The current COVID-19 pandemic is caused by the zoonotic severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)(1, 2). The pandemic has affected almost every aspect of human life, and will continue to do so until effective vaccines or therapeutics are developed. SARS-CoV-2 infection is initiated when the trimeric 'spike' glycoprotein on the virion surface binds angiotensinconverting enzyme 2, allowing viral entry and initating viral replication (3) . After an asymptomatic incubation period, the infection can cause highly heterogenous clinical outcomes, from negligible or mild symptoms to critical disease resulting in death (4) . One of the main risk factors for severe disease and death is age (4) (5) (6) . Therefore, successful interventions to limit the severe health outcomes of COVID-19 disease should aim to be effective in older adults.
Vaccines have been one of the most successful medical interventions to date (7, 8) and represent a potential solution to control the COVID-19 pandemic (9, 10) . However, age-related changes in the immune system mean that older individuals often do not generate protective immunity after vaccination (11) (12) (13) (14) . As older persons are more at risk of severe health outcomes after infection, significant effort has been made to specifically tailor vaccines to be more effective in people aged 65 years or older. Modifications to vaccines include increasing the antigen dose and the use of adjuvants, which can support then enhancement of immune responses in older people compared to standard formulations of vaccines (15, 16) . This indicates that the success of a SARS-CoV-2 vaccine may require alterations to the dosing regimen, including an increase in the dose or the number of doses in order to induce a sufficient immune response in older people.
The effectiveness of COVID-19 vaccine candidates in older adults will ultimately be determined in clinical trials. However, studies in aged animals can be used to test alternative vaccine strategies or dosing regimens, and can therefore be used to inform clinical strategy. This pre-clinical work may therefore expedite the process of developing an effective vaccine against COVID-19 for older adults. Despite ageing occurring on different time scales in mice and people, many of the cellular and molecular changes that occur with ageing are conserved between the species (17) , including those in the immune system such as thymic involution and the loss of naïve T cells (18) . The response to vaccination is no exception (19, 20) ; after vaccination, both aged mice (>20 months old) and older humans have reduced vaccine-specific antibody formation, an impaired type I interferon response and fewer T follicular helper cells (21) (22) (23) (24) . This impaired immune response to vaccination in older mice and humans has been linked with reduced protection against subsequent infection (25) (26) (27) (28) . Importantly, interventions that enhance immunogenicity of vaccines in aged mice are also effective in humans (21, 27, 29, 30) , demonstrating that aged mice are a relevant pre-clinical model for testing the immunogenicity of new vaccine candidates.
ChAdOx1 nCoV-19 is a chimpanzee adeno (ChAd)-vectored vaccine that encodes the full-length spike protein of SARS-CoV-2. Vaccination with ChAdOx1 nCoV-19 elicits robust cellular and humoral immunity in mice, pigs and macaques (31, 32) . Importantly, a single vaccination prevents viral pneumonia and reduces the viral load in rhesus macaques after high-dose SARS-CoV-2 challenge (31) . The phase I/II ChAdOx1 nCoV-19 trial demonstrated that this vaccine has an acceptable safety profile and is immunogenic in adults between 18 and 55 years of age, stimulating virus-neutralising antibodies and spike-reactive T cells (33) . In macaques, pigs and people, a homologous booster dose of ChAdOx1 nCoV-19 increased anti-spike antibody responses, indicating this is a safe and effective strategy to enhance immunogenicity (31) (32) (33) .
Here, we demonstrate that a single dose of ChAdOx1 nCoV-19 elicits a B and T cell response in 3month-old adult mice, with formation of plasma cells, germinal centres and T follicular helper cells contributing to anti-spike antibody production. The development of humoral immunity is complemented by the formation of polyfunctional vaccine-specific Th1 cells and CD8 + T cells. In aged 22-month-old mice a single dose of ChAdOx1 nCoV-19 induced the formation of Th1 cells, vaccinereactive CD8 + T cells, a germinal centre response and vaccine-specific antibodies. However, the cellular and humoral response was reduced in magnitude in 22-month-old mice compared to 3-monthold adult mice, with antibody isotypes and subclasses produced being of a similar profile. Administration of a second dose enhanced the germinal centre response and antibody titre in aged mice, and also boosted the numbers of granzyme B producing CD8 + T cells. Together, this indicates that the immunogenicity of ChAdOx1 nCoV-19 can be enhanced in older individuals through the use of a prime-boost vaccination strategy.
To map where antigen drains upon intramuscular immunisation, we immunised mice with fluorescent nanoparticles that freely drain into lymphoid organs (34) . The nanoparticles were injected into the right quadriceps muscle of 3-month-old adult mice. Twenty-four hours after injection, confocal microscopy revealed fluorescent nanoparticles in the aortic lymph node (aLN) and the spleen of all immunised mice ( Fig. 1a and Sup. Fig. 1a) , while there was negligible detection of fluorescent nanoparticles in the inguinal and popliteal lymph nodes (data not shown). These data suggest the aLN and spleen are the main sites of antigen drainage upon intramuscular immunisation. As such, this study focused on the immune response induced by the ChAdOx1 nCoV-19 vaccine in these two immune compartments.
To characterise the early events of the immune response to ChAdOx1 nCoV-19 vaccination, we immunised adult C57BL/6 mice and assessed the induction of interferon signalling as well as the expansion of antigen presenting cell populations at day 1 and 2 post immunisation. At the site of immunisation, the right quadriceps muscle, expression of the type I interferon inducible gene Mx1 and the type 2 interferon inducible gene Gbp2 was detected at day 1 in ChAdOx1 nCoV immunised mice compared to phosphate buffered saline (PBS) immunised controls (Fig. 1b) . The expression of both genes was significantly reduced by day 2, indicating that the ChAdOx1 nCoV-19 vaccine induces a short-lived interferon response at the site of injection. In the draining aLN and spleen, the expansion and activation of macrophages, type 2 conventional dendritic cells (cDC2s), Langerhans cells and cDC1s was quantified by flow cytometry (Sup. Fig. 1b ; gating strategy from (35) ). In both the aLN and spleen of ChAdOx1 nCoV immunised mice there was a significant increase in the total number of CD11c + macrophages at day 2, while the total number of cDC2s, Langerhans and cDC1s remained unperturbed compared to PBS-immunised controls (Sup. Fig. 1c, d) . However, two days after immunisation, there was an increase in the total number of activated CD40 + CD86 + cells in all antigen presenting cell populations compared to PBS-immunised controls, in both the aLN and spleen (Fig.  1c, d) . The induction of adaptive immunity through T cell priming relies on the activation of these professional APCs; of which cDC2s have been implicated as the main subset to activate Tfh cells while the cDC1 subset is able to cross-present exogenous antigens to activate CD8 + T cells (21, 36, 37) . These data demonstrate that the ChAdOx1 nCoV-19 vaccine activates several antigen presenting cell populations required for the initiation of cell-mediated and humoral immunity.
To assess the adaptive immune response to ChAdOx1 nCoV-19 in detail we used a high-dimensional flow cytometry panel that contained antibodies that recognise multiple molecules used to define different lymphocyte subsets and their activation status (Table 1) . Mice were immunised with either ChAdOx1 nCoV-19 or PBS, and the immune response was assessed seven, fourteen and twentyone days later. t-distributed Stochastic Neighbour Embedding (tSNE) and FlowSOM analysis grouped B cells into five clusters (Fig. 2a) . A cluster that corresponded to naïve B cells which were present in both ChAdOx1 nCoV-19 and PBS immunised mice, and four additional clusters which were overrepresented in ChAdOx1 nCoV-19 immunised mice. These clusters are CD69 + and CD86 + activated B cells, CD138 + IRF4 + antibody-secreting plasma cells and Bcl6 + GL-7 + germinal centre B cells. Manual biaxial gating of these populations (Sup. Fig. 2a) showed that ChAdOx1 nCoV-19 immunisation induced a robust B cell response in the draining lymph node and spleen (Fig. 2b, c) , with early B cell activation and plasma cell formation occurring in the first week after immunisation, and the germinal centre response persisting over the three weeks assessed (Fig. 2b-g) . Formation of T follicular helper and T follicular regulatory cells accompanied the germinal centre response (Fig. 2h, i) . Antibodies binding the SARS-CoV-2 spike protein were induced by vaccination, and, as expected, the temporal induction of anti-spike IgM was faster than that of IgG, and serum IgA antibodies were not observed at high titre (Fig. 2j) . A mix of anti-spike IgG antibody subtypes was observed, with IgG2 and IgG1 subclasses persisting at later timepoints. At all timepoints a predominantly Th1 dominant response (IgG2) was measured (Fig. 2k) . The mean (and standard deviation) value for ratio of IgG2/IgG1 on day seven was 4.8 (1.4), on day fourteen 2.3 (0.5), and on day twenty-one 2.8 (1.8). This demonstrates that ChAdOx1 nCoV-19 immunisation stimulates B cell activation and differentiation, culminating in the production of anti-spike antibodies.
To investigate how ChAdOx1 nCoV-19 immunisation affects CD4 + T cells beyond the germinal centreassociated subsets, tSNE analysis followed by manual gating was performed ( Fig. 3a-c, Sup. Fig.  2b, c) . tSNE/FlowSOM clustering showed that vaccination induced populations of T cells that express markers associated with T follicular helper cells and Th1 cells (Fig. 3a) . The conventional CD4 + T cell response to ChAdOx1 nCoV-19 was characterised by early activation and proliferation, as well as the formation of CXCR3 + Th1 cells ( Fig. 3a-f ). The Foxp3 + regulatory T cell response was characterised by early expression of Ki67, indicative of proliferation, and the differentiation of CXCR3 + Th1-like cells (Fig. 3c, g) . To further characterise CD4 + T cell responses after vaccination, cells from the aLN were restimulated with PdBu/ionomycin and cytokine production was assessed. IL-2, TNFa, IL-10 and IFNg were induced by cells isolated from animals that had been ChAdOx1 nCoV-19 vaccinated, with no difference observed in Th17-associated IL-17 or the Th2-associated cytokines IL-4 and IL-5 (Fig. 3h,  Sup. Fig. 3a, b) . Furthermore, restimulation of splenocytes with peptide pools from SARS-CoV-2 identified antigen-specific IL-2, TNFa, and IFNg producing cells, with triple producers persisting 21days after immunisation (Fig. 3i, Sup. Fig. 3c) , consistent with previous work (31) . Therefore, ChAdOx1 nCoV-19 vaccination induces an early formation of Th1 cells and Th1-like Treg cells accompanied by the induction of virus-specific Th1-skewed cytokine-secreting cells.
tSNE analysis of CD8 + T cells from the aLNs of ChAdOx1 nCoV-19 immunised and PBS control mice revealed distinct clustering seven days after immunisation. CD8 + T cell clusters that were present in ChAdOx1 nCoV-19 immunised mice, but not in PBS immunised animals expressed high levels of CD44, Ki67, CXCR3, PD-1, IRF4 or CD69, indicative of CD8 + T cell activation (Fig. 4a) . In order to understand how the response evolves over time, manual gating of different CD8 + T cell populations (Sup. Fig. 2d) , including those identified in the tSNE analysis, was done on samples taken seven, fourteen and twenty-one days after immunisation. In the aLN and spleen, the CD8 + T cell response to ChAdOx1 nCoV-19 was characterised by an increase in Ki67 expression, the upregulation of the activation markers CD69, CXCR3 and PD-1, as well as the formation of CD44 + CD62L -T effector memory cells (Fig. 4b-f ). To characterise the overall production of granzyme B and cytokines by CD8 + T cells seven days after vaccination, cells from the aLN were restimulated with PdBu/ionomycin. There was a significant production of granzyme B, IL-2, TNFa and IFNg in ChAdOx1 nCOV-19 vaccinated mice compared to PBS immunised mice (Fig. 4g) . Restimulation of splenocytes with spike protein peptide pools from SARS-CoV-2 showed that antigen-specific cytokine producing CD8 + T cells form in response to immunisation (Fig. 4g, Sup. Fig. 4a, b) . Granzyme B producing CD8 + T cells formed early, and tended not to co-produce cytokines (Fig. 4h) . Both single and multiple cytokine producing cells formed at all time points, with TNFa and IFNg being the dominant cytokines expressed (Fig. 4h, Sup. Fig. 4c ). With this dosing regimen, this data demonstrates that ChAdOx1 nCoV-19 stimulates a robust CD8 + T cell response that peaks around the first week after vaccination.
To assess the CD8 + T cell response to ChAdOx1 nCoV-19 immunisation in the context of ageing, we immunised 3-month-old and 22-month-old mice and enumerated the CD8 + T cell types altered by vaccination (in Fig. 4 ) nine days after immunisation (Fig. 5a) . In the draining aLN, CD8 + T cells from aged mice expressed markers of activation and proliferation in response to ChAdOx1 nCoV-19. Unlike in younger adult mice, the frequency of CXCR3 + cells or T effector memory cells did not increase in aged mice, compared to the PBS vaccinated group (Fig. 5b-d) . At this early timepoint, the frequency of central memory T cells was not altered in either younger adult or aged mice by ChAdOx1 nCoV-19 vaccination (Fig. 5e) . In the spleen, fewer Ki67 + CD8 + T cells were observed in aged mice after ChAdOx1 nCoV-19 vaccination, compared to younger adult mice (Fig. 5f) . The formation of antigen-specific CD8 + T cells was assessed by restimulating splenocytes with SARS-CoV-2 spike protein peptide pools. Aged mice had a near absence of granzyme B producing CD8 + T cells, but production of IFNg and TNFa was not significantly impaired compared to younger mice (Fig.  5g) . IL-2 production was low in both adult and aged mice at this time point (Fig. 5g) , compared to the seven days after immunisation ( Fig. 4g-h) . Despite a trend to lower cytokine production by CD8 + T cells in aged mice, the proportion of polyfunctional CD8 + T cells was not significantly diminished in aged mice after ChAdOx1 nCoV-19 vaccination (Fig. 5h) . This demonstrates that a single dose of ChAdOx1 nCoV-19 induces an altered CD8 + T cell response in aged mice characterised primarily by a failure to form granzyme B-producing effector cells.
To determine whether a second dose could improve this response, we administered a booster dose of ChAdOx1 nCoV-19 one month after prime immunisation (Fig. 5i) . Nine days after boost, an increase in Ki67 + CD4 T cells was not observed in the draining aLN (Fig. 5j) , possibly due to the kinetics of the secondary response being faster than the primary. A significant increase in CXCR3 + CD8 + T cells and effector memory cells was observed in aged mice after boost, with no change in the proportion of central memory cells in either age group (Fig. 5k-m) . Assessment of antigen-specific splenocytes showed that the booster dose of ChAdOx1 nCoV-19 rescued the production of granzyme B producing CD8 + T cells in aged mice (Fig. 5n) . IFNg production and cytokine polyfunctionality were similar to that following prime immunisation (Fig. 5o, p) . This demonstrates that ChAdOx1 nCoV-19 is immunogenic in aged mice, and a booster dose can correct the age-dependent defect in the formation of granzyme B-producing CD8 + T cells.
Nine days after primary immunisation of aged mice (Fig. 6a) , an increase in Ki67 + CD4 + T cells and CXCR3-expressing Th1 cells was observed in the draining lymph node of ChAdOx1 nCoV-19 immunised mice compared to PBS immunised control mice (Fig. 6b, c) . This was accompanied by an increase in the frequency of Th1-like Tregs in both adult and aged mice (Fig. 6d ). An increased frequency of these cell types was likewise observed in the spleen in response to ChAdOx1 nCoV-19 immunisation in both adult and aged mice ( Fig. 6e-g) . It is notable that, by these measurements, the response in aged mice is comparable to that in younger adults. The antigen-specific CD4 + T cell response was assessed by restimulating splenocytes with SARS-CoV-2 spike protein peptide pools. As in young mice (Fig. 3) , the response in aged mice to ChAdOx1 nCoV-19 was Th1 dominated, however there were fewer cytokine producing cells in aged mice nine days after a single immunisation (p<0. 001, Fig 6h, i) .
A booster dose of ChAdOx1 nCoV-19 administered one month after prime (Fig. 6j) stimulated Ki67 expression and the formation of CXCR3 + CD44 + Th1 cells, but not CXCR3 + Th1-like Treg cells in the draining lymph node of aged mice ( Fig. 6k-m) . In the spleen, the booster dose did not enhance the frequency of Ki67 + CD4 + T cells or the formation of CXCR3 + conventional or regulatory T cells in adult or aged mice ( Fig. 6n-p) . In contrast to the response to primary immunisation, the number of antigenspecific cytokine producing cells was comparable in adult and aged mice after booster immunisation (Fig. 6q, r) . Together, this indicates that the CD4 + T cell response to ChAdOx1 nCoV-19 immunisation is largely intact in aged mice, with a slight deficiency in antigen-specific cytokine production that can be enhanced by a booster immunisation.
The majority of clinically available vaccines are thought to provide protection by eliciting humoral immunity. Therefore it was important to quantify the B cell response to ChAdOx1 nCoV-19 vaccination in the context of ageing. Early antibody production after vaccination arises from antibody-secreting cells generated in the extrafollicular plasma cell response, which is fast, but typically short-lived (38) . A comparable early plasma cell response was detected in the aLN of younger adult and aged mice after immunisation (Fig. 7a, b) , although there was an increase in the proportion of IgM + plasma cells in aged mice (Fig. 7c ). An intact plasma cell response was coupled with an increase in serum antibodies nine days after immunization. These were of only slightly lower titre in aged mice, and of similar IgG subclass distribution to younger animals, indicative of a predominantly Th1 dominated response ( Fig. 7d-f ).
Long-lived antibody-secreting cells typically arise from the germinal centre response (39) . The percentage, but not total number, of germinal centre B cells was reduced in aged mice compared to younger adult mice after ChAdOx1 nCoV-19 vaccination (Fig. 7g, h) . Like the plasma cell response, there were more IgM + germinal centre B cells in aged mice (Fig. 7i ). An increase in T follicular helper cells, but not T follicular regulatory cells, accompanied the lymph node germinal centre response in adult and aged mice (Fig. 7j, k) . In the spleen, germinal centres were easily visualised by microscopy in adult mice nine days after ChAdOx1 nCoV-19 vaccination, but were conspicuously absent in aged mice (Fig. 7l , and Sup. Fig. 5 ). Quantification of splenic germinal centres by flow cytometry confirmed impaired germinal centre formation in aged mice (Fig. 7n, o) . This was accompanied by fewer proliferating non-germinal centre B cells and T follicular helper cells in aged mice (Fig. 7p, q) . As in the draining lymph node, splenic T follicular regulatory cells were not induced by ChAdOx1 nCoV-19 vaccination at this time point (Fig. 7r) . The impact of an impaired germinal centre response on vaccine-specific antibodies was observed 28 days after immunisation, with aged mice having lower titres of anti-spike IgM and IgG (Fig. 7s, t) , but a similar profile of IgG subclasses (Fig. 7u) . Together, these data indicate that whilst a single of dose of ChAdOx1 nCoV-19 can induce comparable extrafollicular plasma cell responses between young and aged mice, the germinal centre response is compromised with age.
To test whether a prime-boost strategy can enhance the B cell response in aged mice, a prime-boost approach was taken (Fig. 8a) . Nine days after boost, there were Ki67 + non-germinal centre B cells, plasma cells and germinal centre B cells in the draining lymph nodes of aged mice ( Fig. 8b-h) .
Notably, the magnitude of the germinal centre response was larger in aged mice than in younger adult mice after boost ( Fig. 8f-h) and this was associated with increased T follicular helper and T follicular regulatory cell numbers (Fig. 8i, j) . A germinal centre response was not observed in the spleen of either adult or aged mice nine days after booster immunisation (Fig. 8k) . This demonstrates that a second dose of ChAdOx1 nCoV-19 can enhance the B cell response in aged mice. This improvement in the B cell response corresponded to an increase in anti-spike IgG, without skewing IgG subclasses, antibodies in every aged mouse that was given a booster immunisation ( Fig. 8l-o) . The post boost ratio of IgG2/IgG1 was 3.7 (3.0) in younger adult and 2.6 (1.6) in aged mice (mean and standard deviation). The functional effect of the humoral immunity after both prime and boost immunisations was measured by SARS-CoV-2 pseudotyped virus microneutralisation assay. Nine days after prime immunisation, SARS-CoV-2 neutralising antibodies were at a lower titre in aged mice than measured in adult mice (Fig 8p) . Nine days after boost, neutralising antibodies were detectable in all aged mice and had been boosted eight-fold compared to post-prime, although the titre was significantly lower than in younger adult mice (Fig 8q) . This demonstrates that a booster dose of ChAdOx1 nCoV-19 can improve vaccine-induced humoral immunity in older mice.
The development of an effective anti-SARS-CoV-2 vaccine represents an opportunity to limit the health, social and economic consequences of the current pandemic. Since older adults are more likely to have severe health outcomes after infection, a vaccination strategy that provides protection for this group is particularly desirable. Here, we have used a pre-clinical model to determine the immunogenicity of ChAdOx1 nCoV-19 in older bodies. Whilst this vaccine stimulates an immune response in aged mice, it is of reduced magnitude compared to that seen in younger adult animals, consistent with previous immunisation studies (21, 24, 27) . Importantly, the data presented here show that a second homologous immunisation at the same dose level was able to enhance the B cell, helper T cell and CD8 + T cell response in aged mice. In people, the same prime-boost approach had an acceptable safety profile and enhanced humoral immunity (33) , indicating that this is a rational vaccination approach for use in the older members of our communities -arguably the sector of society most in need of an effective vaccine to prevent COVID-19.
After ChAdOx1 nCoV-19 immunisation of aged mice, a significant decrease in the proportion of classswitched germinal centre B cells and plasma cells was observed compared to young adult mice. This was linked with a lower titre of anti-spike antibodies after primary vaccination, with IgG being more affected than IgM by age. Previous studies, in both humans and mice, have demonstrated that ageing causes a B cell-intrinsic impairment in class-switch recombination. This is attributed to a decrease in the expression of the enzyme activation-induced cytidine deaminase, which initiates class-switch, and its positive regulator, the E2A-encoded E47 transcription factor, in B cells from aged mice and humans (40, 41) . The reduced expression of activation-induced cytidine deaminase in B cells from older persons has been correlated with lower neutralising antibody production after seasonal influenza vaccination (42) . Therefore, vaccination strategies that aim to enhance class-switch recombination may improve vaccine efficacy in older people.
The age-dependent defects in the B cell response were not limited to class-switch recombination. This highlights that either the formation of antibody secreting cells, or their ability to secrete antibody is impaired by ageing. Antibody-secreting cells can form from two cellular pathways, but these are not equal in terms of quality or longevity. The first pathway, the extrafollicular response, produces an initial burst of antibodies early after antigenic challenge. This response is short-lived with no additional diversification of the B cell repertoire and thus its contribution to long-term immunity is minimal (38) . The second pathway, the germinal centre reaction, is a specialised microenvironment that produces memory B cells and long-lived antibody secreting plasma cells with somatically-mutated immunoglobulin genes (39, 43, 44) . The germinal centre is the only cellular source of long-lived plasma cells (45) , and the only site where the B cell response can be altered in response to antigen. Nine days after ChAdOx1 nCoV-19 immunisation, the formation of plasma cells was comparable in young and aged mice, a time at which the majority of plasma cells derive from the extrafollicular response (45) . By contrast, the germinal centre response was diminished in aged mice, which is a well described deficit of the older immune system after vaccination (21, 46) . This indicates that poor germinal centre formation is the major barrier that must be overcome to improve humoral immunity in older individuals. In this study we show that a booster dose of ChAdOx1 nCoV-19 enhanced the magnitude of the germinal centre response in aged mice, to a greater extent than it did in younger animals. This may be because younger mice have produced higher titres of anti-vaccine antibodies, which limits the action of a second dose. A similar phenomenon occurs with the seasonal influenza vaccine, in which individuals with high titres of vaccine reactive antibodies typically have a muted response to secondary vaccination (47) , and has been observed in an adenovirus type-5(Ad5)vectored COVID-19 vaccine in people who have high pre-existing antibody titres to the vaccine vector due to natural exposure to Ad5 (48) .
Another feature of the aged germinal centre reaction is reduced selection of high affinity B cells, resulting in a diminished quality of the response in ageing (23) . Whether the further diversification and affinity-based selection of the B cell receptor is important for protective humoral immunity to SARS-CoV-2 has yet to be established. However, antibodies isolated from spike-specific B cells from convalescent COVID-19 patients can be both potently neutralising and have low levels of somatic hypermutation (49) (50) (51) , indicating that extensive somatic hypermutation is not required for anti-SARS-CoV-2 neutralising antibody formation. Therefore, the main barrier to inducing protective humoral immunity to SARS-CoV-2 in older people may be an issue of enhancing the magnitude, rather than quality, of the germinal centre response. Our study shows that it is possible to enhance the number of germinal centre B cells in aged mice by giving a second dose of the vaccine.
It has been proposed that a failure of the germinal centre reaction in older mice is caused by an increased frequency of suppressive T follicular regulatory cells (52) . Here, we did not see an accumulation of T follicular regulatory cells in ageing, and their formation was only robustly observed later in the response. This result is similar to the delayed formation of T follicular regulatory cells after influenza A infection (53) . Because the germinal centre response requires the coordinated response of many cell types, identifying the causal defect is challenging. In heterochronic parabiosis experiments, the age of the microenvironment, and not the age of the recirculating lymphocytes, underpins the poor germinal centre response in ageing (54) . This implicates non-migratory cells, such as stromal cells or tissue resident cells as causal factors in the diminished germinal centre reaction in ageing.
A rapid recall of memory CD8 + T cells in response to viral infections complements the humoral response by promoting efficient pathogen clearance and this becomes particularly important in scenarios where the protective ability or magnitude of neutralising antibodies is compromised (55) . A single dose of ChAdOx1 nCoV-19 vaccine induces a CD8 + T cell response in both adult and aged mice. In aged mice there was a trend for fewer cytokine producing CD8 + T cells, but this was not statistically significant in either the discovery or replication cohort. There was, however, a profound defect in the generation of spike-reactive CD8 + T cells that produced granzyme B, a key cytotoxic effector molecule produced by effector CD8 + T cells. This failure to generate granzyme B + CD8 + T cells is a common feature in ageing, previously observed in aged mice after West Nile Virus infection (56) and in aged monkeys infected with SARS-CoV(57). A similar trend for fewer granzyme B + CD8 cells in older people was observed after influenza vaccination, although this study was not sufficiently powered for the difference to be statistically significant (58) . Consistent with reduced expression of granzyme B in T cells from older adults, two weeks after influenza vaccination T cells taken from older adults have an impaired ability to kill influenza infected cells ex vivo (59) . In a small study of older people, poor induction of granzyme B after influenza vaccination was associated with a greater likelihood of subsequent laboratory confirmed influenza infection, indicating that enhancing granzyme B + CD8 + T cells may be important for protective immunity against respiratory viruses in older adults (60) . Here we demonstrate that a second dose of ChAdOx1 nCoV-19 was able to correct the defective granzyme B + CD8 + T cell response in aged mice, indicating that two doses of this vaccine is a better approach to enhance the cellular immune response in older bodies.
ChAdOx1 nCoV-19 is currently being trialed in older adults as part of a phase III trial, which will ultimately determine whether it is an effective vaccine for this demographic. Immunisation of people over 55 years of age with Ad5-vectored COVID-19 vaccine resulted in lower antibody titres in the older age group, suggesting that adenoviral vectored vaccine strategies may require more than one dose in older people (48) . However, as pre-existing immunity to Ad5, a naturally occurring human tropic adenovirus, can inhibit the response to this vaccine, it is possible that the reduced immunogenicity of this vaccine is due to an increased prevalence of antibodies to the vaccine vector in this age group. By contrast, the presence of anti-ChAdOx1 antibodies in the general population is low (61) . The work presented here demonstrates that one dose of this vaccine is immunogenic in aged mice, but this response can be significantly improved with a second booster dose. Given that a second dose of ChAdOx1 nCoV-19 is immunogenic with expected reactogenicity profile in humans (33) , this may be a viable strategy to enhance immunogenicity and possibly efficacy in older people.
C57BL/6Babr mice were bred, aged and maintained in the Babraham Institute Biological Support Unit (BSU). No primary pathogens or additional agents listed in the FELASA recommendations(62) were detected during health monitoring surveys of the stock holding rooms. Ambient temperature was ~19-21°C and relative humidity 52%. Lighting was provided on a 12 hr light: 12 hr dark cycle including 15 min 'dawn' and 'dusk' periods of subdued lighting. After weaning, mice were transferred to individually ventilated cages with 1-5 mice per cage. Mice were fed CRM (P) VP diet (Special Diet Services) ad libitum and received seeds (e.g. sunflower, millet) at the time of cage-cleaning as part of their environmental enrichment. All mouse experimentation was approved by the Babraham Institute Animal Welfare and Ethical Review Body. Animal husbandry and experimentation complied with existing European Union and United Kingdom Home Office legislation and local standards (PPL: P4D4AF812). Young mice were 10-12 weeks old, and aged mice 93-96 weeks old when experiments were started. Mice that had tumours, which can occur in aged mice, were excluded from the analysis.
Mice were immunised in the right quadriceps femoris muscle with 50µL of either 1x10 8 infectious units of ChAdOx1 nCoV-19 in phosphate buffered saline (PBS) PBS alone, or 0.02µm yellow-green fluorescent Carboxylate-Modified Microspheres (Invitrogen # F8787) in phosphate buffered saline. At the indicated timepoints post vaccination, blood, the right aortic lymph node, spleen and right quadriceps femoris muscle were taken for analysis.
For T and B cell flow cytometric stains a single cell suspension was prepared from the aortic lymph node and half the spleen was generated by pressing the tissues through a 70 µm mesh in 2% FBS in PBS. Cell numbers and viability were determined using a CASY TT Cell Counter (Roche). 2×10 6 cells were transferred to 96-well plates for antibody staining. Samples were blocked with 100μl of 2.4G2 Fc Block (made in house) for 20 min at 4°C. Cells were then stained with surface antibody mix for 2hrs at 4°C and then were fixed with the eBiosciences Foxp3/Transcription Factor Staining Buffer (#00-5323-00) for 30 min at 4°C. Cells were then washed with 1x Permeabilisation buffer (eBioscience #00-8333-56) twice and stained with intracellular antibody mix in 1x Permeabilisation buffer at 4°C overnight. For cytokine staining, splenic cells were stimulated with a pool of SARS-CoV-2 spike protein immunodominant domain peptides, (Miltenyi Biotec #130-126-700) at a 0.6 µM concentration (approx.1µg/ml), whilst lymph node cells were stimulated with 0.5µg/ml of Phorbol 12,13 dibutyrate (PdBu, Tocris Bioscience, #), 0.75µg/ml of Ionomycin calcium salt (Tocris Bioscience, #), both in warm complete RPMI (10% FCS, 1% Pen/Strep, 1% glutamine, 1% sodium pyruvate, 1% MEM NAA, 1% HEPES and 55μM-2-mercaptoethanol) for 4hrs at 37°C, 5%CO2. Cytokine secretion was then blocked with 22µg/ml of Brefeldin A (Tocris Bioscience, #) in warm complete RPMI for 2hrs at 37°C, 5%CO2. The cells were then stained with surface antibody mix for 20 minutes at 4°C and were subsequently fixed with 2% formaldehyde for 30min at room temperature. After two wash steps with 1x Permeabilisation buffer (eBioscience #00-8333-56), the cells were stained with intracellular antibody mix in 1x Permeabilisation buffer, supplemented with 20% 2.4G2 Fc Block at 4°C overnight. Following overnight staining, samples were washed twice with 1x Permeabilisation buffer and once with 2% FBS in PBS and acquired on a Cytek TM Aurora. Cells for single colour controls were prepared in the same manner as the fully stained samples. The antibodies used for surface and overnight staining are listed in Table 1. For dendritic cell analysis, secondary lymphoid tissues were harvested and incubated with 10 mg/ml Collagenase D (Roche #11088866001) in plain RPMI medium (Gibco #11875093) for 30-45 min at 37 °C, followed by gentle pipetting to disrupt the tissue. Cells were washed with PBS containing 2% FBS, before cell numbers were determined using a CASY TT Cell Counter (Roche). 4×106 cells were transferred to 96-well plates for antibody staining. After a wash with PBS, cells were stained with Live/Dead Fixable Blue Dead Cell Stain (Invitrogen #L23105; diluted 1:1000 in PBS) on ice for 10min after which they were washed in 2% FBS in PBS and treated with unlabelled anti-CD16/32 (eBioscience #14-0161-82; diluted 1:100 in 2% FBS in PBS) for 20min at 4ºC. Surface antibody staining was then performed for 1hr at 4ºC in Brilliant Stain Buffer (BD Biosciences #563794). Samples were acquired on an LSRFortessa 5 with stained UltraComp eBeads Compensation Beads (Invitrogen #01-2222-41) as compensation controls.
Manual gating of flow cytometry data was done using FlowJo v10 software (Tree Star). tSNE, FlowSOM and heatmap analysis were performed on aLN samples from day 7 postvaccination using R (version 4.0.2) using code that has previously been described (63) . The antibodies used for surface staining are listed in Table 1 . For imaging yellow-green fluorescent carboxylate-codified microspheres, slides were first air-dried and then washed in PBS three times after which DAPI staining was performed (Invitrogen #D1306; diluted 1:10 000 in PBS) for 10 minutes at room temperature protected from light. Slides were then washed three times with PBS and coverslips were mounted on the slides using Hydromount mounting medium (National Diagnostics #HS-106). For antibody stains, the slides were first air-dried and then hydrated in 0.5% Tween 20 in PBS (PBS-T). The slides were blocked in PBS containing 2% BSA and 10% goat serum for 2hrs, washed three times with PBS-T and then permeabilised with PBS containing 2% Triton X (Sigma #X100) for 45min. Following three wash steps with PBS-T, the slides were stained with primary antibody mix, which included AF647-conjugated rat anti-mouse IgD (clone 11-26 c.2a, Biolegend; 1:200), FITC-conjugated rat anti-mouse Ki67 (clone SolA15, Invitrogen; 1:100), rat antimouse biotin-conjugated CD21/35 (clone 8D9, ThermoFisher Scientific; 1:400) and hamster antimouse CD3ε (clone 500A2, ThermoFisher Scientific; 1:200), in PBS-T containing 1% BSA at 4°C overnight. The next day, slides were washed three times with PBS-T and incubated with secondary antibody mix, which included AF568-conjugated goat anti-hamster IgG (Thermofisher, 1:1000) and BV421-conjugated Streptavidin (Biolegend, 1:1000), in PBS-T containing 2% goat serum for 2hr at room temperature. Slides were then washed three times with PBS-T, PBS and dH2O and coverslips were mounted using Hydromount mounting medium (National Diagnostics #HS-106). Images were acquired using a Zeiss 780 confocal microscope with 10x and 20x objectives and analysed using ImageJ.
The right quadriceps muscle was dissected and weighed. The muscle tissue was snap frozen in liquid nitrogen and homogenized in 2mL TRIzol reagent (Thermo Fisher Scientific #15596026), then 1 mL of the homogenized muscle in TRIzol was processed following the manufacturer's instructions. RNA concentrations obtained from the RNA isolation were measured using the NanoDrop system (Thermo Fisher Scientific). RT-qPCR was performed using TaqMan Gene Expression Assays for Mx1 (Mm 00487796_m1), Gbp2 (Mm 00494576_g1) and Hprt (Mm 03024075_m1) directly on RNA using Thermo Fisher Scientific's TaqMan RNA-to-CT 1-Step Kit (#4392656) following the manufacturer's protocol. All RT-qPCR reactions were assembled in 384-well plates (Bio-Rad #HSP3805), adding 10ng of template RNA per reaction to 8μl of a master mix containing the appropriate TaqMan Gene Expression Assay. All samples were run in triplicates on a BioRad CFX384 Real-Time System. The 2 -ΔΔCt -method was applied for relative quantification of mRNA levels. Samples from PBS immunised mice were used as calibrators. Cq values were exported from the CFX Manager software (Bio-rad).
Standardised ELISA was performed to detect SARS-CoV-2 FL-S protein -specific antibodies in sera. MaxiSorp plates (Nunc) were coated with 100 or 250 ng/well FL-S protein overnight at 4 °C for detection of IgG or IgM and IgA, respectively, prior to washing in PBS/Tween (0.05% v/v) and blocking with Blocker Casein in PBS (Thermo Fisher Scientific) for 1 h at room temperature (RT). Standard positive serum (pool of mouse serum with high endpoint titre against FL-S protein), individual mouse serum samples, negative and an internal control (diluted in casein) were incubated for 2h at RT for detection of specific IgG or 1h at 37 o C for detection of specific IgM or IgA. Following washing, bound antibodies were detected by addition of AP-conjugated goat anti-mouse IgG (Sigma-Aldrich) for 1h at RT or addition of AP-conjugated goat anti-mouse IgM or IgA (Abcam and Sigma-Aldrich, respectively) and addition of pNPP substrate (Sigma-Aldrich). An arbitrary number of ELISA units were assigned to the reference pool and OD values of each dilution were fitted to a 4-parameter logistic curve using SOFTmax PRO software. ELISA units were calculated for each sample using the OD values of the sample and the parameters of the standard curve. The IgG subclass ELISA were performed according to the protocol described for detection of specific IgM or IgA in the serum. In addition, all serum samples were diluted to 1 total IgG ELISA unit and then detected with anti-mouse IgG subclass-specific secondary antibodies (Southern Biotech or Abcam). The results of the IgG subclass ELISA are presented using OD values instead of the ELISA units used for the total IgG ELISA. The ratio of IgG2/IgG1 was calculated for each animal as sum of OD values (IgG2b + IgG2c) divided by the OD value of IgG1 and represented as mean values with standard deviation (SD).
Lentiviral-based SARS-CoV-2 pseudotyped viruses were generated in HEK293T cells incubated at 37 °C, 5% CO2 as previously described (32) . Briefly, cells were seeded at a density of 7.5 x 10 5 in 6 well dishes, before being transfected with plasmids as follows: 500 ng of SARS-CoV-2 spike, 600 ng p8.91 (encoding for HIV-1 gag-pol), 600 ng CSFLW (lentivirus backbone expressing a firefly luciferase reporter gene), in Opti-MEM (Gibco) along with 10 µL PEI (1 µg/mL) transfection reagent. A 'no glycoprotein' control was also set up using the pcDNA3.1 vector instead of the SARS-CoV-2 S expressing plasmid. The following day, the transfection mix was replaced with 3 mL DMEM with 10% FBS (DMEM-10%) and incubated for 48 and 72 hours, after which supernatants containing pseudotyped SARS-CoV-2 (SARS-CoV-2 pps) were harvested, pooled and centrifuged at 1,300 x g for 10 minutes at 4 °C to remove cellular debris. Target HEK293T cells, previously transfected with 500 ng of a human ACE2 expression plasmid (Addgene, Cambridge, MA, USA) were seeded at a density of 2 × 10 4 in 100 µL DMEM-10% in a white flat-bottomed 96-well plate one day prior to harvesting SARS-CoV-2 pps. The following day, SARS-CoV-2 pps were titrated 10-fold on target cells, and the remainder stored at -80 °C. For micro neutralisation tests, mouse sera were diluted 1:20 in serum-free media and 50 µL was added to a 96-well plate in triplicate and titrated 2-fold. A fixed titred volume of SARS-CoV-2 pps was added at a dilution equivalent to 10 5 signal luciferase units in 50 µL DMEM-10% and incubated with sera for 1 hour at 37 °C, 5% CO2 (giving a final sera dilution of 1:40). Target cells expressing human ACE2 were then added at a density of 2 x 10 4 in 100 µL and incubated at 37 °C, 5% CO2 for 72 hours. Firefly luciferase activity was then measured with BrightGlo luciferase reagent and a Glomax-Multi + Detection System (Promega, Southampton, UK). Pseudovirus neutralization titres were expressed as the reciprocal of the serum dilution that inhibited luciferase expression by 50% (IC50).
All experiments were performed either twice or three times with 3-8 mice per group. Data was first tested for gaussian distribution using a Shapiro-Wilk test. Then data that was consistent with a normal distribution was analysed with either a student's t-test for comparing two data set, or one-way ANOVA test for data with multiple groups. If the data did not follow a normal distribution then a Mann-Whitney test was used for comparing two data sets and a Kruskal Wallis test for multiple comparisons. For ELISA data p-values were generated on log transformed data. All p-values shown are adjusted for multiple comparisons where multiple tests were performed on the same data. Analyses were performed within the Prism v8 software (GraphPad). Crossed boxes indicate that there were none of that cell type for that mouse. Bar charts showing the number of Ki67 + (c), effector memory CD44 + CD62L -(d), CXCR3 + (e) and PD-1 + CD44 + (f) CD8 cells in the aortic lymph node of ChAdOx1 nCoV-19 or PBS immunised mice, at the indicated timepoints post immunisation. g. Analysis of cytokine production six hours after PdBu/ionomycin stimulation of aortic lymph node cells from 3-month-old mice seven days after immunization with ChAdOx1 nCoV-19 or PBS. h. Stacked bar plots show the number of CD8 + cells singly or co-producing Granzyme B, IFNg, IL-2 or TNFa six hours after restimulation with SARS-CoV-2 peptide pools, each bar segment represents the mean and the error bars the standard deviation. In c-f bar height in corresponds to the mean and each circle represents one biological replicate. P-values are calculated using a student's ttest with Holm-Sidak multiple testing correction. and IgG (f) anti-spike antibodies and IgG subclasses (u) 28 days after immunization. Bar height in corresponds to the median and each circle represents one biological replicate. The Shapiro-Wilk normality test was used to determine whether the data are consistent with a normal distribution, followed by either an ordinary one-way ANOVA test for data with a normal distribution or a Kruskal Wallis test for non-normally distributed data alongside a multiple comparisons test, for ELISA data analyses were done on log transformed values. Data are representative of two independent experiments (n=4-8 per group/experiment). Percentage (k) of splenic germinal centre B cells. Serum anti-spike IgM (l), IgG (m) and IgG subclasses (n, o) prior to boost (day 29) and nine days after boost immunization. p-q SARS-CoV-2 neutralising antibody titres in sera were determined by micro neutralisation test, expressed as reciprocal serum dilution to inhibit pseudotyped virus entry by 50% (IC50). Samples below the lower limit of detection (LLoD) are shown as half of the LLoD. Bar height in corresponds to the median and each circle represents one biological replicate. The Shapiro-Wilk normality test was used to determine whether the data are consistent with a normal distribution, followed by either an ordinary one-way ANOVA test for data with a normal distribution or a Kruskal Wallis test for non-normally distributed data alongside a multiple comparisons test. For ELISA data analyses were done on log transformed values. In b-o, are shown from one of two independent experiments (n=4-8 per group/experiment), in p and q the data are pooled from two experiments. Scale bars, 100µm for both aLN and spleen. Data are representative of two independent experiments (n=2-5 per group/experiment). b. Gating strategy used for the identification of antigen presenting cell populations the aLN and spleen of mice immunised with ChAdOx1 nCoV-19 or PBS at one-and twodays post immunisation. Macrophages were defined as CD11c + F4/80 + CD64 + cells. cDC1s were defined as XCR1 + CD172a -CD11c + MHC-II + CD3/CD19 -CD64 -F4/80cells. cDC2s CD24 -CD172a + XCR1 -CD11c + MHC-II + CD3/CD19 -CD64 -F4/80cells. Langerhans cells were defined as CD24 + CD11c + CD172a + XCR1 -CD11c + MHC-II + CD3/CD19 -CD64 -F4/80cells. c-d. Quantification of the absolute number of macrophages, cDC2s, Langerhans cells and cDC1s in the aLN (c) and spleen (d.) Bar height in c-d corresponds to the mean and each circle represents one biological replicate. Pvalues were determined using a Shapiro-Wilk normality test followed by either an ordinary one-way ANOVA test for data with a normal distribution or a Kruskal Wallis test for non-normally distributed data alongside a multiple comparisons test. Data are representative of two independent experiments (n=4-5 per group/experiment).