Vaccine Against Neisseria Meningitidis Infections

The present invention relates to a vaccinal pharmaceutical composition intended for the prevention of meningitis caused by Neisseria meningitidis.

Generally speaking, meningitis is either of viral origin or of bacterial origin. The bacteria mainly responsible are N. meningitidis and Haemophilus influenzae, which are implicated, respectively, in approximately 40 and 50% of cases of bacterial meningitis.

N. meningitidis accounts for approximately 600 to 800 cases of meningitis per annum in France. In the USA, the number of cases amounts to approximately 2,500 to 3,000 per annum.

The species N. meningitidis is subdivided into serogroups according to the nature of the capsular polysaccharides. Although a dozen serogroups exist, 90% of cases of meningitis are attributable to 3 serogroups: A, B and C.

There are effective vaccines based on capsular polysaccharides to prevent meningitis caused by N. meningitidis serogroups A and C. These polysaccharides, as such, exhibit little or no immunogenicity in infants under 2 years of age, and do not induce immune memory. However, these drawbacks may be overcome by conjugating these polysaccharides to a carrier protein.

On the other hand, the polysaccharide of N. meningitidis group B exhibits little or no immunogenicity in man, either in conjugated or in unconjugated form. Thus, it is seen to be highly desirable to seek a vaccine against meningitis induced by N. meningitidis, in particular of serogroup B, other than a vaccine based on polysaccharide.

To this end, various proteins of the outer membrane of N. meningitidis have already been proposed. Special attention has focused on the membrane receptor for human transferrin.

Generally speaking, the large majority of bacteria require iron for their growth, and have developed specific systems for acquiring this metal. As regards N. meningitidis in particular, which is a strict pathogen of man, the iron can be abstracted only from human iron-transport proteins such as transferrin and lactoferrin, since the amount of iron in free form is negligible in man (of the order of 10.sup.-18 M), and in any case insufficient to permit bacterial growth.

Thus, N. meningitidis possesses a human transferrin receptor and a human lactoferrin receptor, which enable it to bind these iron-chelating proteins and thereafter to take up the iron needed for its growth.

The transferrin receptor of N. meningitidis strain B16B6 has been purified by Schryvers et al. (WO 90/12591) from a membrane extract. This protein as purified evidently consists essentially of two types of polypeptide: a polypeptide of high apparent molecular weight of 100 kD and a polypeptide of lower apparent molecular weight of approximately 70 kD, as visualised after polyacrylamide gel electrophoresis in the presence of SDS.

The product of the purification carried out, in particular, by Schryvers is referred to, by arbitrary definition and for the requirements of the present patent application, as the transferrin receptor, and the polypeptides of which it consists are referred to as subunits. In the text below, the subunits of high molecular weight and of lower molecular weight are referred to as Tbp1 and Tbp2, respectively.

FIG. 1 is an SDS-PAGE gel with 7.5% acrylamide, in which columns A and B correspond to the receptors of N. meningitidis strains 2169 and 2394, respectively. The arrows pointing horizontally indicate the position of the reference proteins of known apparent molecular mass (94 kD, phosphorylase B; 67 kD, albumin).

It has now been found that there are at least two types of strain which differ in the constitution of their respective transferrin receptors. This was demonstrated by studying membrane extracts of several tens of strains of N. meningitidis of miscellaneous origins. These membrane extracts were first subjected to SDS-PAGE gel electrophoresis and then electrotransferred onto nitrocellulose sheets. These nitrocellulose sheets were incubated:

a) in the presence of a rabbit antiserum directed towards the transferrin receptor purified from N. meningitidis strain B16B6, also referred to as 2394;

b) in the presence of a rabbit antiserum directed towards the transferrin receptor purified from N. meningitidis strain 2169; or

c) in the presence of human transferrin conjugated to peroxidase.

As regards a) and b), the recognition of the subunits of the transferrin receptor is visualised by adding an anti-rabbit immunoglobulins antibody coupled to peroxidase, followed by addition of the substrate for this enzyme.

Tables I and II hereinbelow show the profile of some representative strains as seen on SDS-PAGE gel containing 7.5% polyacrylamide; the bands are characterised by their apparent molecular weight expressed in kilodaltons (kD):

The results appearing in the first 2 lines of the tables show that there are two types of strains:

The first type (Table I) corresponds to strains which possess a receptor in which both of the subunits are recognised by anti-receptor 2394 antiserum while only the high molecular weight subunit is recognised by anti-receptor 2169 antiserum.

The second type (Table II) corresponds to strains which possess a receptor in which both of the subunits are recognised by anti-receptor 2169 antiserum while only the high molecular weight subunit is recognised by anti-receptor 2394 antiserum.

Consequently, there exists an antigenic diversity in respect of the lower molecular weight subunit. This diversity is, however, limited since it resolves into 2 major types, in contrast to the suggestion made by Griffiths et al., FEMS Microbiol. Lett. (1990) 69: 31.

[Moreover, it will be noted that, irrespective of the type of strain, the subunit capable of binding to transferrin is always the lower molecular weight subunit (Tables I and II, third line of results).]

In accordance with these findings, it had been tempting to conclude that an effective vaccine against all N. meningitidis infections could be adequately composed of a transferrin receptor or exclusively of its high molecular weight subunit, irrespective of the strain of origin of the receptor, since this subunit is recognised by both types of antiserum.

Surprisingly, this has now been found not to be the case, inasmuch as the high molecular weight subunit does not appear to be capable of inducing the production of neutralising type antibodies. Only the smaller of the 2 subunits of the receptor appears to be capable of fulfilling this function. Since this lower molecular weight subunit is characterised by a significant antigenic variation from the first type to the second type of strain, a single type of transferrin receptor should not be sufficient for vaccinating against all N. meningitidis infections.

Thus, the invention provides for:

i) A vaccinal pharmaceutical composition which comprises, as therapeutic agents, at least a first and a second molecule capable of binding to human transferrin; the said first molecule originating from a first strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit (Tbp1) and a lower molecular weight subunit (Tbp2), and in which the lower molecular weight subunit (Tbp2) is recognised by an antiserum to the receptor of N. meningitidis strain 2394 (receptor 2394) and is not recognised by an antiserum to the receptor of N. meningitidis strain 2169 (receptor 2169); and the said second molecule originating from a second strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit (Tbp1) and a lower molecular weight subunit (Tbp2), and in which the lower molecular weight subunit (Tbp2) is recognised by an anti-receptor 2169 antiserum and is not recognised by an anti-receptor 2394 antiserum;

ii) A vaccination kit containing:

a) A pharmaceutical composition which comprises, as therapeutic agent, at least a first molecule capable of binding to human transferrin; the said first molecule originating from a first strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an antiserum to the receptor of N. meningitidis strain 2394 (receptor 2394) and is not recognised by an antiserum to the receptor of N. meningisidis strain 2169 (receptor 2169);

b) A pharmaceutical composition which comprises, as therapeutic agent, at least a second molecule capable of binding to human transferrin; the said second molecule originating from a second strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an anti-receptor 2169 antiserum and is not recognised by an anti-receptor 2394 antiserum; and

c) Instructions for the concomitant or consecutive administration of the compositions a) and b);

iii) The combined therapeutic use of at least a first and a second molecule capable of binding to human transferrin; the said first molecule originating from a first strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an antiserum to the receptor of N. meningitidis strain 2394 (receptor 2394) and is not recognised by an antiserum to the receptor of N. meningisidis strain 2169 (receptor 2169); and the said second molecule originating from a second strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an anti-receptor 2169 antiserum and is not recognised by an anti-receptor 2394 antiserum; and

iv) A method of vaccination against N. meningitidis infections, which comprises the act of administering an effective amount, from a therapeutic standpoint, of at least a first and a second molecule capable of binding to human transferrin, concomitantly or consecutively, to a subject requiring such a vaccinal treatment; the said first molecule originating from a first strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an antiserum to the receptor of N. meningitidis strain 2394 (receptor 2394) and is not recognised by an antiserum to the receptor of N. meningitidis strain 2169 (receptor 2169); and the said second molecule originating from a second strain of N. meningitidis which possesses a human transferrin receptor at least consisting of a high molecular weight subunit and a lower molecular weight subunit, and in which the lower molecular weight subunit is recognised by an anti-receptor 2169 antiserum and is not recognised by an anti-receptor 2394 antiserum.

"Molecule capable of binding to human transferrin" is understood to mean either a human transferrin receptor originating from N. meningitidis (that is to say a molecule comprising, in particular, 2 types of subunit), or exclusively the subunit of the receptor, capable of binding to human transferrin, as well as a fragment or an analogue of this subunit.

A transferrin receptor may be obtained in purified form from a strain of N. meningitidis previously cultured in a medium deficient in iron in free form, in particular according to the method of Schryvers et al., WO 90/12591, described in a similar manner in Schryvers et al., Infect. Immun. (1988) 56 (5):1144. Alternatively, a transferrin receptor originating from a strain of N. meningitidis may be produced by employing genetic engineering techniques. The DNA fragment or fragments coding for the subunits of the receptor may be expressed jointly or separately in a heterologous expression system (e.g. bacterium, yeast, mammalian cell). The subunits, in free form or associated in the form of the receptor, are, in this case, collected from a culture and purified. When the subunits are produced in this way in free form, provision may be made for reassociating them in the form of the receptor by subjecting them to a suitable treatment.

The subunit capable of binding to human transferrin (lower molecular weight subunit) may be obtained in purified form (that is to say dissociated and isolated from the high molecular weight subunit), in particular, from a receptor purified according to the method of Schryvers et al., by subjecting the receptor to the action of a strongly denaturing agent such as 8M urea or 6M guanidine HCl, and then separating the dissociated subunits by standard chromatographic methods such as ion exchange chromatography or gel filtration. Alternatively, the subunit may be produced according to genetic engineering methods. These methods are, in addition, entirely suited to the production of fragments or analogues of the subunit.

As an example, the subunits Tbp1 and Tbp2 of the strains 2394 and 2169 are described by reference to their amino acid sequences as shown in the sequence identifiers Nos. 1 to 4 (SEQ ID No. 1 to 4).

"Fragment of the subunit capable of binding to human transferrin" is understood to mean a peptide having an amino acid sequence which is included in the sequence of the subunit. "Analogue of the subunit capable of binding to human transferrin" is understood to mean a protein having an amino acid sequence which exhibits an at least 80%, preferably at least 90% and, as an absolute preference, at least 95% homology with the sequence of the subunit. For the purposes of the present invention, it should be clearly understood that such a fragment or such an analogue must retain the immunogenic properties of the subunit.

N. meningitidis strains 2394 (B:2a:P1.2:L2.3) and 2169 (B:9:P1.9:L3.7), which are commonly used in laboratories, are available to the public from the Collection of the Pasteur Institute, 25 rue du Dr Roux 75015 Paris, under the respective registration numbers CIP 7908 and CIP 7917.

Furthermore, the anti-receptor antisera which are required in order to distinguish the strains of N. meningitidis may be obtained as follows:

A receptor is first purified from an initial strain (2394 or 2169) according to the method of Schryvers et al. Albino New Zealand rabbits receive subcutaneously and intramuscularly 100 .mu.g of the receptor in the presence of Freund's complete adjuvant. 21 days and 42 days after the first injection, the rabbits again receive 100 .mu.g of the purified receptor, but on these occasions in the presence of Freund's incomplete adjuvant. 15 days after the last injection, the animals' serum is withdrawn, then decomplemented and filtered through a membrane of porosity 0.45 .mu.m. The filtrate is thereafter exhaustively extracted by contact with the initial strain which, to this end, has been cultured beforehand in the presence of iron (under these conditions, synthesis of the transferrin receptor is repressed). The contacting procedure is as follows: 10 ml of the filtrate are added to 10.sup.10 cfu (colony forming units) of a culture of the initial strain. Adsorption is carried out overnight at 4.degree. C. with agitation. The bacteria are then removed by centrifugation. The supernatant is recovered and then subjected again to 2 successive adsorption operations as described above.

The type of a strain (with respect to the nature of its transferrin receptor) may be identified from membrane extracts derived from cultures deficient in iron in free form, employing conventional techniques such as SDS-PAGE gel electrophoresis, followed by immunoblotting using an antiserum such as that described above.

The first molecule participating in the vaccinal composition originates from a first strain of N. meningitidis which possesses a transferrin receptor essentially consisting of (i) a subunit of high molecular weight, advantageously of 100 to 90 kD and preferably of 93-95 kD approximately, and (ii) a subunit of lower molecular weight, advantageously of 75 to 60 kD and preferably of 72 to 65 kD, and, as an absolute preference, respectively (i) of 93 kD and (ii) of 67-70 kD approximately.

The second molecule participating in the vaccinal composition originates from a second strain of N. meningitidis which possesses a transferrin receptor essentially consisting of (i) a subunit of high molecular weight, advantageously of 100 to 90 kD, preferably of 100 to 95 kD and, as an absolute preference, of 98 kD approximately, and (ii) a subunit of lower molecular weight, advantageously of 90 to 80 kD, preferably of 87 to 85 kD and, as an absolute preference, of 87 kD approximately.

The molecular weights stated above are apparent molecular weights as visualised after electrophoresis of a receptor purified on SDS-PAGE gel. Such an electrophoresis may be carried out according to the method of Laemmli illustrated as follows:

A polyacrylamide gel (16 cm.times.20 cm.times.1 mm in thickness) comprising a 5% pregel and a 7.5% separating gel is first prepared in electrophoresis buffer (Tris 6 g/l, glycine 28.8 g/l, 0.1% SDS).

In addition, 50 .mu.l of sample buffer (62 mM Tris-HCl pH 6.8, 2% SDS, 5% .beta.-mercaptoethanol, 1% glycerol, 0.001% bromophenol blue) are added to 50 .mu.l of a solution of purified receptor at a concentration of 0.6 mg/ml (in 50 mM phosphate buffer pH 8.0 containing 0.05% Sarkosyl). The mixture is incubated for 5 min in a boiling water bath. 17 .mu.l (equivalent to 5 .mu.g of protein) of the sample thus prepared are placed in a well in the gel. A sample prepared in a similar manner containing molecular weight markers is added in parallel. Electrophoresis is carried out in electrophoresis buffer at 50 volts for 15 hours. The gel is fixed and stained with Coomassie blue.

Generally speaking, the first or the second molecule which is useful for the purposes of the present invention can originate from a strain of N. meningitidis of any serogroup. Advantageously, the first or the second molecule originates from a strain of N. meningitidis serogroup B. Preferably, the first and second molecules originate respectively from a first and a second strain of N. meningitidis serogroup B.

According to an absolutely preferred aspect of the invention, the first molecule originates from the strain 2394 while the second molecule originates from the strain 2169.

A pharmaceutical composition according to the invention may be manufactured in a conventional manner. In particular, the therapeutic agent or agents according to the invention is/are combined with a diluent or vehicle which is acceptable from a pharmaceutical standpoint. A composition according to the invention may be administered by any conventional route in use in the vaccine field, especially subcutaneously, intramuscularly or intravenously, for example in the form of an injectable suspension. The administration can take place in a single dose or in a dose repeated one or several times after a certain time interval. The appropriate dosage varies in accordance with various parameters, for example with the individual being treated or with the mode of administration.

The invention is described in greater detail in the examples below and with reference to FIG. 1, which illustrates an electrophoresis on SDS-PAGE gel with 7.5% polyacrylamide, in which the columns A and B correspond to the receptors of N. meningitidis strains 2169 and 2394, respectively. The arrows pointing horizontally indicate the position of the reference proteins of known apparent molecular mass (94 kD, phosphorylase B; 67 kD, albumin).

1A--Culture

A lyophilisate of N. meningitidis strain 2394 is taken up in approximately 1 ml of Mueller-Hinton broth (MHB, Difco). The bacterial suspension is then plated out on Muller-Hinton solid medium containing cooked blood (5%).

After 24 h of incubation at 37.degree. C. in an atmosphere containing 10% of CO.sub.2, the bacterial lawn is collected in order to inoculate 150 ml of MHB pH 7.2, distributed in three 250-ml Erlenmeyers. Incubation is carried out for 3 h at 37.degree. C. with agitation. Each of the 3 cultures so produced permits the inoculation of 400 ml of MHB pH 7.2 supplemented with 30 .mu.M of ethylenediaminedi(o-hydroxyphenylacetic acid) (EDDA, Sigma), which is a chelating agent for iron in free form.

After 16 h of culture at 37.degree. C. with agitation, the cultures are monitored for their purity by microscopic observation after Gram staining. The suspension is centrifuged and the pellet containing the microbes is weighed and stored at -20.degree. C.

1B--Purification

The purification method is essentially that described by Schryvers et al. (supra).

The bacterial pellet obtained in 1A is thawed and then resuspended in 200 ml of 50 mM Tris-HCl buffer, pH 8.0 (buffer A). The suspension is centrifuged for 20 min at 15,000 x g at 4.degree. C. The pellet is recovered and then resuspended in buffer A at a final concentration of 150 g/l. 150-ml fractions are treated for 8 min at 800 bars in a cell lyser working under high pressure (Rannie, model 8.30H). The cell lysate thereby obtained is centrifuged for 15 min at 4.degree. C. at 15,000 x g. The supernatant is recovered and then centrifuged for 75 min at 4.degree. C. at 200,000 x g.

After removal of the supernatant, the pellet is taken up in buffer A and, after protein assay by the Lowry method, the concentration of the suspension is adjusted to 5 mg/ml.

1.75 mg of biotinylated human transferrin are then added to 1.4 ml of the membrane suspension according to the method described by Schryvers. The final concentration of the membrane fraction is 4 mg/ml. The mixture is incubated for 1 hour at 37.degree. C. and then centrifuged at 100,000 x g for 75 minutes at 4.degree. C. The membrane pellet is taken up with buffer A containing 0.1M NaCl, and incubated for 60 min at room temperature.

After solubilisation, a certain volume of 30% (w/v) N-LauroylSarkosine and of 500 mM EDTA are added to this suspension so that the final concentrations of Sarkosyl and EDTA are 0.5% and 5 mM, respectively. After incubation for 15 min at 37.degree. C. with agitation, 1 ml of streptavidin-agarose resin (Pierce), previously washed in buffer A, is added. The suspension is incubated for 15 min at room temperature and then centrifuged at 1,000 x g for 10 min. The resin is then packed in a column and the direct eluate is discarded.

The resin is washed with 3 column volumes of 50 mM Tris-HCl buffer pH 8.0 containing 1M NaCl, 10 mM EDTA, 0.5% Sarkosyl (buffer B), and then with one column volume of buffer B containing 750 mM guanidine HCl. The transferrin receptor is then eluted with buffer B containing 2M guanidine HCl, 0.05% Sarkosyl. The eluate is collected in fractions whose volume corresponds to 1 Vol. in Lubes containing 1 Vol. of 50 mM Tris-HCl pH 8.0, 1M NaCl. The optical density of the eluate at 280 nm is measured at the column outlet using a UV detector.

The fractions corresponding to the elution peak are collected, dialysed against 10 mM phosphate buffer, pH 8.0 containing 0.05% Sarkosyl and lyophilised. The lyophilisate is taken up in water at a 10-fold higher concentration. The solution is dialysed a second time against 50 mM phosphate buffer pH 8.0 containing 0.05% Sarkosyl (buffer C), and the solution is then filtered through a membrane of porosity 0.22 .mu.m.

The protein content is determined and adjusted to 1 mg/ml by adding buffer C, under aseptic conditions. This preparation is stored at -70.degree. C.

Culturing of the strain 2169 and purification of the transferrin receptor are performed under conditions identical to those described in Example 1.

The sterile solutions obtained in Examples 1 and 2 are thawed. In order to prepare one liter of vaccine containing 100 .mu.g/ml of each of the active principles, the following solutions are mixed under sterile conditions:

Albino New Zealand rabbits receive subcutaneously and intramuscularly 100 .mu.g of the receptor 2394 or 2169 (as obtained in Example 1 or 2), in the presence of Freund's complete adjuvant. 21 and 42 days after the first injection, the rabbits again receive 100 .mu.g of the purified receptor, but this time in the presence of Freund's incomplete adjuvant. 15 days after the last injection, the animals' serum is withdrawn, then decomplemented and filtered through a membrane of porosity of 0.45 .mu.m. The filtrate is thereafter exhaustively extracted by contact with the initial strain (2394 or 2169) which, to this end, has been cultured beforehand in the presence of iron in free form (under these conditions, synthesis of the transferrin receptor is repressed). The contacting procedure is as follows: 10 ml of the filtrate are added to 10.sup.10 cfu (colony forming units) of a culture of the initial strain. Adsorption is carried out overnight at 4.degree. C. with agitation. The bacteria are then removed by centrifugation. The supernatant is recovered and then subjected again to 2 successive adsorption operations as described above.

A dilution series of each of the antisera, anti-receptor 2394 and anti-receptor 2169, is prepared in M199 medium (Gibco). 200 .mu.l of each dilution are placed in the wells of a microtitration plate (8.times.12 in.). A control test is carried out with 200 .mu.l of M199 medium. Into each of the wells there are added (i) 100 .mu.l of a culture in the exponential growth phase of a strain of N. meningitidis, in Mueller-Hinton medium supplemented with 30 .mu.M EDDA and (ii) 100 .mu.l of complement (young rabbit serum, diluted).

After 30 min of incubation at 37.degree. C. with gentle agitation, 1 ml of Mueller-Hinton medium containing 1 ml of Noble agar in the supercooled state is added into each well. After solidification of the medium, incubation is carried out for 18-24 hours at 37.degree. C.; the number of colony forming units in each well is then evaluated. The reciprocal of the final dilution of antiserum in the presence of which a 50% lysis is observed relative to the control corresponds to the bactericidal titre.

The results are presented in Table III below:

The anti-receptor 2394 antiserum has bactericidal activity exclusively against strains of the first type as defined in the present application (2394, 2228, 2154, 2234 and 2448), while the anti-receptor 2169 antiserum has bactericidal activity exclusively against strains of the second type (2169 and 876). This strongly suggests that the production of neutralising antibodies is essentially induced by the lower molecular weight subunit which carries the antigenic variability.