Accounting for vaccines specificities in the Joint Clinical Assessment (JCA): a proposal for guiding principles

Issue/problem

Vaccines are an important public health intervention protecting the population against infectious diseases. The value of vaccines is broad ranging from individual to societal as achieving community immunity protects the unvaccinated, minimizes the risk of outbreaks, reduces the emergence of antimicrobial resistance, and leads to broader societal benefits.

Description of the problem

Vaccines’ market access processes are characterized by the development of recommendations by NITAGs followed by the assessment of health technology assessment (HTA) bodies in less than half of 27 EU member states. Despite that HTA for therapeutic drugs is well established, there is very limited experience in applying HTA methodologies to vaccines, especially for clinical assessments, as HTA methods and frameworks are traditionally geared toward therapeutics. However, following the adoption of the EU regulation on HTA, Joint Clinical Assessments (JCAs) of vaccines are expected.

Results

To support a discussion on how to account for vaccine specificities in the JCA, Vaccines Europe has performed a project which aimed at developing a proposal for high-level guiding principles on processes and methodologies for clinical HTA for vaccines. The proposal is informed by findings of literature reviews on currently applied processes, methods, and clinical assessment frameworks of vaccines as well as the outcomes of an advisory board with scientific experts.

Lessons

A proposal for high-level guiding principles for clinical HTA for vaccines is being developed based on both evidence and the advice from scientific experts which focuses on processes (e.g., horizon scanning, early advice, consideration of vaccine-specific expertise) and methods (e.g., unmet need, safety, efficacy/effectiveness, real-world evidence and technical characteristics of the technology). Lastly, the implementation of vaccines specificities in JCA represents a call for action.

Steel factor controls midline cell death of primordial germ cells and is essential for their normal proliferation and migration

During germ-cell migration in the mouse, the dynamics of embryo growth cause many germ cells to be left outside the range of chemoattractive signals from the gonad. At E10.5, movie analysis has shown that germ cells remaining in the midline no longer migrate directionally towards the genital ridges, but instead rapidly fragment and disappear. Extragonadal germ cell tumors of infancy, one of the most common neonatal tumors, are thought to arise from midline germ cells that failed to die. This paper addresses the mechanism of midline germ cell death in the mouse. We show that at E10.5, the rate of apoptosis is nearly four-times higher in midline germ cells than those more laterally. Gene expression profiling of purified germ cells suggests this is caused by activation of the intrinsic apoptotic pathway. We then show that germ cell apoptosis in the midline is activated by down-regulation of Steel factor (kit ligand) expression in the midline between E9.5 and E10.5. This is confirmed by the fact that removal of the intrinsic pro-apoptotic protein Bax rescues the germ-cell apoptosis seen in Steel null embryos. Two interesting things are revealed by this: first, germ-cell proliferation does not take place in these embryos after E9.0; second, migration of germ cells is highly abnormal. These data show first that changing expression of Steel factor is required for normal midline germ cell death, and second, that Steel factor is required for normal proliferation and migration of germ cells.

BMP signaling regulates PGC numbers and motility in organ culture

Members of the bone morphogenetic protein (BMP) family play diverse roles in multiple developmental processes. However, in the mouse, mutations in many BMPs, BMP receptors and signaling components result in early embryonic lethality making it difficult to analyze the role of these factors during organogenesis or tissue homeostasis in the adult. To bypass this early lethality, we used an organ culture system to study the role of BMPs during primordial germ cell (PGC) migration. PGCs are the embryonic precursors of the sperm and eggs. BMPs induce formation of primordial germ cells within the proximal epiblast of embryonic day 7.5 (E7.5) mouse embryos. PGCs then migrate via the gut to arrive at the developing gonads by E10.5. Addition of BMP4 or the BMP-antagonist Noggin to transverse slices dissected from E9.5 embryos elevated PGC numbers or reduced PGC numbers, respectively. Noggin treatment also slowed and randomized PGC movements, resulting in a failure of PGCs to colonize the urogenital ridges (UGRs). Based on p-Smad1/5/8 staining, migratory PGCs do not respond to endogenous BMPs. Instead, the somatic cells of the urogenital ridges exhibit elevated p-Smad1/5/8 staining revealing active BMP signaling within the UGRs. Noggin treatment abrogated p-Smad staining within the UGRs and blocked localized expression of Kitl, a cytokine known to regulate the survival and motility of PGCs and Id1, a transcription factor expressed within the UGRs. We propose that BMP signaling regulates PGC migration by controlling gene expression within the somatic cells along the migration route and within the genital ridges.

The roles of FGF signaling in germ cell migration in the mouse

Fibroblast growth factor (FGF) signaling is thought to play a role in germ cell behavior. FGF2 has been reported to be a mitogen for primordial germ cells in vitro, whilst combinations of FGF2, steel factor and LIF cause cultured germ cells to transform into permanent lines of pluripotent cells resembling ES cells. However, the actual function of FGF signaling on the migrating germ cells in vivo is unknown. We show, by RT-PCR analysis of cDNA from purified E10.5 germ cells, that germ cells express two FGF receptors: Fgfr1-IIIc and Fgfr2-IIIb. Second, we show that FGF-mediated activation of the MAP kinase pathway occurs in germ cells during their migration, and thus they are potentially direct targets of FGF signaling. Third, we use cultured embryo slices in simple gain-of-function experiments,using FGF ligands, to show that FGF2, a ligand for FGFR1-IIIc, affects motility, whereas FGF7, a ligand for FGFR2-IIIb, affects germ cell numbers. Loss of function, using a specific inhibitor of FGF signaling, causes increased apoptosis and inhibition of cell shape change in the migrating germ cells. Lastly, we confirm in vivo the effects seen in slice cultures in vitro,by examining germ cell positions and numbers in embryos carrying a loss-of-function allele of FGFR2-IIIb. In FGFR2-IIIb-/- embryos,germ cell migration is unaffected, but the numbers of germ cells are significantly reduced. These data show that a major role of FGF signaling through FGFR2-IIIb is to control germ cell numbers. The data do not discriminate between direct and indirect effects of FGF signaling on germ cells, and both may be involved.

Vg 1 is an essential signaling molecule in Xenopus development

Xenopus Vg 1, a transforming growth factor beta (Tgfbeta) family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg 1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg 1 transcript does not rescue Vg 1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg 1 function, and shows it to be an essential maternal regulator of embryonic patterning.

Transcriptional profiling identifies genes differentially expressed during and after migration in murine primordial germ cells

Mouse primordial germ cells (PGCs) are migratory until they colonize the genital ridges, assemble with the somatic tissue, and start to differentiate into oocytes or spermatogonia. Using cell transplantation experiments, we show here that germ cells isolated during migration (at E10.5) will migrate actively to the genital ridges, whereas post-migratory PGCs isolated from E12.5 embryos are non-motile even when transferred into a permissive environment (e.g. E10.5 host tissue). Major transcriptional changes must take place between E10.5 and E12.5 that convert germ cells from a migratory to a non-migratory state. To identify the genes involved, we have performed transcriptional profiling of motile and non-motile populations of PGCs. We have identified 55 transcripts that are expressed in E10.5 PGCs at levels at least 3 x their expression at E12.5, and 48 transcripts with the reciprocal expression levels. Additionally, 309 transcripts were found to be expressed in both populations. Many of the E10.5 transcripts encode proteins involved in controlling cytoskeletal and adhesive interactions implicated in cell motility. Many of the E12.5 transcripts encode proteins implicated in germ cell differentiation.

The pro-apoptotic gene Bax is required for the death of ectopic primordial germ cells during their migration in the mouse embryo

In the mouse embryo, significant numbers of primordial germ cells (PGCs)fail to migrate correctly to the genital ridges early in organogenesis. These usually die in ectopic locations. In humans, 50% of pediatric germ line tumors arise outside the gonads, and these are thought to arise from PGCs that fail to die in ectopic locations. We show that the pro-apoptotic gene Bax,previously shown to be required for germ cell death during later stages of their differentiation in the gonads, is also expressed during germ cell migration, and is required for the normal death of germ cells left in ectopic locations during and after germ cell migration. In addition, we show that Bax is downstream of the known cell survival signaling interaction mediated by the Steel factor/Kit ligand/receptor interaction. Together, these observations identify the major mechanism that removes ectopic germ cells from the embryo at early stages.

GP130, the shared receptor for the LIF/IL6 cytokine family in the mouse, is not required for early germ cell differentiation, but is required cell-autonomously in oocytes for ovulation

GP130 is the shared receptor for members of the IL6 family of cytokines. Members of this family have been shown to enhance the survival of migratory (E10.5) or postmigratory (E12.5) murine primordial germ cells (PGCs) in culture; however, it is uncertain what role these cytokines play during PGC development in vivo. We have examined PGC numbers in E13.5 GP130-deficient mouse embryos and found that males exhibited a slight decrease in PGC numbers; females were normal. Also, we used the Cre-loxP system to inactive GP130 specifically in germ cells and found that this resulted in a fertility defect in females. These animals were found to have a slight reduction in the number of primary follicles and a major defect in ovulation. This data suggests that GP130 is required in female germ cells for their normal function, but is dispensable in male germ cells.

The chemokine SDF1/CXCL12 and its receptor CXCR4 regulate mouse germ cell migration and survival

In mouse embryos, germ cells arise during gastrulation and migrate to the early gonad. First, they emerge from the primitive streak into the region of the endoderm that forms the hindgut. Later in development, a second phase of migration takes place in which they migrate out of the gut to the genital ridges. There, they co-assemble with somatic cells to form the gonad. In vitro studies in the mouse, and genetic studies in other organisms, suggest that at least part of this process is in response to secreted signals from other tissues. Recent genetic evidence in zebrafish has shown that the interaction between stromal cell-derived factor 1 (SDF1) and its G-protein-coupled receptor CXCR4, already known to control many types of normal and pathological cell migrations, is also required for the normal migration of primordial germ cells. We show that in the mouse, germ cell migration and survival requires the SDF1/CXCR4 interaction. First, migrating germ cells express CXCR4, whilst the body wall mesenchyme and genital ridges express the ligand SDF1. Second, the addition of exogenous SDF1 to living embryo cultures causes aberrant germ cell migration from the gut. Third, germ cells in embryos carrying targeted mutations in CXCR4 do not colonize the gonad normally. However, at earlier stages in the hindgut, germ cells are unaffected in CXCR4(-/-) embryos. Germ cell counts at different stages suggest that SDF1/CXCR4 interaction also mediates germ cell survival. These results show that the SDF1/CXCR4 interaction is specifically required for the colonization of the gonads by primordial germ cells, but not for earlier stages in germ cell migration. This demonstrates a high degree of evolutionary conservation of part of the mechanism, but also an area of evolutionary divergence.

The roles of three signaling pathways in the formation and function of the Spemann Organizer

Since the three main pathways (the Wnt, VegT and BMP pathways) involved in organizer and axis formation in the Xenopus embryo are now characterized, the challenge is to understand their interactions. Here three comparisons were made. Firstly, we made a systematic comparison of the expression of zygotic genes in sibling wild-type, VegT-depleted (VegT–), β-catenin-depleted (β-catenin–) and double depleted (VegT–/β-catenin–) embryos and placed early zygotic genes into specific groups. In the first group some organizer genes, including chordin, noggin and cerberus, required the activity of both the Wnt pathway and the VegT pathway to be expressed. A second group including Xnr1, 2, 4 and Xlim1 were initiated by the VegT pathway but their dorsoventral pattern and amount of their expression was regulated by the Wnt pathway. Secondly, we compared the roles of the Wnt and VegT pathways in producing dorsal signals. Explant co-culture experiments showed that the Wnt pathway did not cause the release of a dorsal signal from the vegetal mass independent from the VegT pathway. Finally we compared the extent to which inhibiting Smad 1 phosphorylation in one area of VegT–, or β-catenin– embryos would rescue organizer and axis formation. We found that BMP inhibition with cm-BMP7 mRNA had no rescuing effects on VegT– embryos, while cm-BMP7 and noggin mRNA caused a complete rescue of the trunk, but not of the anterior pattern in β-catenin– embryos.

Time-lapse analysis of living mouse germ cell migration

In mouse embryos, the primordial germ cells arise during gastrulation prior to, and distant from, the prospective gonads. Observations of PGCs in culture, and in fixed sections, have suggested, but not proved, that they migrate to the gonad by a process of active migration. The opaque nature of the early mouse embryo has precluded direct observation. Using confocal microscopy, we have filmed living PGCs expressing eGFP in tissue slices from mouse embryos at different stages of development. We find four clearly distinct phases of PGC migration. First, until E9.0-E9.5, PGCs are already highly motile, but do not leave the gut. Second, in the E9.0-E9.5 period, before the mesentery forms, PGCs very rapidly exit the gut, but do not migrate towards the genital ridges. Third, during the E10.0-E10.5 period, PGCs migrate directionally from the dorsal body wall into the genital ridges. In contrast to the prevailing model of germ cell migration, very few, if any, PGCs found in the gut mesentery at E10.5 migrate into the genital ridges. Finally, at E11.5, PGCs are slowing and the direction of movement is dependent on the sex of the embryo. This allows, for the first time, a formal description of the events of PGC migration in the mouse.

The role of maternal axin in patterning the Xenopus embryo

Regulation of the stability of beta catenin protein is a critical role of Wnt signaling cascades. In early Xenopus development, dorsal axis specification depends on regulation of beta catenin by both cytoplasmic and nuclear mechanisms. While the cytoplasmic protein axin is known as a key component of the cytoplasmic beta catenin degradation complex, loss-of-function studies are needed to establish whether it is required for dorso-ventral patterning in the embryo, and to test where in the embryo it carries out its function. Here, we show that embryos lacking maternal axin protein have increased levels of soluble beta catenin protein and increased nuclear localization of beta catenin in ventral nuclei at the blastula stage. These embryos gastrulate abnormally and develop with excessive notochord and head structures, and reduced tail and ventral components. They show increased expression of dorsal markers, including siamois, Xnr3, chordin, gsc, Xhex, and Otx2, decreased expression of Xwnt 8 and Xbra, and little alteration of BMP4 and Xvent1 and -2 mRNA levels. The ventral halves of axin-depleted embryos at the gastrula stage have dramatically increased levels of chordin expression, and severely decreased levels of Xwnt 8 mRNA expression, while BMP4 transcript levels are only slightly reduced. This dorso-anterior phenotype is rescued by axin mRNA injected into the vegetal pole of axin-depleted oocytes before fertilization. Interestingly, the phenotype was rescued by ventral but not dorsal injection of axin mRNA, at the 4-cell stage, although dorsal injection into wild-type embryos does cause ventralization. These results show directly that the localized ventral activity of maternal axin is critical for the correct patterning of the early Xenopus embryo.

Expression of the homophilic adhesion molecule, Ep-CAM, in the mammalian germ line

During normal embryonic development, mammalian germ cells use both cell migration and aggregation to form the primitive sex cords. Germ cells must be able to interact with their environment and each other to accomplish this; however, the molecular basis of early germ cell adhesion is not well characterized. Differential adhesion is also thought to occur in the adult seminiferous tubules, since germ cells move from the periphery to the lumen as they differentiate. In a screen for additional adhesion molecules expressed by the germ line, expression of the homophilic adhesion molecule, Ep-CAM, was identified in embryonic, neonatal and adult germ cells using immunocytochemistry and flow cytometry with an Ep-CAM-specific monoclonal antibody. At embryonic stages, germ cells were found to express Ep-CAM during migration at embryonic day 10.5 and early gonad assembly at embryonic day 12.5. Expression of Ep-CAM was also found on neonatal male and female germ cells. In the adult testis, Ep-CAM was detected only on spermatogonia, and was absent from more differentiated cells. Finally, embryonic stem cells were shown to express this receptor. It is proposed that Ep-CAM plays a role in the development of the germ line and the behaviour of totipotent cells.

Mouse primordial germ cells lacking beta1 integrins enter the germline but fail to migrate normally to the gonads

Primordial germ cells are the founder cells of the gametes. They are set aside at the initial stages of gastrulation in mammals, become embedded in the hind-gut endoderm, then actively migrate to the sites of gonad formation. The molecular basis of this migration is poorly understood. Here we sought to determine if members of the integrin family of cell surface receptors are required for primordial germ cell migration, as integrins have been implicated in the migration of several other motile cell types. We have established a line of mice which express green fluorescent protein in germline cells that has enabled us to efficiently purify primordial germ cells at different stages by flow cytometry. We have catalogued the spectrum of integrin subunit expression by primordial germ cells during and after migration, using flow cytometry, immunocytochemistry and RT-PCR. Through analysis of integrin beta1(−/−)-->wild-type chimeras, we show that embryonic cells lacking beta1 integrins can enter the germline. However, integrin beta1(−/−) primordial germ cells do not colonize the gonad efficiently. Embryos with targeted deletion of integrin subunit alpha3, alpha6, or alphaV show no major defects in primordial germ cell migration. These results demonstrate a role for beta1-containing integrins in the development of the germline, although an equivalent role for * integrin subunit(s) has yet to be established.

Prostaglandin endoperoxide synthase substituted with manganese protoporphyrin IX. Formation of a higher oxidation state and its relation to cyclooxygenase reaction

The heme in prostaglandin endoperoxide synthase (PGH synthase) was substituted with Mn(III)-protoporphyrin IX. The resulting enzyme, Mn-PGH synthase, showed full cyclooxygenase activity but only 0.9% of the peroxidase activity of the native iron enzyme. During the reaction with exogenous or endogenously produced hydroperoxides, a spectral intermediate of Mn-PGH synthase was observed. The electronic absorption bands of the resting enzyme at 376, 472, and 561 nm decreased, and the intermediate's bands at 417, around 513, and 625 nm appeared. The rate constant of the formation of the intermediate was about 10(4) M-1.s-1 at 22 degrees C, three orders of magnitude lower than with the iron enzyme. Spectral properties, conditions of formation, and the suppressed formation in the presence of electron donors provide evidence for a higher oxidation state of Mn-PGH synthase, tentatively a Mn(IV) species. This species was assigned to an intermediate in the peroxidase reaction of Mn-PGH synthase, the low activity of which was explained by the rate-limiting slow reaction of Mn-PGH synthase with hydroperoxides. The findings and interpretation are consistent with the published properties of other manganese-substituted peroxidases. Although the cyclooxygenase activity was similar to that of Fe-PGH synthase, the cyclooxygenase reaction of Mn-PGH synthase showed distinct differences in comparison with Fe-PGH synthase. A longer activation phase was observed which resembled the time course of the formation of the higher oxidation state. Glutathione peroxidase with glutathione, a hydroperoxide-scavenging system, inhibited the cyclooxygenase of Mn-PGH synthase at concentrations where the activity of Fe-PGH synthase was not affected. It is demonstrated that Mn-PGH synthase requires higher concentrations of hydroperoxides for the activation of the cyclooxygenase. These findings suggest that the substitution of iron with manganese in PGH synthase does not change the mechanism of the enzyme. The main difference is the much lower rate of the reaction with hydroperoxides which affects both the peroxidase activity and the hydroperoxide-dependent activation of the cyclooxygenase. A reaction scheme for Mn-PGH synthase is proposed analogous to that suggested for Fe-PGH synthase (Karthein, R., Dietz, R., Nastainczyk, W., and Ruf, H. H. (1988) Eur. J. Biochem. 171, 313-320).