BACKGROUND OF THE INVENTION 
     Gonadotropin Releasing Hormone (&#34;GnRH&#34;, also known as Luteinizing Hormone Releasing Hormone, or &#34;LHRH&#34;), is of central importance to the regulation of fertility. Johnson M., Everitt B. Essential Reproduction, 3rd Edn. Blackwell Scientific Publications, 1988. In males and females, GnRH is released from the hypothalamus into the bloodstream and travels via the blood to the pituitary, where it induces the release of the gonadotropins, luteinizing hormone and follicle stimulating hormone, by gonadotroph cells. These two gonadotropins, in turn, act upon the gonads, inducing steroidogenesis and gametogenesis. Steroids released from the gonads into the circulation subsequently act upon various tissues. 
     The gonadotropin hormonal cascade can be halted by neutralization of the biological activity of GnRH. Fraser H. M. Physiological Effects of Antibody to Leutenizing Hormone Releasing Hormone. In: Physiological Effects of Immunity Against Reproductive Hormones, Edwards and Johnson, Eds. Cambridge University Press, 1976. As a consequence of GnRH neutralization, the gonadotropins and gonadal steroids are not released into the blood and their biological activities are thereby eliminated. By eliminating the biological activity of GnRH, the hormonal regulation of fertility is interrupted and gametogenesis ceases. GnRH neutralization halts the production of gametes. GnRH neutralization is thus an effective means of contraception. 
     A number of important diseases are affected by gonadotropins and gonadal steroid hormones, particularly the gonadal steroids. Such diseases include breast cancer, uterine and other gynecological cancers, endometriosis, uterine fibroids, prostate cancer and benign prostatic hypertrophy, among others. Removal of the gonadal steroid hormonal stimuli for these diseases constitutes an important means of therapy. An effective method of accomplishing this is by neutralizing GnRH, the consequence of which is the elimination of gonadal steroids that induce and stimulate these diseases. McLachlan R. I., Healy D. L., Burger G. B. 1986. Clinical Aspects of LHRH Analogues in Gynaecology: a Review, British Journal of Obstretics and Gynaecology, 93:431-454. Conn P. M., Crowley W. F. 1991. Gonadotropin-Releasing Hormone and Its Analogs, New England Journal of Medicine. 324:93-103. Filicori M., Flamigni C. 1988. GnRH Agonists and Antagonists, Current Clinical Status. Drugs. 35:63-82. 
     One effective means of neutralizing GnRH is the induction or introduction of anti-GnRH antibodies in the host or patient. Such antibodies can be induced by active immunization with GnRH immunogens or by passive immunization by administering anti-GnRH antibodies. Fraser H. M. Physiological Effects of Antibody to Leutenizing Hormone Releasing Hormone. In: Physiological Effects of Immunity Against Reproductive Hormones, Edwards and Johnson, Eds. Cambridge University Press, 1976. Since anti-GnRH antibodies can neutralize the biological activity of GnRH, immunization constitutes an important approach towards treating diseases dependent upon gonadal steroids and other reproductive hormones as well as a means to regulate mammalian fertility. 
     GnRH has the same amino acid sequence in all mammals (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-GlyNH 2 ) (SEQ ID NO.: 1 in the Sequence Listing), thus a single immunogen would be effective in all mammalian species, including humans. Active immunization against GnRH, however, has not been practicable due to deficiencies associated with the GnRH immunogens. The prior art anti-GnRH immunogens are not of sufficient potency, and therefore must be administered repeatedly to induce effective levels of anti-GnRH antibodies. In addition, they have not proven to be reliable, in terms of inducing anti-GnRH antibodies in an acceptable portion of the immunized population. 
     SUMMARY OF THE INVENTION 
     The present invention, concerns improved immunogens against GnRH that induce neutralizing liters of anti-GnRH antibodies in response to a single administration of immunogen in all of the immunized populations that we have studied. The immunogens of the invention may thus be used to treat steroid dependent diseases and may also be used as immunocontraceptives to regulate fertility. 
     The immunogens of the present invention are peptides composed of two functional regions: the immunomimic region and a spacer region. The function of the immunomimic which immunologically crossreacts with GnRH is to induce antibodies that bind to the targeted hormone. The spacer element of the peptide serves as a link through which the immunomimic is attached to an immunological carrier, such as diphtheria toxoid (&#34;DT&#34;) tetanus toxoid and also affects the immune response generated by the vaccinated mammal against the immunomimic. For example, in a specific embodiment of the invention, the immunogen peptide has the sequence: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-Arg-Pro-Pro-Pro-Pro-Cys (SEQ ID NO: 2 in the Sequence Listing). In this (&#34;GnRH(1-10)-Arg10&#34;) peptide, the sequence pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly- (SEQ ID NO: 3 in the Sequence Listing), comprises the immunomimic of GnRH. The remainder of the peptide&#39;s sequence, -Arg-Pro-Pro-Pro-Pro-Cys (SEQ ID NO: 4 in the Sequence Listing), constitutes the spacer, which is attached to amino acid number 10 of the GnRH immunomimic. 
     A preferred embodiment of the invention concerns two peptide immunomimics of GnRH that are associated with four spacer sequences. Methods of coupling these peptides to immunological carriers, such as DT, to yield anti-GnRH immunogens are provided. The immunogens may be used singly or in combination to induce anti-GnRH antibody responses in the vaccinated mammal. As compared to the prior art anti-GnRH immunogens, the immunogens of the present invention induce a biologically effective immune response following a single administration of immunogen in all of the immunized animals tested. The immunogens can be administered in different physical forms, including soluble and precipitate. The immunomimic spacer peptides of this invention can be coupled to immunological carriers over a wide range of peptide to carrier substitution ratios and yield effective immunogens. 
     The invention also concerns methods of treating gonadotropin and gonadal steroid hormone dependent diseases and cancers by immunization with the immunogens of the invention. A specific embodiment of the invention concerns a method of immunological contraception in mammals comprising the administration of the inventive immunogens. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1: Depicts anti-GnRH antibody responses to the administration of the inventive immunogens comprising peptides 1-4 and the comparative prior art anti-GnRH immunogen, peptide 5 as measured by mean antigen binding capacities (&#34;ABC&#34;) in pico moles per milliliter with respect to days after immunization in immunized rabbits. 
     FIG. 2: Depicts the antibody response to immunization with an immunogen comprising a mixture of peptides 3 and 4 as measured by mean ABC with respect to days after immunization. 
     FIG. 3: Depicts the results of immunizations in mice as measured by a mean ABC with respect to days after immunization after immunization with fractions of a preparation of peptide 2 immunogens fractionated on the basis of solubility. 
     FIG. 4: Depicts antibody responses in mice as measured by mean ABC with respect to days after immunization when immunized with various conjugates of peptides 1 and 2 at different peptide: DT substitution ratios. 
     FIG. 5: Depicts antibody responses of male rabbits as measured by mean ABC with respect to days after immunization when immunized with a mixture of conjugates of peptides 1 and 2. Serum testosterone levels in these male rabbits over the course of the immunization test period are shown. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLE 1 
     Peptides with the amino acid sequences listed in Table 1 were synthesized and prepared by standard solid phase synthesis methods. Each peptide was characterized as to amino acid content and purity. 
     
                                           TABLE 1__________________________________________________________________________PeptideDesignation         Amino Acid Sequence__________________________________________________________________________1    GnRH(1-10)-Ser1         Cys--Pro--Pro--Pro--Pro--Ser--Ser--Glu--His--         Trp--Ser--Tyr--G1y--Leu--Arg--Pro--         Gly(NH.sub.2)(SEQ ID NO: 5 in the Sequence Listing)2    GnRH(1-10)-Ser10         pGlu--His--Trp--Ser--Tyr--Gly--Leu--Arg--         Pro--Gly--Ser--Ser--Pro--Pro--Pro--Pro--Cys         (SEQ ID NO: 6 in the Sequence Listing)3    GnRH(1-10)-Arg1         Cys--Pro--Pro--Pro--Pro--Arg--Glu--His--Trp--         Ser--Tyr--G1y--Leu--Arg--Pro--Gly(NH.sub.2)         (SEQ ID NO: 7 in the Sequence Listing)4    GnRH(1-10)-Arg10         pGlu--His--Trp--Ser--Tyr--Gly--Leu--Arg--         Pro--Gly--Arg--Pro--Pro--Pro--Pro--Cys         (SEQ ID NO: 2 in the Sequence Listing)__________________________________________________________________________ 
    
     Each of peptides 1-4 contains an immunomimic of GnRH that is either preceded by or followed by a spacer. Two immunomimics of GnRH were used: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly- (SEQ ID NO: 3 in the Sequence Listing), (peptides 2 and 4 Table 1) wherein the spacer was attached through the carboxy terminal end of GnRH (amino acid #10); and, -Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly(NH 2 ) (SEQ ID NO: 8 in the Sequence Listing), (peptides 1 and 3 Table 1) wherein the spacer was attached at the amino terminal end of GnRH (amino acid #1). 
     The four spacers set forth in Table 2 were used. 
     
                       TABLE 2______________________________________Spacer Designation         Amino Acid Seguence______________________________________Ser 1         Cys--Pro--Pro--Pro--Pro--Ser--Ser--         (SEQ ID NO: 9 in the Sequence Listing)Ser 10        --Ser--Ser--Pro--Pro--Pro--Pro--Cys         (SEQ ID NO: 10 in the Sequence Listing)Arg 1         Cys--Pro--Pro--Pro--Pro--Arg--         (SEQ ID NO: 11 in the Sequence Listing)Arg 10        --Arg--Pro--Pro--Pro--Pro--Cys         (SEQ ID NO: 4 in the Sequence Listing)______________________________________ 
    
     The numerals 1 and 10 in the spacer designation refer to the GnRH amino acid number to which the spacer is attached. While these spacer regions of the molecules have been set forth separately in Table 2, in the preferred embodiment of the invention the peptide is synthesized as one continuous peptide sequence molecule. 
     Each of these peptides 1-4 of Table 1 was conjugated to amino groups present on a carrier such as Diphtheria Toxoid (&#34;DT&#34;) via the terminal peptide cysteine residue utilizing heterobifunctional linking agents containing a succinimidyl ester at one end and maleimide at the other end of the linking agent. 
     To accomplish the linkage between any of the Peptides 1-4 above and the carrier, the cysteine of the peptide was first reduced. The dry peptide was dissolved in 0.1M sodium phosphate buffer (degassed), pH 8.0, with a thirty molar excess of dithiothreitol (&#34;DTT&#34;). The solution was stirred under a water saturated nitrogen gas atmosphere for three hours at room temperature. An additional 15 molar excess DTT was added and the mixture was stirred an additional hour at room temperature under water saturated nitrogen gas. The peptide containing reduced cysteine was separated from the other components by chromatography at 4° C. over a G10 Sephadex column equilibrated with 0.2M acetic acid. The peptide was lyophilized and stored under vacuum until used. 
     The DT was activated for coupling to the peptide by treatment with the heterobifunctional linking agent epsilon-maleimidocaproic acid N-hydroxysuccinimide ester (&#34;EMCS&#34;), in proportions sufficient to achieve activation of approximately 25 free amino groups per 10 5  molecular weight of DT. In the specific instance of DT, this amounted to the addition of 6.18 mg of EMCS (purity 98%) to each 20 mg of DT. 
     Activation of DT was accomplished by dissolving each 20 mg aliquot of DT in 1 ml of 0.5M sodium phosphate buffer, pH 6.6. Aliquots of 6.18 mg EMCS were dissolved into 0.2 ml of dimethylformamide. Under darkened conditions, the EMCS was added dropwise in 50 microliter (&#34;μl&#34;) amounts to the DT with stirring. After 90 minutes incubation at room temperature in darkness, the mixture was chromatographed at 4° C. on a G50 Sephadex column equilibrated with 0.1M sodium citrate buffer, pH 6.0, containing 0.1 mM ethylenediaminetetraacetic acid disodium salt (&#34;EDTA&#34;). (Column=1.5×120 cm; flow rate=8 ml/hr; fraction size=2 ml). The fractions&#39; A 260  were determined using a spectrophotometer, enabling the fractions containing DT to be identified. 
     Fractions containing the EMCS activated DT were pressure concentrated over a PM 10 ultrafiltration membrane under nitrogen gas in conditions of darkness. The protein content of the concentrate was determined by the BCA method (PIERCE, IL, USA). The EMCS content of the carrier was determined by incubation of the activated DT with cysteine-HCl followed by reaction with 100 μl of 10 mM Elman&#39;s Reagent (5,5, dithio-bis (2-nitrobenzoic acid)). The optical density difference between a blank tube containing cysteine-HCl and the sample tube containing cysteine-HCl and carrier was translated into EMCS group content by using the molecular extinction coefficient of 13.6×10 3  for 5-thio-2-nitro-benzoic acid at 412 nm. 
     The reduced cysteine content (&#34;-SH&#34;) of the peptide was also determined utilizing Elman&#39;s Reagent. Approximately 1 mg of peptide was dissolved in 1 ml of nitrogen gas saturated water and a 0.1 ml aliquot of this solution was reacted with Elman&#39;s Reagent. Utilizing the molar extinction coefficient of 5-thio-2-nitro-benzoic acid (13.6×10 3 ), the free cysteine -SH was calculated. 
     The reduced peptide was then coupled to the activated DT. An amount of peptide containing sufficient free -SH to react with a selected proportion of the EMCS activated amino groups on the DT was dissolved in 0.1M sodium citrate buffer, pH 6.0, containing 0.1 mM EDTA, and added dropwise to the EMCS activated DT under darkened conditions. After all the peptide solution had been added to the activated DT, the mixture was incubated overnight in the dark under a water saturated nitrogen gas atmosphere at room temperature. 
     The conjugate of the peptide linked to DT via EMCS was separated from other components of the mixture by low pressure chromatography at 4° C. over a G50 Sephadex column equilibrated with 0.2M ammonium bicarbonate (column=1.5×120 cm, flow rate=1.8 ml/15 min., fraction size=1.8 ml). The conjugate eluted in the column void volume (detected by A 280  measurements) and was lyophilized and stored desiccated at -20° C. until used. 
     The conjugate may be characterized as to peptide content by a number of methods known to those skilled in the art including weight gain, amino acid analysis, etc. Various substitution ratios of peptide to DT were accurately and reproducibly obtained by (1) varying the quantity of EMCS added to activate the DT, and/or, (2) varying the quantity of reduced peptide added to the EMCS activated DT. For example, the activation of DT with a ratio of 31 moles EMCS to 1 mole of 100,000 molecular weight DT adds 12±2 EMCS groups per 100,000 molecular weight of DT. The addition of 14 peptide groups per 100,000 molecular weight of this activated DT resulted in a substitution ratio of 12±2 peptides per 100,000 molecular weight of DT. Conjugates of Peptides 1-4 to DT produced by these methods were determined by amino acid analysis to have 4-30 moles of peptide per 10 5  MW of DT. All of the conjugates were considered suitable as immunogens for immunization of test animals. 
     EXAMPLE 2 
     For comparative purposes a prior art GnRH immunogen (&#34;peptide 5&#34;) was constructed wherein the peptide immunomimic of GnRH did not contain a spacer element. Peptide 5 had the sequence: Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-GlyNH 2  (SEQ ID NO: 8 in the Sequence Listing). 
     The peptide was activated with m-Maleimidobenzoyl N-Hydroxysuccinimide Ester (&#34;MBS&#34;). 20.0 mg of  glu 1!-GnRH were dissolved in 1.0 ml of N,N-Dimethylformamide (&#34;DMF&#34;). To this solution was added 5.31 mg MBS dissolved in 1.0 ml DMF. The combined solution was stirred overnight at room temperature in the dark. 
     40.0 mg of DT was dissolved in 10.0 ml of Sodium Carbonate Buffer (0.2M, pH=9.0), containing 2.2 mg of 2-Iminothiolane HCl (&#34;2-IT&#34;). The solution containing the MBS-activated GnRH was then slowly added to the DT/2-IT solution, and the mixture was stirred slowly for 8 hours at room temperature in the dark. 
     The conjugate was purified by column chromatography over Sephadex G50 (column: 1.5×100 cm; buffer: Ammonium Bicarbonate, 0.2M; fractions: 2.6 ml, every 15 minutes) with identification of the fractions containing conjugate by spectrophotometry (A 254 ). G50 purified conjugate was lyophilized and stored desiccated at -20° C. until used. The peptide DT substitution ratio of the Immunogen 5 conjugate was determined by amino acid analysis to be 13 peptides per 10 5  molecular weight of DT. 
     EXAMPLE 3 
     Different groups of female rabbits were each immunized with one of the conjugates, peptides 1-5 of Examples 1 and 2. Each conjugate was dissolved to a concentration of 2.0 mg/ml in phosphate buffered saline (0.2M, pH=7.2) containing 200 μg/ml of norMDP adjuvant. The conjugates comprising peptides 1,2,3 and 4 of Example 1 did not completely dissolve in the buffer; the conjugate of peptide 5 of Example 2 did completely dissolve in the buffer. Each mixture was emulsified with an equal volume of Squalene-Arlacel (4:1 ratio, volume of Squalene:volume of Arlacel) to prepare an immunogen formulation which contained 1.0 mg/ml conjugate and 100 μg/ml norMDP. 1.0 ml of immunogen was injected into each rabbit, administered into the rear leg muscles (2 sites, 0.5 ml/site), on day 0 of the test. Blood was collected from each rabbit prior to immunization on day 0, and on selected days thereafter. Serum was prepared from each blood sample and stored frozen at -20° c. until utilized in assays to determine the presence of anti-GnRH antibodies. 
     A liquid phase Radioimmunoassay (RIA) was used to detect and quantify anti-GnRH antibodies. In the RIA, 0.04, 0.2, 1.0 or 5.0 μl aliquots of antiserum were incubated with approximately 150 fmole of 3H labeled GnRH (specific activity=53.2 Ci/mmole) in a total volume of 400 μl. Dilutions were made in FTA Hemagglutination Buffer (BBL, Becton Dickinson Microbiology Systems, MD, USA) containing 1% bovine serum albumin. The antisera were incubated with labeled hormone for 2 hours at room temperature. A 0.1 ml aliquot of heat inactivated (56° C., 30 min) fetal calf serum (cooled to 2°-8° C.) was then added to each tube, following which the antibody-hormone complexes were precipitated by the addition of 0.5 ml of 25% polyethylene glycol (MW=8,000 gm/mole) (cooled to 2°-8° C.). The precipitates were pelleted by centrifugation (30 minutes at 1500×g), the supernatants were discarded, and the pellets were counted by liquid scintillation counting to measure the quantity of radioactivity contained therein. Antigen binding capacities (ABC) for each antiserum were then determined from the amount of radioactive hormone precipitate after substraction of nonspecific background binding (determined by preincubation of the antisera dilution with excess amounts (˜10 5  fold) of the hormone). Inhibition of the antisera with the excess quantity of unlabeled hormone also established the specificity of the antisera for GnRH. Serum taken from the rabbits prior to immunization served as nonimmunized (normal) controls. 
     The mean ABCs measured in the sera from rabbits immunized with the conjugated peptides of Examples 1 and 2 are shown in Table 3 and in FIG. 1. As the results show, a single administration of the immunogens comprising peptides 1,2,3 and 4 of Example 1 induced rapid and potent antibody responses against GnRH. 
     
                                           TABLE 3__________________________________________________________________________RABBIT ANTI-GNRH ANTIBODY RESPONSES INDUCEDBY ONE ADMINISTRATION OF PEPTIDE CONJUGATE__________________________________________________________________________Peptide:DTSubstitution     Rabbit Sera ABC (mean)  pmoles/ml!Peptide    Ratio Day 0         Day 14             Day 21                 Day 28                     Day 36                         Day 44                             Day 56                                 Day 73                                     Day 85                                         Day 105__________________________________________________________________________1   13    0   0.30             10.83                 22.63                     57.23                         68.93                             72.13                                 61.23                                     58.73                                         54.032   13    0   0.27              7.52                 19.83                     57.63                         77.83                             78.73                                 60.83                                     47.90                                         24.935   13    0   0       0    1.78    1.60                                  1.51                                      2.00                                          2.10__________________________________________________________________________     Day 0         Day 15             Day 24                 Day 31  Day 44                             Day 59  Day 79                                         Day 101__________________________________________________________________________3   11    0   1.53             24.59                 58.31   102.71                             118.16  120.99                                         61.004   13    0   1.77              8.90                 26.03   42.88                             38.25   38.30                                         24.35__________________________________________________________________________ n = 5 rabbits for Peptides 1, 2, 3, and 4. n = 6 rabbits for peptide 5. 
    
     By comparison, the anti-GnRH response induced by a single administration of the peptide 5 immunogen of Example 2 induced a minimal response. This is not because the conjugate constructed with peptide 5 is a poor immunogen; when administered in additional booster immunizations several weeks after the first immunization, the peptide 5 conjugate induces effective levels of anti-GnRH antibodies (of approximately 12-18 pmole/ml ABC). In this regard, the peptide 5 conjugate behaves similarly to standard GnRH immunogens. However, the conjugate constructed with peptide 5 requires more than one administration, induces lower levels of anti-GnRH antibodies, and takes a longer time to elicit effective antibody levels than do the conjugates of peptides 1-4 of Example 1. 
     These results also demonstrate the critical contribution of the spacer to the immunogenicity of peptides 1,2,3 and 4 of Example 1. Peptide 5 bears the same immunomimic of GnRH as peptides 1 and 3, yet peptide 5 is inferior as an immunogen. This is because peptide 5 does not contain a spacer sequence, which is present in peptides 1 and 3. Thus, the presence of the spacers in peptides 1,2,3 and 4 of Example 1 makes a critical contribution to their enhanced immunogenicity. 
     EXAMPLE 4 
     Conjugates comprising peptides 3 and 4 of Example 1 were mixed 1:1 to give a protein concentration of 2.0 mg/ml in PBS. The mix was then prepared as immunogen and injected into rabbits, as in Example 3. The sera were tested for anti-GnRH antibody by the RIA of Example 3. The results are shown in Table 4 and FIG. 2. 
     
                       TABLE 4______________________________________RABBIT ANTI-G.sub.N RH RESPONSES INDUCED BY ONEADMINISTRATION OF A MIXTURE OF PEPTIDE CONJUGATESABC (mean ± s.e.)  pmoles/ml!Day of Test   0     15     24    31   44   59    79   101______________________________________ABC     0     4.6±                21.6±                      49.0±                           77.8±                                86.0±                                      74.3±                                           43.0±         0.7    3.3   9.9  13.0 21.0  22.0 12.0______________________________________ 
    
     As can be seen from Table 4, effective levels of antibody were induced by the combined administration of the peptide 3 and 4 conjugates. Both peptide components contributed almost equally to the induction of the anti-GnRH antibodies, as shown by antibody specificity testing. The GnRH (1-10)-Arg1 peptide induced antibodies directed predominantly against the carboxy terminal end of GnRH, while the GnRH(1-10)-Arg 10 peptide induced antibodies directed against the amino terminal end of GnRH. Thus, conjugates comprising these peptides can be mixed to yield immunogens that induce antibodies against both ends of the target molecule. 
     EXAMPLE 5 
     When the peptides of Example 1 are conjugated to DT and prepared as described in Example 1, a proportion of the product is present as a precipitate. The formation of the precipitate is dependent upon various physical parameters, including concentration of conjugate, pH and salt concentration. We prepared a conjugate of peptide 2 of Example 1 to DT as described in Example 1. From this we prepared three fractions of conjugate, based upon solubility. The conjugate was stirred in 0.01M phosphate buffer pH=7.2 and the insoluble material was collected by centrifugation as Fraction #1. To the soluble material we added NaCl (to 0.5M) and adjusted the pH to 6.0 with 0.1M HCl, which yielded additional precipitate that we collected as Fraction #2. The remaining soluble material served as Fraction #3. Each fraction was lyophilized. The percent recoveries (from the 15 mg of starting material) were: Fraction-1, 36%; Fraction-2, 15%; Fraction-3, 27%; lost, 22%. 
     Each of the fractions 1-3 were injected into a group of mice, at 6 mice/group. (100 μg conjugate/mouse, with 25 μg nMDP, in 0.1 ml of a 1:1 mixture of FTA buffer (containing conjugate+adjuvant) to squalene-arlacel, i.p.). The mice received a single injection of immunogen, after which sera samples were obtained at intervals and tested for anti-GnRH antibody by the RIA of Example 3. The results of this test are shown in Table 5 and in FIG. 3. 
     
                                           TABLE 5__________________________________________________________________________ANTI-GnRH RESPONSES OF MICE TOSOLUBILITY FRACTIONS OF CONJUGATEConjugateABC (mean ± s.e.)  pmoles/ml!FractionDay 0    Day 14         Day 21              Day 28                   Day 36                        Day 45                             Day 56__________________________________________________________________________1    0   1.7 ± 0.3         4.5 ± 0.4              4.6 ± 0.4                   5.6 ± 0.4                        5.9 ± 0.5                             5.6 ± 0.42    0   1.7 ± 0.4         4.2 ± 0.3              4.6 ± 0.2                   5.7 ± 0.2                        5.8 ± 0.2                             5.8 ± 0.23    0   1.7 ± 0.3         4.0 ± 0.3              4.5 ± 0.3                   5.3 ± 0.3                        5.3 ± 0.3                             5.0 ± 0.3__________________________________________________________________________ 
    
     As the results show, each mouse group produced equally potent anti-GnRH antibody responses. This demonstrates that despite variances in the solubility of conjugates produced from the peptide of Example 1, the soluble and insoluble forms can be administered as immunogens and are of equivalent immunogenicity. 
     EXAMPLE 6 
     We constructed conjugates of peptides 1 and 2 of Example 1 to DT as described in Example 1. By varying the quantities of reduced peptide added to DT, we constructed conjugates with different peptide:DT substitution ratios. The substitution ratios, determined by amino acid analysis of the conjugates are shown in Table 6: 
     
                       TABLE 6______________________________________Conjugate   Peptide Used                   Peptide:DTNumber      (from Example 1)                   Substitution Ratio______________________________________6.1         1           4.76.2         1           13.16.3         1           25.96.4         2           5.16.5         2           12.86.6         2           30.1______________________________________ 
    
     Mice were immunized with each conjugate preparation. The immunization and subsequent assay procedures were identical to those described in Example 5 (6 mice/group). The results of this test are shown in Table 7 and in FIG. 4. 
     
                                           TABLE 7__________________________________________________________________________ANTI-GnRH RESPONSES OF MICE TO PEPTIDE-CARRIERCONJUGATES WITH A DIFFERENT SUBSTITUTION RATIOSPeptide:DTConjugateSubstitution      ABC (mean ± s.e.)  pmoles/ml!numberRatio Day 0          Day 14               Day 28                    Day 45                         Day 56                               Day 70                                     Day 85                                           Day 105__________________________________________________________________________6.1  4.7   0   0.7 ± 0.1               5.1 ± 0.4                    9.8 ± 0.5                         9.4 ± 0.4                               10.5 ± 0.6                                     11.0 ± 0.8                                           10.0 ± 1.06.1  13.1  0   1.8 ± 0.3               7.4 ± 0.6                    9.7 ± 0.4                         10.1 ± 0.2                               12.2 ± 0.2                                     11.9 ± 0.2                                           11.0 ± 0.26.3  25.9  0   0.4 ± 0               2.1 ± 0.5                    4.9 ± 1.0                         5.1 ± 1.1                               4.7 ± 1.3                                     5.7 ± 1.7                                           4.7 ± 1.66.4  5.1   0   1.7 ± 0.6               4.1 ± 0.6                    4.6 ± 0.7                         5.0 ± 0.6                               6.8 ± 0.9                                     7.3 ± 1.2                                           6.7 ± 1.16.5  12.8  0   1.4 ± 0.1               4.5 ± 0.2                    5.4 ± 0.3                         6.1 ± 0.4                               7.2 ± 0.2                                     8.4 ± 0.3                                           7.9 ± 0.36.6  30.1  0   1.1 ± 0.4               3.9 ± 0.4                    4.6 ± 0.4                         5.4 ± 0.4                               6.6 ± 0.5                                     7.4 ± 0.5                                           7.0 ± 0.5__________________________________________________________________________ 
    
     As the results show, significant anti-GnRH responses were induced by each of the conjugate preparations. This demonstrates that the peptides of Example 1 can be conjugated to carriers over a broad range of peptide:carrier substitution ratios and yield effective immunogens. 
     EXAMPLE 7 
     We constructed conjugates of peptides 1 and 2 of Example 1 to DT as described in Example 1. The peptide:DT substitution ratio for peptide 1 (GnRH(1-10)-Ser1) was 13.1:1 and the ratio for peptide 2 (GnRH(1-10)-Ser10) was 12.8:1. 
     We prepared immunogen by emulsifying aqueous phase (containing a mixture of the two conjugates plus norMDP in PBS) with oily vehicle as described in Example 3. The oily vehicle used was Montanide ISA 703 containing 1.8% aluminum monostearate. &#34;Montanide ISA 703 AMS&#34; is manufactured and sold by SEPPIC, Inc. (Paris, France). The final concentrations of the active components in the immunogen were: GnRH (1-10)-Ser1-DT=0.5 mg/ml; GnRH (1-10)-Ser10-DT=0.5 mg/ml; norMDP=0.1 mg/ml. 1.0 ml of immunogen was injected into each of 3 male rabbits, administered to the rear leg muscles (2 sites/rabbit, 0.5 ml/site), on day 0 of the test. Blood was collected from each rabbit prior to immunization and on selected days thereafter. Serum was prepared from each blood sample and stored frozen at -20° C. until utilized in assays to determine the presence of anti-GnRH antibodies (as described in Example 3). 
     The mean ABC&#39;s measured in the sera from these three male rabbits are shown in Table 8 and in FIG. 5. As the results show, a single immunization with the DT conjugates of peptides 1 and 2 of Example 1 in the Montanide ISA 703 containing 1.8% AMS rapidly induced potent antibody responses against GnRH. These anti-GnRH responses are representative of responses induced by the peptide conjugates (individually or mixtures thereof) of this invention when administered with norMDP in an emulsion comprising equal parts aqueous phase and Montanide ISA 703 containing 1.8% AMS. 
     
                       TABLE 8______________________________________  Mean ABC                Mean ABCDay    (pmol/ml) (±s.e.)                     Day  (pmol/ml) (±s.e.)______________________________________ 0     0.02     (±0.1) 46   543    (±85.0) 7     0.18     (±0)   60   1061   (±368.2)14     3.71     (±0.8) 74   1303.3 (±527.6)24     40.3     (±7.7) 88   1320.7 (±602.9)32     131.5    (±29.1)                     102  1272   (±558.1)40     374.7    (±13.1)                     --   --______________________________________ 
    
     EXAMPLE 8 
     The production of gonadal steroids can be assessed as a measure of GnRH-immunogen efficacy in immunized animals. We measured testosterone levels in the serum samples obtained from the three male rabbits of Example 7. The testosterone levels were determined using a radioimmunoassay kit for testosterone determination (&#34;Coat-a-Count&#34;, purchased from Diagnostic Products Corp., Los Angeles, Calif., USA). The results presented in Table 9 and in FIG. 5 show the immunogen induced levels of anti-GnRH antibodies that totally inhibited the production of testosterone in the male rabbits. 
     Testosterone was undetectable in the sera of 2 animals by day 24 of the test, and in all 3 rabbits by day 32. The drop in testosterone serum coincides with the rise in anti-GnRH Ab titer, as can be seen in FIG. 5. 
     
                       TABLE 9______________________________________Testosterone Levels In Immunized RabbitsDay    Mean T (ng/ml) (±s.e.)                 Day    Mean T (ng/ml) (±s.e.)______________________________________ 0     0.32 (±0.2) 46     0 7     1.37 (±0.1) 60     014     1.21 (±0.5) 74     024     0.1 (±0)    88     032     0              102    040     0              --     --______________________________________ 
    
     
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Immunogenic compositions capable of generating an immune response in mammals against GnRH are disclosed. The immunogenic compositions are effective in methods of treating gonadotropin and gonadal steroid hormone dependent diseases and immunological contraception of mammals.