Opsonization, inflammation, and RBC alloimmunization

In this issue of Blood, Escamilla-Rivera and colleagues show that inflammation can sidestep immunoprophylaxis, with breakthrough red blood cell (RBC) alloimmunization, in a murine model mirroring antenatal administration of Rh immune globulin (RhIg).¹  Successful immunoprophylaxis was associated with early phagocytosis of opsonized RBCs, especially by splenic CD8a⁺ dendritic cells, which was not observed in the presence of inflammation.

Historically, ∼15% of RhD-negative women would develop anti-D during the course of pregnancy, with devastating consequences for subsequent pregnancies. With the introduction of RhIg prophylaxis in the 1960s, the incidence of RhD alloimmunization and hemolytic disease of the fetus and newborn decreased markedly and is considered one of the triumphs of modern medicine. The most recent data show that antenatal and postnatal administration of 300 μg of RhIg can prevent RhD alloimmunization in 98% to 99% of cases.²  RhIg failures or breakthrough RhD alloimmunization occurs in 0.3% to 1% of women and has been attributed to transplacental hemorrhage prior to RhIg administration, inappropriate administration of low-dose (50 μg) RhIg, or insufficient RhIg to cover a large fetal bleed.^2,3 

In this issue of Blood, Escamilla-Rivera and colleagues explore an entirely novel mechanism for breakthrough alloimmunization, building on the known association between RBC alloimmunization and inflammation.^4,5  The authors used a well-described transgenic mouse model expressing the human KEL glycoprotein. To mimic RhIg prophylaxis, wild-type mice were initially transfused with immune anti-KEL immunoglobulin (KELIg) prior to transfusion of KEL⁺ RBCs. As expected, KELIg administration was able to suppress alloimmunization. In contrast, KELIg prophylaxis failed if recipient mice also received interferon or polyinosinic-polycytidylic acid [poly(I:C)] to provoke inflammation prior to RBC transfusion.

The mechanisms underlying KELIg suppression and breakthrough alloimmunization were explored extensively. The authors had previously shown that type I interferon was necessary and sufficient to provoke KEL alloimmunization in their murine model.⁵  Repeating experiments in mice lacking type 1 interferon synthesis (TRF3/5/7^−/−) or the interferon receptor (IFNAR^−/−) suggested a contribution by type 1 interferon, although some degree of breakthrough alloimmunization still occurred.

The authors also looked at the role of T cells and the kinetics of KEL⁺ red cell clearance. Neither KELIg immune suppression nor breakthrough alloimmunization was dependent on CD4⁺ T cells, altered KEL expression, or the clearance rate of KEL⁺ RBCs from circulation. However, there were significant differences in the timing and uptake of opsonized KEL⁺ RBCs by different populations of splenic phagocytes (see figure). In the presence of KELIg alone, there was early phagocytosis of KEL⁺ RBCs by inflammatory monocytes, red pulp macrophages, and, interestingly, CD8⁺ dendritic cells that reside within the white pulp T-cell zone. In animals primed with poly(I:C), erythrophagocytosis was significantly delayed, with uptake of RBCs primarily by inflammatory monocytes and red pulp macrophages, but little to no uptake by CD8a⁺ dendritic cells. The authors speculate that early selective uptake of RBCs by CD8a⁺ dendritic cells may play a critical role in how KELIg and, by inference, RHIg, is able to suppress alloimmunization. Conversely, inflammation and delayed consumption of opsonized red cells by inflammatory monocytes favor alloimmunization, leading to prophylaxis failures.

These findings raise several interesting clinical questions for future study with regard to RhIg prophylaxis and RBC alloimmunization, in general. Because poly(I:C) mimics inflammation associated with viral infections, is there a higher risk for RhIg failure if it is administered to women with active infection? Are women with chronic inflammatory disorders more prone to RhIg failures? What impact do genetic variants in inflammatory response genes play in RhIg efficacy and alloimmunization? Finally, does early uptake by CD8⁺ dendritic cells shift the balance toward immune tolerance to allogeneic RBCs? Baft3-deficient mice, which lack CD8⁺ dendritic cells, are reported to have defects in acquired immune tolerance.⁶  A role for CD8⁺ splenic dendritic cells might have interesting implications for sickle cell patients who are at risk for RBC alloimmunization and have functional asplenia and chronic inflammation.³