the vaccines of the present invention are prepared by culturing cells infected with feline leukemia virus ( felv ), such as feline lymphoblastoid cells replicating felv . the virus produced by the cells should provide antigens of types a , b , and c , as described by sarma et al , virology , 54 , 160 - 169 ( 1973 ). the fl74 feline lymphoblastoid cell line , and cell lines produced therefrom meet this criteria . the fl74 cell line and related cell lines are available in many university laboratories in the united states and other countries . see theilen et al , nature , 222 , 589 - 590 ( 1969 ); essex et al , j . nat . cancer inst . 54 , 631 - 635 ( 1975 ); and jarrett et al , int . j . cancer , 16 , 134 - 141 ( 1975 ). as originally described in the cited publication of theilen et al , the infected lymphoblastoid cells elaborate felv particles into the culture medium in which they are grown , as well as producing pre - viral antigens associated with the cell membranes . immature virus particles grow as buds on the cell membranes , and are released into the media as mature viral particles . the fl74 - derived cell line which is preferred for practicing the present invention was developed by four years of continuous laboratory culture passage . it is identified by the code designation fl74 - ucd - 1 , and is now further identified by deposit with the american type culture collection , rockville , maryland , under atcc no . crl - 8012 ). the virus produced by the original fl74 cell line has been referred to as kt - felv , and correspondingly the virus produced by fl74 - ucd - 1 is identified as kt - felv - ucd - 1 . the kt - felv - ucd - 1 virus has attenuated virulence for cats . however , when inactivated in accordance with the present invention and prepared in vaccine form , the viral particles ( separate from the cells from which they are produced ) are capable of preventing persistent feline leukemia viral infection . the fl74 cells or cell lines derived therefrom may be grown in a suitable aqueous nutrient medium , such as the leibovitz l - 15 medium . see leibovitz , amer . j . hyg . 78 , 173 ( 1963 ). for maximizing the viral yield , the medium preferably contains fetal calf serum . for example , the leibovitz medium may contain from 15 to 30 % ( by volume ) of the fetal calf serum . the culturing can be carried out by standard cell culture techniques , such as a suspension culture in roller bottles . the first phase of the culturing can be carried out over a period of several days , such as three to four days . following the initial phase , the culture medium and cells can be subjected to centrifugation to separate the cells . the pelleted cells are separated from the supernatant culture fluid containing the felv formed in the first phase . however , during the first culturing period , the elaboration of viral particles into the media is at a relatively low level . much greater virus production is obtained by resuspending the pelleted cells in fresh media , and continuing to culture the cells for a second period , such as for a period of three or more days . during the second culture period , very little additional cell growth takes place , but virus production occurs at a high level . the cells and cell debris are pelleted out of the second culture medium by centrifugation , and separated therefrom . the tissue culture fluid from both culturings can then be combined to provide the stock of felv to be used for preparing vaccine . after removal of the cells and large particulate debris , the supernatant fluid can be further purified by centrifugation at higher gravity , such as 6 , 000 × g , thereby pelleting and removing most of the subcellular debris . the felv can then be recovered from the supernatant fluid by centrifugation at a very high gravity force , for example , centrifuging at 20 , 000 rpm in a beckman type 21 batch type rotor for 90 minutes . the resultant pellet will contain all of the virus particles plus a small amount of subcellular debris . the intact viral particles have a buoyant density of 1 . 016 grams per milliliter ( gm / ml ) determined by density gradient separation against sucrose . in preparing the vaccine , the separated viral particles are inactivated . while other known inactivation procedures can be used , formaldehyde inactivation is preferred . when the felv is inactivated in the presence of a sufficient concentration of formaldehyde , the antigenic potency of the vaccine is improved . treatment with aqueous formaldehyde at a concentration of from 0 . 3 to 1 . 8 % is preferred , and optimum results appear to be obtained within the range from about 0 . 5 to 1 . 5 % formaldehyde . in carrying out the inactivation treatment , the separated felv can be resuspended in aqueous formaldehyde at the selected concentration , the volume of the formaldehyde solution being determined so as to correspond with the desired administration concentration of the felv after the formaldehyde solution is combined with a suitable adjuvant . for example , each milliliter ( ml ) of the solution may contain from 0 . 04 to 3 . 2 milligrams ( mg ) of virus on a protein basis . then when 0 . 25 ml of the solution containing the inactivated virus is combined with 0 . 25 ml of an adjuvant , the resulting 0 . 5 ml dose will contain from 0 . 01 to 0 . 8 mg of the inactivated virus . the preferred dose level appears to be about 0 . 02 to 0 . 4 mg of inactivated felv ( protein basis ) per dose . this amount is particularly effective when potentiated by the formaldehyde treatment , and when combined with a suitable adjuvant . preferably , therefore , each vaccine dose should contain at least 0 . 02 mg of the inactivated virus . various standard adjuvants can be used , such as freund &# 39 ; s incomplete adjuvant , or an aluminum hydroxide adjuvant . freund &# 39 ; s incomplete adjuvant can be obtained from difco laboratories , detroit , michigan , or it may be prepared by combining mannide monooleate with pariffin oil in the proportions by volume of 1 . 5 : 8 . 5 . aluminum hydroxide adjuvants can be prepared as described in u . s . pat . no . 3 , 149 , 036 . a commercial form of this adjuvant is sold by merck & amp ; co ., inc ., rahway , n . j ., as merck adjuvant 65 . while the proportions of adjuvant to the virus - containing formaldehyde solution can vary considerably , approximately equal proportions are convenient and desirable . the preparation and use of vaccines prepared in accordance with the present invention are further illustrated by the following examples . the fl74 - ucd - 1 cell line ( atcc no . crl - 8012 ), is cultured for production of the associated kt - felv - ucd - 1 strain of feline leukemia virus . this line was obtained by four years of continuous laboratory culture passage from the original kt - felv replicating lymphoblastoid cells , later referred to as the f74 cell line . see theilen et al , nature , 222 , 589 - 590 ( 1969 ); and jarrett et al , j . nat . cancer inst . 51 , 261 - 263 ( 1973 ). the fl74 - ucd - 1 cell line produces large amounts of feline leukemia virus ( felv ), which it elaborates continuously into the tissue culture media . this elaborated virus is of attenuated virulence for cats . the cells can be grown in leibovitz &# 39 ; s 15 ( l15 ) media with 15 % ( by volume ) fetal calf serum , in roller bottles as a suspension culture . cell numbers are reduced by one - fourth when the cell concentration reaches 5 × 10 6 to 1 × 10 7 cells per ml . maximum virus production is achieved by the following passage schedule : following the initial culture split , cell numbers are allowed to return to around 5 × 10 6 to 1 × 10 7 cells / ml , which is usually after 3 - 4 days in culture . at this time the culture fluid is centrifuged to pellet all of the cells , and the supernatant culture fluid removed . the old culture fluid is replaced with an equal volume of fresh media , and the cells are allowed to grow in the media for three more days . very little additional cell growth takes place during this period , but virus production occurs at a high level . tissue culture fluid harvested from both the split and feeding passages is utilized for virus collection . the kt - felv - ucd - 1 virus can be purified from tissue culture fluid as follows : 1 . cells and large particulate debris are removed by centrifugation at 1 , 200 × g for 10 minutes . 2 . supernatant fluid is then centrifuged at 6 , 000 × g for 45 minutes to remove most of the subcellular debris . 3 . supernatant fluid from step 2 is then centrifuged at 20 , 000 rpm in a beckman type 21 batch type rotor for 90 minutes . this can also be done with oher high speed centrifuge rotors exerting a high g force . the resultant pellet contains all of the virus particles plus a small amount of subcellular debris . one liter of the original tissue culture fluid will yield 1 to 2 mg of purified virus . 1 . the pelleted virus from step 3 of the virus collection procedure , as described above , is resuspended in an isotonic buffered salt solution such as phosphate buffered saline , puck &# 39 ; s saline , tris - saline , or similar solutions ( ph = 7 . 4 ), so that 0 . 1 to 0 . 2 mg of virus ( protein basis ) is present in 1 ml of the buffered salt solution . this concentration is designated 100 ml equivalents ( ml eq )/ ml , indicating that the virus originally present in 100 ml of tissue culture fluid is now present in 1 ml . the amount of virus can be varied from 50 to 800 ml eq / ml , depending on the strength of the vaccine that is desired . 2 . thirty lambda ( μl ) of formalin solution ( 33 % formaldehyde ) is added to each 1 ml of the virus suspension , and the mixture allowed to stand for 12 - 24 hours at 4 ° c . the effective formaldehyde concentration of the mixture is approximately 1 %. after the formaldehyde has reacted with the virus , the mixture is either lyophilized in 0 . 25 ml aliquots or used immediately . the vaccine as prepared for administration can be comprised of 0 . 25 ml of the 1 % formaldehyde inactivated virus suspension , plus 0 . 25 ml of freund &# 39 ; s incomplete adjuvent to give a 0 . 5 ml injection dose . alternatively , 0 . 25 ml of aloh gel can be used as an adjuvant . the amount of actual virus protein in each 0 . 25 ml dose of virus suspension varies depending on the concentration factor . as an example , if the virus suspension was 100 ml eq ( 0 . 1 to 0 . 2 mg virus ) per ml , then 0 . 25 ml would provide 25 ml eq or 0 . 25 to 0 . 050 mg virus protein in each 0 . 5 ml vaccine dose after adjuvant addition . one 0 . 5 ml dose of vaccine in adjuvant , prepared as described above , can be given intramuscularly to kittens at 10 - 16 weeks of age . one dose of vaccine contains 0 . 25 ml of the inactivated virus suspension and 0 . 25 ml of the adjuvant . a second dose of the same vaccine is similarly given 3 - 4 weeks later . a booster injection given at one - year intervals also may be advantageous . in a pilot experiment to determine the effect of using different concentrations of formaldehyde for the felv inactivation , equal amounts of felv , prepared as described in example i , were treated with different formaldehyde solutions . the dose level amounts of the virus thus treated were not precisely deterimined , but were estimated to contain from 0 . 2 to 0 . 4 mg of virus on a protein basis . the equal dose level amounts of felv were suspended in 0 . 5 ml of aqueous formaldehyde at respective concentrations of 0 . 1 , 0 . 5 , 1 . 0 , and 2 . 0 % formaldehyde . after holding in the formaldehyde solutions for 24 hours at 4 ° c ., the 0 . 5 ml dose quantities were mixed with 0 . 5 ml of freund &# 39 ; s incomplete adjuvant , thereby providing injection doses of approximately 1 . 0 ml . individual kittens were then given three successive doses of vaccine at the same formaldehyde treatment level with three week intervals between doses . two kittens were used for the 1 . 0 and 2 . 0 % formaldehyde levels , and one kitten for the 0 . 1 and 0 . 5 % levels . antibody titers were measured 2 weeks after the third dose of vaccine . the focma titer was determined by the method of essex et al , nature , 233 , 195 - 196 ( 1971 ), and the virus neutralizing antibody titer ( vn ) was determined by the modified method described in pedersen et al , am . j . vet . res ., 38 , 1523 - 1531 ( 1977 ). the results are summarized below in table a . table a______________________________________ concentration of focma vn antibodyformaldehyde in vaccine antibody titer titer______________________________________0 . 1 % negative negative0 . 5 % + & gt ; 1 : 5 + 1 : 401 . 0 % + & gt ; 1 : 5 + 1 : 1601 . 0 % + & gt ; 1 : 5 + 1 : 1602 . 0 % + & gt ; 1 : 5 + 1 : 1602 . 0 % + & gt ; 1 : 5 ± 1 : 10______________________________________
a vaccine in parenteral dose form for preventing persistent feline leukemia viremia in cats is characterized by containing as the immunizing agent inactivated feline leukemia virus derived from the fl74 feline lymphoblastoid cell line .