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FcγR and SCD alloimmunization: a nonclass(ical) act

Karina Yazdanbakhsh

In this issue of Blood, Meinderts et al1 found a protective association of a genetic variant in the FCGR gene locus with decreased risk of red cell (RBC) alloimmunization in patients with sickle cell disease (SCD). If validated in a larger study, the identified polymorphism may be used as a prognostic marker to predict in advance patients with SCD at lower risk of alloimmunization, with implications for transfusion management in this patient population.

Structure-function relationship of FCGR2/3 genetic variants. (A) Schematic representation of the FCRG2/3 locus and the relative location of 9 SNPs and haplotypes. (B) Schematic gene structure of FCGR2C and associated SNPs in the 3 FCGR2C alleles. In whites, FCGR2C-STOP is the most common allele and is not expressed due to a premature stop codon in exon 3. In some individuals, a SNP in exon 3 leads to an open reading frame, resulting in the expression of the FcγRIIc protein. In the study by Meinderts et al, 9 out of 272 patients with SCD carried this allele, which is also referred to as the classical FCGR2C-ORF. The nonclassical FCGR2C.nc-ORF allele differs from the classical haplotype by an additional splice site mutation near exon 7 that leads to a stop codon and thus no protein expression. The FCGR2C.nc-ORF haplotype is associated with a decreased risk of alloimmunization in patients with SCD. IgG, immunoglobulin G; ORF, open reading frame. Illustration by Julia Bonnell, New York Blood Center. The figure has been adapted from Figures 2 and 3 in the article by Meinderts et al that begins on page 2121.

Transfusions remain a critical part of treatment of patients with SCD. However, patients can develop alloantibodies against transfused RBCs, causing difficulty in finding matched units and serious life-threatening complications, including delayed hemolytic transfusions reactions and hyperhemolysis, in which both the transfused RBCs and the patient’s own RBCs are destroyed.2 Patients with SCD are at the greatest risk of alloimmunization in part due to differences in minor (non-ABO) blood group antigens between mostly white blood donors and transfusion recipients of African descent. Transfusion guidelines recommending matching for the highly immunogenic C, E, and K blood groups (CEK) for patients with SCD have significantly reduced alloimmunization incidence. However, this still leaves patients at risk of developing alloantibodies to other immunogenic RBC antigens, such as Duffy, Kidd, MNS, and Dombrock.3,4 Even when donors and recipients are ethnically alike, SCD patients can develop alloantibodies against variant Rh antigens due to the highly polymorphic nature of RH genes in individuals of African descent.3,4 Extended prophylactic matching beyond CEK significantly reduces alloimmunization rates; however, the associated cost, logistical and economic issues with obtaining matching units, and/or inventory management are significant.5 Thus, it has been argued that prophylactic matching, including the use of genotype-matched RBCs,6 should be reserved for those SCD patients most at risk of alloimmunization. Currently, the SCD alloimmunization at-risk group includes those who have already made alloantibodies (ie, responders),7 but the quest for predicting whether or not a transfused patient will be a responder has been one of the Holy Grails in transfusion medicine. Murine studies continue to be instrumental in teasing apart mechanisms of alloimmunization.8 For human SCD alloimmunization risk determination, comparative immune response profiling in responders vs non responders has identified potentially altered molecular pathways in SCD alloimmunization, including aberrant innate immune response to heme, impaired follicular T-cell subset signaling, and defective regulatory T-cell suppressive pathways.9 Longitudinal, prospective studies are needed to determine whether these immunological alterations are the cause or effect of SCD alloimmunization. While the latter may help identify possible novel and more targeted therapies to reverse or reduce SCD alloimmunization, the former will bring us closer to identification of SCD alloimmunization biomarkers. With regards to genetic predisposition to SCD alloimmunization, association studies have identified HLA types as well as polymorphisms in TRIM21 and CD81 as potential risk factors.8

Meinderts et al examined the association between polymorphism within the gene cluster encoding the low-affinity Fcγ receptors (FcγRs) II and III and the risk of SCD alloimmunization. These low-affinity FcγRs are expressed by immune effector cells such as macrophages, monocytes, and B cells and can deliver both pro- and anti-inflammatory signals that help shape the immune response. Polymorphisms within the genes are associated with allo- and autoimmune diseases,10 and this study sought to determine whether a similar association was present with SCD alloimmunization. The authors performed a retrospective case-control study of 272 transfused SCD patients consisting of 142 cases (alloimmunized) and 130 controls (nonalloimmunized) who had received at least 20 units sourced from 2 patient cohorts (one from France and the other from The Netherlands). Using a FCGR-specific multiplex ligation–dependent probe amplification assay that allows detection of multiple single-nucleotide polymorphisms (SNPs) and copy-number variants in the FCGR2/3 locus, they found that a nonexpressed allelic form of the FCGR2C gene referred to as the FCGR2C nonclassical open reading frame (FCGR2C.nc-ORF) allele is strongly associated with a decreased risk of alloimmunization (see figure) (odds ratio, 0.26; 95% confidence interval, 0.11-0.64). This threefold lower risk of alloimmunization persisted when the analysis was restricted to nonresponder controls who had received >100 units, further validating the association. Interestingly, the protective effect was even stronger when excluding cases with Rh or K alloantibodies only. In fact, the FCGR2C.nc-ORF haplotype was not protective against alloimmunization exclusively to Rh or K antigens, underscoring the highly immunogenic nature of these antigens, lending further support for the standard care of CEK matching for all SCD patients, and highlighting the need for RH genotype matching for this population.6

To explore the immunologic/functional consequence of these variants, expression levels of FcγRs in patients with the FCGR2C.nc-ORF haplotype were also examined. No differences were found in FcγRII or FcγRIII levels on neutrophils, monocytes, T cells, or natural killer cells. However, levels of FcγRIIb/c on B cells from patients with the FCGR2C.nc-ORF were lower. FcγRIIb is a well-characterized negative regulator of B-cell activation, inhibiting antibody production. However, reduced FcγRIIb expression, which is expected to release the brake on immune activation, was associated with protection against alloimmunization in patients with the FCGR2C.nc-ORF. Further studies, including more in-depth phenotypic characterizations of FcγRIIb/c expression, are needed to understand the significance (if any) of reduced FcγRIIb expression in these patients. Similar to the most frequent FCGR2C allele, which is a pseudogene, FCGR2C.nc-ORF is a nonexpressed gene, raising questions about whether the observed protective effect of this polymorphism on alloimmunization is mediated directly by itself or, as the authors propose, by another yet-to be identified linked functional variant.

In summary, these data provide for the first time a genetic variant associated with protection against RBC alloimmunization. Since alloimmunization to Rh remains an important clinical problem in SCD,3 the fact that FCGR2C.nc-ORF was not protective for alloimmunization to Rh limits the clinical implications of the study’s findings. Nevertheless, this marker, in combination with other reported and to-be-identified genetic markers, may help to risk-stratify SCD patients and thus significantly improve transfusion management for this highly vulnerable patient population by ensuring that the use of extended and genotype-matched RBCs is reserved for those at highest risk of alloimmunization.

Footnotes

  • Conflict-of-interest disclosure: The author declares no competing financial interests.

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