Patent Abstract:
the present invention relates to agents capable of binding sialic acid - binding immunoglobulin - like lectin - 9 and their use in the treatment of cell proliferation and differentiation disorders . furthermore , the present invention provides associated pharmaceutical formulations and methods .

Detailed Description:
the present invention will now be described further by way of example and with reference to the figures which show : fig1 . expression of cd33 - related siglecs on aml cells . mononuclear aml bone marrow cells from sample xxi ( see table 2 ) were stained with anti - cd33 - biotin mab and the indicated fitc - labelled anti - siglec mabs , followed by streptavidin - apc and analysed by flow cytometry . the non - viable cells labelled with 7 - aad were not included in the analyses . the left quadrants were set to include more than 99 % of cells labelled by the isotype control . the percentages of cd33 + / siglec + cells are shown . fig2 . co - expression of siglec - 7 and siglec - 5 on the siglec - 9 - positive subset of aml cells . mononuclear aml bone marrow cells from samples i ( top panels ) and xxi ( lower panels ) ( see table 2 ) were stained with anti - siglec - 9 - fitc mab and either anti - siglec - 5 - biotin or anti - siglec - 7 - biotin mabs , followed by streptavidin - apc and analysed by flow cytometry . fig3 . phenotypic characterisation of siglec - 9 - positive aml cells by flow cytometry . a . mononuclear aml bone marrow cells from sample ii ( see table 2 ) were stained with anti - siglec - 9 - fitc mab in conjunction with one of the following mabs : anti - cd38 - biotin , anti - cd123 - biotin ( both followed by streptavidin - apc ), anti - cd117 - pe or anti - cd14 - pe . b . aml cells were stained with anti - cd33 - fitc or anti - siglec - 9 - fitc mabs in conjunction with anti - cd34 class ii - biotin followed by streptavidin - apc . examples of 3 aml , samples ( table 2 ) are shown ( top panel , sample i , 29 % cd34 + ; middle panel , sample ii , 2 . 5 % cd34 ; bottom panel sample xix , 31 % cd34 + ). the percentages of double - positive cells are shown for each dot - plot . fig4 . may - grunwald - giemsa staining of aml cells and normal bone marrow cells . mononuclear aml bone marrow cells ( a ) and mononuclear normal bone marrow cells ( b ) were immunomagnetically sorted into siglec - 9 - positive and - negative fractions and cytospins stained using may - grunwald - giemsa . cells within the siglec - 9 - positive fractions are enriched for cells of the monocytic lineage . fig5 . expression of cd33 - related siglecs on normal bone marrow cells . mononuclear bone marrow cells were stained with the indicated fitc - labeled mabs and analysed by flow cytometry . a . a plot of forward scatter ( fsc ) versus side scatter ( ssc ) permits definition of three populations : r2 , ssc low ; r3 , ssc medium ; r4 , ssc high , b . the grey histograms show expression of cd33 - related siglecs using the indicated mabs on the three subsets of cells defined above . white histograms show staining with the isotype control . fig6 . characterisation of siglec - positive subsets in normal bone marrow . a . bone marrow cells were labeled with anti - cd33 - apc and anti - siglec - 9 - fitc together with either anti - siglec - 5 - biotin or anti - siglec - 7 - biotin followed by streptavidin - pe . two siglec - 9 positive populations are defined : gate 1 , cd33 high , siglec - 9 + ; gate 2 , cd33 medium , siglec - 9 + . the histograms show expression of siglec - 5 and siglec - 7 on each gated subpopulation . b . bone marrow cells were labeled with anti - siglec - 9 - fitc together with either anti - cd38 - biotin or anti - cd123 - biotin followed by streptavidin - apc , or anti - cd14 - pe . the percentages of single and double positive cells are shown . c . bone marrow cells were stained with anti - cd34 - biotin and either cd33 - fitc , anti - siglec - 5 - fitc , anti - siglec - 7 - fitc or anti - siglec - 9 - fitc followed by streptavidin - apc . the cd34 + cells were gated ( m1 , grey histogram , left panel ) and analysed for expression of cd33 , and siglecs - 5 , - 7 and - 9 ( grey histograms , right panels ). white histograms show labeling of cd34 + cells with isotype matched control . fig7 . internalisation of anti - siglec - 9 mab . a . mononuclear aml bone marrow cells from samples xvi , xix and xxi ( left panel ) or rbl cells stably transfected with wild type ( wt ) or ( y1f ) or ( y2f ) mutant forms of siglec - 9 ( right panel ) were labelled with anti - siglec - 9 - alexa - 488 mab for 45 min on ice , washed and then incubated for 40 or 240 min at 37 ° c . the remaining surface anti - siglec - 9 mab was detected using goat anti - mouse - apc . the graphs show the anti - siglec - 9 mab remaining at the surface expressed as a percentage of the starting values b . internalisation assays were carried out as described in ( a ) and the levels of total cell associated anti - siglec - 9 - alexa - 488 was measured at each time point . the increases seen in the right panel reflect a time - dependent gain in autofluorescence of rbl cells detected on the fl - 1 channel . the graphs show the total remaining anti - siglec - 9 mab expressed as a percentage of the starting values . fig8 . confocal microscopy analysis of internalisation of anti - siglec - 9 mab by transfected rbl cells . wt ( a , b ) or y1f ( c ) or y2f ( d ) mutant forms of siglec - 9 - transfected rbl cells were incubated on ice with anti - siglec - 9 - alexa - 488 mab for 1 h , washed and incubated for 1 h at 37 ° c . ( b , c , d ) or for 1 h on ice ( a ). internalization was stopped by chilling on ice and the plasma membrane labeled with cholera toxin b subunit - alexa - 594 . after washing , the cells were fixed with 4 % paraformaldehyde and examined by confocal microscopy . bone marrow aspirates from aml patients and bone marrow samples from otherwise healthy patients undergoing hip replacement surgery were obtained after informed consent . aml samples were stored in the tayside cancer tissue bank . the study was approved by the tayside cancer tissue committee ( ref . 04 / s1401 / 85 ), which represents the tayside committee for medical research ethics for studies involving banked tissue . mononuclear cells ( mnc ) were purified by ficoll - paque ™ plus ( amersham biosciences , bucks , uk ) density gradient centrifugation . aml cell aliquots were stored in liquid nitrogen and normal mnc were directly used for the experiments . cryopreserved samples were thawed and incubated in rpmi 1640 ( with l - glutamine ) medium supplemented with 20 % pcs , 1 % penicillin / streptomycin , 10 mm hepes buffer ( all reagents were from invitrogen gibco , paisley , uk ) for 90 min at 37 ° c ., 5 % co 2 prior to experiments . bone marrow plasma was prepared by centrifugation of anticoagulated whole bone marrow aspirates and stored at − 80 ° c . blood samples for preparation of serum were collected from laboratory volunteers according to local ethical guidelines , specific monoclonal antibodies ( mabs ) to the following cd33 - related siglecs were produced in our laboratory : cd33 ( 6c5 ), siglec - 5 ( 1a5 ) 6 , siglec - 7 ( s75a ) 7 , siglec - 8 ( 7c9 ) 9 , siglec - 9 ( kalli ) 11 , siglec - 10 ( 5g6 ) 12 and siglec - 11 ( 4c4 ) 13 . all are mouse igg1 except 4c4 which is igg2c . iggs were purified from tissue culture supernatants using protein g sepharose ( sigma , dorset , uk ) and labelled with fluorescein - 5 - isothiocyanate ( isomer 1 ) ( fitc ) ( invitrogen , paisley , uk ). anti - siglec - 5 and - siglec - 7 iggs were labelled with ez - link biotin ( pierce , rockford ) and anti - cd33 , - siglec - 8 and - siglec - 9 iggs were labelled with alexa fluor 488 ( invitrogen ). the following commercial abs were also used : anti - cd33 - biotin ( wm53 , serotec , oxford , uk ), anti - cd33 - apc ( wm53 , serotec ), anti - siglec - 6 ( e20 - 1232 , bd pharmingen , oxford , uk ), anti - cd34 - biotin ( qbend , serotec ), anti - cd14 - pe ( caltag - medsystems , buckingham , uk ), anti - cd38 - biotin ( hit2 , caltag - medsystems ), anti - cd123 - biotin ( 6h6 , ebioscience , san diego , usa ), anti - cd117 ( 104d2 , caltag - medsystems ), anti - mouse immunoglobulin - fitc ( dako , ely , uk ). 3 − 4 × 10 5 cells were stained with saturating concentrations of the indicated mabs for 45 min on ice . after washing , the cells were incubated with streptavidin - apc or - pe for 30 min on ice and after an additional washing step the cells were resuspended in pbs containing 0 . 25 % bovine serum albumin , 10 mm sodium azide and 7 - amino actinomycin d ( 7 - aad ). for siglec - 6 staining , cells were incubated with saturating levels of purified siglec - 6 mab , followed by anti - mouse igg - fitc . all flow cytometry analysis was performed using either a facs calibur or lsr flow cytometer ( 3d biosciences ) and celiquest software . in each case the negative quadrant was set to include more than 99 % of the isotype control - labeled cells and the non - viable 7 - aad - positive cells were routinely excluded from all facs analyses . aml cells and normal bone marrow cells were labeled with anti - siglec - 9 igg - fitc followed by anti - fitc - coupled paramagnetic microbeads and magnetically sorted into positive and negative fractions using an automacs system ( miltenyi biotech , bisley uk ), according to the manufacturer &# 39 ; s instructions . the purity of the separated cell fractions was approximately 90 % as assessed by flow cytometry . cytospins were prepared and stained with may - grunwald - giemsa . images from randomly selected fields were taken with an axioskop microscope ( zeiss , jena , germany ), using a 63 × l25 oil lens ( zeiss ) and axiovision 3 . 0 software . black and white reference was set according to the manufacturer &# 39 ; s instructions and the exposure time was 1273 ms . aml cells and normal bone marrow cells were labeled with anti - siglec - 9 - fitc and sorted into positive and negative fractions using a facs vantage se ( bd biosciences ). the purity of the positive fractions was consistently greater than 95 %. to control for the potential effect of anti - siglec - 9 mab on colony forming ability , all experiments included unsorted cells that had been incubated or not with anti - siglec - 9 - fitc . facs sorted cells and control ab incubated cells were cultured in methylcellulose media ( hsc - cfu complete with erythropoietin , miltenyi biotech ) for 14 days at 37 ° c ., 5 % co 2 , and numbers of cfu - e , bfu - e , cfu - g , cfu - m , cfu - gm and cfu - gemm scored according to the manufacturer &# 39 ; s instructions . the cfu - blast assay was carried as described 19 , using the same culture conditions as for normal bone marrow cells . cells were labeled with anti - siglec - 9 - alexa - 488 mab for 45 rain on ice . the cells were washed and either stored on ice or incubated for 60 or 240 min at 37 ° c ., 5 % co 2 in complete medium . at each time point , internalization was stopped by placing the tubes on ice . at the end of all incubations , the levels of anti - siglec - 9 - alexa - 488 mab remaining at the cell surface were detected in triplicate using goat anti - mouse - igg - apc ( caltag - medsystems ) on the fl - 4 channel . the total cell - associated alexa - 488 - labelled anti - siglec - 9 mab ( surface + internalized ) was measured on the fl - 1 channel . anti - siglec - 8 - alexa - 488 mab was used as an isotype control . internalisation was quantified by subtracting the fl - 4 median fluorescence intensity ( mfi ) obtained with anti - siglec - 8 from the fl - 4 mfi obtained with anti - siglec - 9 . the fl - 4 mfi values of cells kept on ice throughout the experiment were considered as 100 %. the total cell - associated anti - siglec - 9 mab was calculated similarly using the corresponding fl - 1 mfi values . adherent rat basophil leukemia ( rbl ) cells expressing wild - type or tyrosine to phenylalanine mutant forms of siglec - 9 15 were cultured in 8 - well chamber slides ( nalgenunc international , vwr , leics , uk ) and incubated with anti - siglec - 9 - alexa - 488 mab for 1 h on ice . after washing , the cells were incubated in complete medium on ice or for the indicated period of time at 37 ° c ., 5 % co 2 . internalization was stopped by putting the cells on ice and the plasma membrane labeled with 5 μg / ml cholera toxin b subunit - alexa - 594 ( invitrogen ). after washing , the cells were fixed with 4 % paraformaldehyde for 10 min at room temperature and mounted in vectashield mounting medium dapi ( vector laboratories , peterborough , uk ). slides were examined with a leica sp2 aobs confocal microscope equipped with a 63 × 1 . 4 oil lens ( leica , heidelberg , germany ). excitation / emission settings for different fluorescent labels were as following : alexa - 488 , excitation : 488 nm / detected emission : 503 - 585 nm ; alexa - 594 , excitation : 594 nm / detected emission : 611 - 689 nm and dapi excitation : 405 nm , detected emission : 410 - 527 nm . all images were processed using adobe photoshop cs . levels of soluble siglec - 5 and siglec - 9 were measured using enzyme linked immunosorbent assay ( elisa ) kits according to the manufacturer &# 39 ; s instructions ( r & amp ; d systems , abingdon , uk ). an in - house elisa was developed to measure cd33 . immulon 4 elisa plates ( dynatech , chantilly , va .) were coated overnight at 4 ° c . with purified 6c5 anti - cd33 mab at 4 μg / ml in carbonate buffer ph 9 . 6 , followed by either the test sample or a cd33 standard , comprising the cd33 extracellular region fused to enhanced green fluorescent protein , after 1 h at room temperature , wells were washed and incubated with affinity - purified rabbit anti - cd 33 , followed by goat anti - rabbit - horse radish peroxidase , elisas were developed using o - phenylamine diamine and absorbances measured at 450 μm . all samples were analysed in triplicate at 2 different dilutions in 2 independent assays . to assay expression of the cd33 - related siglecs on a collection of 21 cryopreserved aml cell samples , in - house generated anti - siglec mabs were labelled with fitc and used at a saturating concentration as determined by staining of normal blood leukocytes . for each anti - siglec mab , two - colour staining was performed using a biotinylated commercial anti - cd33 mab detected with streptavidin - apc and samples analysed by flow cytometry ( table 2 ). as expected , 1 - step staining with fitc - labelled anti - cd33 mab gave lower levels of labelling compared with the 2 - step staining , reflecting a generally lower sensitivity of directly labelled mabs ( table 2 ). a large variation in the percentage of siglec - positive cells was observed , but each siglec was invariably expressed within the cd33 + subset of aml cells ( fig1 ). using fitc - labelled mabs and taking a percentage cut - off of 5 %, 17 of 21 samples expressed cd33 ( median %, 37 ; median mfi , 25 ), 12 expressed siglec - 5 ( median %, 13 . 5 ; median mfi , 27 . 5 ), 11 expressed siglec - 9 ( median % 25 ; median mfi , 37 ), 5 expressed siglec - 7 ( median %, 28 . 5 , median mfi , 22 . 5 ) and 2 expressed siglec - 10 ( median % 10 . 4 ; median mfi , 21 ). no expression of siglec - 8 or siglec - 11 was seen on any sample analysed , and low levels of expression were seen for siglec - 6 in one case ( table 2 ). apart from cd33 , siglec - 9 was the most strongly expressed of the cd33 - related siglecs , both in terms of the percentages of positive cells and the mfi values of the positive subsets . the analysis showed that in seven cases it was expressed on a similar or even higher percentage of aml cells than cd33 ( table 2 ). a representative sample ( xxi , table 2 ) in which several cd33 - related siglecs were clearly detected on aml cells is shown in fig1 . myelomonoblastic ( fab : m4 ) and monoblastic ( fab : m5 ) aml cells showed an increased expression of siglecs - 5 , - 7 , - 9 and - 10 when compared to aml cells with a more immature phenotype ( fab : m0 , m1 , m2 ) ( table 2 ). this result is consistent with earlier studies on expression of siglec - 5 and siglec - 7 on aml cells 17 , 18 . the expression of siglecs - 5 , - 7 and - 9 on cd33 - positive subsets of cells raised the possibility that these molecules were coexpressed on the same subset of aml cells rather than on separate subsets . this was confirmed for two different aml samples using multi - parameter flow cytometry ( fig2 ). the relatively high expression of siglec - 9 in several cases of aml suggests that this siglec might serve as both a useful marker and a potential therapeutic target in certain subtypes of aml . to further characterise the siglec - 9 + aml subsets , additional phenotypic analyses were carried out , demonstrating that the majority of siglec - 9 + cells were cd38 − , cd123 +/− , cd117 + and cd14 + ( fig3 a ). in addition , cd33 + and siglec - 9 + cells were compared for the expression of cd34 ( class ii ) which is known to be expressed on leukemic stem cells ( lsc ) 20 . the result showed that in 8 of 10 samples analysed , only very few siglec - 9 + cells expressed cd34 , unlike cd33 , which was detected on a higher percentage of cd34 + cells ( fig3 b ). taken together , the immunophenotype of siglec - 9 + cells was consistent with monocytic cells . this was confirmed by may - grunwald - giemsa staining of sorted siglec - 9 + and siglec - 9 − aml cells ( fig4 a ). although the majority of siglec - 9 + cells lacked expression of cd34 ( fig3 b ) the few siglec - 9 + / cd34 + cells could include leukemic cells with blast colony forming potential 19 . to investigate this possibility siglec - 9 + and siglec - 9 − aml cells were purified by facs sorting and cfu - blast assays in methylcellulose performed for three different aml samples . in all cases , no cfu - blast were detected within the siglec - 9 + fractions , whereas the siglec - 9 − fractions had 2 , 200 and 238 cfu - blast colony initiating cells per 105 cells in the three patients &# 39 ; samples analysed . control experiments with unsorted cells incubated with or without anti - siglec - 9 mab showed that there was no effect of the mab on cfu - blast formation ( data not shown ). apart from cd33 and siglec - 5 , there have been no detailed reports on the expression profiles of the cd33 - related siglec family on normal bone marrow cells . this is important in the context of ab - mediated targeting of aml cells in which it would be desirable to spare normal progenitor cells from cytotoxic effects . three subpopulations of normal bone marrow cells were defined by flow cytometry according to side scatter ( ssc ) properties , respectively ssc low ( fig5 , r2 ), ssc medium ( fig5 , r3 ) and ssc high ( fig5 , r4 ) 21 . in four normal bone marrow samples analysed , the expression of the cd33 - related siglecs was mostly confined to the ssc medium and ssc high populations and a representative example is shown in fig5 . the majority of ssc medium cells were strongly positive for cd33 and siglec - 9 and weakly positive for siglecs - 5 and - 7 . in comparison , the ssc high cells were weakly positive for cd33 and siglec - 5 , but mostly negative for siglec - 9 ( fig5 ). multicolour labeling showed that the ssc medium , cd33 high , siglec - 9 + subpopulation ( fig6 a , gate 1 ) also co - expressed siglecs - 5 and - 7 . in contrast , the ssc high , cd33 low , siglec - 9 + , ( fig6 a , gate 2 ) cells were only positive for siglec - 5 , but negative for siglec - 7 . the siglec - 9 + subpopulation was further defined as cd38 − , cd123 +/− , cd14 + and cd34 − ( fig6 b , c ). taken together with may grunwald giemsa staining of purified siglec - 9 + cells ( fig4 b ), these results indicate that siglec - 9 + cells in normal bone marrow are predominantly immature cells of the monocytic lineage . examination of cd34 + cells showed that , similar to siglec - 9 , siglec - 7 was mostly absent whereas siglec - 5 was detected on ˜ 5 % and cd33 on ˜ 14 % of cd34 + cells ( fig6 c ). myeloid progenitor cells characteristically express cd34 and cd33 . although the majority of siglec - 9 + cells in normal marrow were cd34 − cd33 + , a small fraction (˜ 1 %) of the cd34 + cells were siglec - 9 + ( fig6 c ). to investigate whether a subset of progenitors expressed siglec - 9 , normal bone marrow cells were sorted into siglec - 9 + and siglec - 9 − cell fractions by flow cytometry and their colony forming ability was measured using a standard methylcellulose - based clonogenic assay . two independent experiments showed that the siglec - 9 + cell fraction contained no colony forming cells , in contrast to the negative fraction , which contained the expected levels of bfu - e , cfu - e , cfu - g and cfu - gm cells ( table 3 ). * the data show the mean of duplicate colonies per well from 1 × 10 4 cells incubated for 14 days in methylcellulose medium supplemented with stem cell factor , gm - csf , g - csf , il - 6 , il - 3 and erythropoietin . the brackets show the individual counts for both duplicates . similar results were obtained in two independent experiments using normal bone marrow from two different donors . in conclusion , analysis of normal bone marrow showed that siglecs - 5 , - 7 and - 9 are expressed on differentiating cells of the myeloid lineages . in the case of siglec - 9 , this receptor is absent from myeloid progenitors in contrast to cd33 . overall , the phenotype of aml cells and primary bone marrow cells that express cd33 - related siglecs are similar , suggesting that the cd33 - related siglecs are not expressed aberrantly in aml . anti - siglec - 9 mab is rapidly internalised by aml cells and siglec - 9 - transfected rat basophil leukemia cells the relatively high expression of siglec - 9 on monocytic aml cells and its absence from myeloid progenitors makes it a potential new candidate for mab based therapy , aimed at ablating blast cells and lowering the leukemic cell burden in the patient via toxin delivery . for this to be effective , it would be essential that anti - siglec - 9 mab is internalized upon binding to the cell surface . to analyse mab internalization , a flow cytometric assay was developed in which cells were labelled with alexa 488 - labelled anti - siglec - 9 mab on ice and then incubated for varying periods at 37 ° c . the amount of bound mab remaining was then measured using apc - labelled anti - mouse ig followed by flow cytometry . the results of three independent experiments with primary aml samples showed that 30 - 50 % of bound anti - siglec - 9 mab was internalized within 40 min at 37 ° c . and up to 90 % was internalised by 240 min ( fig7 a , left panel ). the loss of surface anti - siglec - 9 mab was due to internalization rather than shedding since the amount of cell - associated alexa - 488 - labelled anti - siglec - 9 mab remained constant during the time - course of the experiment ( fig7 b , left panel ). to demonstrate directly that siglec - 9 could mediate internalization of anti - siglec - 9 mab and investigate the role of the two cytoplasmic tyrosine - based signalling motifs ( y1 and y2 ), 15 we examined the internalization of anti - siglec - 9 ab in siglec - 9 stably - transfected rbl cells by flow cytometry and confocal microscopy . using a similar assay as described above for the aml cells , rbl cells expressing wild - type or y2f ( membrane distal tyrosine changed to phenylalanine ) mutant siglec - 9 showed similar rates of endocytosis such that by 40 min , 40 - 50 % of siglec - 9 was internalized ( fig7 a , right panel ). in comparison , the siglec - 9 y1f ( membrane proximal tyrosine changed to phenylalanine ) mutant was internalized more slowly ( fig7 a , right panel ). the total cell - associated anti - siglec - 9 - alexa - 488 appeared to increase slightly due to a gain in autofluoresence over the time - course of the experiment ( fig7 b , right panel ). internalization by rbl was confirmed by confocal microscopy in which wild type or y2f forms of siglec - 9 showed high levels of internalization after 1 h incubation , whereas the siglec - 9 y1f mutant remained mostly at the cell surface ( data supplement figure ), these experiments showed that the membrane proximal itim of siglec - 9 is required for optimal endocytosis and is consistent with previous studies on cd33 16 . soluble forms of siglec - 9 are either low or undetectable in aml bone marrow plasma whereas siglec - 5 is present at high levels for an anti - siglec - 9 mab to function effectively in targeting aml cells in vivo , it is important that high levels of soluble siglec - 9 are absent from plasma , since this might neutralise injected ab and reduce therapeutic efficacy . indeed , a high antigenic load of cd33 in blood may significantly affect the clinical outcome following mylotarg treatment 22 . we therefore investigated the levels of soluble siglec - 9 in bone marrow plasma collected from eight aml patients , including several with m4 / m5 fab status and one patient ( ap3 , table 4 ) shown to have high levels of siglec - 9 + aml cells ( sample xiii , table 2 ). we also measured soluble cd33 and siglec - 5 for comparison , undetectable levels of soluble siglec - 9 ( limit of detection 1 . 25 ng / ml ) were present in plasma from six patients and very low levels (˜ 4 ng / ml ) were detected in two patients ( table 4 ), in comparison , low to intermediate levels ( 4 - 30 ng / ml ) of soluble cd33 were seen in all patients and siglec - 5 was readily detectable in 7 of 8 patients examined , with levels up to ˜ 500 ng / ml ( table 4 ). no siglec - 9 could be detected in normal bone marrow plasma or normal blood serum samples , whereas soluble siglec - 5 was seen in all but one . cd33 was detected in one of three normal bone marrow plasma samples and in all normal blood serum samples . in general , levels of siglec - 5 and cd33 were higher in the aml samples than controls and these correlated to some extent with the numbers of circulating blood leukocytes in each patient ( table 4 ). in this paper we describe the first comprehensive analysis of cd33 - related siglec expression in aml . the aim of this screen was to determine if there are additional cd33 - related siglecs that could be used for clinical purposes , either as markers to monitor disease or as therapeutic targets . apart from cd33 , siglec - 9 stood out as an interesting new candidate and was shown to be present at similar levels to cd33 in 7 out of 21 aml cases analysed . we also compared the cd33 - related siglecs for their expression profile on normal bone marrow cells and demonstrated that , in contrast to cd33 , siglec - 9 is absent from myeloid progenitors but was present at similar levels to cd33 on immature cells of the monocytic lineage . taken together with our demonstration that anti - siglec - 9 is rapidly internalised by aml cells and is not present in plasma at significant levels , these findings raise the possibility that anti - siglec - 9 antibodies could be exploited for therapeutic purposes in aml , either alone or in conjunction with other treatments , including anti - cd33 mab - based therapy . a characteristic feature of the cd33 - related siglecs is their lineage - restricted expression pattern . for example , siglecs - 5 and - 9 are mostly found on monocytes and neutrophils 6 , 11 , 23 , 24 siglec - 7 is predominantly expressed on monocytes and nk cells 7 , siglec - 8 is restricted to eosinophils 9 , 10 and siglec - 11 is expressed in tissue macrophages but is absent from circulating leukocytes 13 . this differential staining on normal blood leukocytes is consistent with the pattern of expression observed here using a diverse collection of aml samples ranging in fab classification from m0 to m6 . thus , siglecs - 8 and - 11 were not detectable on any leukemic samples analysed whereas siglecs - 5 , - 7 , and - 9 were variably present on most samples . from side - by - side comparisons of all cd33 - related siglecs , it is clear that cd33 is expressed at high levels on the majority of aml samples irrespective of fab status , ( consistent with many previous studies ), whereas the other cd33 - related siglecs are only expressed at significant levels on subsets of cd33 + aml cells with features of myelomonocytic differentiation . an important question from a therapeutic perspective is whether siglecs - 5 , - 7 and - 9 are expressed on the same or on separate subsets of aml cells . we demonstrated here by multiparameter labelling that all 3 siglecs were co - expressed on the same subsets , consistent with the notion that expression of these molecules is co - ordinately regulated during aml cell differentiation . in this study , we showed that siglec - 9 was absent from all progenitors assayed , including cfu - g , cfu - m and cfu - gm and our observations , suggest that siglecs - 5 and - 7 are also absent from myeloid progenitors . therefore , it is likely that siglecs - 5 , - 7 and - 9 are first expressed on cd33 + cells of the myelomonocytic lineages once they have lost the ability to form colonies in response to growth factors . interestingly , expression of siglec - 9 on immature bone marrow neutrophils ( defined by high side - scatter , fig5 ) was weak or absent , suggesting that siglec - 9 is upregulated on these cells following their exit from the marrow . in contrast , cd33 is readily detectable on immature neutrophils and is downregulated on maturation . the lsc is currently considered as a key target for treatment of aml 1 , 25 and is found within the cd34 + cd38 − population 20 , 26 . the absence of siglec - 9 from myeloid progenitors suggests that it is also likely to be absent from lscs . consistent with this possibility , we demonstrated that cells sorted on the basis of siglec - 9 expression did not include any aml blast colony forming cells . therefore it is unlikely that anti - siglec - 9 mab used alone would be capable of directly ablating the lscs , but may be effective in targeting radioisotopes to the bone marrow for bystander toxicity to these rare cells . anti - siglec - 9 mab may also be useful in conjunction with anti - cd33 abs or other therapies , for example in reducing the leukemic burden in certain cases of myelomonoblastic leukemia . there are several potential advantages of targeting siglec - 9 for this purpose compared with siglec - 5 and siglec - 7 . first , siglec - 9 had the highest expression levels as revealed by mfi values ( table 1 and fig1 ). this , combined with the rapid uptake of bound ab shown here , would be expected to lead to high levels of endocytosed ab conjugates into leukemic cells , resulting in efficient cell death . second , concentrations of soluble siglec - 9 in bone marrow plasma from aml patients and controls was low or undetectable , whereas siglec - 5 was present at high concentrations and could possibly neutralise a significant fraction of injected ab . the analysis of soluble cd33 showed significant levels of this molecule also , and the elevation of both cd33 and siglec - 5 in aml samples clearly correlated with the number of circulating white blood cells in each patient . this suggests that the increased pool of soluble siglecs is leukemia cell - derived . although the molecular properties of soluble siglecs are currently unknown , a previous report described the existence of multiple splice variants of siglec - 5 , including one encoding a soluble , secreted form 24 . in conclusion , our findings demonstrate that siglec - 9 is expressed on ( myelo ) monoblastic leukemias and provides a potential novel therapeutic target , especially if considered as a tailored therapy , used in conjunction with conventional cytotoxic agents or novel agents including anti - cd33 directed strategies . 1 . jordan c t , guzman m l . mechanisms controlling pathogenesis and survival of leukemic stem cells . oncogene . 2004 ; 23 : 7178 - 7187 . 2 , bennett j m , catovsky d , daniel m t , flandrin g , galton d a , gralnick h r , sultan c . proposals for the classification of the acute leukaemias . french - 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