Differences in specificities of anti-C3d sera raised to C3d antigens prepared in different ways

Clinical application of a flow cytometric direct antiglobulin test

BACKGROUND

The clinical application of flow cytometric direct antiglobulin test (FC-DAT) has rarely been evaluated for patients with various diseases including immune and nonimmune hemolytic anemia.

STUDY DESIGN AND METHODS

Blood samples from 380 patients with a variety of diseases were studied using the tube direct DAT and FC-DAT. The results of tube DAT and FC-DAT were compared. The predictive values of DAT for hemolysis were evaluated.

RESULTS

Of 57 patients with autoimmune hemolytic anemia (AIHA), 6 of the 17 with a negative tube DAT (immunoglobulin G [IgG]) had a positive FC-DAT (IgG) and 23 of the 36 patients with a negative tube DAT (complement 3d [C3d]) had a positive FC-DAT (C3d). In 57 patients with AIHA, the incidence of positive results of FC-DAT (IgG) and tube DAT (IgG) were similar (42 positive vs. 40 positive); but in 323 patients without AIHA, the incidence of positive FC-DATs (IgG) was higher than that of tube DAT (IgG; 47 positive vs. 9 positive). The higher incidence of positive FC-DAT (C3d) than that of tube DAT (C3d) was seen in patients with AIHA (42 positive vs. 21 positive) as well as in patients without AIHA (61 positive vs. 5 positive). Both DAT (IgG) and DAT (C3d) positive has highest positive predictive value for hemolysis, followed by DAT (IgG) alone positive and DAT (C3d) alone positive.

CONCLUSIONS

FC-DAT is a complementary test for diagnosing AIHA. There is a synergistic effect of the red blood cell-bound IgG and complement in predicting hemolysis.

Unexplained periparturient thrombocytopenia

Since the advent of routine automated platelet counting we have observed unexplained periparturient thrombocytopenia (PPT) in an unexpected number of periparturient women, ie, during labor or within 24 hr postpartum. Mean +/- SD platelet count in 686 random blood donors was 236 +/- 50 X 10(9)/L and 1.02% had a platelet count less than 136 X 10(9)/L; in 2,204 random prenatal and postpartum women mean count was significantly higher (275 +/- 86 X 10(9)/L; p less than 0.001). Of 1,621 periparturient women, 74 (4.6%) had unexplained PPT (mean +/- SD platelet count 122 +/- 24 X 10(9)/L, range 21-135 X 10(9)/L, N = 74). Platelet count in PPT usually rose to normal within 1 week of delivery; in 10% thrombocytopenia persisted greater than 6 months. PPT occurred in successive pregnancies with normal intervening platelet counts. Nine of 34 newborns of mothers with PPT were thrombocytopenic; there was no correlation between mother's and baby's platelet counts. In no case of PPT was there excessive bleeding in mother or infant. Positive indirect platelet radioactive antiglobulin tests (PRAT) were seen in 11% of normal postpartum women and in 90% of 22 women with PPT; 65% of the positive tests in PPT were due to reactions with anti-C3 only. In contrast, pregnant women with autoimmune thrombocytopenic purpura (AITP) had positive PRAT primarily because of anti-IgG (+/- anti-C3); only 10% were positive only with anti-C3. Results were concordant in all of eight women with PPT tested by both indirect and direct PRAT. Amount of C3 bound per platelet in direct or indirect PRAT was not predictive of degree of thrombocytopenia, but there was correlation of fg C3 per platelet detected by the two assays in individual patients (r = 0.8). Mean levels of serum C3, C4, and factor B in women with PPT did not differ from normal; individual patients had abnormal serum complements but no characteristic pattern was observed. Increased immune complexes were observed in 6% of normal subjects and 33% of women with PPT. Etiology and mechanism of PPT is unclear. Despite lack of clinical evidence in women with PPT of syndromes associated with increased platelet destruction, the presence of preeclampsia cannot be absolutely excluded. Similarly, although the pattern of antiglobulin sensitization in PPT differed markedly from that seen in AITP, autoimmune disorder cannot be excluded. Alloantibodies did not appear to be responsible for PPT. While PPT is usually benign, some patients had a markedly reduced platelet count. Recognition of the phenomenon may be important in obstetrics.

Effect of complement on the viscoelastic properties of human erythrocyte membrane

Using the micropipette technique, we examined the viscoelastic properties of the red blood cell (RBC) membrane which had been strongly coated with various components of complement (primary C3b, C3d, C4b, C4d) in vitro. The membrane elastic modulus (E), the viscosity index of the initial rapid phase of deformation (eta D1), the viscosity of the later slow phase of deformation (eta D2) and the viscosity of the recovery phase (eta R) were determined. Compared to control non-complement coated RBCs, RBCs coated with C3d, either alone or with other complement components, showed significant increase in the values for elasticity and viscosities. Thus C3d fixation resulted in decreased membrane deformability. Changes in membrane viscoelasticity due to bound C3d were not enhanced by bound C4b, C4d, C5, factor Bb or p; presence on RBC membrane of the latter two complement components may partially reverse the effect of C3d fixation. Lipid fluidity of RBC membrane, examined by fluorescence depolarization, increased with fixation of all complement components except C5. These complement-induced changes in membrane viscoelastic properties have potential pathophysiological and clinical implications. The data suggest that extravascular sequestration of human RBCs may be explained in part by increased membrane rigidity resulting from C3d fixation.

Use of the indirect platelet radioactive antiglobulin test with anti-IgG and anti-C3 in immune and nonimmune thrombocytopenias

Indirect platelet radioactive antiglobulin tests using anti-IgG and anti-C3 were performed in 483 individuals. The subjects' diagnoses and clinical states were correlated with sensitizing protein (IgG and/or C3). Sensitivity of the technique was increased by increasing the serum to cell ratio and there was good correlation between direct and indirect antiglobulin tests in 56 patients. Seven percent of normal healthy non-thrombocytopenic untransfused males and 88% of patients in whom autoimmune thrombocytopenia was clinically suspected showed a positive test result. In the indirect antiglobulin test, mean +/- SD fg anti-C3d/platelet was 1.75 +/- 0.28 (N = 58) with serum from normal subjects, 3.91 +/- 1.86 (N = 123) with serum from patients with idiopathic thrombocytopenic purpura and systemic lupus erythematosus, and 2.78 +/- 0.91 (N = 52) with serum from patients with lymphoproliferative disease. The differences were significant with P less than 0.001. Levels of bound anti-IgG were in general accord with those of other reports; there was no significant relationship with serum immunoglobulin levels. When the indirect antiglobulin tests were positive for both IgG and C3, there was a significant correlation between IgG and C3 bound to normal platelets (r = 0.671; P less than 0.01). Although sera from 39% of patients whose thrombocytopenia was not initially suspected to be on an autoimmune basis gave a positive test result, indirect platelet radioactive antiglobulin tests using anti-IgG and anti-C3 may be convenient tools in the evaluation and follow-up of immune thrombocytopenias.

Membrane-bound immunoglobulins and complement components on young and old red cells

In order to characterize changes in membrane-bound immunoglobulins and complement components, red cells (RBCs) were separated into young and old populations by simple centrifugation. Old RBCs had reduced mean corpuscular hemoglobin volume, increased mean corpuscular hemoglobin concentration, and reduced sialic acid. Using radioactive anti-antiglobulin techniques, old RBCs were shown to have more IgG, IgM, IgA, and C3d on their surfaces than did young RBCs; there was no increase on old RBCs of C3b, factor B, C4b, or C5. Similar results were observed with RBCs strongly coated with C3d in vivo from a patient with cold agglutinin disease. RBCs taken into ethylenediamine tetraacetate, washed thoroughly in saline, and then stored for prolonged periods in Alsever's solution or kept in autologous ethylenediamine tetraacetate plasma, at 4 degrees C, showed no increase in RBC-bound C3d with increased storage time. If, however, blood was taken into citrate-phosphate-dextrose and maintained at 4 degrees C in autologous plasma, a significant increase in RBC-bound C3d was observed in the mixed-cell population with prolonged storage time. Order donor blood units, taken into citrate-phosphate-dextrose and stored at 4 degrees C as packed red cells, showed higher levels of RBC-bound C3d in the mixed-cell population than did units stored for a shorter time. In no case did donor unit RBCs give a positive direct antiglobulin test on serologic testing with anti-C3d. The findings complement data already collected on membrane and cytoplasmic changes in aging RBCs and may contribute to an understanding of RBC senescence.

Enhanced prothrombin-converting activity and factor Xa binding of platelets activated by the alternative complement pathway

Platelet prothrombin-converting activity and factor Xa binding were studied after exposure of human platelet rich plasma (PRP) to various conditions leading to platelet activation. Zymosan resulted in increased platelet-bound C3, enhanced prothrombin-converting activity and increased factor Xa binding. Similar findings were observed with normal platelets resuspended in factor XII-deficient plasma. The combined use of zymosan and thrombin to activate platelets resulted in synergistic prothrombin-converting activity and factor Xa binding. In contrast, no synergism was obtained with the concomitant use of zymosan and collagen, suggesting that collagen and zymosan share the same pathway for platelet activation. Heterologous antibody to factor V completely inhibited the platelet prothrombin-converting activity for all modes of platelet activation, indicating that this activity is mediated by factor V.

Prospective platelet crossmatching for selection of compatible random donors

An indirect platelet radioactive antiglobulin test was used in a prospective crossmatch study in 29 patients for selecting compatible random donors for platelet transfusion. Patients received crossmatch-compatible, crossmatch-incompatible and uncrossmatched platelets. A successful outcome for transfusion was defined as a 24 h post-transfusion corrected platelet increment of greater than 4.5 X 10(9)/l. Of 47 predictions based on crossmatch results, the technique successfully predicted the outcome of the transfusion in 90% of cases. In all patients, transfusion of crossmatch-compatible platelets resulted in a significantly higher 24 h corrected platelet increment than was obtained following transfusion of incompatible platelets. The mean +/- SEM 24 h corrected post-transfusion increment (X 10(9)/l) was 17.79 +/- 2.01 in 26 patients who received crossmatch-compatible pooled random donor platelets; in 21 patients who received crossmatch-incompatible platelets, it was 1.19 +/- 0.56; and in 25 patients who received uncrossmatched platelets, 4.42 +/- 0.97. The differences were highly significant. There was an 83% correlation of results with those obtained using an indirect platelet suspension immunofluorescence technique.

Quantitative quality control of antiglobulin reagents

Double antibody radioimmunoassays have been developed for the quantification of anti-IgG, anti-C3, anti-C3c, anti-C3d and anti-C4 antibodies and for the determination of their binding constants. Assays were undertaken on 53 polyspecific antiglobulin reagents obtained from a variety of commercial and public sources. Concentrations of anti-IgG varied from 1.2 to 12.8 micrograms/ml in commercial products and from 0.4 to 6.0 micrograms/ml in public products. Concentrations of anti-C3 and anti-C3c varied from 0.1 to 1.0 micrograms/ml in most commercial products but in public products concentrations varied by more than 100-fold from 0.02 to 6.5 micrograms/ml. Concentrations of anti-C3d varied from 0.05 to 0.7 micrograms/ml in most commercial products and from less than 0.01 to 1.3 micrograms/ml in public products. Concentrations of anti-C4 varied from less than 0.01 to 0.18 micrograms/ml in commercial products and from less than 0.01 to 0.08 micrograms/ml in public products. Mean binding constants for commercial products were: anti-IgG 6.6 x 10(9) l/mol, anti-C3 4.6 x 10(9) l/mol, anti-C3c 5.3 x 10(9) l/mol, anti-C3d 0.4 x 10(9) l/mol and anti-C4 4.9 x 10(9) l/mol. Relationships were found between results obtained in quantitative assays of specific antibodies and independently performed serological assessments of potency. Anti-IgG was present in suboptimal concentrations for agglutination in several public products and anti-C3 and anti-C3c were in suboptimal concentrations for agglutination in many public and commercial products.