Abstract:
A system for separating nucleated cells from a blood sample includes a charge-flow separator (CFS), which separates blood into fractions according to the surface charge density characteristics coupled with an affinity-filtration separator which either outputs a separated blood fraction to the CFS or receives a separated blood fraction from the CFS. The system permits separation of nucleated fetal red blood cells, erythroid progenitor cells and other nucleated cells found in blood samples.

Description:
CROSS-REFERENCES TO PRIOR APPLICATIONS 
     This application is a divisional of corresponding patent application Ser. No. 08/899,283, filed Jul. 23, 1997, U.S. Pat. No. 5,906,724 which is a divisional application of corresponding application, Ser. No. 08/327,483, filed Oct. 21, 1994, which subsequently issued as U.S. Pat. No. 5,662,813 on Sep. 2, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to separation of fetal erythrocytes from maternal blood samples. More particularly, the present invention provides a system and non-invasive method for enriching the population of nucleated fetal erythrocytes or nucleated fetal red blood cells (“NRBCs”) obtained from maternal blood samples by separating the NRBCs from the mother&#39;s erythrocytes, leukocytes and other blood components. More specifically, the present invention offers a system and method for enriching the population of NRBCs from a maternal blood sample which concentrates the NRBCs by electrophoresis and/or adsorption-filtration or affinity filtration. 
     Physicians have long sought to develop non-invasive methods for prenatal diagnosis because the available methods, amniocentesis and chorionic villus sampling (CVS) are potentially harmful to the mother and to the fetus. The rate of miscarriage for pregnant women undergoing amniocentesis is increased by 0.5-1%, and that figure is be slightly higher for CVS. Because of the inherent risks posed by amniocentesis and CVS, these procedures are offered primarily to older women, i.e., those over 35 years of age, who have a statistically greater probability of bearing children with congenital defects. 
     Some non-invasive methods have already been developed to diagnose specific congenital defects. For example, maternal serum alpha-fetoprotein, and levels of unconjugated estriol and human chorionic gonadotropin can be used to identify a proportion of fetuses with Downs syndrome. Similarly, ultrasonography is used to determine congenital defects involving neural tube defects and limb abnormalities. 
     Separation of nucleated fetal erythrocytes from maternal blood has been proposed as a viable method for facilitating prenatal diagnosis of genetic disorders. Fetal NRBCs have been separated from maternal blood by flow cytometry using a lysing reagent (European Published Patent Application No. 582736, published Feb. 16, 1994); by triple gradient discontinuous gradient gel electrophoresis (Bhat, et al, U.S. Pat. No. 5,275,933, issued Jan. 4, 1994); by separation from nucleated cells using leukocyte depletion and lysis of non-nucleated erythrocytes using ammonium chloride (Goldbard, PCT Publication WO 9417209, published Aug. 4, 1994); by use of anti-CD71 monoclonal antibody and magnetic beads and in-situ fluorescence hybridization (FISH) (Ahlert, et al, German Published Patent Application No. 4222573, published Aug. 12, 1993) or by other antibodies specific to a fetal erythrocyte antigen (Bianchi, PCT Publication WO 9107660, published May 30, 1991). 
     SUMMARY OF THE INVENTION 
     The present invention demonstrates that fetal nucleated red blood cells exhibit consistent migration patterns in an electric field according to surface charge density which are different and distinct from the migration patterns of adult enucleated red blood cells. By using a novel charge-flow separation (CFS) method, described hereinafter, the present invention is able to divide maternal blood into fractions according to the surface charge density characteristics of each cell type. As the blood cells in a maternal blood sample move through the CFS apparatus, they are focused into compartments by opposing forces, namely buffer counterflow and electric field, and then are directed into waiting collection tubes. An apparatus and method of counterflow focusing suitable for use with the system and method of the charge-flow separation of the present invention are disclosed in our U.S. Pat. Nos. 5,336,387 and 5,173,164, which are hereby incorporated by reference. A preferred embodiment of a CFS apparatus and method will be described in greater detail hereinafter. 
     The inventive CFS system and method has been successfully used to recover nucleated red blood cells from the peripheral circulation of pregnant women. The recovered NRBCs were identified histologically. The NRBCs exhibited consistent migration patterns whether they came from maternal blood or from umbilical cord blood collected at birth. No NRBCs were found in blood from nulliparous women. 
     Because the inventive system and method of charge-flow separation of NRBCs from maternal blood is based on the intrinsic physical properties of the NRBCs, there is little need for extensive preparation of the maternal blood sample. The cells may be processed at greater than or equal to 60,000 cells per second and specialized training is not required. When the inventive charge-flow separation system and method is used, the recovered cells are viable, thus raising the possibility of further enrichment by cell culture. 
     In conjunction with or in addition to the charge-flow separation system and method, the present invention also includes an affinity separation method for separating NRBCs from other cell populations in a maternal blood sample. The adsorption-filtration affinity method of the present invention entails layering a maternal blood sample onto a fibrous adsorption-filtration filter medium having a nominal pore size of about 8 microns and being capable of 40-80% leukocyte immobilization, with a 70-80% post-wash leukocyte retention rate, and which is extremely hydrophilic, being capable of wetting with solutions having surface tensions of up to 85-90 dynes, which has a hold up volume of 40-70 μl/cm 2  for a single layer of adsorption-filtration filter medium and which is characterized by low to medium protein binding. The preferred adsorption-filtration separation filter medium is that sold by Pall Corporation under the trademarks “LEUKOSORB” TYPES A and B or that described in U.S. Pat. Nos. 4,923,620, 4,925,572, or European Patent No. 313348, each of which is hereby incorporated by reference. 
     The charge-flow separation system and method and the adsorption-filtration separation system and method of the present invention may be used separately or may be used in conjunction with one another to achieve enrichment of the nucleated fetal red blood cell population in a maternal blood sample. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a diagrammatic view of a first embodiment of the charge-flow separation apparatus according to the present invention. 
     FIG. 2 is a perspective view of a separation cell according to the present invention. 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a cross-sectional view illustrating multiple adjacent separation cells according to the present invention. 
     FIG. 5 is a diagrammatic view of a second embodiment of the charge-flow separation apparatus according to the present invention. 
     FIG. 5A is a perspective view of an end lateral buffer flow cell in accordance with a second embodiment of the charge-flow separation apparatus according to the present invention. 
     FIG. 6 is a flow diagram illustrating a first embodiment of the adsorption-filtration separation method of the present invention. 
     FIG. 7 is a flow diagram illustrating a second embodiment of the adsorption-filtration separation method of the present invention. 
     FIG. 8 is a flow diagram illustrating a third embodiment of the adsorption-filtration separation method of the present invention. 
     FIG. 9 is a flow diagram illustrating a fourth embodiment of the adsorption-filtration separation method of the present invention operated in conjunction with the charge-flow separation method of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIRST EMBODIMENT 
     Charge-flow Separator 
     Turning to the accompanying Figures, and with particular reference to FIGS. 1-4, there is shown a multi-compartment charge-flow separator  10  according a preferred embodiment of the present invention. The charge-flow separator is comprised of an array of subcompartments  12  with two electrode compartments  30 ,  32  on either end of the array. All subcompartments  12  for this electrophoretic separation cell are identical. Each subcompartment  12  consists generally of a planar body which has a narrow channel  16  formed within the planar body. Channel  16  acts as the separation chamber for processing the sample. The subcompartments  12  are adjacently arrayed in a co-planar fashion. Membranes  17  are interdisposed between the individual subcompartments  12 . Membranes  17  which serve as delimiting inter-subcompartmental boundaries for the narrow channel  16  and effectively separate each adjacent channel  16  into discrete separation chambers. Each subcompartment  12  is provided with a linear array of openings  20  which receive a bolt or other fastening means to clamp together the array of aligned subcompartments  12 . Each subcompartment also is provided with a linear array of alignment openings  22 , which receive alignment pins to maintain a uniform width of the channel  16  along its longitudinal aspect. The array is bolted or clamped together, whereby the electrode compartments function as end-plates which provide a rigid structural support. With the electrode compartments  30 ,  32  provided on either end of the array, an electrical field applied to the electrophoretic separator runs perpendicular to the planar aspect of subcompartments  23  and the membranes  17 . Appropriate electrolytes are pumped into their respective electrode compartments. Continuous recycling of the electrolytes removes electrolytic gases. 
     The membranes  17  reduce inter-subcompartmental fluid convection and mixing of the separated sample zones. A parallel array of membranes  17 , oriented perpendicular to the separation axis, confines fluid convection to the individual subcompartments  12 . The parallel array of membranes  17  benefit the separation process while preventing detrimental fluid convection between subcompartments  12 . 
     Each subcompartment  12  is provided with a heat exchanger  14  to dissipate Joule heat generated during the electrophoretic separation. The heat exchanger  14  is disposed in the channel  16  of each subcompartment  12  such that at least a substantial portion of the longitudinal inner walls of channel  16  formed by the planar body  12 , are associated with heat exchanger  14 . According to the preferred embodiment of the invention, heat exchanger  14  comprises either a quadrilateral or circular cross-sectional tubing having a depth or diameter corresponding to the thickness of the planar body  12 . The heat exchanger  14  connects to inlet and outlet ports  18  at the ends of the length of the cavity. Inlet and outlet ports  18  are preferably formed by providing co-axially aligned inlet and outlet ports  18  in adjacent arrayed subcompartments  12 , each port  18  having fluid conduit  19  between the port  18  and the heat exchanger  14 . Fluid conduit  19  may be an integral with heat exchanger  14  or a discrete component. 
     Sample inlet  11  and collection  13  ports are also provided at each end of channel  16  and are in fluid flow communication therewith. Both inlet port  11  and collection port  13  are connected to a multichannel pump  60  which pumps the sample solution into channels  16  of individual subcompartments  12  and concurrently removes separated solution from the collection ports  13 . 
     The volume of the electrophoretic separator according to the present invention is determined by the number of subcompartments  12  and the length of channel  16  in each subcompartment  12 . 
     According to the best mode contemplated for the invention, each subcompartment  12  is preferably 0.1 to 0.4 cm in thickness, and may be die cut, extruded, molded or otherwise formed as may be known in the art. Regardless of the length of channel  16  or number of subcompartments  12 , the thickness of the each subcompartment  12 , and, hence, channel  16 , i.e., the distance between two membranes  17 , should be about 0.1 to about 0.4 cm. Further, an effective dimension of the channel  16  which forms the separation chamber, should be about 50 cm in length, 0.2 cm in width and 0.3 cm in thickness, which yields an effective total volume of 3 ml within channel  16 . 
     Because of its dielectric properties, a silicone rubber material is well suited for sample containment in an electrophoretic separator, however, other electrically insulating materials are also effective. Because of its elastic properties, silicone rubber, together with the membranes  17 , acts as a gasket between adjacent subcompartments  12  when the subcompartment array (FIG. 1) is bolted or clamped together. 
     The membranes used in the embodiment of the present invention are preferably woven polytetrafluoroetylene, polyvinylidone fluoride (FVDP), cellulose, nitrocellulose, polycarbonate, polysulfone, microporous glass or ceramics, or woven monofilament nylon screens. The screens may be formed of a woven filament, or may be microporous non-woven materials, or solid matrices with opening formed by sacrificial sites imparted by irradiation by laser or other energy sources such as gamma irradiation. 
     The electrode compartments  30 ,  32 , provided at each end of the subcompartment array, provide a cavity for a platinum electrode. The electrode compartments  30 ,  32  are constructed from a suitable plastic material, preferably acrylic, and serve as a rigid end plate allowing the subcompartment array to be bolted together. In order to remove electrolytic gases from the electrolytic cavity, they are equipped with inlet and exit ports for the flow-through operation of the electrolytes. The electrolytes may be recirculated within each compartment  30  or  32 , may be flowed through each compartment  30 ,  32  in a single pass or the anolyte may be mixed with the catholyte. 
     To facilitate assembly of the apparatus  10 , the subcompartments  12 , and the electrode compartments  30 ,  32 , have a plurality of openings perpendicular to their flat faces. Bolts are inserted into the openings, which form one long bore hole upon the co-axial alignment of the components, which tightened to seal the subcompartments and electrode compartments against leakage. Additionally, a plurality of alignment openings  22  are provided in the planar bodies  12  and adjacent to or in close proximity to the channel  16 . The alignment openings  22  serve to maintain uniform longitudinal alignment of channel  16 , by engagement upon alignment pins (not shown). 
     Sample inlet  11  and collection  13  ports may be disposed within the silicone rubber material and pass from each end of the channel  16  and extend external to the planar body of the subcompartment  12 . Each of the sample inlet  11  and collection  13  ports are connected to the multichannel pump  60 . 
     Continuous-flow processing requires the provision of means for dissipating the Joule heat generated by the applied electric field. The apparatus of the present invention has the heat exchange means  14  for the recirculation of a coolant either attached to, or made as an integral part of the planar body of each subcompartment  12  such that the heat exchange means  14  forms side walls of channel  16 . The heat exchange means  14  according to the preferred embodiment of the invention, consists of cooling tubes  14  having the same cross-sectional diameter or depth as that of the channel  16 . Except as hereinafter described otherwise, each subcompartment  12  has associated with it a parallel pair of cooling tubes  14 , each of which has inlet and exit ports  18  provided at the ends of the longitudinal axis of the subcompartment  12 . The inlet and outlet ports  18  of all subcompartments  12  open into the same feed lines running perpendicular to the planar aspect of each subcompartment  12  through the array. The feed lines on either end of the silicone rubber spacer are formed by openings perpendicular to the planar aspect of the planar body of each subcompartment  12  and are in fluid flow communication upon bolting or clamping of the subcompartment array. The coolant is supplied from a coolant supply  50  and is continuously recirculated by a pump  52 . 
     The cooling tubes  14  may be fashioned of any tubular structure which is electrically insulated from the applied electrical field. Cooling tubes  14  may be discrete from or integral with the planar body of each subcompartment  12 . For example, cooling tubes  14  may be made of a plastic material which has a high dielectric strength to avoid electrical conduction to the coolant and has thin walls to facilitate high thermal transfer rates. Suitable plastic materials are tetrafluoroethylene or fluorinated ethylpropylene resins marketed under the trademark TEFLON or polypropylene co-polymers, polyethelyene or silicone. Alternatively, the cooling tubes  14  may be made of a ceramic or glass having high thermal transfer properties, and the glass or ceramic tubes may have metalization layers on luminal surfaces thereof, which is thereby electrically isolated from the electric field and which facilitates heat transfer to the cooling medium. Similarly, cooling tubes  14  which are integral with the planar body of each subcompartment  12  may consist of chambers within each planar body for circulating a coolant medium therethrough, or may consist of inter-subcompartmental channels formed in the planar surface of each planar body and which reside between adjacent subcompartments  12 . Regardless of the configuration of the cooling tubes  14 , they must be in thermal communication with the separation channel  16  and capable of conducting Joule heat away from the separation channel  16 . 
     Cooling tubes  14  may have any suitable cross-sectional shape, but are preferably circular or quadrilateral. Cooling tubes  14  preferably have a cross-sectional dimension which corresponds to the thickness of the planar body forming the subcompartment  12 , and, therefore, have substantially the same width or depth as that of the channel  16 . Non-integral cooling tubes  14  may be affixed to the side walls of the channel  16  by gluing, welding, or other suitable method of affixation as may be known in the art, the cooling tubes may be molded directly into the material forming the planar body of the subcompartment  12 , or the cooling tubes  14  may be extruded or otherwise formed as an integral part of the planar body of the subcompartment  12 . 
     Sufficient cooling of the sample volume in the cavity is achieved only when the entire volume of the sample fluid is in close proximity to the surface area of the heat exchange means  14 . It has been found preferable to have the sample fluid within a range of about 0.05 cm to about 0.15 cm away from any heat exchange means  14  surface to provide effective cooling of the sample fluid. Thus, the desirable width of channel  16 , as measured by the lateral distance between the two parallel cooling tubes  14  is from about 0.1 cm to about 0.3 cm. 
     Providing internal cooling adjacent to the separation chamber permits use of a higher applied potential for the separation process which increases both the resolution and the speed of the separation. An advantage of the apparatus according to the present invention over designs of the prior art is the configuration of a long narrow separation chamber having internal cooling. As long as the ratio of cooling surface area to process volume remains constant or is increased, the device can be scaled to any sample cavity volume desired, simply by increasing the length and the number of subcompartments  12 , without loss of resolution. 
     SECOND EMBODIMENT 
     Laterally Introduced Buffer Counterflow Gravity Biased Charge-Flow Separator 
     Turning now to FIGS. 5 and 5A, there is illustrated a second embodiment of a charge-flow separator apparatus  60  which employs a laterally introduced buffer counterflow and a gravity biased sample flow. The charge-flow separator apparatus  60  (CFS  60 ) is largely identical to that of the first embodiment of the charge-flow separator apparatus  10  (CFS  60 ), described above. Specifically, the CFS  60  includes a plurality of planar spacer members  62  formed in a parallel array, identified as 1-12 in FIG. 5, with screen members interdisposed therebetween (not shown) as described above with reference to the CFS  10 . Each of the plurality of planar spacer members  62  are identical to those described above with reference to FIG.  3  and as described in U.S. Pat. Nos. 5,336,387 and 5,173,164, incorporated herein by reference. Two planar end-spacer members  90 , identified as 0 and 00 in FIG. 5, form end boundary fluid flow members for introduction and withdrawal of a buffer counterflow  75  through the parallel array of planar spacer members  62 . Electrodes  61  and  64  form an anode and cathode, respectively, at opposing ends of the parallel array of planar spacer members  62  and the planar end-spacer members  90 . Electrodes  61  and  64  are electrically connected to an appropriate variable power supply (now shown) to provide an electromotive force within the separation chamber. Each of the electrodes  61  and  64  are in fluid flow communication with an electrode buffer reservoir  70  and an electrode buffer pump  68  to recirculate an electrode buffer through the electrode compartments as hereinbefore described with reference to the CFS  10 . 
     A throughput sample flow  63  is introduced into the parallel array of planar spacer members  62  from a sample container  58  under the influence of a throughput pump (T-Pump)  72 . 
     The T-Pump  72  is capable of introducing the throughput sample flow  63  into a first end of a selected one or more of the planar spacer members  62 . A separated sample flow  65  is withdrawn from a second end of each of the planar spacer members  62  under the influence of a fraction pump (F-Pump)  73  which feeds the separated samples from each of the plurality of planar spacer members into a fraction collector  74 . The operational parameters of sample flow  63  and separated sample flow  65 , T-Pump  72 , F-Pump  73 , and fraction collector  74  are more full set forth above with reference to FIGS. 1-4 and in U.S. Pat. Nos. 5,336,387 and 5,173,164, incorporated herein by reference. 
     Two significant areas of difference exist between CFS  60  and CFS  10 . The first of these differences results from the CFS  60  being oriented such that sample inflow  63  occurs at the bottom each of a plurality of separation chambers  62  and separated sample outflow  65  occurs at the top of each of the plurality of separation chambers  62 . In this manner, the sample flow through the plurality of separation chambers  62  is biased against ambient gravity  85 . Biasing the sample flow through the plurality of separation chambers  62  reduces zone sedimentation effects on cell populations normally found in sample flows which are normal to the gravity vector  85 . By reducing the zone sedimentation effect on the cell populations, the gravity biasing of the present invention effectively increases throughput with correspondingly longer residence times in the separation chamber. 
     The second of these differences is a lateral orientation of the buffer fluid flow  77  and  79 . As best illustrated in FIG. 5A, the lateral buffer fluid flow  77  and  79  is facilitated by modification of the spacer members  12  described above with reference to FIG.  3 . Turning to FIG. 5A, a generally rectilinear planar end-spacer member  90  is provided with a fluid flow opening  96  longitudinally oriented therein. The fluid flow opening  96  is bounded on an upper, a lower and lateral surfaces thereof by the spacer member  90 , but is open to frontal and rearward planar surfaces of the planar end-spacer member  90 . A cooling member  94 , similar to the cooling member  14  described above with reference to the CFS  10  is disposed along one lateral surface of the fluid flow opening  96  forming one lateral boundary of the fluid flow opening. The cooling member  94  is connected, in fluid flow communication with inlet and outlet ports  98  also disposed in the planar end-spacer member  90 . Inlet and outlet ports  98 , in turn, communicate with an external fluid cooling medium and heat exchanger (not shown, but identical to that of the CFS  10 , heretofore described). 
     A plurality of buffer ports  93  are disposed on a second lateral surface of the fluid flow opening  96 , which open into the fluid opening  96  in the direction of and opposite to the cooling member  94 . Each of the plurality of buffer ports  93  are connected, through a first lateral wall  95  of the planar end-spacer member  90 , to a plurality of tubular members  91 . Tubular members  91  serve either to convey a buffer fluid flow  77  from an external buffer reservoir  80  to the plurality of buffer ports  93  under the influence of a multichannel counterflow buffer pump (C-Pump)  76  or from the plurality of buffer ports  93  to either a second fraction collector or to waste  82  under the influence of a second multichannel fraction pump (F-Pump′)  78 . Thus planar end-spacer  0  serves to receive the buffer inlet flow  77  from the C-Pump  76 , while planar end-spacer  00  serves to convey the buffer outlet flow  79  to either fraction collector or waste  82  under the influence of F-Pump′  78 , thereby creating the buffer counterflow  75  across the parallel array of planar spacer members  62  in a direction opposing the applied electrical field  71 . 
     The buffer inlet flow  77  and the buffer outlet flow  79  are, therefore, laterally oriented relative to the planar end-spacer member  90  used as the buffer inlet member and the fluid opening  96  within the planar end-spacer member  90 . By orienting the buffer ports  93  laterally relative to the planar end-spacer member  90 , the buffer inlet flow  77  and the buffer outlet flow  79  are perpendicular to the axis of the buffer counterflow  75  through the plurality of planar spacer members  62  and the axis of the applied electrical field  71 . In this manner a generally laminar buffer counterflow  75  through the plurality of planer spacer member  62  results. 
     It has been found that the CFS  60 , described above, is especially well suited for enriching fetal NRBCs from maternal blood samples. Because the CFS  60  employs a horizontal crossflow fluid gradient  75  opposing the mobility induced by the applied electric field  71 , cells flowing vertically accumulate into particular compartments when the electrophoretic mobility of the cells is countered by the buffer counterflow. 
     EXAMPLE I 
     Ten whole blood samples (10-40 ml in anticoagulant) were obtained from 9 pregnant women and one postpartum female by venipuncture. Cord blood specimens, adult male whole blood specimens and whole blood specimens from nulliparous women served as controls. The blood samples were layered on a Ficoll gradient and centrifuged. After centrifugation, the nucleated cell layer was removed, washed and re-suspended in buffer (triethanolamine, 10 mM; glycine, 280 mM; glucose, 1 mM, acetic acid to pH 7.2). The re-suspended cells were processed the a CFS  60  configured with 12 separation channels, 10 counterflow input channels and 4 counterfiow output channels. The resuspended cells were introduced in an sample input flow, with the sample buffer input having a flow rate of 0.270 ml/min/channel, with a buffer output flow rate of 0.220 ml/min/channel and a counterflow rate of 1.8 ml/min, in the presence of an applied electric field of 250 V at 68-72 mA. 
     After processing, the fetal NRBCs were collected and identified histologically after staining with Giesma and benzidine to stain the nuclei blue and the cytoplasm brown. The cells were enumerated at 400× under light microscopy in a grid reticule of 0.06 mm 2 . A Hettich cytocentrifuge was used to prepare slides. 
     In each case, 1-7 NRBC&#39;s per 1000 maternal cells were counted, with a mean of 3.8±1.75 (SD) cells. Among 5 cases, 265-1430 NRBCs per slide were counted, with a mean of 531.6±504.2 (SD) cells. These cells were present in the blood of the pregnant and postpartum females and cord blood, but were not found in the control adult males or nulliparous females. There does not appear to be a correlation between gestational age and the frequency of NRBCs in maternal blood. The data are summarized in Table 1, below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Gestastion 
                   
                 NRBC per 1000 
                   
               
               
                 Case 
                 (Weeks) 
                 Fetal Sex 
                 WBC 
                 NRBC per slide 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 15.5 
                 XY 
                 7 
                   
               
               
                 2 
                 16.0 
                 NR 
                 5 
               
               
                 3 
                 18.0 
                 NR 
                 3 
                 265 
               
               
                 4 
                 20.0 
                 XY 
                 5 
               
               
                 5 
                 20.5 
                 XY 
                 3 
                 321 
               
               
                 6 
                 21.5 
                 XY 
                 2 
                 1430  
               
               
                 7 
                 24 
                 NR 
                 3 
               
               
                 8 
                 26 
                 XY 
                 1 
                 374 
               
               
                 9 
                 28 
                 XY 
                 4 
               
               
                 10  
                 30.5 
                 XY 
                 5 
                 268 
               
               
                   
               
             
          
         
       
     
     EXAMPLE II 
     Whole blood samples were obtained on anticoagulant from 6 pregnant females by venipuncture and separated in a charge-flow separator under conditions identical to those in Example I above. Separated samples were obtained from the charge-flow separator and processed as described in Example I. Table II summarizes the results: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                 No. NRBCs/WBC in Peak 
               
               
                 Case 
                 Gestation (Weeks) 
                 Fraction 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  A-11 
                 11 
                 {fraction (7/993)} 
               
               
                  A-13 
                 12 
                 {fraction (5/995)} 
               
               
                 A-8 
                 15.5 
                 {fraction (67/993)}  
               
               
                 A-6 
                 24 
                 {fraction (3/997)} 
               
               
                 A-5 
                 9.0 
                 {fraction (12/988)}  
               
               
                 A-9 
                 12.0 
                 {fraction (8/992)} 
               
               
                   
               
             
          
         
       
     
     EXAMPLE III 
     Whole blood samples were collected on anticoagulant from 3 pregnant women by venipuncture. The blood samples were processed by charge-flow separation in the buffer described in Example I, operated under the following conditions: the sample buffer input having a flow rate of 0.220 ml/min/channel, with a buffer output flow rate of 0.270 ml/min/channel and a counterflow input rate of 4.04 ml/min and a counterflow output flow rate of 1.48 ml/min, in the presence of an applied electric field of 325 V at 90-100 mA. To achieve T-Pump rate less than the F-Pump rate, in conjunction with a differential counterflow in and counterflow out rate, a counterflow buffer inflow line was added from the C-Pump into the bottom of buffer inflow spacer  0 , and a counterflow buffer outflow line was added from the top of buffer outflow spacer  00  to the Fpump′  78  as indicated in phantom on FIG.  5 . 
     The results of this separation are detailed in Table III, below: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                   
                   
                 No. NRBCs/WBC in Peak 
               
               
                 Case 
                 Gestation (Wks) 
                 Fraction 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A-24 
                 7.5 
                 {fraction (55/945)}  
               
               
                 A-25 
                 12.0 
                 {fraction (3/997)} 
               
               
                 A-27 
                 20.5 
                 {fraction (3/997)} 
               
               
                   
               
             
          
         
       
     
     Alternative operating conditions may be employed, such as different buffers, different separation times, different fluid flow rates, different voltage and wattage have been found useful while realizing enrichment of the fetal NRBCs from a maternal whole blood sample. For example, a buffer consisting of 5 mM triethanolamine, 22 mM glucose, 280 mM glycine and acetic acid to pH 7.33 has been used in a CFS  60  in which the sample input flow rate was 0.22 ml/min/channel, the sample output flow was 0.37 ml/min/channel and the crossflow pump was operated at 3.75 ml/min, in the presence of an electrical field generated at 325 volts and 55 mA, with comparable enrichment of fetal NRBCs as found in the examples stated above. 
     The foregoing examples should be understood to be merely as non-limiting examples of the operation of the charge-flow separation system and method of separating fetal nucleated red blood cells from a maternal blood sample. Those skilled in the art will understand and appreciate, that other operational parameters, including flow rates, applied voltage, buffer constitutions or pre-processing methods for separating nucleated from enucleated cells in a whole blood samples may be employed without departing from the spirit and scope of the present invention. 
     It has been found that the mobility of fetal NRBCs in an electric field is intermediate between the fastest enucleated red blood cells and the slowest white blood cells. Thus, for example, where enucleated red blood cells elute in fractions 4-7, and leukocytes elute from fractions 6-8, it has been found that the nucleated fetal red blood cells elute in fractions 6-8 with the peak in fraction 7. The CFS  60  is well suited to run in a tandem or in-series mode in which a first run is conducted to conduct a gross separation as noted above, and the fractions of interest, e.g., fractions 6-8, which contain the peak nucleated fetal red blood cells, but also the fastest enucleated red blood cells and the slowest leukocytes, can be conducted to a second CFS  60  set to run at voltage, wattage, T-Pump, F-Pump, and C-Pump flow rates which are set different from the first run CFS in order to amplify the 3 fraction sample across a larger number of channels, e.g., 10-12. This second, or tandem, CFS  60  run then provides a finer separation of the fetal NRBCs than that obtained from the first run. 
     THIRD EMBODIMENT 
     Adsorption-Filtration And Affinty Matrix Charge-Flow Separation 
     Turning now to FIGS. 6-9, there is disclosed the use of at least one leukocyte-depleting matrix which preferentially captures leukocytes by at least one of adsorption, filtration and affinity as described in U.S. Pat. No. 4,925,572 or PCT Publication No. WO 94/17209, incorporated herein by reference. The preferable leukocyte-depleting matrices are “LEUKOSORB A” and “LEUKOSORB B” both manufactured and sold by Pall Corporation. For purposes of illustration only, the leukocyte-depleting matrices depicted in FIGS. 6-9 are identified by LEUKOSORB A and LEUKOSORB B, but any type of adsorption, filtration or affinity matrix which functions to preferentially pass nucleated red blood cells or white blood cells and red blood cells in a manner as described in FIGS. 6-9 is contemplated as being useful. 
     The adsorption-filtration affinity method of the present invention entails layering a maternal blood sample onto a fibrous adsorption-filtration filter medium having a nominal pore size of about 8 microns and being capable of 40-80% leukocyte immobilization, with a 70-80% post-wash leukocyte retention rate, and which is extremely hydrophilic, being capable of wetting with solutions having surface tensions of up to 85-90 dynes, which has a hold up volume of 40-70 μl/cm 2  for a single layer of adsorption-filtration filter medium and which is characterized by low to medium protein binding. The preferred adsorption-filtration separation filter medium is that sold by Pall Corporation under the trademarks “LEUKOSORB” TYPES A and B or that described in U.S. Pat. Nos. 4,923,620, 4,925,572, or European Patent No. 313348, each of which is hereby incorporated by reference. 
     Those skilled in the art will appreciate, from the foregoing, that while the present invention has been described with reference to its preferred embodiments, that the spirit and scope of the present invention is not intended to be limited thereto, but by the claims appended hereto.