The limitations of site-directed mutagenesis in the localization of Rh D epitopes

Recapitulation of posttransfusion purpura by cross-strain platelet immunization in mice

Posttransfusion purpura (PTP) is an uncommon but life-threatening condition characterized by profound thrombocytopenia occurring ∼1 week after a blood transfusion. The hallmark of PTP is a potent immunoglobulin G antibody specific for a transfused platelet-specific alloantigen, usually located on glycoprotein IIb/IIIa (GPIIb/IIIa; αIIb/β3 integrin). It is widely thought that this alloantibody somehow causes the thrombocytopenia, despite absence from host platelets of the alloantigen for which it is specific. In studies described here, we found that cross-strain platelet immunization in mice commonly induces GPIIb/IIIa-specific alloantibodies combined with platelet-specific autoantibodies and varying degrees of thrombocytopenia, and we identified 1 strain combination (129S1Svlm/PWKPhJ) in which 95% of immunized mice made both types of antibody and developed severe thrombocytopenia. There was a strong inverse correlation between autoantibody strength and platelet decline (P < .0001) and plasma from mice that produced autoantibodies caused thrombocytopenia when transfused to syngeneic animals, arguing that autoantibodies were the cause of thrombocytopenia. The findings define a model in which a routine alloimmune response to platelets regularly transitions to an autoimmune reaction capable of causing severe thrombocytopenia and support the hypothesis that PTP is an autoimmune disorder.

The restricted use of IGHV3 superspecies genes in anti-Rh is not limited to hyperimmunized anti-D donors

BACKGROUND

Antibodies produced against the D antigen make use of IGHV genes restricted to the IGHV3 superfamily. These findings are based on the IGHV gene analysis in anti-D-producing B cells from hyperimmunized donors, however, and therefore the restriction might be due to the hyperimmunization. In this study the IGHV gene usage of anti-Rh-producing B cells in a woman who was immunized in the last trimester of her pregnancy was analyzed.

STUDY DESIGN AND METHODS

Serologic analysis was performed by absorption and elution. Antibody-dependent cellular cytotoxicity (ADCC) of the different anti-Rh was determined. A phage display library was constructed from 2.2 x 10(6) isolated B cells and pannings were performed with red cells of the r'r, R1R1, and R2R2 phenotype.

RESULTS

A plasma sample of the immunized person showed high levels of both anti-D and anti-G and low levels of anti-C. Anti-D and anti-G contributed equally strong to the ADCC whereas anti-C did not. Eighteen anti-D-, 5 anti-G-, and 1 anti-C-specific phage clones were found, of which 16, 2, and 1 used the IGHV3s genes, respectively.

CONCLUSION

For the first time a restriction to the IGHV3s genes in anti-D in a naturally immunized pregnant woman is shown. Moreover, the use of IGHV3s genes appears to be present in anti-C and anti-G as well. Therefore, it is concluded that restricted IGHV3s gene usage in anti-D is not due to hyperimmunization but due to characteristics of the Rh antigens and the intrinsic binding capacities of IGHV3s genes, supporting the common Rh footprint hypothesis.

The RHCE allele ceRT: D epitope 6 expression does not require D-specific amino acids

BACKGROUND

False-positive D typing in patients may lead to anti-D immunization caused by D+ transfusions or by omission of anti-D prophylaxis. Known causes of such errors are RhCE variants carrying RhD-specific amino acids and cold agglutinin activity of some frequently used monoclonal anti-D.

STUDY DESIGN AND METHODS

The molecular basis of eight samples referred because of "false-positive" reactions with some commercial monoclonal anti-D was investigated by PCR and nucleotide sequencing from genomic DNA. PCR with sequence-specific priming was developed to specifically detect the underlying aberrant RHCE allele. The D epitope profile of the allele was determined by serology.

RESULTS

The aberrant reactivity of the samples was caused by the RHCE allele RHCE(R154T) that occurred in a cde haplotype. The phenotype dubbed ceRT expressed the important D epitope 6, which is the target epitope of most monoclonal anti-D used in routine typing.

DISCUSSION

The characterization of ceRT demonstrated a previously unknown mechanism of antigen D expression that does not require any D-specific amino acid. At least for some D epitopes, D-like structures may be mimicked by RhCE proteins carrying amino acid substitutions not representative for RhD.

Laboratory, morphologic, and immunophenotypic correlates of surface immunoglobulin heavy chain isotype expression in B-cell chronic lymphocytic leukemia

We compared surface immunoglobulin heavy chain isotype expression with a number of laboratory, morphologic, and immunophenotypic features in a series of 76 cases of B-cell chronic lymphocytic leukemia (B-CLL). Fifty-five cases were IgM+/IgD+, a phenotype associated with antigenically naïve B cells; 16 cases expressed IgD without IgM, a phenotype seen in a subset of normal B cells with extensive somatic immunoglobulin variable region (IgV) gene mutations; and 5 cases were IgD-, a phenotype associated with memory B cells. WBC count, atypical morphologic features, atypical immunophenotypic characteristics, and CD38 expression were nonrandomly distributed among the 3 categories of heavy chain isotype expression. Moreover, a WBC count more than 30,000/microL (30 x 10(9)/L), atypical morphologic features, and CD38 expression in more than 30% of neoplastic cells (all adverse prognostic factors in B-CLL) were less common among IgD-only cases than among IgM+/IgD+ and IgD- cases. These data demonstrate that surface immunoglobulin heavy chain isotype expression is associated with several laboratory, morphologic, and immunophenotypic features in B-CLL. The subset of B-CLL with the IgD-only phenotype is associated with several favorable prognostic factors, suggesting the possibility that IgD-only B-CLL may be associated with a favorable prognosis.