KNMB, a novel Knops blood group antigen located on LHR-C

Background

The complement receptor 1 (CR1) carries the antigens of the Knops blood group system (KN; ISBT 022). They are located on the long homologous repeats (LHR) C and D of the CR1. Antibodies to Knops antigens are not clinically significant but they are common in patients. In a previously transfused female patient of Ethiopian origin with a severe COVID-19 pneumonia we found an antibody to a new high prevalence antigen which we suspected to belong the Knops blood group system.

Methods

Antibody identification was performed in IAT in gel technique (BioRad, CH) using different commercial panels as well as untreated and papain treated red cells negative for high prevalence antigens. Different recombinant blood group proteins (imusyn, Germany) and specially produced recombinant proteins for LHR-C, LHR-C_1097Pro, LHR-C_1100Gly and LHR-C_1097Pro-1100Gly were used for inhibition assays. Massive parallel sequencing (MPS) of a blood group gene panel including all exons of the genes encoding the blood group systems ISBT001 to 043 was performed on the patient and a single non-related African patient non-reactive with patient´s plasma. Sequence-specific PCR (PCR-SSP) methods were established to confirm the mutations in CR1.

Results

The antibody was reactive with all test cells of the antibody identification panels and 29 test red cells negative for different high prevalence antigens. It was non-reactive with papain-treated red cells. Recombinant DACY protein inhibited, recombinant Kn(a) protein did not inhibit the antibody, indicating that the antigen is located on LHR-C. Molecular analysis revealed 2 unknown variants in CR1 exon 21: c.3290T>C (p.Leu1097Pro; rs200111726) and c.3298A>G (p.Arg1100Gly; rs202070239). Further assays showed inhibition of the antibody with LHR-C and LHR-C_1100Gly recombinant proteins, but not with LHR-C_1097Pro and LHR-C_1097Pro-1100Gly. This proved that the antibody is directed against p.1097Leu.

Conclusion

Using an antibody to a high prevalence antigen found in a previously transfused patient of Ethiopian origin we identified a new Knops blood group antigen located on LHR-C region of the Knops protein. The provisional antigen name (KNMB) was derived from the initials of the antibody producer. KNMB is defined by p.1097Leu and homozygosity for p.1097Pro caused the KNMB-negative phenotype. The CR1 variant rs200111726T>C is rare in the European population but frequent in the African population.

Offenlegung Interessenkonflikt:

no conflict of interest