before providing detailed working examples of the invention , certain parameters of the invention will be described generally . a variety of methods are suitable for producing the tumor - or pathogen - derived rna that can be used in the invention . as the following examples illustrate , it is not necessary that the rna be provided to the apc in a purified form . preferably , the rna sample ( i . e ., the fractionated tumor preparation or ivt rna sample ) is at least 50 %, more preferably 75 %, 90 %, or even 99 % rna ( wt / vol ). in practicing the invention , antigen - presenting cells , preferably professional apc such as dendritic cells and macrophage , are used . such cells can be isolated according to previously - described procedures . any of a variety of methods can be used to produce rna - containing tumor preparations . for example , the tumor preparations can be produced by sonicating tumor cells in a mammalian cell culture medium such as opti - mem or a buffer such as phosphate buffered saline . similarly , pathogen - derived rna can be produced by sonicating pathogenic bacteria or cells containing a pathogenic virus . other methods for disrupting cells also are suitable , provided that the method does not completely degrade the tumor - or pathogen - derived rna . typically , the rna preparation has 10 6 to 10 8 cells / ml ; most preferably 10 7 cells / ml . as alternatives , or in addition , to sonication , the tumor - or pathogen - derived rna can be prepared by employing conventional rna purification methods such as guanidinium isothiocyanate methods and / or oligo dt chromatography methods for isolating poly a + rna . ivt rna , synthesized according to conventional methods , can be used in lieu of rna in tumor preparations . for example , rna from a tumor or pathogen can be reverse transcribed into cdna , which then is amplified by conventional pcr techniques to provide an essentially unlimited supply of cdna corresponding to the tumor or pathogen rna antigen . conventional in vitro transcription techniques and bacterial polymerases then are used to produce the ivt rna . as an alternative , the ivt rna can be synthesized from a cloned dna sequence encoding a tumor or pathogen polypeptide antigen . methods for identifying such antigens are known in the art ; for example , several melanoma peptide antigens have been identified . rna transcribed in vitro from cdna encoding identified peptide antigens can serve as tumor - or pathogen - specific rna in the invention . as an alternative , rna can be transcribed from “ minigenes ” consisting of a portion of the tumor antigen cdna that encodes an epitope . tumor - or pathogen - specific rna can also be produced by employing conventional techniques for subtractive hybridization . for example , an rna sample from tumor cells and non - tumor cells can be used in the subtractive hybridization method to obtain tumor - specific rna . art - known transfection methods are suitable for introducing the tumor - or pathogen - derived rna into an antigen - presenting cell . for example , 5 - 50 μg of rna in 500 μl of opti - mem can be mixed with a cationic lipid at a concentration of 10 to 100 μg , and incubated at room temperature for 20 to 30 minutes . other suitable lipids include lipofectin ™ ( 1 : 1 ( w / w ) dotma : dope ), lipofectamine ™ ( 3 : 1 ( w / w ) dospa : dope ), dodac : dope ( 1 : 1 ), chol : dope ( 1 : 1 ), dmeda , chol , ddab , dmeda , dodac , dope , dori , dorie , dospa , dotap , and dotma . the resulting rna - lipid complex is then added to 1 - 3 × 10 6 cells , preferably 2 × 10 6 , antigen - presenting cells in a total volume of approximately 2 ml ( e . g ., in opti - mem ), and incubated at 37 ° c . for 2 to 4 hours . alternatively , the rna can be introduced into the antigen presenting cells by employing conventional techniques , such as electroporation or calcium phosphate transfection with 1 - 5 × 10 6 cells and 5 to 50 μg of rna . typically , 5 - 20 μg of poly a + rna or 25 - 50 μg of total rna is used . when the rna is provided as a tumor or pathogen preparation , the preparation typically is fractionated or otherwise treated to decrease the concentration of proteins , lipids , and / or dna in the preparation , and enrich the preparation for rna . for example , art - known rna purification methods can be used to at least partially purify the rna from the tumor cell or pathogen . it is also acceptable to treat the rna preparation with proteases or rnase - free dnases . the rna - loaded antigen - presenting cells of the invention can be used to stimulate ctl proliferation in vivo or ex vivo . the ability of the rna - loaded antigen - presenting cells to stimulate a ctl response can be measured by assaying the ability of the effector cells to lyse target cells . for example , the commonly - used europium release assay can be used . typically , 5 - 10 × 10 6 target cells are labeled with europium diethylenetriamine pentaacetate for 20 minutes at 4 ° c . after several washes 10 4 europium - labeled target cells and serial dilutions of effector cells at an effector : target ratio ranging from 50 : 1 to 6 . 25 : 1 are incubated in 200 μl rpmi 1640 with 10 % heat - inactivated fetal calf serum in 96 - well plates . the plates are centrifuged at 500 × g for 3 minutes and the incubated at 37 ° c . in 5 % co 2 for 4 hours . a 50 μl aliquot of the supernatant is collected , and europium release is measured by time resolved fluorescence ( volgmann et al ., j . immunol . methods 119 : 45 - 51 , 1989 ). the following working examples are meant to illustrate , not limit , the invention . first , the methods used in these examples are described . seven to eight weeks old and retired breeder female c57bl / 6 mice ( h - 2 b ) were obtained from the jackson laboratory ( bar harbor , me .). the f10 . 9 clone of the b16 melanoma of c57bl / 6 origin is a highly metastatic , poorly immunogenic , and low class i expressing cell line . f10 . 9 / k1 is a poorly metastatic and highly immunogenic cell line derived by transfecting f10 . 9 cells with class i molecule , h - 2k b cdna . rma and rma - s cells are derived from the rauscher leukemia virus - induced t cell lymphoma rbl - 5 of c57bl / 6 ( h - 2 b ) origin . other cell lines used were el4 ( c57bl / 6 , h - 2 b , thymoma ), e . g7 - ova ( el4 cells transfected with the cdna of chicken ovalbumin ( ova ), a20 ( h - 2 d b cell lymphoma ) and l929 ( h - 2 k fibroblasts ). cells were maintained in dmem supplemented with 10 % fetal calf serum ( fcs ), 25 mm hepes , 2 mm l - glutamine and 1 mm sodium pyruvate . e . g7 - ova cells were maintained in medium supplemented with 400 μg / ml g418 ( gibco , grand island , n . y .) and f10 . 9 / k1 cells were maintained in medium containing 800 μg / ml g418 . splenocytes obtained from naive c57bl / 6 female retired breeders were treated with ammonium chloride tris buffer for 3 minutes at 37 ° c . to deplete red blood cells . splenocytes ( 3 ml ) at 2 × 10 7 cells / ml were layered over a 2 ml metrizamide gradient column ( nycomed pharma as , oslo , norway ; analytical grade , 14 . 5 g added to 100 ml pbs , ph 7 . 0 ) and centrifuged at 600 g for 10 minutes . the dendritic cell - enriched fraction from the interface was further enriched by adherence for 90 minutes . adherent cells ( mostly dendritic cells ( dc ) and a few contaminating macrophage ( m ) were retrieved by gentle scraping , and subjected to a second round of adherence at 37 ° c . for 90 minutes to deplete the contaminating m . non - adherent cells were pooled as splenic dc and facs analysis showed approximately 80 %- 85 % dc ( mab 33d1 ), 1 - 2 % m  ( mab f4 / 80 ), 10 % t cells , and & lt ; 5 % b cells ( data not shown ). the pellet was resuspended and enriched for m  by two rounds of adherence at 37 ° c . for 90 minutes each . more than 80 % of the adherent population was identified as m  by facs analysis , with 5 % lymphocytes and & lt ; 55 % dc . b cells were separated from the non - adherent population ( b and t cells ) by panning on anti - ig coated plates . the separated cell population , which was comprised of & gt ; 80 % t lymphocytes by facs analysis was used as responder t cells . total rna was isolated from actively growing tissue culture cells as previously described ( chomczynski and sacchi , 1987 , analy . biochem . 162 : 156 - 159 ). briefly , 10 7 cells were lysed in 1 ml of guanidinium isothiocyanate ( gt ) buffer ( 4 m guanidinium isothiocyanate , 25 mm sodium citrate , ph 7 . 0 ; 0 . 5 % sarcosyl , 20 mm edta , and 0 . 1 m 2 - mercaptoethanol ). samples were vortexed , and followed by sequential addition of 100 μl 3 m sodium acetate , 1 ml water - saturated phenol and 200 μl chloroform : isoamyl alcohol ( 49 : 1 ). suspensions were vortexed and then placed on ice for 15 minutes . the tubes were centrifuged at 10000 × g , at 4 ° c . for 20 minutes , and the supernatant was carefully transferred to a fresh tube . an equal volume of isopropanol was added and the samples were placed at − 20 ° c . for at least 1 hour . rna was pelleted by centrifugation as above . the pellet was resuspended in 300 μl gt buffer , and then transferred to a microcentrifuge tube . rna was again precipitated by adding an equal volume of isopropanol and placing the tube at − 20 ° c . for at least 1 hour . tubes were microcentrifuged at high speed at 4 ° c . for 20 minutes . supernatants were decanted , and the pellets were washed once with 70 % ethanol . the pellets were allowed to dry at room temperature and then resuspended in te ( 10 mm tris - hcl , 1 mm edta , ph 7 . 4 ). possible contaminating dna was removed by incubating the rna sample in 10 mm mgcl 2 , 1 mm dtt and 5 u / ml rnase - free dnase ( boehringer - mannheim ) for 15 minutes at 37 ° c . the solution was adjusted to 10 mm tris , 10 mm edta , 0 . 5 % sds and 1 mg / ml pronase ( boehringer - mannheim ), followed by incubation at 37 ° c . for 30 minutes . samples were extracted once with phenol - chloroform and once with chloroform ; rna was again precipitated in isopropanol at − 20 ° c . following centrifugation , the pellets were washed with 70 % ethanol , then air dried and resuspended in sterile water . total rna was quantitated by measuring the optical density ( od ) at 260 and 280 nm . the od 260 / 280 ratios were typically 1 . 65 - 2 . 0 . the rna was stored at − 70 ° c . poly a + rna was isolated either from total rna using an oligotex ™ poly a + purification kit ( qiagen ), or directly from tissue culture cells using the messenger rna isolation kit ( stratagene ) as per the manufacturer &# 39 ; s protocols . if desired , alternative , conventional methods can be used to prepare poly a + rna . the 1 . 9 kb ecori fragment of chicken ovalbumin cdna in puc18 ( mcreynolds et al ., 1978 , nature 273 : 723 ) containing the coding region and 3 ′ untranslated region , was cloned into the ecori site of pgem4z ( promega ). clones containing the insert in both the sense and anti - sense orientations were isolated , and large scale plasmid preps were made using maxi prep kits tm plasmid preparation kit ( qiagen ). plasmids were linearized with bamhi for use as templates for in vitro transcription . transcription was carried out at 37 ° c . for 3 - 4 hours using the megascript in vitro transcription kit tm ( ambion ) according to the manufacturer &# 39 ; s protocol and adjusting the gtp concentration to 1 . 5 mm and including 6 mm m 7 g ( 5 1 ) ppp ( 5 1 ) g cap analog ( ambion ). other , conventional in vitro transcription methods also are suitable . template dna was digested with rnase - free dnase 1 , and rna was recovered by phenol : chloroform and chloroform extraction , followed by isopropanol precipitation . rna was pelleted by microcentrifugation , and the pellet was washed once with 70 % ethanol . the pellet was air - dried and resuspended in sterile water . rna was incubated for 30 minutes at 30 ° c . in 20 mm tris - hcl , ph 7 . 0 , 50 mm kcl , 0 . 7 mm mncl 2 , 0 . 2 mm edta , 100 μg / ml acetylated bsa , 10 % glycerol , 1 mm atp and 5000 u / ml yeast poly ( a ) polymerase ( united states biochemical ). the capped , polyadenylated rna was recovered by phenol : chloroform and chloroform extraction followed by isopropanol precipitation . rna was pelleted by microcentrifugation , and the pellet was washed once with 70 % ethanol . the pellet was air - dried and resuspended in sterile water . rna was quantitated by measuring the od at 260 and 280 nm , and the rna stored at − 70 ° c . the procedure used for rnase h site - specific cleavage of ovalbumin mrna was adapted from those previously described ( donis - keller , 1979 , nucl . acid . res . 7 : 179 - 192 ). briefly , 5 - 10 μg mrna from e . g7 - ova cells was suspended in 20 mm hepes - koh , ph 8 . 0 , 50 mm kcl , 4 mm mgcl 2 , 1 mm dtt , 50 μg / ml bsa and 2 μm of either the oligodeoxynucleotide 5 ′ cag ttt ttc aaa gtt gat tat act - 3 ′ seq id no : 1 , which hybridizes to sequence in ova mrna that codes for the ctl epitope siinfekl , or 5 ′- tca tat tag ttg aaa ctt ttt gac - 3 ′( seq id no : 2 ) ( oligos , etc . ), which serves as a negative control . the samples were heated to 50 ° c . for 3 minutes followed by incubation at 37 ° c . for 30 minutes . rnase h ( boehringer - mannheim ) was added at 10 u / ml , and digestion proceeded for 30 minutes at 37 ° c . rna was recovered by phenol : choloroform and chloroform extraction , followed by isopropanol precipitation . rna was pelleted by microcentrifugation , and the pellet was washed once with 70 % ethanol . the pellet then was air - dried and resuspended in sterile water . cleavage of ova mrna was confirmed by oligo dt primed reverse transcription of test and control samples , followed by pcr with ova specific primers that flank the cleavage site . pcr with actin - specific primers was used to control between test and control samples . apc were washed twice in opti - mem medium ( gibco , grand island , n . y .). cells were resuspended in opti - mem medium at 2 - 5 × 10 6 cells / ml , and added to 15 ml polypropylene tubes ( falcon ). the cationic lipid dotap ( boehringer mannheim biochemicals , indianapolis , ind .) was used to deliver rna into cells ( walker et al ., 1992 , proc . natl . acad . sci . usa 89 : 7915 - 7918 ). rna ( in 250 - 500 μl opti - mem medium ) and dotap ( in 250 - 500 μl opti - mem medium ) was mixed in a 12 × 75 mm polystyrene tube at room temperature ( rt ) for 20 minutes . the rna to dotap ratio routinely used was 1 : 2 , and varied in certain experiments between 2 : 1 to 1 : 2 . the complex was added to the apc ( 2 - 5 × 10 6 cells ) in a total volume of 2 ml and incubated at 37 ° c . in a water - bath with occasional agitation for 2 hours . the cells were washed and used as stimulators for primary ctl induction in vitro . the synthetic peptide encoding the ctl epitope in chicken ovalbumin ova , aa 257 - 264 siinfekl ( h - 2k b ), was used for peptide pulsing . the peptide had unblocked ( free ) amino and carboxyl ends ( research genetics , birmingham , ala .). peptides were dissolved in serum - free imdm and stored at − 20 ° c . t cells ( 5 × 10 6 cells / ml ) and rna or peptide pulsed apc ( 2 . 5 × 10 5 cells / ml ) were cultured in imdm with 10 % fcs , 1 mm sodium pyruvate , 100 iu / ml penicillin , 100 mg / ml streptomycin , and 5 × 10 − 5 m β - mercaptoethanol in 96 well u - bottom plates to give an r / s ratio of 20 : 1 . after 5 days , cells were used as effectors in a standard 4 hours europium release assay . in these assays , 5 - 10 × 10 6 target cells were labeled with europium diethylenetriamine pentaacetate for 20 minutes at 4 °. after several washes , 10 4 europium - labeled targets and serial dilutions of effector cells at effector : target ratios of 50 : 1 to 6 . 25 : 1 were incubated in 200 μl of rpmi 1640 with 10 % heat - inactivated fcs in 96 - well v - bottom plates . the plates were centrifuged at 500 g for 3 minutes and incubated at 37 ° c . and 5 % co 2 for 4 hours . 50 μl of the supernatant was harvested , and europium release was measured by time resolved fluorescence ( delta fluorometer , wallace inc ., gaithersburg , md .). spontaneous release was less than 25 %. standard errors ( se ) of the means of triplicate cultures was less than 5 %. c57bl / 6 mice were immunized once with irradiated , rna - pulsed apc ( 2 × 10 6 cells / mouse ) or 5 × 10 6 e . g7 - ova or el4 cells . at 10 - 14 days post - immunization , mice were challenged with 2 × 10 7 live e . g7 - ova cells injected sub - cutaneously in the flank region . mice were monitored on a regular basis for tumor growth and size . mice with tumor sizes & gt ; 3 . 5 cm were sacrificed . all survivors were sacrificed at 40 days post - challenge . mice were received by intrafootpad injection 2 × 10 5 f10 . 9 cells . the post - surgical protocol was essentially as described previously ( porgador et al ., 1995 , cancer res . 55 : 4941 - 4949 ). the legs of the mice were amputated when the local tumor in the footpad was 5 . 5 - 7 . 5 mm in diameter . post - amputation mortality was less than 5 %. at two days post - amputation , the mice were immunized intraperitoneally , followed by weekly vaccinations twice , for a total of three vaccinations . the mice were sacrificed based on the metastatic death in the non - immunized or control groups ( at 28 - 32 days post - amputation ). the metastatic loads were assayed by weighing the lungs and by counting the number of metastatic nodules . induction of a primary ctl response in vitro using dendritic cells transfected with chicken ovalbumin rna the ability of rna pulsed splenic dendritic cells ( dc ) derived from c57bl / 6 ( h - 2k b ) mice to induce a primary ctl response in vitro was demonstrated in the e . g7 - ova tumor system . e . g7 - ova cells were derived from the el4 tumor cell line ( h - 2k b haplotype ) by transfection with the chicken ovalbumin cdna ( moore et al ., 1988 , cell 54 : 777 - 785 ). the chicken ovalbumin encodes a single dominant epitope ( aa 257 - 264 ) in c57bl / 6 mice ( rotzschke et al ., 1991 , euro . journal immunology , 21 : 2891 - 2891 ). dendritic cells pulsed with the ova peptide ( aa 257 - 264 ) incubated with t cells from naive mice induce a potent ctl response in vitro ( fig1 ). this example demonstrates that rna can be used as a source of antigen to sensitize dc to present antigen to cd8 + t cells . splenic dc were isolated from c57bl / 6 mice and pulsed with ova peptide or incubated with rna synthesized in vitro ( ova ivt rna ) from a plasmid encoding the chicken ovalbumin cdna , and used to stimulate an ova - specific primary ctl response in vitro . as shown in fig1 both ova peptide as well as ova ivt rna pulsed dc were capable of inducing an ova specific primary ctl response ( fig1 ). rna pulsed dc were consistently more effective stimulators than peptide pulsed dc . to test whether rna isolated from e . g7 - ova cells was capable of sensitizing dc to stimulate a primary , ova - specific , ctl response , total rna or poly a + rna was isolated from e . g7 - ova or el4 cells and incubated with dc . as shown in fig1 dc pulsed with either total or poly a + rna from e . g7 - ova cells but not from el4 cells , were capable of inducing a strong ova specific ctl response . surprisingly , dc pulsed with unfractionated rna , total or poly a + , were as potent inducers of a primary ctl response as dc pulsed with the ova peptide encoding a defined ctl epitope . stimulation of a ctl response by ( total or poly a + ) el4 rna pulsed dc was only marginally above background and statistically not significant ( compare to lysis of el4 targets by ctl stimulated with ova peptide or ova ivt rna pulsed dc ), reflecting the immunodominance of the ova epitope and the relative weakness of the el4 encoded antigens . as is illustrated by fig2 total , as well as poly a + , but not poly a − , rna isolated from e . g7 - ova cells is capable of sensitizing dc to stimulate a primary ctl response . to prove that sensitization of dc is indeed mediated by rna , poly a + rna from e . g7 - ova cells was incubated with either an antisense oligonucleotide spanning the sequence encoding the single ctl epitope present in the chicken ovalbumin gene or with a control oligodeoxynucleotide , and then treated with rnase h to remove any rna sequence to which the oligodeoxynucleotide probe has hybridized . as shown in fig2 induction of a primary , ova - specific ctl response was abolished when the poly a + rna was incubated with the antisense , but not with the control , oligodeoxynucleotide . fig2 also shows that cells expressing the complete ovalbumin gene , e . g7 - ova cells , and rma - s cells pulsed with the 8 amino acid long ova peptide encoding the single dominant ctl epitope are lysed to a similar extent following stimulation with total or poly a + e . g7 - ova rna pulsed dc . this indicates , therefore , that the majority of epitopes presented by e . g7 - ova rna pulsed dc correspond to the previously defined single dominant ctl epitope encoded in the chicken ovalbumin gene . this example demonstrates that vaccination of mice with ova rna pulsed dc provided protection against a challenge with e . g7 - ova tumor cells . mice were immunized once with 2 × 10 6 rna pulsed dc or with 5 × 10 6 irradiated e . g7 - ova cells . ten days later , mice were challenged with a tumorigenic dose of e . g7 - ova cells . appearance and size of the tumor were determined on a regular basis . fig3 shows the size of the tumors at 37 days post - tumor implantation . the average tumor size in mice immunized with irradiated el4 cells was 25 cm , while the average tumor size in animals immunized with the ova expressing el4 cells ( e . g7 - ova ) was only 7 . 03 cm . this difference is a reflection of the high immunogenicity of the chicken ova antigen expressed in el4 cells and the poor immunogenicity of the parental , el4 , tumor cell line . vaccination with dc pulsed with rna ( total or poly a + fraction ) derived from e . g7 - ova cells was as effective as vaccination with the highly immunogenic e . g7 - ova cells ( average tumor size 7 cm ). vaccination with dc incubated with total or poly a + rna derived from el4 tumor cells had a slight protective effect ( average tumor size : 22 cm and 19 . 5 cm , respectively ) which was not statistically significant , consistent with poor to undetectable immunogenicity of el4 - derived antigens . consistent with the primary ctl induction data ( fig1 ), vaccination of mice with ova ivt rna pulsed dc provided the most effective antitumor response ( average tumor size : 3 . 9 cm ), while vaccination with the control antisense ova ivt rna did not elicit a significant protective response . the potency of dc pulsed with tumor - derived rna was further evaluated in the b16 / f10 . 9 ( h - 2 b ) melanoma metastasis model . the b16 / f10 . 9 melanoma tumor is poorly immunogenic , expresses low levels of mhc class i molecules , and is highly metastatic in both experimental and spontaneous metastasis assay systems ( porgador et al ., 1996 , j . immunology 156 : 1772 - 1780 ). porgador et al . have shown that , when vaccinations are carried out after the removal of the primary tumor implant , only irradiated tumor cells transduced with both the il - 2 and the h - 2k b genes , are capable of significantly impacting the metastatic spread of b16 / f10 . 9 tumor cells in the lung ( porgador et al . 1995 , cancer research 55 : 4941 - 4949 ). thus , the b16 / f10 . 9 melanoma model and the experimental design used by porgador et al . constitutes a stringent and clinically relevant experimental system to assess the efficacy of adjuvant treatments for metastatic cancer . to demonstrate that immunization with tumor rna pulsed dc , in accordance with the invention , was capable of causing the regression of preexisting lung metastases , primary tumors were induced by implantation of b16 / f10 . 9 tumor cells in the footpad . when the footpad reached 5 . 5 - 7 . 5 mm in diameter , the tumors were surgically removed . two days later , mice were immunized with irradiated b16 / f10 . 9 cells , irradiated b16 / f10 . 9 cells transduced with the h - 2k b gene ( f10 . 9k1 ), or with rna pulsed dc preparations ( fig4 ). the mice received a total of three vaccinations given at weekly intervals . the average lung weight of a normal mouse is 0 . 18 - 0 . 22 g . mice treated with pbs ( a negative control ) were overwhelmed with metastases . the mean lung weight of mice in this treatment group was 0 . 81 g ; approximately three - quarters of the weight was contributed by the metastases , which were too many to count (& gt ; 100 nodules ). a similar metastatic load was seen when mice were treated with irradiated b16 / 10 . 9 cells ( data not shown ), which confirms numerous previous observations that treatment with irradiated b16 / f10 . 9 tumor cells alone has no therapeutic benefit in this tumor model . as also previously shown , immunization with h - 2k b expressing b16 / f10 . 9 cells ( f10 . 9k1 , as a positive control ) had a modest therapeutic benefit , as indicated by a statistically significant decrease in the average lung weight of the animals in this treatment group . a dramatic response , however , was seen in animals treated with dc that were pulsed with total rna derived from f10 . 9 cells in accordance with the invention . the mean lung weight of mice in this treatment group was 0 . 37 g . a significant dramatic response also was seen in mice treated with dc pulsed with poly a + rna derived from f10 . 9 cells in accordance with the invention ( average lung weight : 0 . 42 g ). by contrast , no statistically significant decrease in metastatic load was seen in mice treated with dc that were pulsed with either the poly a - rna fraction derived from f10 . 9 cells or with total rna isolated from el4 tumor cells . the observation that cells expressing the ova protein ( e . g7 - ova ) or cells pulsed with the ova peptide were efficiently lysed by ctl , and the sensitization of dc fractionated with poly a + rna , strongly suggest that rna - mediated stimulation of ctl occurs via translation of the input rna and generation of the predicted class i restricted epitopes , in this case a single dominant epitope encoded in the chicken ova peptide . these data show that rna mediated sensitization of dc is more effective than pulsing with peptide because the transfected rna can serve as a continuous source for the production of antigenic peptides . the invention can be used to treat or prevent tumor formation in a patient ( e . g ., melanoma tumors , bladder tumors , breast cancer tumors , colon cancer tumors , prostate cancer tumors , and ovarian cancer tumors ). similarly , the invention can be used to treat or prevent infection in a patient with a pathogen such as a bacterium ( e . g ., salmonella , shigella , or enterobacter ) or a virus ( e . g ., a human immunodeficiency virus , a herpes virus , an influenza virus , a poliomyelitis virus , a measles virus , a mumps virus , or a rubella virus ). in treating or preventing tumor formation or pathogen infection in a patient , it is not required that the cell ( s ) that is administered to the patient be derived from that patient . thus , the antigen - presenting cell can be obtained from a matched donor , or from a culture of cells grown in vitro . methods for matching haplotypes are known in the art . similarly , it is not required that the rna be derived from the patient to be treated . rna from a donor can be used . it is preferable that treatment begin before or at the onset of tumor formation or infection , and continue until the cancer or infection is ameliorated . however , as the examples described herein illustrate , the invention is suitable for use even after a tumor has formed , as the invention can cause a regression of the tumor . in treating a patient with a cell or vaccine produced according to the invention , the optimal dosage of the vaccine or cells depends on factors such as the weight of the mammal , the severity of the cancer or infection , and the strength of the ctl epitope . generally , a dosage of 10 5 to 10 8 rna - loaded antigen - presenting cells / kg body weight , preferably 10 6 to cells / kg body weight , should be administered in a pharmaceutically acceptable excipient to the patient . the cells can be administered by using infusion techniques that are commonly used in cancer therapy ( see , e . g ., rosenberg et al ., new eng . j . of med . 319 : 1676 , 1988 ). the optimal dosage and treatment regime for a particular patient can readily be determined by one skilled in the art of medicine by monitoring the patient for signs of disease and adjusting the treatment accordingly . where the antigen - presenting cell is used to induce a ctl response in vitro , the resulting effector ctls can subsequently be administered to a mammal in a ctl - based method of therapy ( see , e . g ., pct / us91 / 06441 ). ctl produced in vitro with the antigen - presenting cells of the invention can be administered in a pharmaceutically acceptable excipient to a mammal by employing conventional infusion methods ( see , e . g ., rosenberg et al ., supra ). typically , 10 9 - 10 10 cells are administered over the course of 30 minutes , with treatment repeated as necessary . such a ctl - based method of therapy may be combined with other methods , such as direct administration of the antigen - presenting cells of the invention . the ctl and antigen - presenting cells may be autologous or heterologous to the patient undergoing therapy . if desired , the treatment may also include administration of mitogens ( e . g ., phyto - hemagglutinin ) or lymphokines ( e . g ., il - 2 or il - 4 ) to enhance ctl proliferation .