IgG-Mediated Immune Suppression to Erythrocytes by Polyclonal Antibodies Can Occur in the Absence of Activating or Inhibitory Fcγ Receptors in a Full Mouse Model

Abstract

Polyclonal anti-D has been used to prevent RhD-negative mothers from becoming immunized against RhD positive fetal erythrocytes, and this mechanism has been referred as Ab or IgG-mediated immune suppression (AMIS). Although anti-D has been highly successful, the inhibitory mechanisms remain poorly understood. Two major theories behind AMIS involve the binding of IgG to activating or inhibitory FcγR, which can induce either erythrocyte clearance or immune inhibition, respectively. In this work, we explored the absolute role of activating and inhibitory FcγR in the AMIS mechanism using the HOD mouse model of RBC immunization. HOD mice contain a RBC-specific recombinant protein composed of hen egg lysozyme (HEL), OVA and human transmembrane Duffy Ag, and erythrocytes from HOD mice can stimulate an immune response to HEL. To assess the contribution of activating and inhibitory FcγR to AMIS, C57BL/6 versus FcRγ-chain−/− or FcγRIIB−/− mice were used as recipients of HOD-RBC alone or together with anti-HEL Abs (i.e., AMIS) and the resulting immune response to HEL evaluated. We show that anti-HEL polyclonal Abs induce the same degree of AMIS effect in mice lacking these IgG binding receptors as compared with wild-type mice. In agreement with this, F(ab′)2 fragments of the AMIS Ab also significantly reduced the Ab response to the HOD cells. In conclusion, successful inhibition of in vivo Ab responses to HOD-RBC by polyclonal IgG can occur independently of activating or inhibitory FcγR involvement. These results may have implications for the understanding of RhD prophylaxis.

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

“…These receptors are unlikely to be of critical importance in the KEL model, however, given results recently published in other systems. 18,19 In conclusion, immunoprophylaxis with anti-KEL sera did not prevent alloimmunization to transfused KEL RBCs in double-KO recipients lacking both FcgR and C3, despite its efficacy in wildtype recipients and in single-KO recipients lacking either FcgR or C3. The observation that modulation of the KEL glycoprotein antigen did not occur in the peritransfusion period in double-KO mice provides new insight into the potential mechanism(s) of action of anti-RBC immunoprophylaxis.…”

“…30 In a full mouse model, another group has recently described that IgG-mediated immune suppression to murine RBCs expressing the model HOD antigen occurs in the absence of activating or inhibitory FcgRs. 18 Given these past sheep RBC and murine RBC studies, our finding that FcgRs are not required for effective immunoprophylaxis against transfused KEL RBCs was not unexpected. However, the fact that anti-KEL sera was not able to prevent alloimmunization in double-KO mice suggests that FcgRs are, in fact, important in some way, albeit in a way that appears to be fully compensated when C3 is present.…”

“…17 Findings in these models argue against the importance of RBC clearance or "epitope masking" in the mechanism of action of antibody-mediated immune suppression. 16 Further, findings in these models argue against the sole importance of inhibitory or activating Fcg receptors (FcgRs) 18 or complement or complement receptors. 19 Thus, the mechanism of action through which monoclonal or polyclonal antibodies prevent alloimmunization remains elusive.…”

Epitope-Specific Suppression of IgG Responses by Passively Administered Specific IgG: Evidence of Epitope Masking

“…Remarkably, no knockout mouse strain has been found in which suppression does not work. It operates well in mice lacking the activating Fc gamma receptors FcγRI, FcγRIII, and FcγRIV (FcRγ KO), the inhibitory FcγRIIB (FcγRIIB KO), the neonatal Fc receptor (FcRn) (β 2 -microglobulin KO) (10, 11, 20, 21), as well as complement factor C1q (C1q KO), complement factor C3 (C3 KO), or complement receptors 1 and 2 (CR1/2 KO) (21). Moreover, a monoclonal IgG1 antibody, which is unable to activate complement, suppresses to the same degree as a complement-activating IgG1 antibody (22).…”

Hemolytic Disease of the Fetus and Newborn: Modern Practice and Future Investigations

“…58 Multiple subtypes of monoclonal antibodies as well as polyclonal antibodies have been investigated, with some but not all being capable of preventing alloimmunization altogether or of decreasing the magnitude of an alloimmune response [59][60][61] . In at least one model, the efficacy of passively infused antibodies at preventing an alloimmune response to transfused RBCs has been shown to be independent of the inhibitory FcRIIb receptor 60 .…”