Determinants of ABH expression on human blood platelets

Prevalence, serologic and genetic studies of high expressers of the blood group A antigen on platelets*

OBJECTIVE/AIM

The aim of this study is to describe the distribution of the platelet blood group A antigenicity in Euro-Brazilians (EUBs) and Afro-Brazilians (AFBs).

BACKGROUND

A small but significant proportion of individuals express high levels of A or B antigen on their platelets corresponding to the erythrocyte ABO group. The mechanism of increased antigen expression has not been elucidated.

MATERIAL/METHODS

A cohort of 241 blood group A donors was analysed by flow cytometry. Although mean fluorescence intensity (MFI) is a typical continuous variable, platelets were screened and divided into two categories: low expressers (LEs) and high expressers (HEs). A three-generation family was investigated looking for an inheritance mechanism.

RESULTS

The prevalence of the HE platelet phenotype among group A(1) donors was 2%. The mean of MFI on platelets of A(1) subgroup of EUBs differs from that of AFBs (P = 0·0115), whereas the frequency of the HE phenotype was similar between them (P = 0·5251). A significant difference was found between sexes (P = 0·0039). Whereas the serum glycosyltransferase from HE family members converted significantly more H antigen on group O erythrocytes into A antigens compared with that in LE serum, their ABO, FUT1 and FUT2 genes were consensus. The theoretically favourable, transcriptionally four-repeat ABO enhancer was not observed.

CONCLUSION

The occurrence of HE in several members suggests familial aggregation. Indeed, in repeated measures, stability of the MFI values is suggesting an inherited condition. Factors outside the ABO locus might be responsible for the HE phenotype. Whether the real mechanism of inheritance is either of a polygenic or of a discrete Mendelian nature remains to be elucidated.

Neonatal alloimmune thrombocytopenia associated with maternal-fetal incompatibility for blood group B

BACKGROUND

Blood group A and B antigens are expressed only weakly on platelets (PLTs) of most individuals but are very strongly expressed on PLTs from approximately 1 percent of normal subjects (Type II high expressers). The implications of this trait for transfusion medicine are undefined.

STUDY DESIGN AND METHODS

A family was studied in which two Group B infants were born with neonatal thrombocytopenia, whereas a third infant whose blood group was A(2) had a normal PLT count at birth.

RESULTS

Serologic studies demonstrated a maternal antibody that reacted strongly with PLTs from the father and the two group B children in flow cytometry and with GPIIb/IIIa from their PLTs in solid-phase assays. No PLT-specific antibodies were detected in maternal serum sample, but it contained a high-titer immunoglobulin G antibody specific for blood group B. All PLT-reactive antibody in the mother's serum was removed by absorption with pooled, washed group A and B red cells (RBCs). Studies with monoclonal anti-B and measurement of serum B-glycosyltransferase activity showed that the father and both group B children were Type II high expressers of blood group B.

CONCLUSIONS

The findings indicate that high-titer blood group antibodies acquired from the mother can cause thrombocytopenia in infants possessing the Type II high-expresser phenotype despite competition for antibody binding by blood group antigens expressed on RBCs and other tissues.

In vitro evaluation of prestorage pools consisting of mixed A and O platelet concentrates

BACKGROUND

Current FDA regulations allow the prestorage pooling of whole blood-derived platelet concentrates (PCs) of identical ABO type in a recently cleared platelet (PLT) pooling bag (Acrodose, Pall Medical). It is unclear how ABO-mixed PC pools would store if pooled before storage.

STUDY DESIGN AND METHODS

Pools consisting of ABO-identical PLTs and mixed A and O PLTs in varying proportions were evaluated on Days 1, 5, and 7 of storage with measures of the PLT storage lesion, lymphocyte activation, and activation of the complement and coagulation system. Data were analyzed by analysis of variance.

RESULTS

Pools did not differ on Day 7 in pH (p = 0.63), hypotonic shock response (p = 0.25), extent of shape change (p = 0.26), morphology score (p = 0.18), or white cell count (p = 0.79), but surface P-selectin expression was more evident in the ABO-mixed pools (p = 0.02). Small microscopic clumps of PLTs were observed in all pools, but were more prominent in the ABO-mixed pools (p < 0.01). PLT counts, however, did not differ between pools (p = 0.93), indicating that only a small proportion of PLTs were clumped. Surface A-antigen expression was proportional to the number of A PCs in each pool and did not vary between study days. Anti-A(1) titers were either unchanged or decreased by one dilution. Complement and coagulation activation markers did not differ between pools.

CONCLUSION

Pooling A and O PCs is associated with evidence of increased microscopic PLT clumping and activation, but these differences are not exacerbated with 7-day storage. Other major measures of PLT quality do not differ, and there was no evidence of a mixed lymphocyte reaction.

Transfusion of platelets containing ABO-incompatible plasma: a survey of 3156 North American laboratories

CONTEXT

Hemolytic transfusion reactions due to platelet transfusions containing ABO-incompatible plasma (ie, group O platelets into a non-group O patient) have been reported in the literature. However, limited data describe the extent to which transfusion services manage such platelet transfusions or the methods used to limit the risk of such reactions.

OBJECTIVE

To determine transfusion services' current practices regarding the use of platelets containing ABO-incompatible plasma.

DESIGN

In a College of American Pathologists' Transfusion Medicine Proficiency Testing Survey, supplemental questions asked participants whether a policy existed for the use of platelets containing ABO-incompatible plasma and, if a policy existed, what elements were part of the policy.

RESULTS

Of 3156 laboratories that transfused platelets, 3152 responded to the question of whether they had a policy. Of these respondents, 83% (n = 2623) had a policy. One or more elements were reported for transfusions in adults: only ABO-compatible plasma products (n = 1363); only ABO-compatible plasma and platelet products (n = 679); notification of medical director (n = 646); notification of ordering physician (n = 637); volume limit of ABO-incompatible plasma allowed (n = 255); volume-reduction of ABO-incompatible products (n = 168); screening for critical titer of anti-A or anti-B (n = 53). A total of 529 laboratories indicated that they did not have a policy.

CONCLUSIONS

A majority of laboratories have a policy, but most do not include a method to limit the risk of hemolysis if platelets containing ABO-incompatible plasma must be transfused. When such platelets are used, there does not appear to be consensus on a specific method to minimize the transfusion of anti-A or anti-B.

HLAMatchmaker-driven analysis of responses to HLA-typed platelet transfusions in alloimmunized thrombocytopenic patients

This study describes a novel application of HLAMatchmaker to determine platelet compatibility in 16 alloimmunized patients with aplastic anemia refractory to random donor platelet transfusions. HLAMatchmaker is a software algorithm that predicts HLA compatibility by identifying immunogenic epitopes represented by amino acid triplets in antibody-accessible regions of human leukocyte antigen (HLA) molecules and determines the number of triplet mismatches (TMMs) and highly immunogenic triplet mismatches (HIMMs). Corrected count increments (CCIs) and molecular HLA typing were available for 523 transfusions. Conventional compatibility assessment based on cross-reactive group (CREG) determination was not predictive of transfusion outcome. Low HIMMs and TMMs numbers were associated with a higher likelihood of satisfactory (CCIs > or = 8) compared with unsatisfactory (CCIs < 8) outcomes (median HIMMs = 4 vs 6, p2 value < .001; median TMMs = 11 vs 13, p2 value < .001). Although receiver operator characteristic curves revealed that HIMMs or TMMs number are not powerful predictors of individual transfusion outcome, a threshold of at least 3 HIMMs or at least 9 TMMs appeared to be associated with successful transfusions. Triplet-matched transfusions were successful, regardless of CREG matching. Our data validate HLAMatchmaker for platelet transfusions and demonstrate its potential to refine and expand donor selection for HLA-alloimmunized patients.