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

The Complement Receptor 1 (CR1) carries the Knops blood group antigens (KN; ISBT 022).1, 2 CR1 consists of 30 complement control protein (CCP) domains grouped into four long homologous repeats (LHR-A to -D). The antigens KN1 to KN10 are located in LHR-D while the antithetical antigens KN11 and KN12 are located in LHR-C.3 Antibodies to Knops antigens are not clinically significant but they are often found in patients causing problems in ruling out additional relevant antibodies. In a female patient (M.B.; index case) of Ethiopian origin with transfusion history and a severe COVID-19 pneumonia, we found an antibody to a high prevalence antigen. After ruling out antibodies to a number of high prevalence antigens, an antibody to a Knops antigen was suspected, because it could be inhibited by the Knops/DACY recombinant protein. Molecular analysis of CR1 was conducted for M.B. and a nonrelated African patient (Y.U.) who was nonreactive with M.B.'s plasma. Antibody identification was performed by the gel technique in the indirect antiglobulin test using different commercial panels (BioRad, Switzerland) with untreated and papain treated red cells negative for high prevalence antigens. Recombinant proteins Chido, Rodgers, JMH, Kn(a), and DACY (imunsyn GmbH, Hannover, Germany) were used for inhibition assays of M.B.'s plasma. Furthermore, recombinant CR1 proteins for LHR-C, LHR-C_1097Pro, LHR-C_1100Gly, and LHR-C_1097Pro-1100Gly were produced according to previously described procedures and used for inhibition assays.4 Molecular analysis of CR1 was included in targeted next generation sequencing of all exons of the blood group genes encoding the systems ISBT 001 to 043. Genomic DNA from M.B. and Y.U. was sequenced according to standard protocols for amplicon-based library generation with the iSeq 100 system (illumina Inc., Berlin, Germany). For data analysis, the Variant Interpreter (illumina Inc.) and the Integrative Genomics Viewer (IGV) tools were used.5 Genotyping of CR1 c.3290T>C (rs200111726) was performed according to standard PCR-SSP protocol with forward primers for the wild type allele (5′-GTGACCTACCGCTGCAATCT-3′) and the variant allele (5′-GTGACCTACCGCTGCAATCC-3′), a reverse primer (5′-TGGAGGCGTGCATTTGTTAGG-3′), and primers for an internal control amplified from the HBB gene.6 The antibody of M.B. was reactive with all test cells of the antibody identification panels and 29 test red cells negative for different high prevalence antigens. It was nonreactive with Y.U.'s red cells only and with all papain-treated red cells. An antibody to known KN antigens was ruled out. Recombinant Kn(a), Chido, Rodgers and JMH did not inhibit the antibody, whereas, it was inhibited by the DACY recombinant protein, indicating that the corresponding antigen is probably located in LHR-C. Molecular analysis revealed 5 homozygous missense variants in CR1 of both patients: 3 known CR1 variants (c.3623A>G, c.4801A>G, c.4843A>G) and 2 variants in exon 21 c.3290T>C (p.Leu1097Pro; rs200111726) and c.3298A>G (p.Arg1100Gly; rs202070239). PCR-SSP for c.3290T>C confirmed the sequencing results. Inhibition assays using the different recombinant CR1 proteins showed inhibition of the patient's antibody with LHR-C and LHR-C_1100Gly but no inhibition with LHR-C_1097Pro and LHR-C_1097Pro-1100Gly (Figure 1). The inhibition with LHR-C and LHR-C_1100Gly proved that the antibody is directed against p.1097Leu. Using an antibody to a high prevalence antigen found in a previously transfused Ethiopian patient we identified a new Knops blood group antigen located in LHR-C region of the Knops protein. In commemoration of the index patient who died from severe COVID-19 the provisional antigen name (KNMB) was derived from the initials. The antigen number KN13 was assigned by the ISBT working party on Red Cell Immunogenetics and Blood Group Terminology at the ISBT meeting in Gothenburg, Sweden, in June 2023. KNMB is defined by p.1097Leu and homozygosity for p.1097Pro in both patients caused the KNMB-negative phenotype. The underlying CR1 variant rs200111726T>C is rare (0.02%) in the European population, but more frequent (3.9%) in the African population.7 Accordingly, only 1 of 25,000,000 Europeans (0.000004%) but 1 of 625 Africans (0.16%) are expected to be KNMB negative. The authors have disclosed no conflicts of interest. Open Access funding enabled and organized by Projekt DEAL.