Method of binding material to the .beta.-amyloid peptide

The present invention relates to the finding that antibodies bind to the .beta.-amyloid peptide, and that .beta.-amyloid peptide binds the hinge region of the immunoglobulin heavy chain, thereby preserving the ability of the immunoglobulin to bind antigen. Methods for binding compounds such as detectable groups to .beta.-amyloid peptide are accordingly presented.

FIELD OF THE INVENTION
 The present invention relates to Alzheimer's disease, and particularly
 relates to methods of binding antigenic compounds to the .beta.-amyloid
 peptide found in the senile plaques of Alzheimer's disease.
 BACKGROUND OF THE INVENTION
 The senile amyloid plaques and congophilic angiopathic lesions found in
 abundance in brain of patients with Alzheimer's disease are abnormal
 extracellular structures. The biochemical composition of these structures
 has been extensively studied to better understand their possible role in
 the pathogenesis of this dementing disease. The mature amyloid plaque is a
 complex structure, consisting of a central core of amyloid fibrils
 surrounded by dystrophic neurites, axonal terminals and dendrites,
 microglia and fibrous astrocytes. The amyloid core of the senile amyloid
 plaque, and which surrounds blood vessels to produce the congophilic
 angiopathy, is a peptide of 39 to 43 amino acids termed the .beta.-Amyloid
 (.beta.A) peptide, the A.beta. peptide, the A4 protein, or the .beta.A4
 peptide. .beta.A peptide is found in the brain in Alzheimer's disease,
 Down's syndrome, hereditary cerebral hemorrhage of the Dutch type, and in
 old age. See, e.g., K. Kosik Science256, 780-783 (1992).
 E. Kline et al., PCT Appln WO 91/16819 describes a method of treating
 Alzheimer's disease by administering the .beta.-amyloid peptide itself, or
 an active fragment thereof. H. Potter, PCT Appln WO 92/03474, describes a
 therapeutic method of treating individuals, such as Alzheimer's disease
 patients, to prevent the formation of an
 .alpha.-antichymotrypsin-.beta.-amyloid peptide complex by administering
 to the subject a synthetic peptide comprising a fragment of the
 .beta.-amyloid peptide.
 The .beta.A peptide is produced by the abnormal proteolytic processing of a
 larger protein, the amyloid precursor protein (APP). APP itself has been
 identified in the senile amyloid plaque, and additional proteins which
 have been localized to senile amyloid plaques and angiopathic lesions
 include apolipoprotein E, alpha-1-antichymotrypsin, complement factors C1q
 and C3q, APP, and IgG. See, e.g., W. Strittmatter et al., Proc. Natl.
 Acad. Sci. 90, 1977 (1993); Abraham et al., Cell, 52, 487 (1988),
 Eikelenboom et al., Acta Neuropathol., 57, 239 (1982); McGeer et al.,
 Canad. J. Neuro. Sci., 16, 516 (1989), Beyreuther et al., Brain Pathol.,
 1, 241 (1991); Ishii et al., Acta Neuropathol., 36, 243 (1976). The
 mechanisms by which these proteins aggregate in the extracellular space to
 associate with the senile amyloid plaque and congophilic angiopathic
 lesion are not known. Hence, there is an ongoing need for new ways to
 investigate and combat this disorder.
 SUMMARY OF THE INVENTION
 Disclosed is a method of binding a compound to .beta.-amyloid peptide. The
 method is based on the finding that the hinge region of antibodies bind to
 the .beta.-amyloid peptide. The method comprises the steps of contacting a
 linker antibody having a hinge region (and preferably at least one antigen
 binding site) to the .beta.-amyloid peptide so that the hinge region binds
 to the .beta.-amyloid peptide. A compound to be delivered is bound to the
 antibody before, during, or after the contacting step. In a preferred
 embodiment the Fc receptor binding region of the linker antibody is
 deleted.
 Where the compound to be delivered is an antigenic compound, the linker
 antibody is bound with the combining site free, the antibody is selected
 so that it specifically binds to the compound to be delivered at the
 combining site, and the antigenic compound is bound to the antigen binding
 site (before, concurrently with or after the contacting step) so that the
 antigenic compound is bound to the .beta.-amyloid peptide.
 The method is useful, among other things, for histologic and diagnostic
 examination of Alzheimer's disease brain tissue, or brain tissue of a
 patient suspected of being afflicted with Alzheimer's disease, both in
 vitro and in vivo.
 The foregoing and other objects and aspects of the present invention are
 explained in detail in the specification set forth below.

DETAILED DESCRIPTION OF THE INVENTION
 To study the binding of various proteins to the .beta.A peptide, the
 present inventors used a previously developed in vitro assay in which
 .beta.A peptide, or fragments thereof, are covalently immobilized to a
 membrane matrix. See Strittmatter et al., Proc. Natl. Acad. Sci. 90, 1977
 (1993). Using this assay the binding of apolipoprotein E and amyloid
 precursor protein (APP) to synthetic .beta.A peptide has been previously
 characterized (Strittmatter et al., Proc. Natl. Acad. Sci. 90, 1977
 (1993); Strittmatter et al., Exper. Neurol. 122, 327 (1993).
 The present invention is based on the findings that (1) IgG directly and
 avidly binds .beta.A amyloid, (2) it is the domain between amino acids
 12-28 of .beta.A that binds IgG, and (3) .beta.A peptide binds the hinge
 region of the immunoglobulin heavy chain, thereby preserving the ability
 of the immunoglobulin to bind antigen. The term "hinge region" is used
 herein to indicate the area of the heavy chain of the immunoglobulin G
 molecule which lies between the first and second constant region domains
 (CH1 and CH2). See, e.g., Goodman, Immunoglobulin Structure and Function,
 In: Stites and Terr (Eds.), Basic and Clinical Immunology, 7th ed., 1991,
 Appleton & Lange, p. 110. The binding of IgG and .beta.A was found to
 resist dissociation by either sodium dodecyl sulfate or guanidine
 hydrochloride. These findings indicate that proteins which do not directly
 bind to .beta.A peptide may nonetheless become bound to the senile amyloid
 plaque and the angiopathic lesion by interaction with IgG.
 The senile amyloid plaque and congophilic angiopathic lesion are complex
 structures containing many proteins; the recruitment of proteins via the
 IgG-.beta.A peptide link may be important in the pathogenesis of the
 disease. The amyloid cascade hypothesis, discussed by Hardy & Higgins,
 Science, 256, 184 (1992), posits that deposition of amyloid .beta. protein
 is the causative agent of Alzheimer's pathology and that the
 neurofibrillary tangles, cell loss, vascular damage, and dementia follow
 as a direct result of this deposition. The binding of IgG to .beta.A
 provides a sandwich mechanism for the deposition of other plaque
 constituents in Alzheimer's disease.
 As used herein, the term "amyloid plaque" or "senile plaque" refers to the
 extracellular amyloid deposits that are a characteristic feature of
 Alzheimer's disease, and which consist of a central core of amyloid
 fibrils surrounded by dystrophic neurites, axonal terminals and dendrites,
 microglia and fibrous astrocytes. See D. Selkoe Neuron6, 487-498 (1991).
 The amyloid plaque surrounds blood vessels to produce the congophilic
 angiopathy found in Alzheimer's disease.
 The finding that IgG is capable of binding both its specific antigen and
 .beta.A peptide presents a mechanism by which proteins which do not
 directly bind .beta.A can be bound to these structures via IgG, and
 provides a method to detectably label the pathological lesions found in
 Alzheimer's disease. Such labelling may be used in staining anatomical
 specimens and in detecting pathological lesion in situ. In general, the
 present invention provides a means for delivering or binding an antigenic
 compound to the .beta.A peptide for any purpose, whether therapeutic or
 diagnostic.
 Any antibody may be employed in carrying out the present invention so long
 as it has a hinge region capable of binding to the .beta.A peptide. The
 terms "antibody" and "antibodies" as used herein refers to all types of
 immunoglobulins, including IgG, IgM, IgA, IgD, and IgE, including the
 fragments thereof. The term "immunoglobulin" includes the subtypes of
 these immunoglobulins, such as IgG.sub.1, IgG.sub.2, IgG.sub.3, IgG.sub.4,
 etc. Of these immunoglobulins, IgM and IgG are preferred, and IgG is
 particularly preferred. The antibodies may be of any species of origin,
 including (for example) mouse, rat, rabbit, horse, or human, or may be
 chimeric antibodies. See, e.g., M. Walker et al., Molec. Immunol. 26,
 403-11 (1989). The antibodies may be monoclonal antibodies. Such
 monoclonal antibodies are produced in accordance with known techniques.
 The term "antibody" as used herein includes antibody fragments which
 retain the capability of binding to a target antigen, for example,
 F(ab').sub.2 fragments, and the corresponding fragments obtained from
 antibodies other than IgG, retain a hinge region and may be used in the
 present invention. Such fragments are also produced by known techniques.
 The monoclonal antibodies may be recombinant monoclonal antibodies produced
 according to the methods disclosed in Reading U.S. Pat. No. 4,474,893, or
 Cabilly et al., U.S. Pat. No. 4,816,567. The antibodies may also be
 chemically constructed by specific antibodies made according to the method
 disclosed in Segel et al., U.S. Pat. No. 4,676,980 (Applicants
 specifically intend that the disclosure of all U.S. patent references
 cited herein be incorporated herein by reference).
 Monoclonal antibodies may be chimeric antibodies produced in accordance
 with known techniques. For example, chimeric monoclonal antibodies may be
 complementarity determining region-grafted antibodies (or "CDR-grafted
 antibodies"), or "humanized" antibodies produced in accordance with known
 techniques. See, e.g., H. Waldmann, PCT Application WO 93/01289; M. Clark,
 PCT Application WO 92/16562; M. Bendig et al., PCT Application WO
 92/15683; K. Tan, PCT Application WO 92/15699.
 As used herein, the term "antigen-binding portion of an antibody" means the
 portion of the antibody that binds an antigen to which the antibody is
 specific. A preferred embodiment of this method comprises using a fragment
 of IgG having the F.sub.c region deleted, such as the F(ab)'.sub.2
 fragment.
 Where it is desired to bind a compound which is not antigenic to the
 antibody to bind that compound to the .beta.A peptide, that compound may
 be bound to the antibody by direct means (e.g., covalently) or indirect
 means (e.g., via a chelator) by any suitable technique, such as the
 Iodogen method or with N-succinimidyl-3-(tri-n-butylstanyl)benzoate (the
 "ATE method"), as will be apparent to those skilled in the art. See, e.g.,
 M. Zalutsky and A. Narula, Appl. Radiat. Isot. 38, 1051 (1987).
 The compound to be bound to the 6A peptide may be a detectable group.
 Suitable detectable groups include, for example, radiolabels (e.g.,
 .sup.35 S, .sup.125 I, .sup.131 I), enzyme labels (e.g., horseradish
 peroxidase, alkaline phosphates), and fluorescent labels (e.g.,
 fluorescein) as used in accordance with known techniques.
 Linker antibodies used to carry out the present invention, with or without
 the compound to be delivered bound thereto, may be provided in lyophylized
 form in a sterile aseptic container or may be provided in a pharmaceutical
 formulation, such as in combination with a pharmaceutically acceptable
 carrier such as sterile pyrogen-free water or sterile pyrogen-free
 physiological saline solution.
 The step of contacting the linker antibody to the .beta.-amyloid peptide
 may be carried out by any suitable means. The .beta.-amyloid peptide may
 reside in nerve tissue and the contacting step is carried out in nerve
 tissue. The nerve tissue may or may not reside in a patient, depending
 upon whether the method is being carried out for histologic, diagnostic,
 or thereapeutic purposes. Where the technique is carried out on a tissue
 sample for histologic purposes (e.g., staining of the congophilic
 angiopathy), it may be carried out by contacting the tissue to a solution,
 typically an aqueous solution, containing the antibody (e.g., by washing
 the tissue with the solution or by immersing the tissue in the solution).
 The step of binding the compound to be delivered to the antibody may be
 carried out before, during (i.e. concurrently with), or after the step of
 contacting the antibody to the .beta.-amyloid peptide as described above.
 The binding step may be carried out by any of the means for carrying out
 the contacting step as described above.
 For administration to a subject, the antibody will generally be mixed,
 prior to administration, with a non-toxic, pharmaceutically acceptable
 carrier substance (e.g. normal saline or phosphate-buffered saline), and
 will be administered using any appropriate procedure, e.g., intravenous or
 intra-arterial administration, injection into the cerebrospinal fluid). In
 addition, either intrathecal administration or injection into the carotid
 artery are advantageous for therapy of tumors located in the brain.
 Dosage of the antibody will depend, among other things, on the route of
 administration, the nature of the compound being bound to the .beta.A
 peptide, etc. For example, the dosage will typically be about 1 to 10
 micrograms per Kilogram subject body weight.
 The mechanism by which the hinge region of IgG binds .beta.A peptide also
 presents the opportunity to devise methods to specifically and selectively
 block this interaction, and methods to deliver therapeutic molecules to
 the amyloid plaque. A therapeutic protein molecule is delivered to the
 amyloid plaque by raising IgG specific for the therapeutic molecule, then
 binding the therapeutic molecule to the immunoglobulin (or a fragment
 thereof) and delivering this construct to the area of pathological .beta.A
 peptide deposition. Alternatively, the immunoglobulin could first be
 contacted to the .beta.A peptide and then exposed to the therapeutic
 molecule, or these steps may be carried out concurrently. Conjugates of
 IgG and therapeutic protein molecules may be made using a variety of
 bifunctional protein coupling agents as are known in the art.
 The present invention is also useful for blocking the pathological
 deposition of proteins which bind to .beta.-amyloid peptide via IgG,
 providing a method of combatting diseases in which abnormal protein
 deposition on .beta.A peptide occurs. Such a method would involve
 inhibiting the binding of IgG to .beta.A peptide by contacting to the
 .beta.A peptide a fragment of IgG capable of binding to the .beta.A
 peptide but incapable of binding other proteins, for example, a fragment
 of IgG comprising the hinge region, but lacking the antigen-binding
 portion of the antibody.
 The present invention also provides a method to screen compounds for the
 ability to block the IgG-.beta.A peptide interaction. Generally, a
 screening method involves providing an aqueous solution containing .beta.A
 peptide or a fragment thereof, adding to this aqueous solution a test
 compound suspected of inhibiting IgG binding, then adding IgG to the
 aqueous solution, and then detecting the presence of absence of bound IgG.
 Another embodiment of this method would utilize a .beta.A peptide
 construct immobilized on a solid support, as described above (see
 Strittmatter et al. , Proc. Natl. Acad. Sci. 90, 1977 (1993); also
 disclosed in co-pending patent application Ser. No. 07/959,251, filed Oct.
 13, 1992). Detection of bound IgG may be carried out using antibodies with
 detection groups attached or by any suitable means, such as staining,
 affinity binding, competitive binding assay, etc., as are known in the
 art. The .beta.-amyloid peptide fragment used in the in vitro assays
 described above could comprise .beta.A.sub.(1-28) or .beta.A.sub.(12-28),
 although other fragments that bind IgG may also be employed.
 The following examples are provided to illustrate the present invention,
 and should not be construed as limiting thereof. In these examples, PBS
 means phosphate buffered saline; CSF means cerebrospinal fluid; SDS means
 sodium dedecyl sulfate; M means molar; mM means millimolar; mg means
 milligram; .mu.g means microgram; ng means nanogram; ml means milliliter;
 .mu.l means microliter; and .degree.C. means degrees Centigrade.
 EXAMPLE 1
 MATERIALS AND METHODS
 Affinity purified human IgG and IgG fragments (Fab, Fc, and F(ab)'.sub.2)
 were purchased from Cappel-Organon Teknika, Durham, N.C.
 Peroxidase-conjugated goat antibodies to human IgG (heavy and light
 chains) were purchased from Pierce, Rockford, Ill. Sheep anti-human serum
 albumin (HSA) IgG, sheep anti-ferritin IgG, and peroxidase-conjugated
 sheep IgG against human serum albumin were purchased from The Binding
 Site, Ltd., San Diego, Calif. .beta.A peptides (.beta.A.sub.(1-40),
 .beta.A.sub.(1-28), and .beta.A.sub.(12-28) were obtained from Bachem,
 Torrance, Calif., US. The synthesis of peptides E.H. ("even-hydro", as
 explained below) and H.M. ("hydro-mimic" as explained below) have been
 described previously (Strittmatter et al., Proc. Natl. Acad. Sci. 90, 1977
 (1993)). Human cerebrospinal fluid (CSF), from diagnostic lumbar punctures
 was obtained from the Kathleen Bryan Brain Bank at Duke University Medical
 Center, Durham, NC.
 .beta.A peptides, other peptides, or ethanolamine were covalently bound to
 13 mm Immobilon AV affinity membrane (Millipore) discs as described
 previously, with 100 .mu.g peptide. Strittmatter et al., Proc. Natl. Acad.
 Sci. 90, 1977 (1993). The membrane is a chemically activated hydrophilic
 microporous membrane which covalently immobilizes peptides and proteins
 through amino and thiol groups.
 EXAMPLE 2
 Binding of IgG to Immobilized Peptides
 To characterize the binding of immunoglobulins to selected immobilized
 peptides, affinity-purified human IgG (0.3 ng), Fab (0.45 ng), Fc (0.15
 ng) or F(ab)'.sub.2 (0.45 ng) in 150 .mu.l PBS, pH 7.4, were incubated
 with immobilized .mu.A.sub.(1-28) peptide, other peptides, or with
 ethanolamine at room temperature for 30 minutes. The membranes were washed
 in a Millex filter holder (Millipore) with 3.0 ml PBS, followed by 700
 .mu.1 10% sodium dodecyl sulfate (SDS). Retained proteins were then eluted
 from the membranes by boiling five minutes in 150 .mu.l Laemmli with
 .beta.-mercaptoethanol. 45 .mu.l of each sample was loaded on a 12%
 polyacrylamide gel. Electrophoresis and Western transfer were performed as
 previously described (Strittmatter et al., Proc. Natl. Acad. Sci. 90, 1977
 (1993)). The Western transfer membrane was blocked with 40 ml Blotto (5%
 dried milk in Tris buffered saline pH 7.6) with 0.1% Tween 20 at room
 temperature for one hour. The membrane was incubated with
 peroxidase-conjugated goat anti-human IgG antibody which recognizes both
 heavy and light chains (diluted 1:4000 in Blotto) at 4.degree. C.
 overnight. The membrane was rinsed four times with 20 ml Blotto and was
 then washed three times with 40 ml Blotto for five minutes. Antibody was
 visualized using an enhanced chemoluminescence detection kit (Amersham)
 and exposure of the membrane to Hyperfilm (Amersham), as described
 previously (Strittmatter et al., Proc. Natl. Acad. Sci. 90, 1977 (1993)).
 Results: Purified IgG was found to bind directly to .beta.A.sub.(1-28)
 peptide, and binding required the hinge region of the heavy chain. As
 shown in FIG. 1, panel 1, purified IgG binds to immobilized
 .beta.A.sub.(1-28) peptide, and does not bind to the ethanolamine-blocked
 control membrane. Various domains of the immunoglobulin molecule were
 examined for binding to immobilized .beta.A peptide. Fab (Panels 2 and 4)
 and Fc (panel 3) did not bind to .beta.A.sub.(1-28) peptide. F(ab)'.sub.2,
 containing the hinge region of the heavy chain, bound to immobilized
 .beta.A.sub.(1-28) peptide (Panel 5). To further characterize the avidity
 of .beta.A peptide binding, F(ab)'.sub.2 was incubated with immobilized
 .beta.A.sub.(1-28) or with immobilized ethanolamine (FIG. 2A), washed with
 PBS (Lane 1), and then followed by 10% SDS (Lane 2), 4 M urea (Lane 3), or
 6 M guanidine hydrochloride (Lane 4). The F(ab)'.sub.2 that bound to
 .beta.A.sub.(1-28) was not eluted by urea, and was only partially eluted
 by SDS or guanidine hydrochloride. Similar results were obtained with IgG
 in CSF, shown in FIG. 2B. Incubation of cerebrospinal fluid, which
 contains IgG, with immobilized .beta.A.sub.(1-28) or with immobilized
 ethanolamine, and washed under the conditions shown in FIG. 2A, revealed
 that cerebrospinal fluid (CSF) IgG avidly bound .beta.A.sub.(1-28). In a
 previous study, Pardridge et al. (1987), examined human cerebrospinal
 fluid for protein immunoreactive with an antibody against synthetic
 .beta.A.sub.(1-28), and demonstrated that CSF IgG was identified by this
 antibody, suggesting either cross-reactivity or the presence of .beta.A
 peptide on IgG.
 Both IgG and F(ab)'.sub.2 bind with high avidity to full length .beta.A
 peptide (.beta.A.sub.(1-40)). As illustrated in FIG. 3, affinity-purified
 IgG bound to .beta.A.sub.(1-40), .beta.A.sub.(1-28), and
 .beta.A.sub.(12-28) Thus the binding requires amino acids 12-28 of
 .beta.A. Peptide H.M. ("hydro-mimic" peptide) is a 17 amino acid peptide
 with a hydropathy profile similar to .beta.A.sub.(12-28), but with
 different amino acids, while peptide E. H. ("evenhydro" peptide) is a 17
 amino acid peptide containing the same amino acids as .beta.A.sub.(12-28)
 but with a scrambled sequence (Strittmatter et al., Proc. Natl. Acad. Sci.
 90, 1977 (1993)). IgG bound to peptide H.M. but only bound minimally to
 peptide E.H. FIG. 3. Similar results were obtained with purified
 F(ab)'.sub.2 (data not shown). The lack of binding of IgG and F(ab)'.sub.2
 to the scrambled amino acid sequence of peptide E.H. suggests a certain
 degree of specificity of interaction. Since IgG and F(ab)'.sub.2 bound to
 peptide H.M., which has the same hydropathic profile as
 .beta.A.sub.(12-28), binding may require specific steric, hydrophobic, or
 charge interactions.
 These studies show that IgG binds directly and avidly to .beta.A peptide;
 that the hinge region of the heavy chain is required for binding; and that
 binding requires amino acids 12-28 of .beta.A. With .beta.A bound to the
 hinge domain, the antigen-binding portion of IgG remains free to interact
 with other antigens.
 EXAMPLE 3
 Binding of IgG-Antigen Pairs to .beta.A Peptide
 Studies examining the binding of IgG to both antigen and .beta.A.sub.(1-28)
 were conducted by incubating 1.0 .mu.l cerebrospinal fluid (which contains
 albumin) with either 0.5 mg sheep anti-human serum albumin (anti-HSA)
 antibody or sheep antiferritin antibody (control) in 150 .mu.l PBS for
 thirty minutes at room temperature. The mixtures were then incubated 30
 minutes with either immobilized .beta.A.sub.(1-28) peptide or immobilized
 ethanolamine (control) at room temperature. The membranes were first
 washed with 3.0 ml PBS, followed by 1.0 ml 6 M guanidine hydrochloride,
 and 2.0 ml PBS. The retained proteins were then eluted by boiling five
 minutes in 150 .mu.l Laemmli buffer. Proteins were then electrophoresed
 and transferred to Immobilon P membrane as described above.
 The Western transfer membrane was incubated with peroxidase-conjugated
 sheep anti-HSA antibody (1:10,000 dilution in Blotto) overnight at
 4.degree. C. After washing the membranes as described above, immunolabeled
 albumin was visualized by chemoluminescence. All of the experiments shown
 in the FIGS. 4 and 5 have been replicated at least once (duplicate data
 not shown).
 To test whether IgG can deliver an antigen to the senile plaque by first
 binding its specific antigen and then binding to .beta.A peptide, the
 ability of albumin to bind to .beta.A peptide directly and to bind to
 .beta.A peptide indirectly through anti-albumin IgG was examined. As shown
 in FIG. 4, panel 1, albumin in cerebrospinal fluid was weakly bound to
 both immobilized .beta.A peptide or to ethanolamine-bound control
 membranes. Cerebrospinal fluid was then first incubated with either an
 anti-albumin IgG or with an anti-ferritin IgG (as a control) and was then
 incubated with immobilized .beta.A peptide. Prior incubation with
 anti-albumin IgG markedly increased the amount of albumin bound to the
 immobilized .beta.A peptide (FIG. 4, panel 4) in contrast to
 pre-incubation with the anti-ferritin IgG control (FIG. 4, panel 5).
 Neither of the antibodies were immunoreactive for albumin (FIG. 4, panels
 2 and 3) and both antibodies bound equally well to immobilized .beta.A
 peptide (data not shown).
 These studies show that albumin, which binds weakly to .beta.A peptide, can
 be targeted to .beta.A peptide via IgG.
 EXAMPLE 4
 Binding of Antigen to IgG-.beta.A Peptide Pairs
 The following experiments indicate that IgG bound to .beta.A.sub.(1-28)
 peptide can still recognize and bind antigen. Both anti-albumin IgG and
 anti-ferritin IgG were first bound to immobilized .beta.A.sub.(1-28)
 peptide, and were then incubated with cerebrospinal fluid. The
 anti-albumin IgG continued to bind albumin (FIG. 5, lanes 2 and 5) while
 the anti-ferritin antibody did not (FIG. 5, Lane 3 and 6). The binding of
 albumin to .beta.A peptide via specific IgG was maintained even after wash
 with 6 M guanidine hydrochloride (FIG. 5, Lane 5).
 Data in FIGS. 4 and 5 indicate that IgG is capable of high avidity binding
 to antigen and to .beta.A peptide simultaneously, and that the temporal
 sequence of interaction is not important.
 The foregoing is illustrative of the present invention and is not to be
 construed as limiting thereof. The invention is defined by the following
 claims, with equivalents of the claims to be included therein.