the present invention provides the gag chimeric protein of hiv , which retains both antigenic and immunogenic properties of gag , and a portion of an env protein . the present inventors reported previously that expression of the gag coding sequences of hiv - 2 , lacking the protease gene , in insect cells produced virus - like particles ( virology 179 , 874 - 880 , 1990 ). the c - terminus of the gag protein , including the zinc finger domain , is not necessary for particle formation ( fig1 ). therefore , it is possible to replace the c - terminus of the gag precursor protein with other sequences without losing the ability of the gag protein to form virus - like particles . the gag protein has the unique ability to form particles in the absence of all other components of the virus , and the chimeric gag particles are devoid of genomic rna . formation of chimeric gag particles containing the major neutralizing epitope ( v3 ) and / or the cd4 - binding domain ( cd4bd ) of gp120 may allow effective generation of hiv - neutralizing antibodies ; antigens presented in a particulate form may enhance immunogenicity of the epitopes and multiple copies of specific epitopes can be presented . furthermore , secreted chimeric particles can be safely and easily collected and purified from cell culture media by centrifugation . in this application , we show mapping of essential domains of gag protein for particle formation and construction of six different chimeric gag genes containing either the v3 loop ( v3 ) or the v3 loop plus the cd4 binding domain ( v3 + cd4bd ) of gp120 from hiv - 1 or hiv - 2 . these constructs were expressed in insect cells using a baculovirus expression vector . the minimum length of hiv - 2 gag to form particles is 376 amino acids and 373rd position proline is essential for the particle formation . the chimeric gene construction reveals that only certain combinations of fusion proteins are expressed , assembled as virus - like particles , and retain antigenicity and immunogenicity of both gag and env epitopes . the present invention provides a map of the minimum length of hiv - 2 gag gene sequences which are required for the gag particle formations using deletion mutagenesis . an hiv - 2 gag gene sequence which is deleted up to 143 amino acids at the c - terminus from the 519 amino acid sequence , and which retains 376 amino acids at the n - terminus is the minimum protein for the gag particle formation of hiv - 2 . in contrast , deletion of four more amino acids to 372 amino acids at the n - terminus , eliminated gag particle formation . site - directed mutagenesis revealed that the proline at amino acid position 373rd , but not 375th and 377th , is essential for the particle formation . to map the gag protein domain ( s ) which is essential for the particle formation , both deletion and site - directed mutagenesis were carried out . fig1 clearly demonstrates that 376 amino acids at the n - terminus are required for the particle formation . in contrast to previous reports , the zinc - finger domain at the c - terminus is not required for gag particle formation . furthermore , the present site - specific mutants showed that the 373rd position proline is essential for the particle formation as shown in fig2 . the results in fig1 and fig2 indicate that one must retain uninterrupted 1 , 128 base pairs of hiv - 2 gag reading frame is essential to package foreign epitopes into gag particles . the present inventors have constructed six different combinations of chimeric genes by coupling the truncated hiv - 2 gag gene which will code for 425 amino acids to the neutralizing domain ( v3 ) on to the neutralizing domain ( v3 ) and the cd4 binding domains ( v3 + cd4bd ) of gp 120 env gene sequences from hiv - 1 or hiv - 2 . such a preparation has led to the present invention . the env gene sequences were either inserted into the middle of the gag gene or at the 3 &# 39 ; terminus of the gag gene . virus - like particles were formed by chimeric gene products only when the env gene sequences were linked to the 3 &# 39 ; terminus of the gag gene . insertion of env gene sequence in the middle of the gag gene resulted in high level chimeric gene expression but without the formation of virus - like particles . in particular , three different chimeric proteins : ( 1 ) gag ( 425 amino acids ) with hiv - 1 v3 ( 91 amino acids ), ( 2 ) gag with hiv - 2 v3 ( 90 amino acids ) and ( 3 ) gag with hiv - 2 v3 + cd4bd ( 198 amino acids ) formed virus - like particles that were secreted into the cell culture medium ( cf . table 1 ). table 1______________________________________comparison of particle formation and yield of chimericfusion proteins particle sucroserecombinant for - concen - yield / 5 × 10 . sup . 8 cell / viruses mation * tration ** liter______________________________________acnpv - hiv - 2gag + 40 % 30 mgac - gagm - 1v3 - -- -- ac - gagc - 1v3 + 50 % 6 mgac - gagm - 2v3 - -- -- ac - gagc - 2v - 3 + 50 % 25 mgac - gagc - 2v3 + + 60 % 2 mg2cd4bd______________________________________ * plus sign (+) denotes viruslike particles were recovered from the cell culture media . ** viruslike particles were banded on top of the sucrose solutions after ultracentrifugation . the present invention also provides a recombinant baculovirus containing chimeric gene . recombinant baculovirus of the present invention can be prepared by using autographa californica nuclear polyhedrosis virus ( acnpv ) or bombyx iridexcent virus . the recombinant virus is obtained by inserting the chimeric gene into acnpv genome and purified by the consecutive plaque assays to select polyhedrin negative recombinant virus . the acnpv inserted with gag of hiv - 2 is deposited with the atcc as a deposit number vr2314 , acnpv inserted with gag of hiv - 2 and v3 of hiv - 1 as a number vr2316 , acnpv inserted with gag of hiv - 2 and v3 of hiv - 2 as a number vr2317 , on feb . 26 , 1991 . these are guaranteed for furnishing for the purpose of research . the insect cells selected from the group consisted of spodoptera frugiperda cells , mamestra brassica cells , trichoplusia ni cells , bombyx mori cells and bombyx mori silkworm cells were infected with recombinant baculovirus obtained as above to express gag chimeric protein . the chimeric gag particles of the invention elicit neutralizing antibodies in the rabbits which completely block hiv infection . the chimeric gag particles of the invention can be used for the antigen of a potential anti - aids vaccine or as a diagnostic reagent . a vaccine prepared with the chimeric gag particles according to the present invention would be useful to induce a specific immune reaction against an immunogen when the vaccine is used at pre - or post - exposure of hiv to assist in preventing an hiv infection , or may be used for immunotherapeutic treatment . the vaccine of the invention may include a conventional absorbent , stabilizer , adjuvant , carrier and the like , and may be administered through a conventional route . other features of the invention will become apparent in the course of the following description of the exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof . autographa californica nuclear polyhedrosis virus ( acnpv ) and recombinant acnpv were grown and assayed in spodoptera frugiperda ( sf9 ) cell monolayers using complete tnm - fh medium containing 10 % fetal bovine serum at 27 ° c . as described in adv . virus res . 35 , 177 - 192 , 1988 . wild type acnpv dna was purified by the method of smith and summers ( virology 89 , 517 - 527 , 1978 ). plasmids phxb - 2d and p1bm containing the entire hiv - i hxb2d and hiv - 2 nihz genomes respectively were obtained from dr . r . gallo ( national institutes of health , bethesda , md .). the recombinant plasmids puc19 - gp120 - nss , puc18 - gp120 and puc19 - gag containing full - length cdna copies of the hiv - 1 env , hiv - 2 env and a truncated gag gene of hiv - 2 ( which codes 425 amino acids ) respectively were subcloned from phxb - 2d and p1bm as described by the inventors in virology 179 , 874 - 880 , 1990 . to subclone the v3 and v3 + cd4b of the hiv - 1 and hiv - 2 env genes into puc19 - gag recombinant plasmid puc19 - gp120 - nss and puc19 - gp120 were used as templates and the specific regions were amplified with appropriate oligonucleotide primers . that is to say , plasmids puc19 - gp120 - nss ( hiv - 1 ) and pucls - gp120 ( hiv - 2 ) were used as templates for polymerase chain reaction ( pcr ) amplification of an hiv - 1 dna fragment corresponding to the v3 domain ( 91 amino acids , position 273 - 363 ) and hiv - 2 dna fragments corresponding to a v3 equivalent domain ( 90 amino acids , position 294 - 383 ) and v3 + cd4bd ( 19 amino acids , position 294 - 491 ). all primers were designed to create a bglii restriction site at the 5 &# 39 ; end so that gp120 dna fragments could be inserted into the bglii site of puc19 - gag . the hiv - 1 v3 dna fragment was amplified using l - 1 :( 5 &# 39 ;- cgaagatctgtcaatttcacgg - 3 &# 39 ;) [ seq . id no : 1 ] and l - 2 ( 5 &# 39 ;- gcagatctttgcttaaagatta - 3 &# 39 ;) [ seq . id no : 2 ] primers , which are complementary to nucleotides 816 through 836 and 1075 through 1089 , respectively . the v3 + cd4bd dna fragment of hiv - 1 was generated from puc19 - gp120 - nss bglii digestion at nucleotide positions 816 and 1398 of gp120 . primers l - 3 :( 5 &# 39 ;- cgagatctcattgttaagaggcc - 3 &# 39 ;) [ seq . id no : 3 ] and l - 4 ( 5 &# 39 ;- gcagatcttctgcagttagtcc - 3 &# 39 ;) [ seq . id no : 4 ], complementary to nucleotides position 879 through 893 and 1135 through 1149 of hiv - 2 gp 120 , respectively , were used to synthesize the hiv - 2 v3 dna fragment . primers l - 3 and l - 5 :( 5 &# 39 ;- gcagatctctttactgatgtag - 3 &# 39 ;) [ seq . id no : 5 ], which is complementary to nucleotides 1459 through 1473 of hiv - 2 gp120 were used to synthesize the hiv - 2 v3 + cd4bd dna fragment . pcr was performed according to the procedures provided with the geneamp kit ( norwalk , conn .). briefly , 20 ng of linearized gp120 dna was added to 100 μl pcr reaction mixture ( 10 mm tris - hcl , 50 mm kcl , 1 . 5 mm mgcl 2 and 0 . 01 % gelatin ) containing 20 μm each dntp , 2 . 5 units of taq polymerase , and 20 μm of each oligonucleotide primer . the v3 and v3 + cd4bd genes were amplified for 30 pcr cycles ( 94 ° c . for 1 min ., 45 ° c . for 3 min .). the chimeric gag gene constructs containing the v3 and v3 + cd4bd fragments were confirmed by dideoxy dna sequencing of double stranded dna , using the sequence kit ( trademark of united states biochemical corporation ). gagm - 1v3 , gagc - 1v3 , gagc - 1v3 + 1cd4bd , gagm - 2v3 , gagc - 2v3 , gagc - 2v3 + 2cd4bd chimeric genes were prepared by the above method . the numbers 1 and 2 in front of v3 and cd4bd denote genes from hiv - 1 and hiv - 2 respectively , gagm denotes the chimeric gene inserted into the middle of the gag gene , and gagc denotes the chimeric gene inserted into the terminus of the gag gene . the procedure for generation of the present chimeric gene is shown briefly in fig3 . the chimeric genes obtained from the example 2 were isolated after digestion with bamhi and inserted into the baculovirus transfer vector pacym1 . sf9 cells were co - transfected with mixtures of infectious wild type acnpv dna ( 1 μg ) and recombinant plasmid dnas ( 4 μg ) using standard procedures ( adv . virus res ., 35 , 177 - 192 , 1988 ). among the recombinant baculoviruses obtained from the above procedure , acnpv carrying gag of hiv - 2 ( hereinafter called ` achiv2gagyk &# 39 ;) was deposited as deposit number vr2314 , acnpv carrying gag of hiv - 2 and v3 of hiv - 1 ( hereinafter called ` achiv2gag + 1v3yk &# 39 ;) as deposit number vr2316 , and acnpv carrying gag of hiv - 2 and v3 of hiv - 2 ( hereinafter called ` achiv2gag + 2v3yk &# 39 ;) as deposit number vr2317 with the american type culture collection ( atcc ) on feb . 26 , 1991 . the deposits are guaranteed for furnishing of samples for the purpose of research . after incubation of recombinant baculovirus at 27 ° c . for 4 days , culture supernatants were harvested and titrated on 80 % confluent monolayers of sf9 cells . to obtain polyhedrin - negative recombinant acnpv , the plaques lacking polyhedra were picked , purified by three consecutive plaque assays , and used to produce virus stock of 2 × 10 8 pfu / ml . sf9 cells were infected with either wild type acnpv or recombinant acnpv carrying chimeric genes at multiplicities of infection of 5 pfu / cell and incubated at 27 ° c . after appropriate incubation times ( usually 3 to 4 days ), cells were harvested and washed twice with pbs . whole cell lysates were prepared by resuspending the cell pellet in water and adding an equal volume of 2 × dissociation buffer ( 10 % beta - mercaptoethanol , 10 % sds , 25 % glycerol , 100 mm tris - hcl , ph 7 . 0 , 0 . 04 % bromophenol blue ). cell lysates were analyzed by electrophoresis in 12 % polyacrylamide gel containing sds ( sds - page ) and the protein bands were visualized by staining with coomassie blue ( nature 227 , 283 - 303 , 1987 ). western blot analyses were then performed using sera from aids patients and the bio - rad immune blot ak system . the results are shown in fig4 . fig4 a shows the proteins produced by recombinant baculoviruses ac - gagm - 1v3 , ac - gagc - 1v3 and ac - gagc - 1v3 + 1cd4bd . since the original hiv - 2 gag protein has 93 amino acids deleted from the c - terminus , an insertion of v3 ( 91 amino acids ) or v3 + cd4bd ( 194 amino acids ) of hiv - 1 gp120 into the gag gene would be expected to produce proteins of 55 kda and 66 kda , respectively . as shown in fig4 a , strongly stained protein bands migrating at either 55 kda or 66 kda were observed in the lysates of sf9 cells infected with ac - gagm - 1v3 ( lane 2 ), ac - gagc - 1v3 ( lane 3 ) and ac - gagc - 1v3 + 1cd4bd ( lane 4 ) but were not present in lysates of mock or wild type baculovirus - infected cells . western blot analysis revealed that both the p55 and p66 fusion proteins were recognized by pooled hiv - 1 positive human sera ( fig4 b , lanes 2 , 3 , and 4 ). no specific reaction was observed with mock - and wild type acnpv - infected cells . the results clearly demonstrate that insertion of either hiv - 1 v3 or v3 + cd4bd into the gag protein resulted in expression of proteins at levels at least as high as that of recombinant acnpv - hiv - 2gag alone ( fig4 a , lane 1 ). similar results were obtained when three other recombinant baculoviruses containing the v3 and v3 + cd4bd genes from hiv - 2 gp120 , ac - gagm - 2v3 , ac - gagc - 2v3 , and ac - gagc - 2v3 + 2cd4bd were analyzed . the cell culture fluids obtained from the example 4 were collected after centrifugation at 1 , 000 × g for 20 min . chimeric particles in the culture supernatant were collected by ultracentrifugation in a beckman sw 28 rotor at 80 , 000 × g for 1 hr and resuspended in pbs containing 0 . 1 % tween 20 , 10 μg / ml aprotinin and stored at 4 ° c . a band containing gag particles was collected from a 20 - 60 % discontinuous sucrose gradient , diluted at least 10 - fold with pbs and pelleted in an sw28 rotor at 80 , 000 × g for 1 hour . the pellet was gently suspended in pbs . gagc - 1v3 and gagc - 2v3 particles were recovered from the 50 % sucrose cushion , whereas gagc - 2v3 + 2cd4bd chimeric particles banded on top of the 60 % sucrose cushion . the results of the morphology of sucrose gradient purified particles by transmission electron microscopy using uranyl acetate staining are shown in fig5 . hiv - 2 gag particle were spherical ( fig5 a ), had diameters of approximately 100 nm , and were similar to those of mature hiv - 1 particles budding from hiv - 1 infected cells . the chimeric particles gagc - 1v3 ( fig5 b ), gagc - 2v3 ( fig5 c ) and gagc - 2v3 + 2cd4bd ( fig5 d ) exhibited morphologies similar to the gag particles . the peripheral granular material frequently showed striations with a periodicity suggesting a helical arrangement . it formed a shell - like layer probably inside a lipid membrane and was presumably rigid . the only difference between the gag particles and chimeric gag particles was that the chimeric gag particles were slightly larger with approximate diameters of 130 nm , similar to the diameter of mature hiv - 1 particles . antigenicity of chimeric gag particles was investigated by immunoblot analysis . purified chimeric gag particles were subjected to sds - page and analyzed by western blotting with rabbit antisera directed against hiv - 1 and hiv - 2 gp120 and 125 i - labeled protein a . the results are shown in fig6 . in this fig6 lane 1 is hiv - 2 gag protein ; lane 2 , gp120 protein ; lane 3 , chimeric gagc - 1v3 protein ; lane 4 , chimeric gagc - 1v3 + 1cd4 protein ; c , cell control ; and w is wild type acnpv - infected cell control . the hiv - 2 gag protein was not recognized by anti - gp120 sera ( fig6 a and b , lane 1 ) while the nonglycosylated forms of gp120 of hiv - 1 and hiv - 2 showed strong reactivity with their corresponding antisera ( fig6 a and b , lane 2 ). the 55 kda and 66 kda fusion proteins were specifically recognized by rabbit antisera against hiv - 1 or hiv - 2 gp120s ( fig6 a and b , lanes 3 and 4 ). the results clearly demonstrate that the chimeric proteins can be detected by antiserum specific for gp120 and reaffirm that inserted sequences of v3 or v3 + cd4bd are antigenic . in order to examine the capacity of particles to induce antibodies to both gag and env proteins of hiv , rabbits were immunized four times at 4 week intervals with purified gagc - iv3 and gagc - 2v3 chimeric particles . the immune rabbit sera were collected 2 weeks after the last immunization and tested for their ability to recognize gp120 of hiv - 1 and hiv - 2 . as shown in fig7 the antisera made against both gagc - 1v3 and gagc - 2v3 chimeric particles recognize not only carrier hiv - 2 gag protein ( fig7 a , b , lanes 1 ) but also non - glycosylated gp 120 of hiv - 1 ( fig7 a , lane 2 ) and hiv - 2 ( fig7 b , lane 2 ). in contrast , neither wild type acnpv infected sf9 cells nor uninfected sf9 cells contain proteins which were recognized by the antisera . these results clearly demonstrate that the v3 loop domain in chimeric gagc - 1v3 and gagc - 2v3 particles retain both antigenic and immunogenic properties . & lt ; immune sera against chimeric gag - v3 particles of hiv - 1 or hiv - 2 neutralize virus infectivity in vitro & gt ; rabbit anti - sera directed against chimeric particles were used to neutralize the infectivity of hiv as assayed by reverse transcriptase ( rt ) activity and gag p24 production . rabbits were immunized four times at one month intervals with intramuscular injections of 25 μg of the density gradient - purified chimeric gag particles . rabbit anti - gagc - 1v3 and anti - gagc - 2v3 sera ( 25 μl ) were mixed with 100 μl of virus which represented 5000 tcid 50 of hiv - 1 iiib or 8000 tcid 50 of hiv - 2 rod , respectively , and the mixtures were used to infect h9 cells . the amount of p24 gag protein and reverse transcriptase ( rt ) activity in the culture media were assayed as quantitation of virus production on different days after infection ( days 1 - 16 ) and the levels were compared with those of control samples in which virus was incubated with preimmune sera or rabbit anti - gp120 serum . fig8 shows that both rabbit anti - gagc - 1v3 and anti - gagc - 2v3 sera contained antibodies capable of neutralizing hiv infection of h9 cells . by day 9 postinfection , antisera to gagc - 1v3 and gagc - 2v3 chimeric particles completely blocked the production of hiv - 1 and hiv - 2 , respectively . however , at day 12 post - infection , a small amount of hiv - 1 was detected in cultures treated with anti - gagc - 1v3 serum ( fig8 a and b ). in contrast , the antisera to gag - 2v3 chimeric particles completely neutralized hiv - 2 infectivity ( fig8 c and d ). no reduction in rt and p24 gag protein production were observed with pre - immune sera . rabbit anti - gp120 sera of hiv - 2 showed stronger neutralizing activity of hiv - 1 rod than rabbit anti - gp120 sera of hiv - 1 against hiv - 1 iiib ( fig8 ). from the above , it is recognized that the chimeric protein according to the present invention showed excellent neutralizing activity against hiv - 1 and hiv - 2 . __________________________________________________________________________sequence listing ( 1 ) general information :( iii ) number of sequences : 5 ( 2 ) information for seq id no : 1 :( i ) sequence characteristics :( a ) length : 22 base pairs ( b ) type : nucleic acid ( c ) strandedness : single ( d ) topology : linear ( ii ) molecule type : dna ( genomic )( vi ) original source :( a ) organism : hiv - 1 ( ix ) feature :( a ) name / key : l - 1 oligonucleotide primer complementaryto nucleotides 816 through 836 of hiv - 1v3 dna fragment ( b ) location : 816 .. 836 ( xi ) sequence description : seq id no : 1 : cgaagatctgtcaatttcavgg22 ( 2 ) information for seq id no : 2 : ( i ) sequence characteristics :( a ) length : 22 base pairs ( b ) type : nucleic acid ( c ) strandedness : single ( d ) topology : linear ( ii ) molecule type : dna ( genomic )( vi ) original source :( a ) organism : hiv - 1 ( ix ) feature :( a ) name / key : l - 2 oligonucleotide primer complementaryto nucleotides 1075 through 1089 of hiv - 1 v3 dna fragment ( b ) location : 1075 .. 1089 ( xi ) sequence description : seq id no : 2 : gcagatctttgcttaaagatta22 ( 2 ) information for seq id no : 3 :( i ) sequence characteristics :( a ) length : 22 base pairs ( b ) type : nucleic acid ( c ) strandedness : single ( d ) topology : linear ( ii ) molecule type : dna ( genomic )( vi ) original source :( a ) organism : hiv - 2 ( ix ) feature :( a ) name / key : l - 3 oligonucleotide primer complementaryto nucleotides 879 through 893 of hiv - 2v3 dna fragment ( b ) location : 879 .. 893 ( xi ) sequence description : seq id no : 3 : cgagatctcattgtaagaggcc 22 ( 2 ) information for seq id no : 4 :( i ) sequence characteristics :( a ) length : 22 base pairs ( b ) type : nucleic acid ( c ) strandedness : single ( d ) topology : linear ( ii ) molecule type : dna ( genomic )( vi ) original source :( a ) organism : hiv - 2 ( ix ) feature :( a ) name / key : l - 4 oligonucleotide primer complementaryto nucleotides 1135 through 1149 of hiv - 2 v3 dna fragment ( b ) location : 1135 .. 1149 ( xi ) sequence description : seq id no : 4 : gcagatcttctgcagttagtcc22 ( 2 ) information for seq id no : 5 :( i ) sequence characteristics :( a ) length : 22 base pairs ( b ) type : nucleic acid ( c ) strandedness : single ( d ) topology : linear ( ii ) molecule type : dna ( genomic )( vi ) original source :( a ) organism : hiv - 2 ( ix ) feature :( a ) name / key : l - 5 oligonucleotide primer complementaryto nucleotides 1459 through 1473 of hiv - 2 gp120 ( b ) location : 1469 .. 1473 ( xi ) sequence description : seq id no : 5 : gcagatctctttactgatgtag22