Research just published by the Linterman lab shows that the immune system of older mice can be given a helping hand by applying immunology expertise and some genital wart treatment (don’t try this at home just yet)!
Mice and humans show similar age-dependent changes in their immune system so this finding offers hope for easily increasing the robustness of vaccination response in the older population.
As we age, the function of our immune system declines, rendering us more susceptible to infections, and making us less able to generate protective immunity after vaccination.
By understanding the cellular and molecular mechanisms that underpin this poor response in older individuals, researchers in the Linterman lab were able to repurpose an existing treatment for genital warts, and demonstrate that this was effective in overcoming the age-related effects on two of the many cell types making up our immune system.
The research is published online in the journal eLife.
Dr. Michelle Linterman, a group leader in the Institute’s Immunology research programme, said: “The current coronavirus pandemic highlights that older members of our families and communities are more susceptible to the morbidity and mortality associated with infectious diseases.
Therefore, it is imperative that we understand how the immune system in older people works, and to explore how we might be able to boost their immune responses to vaccines to ensure they work well in this vulnerable part of our society.”
Vaccines work by generating antibodies that are able to block the ability of pathogens to infect us.
Antibody secreting cells are produced in the germinal centre, immune reaction hubs that forms after infection or vaccination. With age, the magnitude and quality of the germinal centre reaction declines.
Immune cells called T follicular helper cells are essential to the germinal centre response. In this study the team used mice and humans to investigate why T follicular helper cell numbers decline with age, and if there is a way to boost them upon vaccination.
“The germinal centre response is a highly collaborative process that requires multiple cell types to interact at the right place and the right time.
Therefore, it made sense to us that defects in one or more of these cell types could explain the poor germinal centre response observed in older individuals after vaccination,” explains Dr. Linterman.
The researchers found that older mice and humans form fewer T follicular helper cells after vaccination, which is linked with a poor germinal centre response and antibody response.
By developing our understanding of the cellular and molecular events occurring in the germinal centre after vaccination, the researchers identified that T follicular helper cells in older mice and people received less stimulatory interactions from their immune system co-workers.
By using a cream (imiquimod, currently used to treat genital warts in humans) on the site of immunisation to boost the number of stimulatory cells, they were able to restore the formation of T follicular helper cells in older mice and also rescue the age-dependent defects in another immune cell type (dendritic cells).
Encouragingly, this demonstrates that the age-related defects in T follicular helper cell formation in ageing are not irreversible, and can be overcome therapeutically.
The full picture and evaluation of whether this approach will work as an intervention in humans requires more research into why the germinal centre response changes with age, and what can be done to overcome this.
Once achieved, it could be that clinical trials are established to incorporate this knowledge into new vaccine formulations for older people.
The humoral immune response to vaccination is diminished in older individuals, at least in part due to defects in the GC response (Linterman, 2014; Gustafson et al., 2018). Here, we show that the formation of Tfh cells is impaired in older persons and aged mice.
In mice, we show topical application of the TLR7 agonist imiquimod corrected the age-dependent defects in cDC2s and increased the formation of Tfh and GC B cells in aged mice upon immunisation.
This demonstrates that the defect in the Tfh cell response in aged individuals is not irreversible, and can be corrected by exogenous stimuli. Supporting our data, a clinical trial has demonstrated that topical imiquimod treatment at the time of seasonal influenza vaccination enhances antibody responses to the vaccine in older persons (Hung et al., 2014; Hung et al., 2016).
This, together with the data presented here, demonstrates that changing vaccination approaches is a rational strategy for improving vaccine efficacy in older persons, a challenge of increasing importance with the rising population age world-wide.
Ageing has been reported to impair DC activation (Agrawal et al., 2007; Moretto et al., 2008), the production of IFN-I cytokines by pDCs in both humans and mice (Stout-Delgado et al., 2008; Panda et al., 2010; Sridharan et al., 2011; Agrawal, 2013; Agrawal et al., 2017), and the formation of Tfh cells (Garcia and Miller, 2001; Eaton et al., 2004; Lefebvre et al., 2012; Linterman, 2014; Gustafson et al., 2018; Nikolich-Žugich, 2018).
In addition, Brahmakshatriya and colleagues have demonstrated that transferring activated, in vitro BM-derived DCs into aged mice can boost both the GC and Tfh cell response upon immunisation (Brahmakshatriya et al., 2017). Our study demonstrates that age-associated defects in the early induction of IFN-I expression, probably by pDCs and macrophages, results in impaired expression of co-stimulatory molecules on cDC2s.
The impact of reduced IFN-I stimulation on cDC2s in aged mice can be multifactorial: in DCs, IFN-Is promote the expression of co-stimulatory molecules and the cytokines IL-6, IL-27 and IL-1β (Luft, 1998; Moretto et al., 2008; Cucak et al., 2009; Batten et al., 2010; Gringhuis et al., 2014; Hassanzadeh-Kiabi et al., 2017).
These molecules have been shown to enhance the expression of Bcl6, CXCR5 and ICOS by CD4+ T cells, thereby supporting differentiation towards the Tfh cell phenotype (Cucak et al., 2009; Barbet et al., 2018). In particular, IL-6 production is reduced in DCs from aged mice (Brahmakshatriya et al., 2017), which may act together with reduced co-stimulation to contribute to poor Tfh cell priming in aged mice.
In an attempt to enhance cDC2 and Tfh cell responses upon vaccination, we applied a cream containing the TLR7-agonist imiquimod, which has been shown to induce IFN-Is (Chen, 1988; Bottrel et al., 1999; Sauder, 2003) to the skin of mice upon immunisation.
Treatment with the TLR7 agonist imiquimod at the time of immunisation restored IFN-I signalling in cDC2s in aged mice, increased their numbers in the draining lymph node and enhanced CD80/CD86 expression on their surface.
This demonstrated that IFN-I is an important signalling pathway to target to improve cDC2 functions in ageing. However, IFN-I signalling in cDC2s was not uniquely required for the imiquimod-mediated increase in Tfh cell numbers in young mice.
This suggests that other, non-IFN driven effects of imiquimod, such as IL-6 production, can support Tfh cell formation (Schön and Schön, 2007; Walter et al., 2013). Alternatively, it may be that the induction of IFN-I signalling in cells other than DCs, such as B cells or Tfh cells themselves is sufficient to promote Tfh cells (Le Bon et al., 2006; Hervas-Stubbs, 2011; Nakayamada et al., 2014; Riteau et al., 2016; Li et al., 2018).
Together, this indicates that the TLR7 agonist imiquimod can boost cDC2 and Tfh cells using more than one mechanism, reinforcing its potential as a vaccine adjuvant.
In our experiments, imiquimod treatment rescued the age-dependent defects in cDC2s and Tfh cell differentiation but did not fully restore GC B cell numbers or early antigen-specific antibody responses in aged mice to levels observed in younger adult mice.
The most obvious hypothesis to explain a poor GC response is that ageing results in cell-intrinsic defects in the GC B cells themselves. However, adoptive transfers of aged B cells into young hosts show that B cells from aged mice are capable of forming GCs in a young environment, suggesting that a B cell-intrinsic defect is not the cause of the poor GC response in aged mice (Yang et al., 1996).
These data therefore implicate B cell-extrinsic factors as additional contributors to the poor GC response in ageing. It is noteworthy that both T follicular regulatory (Tfr) cells and FDCs have been linked to the age-dependent diminution of the GC response. The GC response is negatively regulated by Tfr cells (Stebegg et al., 2018), that are reported to be increased in number in aged mice and this overrepresentation of Tfr cells may result in excessive suppression of the GC response in older animals (Sage et al., 2015).
There is also evidence that FDCs, stromal cells which are essential for the maintenance of the GC, are impaired in ageing (Wang et al., 2011). FDCs in aged mice form smaller networks and present fewer antigen-containing immune complexes on their surfaces after immunisation (Aydar et al., 2003; Turner and Mabbott, 2017).
This is likely to affect the ability of B cells to capture antigen for presentation to Tfh cells, which in turn provide B cell growth and differentiation cues. This suggests that the age-associated defect in GC B cell expansion in mice is linked not only with a defect in T cell priming but also with other factors such as reduced antigen retention on FDCs and increased suppression by Tfr cells.
Several strategies are currently being used to enhance the response to vaccination in older persons, including modifications of adjuvants (Frech et al., 2005) or administration of increased antigen doses (Remarque et al., 1993). Hung and co-workers have shown that topical imiquimod treatment at the time of vaccination enhances the antibody responses to influenza vaccination in both younger and older persons (Hung et al., 2014; Hung et al., 2016).
We have previously shown that the poor gut GC response in aged mice can be boosted by replenishing the gut microbiome with that of a younger animal (Stebegg et al., 2019). Together with the data presented here, this demonstrates that age‐related defects in the GC response are not irreversible and can be targeted therapeutically to improve immune responses in older individuals. B
ecause imiquimod can correct defective IFN-I signalling and the associated cellular defects in cDC2s, and can also boost Tfh cell formation in aged animals, this compound could play a key role in improving T-dependent vaccine responses, especially in the older members of our communities.
More information: Marisa Stebegg et al, Rejuvenating conventional dendritic cells and T follicular helper cell formation after vaccination, eLife (2020). DOI: 10.7554/eLife.52473