Assessment of complement binding by anti-D and anti-M antibodies employing labelled antiglobulin antibodies

Antigen modulation as a potential mechanism of anti-KEL immunoprophylaxis in mice

Red blood cell (RBC) alloimmunization is a serious complication of transfusion or pregnancy. Despite the widespread use of Rh immune globulin to prevent pregnancy associated anti-D alloimmunization, its mechanism of action remains elusive. We have previously described a murine model in which immunoprophylaxis with polyclonal anti-KEL sera prevents alloimmunization in wild-type recipients transfused with transgenic murine RBCs expressing the human KEL glycoprotein. To investigate the mechanism of action, we have now evaluated the outcome of immunoprophylaxis treatment in mice lacking Fcγ receptors (FcγRs), complement (C3), both, or none. Whereas polyclonal anti-KEL sera completely prevented alloimmunization in wild-type and single-knockout (KO) mice lacking FcγRs or C3, double-KO mice lacking both FcγRs and C3 became alloimmunized despite immunoprophylaxis. Rapid clearance of essentially all transfused RBCs with detectable KEL glycoprotein antigen occurred within 24 hours in wild-type and single-KO recipients treated with immunoprophylaxis, with the transfused RBCs remaining in circulation having minimal KEL glycoprotein antigen detectable by flow cytometry or western blot. In contrast, transfused RBCs with the KEL glycoprotein antigen fully intact continued to circulate for days in double-KO mice despite treatment with immunoprophylaxis. Further, in vitro phagocytosis assays showed no consumption of opsonized murine RBCs by double-KO splenocytes. Taken in combination, our data suggest that modulation of the KEL antigen (and potentially RBC clearance) by redundant recipient pathways involving both FcγRs and C3 may be critical to the mechanism of action of polyclonal anti-KEL immunoprophylaxis. These findings could have implications for the development of immunoprophylaxis programs in humans.

Mechanisms of anti-D action in the prevention of hemolytic disease of the fetus and newborn: what can we learn from rodent models?

PURPOSE OF REVIEW

Hemolytic disease of the fetus and newborn can be effectively prevented by administration of anti-D to the mother. In this setting, the IgG purified from the plasma of D-alloimmunized donors prevents the maternal immune response to D-positive red blood cells (RBC). Several monoclonal anti-D antibodies have recently been developed for potential use in the setting of hemolytic disease of the fetus and newborn; the functional assays used to assess the potential success of these antibodies have often assumed antigen clearance as the predominant mechanism of anti-D. Unfortunately, the in-vivo success of these monoclonal antibodies has thus far been limited. A similar inhibitory effect of IgG has been observed in animal models with a vast array of different antigens, referred to as antibody-mediated immune suppression (AMIS). Here, studies of AMIS are reviewed and the relevance of these findings for anti-D-mediated immunoprophylaxis is discussed.

RECENT FINDINGS

In animal models of AMIS, IgG-mediated antigen clearance was not sufficient for prevention of the antibody response to RBC. Furthermore, anti-RBC IgG inhibited B-cell priming to foreign RBC, but failed to prevent a T-cell response and immunological memory.

SUMMARY

The applicability of AMIS models for determining the true mechanism of anti-D, though uncertain, may nevertheless provide knowledge as to potential mechanisms of action of anti-RBC antibodies.

Mechanisms of anti-D action in the prevention of hemolytic disease of the fetus and newborn

Anti-D is routinely and effectively used to prevent hemolytic disease of the fetus and newborn (HDFN) caused by the antibody response to the D antigen on fetal RBCs. Anti-D is a polyclonal IgG product purified from the plasma of D-alloimmunized individuals. The mechanism of anti-D has not been fully elucidated. Antigenic epitopes are not fully masked by anti-D and are available for immune system recognition. However, a correlation has frequently been observed between anti-D-mediated RBC clearance and prevention of the antibody response, suggesting that anti-D may be able to destroy RBCs without triggering the adaptive immune response. Anti-D-opsonized RBCs may also elicit inhibitory FcgammaRIIB signaling in B cells and prevent B cell activation. The ability of antigen-specific IgG to inhibit antibody responses has also been observed in a variety of animal models immunized with a vast array of different antigens, such as sheep RBCs (SRBC). This effect has been referred to as antibody-mediated immune suppression (AMIS). In animal models, IgG inhibits the antibody response, but the T-cell response and memory may still be intact. IgG does not mask all epitopes, and IgG-mediated RBC clearance or FcgammaRIIB-mediated B-cell inhibition do not appear to mediate the AMIS effect. Instead, IgG appears to selectively disrupt B cell priming, although the exact mechanism remains obscure. While the applicability of animal models of AMIS to understanding the true mechanism of anti-D remains uncertain, the models have nevertheless provided us with insights into the possible IgG effects on the immune response.

Anti-M: Report of two cases and review of literature

Anti-M is a fairly common naturally occurring antibody with rarely causing hemolytic transfusion reactions or hemolytic disease of the newborn. Most anti-M are not active at 37 degrees C and can generally be ignored in transfusion practice. However, we did not find this antibody to be fairly common and detected only two cases of anti-M in the past three years. We describe these two cases; one 'immunizing' type and other 'naturally occurring' and review the literature. The immunizing type was reactive at 37 degrees C as well as AHG phase of testing with IgG component, and showing dosage effect while the other was 'naturally occurring' reactive well below 37 degrees C. Though rare, sometimes these antibodies can be of clinical significance when the antibody detected is reactive at 37 degrees C and AHG phase. When the antibody is active at 37 degrees C, M antigen negative cross match compatible red cell unit should be given.

The significance of complement in immunohematology

In vitro detection of red cell-bound complement can be important in the differential diagnosis of autoimmune and drug-induced immune hemolytic anemias; it can also be a sensitive test for the detection of complement-binding alloantibodies, e.g., in compatibility testing. Red cell-bound complement can be detected by the antiglobulin test if suitable antiglobulin sera (AGS) are utilized. In 1971, the Federal Standards for AGS were criticized because so-called broad spectrum AGS used routinely in blood banks were shown to often be deficient in anticomplement reactivity. In the new few years commercial regents changed with regard to the quantity and specificity of their anticomplement components. A great deal of controversy developed as to the true importance of detecting red cell-bound complement, the particular fragments of complement that should be detected, and the causes of nonspecific reactions that seemed to be occurring, especially with the increasing usage of new techniques utilizing low ionic strength media. When monospecific anti-IgG and anti-C3 became available commercially, the controversy regarding diagnostic testing was resolved as direct antiglobulin testing could be performed with these reagents rather than the broad spectrum reagents. Two main questions remained: how rare are alloantibodies that are only detectable by the anticomplement component of AGS? How clinically significant are such antibodies? The results of our 3-year study indicated that such antibodies (usually anti-Kidd) occurred with a incidence of 1/8000 sera tested. Some of these antibodies seemed capable of shortening the life span of transfused red cells, as determined by 51Cr survival studies; some showed negligible cell destruction. Severe transfusion reactions due to such antibodies would seem unlikely. Individual laboratories will have to balance the risk of missing some complement-dependent antibodies of possible clinical significance with the increased nonspecificity encountered in their own laboratory with AGS containing anticomplement.

Quantitation of red blood cell-bound C3d in normal subjects and random hospitalized patients

A sensitive radiolabelled anti-antiglobulin method was devised and applied to quantitating red blood cell-bound C3d (RBC-C3d) in samples from 174 normal blood donors. C3d was demonstrable on all RBC examined; 98% of values fell over a broad range, with the highest values being approximately 3.5 X the lowest values (equivalent to 50-160 molecules of C3d per cell). RBC-C3d did not correlate with sex or age (over 18-65 years); indirect evidence suggests that values for the paediatric age group will fall in the same normal range. Studies on samples obtained weekly for 10-12 weeks from six adult males and six adult females indicated stable levels of RBC-C3 for individual subjects; i.e. high normals, mid normals and low normals remained in their characteristic range levels over the period of observation. For comparison, RBC-C3d was measured in samples from 313 randomly selected hospitalized adult patients. 33% of the values were above the normal range; 8% were elevated to a level likely to have been detectable by a direct anti-C3d antiglobulin test. The great majority of elevated values occurred in patients not ordinarily considered to have autoimmune conditions. The results provide background for studies of the aetiology and significance of RBC-C3d in health and disease.