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The Effect of Ph and Ionic Strength on the Reaction Between Anti-D and Erythrocytes

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The Effect of Ph and Ionic Strength on the Reaction Between Anti-D and Erythrocytes Immunology, 1964, 7, 72. The Effect of pH and Ionic Strength on the Reaction between Anti-D and Erythrocytes N. C. HUGHES-JONES, BRIGITTE GARDNER AND RACHEL TELFORD Medical Research Council's Experimental Haematology Research Unit, Wright-Fleming Institute, St. Mary's Hospital, London, W.2 (Received 10th June 1963) Summary. The effect of pH and ionic strength on the reaction between anti-D and erythrocytes was investigated using 131I-labelled antibody. The following observations were made. 1. The association constant of the reaction was influenced by the pH of the suspending medium. The highest values were obtained in the pH range 60-8&0. The association constant was considerably reduced above and below these values. 2. There was considerable heterogeneity of the rate of dissociation of the antigen-antibody complex at pH values below 6-0 3. The antibody was separated into fractions which, at neutral pH, had different rate constants for dissociation. 4. The rate ofassociation between anti-D and erythrocytes was greatly increased by a reduction in the ionic strength of the suspending medium. INTRODUCTION The reaction between antibody and antigen can be interpreted in terms of the law of mass action (for review see Talmage and Cann, 1961). This not only applies to reactions between antibody and haptens or protein antigens, but also to antibodies against the blood group antigens, A, B, c and D (Mavrides, 1954; Wurmser and Filitti-Wurmser, 1957; Hughes-Jones, Gardner and Telford, 1962, 1963). Both hydrogen ion concentration and ionic strength of the medium influence the reaction. For instance, Singer and Camp- bell (1955) found that the binding constant for a rabbit anti-albumin antibody system diminished over the pH range 4*5-2-4, and concluded that one carboxyl group was involved in the reaction. Similarly, Epstein and Singer (1958) found a reduction in the binding constant for anti-benzenearsonic acid antibody over the pH range 8-2-10-6, and concluded that one amino group was involved in the reaction. The effect of salt concentration on the rate of association between antigen and antibody was first described byJerne and Skovsted (1953), who found that the rate ofneutralization of bacteriophage was increased 1000-fold when the salt concentration was reduced from 1-0 M to 0-001 M. Cann and Clark (1956) and Tsuji, Davis and Gindler (1962) also found that a reduction in salt concentration increased the rate of association of an anti-bacterio- phage and anti-luciferase antibody respectively. Both groups have interpreted this effect as due to the interaction of oppositely charged ionic groups at the combining sites. 72 Reaction between Anti-D and Erythrocytes 73 We have recently described the production and purification of 131I-labelled anti-D (Hughes-Jones, Gardner and Telford, 1963). We have now studied the effect of hydrogen ion and salt concentration on the reaction between 131I-labelled anti-D and erythrocytes. METHODS Red Cells Rh-positive group 0 red cells of probable genotype CCDee (R1Rl) were used throughout. When necessary, Rh-negative control cells of the genotype ccddee were also used. Antibody The anti-D used was obtained from the same donor as that described as serum Av in a previous publication (Hughes-Jones et al., 1963). The labelling with 131I and the purifica- tion took place in three stages. Stage 1. Four ml. of serum (titre 1000) was added to 100 mg. of freeze-dried haemo- globin-free RR' red cell stroma at 370 for 30 minutes. The stroma was then washed three times with saline (0.17 M NaCi, 0 003 M NaH2PO4-Na2HPO4, pH 6.7) at 20 and the antibody dissociated at pH 3-5, into 5 ml. ofsaline containing 10 mg. of y-globulin at 200. This y-globulin solution was prepared by using 6-9-diamino-2-ethoxyacridine (Horiejsi and Smetana, 1956); this preparation also contains p,-globulin as impurity. Stage 2. The y-globulin solution containing the antibody was iodinated by the method of McFarlane (1958). Unbound l31J was removed by passage through an anion exchange column (Amberlite 401; Cl) followed by dialysis for 16 hours against saline. Stage 3. Purification was carried out in the sense that 131I-labelled antibody was freed from other l3lI-labelled compounds. This was achieved as follows. Two ml. ofconcentrated serum protein (30 per cent w/v) were added to the 131I-labelled y-globulin solution con- taining anti-D and mixed with 2 ml. ofR1R1 red cells for 30 minutes at 370. The cells were then washed eight times at 20, using for each wash 10 volumes of saline containing serum protein, 1 g. per 100 ml. Lysis of the cells was achieved by freezing at 790 and the haemoglobin removed from the stroma by washing with saline at 20 (usually two washes). The antibody was then dissociated at pH 3-5 at 200 into 5 ml. ofa 1 in 5 dilution ofhuman serum in saline. Specific Activity of 131I-Labelled Antibody It was assumed that the specific activity of the antibody was the same as that of the 7S y-globulin fraction. The method used was the same as that previously described (Hughes-Jones et al., 1962) except that y-globulin was obtained by fractionation of the impure l3ll-labelled y-globulin on a diethylaminoethyl cellulose column, using the method described by Adinolfi, Polley, Hunter and Mollison (1962). Calculations The concentrations ofreactants, equilibrium constants and rate constants for association and dissociation were calculated as previously described (Hughes-Jones et al., 1962, 1963). The Effect ofpH on the Equilibrium Constant The concentration of the l3lI-labelled anti-D and the volume of red cells were adjusted so that approximately half the antibody was bound at equilibrium. The required pH was 74 4N. C. Hughes-Jones, Brigitte Gardner and Rachel Telford obtained by addition of0-1 N HC1 or 0-1 N NaOH and the system brought into equilibrium by incubation at 370 for 2 hours. Initial experiments had shown that equilibrium was reached during this time. When lysis occurred, stroma was separated from the supernatant by centrifuging at 10,000 g. The Effect ofpH on the Rate of Dissociation of Antigen-Antibody Complex Measurement of the rate of dissociation of antigen-antibody complex (approximately 4-0 PmM.) was carried out at 370 at various pH values, using serum protein (2 g. per 100 ml.) as a buffer. Red cell stroma was used for experiments at pH values below 7 0 and dissociation was stopped by rapid freezing; the suspensions were allowed to thaw while centrifuging at 10,000 g. Whole red cells were used at pH values above 7 0, the reaction being stopped by centrifuging at 3000 g. The pH of suspension did not fall by more than 0-2 units during the experiments. The Dissociation of Antibody from a Stroma-Celite Column at Acid pH Values Three ml. of red cells which had taken up 2-5 x 10-10 moles of antibody were lysed by freezing at -79° and the stroma washed twice at 20 with saline. The stroma was then added to 0-8 g. of diatomaceous earth (celite, 512) and packed in a column 1 cm. in diameter. Dissociation of the complex was carried out at 40 using a solution of serum protein (2 g. per 100 ml.) as eluant. The pH value ofeach succeeding 9 ml. ofeluant was decreased by 0-2 pH units. The rate offlow was 3 ml./min. The eluate was collected in 3 ml. aliquots and the 1311 content and the pH value of each was estimated. The pH of the eluate was then adjusted to 6-7 and the antibody from selected eluates was recombined with 20 mg. of dry red cell stroma (95 per cent uptake was obtained in each case). The antibody-stroma complex was then used for further dissociation at acid pH values under similar conditions. Other antibody-containing fractions obtained in a similar manner by dissociation at acid pH values were also recombined with red cells and the rate of dissociation then determined at 370, pH 6-7, in the manner previously described (Hughes-Jones et al., 1963). Excess unlabelled antibody was added to prevent reassociation of the 131I-labelled antibody. The Relationship between Ionic Strength and the Rates of Association and Dissociation 131I-labelled anti-D solutions at pH 6-7 were diluted with a solution of glucose (7 g. per 100 ml.) to give final ionic strengths ranging from 0 037 to 0 17 L Ionic strengths below 0*037 I were obtained by gel filtration (Sephadex G 25). The solutions were then left for 1 hour at 4°. Precipitated protein was removed by centrifuging at 10,000 g. The final concentration of antibody in each solution was approximately 1-3 jimM. Each solution was then added to red cells at 4°. The volume of red cells used was predetermined to give a rate of formation of antigen-antibody complex that could conveniently be estimated. The amount of antibody combined with red cells at 5, 10, 20, 40 and 60 minutes was determined, the cells being separated by centrifuging. Correction was made for non- specific uptake of 131I, using Rh-negative control cells. The ionic strength of the 5 minute samples was estimated using a conductivity meter, and this value used in presenting the results. The amount of antigen-antibody complex formed was plotted against time, and the rate constant for association calculated from the initial slope of the curve. Reaction between Anti-D and Erythrocytes 75 The rate of dissociation of the antigen-antibody complex was estimated by allowing the complex to dissociate at 370 in solutions of ionic strength ranging from 003 to 017 I in the presence of excess unlabelled antibody.
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