VACCINE FOR SUSCEPTIBLE HOSTS AGAINST NONADAPTED PARASITES

Abstract: It has been found that hosts which are susceptible to nematode parasite infections can readily be protected from such infections when the parasites are not adapted for a parasite/host relationship to this host. In particular, feline hosts were immunized against heartworm using a variety of antigens derived from Dirofilaria immitis and related nematodes. Because cats are hosts susceptible to this nonadapted parasite, such antigens are successfully protective.

Description: VACCINE FOR SUSCEPTIBLE HOSTS AGAINST NONADAPTED PARASITES Technical Field The invention is directed to methods to protect susceptible animals against infection by nematode parasites where the parasites are generally nonadapted to a long-term parasite/host relationship with the host. In particular, the invention concerns protection of cats against heartworm infection. Background Art Parasitic infections generally depend on the ability of the parasite to establish a relationship with the host which, while detrimental to the robust good health of the host, is sufficiently benign to assure the parasite of a sufficiently long-term relationship to continue its own life cycle. An example of such a parasitic relationship is that of heartworm in dogs. Heartworm infection is very common and caused by the nematode, Dirofilaria immitis. Heartworm primarily affects dogs; indeed, in some geographical areas many canines are carriers of this parasite. On the other hand, only a relatively small number of cats in enzootic areas, probably less than 15%, harbor this nematode. It is currently not understood why cats, while susceptible, appear to react to infection differently from their canine counterparts. Heartworm infection in dogs occurs through passage of the third stage larvae (L3) of D. immitis into the subcutaneous tissue as a result of a mosquito bite. In the subcutaneous tissue, the L3 larvae molt into a fourth larval stage (L4) which then migrates toward the thoracic cavity. A subsequent molt to an immature adult occurs and the immature adult gains access to the heart and pulmonary arteries where maturity into adulthood occurs. The L3 remain near the site of infection; L4 migrate through the cutaneous tissue and muscle and do not molt to the fifth stage until 50-70 days after infection, "the adult D. immitis are on the order of 12- 20 cm (males) and 25-31 cm (females) and produce motile vermiform embryos called microfilariae which are only 0.3 mm long. These embryos traverse capillary beds and circulate in the vascular system. There, the microfilariae are ingested by mosquitoes and continue their life cycle through L3 in the mosquito vector until infection of the host subsequent to the mosquito bite. As heartworm infection is a recognized problem in dogs, various attempts have been made to prevent or treat this condition. While there have been a number of attempts to prepare vaccines against heartworm infection in dogs, it is clear that this is not a straightforward matter and that the success of a particular vaccine depends on the proper selection of the appropriate antigen. The nature of this antigen is not yet completely established. Because vaccines have not been practical, dogs have been treated with parasiticides after infection. Cats do not ordinarily harbor heartworm parasites, and when they do, they are likely to die from the infection. It has not been possible to treat infected cats with parasiticides, because while treatment with parasiticides, in particular adulticides, is commonly practiced in dogs, it may be fatal in cats. In general, it is recommended that cats without overt signs of heartworm should not be treated (Hawkins, E.C. Kal Kan Forum (1989) .8:2-7) . Cats exhibit severe post-adulticide reactions which often must be treated with oxygen supplementation and corticosteroid administration. Follow up treatment when microfilariae are present using a microfilaricide is also often necessary. Preventatives used in dogs have also been less successful in cats, are not currently approved for use in the United States and are generally not used. In view of the substantial incidence of harmful heartworm infection in cats, and in view of the great risks associated with post-infection treatment, it would be highly desirable to produce a vaccine which would be protective against heartworm infection in felines, as well as other animals susceptible to but not adapted to a parasite/host relationship with heartworm. Cats, with respect to infection by D. immitis, are a model system of a susceptible host and a parasite which is not adapted to that host. As demonstrated herein, presumably because cats are not adapted to accommodate the heartworm parasite as are dogs, generic stimuli of the immune system are protective against infection. These stimuli include, for example, the administration of cytokines or of immunogens, e.g., proteins or other antigens associated with the various life stages of D. immitis and related nematodes, or mild viral infection. Disclosure of the Invention The invention provides protocols and vaccines capable of protecting susceptible hosts against infection by nonadapted nematode parasites. In particular, cats are protected from infection by the nonadapted parasite D. immitis. Presumably because of the nonadaptation of the parasite, a spectrum of protocols to stimulate the immune system, such as immunization with antigenic substances derived from the parasite or substances related to these, is effective. In particular, for cats, antigens from D. immitis and related nematodes may be used as the active components of a vaccine. Thus, in one aspect, the invention is directed to a method to protect susceptible hosts against infection by nonadapted nematode parasites, which method comprises stimulating the immune system of the host by administration of immunostimulants, such as cytokines, or by administering to the host a protective amount of an antigen derived from the nonadapted nematode parasite or other nematode. In the case of cats, Dirofilaria immitis represents a suitable nonadapted parasite. In other aspects, the invention is directed to pharmaceutical or vaccine compositions useful in the invention method. Brief Description of the Drawings Figure 1 shows the DNA encoding the immunogenic protein Di22 and the deduced amino acid sequence of this protein. Modes of Carrying Out the Invention The invention is based on the discovery that hosts which are " susceptible to infection by a nematode parasite, but for which host an effective parasite/host relationship has not been developed, can be protected altogether from infection by these nonadapted parasites by the use of generalized immune stimulants such as a wide spectrum of antigens derived from this parasite or other nematodes. Thus, for example, while prevention of heartworm infection in dogs requires specific antigens capable of providing the most effective immune responses, hosts which are not ordinarily infected by heartworm, such as cats, can easily be protected by immunization with undefined extracts of the nonadapted nematode. The inventors herein have thus established that immunization i>γ .ntigens in general derived from parasites not adapted for infection of a particular host, but wherein the host is susceptible to infection by the parasite, are effective in preventing infection in these hosts. Thus, as used herein, the terms "susceptible" host and "nonadapted" parasites are used in conjunction with each other. The susceptible host is capable of being infected by the parasite, but the parasite is not adapted to the susceptible host in such a way that a long-term parasite/host relationship can be established as in the case of "normal" hosts. Thus, the susceptible hosts are infrequently infected, but when infection occurs, it is serious and often fatal. The method of the invention is grounded in the discovery that because the feline host is not adapted to support the growth of the heartworm parasite, a wider range of immunogens can be used to confer immunity than would be possible for hosts adapted to the growth of the parasite. Thus, in a generalization of this finding, susceptible hosts may be protected from infection by nematode parasites by any protocol that operates as a general immune system stimulant. In one approach, the immune system can be stimulated to operate at an elevated level at the time of challenge by the parasite by administering to the host generic immunostimulants such as cytokines or other medicaments known to elevate the response of the immune system generally. Such immunostimulants include, for example, interleukins such as IL-2 and colony-stimulating factors such as GCSF and GMCSF. Methods for elevating immune system performance are known generally, and such protocols may be used to advantage in the present invention. In the alternative, or in addition, the susceptible host may be immunized using antigens derived from nematodes in general, including both parasitic and free-living forms. It appears to be unnecessary that the nematode be related to the nematode parasite against which protection is desired; however, in a preferred embodiment, the parasites to which the hosts are susceptible but to which they are not adapted, or nematodes related to these parasites, are preferred. (It should be noted that although "nonadapted" as an adjective is generally applied to the parasite, it is actually a mutual adaptation that is being described. Thus, "nonadapted" parasite includes adaptations which are made in the metabolism of the host, as well as in the metabolism of the parasite.) By "derived from" a nematode is meant that the counterpart of the antigen exists in the nematode, and does not necessarily imply that the antigen as used is, in fact, physically extracted from the nematode in question. For purposes of illustration, cats are used herein as the susceptible host, and heartworm infection caused by D. immitis is the corresponding nonadapted parasite. The antigens useful in the method of the invention using cats as hosts are derived from Dirofilaria immitis or related nematodes. They can be isolated from parasite culture, necropsy specimen or can be synthetically or recombinantly produced. By "derived from" is meant that the antigen in question exists in D. immitis or a related nematode. Its actual production need not be by isolation therefrom. It has been found that, in addition to vaccine components identifiable in larval stages of D. immitis which are effective in dogs, it is also possible to substitute larval immunogens which are not effective in dogs or immunogens that may be isolated from additional life cycle stages of this nematode, including the adult form, as well as the various life stages of related nematodes. U.S. patent application Serial No. 07/683,202, filed 8 April 1991 and incorporated herein by reference, describes a particular protein, designated therein "Di22". Proteins such as Di22 can be used in cats. In addition, as set forth below, U.S. Serial No. 07/792,209, filed 12 November 1991, describes proteases isolatable from the L3 and L4 larval stages of the D. immitis parasite. These vaccine components are useful in both dogs and cats. On the other hand, the active immunogen in vaccines effective against cats may be derived from LI, L2, L5 or adult of D. immitis or may be derived from related nematodes -- specifically, those nematodes which are capable of infecting mammalian hosts. Such nematodes include Onchocerca cervipedis, 0. cervicalis, Brugia malayi , Trichostrongylus , Haemonchus, Ostertagia, Nematodirus, Parascaris equorum, Ascaris suu , Toxascaris leonina, Toxocara canis, Toxocara cati , Ancylostoma, Uncinaria, Strongyloides , or any other nematode capable of sustaining a parasitic relationship with a host or a free-living nesaatode such as Caenorhabditis elegans. In general, the active vaccine component(s) may be derived from related nematodes such as those set forth above, or any nematode capable of sustaining a parasitic relationship with a host, or even free-living nematodes. For isolation of an antigen or DNA encoding it, the nematodes can be cultured in vitro under suitable conditions to provide various larval stages as described by Abraham, D. et al. J Parasitol (1987) 73.:377-383. Briefly, for D. immitis, the mosquito Aedes aegypti Liverpool (black eyed strain) are infected with D. immitis by feeding on microfilaremic blood obtained from experimentally infected animals. 15 days after feeding, the mosquitoes are anaesthetized, surface sterilized, and placed on screens in funnels filled with a 1:1 mixture of NCTC-135 and Iscove's modified Dulbecco medium (Sigma) containing 2.5 μg/ml amphotericin B; 100 μg/ml gentamycin; 50 μg/ml sulfadiazine; and 10 μg/ml trimethoprim. The larvae are collected from funnels 90 minutes post-incubation. The cultures are maintained at 10 L3 organisms per ml of medium at 5% C0 2 and saturated humidity. The L3 larvae are cultured at 37°C in the foregoing medium, supplemented with 20% fetal calf serum for 1-8 days. Alternatively, 10 days after feeding, the mosquitoes are anesthetized and the worms are recovered by dissecting the heads and allowing the worms to emerge into medium with 20% Seru-max (Sigma) to induce molting. After 48 hours, the worms are recovered, washed 5 times in medium which does not contain Seru-max, and recultured. The L3 larval stage is maintained for about 96 hours in culture on Seru-max-free medium; molting to L4 then occurs and L4 is maintained up to 144 hours. These L3 and L4 larval stages are particularly useful as sources for antigens to provide the active ingredients for the invention vaccines. For example, protease components of the L3 and L4 larval stages are described in copending application U.S. Serial No. O7/792,209, filed 12 November 1991 and incorporated herein by reference. General methods to identify antigenic components are also described in copending application serial no. 07/654,226, filed 12 February 1991 and incorporated herein by reference. In addition to direct isolation of native antigens from the various larval stages of D. immitis or a related nematode, the antigens can be recovered using cDNA libraries prepared therefrom and produced recombinantly. Particularly useful components of the larvae can be identified by preparing the cDNA as an expression library, for example in λgtll and screening the library with antibodies obtained from infected dogs or by labeling the proteins produced in these libraries. As another alternative, antigens may be prepared from adult worms. Following humane euthanasia of dogs infected with Dirofilaria immitis the heart and lungs are removed, dissected and antigens are prepared from them. Adult female and male D. immitis are collected and separated by gender. Adult, exclusively male, D. immitis antigens are preferred for immunization. The antigens, once isolated and characterized, may also be produced synthetically. A single antigen may be employed, or a crude or defined mixture of antigens may be utilized. The antigens are often proteins, but alternative antigens include other biomolecules derived from nematodes such as carbohydrates, lipids, and the like. The vaccines of the invention are administered in a manner consistent with the nature of the vaccine; protein antigens are administered with conventional excipients for injection or other systemic administration. In addition to injection, transmucosal or transdermal delivery into the bloodstream may also be used using stand rd formulations for this purpose such as those described in Remington's Pharmaceutical Sciences. latest edition, Mack Publishing Company, Easton, PA. The mode of administration, if properly formulated, may also be oral; however, systemic injection using standard adjuvants and excipients is preferred. Of course, the active ingredient may be a single antigen or a mixture of two or more. Similarly, immunostimulants can be formulated in ways generally understood in the art and suitable for the immunostimulant of choice. Administration protocols are optimized for the specified host and the active ingredient. The following examples are intended to illustrate but not to limit the invention. Example 1 Immunization of Cats with L4 Antigens A. A general vaccine was prepared using D. immitis L4 larvae which have been cultured as described above. Frozen, thawed L4s were homogenized in phosphate buffered saline with 0.1% Triton X-100. The homogenate contained about 1,000 L4/0.05 ml. 24 cats, 16 female and 8 male, were divided into a control group and an experimental group. The control group received 0.05 ml on day 0 in each hindlimb of equal volumes of Titermax (CytRx Corporation, Norcross, Georgia) adjuvant and PBS; the immune group received similar amounts of Titermax and homogenate. On day 28, the animals were boosted with injections as on day 0, except those animals immunized with larvae received half the dose of L4 (500 L4/0.05 ml) . On day 35, the animals were challenged with 100 L3 larvae per cat by subcutaneous inoculation in the inguinal region. On day 181, 4 immune and 4 control animals were necropsied, but showed little sign of infection. On day 205, 8 controls and 7 immunized animals were necropsied (one had died previously) and assessed for heartworm infection. The results showed that 5 of the 12 control cats were infected with at least one parasite; however, none of the immunized cats were infected. B. In a second trial, 36 cats were divided into three groups, 12 cats per group, and each cat received 0.1 ml intramuscularly of either (1) a mixture of Titermax with extract prepared from 1,000 D. immitis L4 which had been homogenized as described in paragraph A in PBS/0.1% Triton X-100; (2) a second group received Titermax mixed with PBS/0.1% Triton X-100; and the (3) third group received only PBS/0.1% Triton X-100 on day 0. On day 28, each cat in group 1 received 0.1 ml Titermax plus the extract of 500 L4 D. immi tis prepared as above; the second group received 0.5-0.75 ml of Titermax and PBS/0.1% Triton X-100; the third group received no injections. On day 43 the cats were challenged with 100 D. immi tis L3 larvae and examined 24 weeks, after the challenge. The results showed 7 of 12 cats infected (58%) in Group 1 (L4/adjuvant) , 9 of 12 cats infected (75%) in Group 2 (adjuvant control) and 11 of 12 cats infected (92%) in Group 3 (PBS) . The combination of L4 and adjuvant was therefore successful in reducing the incidence of infection. Example 2 Immunization of Cats with Male Adult Antigens To prepare the vaccine, worms collected as described above are thoroughly washed in phosphate- buffered saline, drained, snap-frozen in liquid nitrogen and ground to a fine powder. This powder is resuspended in phosphate-buffered saline and sonicated. To determine dose, an aliquot is lyophilized and weighed. A total of 200 micrograms is administered twice, at Day 0 and Day 28. In a scaled-up procedure any method to produce a fine suspension of adult worm homogenate may be used. Experimental animals (cats) are divided into control and test groups and administered antigen and adjuvant, adjuvant with PBS, or antigen alone. The adjuvant used in this protocol is Titermax. Groups include: Group 1 Group 2 Group 3 Group 4 Group 5 - 9 9 10 10 10 Antigen Adjuvant Antigen Antigen Adjuvant +adjuvant +adjuvant Repeat Day 0 Challenge Challenge - Challenge Challenge Challenge Necropsy Necropsy - - - Necropsy Necropsy Necropsy Necropsy to assess worm burdens is performed at 169-170 after challenge as shown above. The results showed 3 of 9 cats infected (33%) in Group 1 (antigen/adjuvant) , 9 of 12 cats infected (67%) in Group 2 (adjuvant control) , 3 of 10 cats infected (30%) in Group 3 (antigen only) , 2 of 10 cats infected (20%) in Group 4 (antigen/adjuvant) and 6 of 9 cats infected (67%) in Group 5 (adjuvant control) . Thus antigen or antigen/adjuvant showed increased protection over the adjuvant control. Example 3 Immunization of Cats with Antigens from Free-Living Nematodes C. elegans are grown in bulk by standard procedures (The Nematode Caenorhabdi is elegans, Cold Spring Harbor Laboratory, William B. Wood, ed. , 1988), thoroughly washed and sonicated in M-9 buffer. To determine dose, an aliquot is lyophilized and weighed. A total of 200 micrograms is administered twice, at Day 0 and Day 28. Experimental animals (cats) are divided into control and test groups and administered antigen and adjuvant, adjuvant and M-9 buffer or antigen alone. The adjuvant used is Titermax. Groups of 10 cats each include: Day Group 1 Group 2 Group 3 Group 4 Group 5 0 Antigen Adjuvant Antigen Antigen Adjuvant +adjuvant +adj vant 28 Repeat Day 0 112 Challenge Challenge - 196 - - Challenge Challenge Challenge Necropsy to assess worm burdens is performed at 6 months after challenge. Example 4 Effectiveness of Sindbis Constructs The Sindbis constructs were prepared as follows: Subcloning of RA cDNA Di22 DNA (referenced hereinabove) contains a 399 base pair repeat which encodes a 15 Kd peptide designated RA. The DNA region corresponding to RA was amplified from Di22 DNA by polymerase chain reaction (PCR) using two synthetic oligonucleotides, P12 and P14, as described by Sambrook, J., et al. , Molecular Cloning (1989) . P12 : Xbal 5' GTC GAC CCC GGG TCTAGA ACCATGG CTC TCA GTG AAA TCA 3' P14 : Xhol 5' ACG CGT GGT AC CTCGAG TCA TCT GCA GCC TTC TTG AA 3' These two oligonucleotides represent sequences complementary to the opposite strands of RA-encoding DNA at the 5' end of each strand flanked with restriction sites Xbal and Xhol. After double digestion with Xbal and Xhol, respectively, and gel purification, RA-encoding DNA was subcloned into Bluescript II sk (+) (Stratagene) designated BHA. This subclone was confirmed by restriction enzyme mapping. Cloning RA Sequence Into Sindbis Vector The Xbal to Xhol fragment which contains RA- encoding sequence of the plasmid BHA DNA was gel- purified. The purified RA-encoding DNA was ligated into the Sindbis expression vector Toto 2J1 designated HJA, so that when the Sindbis vector is expressed in baby hamster kidney (BHK) cells or infects a host, it produces the RA polypeptide encoded by the insert, as described by Xiong, C, et al., Science (1989) 243:188-1191 (See also, Huang et al. U.S. Serial No. 07/296,454, incorporated herein by reference) . Toto2Jl sp6 p romoter ns r s r CAT nolvA Xbal t Xhol RA Xbal Xhol HJA sp6 promoter ns r s - RA nolvA Cats were divided into five groups, and each cat received 1 ml subcutaneously into the nape of the neck: Group 1 received 5 x 10' pfu HJA Sindbis construct (8 cats) ; Group 2 is a duplicate of Group 1; Group 3 received 5 x 10' pfu wild-type Sindbis (9 cats) ; Group 4 received HBSS plus 1% FCS diluent (8 cats) ; and 7 Group 5 received 5 x 10' pfu first-passage HJA Sindbis construct (2 cats) . As a secondary immunization on day 28, the protocol for Group 1 was repeated; Group 2 received 0.1 ml Titermax plus 80 μg GST-22 fusion protein 1 intramuscularly; Group 3 received 0.1 ml Titermax and HBSS plus 1% FCS; Groups 4 and 5 repeated their previous protocols. The cats were challenged 56 days after day 0 with 100 D. immi tis L3 subcutaneously. Examination for infection was conducted 24 weeks after challenge. Sera were collected preimmunization, 1 month postprimary and secondary immunizations, and monthly postchallenge. On days 169 and 170 postchallenge, all cats were euthanatized, necropsied, and inspected for the presence of adult D. immi tis . The necropsy results were as follows: 0% of the cats in Groups 1, 3 and 5 had worms, 12.5% (1 cat out of 8 cats) in Group 2 had a single worm, and 50% (4 cats out of 8 cats) in Group 4 had one to three worms. In accordance with these results, it can be concluded that use of Sindbis alone (Group 3) as well as the Sindbis construct containing the D. immitis-derived antigen is effective in protecting the subject host. GST-22 fusion protein refers to a protein recombinantly produced in E. coli by expression of the insert into the expression vector pGEX-3X of a 1300- nucleotide fragment encoding the protein shown in Figure 1. This fragment was recovered as described in detail in U.S. Serial No. 07/683,202, filed 8 April 1991 and incorporated herein by reference.

Claims: Claims 1. A method to protect susceptible hosts against infection by nonadapted nematode parasites, which method comprises subjecting said host to a protocol which results in stimulation of the immune system of the host to elevated levels. 2. The method of claim 1 wherein said susceptible host is a feline and said parasite is D. immitis. 3. The method of claims 1-2 wherein said protocol comprises administering to said host a protective amount of at least one antigen derived from an adult or larval stage of a nematode. 4. The method of claim 3 wherein the nematode is selected from the group consisting of D. immitis, C. elegans, Onchocerqa cervipedis, O. cervicalis, Brαgia malayi, Trichostrongylus , Haemonchus, Ostertagia, Nematodirus, Parascaris equorum, Ascaris s um, Toxascaris leonina, Toxocara canis, Toxocara cati, Ancylostoma, Uncinaria, and Strongyloides . 5. The method of claims 1-2 wherein said protocol comprises administering to said host an effective amount of at least one cytokine. 6. A vaccine composition useful in the method of claim 1, which vaccine comprises at least one antigen derived from said nonadapted nematode or a nematode related thereto, said antigen not protective against infection by said nematode parasite in normal hosts, in admixture with a pharmaceutically acceptable excipient. 7. The vaccine of claim 6 wherein the nonadapted nematode is D. , immitis and the related nematode is C. elegans, Onchocerca cervipedis, O. cervicalis , Brugia malayi, Trichostrongylus, Haemonchus, Ostertagia, Nematodirus, Parascaris equorum, Ascaris suum, Toxascaris leonina, Toxocara canis, Toxocara cati , Ancylostoma, Uncinaria, or Strongyloides .

Inventors: FRANK GLENN, GRIEVE ROBERT B, FRANK, GLENN, GRIEVE, ROBERT, B.
Applicants: UNIV COLORADO RES, COLORADO STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 1993
Main classification: A61K39
IPC codes: A61K39, A61P33, C07K14