Preventive effects of prenatal administration of OM‐85/BV on asthma and respiratory infection risk in the offspring: A review of animal models

Asthma is the most common chronic disease in childhood affecting the daily lives of many patients despite current treatment regimens. Therefore, the need for new therapeutic approaches is evident, where a primary prevention strategy is the ultimate goal. Studies of children born to mothers in farming environments have shown a lower risk of respiratory infections and asthma development. Already at birth, these newborns have demonstrated accelerated maturation and upregulation of host defense immune functions suggesting a prenatal transplacental training of the innate immune system through maternal microbial exposure. This mechanism could possibly be utilized to help prevent both respiratory infections and asthma in young children. Human studies exploring the potential preventative effects of pregnancy bacterial lysate treatment on asthma and respiratory infections are lacking, however, this has been studied in experimental studies using mice through administrations of the bacterial lysate OM‐85. This review will present the current literature on the immunomodulatory effects relevant for respiratory infections and asthma in the offspring of mice treated with OM‐85 throughout pregnancy. Further, the review will discuss the cellular and molecular mechanisms behind these effects. In conclusion, we found promising results of an accelerated immune competence and improved resistance to airway challenges as a result of prenatal bacterial lysate treatment that may pave the way for implementing this in human trials to prevent asthma and respiratory infections.


| INTRODUC TI ON
The airways are constantly exposed to inhaled antigens from the external environment during respiration.Most of these antigens are harmless, while some are pathogenic and require a sufficient immune response to uphold the survival of the host.The constant discrimination between harmless and pathogenic agents is crucial, as the consequences of too frequent elicitation of the immune responses can result in chronic airway inflammation and damage to the vital function of the lungs.When the immunological homeostasis is out of balance asthma can be the result. 1 In children, this presents as coughing, wheezing, shortness of breath, and increased airway resistance, 2 resulting in a marked impact on health in affected children, as emphasized by asthma ranking among the top 20 conditions worldwide for disability-adjusted life years in this population group. 3Childhood asthma is recognized as the most common chronic disease in children, 2 and despite attempting to control the symptoms with inhalation of corticosteroids, some children still experience severe complications. 4 This makes the need for new approaches evident, while primary prevention of the disease is desirable.
Although the complex immunological pathogenesis of asthma is not fully understood, early asthma symptoms during pre-school age are often caused by airway infections including respiratory syncytial virus (RSV) that also associated with increased asthma risk later in life, whereas the more prevalent allergic asthma in schoolage and beyond is often triggered by airway allergens and are considered to be more genetically predisposed. 5,6The neonatal period has shown to be a critical window of opportunity for the prevention of respiratory infections and asthma through the maturational immunomodulatory response to the environmental stimuli in this period. 5,7This essential time span seems to be further extended to the prenatal period, as studies revealed that children of mothers in farming families, who had continuous microbial exposure throughout their pregnancy, already at birth demonstrated upregulation of host defense functions usually seen postnatally and accelerated maturation from the Th2-skewed immunophenotype, 8,9 suggesting a transplacental training of the innate immune system. 8,10Utilizing this transplacental immune training could possibly help prevent both respiratory infections and allergic and non-allergic asthma.
While human trials are still lacking, studies to further understand the potential effects of prenatal microbial exposure on the development of respiratory infections and asthma have been conducted in pregnant mice, using the bacterial lysate OM-85.2][13] Based on a literature search, we will provide an overview of the available evidence on the immunomodulatory effects relevant for respiratory infections and asthma in the offspring of mice treated with OM-85 throughout the pregnancy, and discuss the cellular and molecular mechanisms behind these effects.

| Search strategy
A literature search was performed using the PubMed online database on 10 June 2023.Initially, a MeSH (Medical Subject Headings) search was performed.Since bacterial lysate or equivalents of this, is not yet available in the MeSH term repertoire, no results showed up (#1).In the attempt to broaden the search, the MeSH search was followed by an All Fields search (#2), and more search terms were added with the function "OR" to expand the search (#3).In the process of reading through the abstracts and articles, it became clear that asthma is not necessarily a key word, as the search on OM-85 or Broncho-Vaxom implies that the prevention is aimed at respiratory inflammatory diseases and infections.Therefore, an additional search was performed without the asthma term (#4); see Table 1.

| Study selection and quality assessment
The results of searches #2, #3, and #4 were assessed and duplicates were removed.The remaining articles were then evaluated for relevance for the predefined research question according to the following inclusion and exclusion criteria.The assessment of eligibility was performed by two authors, and disagreement was resolved by consensus.The relevant articles were narrowed down by first reading titles, then abstracts, and eventually full-text articles.

| Inclusion
Written in English, Administration of OM-85/BV in pregnant mice, outcome evaluated in the offspring, outcome measured

Key message
The studies presented in this review point toward promising results of prenatal bacterial lysate treatment as primary prevention, which could pave the way for physicians implementing this in human trials against asthma and respiratory infections.
immunomodulatory effects with relevance for development of asthma, allergies, or respiratory infections.

| Exclusion
Publication not available in online databases, in vitro studies, not original research articles, administration of other types of bacterial lysates than OM-85/BV, administration of OM-85/BV to human children or adults for secondary prevention, administration of OM-85/ BV to human infants for primary prevention.

| Data extraction
All nominally significant results (p < .05)relevant to the research question have been extracted.Because most of the articles presented their data through graphs, numerical values have been read of these.The comparable values have been presented against each other in Tables 2, 3, and 4. The main search resulted in 61 studies of which four studies met the eligibility criteria.The process is summarized in the flow chart (Figure 1).The main trends for the excluded articles were: Administration of other types of bacterial lysates to prevent other types of infectious/inflammatory disease (primarily urinary tract infections), administration of OM-85/BV to human children/(non-pregnant) adults for secondary prevention, administration of OM-85/BV to human infants for primary prevention.Four studies were included and the characteristics are summarized in Table 2. Due to heterogeneity in existing studies and a low number of studies in total, we present each study in detail in the following section followed by an overall discussion.

| MIN CHAM K T E T AL . (2 018)
Mincham KT et al. 14 conducted a study in pregnant mice treated with OM-85, where the main purpose was testing the possible enhancement of immunocompetence and protection against allergic airway inflammation in the offspring during the postnatal weaning period.This was done by daily orally administering OM-85 to the time-mated pregnant mice through gestation day (GD) 9.5-17.5.
Controls were left untreated.An experimental model of allergic airway inflammation was created and challenged as summarized in Figure 2. Fetal bone marrow (fBM) tissue was collected at GD 18.5.
Three-week-old offspring were sacrificed and bronchoalveolar lavage (BAL)-fluid, lungs, and hind legs were collected.Six-week-old offspring were sacrificed 24 hours after the last challenge.BAL-fluid, lungs, hindlegs, airway draining lymph nodes, and blood were collected from the 6-week-old offspring.

| Baseline characteristics of the allergic inflammation model
The model showed to mirror some of the hallmark features of allergic asthma by intense infiltration of inflammatory cells in the respiratory tract, increased levels of Th2 cytokines in lung tissue, and increased airway resistance in response to methacholine (MCh) inhalation.

| Effects of maternal OM-85 treatment in offspring airways
The prenatally exposed offspring overall demonstrated less susceptibility to the ovalbumin (OVA)-induced allergic airway inflammation.The infiltration of inflammatory cells and hyperresponsiveness to MCh were significantly reduced.Upon challenge, an intensification of the decrease of Tregs in the airway draining lymph nodes and increase in the airway mucosa, was seen.Moreover, the Tregs found in airway tissue, showed higher relative expression of the functions-associated molecules (FAM) CD25, CTLA-4, FoxP3, and proliferation-associated markers Ki67 and CD69.The level of conventional dendritic cells (cDCs) in the airways was generally reduced.While the MHC-II (IAIE) expression in the trachea and airway draining lymph nodes was attenuated upon challenge, there was an increase in IAIE expression in DCs in lung tissue.The OM-85 effects on cellular responses in lung tissue are summarized and compared in Table 3.

Y Li et. al (China, 2020)
To evaluate the effects of a high microbial load in pregnancy on experimentally induced asthma in the offspring using BTT and Broncho-Vaxom.The pregnant mice were administered 1 mg/ kg body weight Broncho-Vaxom from 10 days before mating until birth.The asthma models were created as seen in Figure 2 Weight, mortality, and sex ratio.

| Effects of maternal OM-85 treatment in offspring BM
The immunological changes in the respiratory tract were paralleled by changes in the bone marrow.At the challenge, the BM of offspring of untreated mothers showed expansion of all precursor stages spanning from myeloid progenitor (MP) to granulocyte-macrophage progenitor (GMP) and macrophage-dendritic cell progenitor (MDP), and depletion of the pre-cDCs and cDCs.Both reactions were significantly attenuated in offspring from treated mothers.The OM-85 effects on cellular responses in BM are summarized and compared in Table 4.

| Effects of maternal OM-85 treatment in offspring at earlier stages
The 3-week-old mice of treated mothers showed increased levels of cDCs in lung tissue, as well as higher levels of expression of IAIE and higher cDCs yields of the BM.Similarly, analysis of the fBM showed

| Broncho-Vaxom treatment effects in maternal tissue
Increased levels of Tregs and expression of TLR2/4 were seen in the intestinal epithelium and increased levels of Tregs in the spleen.

| Safety evaluation of Broncho-Vaxom in pregnancy
No significant differences in maternal and offspring weight, neonatal mortality, and sex ratio were found.In summary, the study proposes the safe use of Broncho-Vaxom in pregnancy and reduced inflammatory response in prenatally exposed offspring.

| MIN CHAM K T E T AL . (2 02 0)
Mincham KT et al. 16  Fetal hind legs were collected and prepared for flow cytometry and RNA-sequencing.

| Effects of maternal OM-85 treatment on the cellularity of the fBM
A significant increase was seen in the cellularity of the fBM, especially the cDCs, which also showed enhanced expression of IAIE.
Additionally, a significant increase in the DC progenitor populations (MP, GMP, and MDP) was seen.

| Effects of maternal OM-85 treatment on gene expression in the fBM
The analysis disclosed that 152 genes were expressed differently, compared to the untreated group.The genes were involved in mul- In summary, the study indicates that prenatal OM-85 exposure facilitates accelerated expansion and maturation of the myeloid compartment of the fBM and upregulation of molecular pathways likely in-part responsible for the increased myelopoiesis.

| Effects of maternal OM-85 treatment in offspring without infection
An increased rate of maturation in the myeloid respiratory compartment was seen in exposed offspring, represented by the accumulation of plasmacytoid dendritic cells (pDCs), macrophages, monocytes, Tregs, and myeloid-derived suppressor cells (MDSC) in the lung.An upregulation of IAIE on DCs was seen during PND 3-7.
A significant upregulation in the expression of caspase-1, IL-1α, and IL-1β, was seen at PND 22, while NLRP3 was found to be upregulated at all time points.Furthermore, the treated group showed an increased baseline expression of TLR4 and TLR7.

| Effects of maternal OM-85 treatment in offspring upon infection
Maternal OM-85 treatment resulted in a significant 100% protection the study suggests that prenatal OM-85 exposure enhances the immunocompetence through maturation of myeloid cells in the lungs, significantly improving resistance toward viral infection without increased airway inflammation.

| DISCUSS ION
The number of available studies on the topic is limited and the four studies included in this review, all evaluated slightly different outcomes under slightly different circumstances.However, similar trends of improved resistance to inflammatory/infectious airway challenges through maturation of DC networks and upregulation of Tregs are seen throughout the studies.The Mincham, 14 and the Lauzon-Joset, 17 study both tested the cellular response in lung tissue at baseline and after inflammatory/infectious challenge in offspring from OM-85-treated mothers.In both studies, the baseline level of DCs (pDCs, cDC1, and cDC2) were increased in the treatment group and the levels of MHC-II (IAIE) expression on the cDCs were higher.Regarding the Tregs, they showed to be increased at baseline in the Lauzon-Joset study, while no significant changes were seen in the Mincham study.Considering, that the DC networks usually are compromised in the first period of life in both humans 18 and animals, 19,20 the present findings of expansion and maturation of the DC networks in the lungs within the 3 postnatal weeks, indicate an OM-85-mediated accelerated immune competence in the prenatally exposed offspring.
After the inflammatory/infectious challenge was applied to the offspring, the influx of inflammatory cells in the lung tissue 24 h after challenge was reduced in the treated group in both studies.This is also supported by the histopathologic evaluation of lung tissue in Li's study, 15 that also showed a decrease in inflammatory cell infiltration, suggesting a more quiescent immunological response because of OM-85 treatment.In Mincham's study, a significant decrease of both pDCs and cDCs was already seen 24 h post aerosol-challenge in the treated group, while a corresponding change was seen in Lauzon-Joset's study, but this only reached significance 5 days after the inoculation with vMC 0 .Likewise, an increase in number of Tregs available in the lung tissue was seen 5 days after inoculation, while the increase did not reach significance in lung tissue in the Mincham study.However, significant increases in Tregs were seen in trachea and airway draining lymph nodes, and overall, the Tregs in the Mincham study showed increased expression of FAM upon challenge.The OM-85 effect on the availability of Tregs is further emphasized by the increase found in peripheral blood in Li's study.
Held together, the attenuated cDC recruitment and the accelerated maturation of the DCs, could explain the altered activation and recruitment of the Tregs.In combination these changes points toward a more tightly regulated immune state in the lungs of the offspring from treated mothers, equipping them with favorable conditions for a balanced immune response upon challenge.This is consistent with earlier findings of both DC networks and Tregs playing an essential role in controlling the intensity and duration of the response to inhaled allergen, and that ill-adjusted activity in these cell groups can lead to compromised immunological homeostasis in the respiratory tract, as seen in allergic asthma. 1 In both Mincham's and Lauzon-Joset's studies an upregulation of MHC-II and CD86 on the cDCs were seen upon challenge.While this could be suggestive of a beneficial accelerated maturation, previous findings have shown associations between the upregulation of these surface molecules and the airway inflammation seen in allergic asthma.This has been explained by the transition of the cDCs from a simple antigen-surveillance state to a potent active antigenpresenting state within the lungs. 1 In line with this, the Mincham OM-85 treatment is exerted through changes in the BM.This is in line with the mechanistic explanation given through the gut-bone marrow-lung axis, 21,22 proposing that DCs modified by the influence of OM-85 in the BM, migrate to the airway mucosal lining, forming the improved immune surveilling cDCs networks 19 (see pregnancies by late gestation (from GD 16.5), 24 which coincides with the time the bone marrow becomes the primary site of hematopoiesis. 25While it is not known how this cell passage from mother to fetus is mediated, the findings suggest that the maternal cells persist at this site in adult progeny, 24 which may allow maternal cells to have a functional role in the offspring.
When assessing the potential molecular pathways underlying the cellular changes seen in the prenatally exposed offspring, the included studies present multiple changes in signaling molecules and pathways.The Mincham, 2020 study found that prenatal OM-85 exposure upregulates multiple drivers (SREBF1/2, SCAP, and LDLR) central to the pathways involved in the production of membrane lipids in the Golgi, 26 which is required for the continuous proliferation of progenitor cells in the BM, 27 making the expansion of these cell compartments possible.Furthermore, the Mincham 2020 study found a significant upregulation of key components (XBP1, ATF6β, and ERN1) in the UPR pathway, which is classically known to moderate the effects of ER stress. 28However, other studies have found the XBP1-ERN1 signaling axis to also be crucial in DC development, function, and survival, 29,30 suggesting that the maternal OM-85 treatment could exert its immunomodulatory effects in the offspring through upregulation of this pathway.
The Lauzon-Joset study found a significant upregulation in the expression of NLRP3, caspase-1, and IL-1β, suggesting the formation of an active NLRP3 inflammasome complex at this point. 31ile the activation of the inflammasome complex has been associated with the development of chronic inflammatory disease (including asthma) in multiple studies, [32][33][34] the activation of the NLRP3 inflammasome has also been associated with the activation of the immune training XBP1-ERN1 signaling axis, discussed above. 35Also, reduced production of the downstream molecule IL-1β in human neonates, has been associated with the development of early transient asthma. 7Moreover, IL-1β has been associated with the expansion of myeloid progenitors, 36 further suggestive of prenatal OM-85 treatment exerting its effects through this compartment in the BM.
Additionally, the Lauzon-Joset study showed increased expression of the toll-like receptors (TLR) 4 and 7 in the peripheral lung, paralleling the upregulation of TLR4 (and 2) seen in children from farming families, with a reduced incidence of atopy. 9Whereas impaired TLR responses is part of the explanation for the immunological immaturity seen in newborns, 37 the upregulation seen in the study could be part of the explanation of the improved immunocompetence of the maternally OM-85 exposed offspring.This is further plausible, as the differentiation of myeloid cells is dependent on effective TLR-signaling. 38Supporting these results, the Mincham 2020 study also found an indication of TLR4 upregulation in the fBM, as specific micro-RNA (miR-149-3p), which negatively regulate TLR4-expression, is strongly downregulated in the study. 39nsidering the effects of OM-85 on the cytokine levels measured in the offspring, the Mincham 2020 study showed increased levels of IL-5 in the fBM, while the same was seen following infection in lung tissue in the Lauzon-Joset study accompanied by decreases in IFNβ and IFNγ.Considering that IL-5 is known to play a central role in the pathogenesis of asthma, 40 this finding does not align with the overall hypothesis of an asthma preventative effect of maternal OM-85 treatment.As IFNγ is known to inhibit Th2-cell functioning, and thereby decrease the type 2 cytokines, IL-4, IL-5, and IL-13, 41 this could explain the origin of the enhanced IL-5 expression.Regarding IFNβ, defective production in lung tissue has been associated with asthma exacerbations 42 and inhalation of IFNβ has been associated with beneficial effects in preventing asthma exacerbations following cold symptoms. 43owing that the pathogenesis of asthma is complex, in isolation, the present OM-85-induced changes in cytokine expression do not favor anti-asthmatic effects.The promising results of enhanced resistance toward asthma and respiratory viral infection, reviewed here, could provide the foundation for OM-85 treatment during pregnancy in women, whose offspring has a high a priori F I G U R E 3 Schematic overview of the proposed OM-85-mediated effects.
risk of developing asthma.A study in mice found the use of OM-85 safe for both mother and fetus and even significantly reduced the risk for fetal loss and growth restriction following maternal infection with LPS or influenza. 23Li's study supported this, as no differences in weight, mortality, or sex ratio in the offspring were found.Of pertinent notice, a single case-control study of OM-85 use during pregnancy has been conducted in humans to study the teratogenic potential.Of 38,151 pregnant who had newborns without congenital abnormalities and 22,865 in the case group with congenital abnormalities, respectively, 58 women in the control group and 22 women in the case group had received OM-85 treatment during pregnancy.The study showed no detectable risk for teratogenic effects of OM-85 use during pregnancy (OR with 95% CI: 0.6, 0.4-1.1). 44e findings of this review have certain limitations.The primary limitation is the limited number of relevant studies, making the need for further studies evident to make clear conclusions.Second, the heterogeneity of the included studies makes direct comparison challenging as the dose, duration, and day of gestation for the administration of OM-85 differed between studies.Furthermore, the analyzed tissue, age of the offspring, and airway challenges varied.
The OVA-sensitization and -challenge of the offspring were carried out with a few days difference, while the airway challenge in the Lauzon-Joset study was infectious of nature, which could influence the character of the elicited immune response.Moreover, the evaluated cell counts were assessed as percentages of various types of cell pools in the different studies, making it possible to only compare tendencies in increased or decreased expression of the different cells.This all points toward the need for further studies, with comparable study designs to make stronger conclusions.
In conclusion, through a literature search the immunomodulatory effects of prenatal OM-85 treatment relevant for the prevention of respiratory infections and asthma, have been reviewed.Even though the studies on the topic are limited and heterogenous and further studies are needed for exact suggestions, trends of improved resistance to inflammatory/infectious airway challenge were seen.
The OM-85 effects are likely to be exerted through the BM, which The model showed an increase in effector T-cells, regulatory T-cells (Tregs), and dendritic cells (DCs) in all airway tissue upon challenge, with the only exception of a decrease in Tregs in the airway-draining lymph nodes.The changes in DCs in the peripheral tissue following the challenge were paralleled by changes in the precursor populations of the bone marrow (BM).

a 3 . 5 |
distinct increase in total cDCs together with an expansion in the upstream progenitor MDP.Effects of OM-85 treatment in an independent culture of 6-week-old BM The introduction of the archetypal proinflammatory agent bacterial lipopolysaccharides (LPS) directly to a BM culture triggered very high levels of IAIE and CD86 expression on cDCs.This response was moderated in the treated group.Summarized, this study suggests that prenatal OM-85 exposure enhances immunocompetence of the offspring, seen as reduced influx of inflammatory cells, increased Treg activity, accelerated DC maturation, and reduced depletion of mature precursors in the BM.4 | Y LI E T AL .(2 02 0)Y Li et al.15 conducted a study to investigate the effects of a high microbial maternal load through pregnancy on experimentally induced asthma in the offspring.This was achieved by oral administration of OM-85(1 mg/kg) to pregnant BALB/c mice from 10 days before mating until parturition.Controls were treated with phosphate-buffered saline.Twentyone days after birth, the offspring were sensitized and challenged, as summarized in Figure2.OVA-induced sensitization was used, as seen in the Mincham 2018 study.Half of the offspring from the maternal controls were left unsensitized.The offspring were sacrificed 24 hours after the last challenge, and blood, lungs, spleen, and BAL fluid were collected.

4. 3 |
Effects of maternal Broncho-Vaxom treatment in offspring The exposed offspring showed to have thinner airway smooth muscle, decreased inflammatory cell infiltration in lung and reduced OVA-specific IgE in BAL fluid.Increased levels of circulating Tregs were found in peripheral blood.
led a study to map out the changes in gene expression in fetal bone marrow following maternal treatment with OM-85 in pregnancy.Hereby attempting to identify the immune-metabolic pathways behind the accelerated myelopoiesis seen in the offspring.BALB/c mice were time-mated and treated with oral OM-85 through GD 9.5-17.5.Tissue collection was performed 24 hours after the last OM-85 administration.

1 (
tiple pathways: the endoplasmatic reticulum stress response, the unfolded protein response (UPR), cholesterol synthesis, and lipid metabolism.The most upregulated molecular driver in the fBM, was the X-Box binding protein 1 (XBP1), a transcription factor central to the UPR.Coherent with this, other UPR-genes were found F I G U R E 1 Flowchart illustration of the selection process.*One study from 1983 was found through the references.Unfortunately, the article is in French, and the full article was not available online.45 upregulated, including the driver gene endoplasmatic reticulum to nucleus signaling 1 (ERN1) encoding the inositol-requiring enzyme IRE1α) and activating transcription factor 6 beta (ATF6β).Increased levels of XBP1s were found in the cDCs of the fBM.Furthermore, upregulation of driver genes central in the cellular cholesterol homeostasis was found, including sterol regulatory element binding transcription factors 1 and 2 (SREBF1/2), sterol regulatory element binding protein cleavage-activating protein (SCAP), and low-density lipoprotein receptor (LDLR).
performed a study in pregnant mice with oral OM-85 treatment, to investigate the immune changes in the offspring and their resistance to a lethal respiratory viral infection.This was done by daily administered OM-85 trough GD 0.5-17.5 to pregnant female BALB/c mice.Controls were left untreated.Two days after parturition the neonatal mice were intranasally inoculated with 10 μL (104 PFU) of live attenuated mouse-adapted rhinovirus (vMC 0 ).Offspring were sacrificed at days 1, 5, 10, and 20 postinfection (DPI) (3, 7, 12, and 22 post-natal days, PND).Peripheral lung and brain tissue were collected.Clinical evaluation was performed on the offspring.

F I G U R E 2
from neonatal death induced by vMC 0 -infection compared to a 75% probability of survival in the offspring from untreated mothers 20 DPI.Furthermore, the maternal treatment resulted in a significant reduction in infection-induced weight loss and peak viral titers in both lung and brain.The inflammatory cellular response upon infection was dampened, with a reduced influx of neutrophils, NK-cells, monocytes and DCs (pDC and cDC).Both pDCs and cDCs showed a significant increase in IAIE and CD86-expression at peak viral load.Moreover, Tregs and IL-5 expression in lung tissue were increased, while expression of IFNβ and IFNγ were decreased.In summary, Schematic presentation of the maternal OM-85 treatment regiments (black) and challenges of offspring (blue).A: Mincham 2018 study.Maternal oral administration of 400 mg/kg body weight OM-85 through GD 9.5-17.5.Sensitization of offspring through priming (day 21) and boosting (day 35) with OVA-Alum.Challenged with aerosols.B: Y Li 2020 study.Maternal oral administration of 1 mg/kg body weight Broncho-Vaxom from 10 days before mating until birth.Sensitization of offspring through priming (day 25) and boosting (day 32 + 39) with OVA-Alum.Challenged with aerosols.C: Mincham 2020 study.Maternal oral administration of 400 mg/kg body weight OM-85 through GD 9.5-17.5.Autopsy and collection of fetal tissue before delivery.D: Lauzon-Joset 2021 study.Maternal oral administration of 400 mg/kg body weight OM-85 through GD 0.5-17.5.Offspring inoculated 2 days after birth.Neonates were autopsied and tissue was collected 3, 7, 12, and 22 days after birth.
study showed attenuated upregulation of MHC-II expression on tracheal and lymph node cDCs in the treated group, which could be suggestive of a reduced antigen-presenting activity of the DCs in these compartments, ultimately resulting in a more quiescent and balanced immune state.Merging these findings proposes that maternal OM-85 treatment could facilitate a beneficial immunological state, where exaggerated and inappropriate inflammation is avoided, while immune competence is improved.Both the Mincham, 2018 and Mincham, 2020 studies show, that the OM-85 treatment effects seen in the airway tissue, are paralleled by changes in the myeloid cell compartments of the BM.Following maternal OM-85 treatment, both studies showed significant upregulation of both cDCs and the progenitor MDP in fBM.The Mincham, 2020 study moreover showed increases in overall DCs, the MHC-II expression on the cDCs, and expansion of the progenitor stages MP and GMP.Likewise, an expansion of GMP and upregulation of MHC-II was seen in the 2018 study, only here it was found in the bone marrow of 6-week-old offspring.Since the increase in number of cDCs and their functional maturation is already seen in the fBM without airway challenge, this could indicate that the beneficial effects of prenatal

Figure 3 )
Figure3).This is further supported by the effects on the BM seen post-aerosol-challenge in the 2018 study: The reduced accumulation and activation of cDCs in the airway mucosa upon challenge is reflected in the increased BM reserve of mature stages of progenitors (pre-cDC) and mature cDCs, as the draw from the BM is limited in the more well-balanced immune response.14Moreover, the very convincing attenuation of the surge in MHC-II and CD86 on cDCs seen after the introduction of bacterial LPS directly to the BM-culture in the Mincham, 2018 study, gives further evidence for the BM-driven effects.While the exact mechanisms underlying the transmission of the trained innate immune effects from mother to fetus are yet to be described, a study performed on gestational tissue (including uterus and placenta among others) in pregnant mice treated with OM-85,23 could point toward trafficking of maternal cells across the fetomaternal interface as a possible explanation.The maternal OM-85 give rise to accelerated maturation of DC networks and more effective recruitment of Tregs.The affected molecular pathways showed to be central in proliferation and differentiation of progenitor cells in the BM and in DC development, function, and survival.While the reviewed cytokine expression does not favor anti-asthmatic effects, the overall promising results of OM-85 mediated enhanced resistance toward asthma and respiratory infection, could be a steppingstone toward assessing the preventative effects of maternal OM-85 treatment in offspring in human trials, as it has shown to have beneficial effects on both mother and fetus in mice and no teratogenic effects in humans.

Overview of literature search. Search on PubMed, 6/10/2023 Hits
TA B L E 2 Schematic overview of the reviewed articles.

Baseline characteristics in lysate group compared to untreated Type of challenge Response upon challenge in lysate group compared to untreated group 24 hours after challenge
Comparison of responses in lysate group compared to offspring from non-treated mothers.
TA B L E 3 ↑Tregs (0.30-0.52) %CD45 Note: Baseline characteristics and after inflammatory/infectious challenge of the offspring in lung tissue.Only significant changes are included.All values are median values.↑: an increase compared to untreated group.↓: a decrease compared to untreated group.*Proportion of total peripheral lung cells.**Proportion of CD45 + cells within neonatal peripheral lungs.***From Supplemental data, Mincham 2020 study.TA B L E 4 Comparison of responses in bone marrow in the maternal lysate group compared to offspring from the untreated mothers.Note: Only significant changes are included.All values are median values.↑: an increase compared to untreated group.↓: a decrease compared to untreated group.