Influenza infected newborn and adult monkeys exhibit a strong primary antibody response to hemagglutinin stem

The specificity of antibodies (Abs) generated to influenza A virus (IAV) infection can significantly alter protection and viral clearance. At present, the impact of age upon this process is relatively unexplored. Here, we evaluated the Ab response in newborn and adult African green monkeys (AGM) following infection with IAV using a strain that enables us to determine the immunodominance (ID) hierarchy of the Ab response to hemagglutinin (HA), the principal target of protective Abs. This revealed altered ID patterns in the early IgM antiHA response in newborns versus adults that converged over time. While the IgG ID profiles for HA in newborn and adult monkeys were similar, this was not the case for IgA. Importantly, HA stem-specific Abs were generated robustly and similarly in newborns and adults in terms of quality and quantity. Together these results demonstrate that newborns and adults can differ in the Ab ID pattern established following infection and that the ID pattern can vary across isotypes. In addition, newborns have the ability to generate potent HA stem-specific Ab responses. Our findings further the understanding of the newborn response to IAV antigens and inform the development of improved vaccines for this at-risk population. Research Immunology


Introduction
At birth, infants transition from the protected environment of the womb to the myriad dangers of the outside world. This requires substantial adjustment from the neonatal immune system as it encounters its first antigenic challenges. At early times after birth, the immune system exists in an altered state that has been proposed to provide evolutionary benefit by allowing colonization of commensal microbiota; however, it leaves neonates more susceptible to infection (1,2).
Influenza A virus (IAV) is a common respiratory tract infection that, compared to infection in older individuals, exhibits higher attack rates and higher incidence of serious disease and secondary complications in infants (3)(4)(5). This increased susceptibility, coupled with the lack of an approved influenza vaccine for infants under six months of age, leaves neonates vulnerable to IAV infection (6).
Ineffective neonatal immunity to IAV likely results from multiple defects in innate and adaptive mechanisms (7)(8)(9). These include diminished antibody (Ab) responses, which could reflect deficiencies in B cell activation, somatic hypermutation, and class switching (10)(11)(12)(13)(14). A nearly completely unknown factor is the specificities of neonatal Abs relative to those found in older individuals. IAV has a number of immunogenic proteins, the most important of which for vaccination purposes is the hemagglutinin (HA) molecule, the target of the most potent neutralizing Abs in vitro and protective Abs in vivo.
HA is a homotrimeric glycoprotein with a highly variable globular domain sitting atop a more conserved stalk. The variability of the globular domain accounts for IAV's rapid antigenic drift and necessity for frequent vaccine updates. Previous studies have demonstrated that the H1 HA, as exemplified by the prototypical A/Puerto Rico/8/34 (H1N1) (PR8) strain, has five major antigenic sites in the globular domain recognized by Abs that neutralize viral infectivity (15). Recognition of these five sites can inform Ab function-for example, Abs binding to the Sa and Sb sites near the receptor binding site have been shown to be particularly efficacious in neutralizing viral entry, while Ab to other sites may be higher affinity or inhibit other steps of the viral life cycle. Recently, Ab to Sa was also shown to neutralize the 2009 pandemic strain of H1N1, suggesting that different head epitopes may be more or less susceptible to antigenic drift (16). Using the PR8 strain, Angeletti et al. described a system that enables quantitation of both Abs and B cells specific for each of the five sites (17). Applying this to the mouse model, they reported that Ab immunodominance (ID) hierarchy is consistent within a given mouse strain, evolves over time after immunization, and is altered by the nature of the immunizing event (i.e. vaccination vs. infection). In mice, as in multiple animal models and humans, the globular domain is highly immunodominant over the stem, which in naïve animals exposed to either infection or vaccination elicits a minor Ab response.
As with T cell ID (18), multiple mechanisms likely contribute to Ab ID, the details of which will take a major effort to unravel. With the urgent need to improve IAV vaccines (which under the best circumstances only provide protection in ~60% of recipients), it is critical to better understand Ab ID and how it impacts protection, given that Ab specificity and heavy chain class govern the capacity for protection and cross-strain reactivity (19)(20)(21)(22).
Virtually nothing is known about age related alterations in Ab ID. This is an important topic given the high susceptibility of neonates to IAV and the potential long term effects of childhood vaccination on lifelong IAV immunity, as evidence suggests that first exposure to IAV antigens can mold the lifetime response (23). Here, we investigate HA-specific Ab ID in newborn and adult African green monkeys (AGM) following IAV infection, a model chosen because of the similarities between nonhuman primate (NHP) and human infant immune system development and function (24).

Newborns and adults generate high levels of HA-specific Abs in response to IAV infection.
To characterize age related differences in HA Ab ID, we infected AGM newborns (3-5 days of age) or adults (6-9 years of age) with influenza virus strain PR8 via a combination of intranasal and intratracheal routes, measuring Ab responses at d8 and d14 postinfection (p.i.). For preimmune samples, we collected adult blood at the time of infection; for newborns we used samples from an age-matched naïve cohort. As expected, HA-specific IgG and IgA Abs were not detectable by ELISA using purified HA as antigen in samples collected prior to infection, confirming that all infants and adults were initially influenza-naïve ( Fig. 1). HA-specific IgM was not detected in neonates, but was present at low levels in naïve adults (Fig. 1A). At either d8 or d14 p.i., the amount of plasma HA-specific IgM, IgG, or IgA Ab was similar in newborns and adults ( Fig. 1), consistent with our previous analysis of total IAV specific IgG and IgM Ab responses (9).
As expected, levels of HA-specific IgG and IgA in bronchoalveolar lavage (BAL) corresponded to plasma titers ( Fig. 1B and C). However, the concordance between total virus-specific and HAspecific Ab was not as clear for IgA. While there was a trend towards higher anti-HA IgA Ab in adults ( Fig. 1), this did not reach statistical significance, as it did when total virus-specific IgA was measured in our previous studies (9). This finding suggests the fraction of Abs specific for individual viral proteins differs across Ab isotypes.

The IgM ID profile differs in adults versus newborns.
In a variety of species ranging from lamprey to birds to mammals, nearly all Abs that bind the globular head domain of HA recognize amino acids present in one or more of the 5 major antigenic sites (15,25). To measure specific Ab responses to these sites we performed ELISAs using purified HA from an HAΔ4 panel of 5 viruses that were generated by multiple rounds of selection with neutralizing mAbs to lose antigenicity in 4 of 5 antigenic sites (17). In addition, we used wildtype virus and S12, a strain which has lost antigenicity at all 5 sites due to 12 rounds of sequential selection with different neutralizing mAbs (26). The area under the curve (AUC) for binding to each HAΔ4 construct was normalized to the AUC for the wild-type HA molecule to allow comparison of ID hierarchy between animals.
The IgM response in newborns at d8 p.i. exhibited a distinct ID hierarchy, with Cb and Sb being dominant ( Fig. 2A). By d14 p.i. in neonates, Sb was solely dominant, with responses to the other 4 sites being nearly equal in magnitude (Fig. 2B). Interestingly, in adults none of the sites demonstrated immunodominance at either timepoint (Fig. 2). These findings are consistent with the conclusion that the ID hierarchy for IgM anti-HA Abs differs between newborns and adults.

Plasma IgG and IgA anti-HA Ab responses form clear ID hierarchies at d14 p.i. in both newborns and adults.
We then assessed the ID hierarchy of IgG anti-HA at d14 p.i. IgG ID hierarchy was not assessed at d8 p.i. as IgG Abs were not reliably detectable this early after infection, likely due to the time required to produce high amounts of class-switched Abs. IgG Abs to Cb were clearly immunodominant in both newborns and adults (Fig. 3A). For IgA Abs, by d14 p.i. both newborns and adults responded to all 5 sites based on their values relative to S12 (Fig. 3B). In newborns Sb was dominant, with Ca1 and Cb subdominant, while in adults Sb and Cb were co-dominant ( Fig. 3B). The hierarchy patterns were consistent for all individuals within each age group and isotype.
The ID hierarchy of HA-specific IgG Abs does not correlate with Ab avidity.
We next examined the correlation between IgG Ab avidity for a given site with its position in the ID hierarchy at d14 p.i. To estimate Ab avidity, we measured the amount of the chaotropic reagent NaSCN required to decrease Ab binding by 50% (IC50) in ELISAs using purified HA. While clearly an imperfect measure of Ab avidity, which is difficult to measure in polyclonal Abs, many studies have found a good correlation between binding avidity and Ab avidity (27), and for HA in particular, a direct correlation between resistance to chaotropic agents and Ab off-rates (28).
No correlation between the IC50 for IgG binding and ID was present, i.e. the avidity of the immunodominant Cb-specific Ab was not consistently either higher or lower than other epitopes ( Fig. 4). Ca2 was the only site in adults to which IgG Abs had a significantly lower avidity; however, this difference is not reflected in the ID hierarchy ( Fig. 4 vs. Fig. 2). Interestingly, Abs to the dominant Cb epitope generated by newborns had a significantly lower avidity compared to those from adults (0.67 vs. 0.79, p=0.028). This difference was restricted to the Cb-specific Abs, as it was not observed for Abs to the other epitopes.

Nonhuman primates generate a robust HA stem-specific Ab response.
In addition to Ab directed to epitopes in the HA head region, individuals also produce antibody to the more highly conserved stem region of the HA molecule (29)(30)(31)(32). In humans, these antibodies appear to be subdominant, as they are represented at significantly lower levels than those directed to the head region (31,32). The robust response to S12, which lacks the frequently recognized head epitopes, that was observed in both adult and newborn animals suggested that NHP may generate responses to the HA stem region. Alternatively, the animals could be producing antibodies to alternative epitopes present in the head portion of HA or to contaminating virus proteins in our preparation.
To test this last possibility, we used a recombinant S12 molecule that was produced in 293 cells and thus could not have non-HA influenza virus proteins. This analysis showed the Abs bind similarly to both recombinant and virally derived S12 (Fig. 5). These data also show that the Ab recognizing the recombinant (Fig. 5A) or virion derived (Fig. 5B) S12 is the result of infection as they were absent in plasma from naïve animals but significantly increased at d14 p.i.
Having ruled out the possibility of contaminating proteins in the S12 preparation, we investigated whether Ab present in the plasma from infected animals could recognize the HA stem region. We evaluated IgM, IgG, and IgA Abs at d14 p.i. using a stabilized California/09 HA stem construct lacking the globular head (33). IgM capable of recognizing the stem construct was significantly induced in adults and levels were elevated in newborns, though the increase was just below statistical significance, likely due to the variability among the infants (Fig. 6A). Both newborn and adult animals generated a robust anti-stem IgG response (Fig. 6B). Stem-specific IgG was also present in the BAL of newborns and adults at d14 p.i., albeit at low levels ( Fig. 6C). While there was a trend towards higher levels of stem-specific Ab in the respiratory tracts of adults compared to newborns, the data did not reach statistical significance. These data show for the first time to our knowledge that a newborn can mount an Ab response to the conserved HA stem region following primary infection with IAV. To further define these Abs we used the well characterized stem mAb CR9114 (34) as a competitor for binding to stem or HA. CR9114 inhibited IgG binding to stem in a dose-dependent manner (Fig. 6D), showing that the Abs generated in the infected animals and CR9114 recognize a similar epitope.
We also evaluated IgA Ab to the HA stem region. Interestingly, in the plasma an IgA stem-specific Ab response was observed in only one adult (Fig. 6E) and only 2 of 4 adult animals had detectable IgA Ab to stem in the BAL (Fig. 6F). No HA stem-specific IgA was detected in infants. These data suggest IgA antibody directed to the HA-stem is less efficiently generated compared to IgG antibody.

The HA stem-specific Ab response generated by infection is both neutralizing and high avidity.
We next measured the avidity of the stem-specific IgG Ab as an indicator of Ab quality. No difference in avidity between newborns and adults was observed for IgG binding to the stabilized Ca09 headless stem construct (Fig. 7A), suggesting that at this point in the acute response when infection is actively being cleared, newborn Ab to stem should have similar activity based on avidity. To further examine the quality of these Abs, we assessed the capacity of the elicited stem antibodies to neutralize virus using a chimeric virus expressing an H5 head, H1 PR8 stem, and N2 neuraminidase (H5/H1-N2). The substitution of a heterologous globular head and neuraminidase should restrict Ab binding to the stem, such that any neutralization observed can be attributed to stem-specific Ab. Plasma collected at d14 p.i. from both age groups was able to neutralize the chimeric H5/H1-N2 virus (Fig. 7B), demonstrating that at this timepoint the stemspecific Abs elicited by infection have comparable neutralizing capacity. These Abs did not impact neuraminidase activity as has been reported for some Abs (35) as neuraminidase activity of the chimeric virus was not inhibited as measured via an enzyme-linked lectin assay (data not shown).

Discussion
Our understanding of the immune response generated following IAV infection of newborns remains highly limited. Here we have used an outbred NHP model that closely mimics human newborn immune development and function to probe the breadth and specificity of the Ab response generated following initial exposure to IAV. By using this model, we expand the translatability of these findings to humans and probe the potential for genetic heterogeneity to impact Ab ID.
We found that newborns and adults establish similar IgG dominance profiles to the five main antigenic sites of H1 HA, while exhibiting distinct patterns of ID in the IgA and early IgM response.
Consistent with studies in adult mice (17), avidity was not associated with dominance. We also show for the first time isotype-dependent divergence in ID, with IgA exhibiting a distinct profile compared to IgG in both newborns and in adults. Finally, we report that by 14 days following primary exposure, newborns have generated robust levels of stem-specific Ab that are similar to those generated by adults in avidity and neutralizing potential.
We were intrigued to find a disparity in the HA-specific IgM antibody present in naïve infants versus adults. This was coupled with an early difference in the dominance pattern for IgM between these two groups. Given the reduced control of virus in infants at d2 and d8 p.i. that we have previously observed , it is tempting to speculate that these early differences impact viral load.
Alternatively, it is possible that viral load is driving the divergence in early responses. In mice, natural IgM (nIgM) Abs appear to play an important role in limiting IAV replication (36). These Abs promote viral clearance in part through complement-mediated neutralization (36,37). As nIgM production is attributable to B1 cells, our finding may reflect a deficit in the B1 compartment of newborn primates, consistent with the poor responses to T-independent antigens reported in this age group (38,39). An alternative, but not exclusive, possibility is that there is a difference in the initial recruitment and/or activation of B cells as we would expect extrafollicular B2 cells to be a significant contributor to the IgM present in circulation at d8 (40).
The finding that Cb dominates the IgG response at d14 p.i. in both newborns and adults is striking in its similarity to the pattern reported in a mouse model of PR8 infection (17). Cb-specific Abs have been observed to dominate the early Ab response across a variety of models, preferentially differentiating into extrafollicular plasmablasts rather than adopting a germinal center fate (41,42).
This may be the result of a unique aspect of the response to Cb, whether it be the B cell repertoire as was suggested by Kavaler et al. or an intrinsic property of the antigen. Interestingly, while the majority of Ab specificities were of similar avidity, this dominant Cb-specific IgG response exhibited decreased avidity in newborns compared to adults. It is possible that this has consequences for clearance, considering the established role of Cb binding Abs in inhibiting viral egress following replication (44). In mouse models, the early dominance of Cb gives way to the emergence of Sb-specific IgG at later times (17). Unfortunately, we were unable to assess timepoints beyond d14 in our study as animals were euthanized for examination of lung pathology  (10,13,46,47). Any of these are potential contributors to ID. Identification of those responsible for these differences awaits further study.
There is now intense focus on the development of approaches that can optimally elicit Abs that recognize the relatively conserved stem region of HA, given the capacity of these Abs to provide broad protection (48). Stem-binding Abs can function by inhibiting viral entry or release (49) as well as FcR-mediated clearance (50,51). HA stem-specific responses are challenging to elicit as this region is poorly immunogenic, especially in the context of current vaccines (31,32). However, HA stem-specific Abs in humans are known to increase with age, presumably as a result of multiple exposures to heterologous strains of IAV, supporting the ability of these responses to be boosted (19,31,33,52). Given the challenges associated with studying young human infants and the difficulty in knowing the exposure history in the first years of life, there is limited information regarding the capacity for these individuals to generate the highly sought after stem-specific Abs upon IAV infection. This issue is significant as a lower capacity for HA stem-specific Ab production in newborns would be predicted to put them at significant disadvantage in the event of infection with a novel pandemic strain (53).
We found a robust stem-specific IgG response in the newborns infected with PR8. This is an exciting result and provides rationale for targeting these responses in young infants. In addition, when evaluated at the d14 p.i. timepoint, the avidity of Ab binding to the HA stem from newborns was comparable to Ab binding to the other HA epitopes and was similar to that in adults. Importantly, these Abs were able to neutralize influenza virus.
Abs directed to the stem region of HA isolated from humans preferentially utilize a set of VH regions including VH1-69 for group 1 HA stem and VH1-18, and VH6-1 for cross-group stem reactivity (54)(55)(56)(57)(58). These Abs are characterized by low to moderate amounts of somatic hypermutation (SHM) (59). Pappas et al. reported that a single mutation could confer high affinity for the stem region (54). The B cell repertoire in newborns versus adults is distinct (60-63).
Specifically, neonate B cells have a nearly germline VH repertoire (61) and exhibit a reduced range of IgM HCDR3 lengths, i.e. the CDR3 is significantly longer in adult B cells (62,63). The breadth and complexity of the repertoire of B cells in newborns that can respond to influenza virus is relatively unexplored. However, the limited SHM required by many HA stem-reactive Ab may reduce the barrier to eliciting these Abs in infants and may contribute to the production observed in our study.
It was surprising that circulating IgA Abs against the HA stem were largely absent in both newborn and adult animals. The modest IgA response to stem is particularly noteworthy in light of recent studies demonstrating more potent neutralization by heterosubtypic IgA than IgG that appears to result from intrinsic characteristics of the IgA constant region (64,65). Further studies to identify the mechanism responsible for the poor generation of stem-specific IgA Ab following infection may facilitate design of vaccine regimens with improved capacity to elicit IgA Ab to stem that may provide increased protection.
In summary, the findings presented here show that establishment of HA-specific Ab immunodominance is dependent on age and differs by isotype. The observation that the dominant epitope recognized by HA-specific IgG at d14 p.i. is similar in newborns and adults together with the finding that newborns can generate stem-specific Abs suggest the newborn B cell repertoire does not represent a bottleneck for developing HA stem-specific responses. These data are supportive for continued pursuit of a universal influenza vaccine for use in this population. It will be important for future studies to evaluate the later phases of the Ab response in this model as well as the ID pattern within the memory B cell pool. We also note that these patterns could differ following exposure to non-replicating virus as is the case during vaccination. This will be an important area for future study. Finally, the difference identified in the early IgM response in newborns versus adults emphasizes the need for a generally increased understanding of the early Ab response in newborns. Together, these findings provide new insights into the dynamics and quality of the Ab response that is elicited in the neonatal immune system following infection with influenza virus.

Materials and Methods
Study Design. The main objective of this study was to compare differences in Ab immunodominance to IAV HA between newborn and adult NHP. Newborn animals were chosen based on animals born in the timeframe that would allow concurrent entry into the study. Plasma  (9). In brief, newborns (6-10 days of age) received 1x10 9 (3 infants) or 1x10 8 (1 infant) EID50 A/Puerto Rico/8/34 (H1N1) (PR8) divided equally between intranasal and intratracheal routes. The combined infection approach was selected based on prior NHP IAV challenge studies (66,67). There were no differences in the findings between the infant that received the lower dose versus those receiving the higher dose and thus these infants were grouped for the analyses presented. Adults (6-9 years of age) were infected with 5x10 9 EID50 in the same manner. Adult animals received more virus to adjust for differences in body size. Blood was collected by venipuncture into sodium heparin tubes at d8 and d14 p.i., from which plasma was obtained.
Bronchoalveolar lavage (BAL) using 5ml and 25ml PBS for infants and adults, respectively, was performed at necropsy on d14 p.i. Samples were centrifuged to remove cells and BSA added to a concentration of 0.5%.
Hemagglutinin ELISAs. ELISAs were performed as previously described in Angeletti 2017 with modifications for the NHP model. In brief, plates were coated overnight with 5ng of purified HA probes (17) or a headless California/2009 HA stem construct (33). Plates were blocked with 50µl of casein blocking buffer + 2% goat serum for 1 hour, after which they were washed five times with PBS+0.01% Tween-20 (PBST). Plates were incubated for 3h with 25µl plasma serially diluted in blocking buffer by twofold. Starting dilutions were as stated for each assay. Plates were then washed five times in PBST, and subsequently incubated with 25µl HRP-conjugated goat anti-NHP IgG (Fitzgerald), IgM (LifeSpan Bioscience) or biotinylated goat anti-NHP IgA (AbD Serotec).
For IgA detection, plates underwent an additional wash and incubation with 25µl streptavidinconjugated HRP (Mabtech). All plates were washed five times with PBST and developed for 5 min using TMB Ultra (Fisher), after which the reaction was stopped with 0.1N H2SO4. Binding was expressed as area under the curve (AUC) calculated using GraphPad Prism. Calculated AUC was divided by the PR8 HA AUC value to normalize for differences in absolute HA titer among the animals. Plasma binding to uncoated wells was subtracted for each animal, and limit of detection for the AUC calculation was defined as a value that exceeded the average OD450 of the uncoated wells plus two standard deviations.
Avidity. Avidity assays were performed as the ELISAs with the addition of a sodium thiocyanate (NaSCN) dissociation step following sample incubation. The plasma dilution used was selected for each animal based on the dilution that yielded 50% of the max OD450 in the ELISA binding curve. Following incubation with 25µl plasma, two-fold dilutions of NaSCN starting at 5M were added to the plate for 15 minutes. Plates were then washed five times with PBST and incubated with HRP-conjugated goat anti-IgG for one hour and developed as in the ELISA protocol. The IC50 was calculated using GraphPad Prism software.
Antibody competition. As with the ELISAs, plates were coated with 5ng of HA or Ca09 stem probe overnight and blocked for 1 hour with casein blocking buffer + 2% goat serum. Plates were then washed with PBST and pre-incubated with CR9114 mAb in two-fold serial dilutions for one hour.
Antibody was flicked off and 25µl plasma at a 1:80 dilution in casein blocking buffer was added to each well. After three washes with PBST, a human adsorbed HRP conjugated goat anti-NHP IgG detection antibody (Bethyl Laboratories) was added for one hour. Plates were then washed again in PBST and developed for five minutes with TMB-Ultra substrate and stopped with 0.1N H2SO4 as in the ELISA. The OD450 was used to calculate the percent of signal seen in sample with no CR9114 competition.
Microneutralization. Microneutralization assays were performed on MDCK cells as previously described (17). Briefly, two-fold diluted plasma samples from naïve and infected NHP were incubated with 100 TCID50 of either PR8 or a viral construct with a chimeric hemagglutinin consisting of an H5/Vietnam/04 head and an H1/PR8 stem as well as an N2 neuraminidase from A/Udorn/72/H3N2 in TCID50 media (DMEM+GlutaMAX +1% HEPES and 1ug/mL L-1-tosylamide-2-phenylethul chloromethyl ketone (TPCK)-treated trypsin). Samples were incubated for one hour at room temperature and added in quadruplicate to confluent MDCK cells, then maintained at 37°C. Cytopathic effects were assessed after 3 days by microscopy and confirmed by crystal violet staining following methanol fixation.
Statistics. Statistical significance for pairwise comparisons (e.g., newborns vs adults) was determined using one-tailed unpaired t-tests. Statistical comparisons of antibody binding to HA epitopes (Figs. 2 and 3) were assessed by one-way ANOVA with a Holm-Sidak test for multiple comparisons. Differences in antibody avidity (Fig. 4) were assessed for significance using a  Newborn and adult AGMs (n=4/group) were infected with PR8 by the combined intranasal and intratracheal routes. Following infection, blood was drawn on days 8 and 14. To assess naïve responses, blood was collected from the adult animals at the time of infection and from agematched controls for the newborn samples. IgM, IgG, and IgA Ab (A, B, and C, respectively) to PR8 HA was measured by ELISA using plates coated with 5 ng of PR8 HA. Plasma (left panels) or bronchoalveolar lavage fluid (right panels) collected at d14 p.i. was added starting at 1:10 and 1:5, respectively, with subsequent two-fold dilutions. Total HA binding was quantified by calculating the area under the curve (AUC) for each animal. HA-specific IgA was not detected (ND) in plasma from naïve newborn or adult AGM. Significance was determined by one-tailed unpaired t-test, **p<0.01.

Figure 2. Differences in ID in the early IgM response between newborn and adult AGMs following IAV infection.
To measure Abs to each of the identified neutralizing HA head epitopes, a panel of Δ4 HA constructs was used, each of which had mutations in four of the five antigenic sites. The name designates the remaining epitope. IgM binding to each construct was assessed at d8 (A) or d14 (B) following PR8 infection (n=4/group). The starting dilution of plasma was 1:10. Following AUC calculation, the percent of the response based on the wildtype HA AUC was determined. Significance was determined by one-way repeated measures ANOVA with a Holm-Sidak test for multiple comparisons. Significant differences in binding between epitopes are denoted by lowercase letters corresponding to an index capital letter,e.g. "A" vs "a", where values with the "a" are significantly different (threshold p<0.05) than those with the "A". The proportion of IgG (A) and IgA (B) that binds each of the Δ4 HA molecules relative to wildtype HA-specific in newborn and adult AGM at d14 p.i. was determined (n=4/group). Ab binding to each site was calculated as in Fig. 2. The initial plasma dilution was 1:160 for IgG and 1:20 for IgA. Significance was determined by one-way ANOVA with a Holm-Sidak test for multiple comparisons and is denoted as described in Fig. 2. The threshold for significance was p<0.05. The avidity of IgG binding to each epitope was determined by disruption with NaSCN in plasma collected at d14 p.i. from newborns (A) and adults (B). The Ab dilution used in the assay for each animal was that which gave 50% of maximal binding to wildtype HA to normalize for the total amount of HA-specific Ab in the assay. To measure avidity, increasing concentrations of NaSCN (0.5M) were added to plates to disrupt binding. Shown is the NaSCN concentration giving a 50% reduction in optical absorbance compared to the untreated sample. Significance was assessed by RM one-way ANOVA with Tukey's multiple comparison; *p<0.05. Figure 5. Generation of Ab capable of recognizing S12 in newborn and adult AGMs following IAV infection. (A) Plasma from newborn or adult naïve animals or animals infected with PR8 14 days prior was tested for recognition of the recombinant S12 HA protein. The starting dilution of plasma was 1:80. (B) Plasma from naïve and infected animals was also tested for recognition of virally derived S12 HA. Significance was determined by unpaired t-test; **p<0.01, ***p<0.001 Figure 6. HA stem-specific Ab response in newborn and adult AGM following IAV infection. IgM (A) and IgG (B) binding to the HA stem region in plasma samples from naïve and infected animals (n=4/group) was quantified by ELISA using a headless California/09 HA stem construct. (C) The presence of stem-specific IgG was also assessed in BAL collected at d14 p.i. Pre-and post-infection samples for adults are from the same animals. Naïve newborn samples are from an age-matched control cohort. (D) The capacity of stem mAb CR9114 to competitively inhibit IgG binding to HA and Ca09 was assessed by ELISA in plasma collected at d14 p.i. from newborns and adults. Plates were pre-incubated with serial dilutions of CR9114. Plasma was diluted at 1:80 and added to plates coated with HA or the stem construct a indicated. IgA antibody in the plasma (E) and BAL (F) collected at d14 p.i. was also evaluated for Ab binding to the stem region. Plasma IgM and IgA assays were performed starting at a dilution of 1:10 and 1:20, respectively. The starting dilution for plasma IgG was 1:80 due to the high HA-specific Ab titer at d14 p.i. BAL assays were performed at a starting dilution of 1:5. Significance was determined by unpaired t-test, *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001. (B) Neutralizing stem-specific antibody present at d14 p.i. was assessed using a chimeric virus bearing an H5 head and N2 neuraminidase to limit detectable neutralization to stem binding Abs. The limit of detection for the assay is designated by the dashed line; representative naïve samples were all below threshold.