*4.2.4 Oral polio vaccine*

There are currently only three countries where polio remains endemic. Thus, polio-free, high income countries are introducing the use of the inactivated polio vaccine (IPV). However, there are still many countries that use the live-attenuated oral polio vaccine (OPV). Despite current WHO policy to replace OPV by IPV, there is epidemiological evidence that supports that replacing OPV by IPV might have an impact on overall mortality [96], since OPV has shown strong non-specific beneficial effects even in settings where the incidence of the targeted infection is low. In this regard, campaigns to eliminate polio in West Africa have been associated with lower child mortality rates [127].

#### **4.3 Inactivated vaccines**

As pointed above, most of the vaccines described so far showing non-specific heterologous effects contain live-attenuated microorganisms. Nevertheless, fully inactivated bacterial vaccines have also been described conferring protection against viral infections, and some of them for a fairly long period of time. Interestingly, these vaccines are mucosal preparations that are administered daily for long periods of time (weeks/months) rather than single, or seldom, doses used in live attenuated vaccines. Thus, it seems that the much longer administration of these inactivated mucosal vaccines resembles the effect achieved by live vaccines on heterologous protection associated to trained immunity (**Figure 3**).

#### *4.3.1 Polybacterial whole-cell vaccines*

These vaccines are used for the prevention of recurrent infections in susceptible subjects, mainly associated to the respiratory and urogenital tracts [128–134]. Since they target infections occurring in these tracts, their administration is generally through mucosal tissues to obtain a better mucosal response [135, 136].

#### **Figure 3.**

*Trained immunity window by self-replicating and inactivated TIbVs. Trained immunity-based vaccines (TIbVs) containing live-attenuated self-replicating microorganisms (e.g. BCG) may require fewer administrations to induce an adequate trained immunity window of sufficient intensity, quality and/or duration than vaccines with dead microorganisms. Fully-inactivated TIbVs can be enhanced to induce trained immunity with a multiple dose schedule (e.g. MV130).*

MV130 is a sublingual vaccine used to prevent recurrent respiratory tract infections [128, 129] containing inactivated whole-cell bacteria that are common pathogens in the airways. Its ability immunomodulating DCs has been addressed experimentally *in vitro* and *in vivo*. MV130 triggers the release of cytokines ascribed to trained immunity in different setting, including TNF-α, IL-1β and IL-6 [103, 137, 138]. Sublingual immunization of mice with MV130 induces a systemic Th1/Th17 and IL-10 enhanced responses against unrelated antigens [103]. Similar enhancement was shown in patients treated with MV130 where an increased T cell response to flu antigens were described [128]. MV130 was successfully used in infants with recurrent wheezing, a condition triggered in most cases by viral infections. It is noteworthy that the protective effect was also shown 6 months after discontinuation of treatment, which points to a longlasting effect that fits with the memory ascribed to trained immunity (Nieto et al., under review). In this regard, MV130 has been shown to induce trained immunity and to confer protection against experimental virus infections (Brandi et al., under review). Recent studies have assessed the clinical benefit of MV130 as a TIbV in the context of recurrent respiratory infections in vulnerable populations such as patients with different primary and secondary immunodeficiencies showing a reduced rate of respiratory infections [130, 139] (Ochoa-Grullón et al., in press).

#### *4.3.2 Polybacterial lysates*

Although not considered vaccines but immunostimulants, these bacterial preparations are, like MV130, used for the prevention of recurrent respiratory infections. OM-85, one of the best studied, is composed of chemically treated bacterial lysates for oral administration, acting through the gastro-intestinal mucosa. OM-85 has been shown to be effective in experimental viral infections [140] and in children with recurrent wheezing [141], a condition triggered by viruses as noted above. OM-85 stimulates the release of proinflammatory cytokines such as IL-1β, TNF-α and IL-6 by macrophages [142], typical of trained immunity induction, as well as Th1 cytokines including IFN-γ [143]. It is not known, however, the role of trained immunity in their mechanism of protection. A recent study conducted in infants, the observed protection against respiratory infections under OM-85 treatment stopped when treatment was discontinued [144], which may point against the memory ascribed to trained immunity.
