solvent in OmpT/A8-35 than in OmpT/dodecylmaltoside complexes (Watkinson et al. 2017). To
which extent such differences are relevant to the presentation of MPs to the immune system remains
uncertain, however given that the detergent is expected to desorb once diluted under its critical micellar
concentration (Chap. 2), as happens following injection, and the APol to remain associated only as long
as it is not displaced by other amphiphiles, as is expected to occur upon integration of the protein into
the plasma membrane of a cell (see Chap. 5, § 5.7, and below, § 15.3). A further complicating element is
that most of the immunization trials included Montanide as an adjuvant. Montanide is an oily emulsion,
and whatever happens to MP/APol complexes when mixed with it remains to be examined.
15.2.3 How Could Amphipols Help in Formulating More Efficient Vaccines?
The results summarized in the previous section indicate that an A8-35-trapped MP used as immunogen in a
vaccine preparation can elicit a stronger protection than the same protein kept in detergent solution. It
remains to be determined whether this effect results from a higher stability; from the fact that the APoltrapped protein, at variance with the detergent-solubilized one, is expected to remain soluble following
injection and dilution of the preparation in body fluids; from a different presentation to the immune system;
or from an ensemble of these factors. A second important point is that transfer to A8-35 considerably
stabilizes the protein used in these studies, MOMP, against inactivation by either heat treatment or freeze/
thawing. This is of considerable practical importance for storing vaccines and using them in the field.
Relatively few vaccines to date resort to MPs, but a few such examples have been reported, e.g. against
hepatitis B (Ionescu-Matiu et al. 1983) or papillomavirus (Suzich et al. 1995). It would be of great interest
to examine whether these vaccines could be improved by the use of APols.
1
The two major obstacles to a broader use of MPs in formulating vaccines are (i) the difficulty of
producing them in bulk at an affordable cost and (ii) their instability in detergent solution. As described
in Chap. 6, APols can be used to fold to their native state many MPs after they have been expressed,
simply and economically, as inclusion bodies. Identifying efficient folding conditions appears relatively straightforward for monomeric MPs, like monomeric porins, bacteriorhodopsin, or G proteincoupled receptors (reviewed in Popot 2014). For oligomeric MPs like MOMP, case-by-case development of efficient protocols will be required. As regards the stability issue, most APol-trapped MPs are
stabilized, and often strongly stabilized, against thermal denaturation (see Chap. 5, § 5.5), and the two
of them that have been submitted to freeze/thaw cycles, BR and MOMP, were found to be resistant to
this treatment (respectively C. Le Bon and M. Zoonens, unpublished data cited in Chap. 5, § 5.5, and
Feinstein et al. 2014). There therefore seems to be a case for taking advantage of these developments to
explore more vigorously the use of MPs in the formulation of subunit vaccines.
The potential advantages of APols do not stop there. Indeed, they present two further properties
that could be usefully harnessed for improving the efficiency of vaccines based on MP/APol
complexes: first, they can be chemically modified so as to carry almost any desirable moieties
(Chap. 4, § 4.4); second, because a given protein binds several APol molecules (Chap. 5, § 5.3.1.1),
two or more APols carrying different groups can be mixed so as to provide MP/APol complexes with
multiple functionalities. This has been illustrated in Chap. 13, § 13.2, by the use of complexes that
carried both a tag, allowing their immobilization onto solid supports, and a fluorescent group, allowing
their visualization (see e.g. Della Pia et al. 2014). Among the various APols and various functionalities
that have been developed, three features appear to present a particular interest in the context of vaccine
1 Published too late to be discussed here are vaccine trials in which A8-35-trapped BamA was used to boost the immune
response against Escherichia coli (Vij et al. 2018)
15.2 Using Amphipols to Formulate a Subunit Vaccine Against Chlamydia trachomatis
669
which extent such differences are relevant to the presentation of MPs to the immune system remains
uncertain, however given that the detergent is expected to desorb once diluted under its critical micellar
concentration (Chap. 2), as happens following injection, and the APol to remain associated only as long
as it is not displaced by other amphiphiles, as is expected to occur upon integration of the protein into
the plasma membrane of a cell (see Chap. 5, § 5.7, and below, § 15.3). A further complicating element is
that most of the immunization trials included Montanide as an adjuvant. Montanide is an oily emulsion,
and whatever happens to MP/APol complexes when mixed with it remains to be examined.
15.2.3 How Could Amphipols Help in Formulating More Efficient Vaccines?
The results summarized in the previous section indicate that an A8-35-trapped MP used as immunogen in a
vaccine preparation can elicit a stronger protection than the same protein kept in detergent solution. It
remains to be determined whether this effect results from a higher stability; from the fact that the APoltrapped protein, at variance with the detergent-solubilized one, is expected to remain soluble following
injection and dilution of the preparation in body fluids; from a different presentation to the immune system;
or from an ensemble of these factors. A second important point is that transfer to A8-35 considerably
stabilizes the protein used in these studies, MOMP, against inactivation by either heat treatment or freeze/
thawing. This is of considerable practical importance for storing vaccines and using them in the field.
Relatively few vaccines to date resort to MPs, but a few such examples have been reported, e.g. against
hepatitis B (Ionescu-Matiu et al. 1983) or papillomavirus (Suzich et al. 1995). It would be of great interest
to examine whether these vaccines could be improved by the use of APols.
1
The two major obstacles to a broader use of MPs in formulating vaccines are (i) the difficulty of
producing them in bulk at an affordable cost and (ii) their instability in detergent solution. As described
in Chap. 6, APols can be used to fold to their native state many MPs after they have been expressed,
simply and economically, as inclusion bodies. Identifying efficient folding conditions appears relatively straightforward for monomeric MPs, like monomeric porins, bacteriorhodopsin, or G proteincoupled receptors (reviewed in Popot 2014). For oligomeric MPs like MOMP, case-by-case development of efficient protocols will be required. As regards the stability issue, most APol-trapped MPs are
stabilized, and often strongly stabilized, against thermal denaturation (see Chap. 5, § 5.5), and the two
of them that have been submitted to freeze/thaw cycles, BR and MOMP, were found to be resistant to
this treatment (respectively C. Le Bon and M. Zoonens, unpublished data cited in Chap. 5, § 5.5, and
Feinstein et al. 2014). There therefore seems to be a case for taking advantage of these developments to
explore more vigorously the use of MPs in the formulation of subunit vaccines.
The potential advantages of APols do not stop there. Indeed, they present two further properties
that could be usefully harnessed for improving the efficiency of vaccines based on MP/APol
complexes: first, they can be chemically modified so as to carry almost any desirable moieties
(Chap. 4, § 4.4); second, because a given protein binds several APol molecules (Chap. 5, § 5.3.1.1),
two or more APols carrying different groups can be mixed so as to provide MP/APol complexes with
multiple functionalities. This has been illustrated in Chap. 13, § 13.2, by the use of complexes that
carried both a tag, allowing their immobilization onto solid supports, and a fluorescent group, allowing
their visualization (see e.g. Della Pia et al. 2014). Among the various APols and various functionalities
that have been developed, three features appear to present a particular interest in the context of vaccine
1 Published too late to be discussed here are vaccine trials in which A8-35-trapped BamA was used to boost the immune
response against Escherichia coli (Vij et al. 2018)
15.2 Using Amphipols to Formulate a Subunit Vaccine Against Chlamydia trachomatis
669