acid sequences was very infrequent in unrelated individuals, but highly increased in twins, in particular in CD4 memory T cells. Based on nucleotide sequence identity, a subset of these shared clonotypes appeared to be the progeny of T cells that had been generated during fetal development and had persisted for more than 50 y. Additional shared TCRαß in twins were encoded by different nucleotide sequences, implying that genetic determinants impose structural constraints on thymic selection that favor the selection of TCR α-ß pairs with entire sequence identities.

Signatures of selection in the human antibody repertoire: Selective sweeps, competing subclones, and neutral drift.

Antibodies are created and refined by somatic evolution in B cell populations, which endows the human immune system with the ability to recognize and eliminate diverse pathogens. However, the evolutionary processes that sculpt antibody repertoires remain poorly understood. Here, using an unbiased repertoire-scale approach, we show that the population genetic signatures of evolution are evident in human B cell lineages and reveal how antibodies evolve somatically. We measured the dynamics and genetic diversity of B cell responses in five adults longitudinally before and after influenza vaccination using high-throughput antibody repertoire sequencing. We identified vaccine-responsive B cell lineages that carry signatures of selective sweeps driven by positive selection, and discovered that they often display evidence for selective sweeps favoring multiple subclones. We also found persistent B cell lineages that exhibit stable population dynamics and carry signatures of neutral drift. By exploiting the relationship between B cell fitness and antibody binding affinity, we demonstrate the potential for using phylogenetic approaches to identify antibodies with high binding affinity. This quantitative characterization reveals that antibody repertoires are shaped by an unexpectedly broad spectrum of evolutionary processes and shows how signatures of evolutionary history can be harnessed for antibody discovery and engineering.

SIMON, an Automated Machine Learning System, Reveals Immune Signatures of Influenza Vaccine Responses.

Machine learning holds considerable promise for understanding complex biological processes such as vaccine responses. Capturing interindividual variability is essential to increase the statistical power necessary for building more accurate predictive models. However, available approaches have difficulty coping with incomplete datasets which is often the case when combining studies. Additionally, there are hundreds of algorithms available and no simple way to find the optimal one. In this study, we developed Sequential Iterative Modeling "OverNight" (SIMON), an automated machine learning system that compares results from 128 different algorithms and is particularly suitable for datasets containing many missing values. We applied SIMON to data from five clinical studies of seasonal influenza vaccination. The results reveal previously unrecognized CD4+ and CD8+ T cell subsets strongly associated with a robust Ab response to influenza Ags. These results demonstrate that SIMON can greatly speed up the choice of analysis modalities. Hence, it is a highly useful approach for data-driven hypothesis generation from disparate clinical datasets. Our strategy could be used to gain biological insight from ever-expanding heterogeneous datasets that are publicly available.

Continuous immunotypes describe human immune variation and predict diverse responses.

The immune system consists of many specialized cell populations that communicate with each other to achieve systemic immune responses. Our analyses of various measured immune cell population frequencies in healthy humans and their responses to diverse stimuli show that human immune variation is continuous in nature, rather than characterized by discrete groups of similar individuals. We show that the same three key combinations of immune cell population frequencies can define an individual's immunotype and predict a diverse set of functional responses to cytokine stimulation. We find that, even though interindividual variations in specific cell population frequencies can be large, unrelated individuals of younger age have more homogeneous immunotypes than older individuals. Across age groups, cytomegalovirus seropositive individuals displayed immunotypes characteristic of older individuals. The conceptual framework for defining immunotypes suggested by our results could guide the development of better therapies that appropriately modulate collective immunotypes, rather than individual immune components.

Phylogenetic analysis of the human antibody repertoire reveals quantitative signatures of immune senescence and aging.

The elderly have reduced humoral immunity, as manifested by increased susceptibility to infections and impaired vaccine responses. To investigate the effects of aging on B-cell receptor (BCR) repertoire evolution during an immunological challenge, we used a phylogenetic distance metric to analyze Ig heavy-chain transcript sequences in both young and elderly individuals before and after influenza vaccination. We determined that BCR repertoires become increasingly specialized over a span of decades, but less plastic. In 50% of the elderly individuals, a large space in the repertoire was occupied by a small number of recall lineages that did not decline during vaccine response and contained hypermutated IgD+ B cells. Relative to their younger counterparts, older subjects demonstrated a contracted naive repertoire and diminished intralineage diversification, signifying a reduced substrate for mounting novel responses and decreased fine-tuning of BCR specificities by somatic hypermutation. Furthermore, a larger proportion of the repertoire exhibited premature stop codons in some elderly subjects, indicating that aging may negatively affect the ability of B cells to discriminate between functional and nonfunctional receptors. Finally, we observed a decreased incidence of radical mutations compared with conservative mutations in elderly subjects' vaccine responses, which suggests that accumulating original antigenic sin may be limiting the accessible space for paratope evolution. Our findings shed light on the complex interplay of environmental and gerontological factors affecting immune senescence, and provide direct molecular characterization of the effects of senescence on the immune repertoire.

Diminished B-Cell Response After Repeat Influenza Vaccination.

Annual vaccination with influenza vaccines is recommended for protection against influenza in the United States. Past clinical studies and meta-analysis, however, have reported conflicting results on the benefits of annual vaccination. B-cell responses elicited following repeat influenza vaccinations over multiple seasons have not been examined in detail. We analyzed the B-cell and antibody (Ab) responses in volunteers vaccinated yearly, from 2010 or 2011 through 2014, with seasonal trivalent inactivated influenza vaccines. Statistical analyses were designed to help correct for possible bias due to reduced sample size in the later years of the study. We show that, after the second annual vaccination, the frequency of vaccine-specific plasmablasts and the binding reactivity of plasmablast-derived polyclonal Abs are reduced and do not increase in subsequent years. Similar trends are observed with the serum hemagglutination inhibition Ab response after each annual vaccination, as well as the binding reactivity of plasmablast-derived polyclonal Abs to the hemagglutinin of influenza A virus vaccine components, even with changes in the seasonal vaccine components during the study. Our findings indicate a diminished B-cell response to annual vaccination with seasonal trivalent influenza vaccine. These results emphasize the need for developing improved strategies to enhance the immunogenicity and efficacy of annual influenza vaccination.

Plasmablast antibody repertoires in elderly influenza vaccine responders exhibit restricted diversity but increased breadth of binding across influenza strains.

Seasonal influenza vaccines elicit antibody responses that can prevent infection, but their efficacy is reduced in the elderly. While a subset of elderly individuals can still mount sufficient vaccine-induced antibody responses, little is known about the properties of the vaccine-induced antibody repertoires in elderly as compared to young responders. To gain insights into the effects of aging on influenza vaccine-induced antibody responses, we used flow cytometry and a cell-barcoding method to sequence antibody heavy and light chain gene pairs expressed by individual blood plasmablasts generated in response to influenza vaccination in elderly (aged 70-89) and young (aged 20-29) responders. We found similar blood plasmablast levels in the elderly and young responders seven days post vaccination. Informatics analysis revealed increased clonality, but similar heavy chain V(D)J gene usage in the elderly as compared to young vaccine responders. Although the elderly responders exhibited decreased antibody sequence diversity and fewer consequential mutations relative to young responders, recombinant antibodies from elderly responders bound a broader range of influenza strain HAs. Thus elderly influenza vaccine responders mount plasmablast responses with restricted diversity but with an increased breadth of binding across influenza strains. Our results suggest that the ability to generate plasmablast responses encoding cross-strain binding antibodies likely represents a mechanism important to vaccine responses in the elderly.

Defective T Memory Cell Differentiation after Varicella Zoster Vaccination in Older Individuals.

Vaccination with attenuated live varicella zoster virus (VZV) can prevent zoster reactivation, but protection is incomplete especially in an older population. To decipher the molecular mechanisms underlying variable vaccine responses, T- and B-cell responses to VZV vaccination were examined in individuals of different ages including identical twin pairs. Contrary to the induction of VZV-specific antibodies, antigen-specific T cell responses were significantly influenced by inherited factors. Diminished generation of long-lived memory T cells in older individuals was mainly caused by increased T cell loss after the peak response while the expansion of antigen-specific T cells was not affected by age. Gene expression in activated CD4 T cells at the time of the peak response identified gene modules related to cell cycle regulation and DNA repair that correlated with the contraction phase of the T cell response and consequently the generation of long-lived memory cells. These data identify cell cycle regulatory mechanisms as targets to reduce T cell attrition in a vaccine response and to improve the generation of antigen-specific T cell memory, in particular in an older population.

Accuracy and Discomfort of Different Types of Intranasal Specimen Collection Methods for Molecular Influenza Testing in Emergency Department Patients.

STUDY OBJECTIVE: While development is under way of accurate, point-of-care molecular tests for influenza infection, the optimal specimen type for molecular tests remains unclear. Compared with standard nasopharyngeal swab specimens, less invasive nasal swab and midturbinate swab specimens may cause less patient discomfort and be more suitable for routine emergency department (ED) testing, although possibly at the expense of diagnostic accuracy. We compare both the accuracy of a polymerase chain reaction molecular influenza test and discomfort between these 3 intranasal specimen types. METHODS: A convenience sample of adult and pediatric patients with influenza-like illness and presenting to 2 Northern California EDs and 2 EDs in Santiago, Chile, was prospectively enrolled during the 2015 to 2016 influenza season. Research nurses collected nasopharyngeal swab, midturbinate swab, and nasal swab specimens from each subject and assessed discomfort on a validated 6-point scale. Specimens were tested for influenza A and B by real-time polymerase chain reaction at reference laboratories. Outcome measures were comparison of test performance between nasal swab and midturbinate swab, when compared with a reference standard nasopharyngeal swab; and comparison of discomfort between all 3 specimen types. RESULTS: Four hundred eighty-four subjects were enrolled, and all 3 swabs were obtained for each subject; 14% were children. The prevalence of influenza (A or B) was 30.0% (95% confidence interval [CI] 26.0% to 34.8%). The sensitivity for detecting influenza was 98% (95% CI 94.25% to 99.65%) with the midturbinate swab versus 84.4% (95% CI 77.5% to 89.8%) with the nasal swab, difference 13.6% (95% CI 8.2% to 19.3%). Specificity was 98.5% (95% CI 96.6% to 99.5%) with the midturbinate swab versus 99.1% (95% CI 97.4% to 99.8%) with the nasal swab, difference -0.6% (95% CI -1.8% to 0.6%). Swab discomfort levels correlated with the depth of the swab type. Median discomfort scores for the nasal swab, midturbinate swab, and nasopharyngeal swab were 0, 1, and 3, respectively; the median differences were nasopharyngeal swab-midturbinate swab 2 (95% CI 1 to 2), nasopharyngeal swab-nasal swab 3 (95% CI 2 to 3), and midturbinate swab-nasal swab 1 (95% CI 1 to 2). CONCLUSION: Compared with the reference standard nasopharyngeal swab specimen, midturbinate swab specimens provided a significantly more comfortable sampling experience, with only a small sacrifice in sensitivity for influenza detection. Nasal swab specimens were significantly less sensitive than midturbinate swab. Our results suggest the midturbinate swab is the sampling method of choice for molecular influenza testing in ED patients.

B-cell repertoire responses to varicella-zoster vaccination in human identical twins.

Adaptive immune responses in humans rely on somatic genetic rearrangements of Ig and T-cell receptor loci to generate diverse antigen receptors. It is unclear to what extent an individual's genetic background affects the characteristics of the antibody repertoire used in responding to vaccination or infection. We studied the B-cell repertoires and clonal expansions in response to attenuated varicella-zoster vaccination in four pairs of adult identical twins and found that the global antibody repertoires of twin pair members showed high similarity in antibody heavy chain V, D, and J gene segment use, and in the length and features of the complementarity-determining region 3, a major determinant of antigen binding. These twin similarities were most pronounced in the IgM-expressing B-cell pools, but were seen to a lesser extent in IgG-expressing B cells. In addition, the degree of antibody somatic mutation accumulated in the B-cell repertoire was highly correlated within twin pair members. Twin pair members had greater numbers of shared convergent antibody sequences, including mutated sequences, suggesting similarity among memory B-cell clonal lineages. Despite these similarities in the memory repertoire, the B-cell clones used in acute responses to ZOSTAVAX vaccination were largely unique to each individual. Taken together, these results suggest that the overall B-cell repertoire is significantly shaped by the underlying germ-line genome, but that stochastic or individual-specific effects dominate the selection of clones in response to an acute antigenic stimulus.

Systems analysis of sex differences reveals an immunosuppressive role for testosterone in the response to influenza vaccination.

Females have generally more robust immune responses than males for reasons that are not well-understood. Here we used a systems analysis to investigate these differences by analyzing the neutralizing antibody response to a trivalent inactivated seasonal influenza vaccine (TIV) and a large number of immune system components, including serum cytokines and chemokines, blood cell subset frequencies, genome-wide gene expression, and cellular responses to diverse in vitro stimuli, in 53 females and 34 males of different ages. We found elevated antibody responses to TIV and expression of inflammatory cytokines in the serum of females compared with males regardless of age. This inflammatory profile correlated with the levels of phosphorylated STAT3 proteins in monocytes but not with the serological response to the vaccine. In contrast, using a machine learning approach, we identified a cluster of genes involved in lipid biosynthesis and previously shown to be up-regulated by testosterone that correlated with poor virus-neutralizing activity in men. Moreover, men with elevated serum testosterone levels and associated gene signatures exhibited the lowest antibody responses to TIV. These results demonstrate a strong association between androgens and genes involved in lipid metabolism, suggesting that these could be important drivers of the differences in immune responses between males and females.

Enhanced natural killer-cell and T-cell responses to influenza A virus during pregnancy.

Pregnant women experience increased morbidity and mortality after influenza infection, for reasons that are not understood. Although some data suggest that natural killer (NK)- and T-cell responses are suppressed during pregnancy, influenza-specific responses have not been previously evaluated. Thus, we analyzed the responses of women that were pregnant (n = 21) versus those that were not (n = 29) immediately before inactivated influenza vaccination (IIV), 7 d after vaccination, and 6 wk postpartum. Expression of CD107a (a marker of cytolysis) and production of IFN-γ and macrophage inflammatory protein (MIP) 1ß were assessed by flow cytometry. Pregnant women had a significantly increased percentage of NK cells producing a MIP-1ß response to pH1N1 virus compared with nonpregnant women pre-IIV [median, 6.66 vs. 0.90% (P = 0.0149)] and 7 d post-IIV [median, 11.23 vs. 2.81% (P = 0.004)], indicating a heightened chemokine response in pregnant women that was further enhanced by the vaccination. Pregnant women also exhibited significantly increased T-cell production of MIP-1ß and polyfunctionality in NK and T cells to pH1N1 virus pre- and post-IIV. NK- and T-cell polyfunctionality was also enhanced in pregnant women in response to the H3N2 viral strain. In contrast, pregnant women had significantly reduced NK- and T-cell responses to phorbol 12-myristate 13-acetate and ionomycin. This type of stimulation led to the conclusion that NK- and T-cell responses during pregnancy are suppressed, but clearly this conclusion is not correct relative to the more biologically relevant assays described here. Robust cellular immune responses to influenza during pregnancy could drive pulmonary inflammation, explaining increased morbidity and mortality.

Human B-cell isotype switching origins of IgE.

BACKGROUND: B cells expressing IgE contribute to immunity against parasites and venoms and are the source of antigen specificity in allergic patients, yet the developmental pathways producing these B cells in human subjects remain a subject of debate. Much of our knowledge of IgE lineage development derives from model studies in mice rather than from human subjects. OBJECTIVE: We evaluate models for isotype switching to IgE in human subjects using immunoglobulin heavy chain (IGH) mutational lineage data. METHODS: We analyzed IGH repertoires in 9 allergic and 24 healthy adults using high-throughput DNA sequencing of 15,843,270 IGH rearrangements to identify clonal lineages of B cells containing members expressing IgE. Somatic mutations in IGH inherited from common ancestors within the clonal lineage are used to infer the relationships between B cells. RESULTS: Data from 613,641 multi-isotype B-cell clonal lineages, of which 592 include an IgE member, are consistent with indirect switching to IgE from IgG- or IgA-expressing lineage members in human subjects. We also find that these inferred isotype switching frequencies are similar in healthy and allergic subjects. CONCLUSIONS: We found evidence that secondary isotype switching of mutated IgG1-expressing B cells is the primary source of IgE in human subjects, with lesser contributions from precursors expressing other switched isotypes and rarely IgM or IgD, suggesting that IgE is derived from previously antigen-experienced B cells rather than naive B cells that typically express low-affinity unmutated antibodies. These data provide a basis from which to evaluate allergen-specific human antibody repertoires in healthy and diseased subjects.

Increased Proinflammatory Responses of Monocytes and Plasmacytoid Dendritic Cells to Influenza A Virus Infection During Pregnancy.

Pregnancy-induced alterations in immunity may contribute to the increased morbidity associated with influenza A virus infection during pregnancy. We characterized the immune response of monocytes and plasmacytoid dendritic cells (pDCs) to influenza A virus infection in 21 pregnant and 21 nonpregnant women. In pregnant women, monocytes and pDCs exhibit an exaggerated proinflammatory immune response to 2 strains of influenza A virus, compared with nonpregnant women, characterized by increased expression of major histocompatibility complex class II (approximately 2.0-fold), CD69 (approximately 2.2-fold), interferon γ-induced protein 10 (approximately 2.0-fold), and macrophage inflammatory protein 1ß (approximately 1.5-fold). This enhanced innate inflammatory response during pregnancy could contribute to pulmonary inflammation following influenza A virus infection.

Effects of aging, cytomegalovirus infection, and EBV infection on human B cell repertoires.

Elderly humans show decreased humoral immunity to pathogens and vaccines, yet the effects of aging on B cells are not fully known. Chronic viral infection by CMV is implicated as a driver of clonal T cell proliferations in some aging humans, but whether CMV or EBV infection contributes to alterations in the B cell repertoire with age is unclear. We have used high-throughput DNA sequencing of IGH gene rearrangements to study the BCR repertoires over two successive years in 27 individuals ranging in age from 20 to 89 y. Some features of the B cell repertoire remain stable with age, but elderly subjects show increased numbers of B cells with long CDR3 regions, a trend toward accumulation of more highly mutated IgM and IgG Ig genes, and persistent clonal B cell populations in the blood. Seropositivity for CMV or EBV infection alters B cell repertoires, regardless of the individual's age: EBV infection correlates with the presence of persistent clonal B cell expansions, whereas CMV infection correlates with the proportion of highly mutated Ab genes. These findings isolate effects of aging from those of chronic viral infection on B cell repertoires and provide a baseline for understanding human B cell responses to vaccination or infectious stimuli.

Genetic measurement of memory B-cell recall using antibody repertoire sequencing.

Annual influenza vaccinations aim to protect against seasonal infections, and vaccine strain compositions are updated every year. This protection is based on antibodies that are produced by either newly activated or memory B cells recalled from previous encounters with influenza vaccination or infection. The extent to which the B-cell repertoire responds to vaccination and recalls antibodies has so far not been analyzed at a genetic level--which is to say, at the level of antibody sequences. Here, we developed a consensus read sequencing approach that incorporates unique barcode labels on each starting RNA molecule. These labels allow one to combine multiple sequencing reads covering the same RNA molecule to reduce the error rate to a desired level, and they also enable accurate quantification of RNA and isotype levels. We validated this approach and analyzed the differential response of the antibody repertoire to live-attenuated or trivalent-inactivated influenza vaccination. Additionally, we analyzed the antibody repertoire in response to repeated yearly vaccinations with trivalent-inactivated influenza vaccination. We found antibody sequences that were present in both years, providing a direct genetic measurement of B-cell recall.

Distinct patterns of B-cell activation and priming by natural influenza virus infection versus inactivated influenza vaccination.

BACKGROUND: The human B-cell response to natural influenza virus infection has not been extensively investigated at the polyclonal level. METHODS: The overall B-cell response of patients acutely infected with the 2009 pandemic influenza A(H1N1)pdm09 virus (A[H1N1]pdm09) was analyzed by determining the reactivity of plasmablast-derived polyclonal antibodies (PPAbs) to influenza proteins. Recipients of inactivated influenza vaccine containing the same A(H1N1)pdm09 strain were studied for comparison. RESULTS: During acute infection, robust plasmablast responses to the infecting virus were detected, characterized by a greater PPAb reactivity to the conserved influenza virus nuclear protein and to heterovariant and heterosubtypic hemagglutinins, in comparison to responses to the inactivated A(H1N1)pdm09 vaccine. In A(H1N1)pdm09 vaccinees, the presence of baseline serum neutralizing antibodies against A(H1N1)pdm09, suggesting previous exposure to natural A(H1N1)pdm09 infection, did not affect the plasmablast response to vaccination, whereas repeated immunization with inactivated A(H1N1)pdm09 vaccine resulted in significantly reduced vaccine-specific and cross-reactive PPAb responses. CONCLUSIONS: Natural A(H1N1)pdm09 infection and inactivated A(H1N1)pdm09 vaccination result in very distinct patterns of B-cell activation and priming. These differences are likely to be associated with differences in protective immunity, especially cross-protection against heterovariant and heterosubtypic influenza virus strains.