**1. Introduction**

Pandemics and epidemics of infectious origin are large-scale outbreaks that can greatly increase morbidity and mortality globally or over a wide geographic area, respectively [1]. Pandemics have occurred throughout history and appear to be increasing in frequency in the last centuries. Noteworthy examples include the Black Death at the end of the Middle Ages, Spanish flu in 1918, the 2014 West Africa Ebola epidemic or the current COVID-19 pandemic. The direct impact of pandemics on health can be dramatic. These large outbreaks can disproportionally affect younger or active workers, but vulnerable populations such as the elderly are at a particular high-risk. Pandemics can cause acute, short-term as well as longer-term damage to economic growth due to public health efforts, health system expenditures, and aid to affected sectors. Evidence suggests that epidemics and pandemics can have significant social and political consequences too, by debilitating institutions, amplifying political tensions, stigmatizing minority populations, or encouraging sharp differences between social classes [2].

Outbreaks by respiratory ribonucleic acid (RNA) viruses such as influenza or coronaviruses entail the principal threat due to their ease of spreading among humans, their potential severity and recurrence. However, other RNA viruses such as flaviviruses (Zika) or filoviruses (Ebola) must be taken into consideration due to a great overall burden of morbidity and mortality [3]. Antiviral drugs can help mitigate a viral outbreak by reducing the disease in infected patients or their infectiousness. While these drugs can be very successful against some viruses (*e.g*. hepatitis C virus [HCV]) [4], they are not universally effective as exemplified in the current SARS-CoV-2 pandemic [5]. Nowadays, having effective vaccines may be the only tool to reduce susceptibility to infection and thus, prevent the rate of virus spread [2].

Vaccination has dramatically decreased the burden of infectious diseases. Vaccines have saved hundreds of millions of lives over the years [6]. It has been estimated that approximately 103 million cases of childhood diseases were prevented in the United States through vaccination between 1924 and 2010 [7]. The eradication of smallpox in 1980 through vaccination is considered one of the crown accomplishments of medicine. Despite these achievements, effective vaccines have been developed against just over 30 pathogens among bacteria and viruses. There are many pathogens, including viruses such as human immunodeficiency virus (HIV) or respiratory syncytial virus (RSV), for which all efforts for vaccine development have failed so far. In addition, current available vaccines for worldwide important viral diseases like influenza are suboptimal, especially in the elderly, resulting in vulnerability among billions of at-risk populations [6]. On the other hand, having a new effective and safe vaccine in time to control highly contagious emerging viruses that cause epidemic or pandemic threats is an almost impossible task considering the timeframes for vaccine development. This includes preclinical and clinical research, its approval by the regulatory authorities, as well as its production and distribution [3].

Altogether, it has been postulated that one possibility of filling the gap between the appearance of a viral outbreak by an emerging pathogen and the availability of a specific vaccine is to take advantage of the heterologous protection of some existing vaccines, in order to increase the non-specific resistance of the host through trained immunity [8, 9].

### **2. Specific anti-infectious vaccines**

Conventional (specific) anti-infectious vaccines are biological preparations containing live-attenuated or dead microorganisms, their antigens or nucleic acids encoding for them, designed for specific pathogens. The purpose of vaccination is to induce a long lasting adaptive immune response against key antigens able to confer host resistance for future encounters with the corresponding pathogen. Either the production of antibodies, generation of T helper/effector cells, or both, may play a critical role in such a resistance, which greatly depends on the type of pathogen, the route of entrance and the host-pathogen relationship (*e.g.,* extracellular and/or intracellular) [10]. Successful vaccines are highly effective not only in inducing long-lasting immunity against disease-causing pathogens, but also in providing herd immunity to the community that substantially restricts the spread of infection [6].

Most of the vaccines available today have been developed empirically and used successfully long before their mechanism of action on the immune system was understood. Early protection is associated to induction of antigen-specific antibodies, being their quality (avidity, specificity, or neutralizing capacity) key factors for

#### *Trained Immunity-Based Vaccines: A Ready-to-Act Strategy to Tackle Viral Outbreaks DOI: http://dx.doi.org/10.5772/intechopen.95765*

their efficacy. Long-term protection relies on the persistence of vaccine antibodies and availability of immune memory cells capable of rapid and effective reactivation with subsequent microbial exposure. On the other hand, T cells have a critical role in the induction of high affinity antibodies and immune memory. Furthermore, T cells have a direct role in protection conferred by some vaccines, including the tuberculosis Bacille Calmette-Guérin (BCG) vaccine [11].

Vaccines using whole pathogens have been classically classified as either live attenuated or inactivated (killed). Subunit vaccines contain just selected antigens (*e.g.,* proteins, polysaccharides). Recently, due to a growing availability of bioinformatics and sequencing tools, there has been an increase interest on so-called "rational" vaccine design approaches for subunit vaccines, such as the reverse vaccinology [12]. In this regard, modern vaccines include recombinant proteins or nucleic acids [13]. Rather than administering the antigen itself, DNA and mRNA vaccines targeting dendritic cells (DCs) encode the antigen of interest that will be produced by the vaccinated host, representing a new era in vaccinology [14]. In fact, the first RNA vaccine licensed for humans in Western countries has been recently developed for SARS-CoV-2.

As commented before, a vaccine response is linked to the induction of T and B cell specific responses to the antigens contained in the vaccine. This requires lymphocyte activation, proliferation and differentiation on specialized lymphoid tissues (*e.g* lymph nodes), where antigen presenting cells, like DCs for T cells or follicular dendritic cells (FDCs) for B cells, are present. Mature DCs are recruited into the T cell areas of lymph nodes from the periphery, *e.g.,* at the site of injection of the vaccine. DCs express pattern recognition receptors (PRR) that recognize evolutionary conserved pathogen-associated molecular patterns (PAMPs) that are not contained in self-antigens and are identified as "danger signals" [15]. When immature DCs are exposed to the vaccine-derived antigens at the site of vaccination, they uptake them and become activated [16]. This activation will lead to their maturation with the expression of homing receptors at their surface, triggering DC migration to the draining lymph node through afferent lymphatic vessels, where the activation of T and B lymphocytes will occur. Mature DCs process the up-taken antigens and present them to naïve T cells associated to molecules of the major histocompatibility complex (MHC) within the T cell areas of lymph nodes. On the other hand, unprocessed native antigens, either free or complexed with antibodies or complement, access the B cell areas of lymph nodes (lymphoid follicles) where they are captured by FDCs and displayed from their cell surface to the B cells. Antigen-specific B cells will rapidly proliferate forming a germinal center and differentiate into plasma cells producing low-affinity immunoglobulin (Ig) M antibodies. The B cells will then receive additional signals from activated T cells, undergoing isotype antibody switch from IgM to IgG or IgA and affinity maturation of the antibodies produced.

For a vaccine to be immunogenic enough, DC activation, that can be achieved by adjuvants, is essential. Live attenuated and inactivated whole-cell vaccines are considered "self-adjuvanted" as they naturally present sufficient PAMPs to activate innate immune cells, including DCs; thus, promoting a robust antigen-specific immune response. In contrast, subunit vaccines generally require different types of adjuvants to enhance and/or drive the immune response in the desired direction [15, 17].

#### **2.1 Difficulties for novel specific vaccines in a viral outbreak situation**

Viral outbreaks appear when there is a sufficient number of susceptible individuals within a nearby population. Although susceptibility is a balance between host factors (high/low resistance) and pathogens (high/low virulence), in many cases it reflects a

lack of prior contact with a given pathogen. In general, this is related to the emergence of new viruses or the lack of effective vaccines against known viruses. As pointed above, the development of effective vaccines is not an easy task against certain viruses. We are still lacking vaccines for some of the most lethal viral infections, including HIV and MERS-CoV, among others. These pathogens are difficult to tackle, as we do not fully understand their mechanisms to evade the immune system or how to elicit protective immunity against them [13]. However, encouraging progress is being made against these pathogens and there are currently several "pipeline vaccines" in development, such as RSV, universal influenza vaccine, and SARS-CoV-2 [18–20]. Apart of SARS-CoV-2 for obvious reasons in the current pandemic, there is an urgency to have a universal influenza vaccine that provides a broad and durable protection from influenza virus infection. Yet, the high level of antigenic diversity and variability, and antigenic drift in the surface antigens, enable these viruses to escape antibody-mediating neutralization [21]. On the other hand, there is a number of vaccines currently licensed, including the influenza A virus vaccine, that provide incomplete protection, especially in high-risk groups [22]. Mumps outbreaks observed in Ireland, United Kingdom and United States in vaccinated subjects with Measles Mumps Rubella (MMR) vaccine is another example [23]. Different factors have been postulated to contribute to mumps outbreak, including waning immunity and primary and secondary vaccine failure. Yet, their actual contribution is not fully understood [23].

Vaccine efficacy must consider different target populations as well. Adaptive immune response to vaccines may be limited in newborn and the elderly. Early in life, immune responses are dampened compared to adults [24, 25]. Neonates have underdeveloped germinal centers in lymph nodes and the spleen, and low expression of B cell receptors which in turn results in low levels of primary IgG responses to infections and vaccines [26]. As we age, our immune system undergoes agerelated changes that lead to progressive deterioration of the innate and the adaptive immune responses, this is termed immunosenescence. The most common features of immunosenescence are short-lived memory responses, impaired response to new antigens, increased predisposition to autoimmune diseases and low-grade systemic inflammation (*inflammaging*) [27, 28]. Immunosenesence results in increased susceptibility to infections and deficient response to vaccination causing high hospitalization and mortality rates. For example, influenza vaccine efficiency has been reported to be 17–53% in the elderly, compared with the 70–90% efficacy in young adults [29]; and vaccination with Varicella zoster virus (VZV), also an important pathogen in elderly people, only partially prevents reactivation of herpes zoster [27].

If the difficulties listed above are outlined for existing or developing vaccines, quickly obtaining an effective vaccine to urgently control a new virus outbreak is almost an impossible task in the short-term as pointed above. This is well exemplified by the SARS-CoV-2 vaccine race pushed by the devastating COVID-19, with more than 100 vaccine candidates in the running. It is considered that no less than 1 year will last the time until the first licensed vaccine can provide protection in the best scenario [30]. This, in spite of greatly shortening the usual clinical development time and regulatory obstacles for a new vaccine and, therefore, without knowing its true performance and/ or safety in the medium term compared to other authorized vaccines [31].

## **3. Trained immunity and infections**

It has become evident from epidemiological, clinical and experimental data that some conventional whole-cell vaccines, like BCG and others, also provide resistance to infectious diseases not related with the specific pathogen targeted by the vaccine [32–34]. Much of these non-specific "heterologous" effects appear to depend on

#### *Trained Immunity-Based Vaccines: A Ready-to-Act Strategy to Tackle Viral Outbreaks DOI: http://dx.doi.org/10.5772/intechopen.95765*

the activation of innate immune cells by the PAMPs contained naturally in these vaccines [10], although other mechanisms such as cross-reactive epitopes between different pathogens could also account for this protection in some cases [35].

Immunological memory, understood as the ability to "remember" past encounters with pathogens, has been classically attributed to the adaptive branch of the immune system exclusively, by virtue of the antigen-driven clonal expansion of T and B lymphocytes and exemplified by the mechanism of conventional specific vaccines pointed above. However, the notion that innate immunity was unable to induce immunological memory has been challenged in recent years, particularly from studies in organisms that lack adaptive immunity, such as plants or invertebrates, as well as early studies in mice lacking the adaptive immune system [8, 36]. Altogether, the term 'trained immunity' was coined to define an innate immune memory that lead the innate immune system to an enhanced response to secondary challenges [37]. Importantly, trained immunity seems to be underlying the heterologous effects of an increasing number of vaccines [38–40].

## **3.1 The concept**

*What is trained immunity? -* Trained immunity is defined as the memory of the innate immune system, where an encounter with a first stimulus (*e.g.* a microbial insult) results in a subsequent long-term adaptation and enhanced non-specific response by innate immune cells against a secondary challenge (the same or unrelated), thus providing non-specific, broad-spectrum, long-term protection in case of infection [8, 9, 37, 41].

*Which cells can be trained? -* Trained immunity properties have been defined for distinct cell subsets of the innate immune system [9, 42], including natural killer (NK) cells and innate lymphoid cells [43]. Of note, training of myeloid cells [42], particularly monocytes and macrophages [44, 45], and more recently DCs [46, 47] and hematopoietic stem cells [48], have been extensively studied. Finally, the acquisition of this immunological memory has also been demonstrated to a lesser extent for non-immune cells [49].

*How to get trained? -* A wide variety of stimuli can train innate immune cells, particularly when considering monocytes and macrophages [9, 50]. Among infectious agents, live microorganisms such as the tuberculosis vaccine BCG [51], *Candida spp* [52] or viruses [53, 54]; bacterial components, such as flagellin, lipopolysaccharide, muramyl dipeptide [55], fungal components as β-glucan [52] or even helminth products [56]. In general, microbial ligands engaging some PRR, like C-type lectin receptors (CLRs), nucleotide-binding oligomerization domain-like receptors (NLRs) are well established training inducers, whereas those engaging toll-like receptors (TLRs) may have opposite effects depending on the TLR type and concentration [55, 57]. Intriguingly, not only infectious agents but also endogenous inducers and metabolites such as oxidized low-density lipoprotein or mevalonate can induce trained immunity [50].

*What hallmarks define trained immunity?* - In contrast to adaptive immune responses, epigenetic reprogramming of transcriptional pathways — rather than gene recombination — mediates trained immunity. This training phenomenon comprises three key hallmarks that occur at the intracellular level: increased cytokine production upon rechallenge, changes in the metabolism and epigenetic reprogramming [9, 58, 59], which eventually support increased protection upon infection.

Among those cytokines whose production is augmented after re-exposure in trained cells, proinflammatory molecules such as tumor necrosis factor α (TNF-α), interleukin (IL)-6, IL-1β and interferon γ (IFN-γ) are fairly constant [45, 52, 55, 60, 61]. Modulation of IL-10 varies between studies [45, 52, 56, 62, 63]. A noted

shift from oxidative phosphorylation to aerobic glycolysis (Warburg effect) is the main change in cellular metabolism during the induction trained immunity [64]. Moreover, glutaminolysis, cholesterol synthesis and the tricarboxylic acid cycle are non-redundant pathways required for trained immunity to take place [64, 65]. Epigenetic reprogramming, mainly mediated by histone modifications, is one of the bases for the long-lasting effect of trained immunity [8, 66–68]. Immune pathway activation and changes in metabolism serve as basis for epigenetic rewiring [65]. As a result, epigenetic modifications have been found at the level of important promoters for the training process, which makes chromatin more accessible and conditions gene expression patterns of trained cells upon stimulation with a secondary challenge [69].

As a result of the whole process, enhanced, broad-spectrum, non-specific protection mediated by innate immune cells is found upon infection. This crossprotection has been observed for a wide range of human pathogens including fungi [51, 52], parasites [70, 71] and different bacterial infections [72–75]. Importantly, induction of trained immunity has been proved to be effective against viral infections including yellow fever [76], influenza A virus [77] and others [78, 79]. In this line, the induction of this phenomenon has been also proposed as a tool for reducing susceptibility to emergent SARS-CoV-2 infection, as will be described at the end of the chapter [78, 80].

*How long does trained immunity last?* – Trained immunity phenotypes have been observed for months and up to one year after the training insult. This was initially controversial, as trained immunity properties had been attributed to short-lived myeloid cells such as monocytes or DCs [38]. In this regard, several studies have shown that modulation of bone marrow progenitors is also an integral component of trained immunity, supporting the long-lasting effect of this phenomenon [9, 81]. In this way, trained immunity inducers [82–85] would be able to reprogram and induce expansion of hematopoietic progenitors with a particular bias to the myeloid lineage. Thus, bone marrow-derived mature cells would be also trained [86], showing improved clearance of infection [83].

Complementary to progenitor reprogramming, peripheral trained immunity induction would take place in tissue-resident cells [9]. This is especially relevant at the mucosal level, where cells encounter most of the infectious training inducers. Alveolar macrophage (AM) memory was demonstrated following viral infection [87, 88]. Training of these long-living cells led to increase antimicrobial properties, independently of systemic immunity [87, 89]. This local training of AM was further reproduced following respiratory mucosal administration of tuberculosis vaccine, being crucial for *Mycobacterium tuberculosis* clearance [90]. On the other hand, training of NK cells lead to long-lived, self-renewing, stable expanded cells with memory-like properties, both in an antigen-dependent or independent manner [91–93]. Finally, it was also reported that self-renewing long-living skin epithelial stem cells exhibited local trained immunity, providing faster wound healing in primed mice than in naïve mice [94, 95].

#### **3.2 Trained immunity on ongoing immune responses**

### *3.2.1 Effect on adaptive immunity*

Non-specific effects of vaccines have been extensively studied and reported over the last decades. Although trained innate cells could partially account for these effects, involvement of adaptive immunity has also been suggested [96]. An adaptive immune mechanism of non-specific effects could be heterologous immunity; vaccine antigens can give rise to T cell cross-reactivity against other antigens that may confer some protection against unrelated pathogens [96, 97].

#### *Trained Immunity-Based Vaccines: A Ready-to-Act Strategy to Tackle Viral Outbreaks DOI: http://dx.doi.org/10.5772/intechopen.95765*

However, innate immune cells constitute the bridge between the intrusion of microbial threats and the activation of adaptive immunity. As said before, following sensing of pathogens by PRRs, activated innate immune cells secrete different factors and act as antigen-presenting cells (APCs) to initiate activation of adaptive immunity [98]. Thus, it would not be unexpected that trained innate immune cells, within their acquired enhanced properties, would be able to induce stronger adaptive immune responses [39]. In this regard, BCG vaccine, a well-known trained immunity inducer, has shown to enhance the antibody titer and alter heterologous T cell responses against a wide range of vaccines and unrelated infections [99–101]. In different experimental models, BCG-mediated protection against viral and *Plasmodium* infections was abrogated in the absence of T cells. In these models, BCG vaccination has been mainly associated with modulation of CD4+ T helper (Th) 1 responses. Similar observations have been found in different clinical studies [99]. Of note, BCG vaccinated human volunteers displayed a long-lasting heterologous Th1 and Th17 response upon stimulation with unrelated pathogens and TLR-ligands [38]. To some extent, similar observations have been found in other vaccines such as diphtheria-tetanus-pertussis (DTP) or measles vaccine [99].

As said before, trained immunity properties have been recently described also for DCs. As being the most professional APCs, they emerge as crucial bridge for potentiating adaptive immune responses. In this sense, DCs with high immunostimulatory properties that enhance adaptive immune responses via IL-1β release had been described [102]. More recently, programmed memory DCs have shown to increase Th1/Th17 immunity and confer protection during cryptococcosis [46]. Finally, different polybacterial preparations of whole-cell inactivated bacteria, have shown to prime DCs and induce enhanced Th1, Th17 and IL-10 T cell responses against related and unrelated stimuli [103, 104]. This capability of modulating heterologous T cell responses by APCs have been also described to suppress pathogenic T cell immunity in experimental models of autoimmune encephalomyelitis [56].
