Host Inflammation Increases Alloimmunization to Transfused Red Blood Cells.

Background: Alloimmunization to RBC is a serious sequelae of transfusion, which can result in hemolytic transfusion reactions and difficulty identifying compatible units for future transfusion. Although any given unit of RBC contains numerous alloantigens, only 6% of transfusion recipients mount a measurable alloantibody response. The factors that determine whether a given transfusion recipient will generate antibodies remains undetermined. Activation of the innate immune system through inflammation is known to have a significant effect on immune responses in other settings. Accordingly, we hypothesized that recipient inflammation would modulate humoral responses to transfused RBC.

Methods: Hen egg lysozyme (HEL) is a well studied antigen. In order to convert HEL into a model blood group antigen, we utilized transgenic mHEL mice, which express HEL on the surface of RBC as a transmembrane protein. Leukoreduced mHEL RBC were transfused into B10. BR recipient mice (H-2^k MHC haplotype). To induce host inflammation, recipient mice received an intraperitoneal injection of poly(I:C), which is a double-stranded RNA that mimics inflammation commonly seen with viral infection. The inflammatory effects of poly (I:C) were confirmed by observing activation of macrophages using flow cytometry. Plasma was obtained from recipient mice 7 and 14 days post-transfusion, and anti-HEL antibody titers and isotypes were determined by HEL specific ELISA.

Results: Inflammation of mice with poly (I:C) resulted in a significant increase in the magnitude of anti-HEL IgG synthesis. Whereas mice receiving mHEL RBC alone had only a weak anti-HEL IgG response, treatment with poly (I:C) prior to transfusion resulted in a 15 fold increase in anti-HEL IgG (p value =.0003). In addition, whereas IgG1 and IgG3 were the predominant anti-HEL IgG subtypes in mice receiving mHEL RBC alone, IgG3 and IgG2b predominated in mice treated with poly (I:C) prior to mHEL transfusion. Thus, treatment of transfusion recipients with an agent that mimics inflammation commonly seen in viral infections significantly increased humoral immunity and altered IgG subtype composition.

Discussion: The findings reported here demonstrate that host inflammation with a viral-like agent increases alloimmunization to transfused RBC and suggests that co-existing infections in humans may increase humoral immunization to transfused RBC. In addition, since different IgG subtypes have different hemolytic potentials, the observed deviation may influence the clinical significance of alloantibody formation. These findings provide the rational basis for possible intervention using immunomodulatory agents to prevent alloimmunization or to divert the response to a non-hemolytic IgG subtype.